86191_11

Troubleshooting and Maintenance

Troubleshooting Introduction

Troubleshooting is the art and science of remedying defects after the process has demonstrated the ability to produce acceptable production parts. Most defects respond to one of a variety of process and/or material changes. The goal is to correctly identify which problem is actually causing the defect and to know when a particular solution will work. When making adjustments consider the following recipe: (1) create a mental image of what should be happening, (2) look for obvious differences, (3) make only one change at a time, and (4) allow the process to stabilize after any change is made. Stud- ies have determined that about 60% of defects result from machines and equipment, 20% from molds and dies 10% from material, and 10% from operator error. Software programs, either already installed on the machine’s processor controller or available as a software package, can provide some help (157, 158,255,582).

With all types of equipment, materials, and products, troubleshooting guides are set up (usually required) to take fast, corrective action when products do not meet their performance requirements. This problem-solving approach fits into the overall

fabricating-design interface as summarized in the FALL0 approach (Fig. 1-1). The following provides some guidelines for obtain- ing possible solutions when confronted with common operating problems.

When possible start with feeding low bulk density plastic in a starved fed IMM. To avoid aeration and therefore increased potential for volumetric feed limitation, minimize the free-fall path from the feeder to the feed throat. If a barrel zone on the barrel constantly overrides or requires too much cooling to maintain a set point, it may be that the melting is being concentrated in that section. This can exist either because of screw design or an improper barrel heat profile. A simple and hopeful solution is to increase the melting prior to the “hot zone” of the screw (Chap. 3).

To understand potential problems and solutions (and eliminate myths), it is important to consider the relationships of machine and equipment capabilities, plastics processing variables (Chap. S), and product performances (Chap. 4). A distinction has to be made between machine conditions and processing variables. Machine conditions could include operating temperatures, back pressures, screw rotation speed, die temperature, etc. Processing variables are more specific,

969

970 11 Troubleshooting and Maintenance

such as melt conditions in the plasticator and die, melt flow rate versus temperature, etc.

Throughout this book, we have addressed the topic of “why problems develop” during injection molding and how they can be eliminated or kept to a minimum. To do the best job of eliminating or reducing problems, one must understand the complete molding operation. For example, all that may be required is some degree of incoming inspection of molding material or replacement of a worn-out screw. This book reviews the different parameters and factors that influence molding performance. Different approaches are used so that the reader can understand the complete process.

Plastic Material and Equipment Variables

At first glance the following information might appear to indicate that one cannot design and fabricate reliable products that meet tight tolerances. This is obviously not true because for over a century such reliable products have been produced in production lines and elsewhere. However, lack of familiarity with plastics or lack of proper instruction could lead to deep trouble. There are many schools worldwide that provide the required training. Even though equipment operations have understandable but controllable variables, the usual most uncontrollable

variable in the process can be the plastic material. Proper compounding or blending by the plastic manufacturer, converter, or in-house fabricator is important. Most additives, fillers, and/or reinforcements when not properly compounded will significantly influence processability and molded product performances (Chap. 12, Quality Control Variables).

In order to judge performance capabilities that exist within the controlled variabilities, there must be a reference to measure performance against. For example, the mold cavity pressure profile is a parameter that is easily influenced by variations. Related to this parameter are four groups of variables, which, when put together, influence the profile: (1) melt viscosity and fill rate, (2) boost time, (3) pack and hold pressures, and (4) recovery of plasticator. Another practical example is that the operator should be familiar with the type and degree as well as the appearance (Fig. 11-1) of equipment.

Material variables A very important factor that should not be overlooked by a de- signer, processor, analyst, statistician, etc. is that most conventional and commercial tab- ulated material data and plots, such as ten- sile strength and fatigue strength, are mean values and thereby imply 50% survival rate. Our goal is to obtain some level of reliable

Fig. 11-1 Barrel inspection can prevent major breakdown periods.

11 Troubleshooting and Maintenance 971

accountability for material variations and other variations that can occur during the product design phase or during processing.

Equipment variables A number of factors in equipment hardware and controls cause variabilities: accuracy of machining component parts; method and degree of accuracy during the assembly of component parts; temperature and pressure control capability, particularly when interrelated with time (Chap. 7, Injection Molding Boost Cutoff or Two-Stage Control); and heat transfer uniformity in metal components. Details are reviewed throughout this book, particularly in Chapters 2, 3 , 4, 7, and 10. As reviewed, these variables are controllable within limits to produce useful molded products. Improve- ments made over many decades in equipment have significantly reduced operating variabilities or limitations and will no doubt continue into the future as there seems to be almost no end to our ability to improve the performance of steels and other materials while better methods of controlling such as fuzzy

control are also being exploited, as reviewed in (Chap. 7).

Diagnosing mold imbalances The large number of variables in the injection molding process creates serious challenges to diagnosing and solving problems related to molding quality plastic parts. These problems are significantly compounded within multicavity molds. Here one has the problem of not only shot to shot variations but also variations existing between individual cavities within a given shot. This subject is reviewed in Chap. 4, Correcting Mold Filling Imbalances in Geo- metrically Balanced Runner Systems.

Definitions

When setting up troubleshooting guides, as well as reviewing any problems or even open discussions on the subject of fabricating, it is important that the terms used to identify a problem be understandable, clear, and properly defined. For example, the word “flaw” could have any of the following meanings:

Blush Burn Discoloration Fill-in

Flow marks (plastic)

Flow marks (silk screen)

Glossiness Gouge Haze Inconsistency

Marks Misalignment Nonadhesion Nonuniform

(coverage) Pit Porosity Protrusion Runs Scratches Shrink marks Smearing Speck

Void Weld line

Discoloration caused by plastic flow during molding Discoloration caused by thermal decomposition Any change from original color or unintended, inconsistent part color An excess of ink that alters the form of a screened feature, affecting clarity and

legibility

Wavy or streaked appearance of a surface

Waviness of edge or excessive linear surface texture of screened areas An area of excessive or deficient gloss Indentation that can be felt (dents) Cloudiness of an otherwise transparent part Variation of gloss, thickness of line, or surface texture not called for by master

Pits, sanding, machining, or other marks on part surface that are unacceptable The failure of the screened graphics to align with the part or its features Lack of proper sticking of the coating to the surface (chipping, orange peel)

Areas that have an insufficient or excessive coating Small crater on a surface Holes or voids (blow holes, pits, or underfills) A raised area on a surface (blister, bump, ridge) Excessive coating that causes drips Shallow grooves A depression on a surface The presence of ink on areas not called for by master artwork An included substance that is foreign to its intended composition

Failure of a plastic to completely fill a cavity A visible line or mark on a surface, caused by plastic flow molding

artwork

(bubble, inclusion)


Although we have stretched our definition of the term flaw, this example should highlight the fact that a proper definition can eliminate problems.

Defects

Different terms are used throughout the industry to identify defects in plastic materials, fabricating equipment, and products. They include adhesive stringing, alligatoring, air bubble, applesauce, arrowhead, black speck, bleed, blister, blockage, bloom, blowhole, blush, burn line, chalking, coating defect, cos- metic defect, compressive buckling, crazing, degradation, electrostatic charge, fin, fines, fish-eye, flash, fracture, flaw mark, freeze- off, frosting, gas pocket, gel, globule, hairline, migration, orange peel, paint framing, pim- ple, pin hole, pit, plastic pocket, plate-out, pocket, pock mark, puckering, run, sag, scale, segregation, shark skin, sink mark, speck, splay mark, stain, starved area, streak, stress whitening, striation, surface finish, trim, void, weld line, yellowing, and so on (1).

The usual problem can be resolved with one or just a few changes in the complete molding operation. Simplified guides to troubleshooting are given in Tables 11-1 to 11-4 Table 11-5 lists “errors” in molding and product design that can lead to problems for the molding process and/or the molded part. (See Chaps. 4, 7, and 8 on designing parts and molds.) A guide to processing temperature ranges for injection molding general-purpose grades of thermoplastics is given in Table 11-6. A more detailed guide to troubleshooting is included in the following section.

Remote Controls

To aid the manufacturing plants, remote troubleshooting has been available from different equipment manufacturers and service facilities. Users of certain microprocessor equipment need not be concerned about their plant personnel’s ability to service and maintain the equipment. Via telephone link from your computer controller to the service’s cen- tral commter. a mecialist and/or automatic

device can immediately check out conditions in your controller as well as in the complete production line. This remote diagnostic link can also be used to set up preventative maintenance programs.

Troubleshooting Approaches

It is important to use the proper approach in eliminating molding problems, The following review can help the molder find the cause and probable remedy for problems that result in unsatisfactory molded parts. In the detailed troubleshooting guide presented below, practical possible remedies have been classified according to (1) materials, (2) mold, ( 3 ) molding cycle, and/or (4) machine performance.

Faulty or unacceptable molded parts usually result from problems in one or more of three areas of operation:

1. Premolding. Material handling and stor-

2. Molding. Conditions in the molding

3. Postmolding. Parts handling and finish-

age

cycle

ing operations

Problems occurring in (1) and (3) include those involving contamination, color, static dust collection, painting, and vacuum met- alizing. The solutions to these problems are usually quite obvious, or they are very specialized. This review discusses primarily the solution of problems encountered in the molding cycle. These faults can be attributed to the following:

1. Machine 2. Molds 3. Operating conditions (time, tempera-

ture, pressure) 4. Material 5. Part design 6. Management

The analysis of most molding problems fo- cuses on the molding cycle. The molding cycle can best be described by what happens to the polymer in terms of


Table 11-1 Simplified guide to troubleshooting

Problems

Increase injection pressure X x x - - Decrease injection pressure X x Increase stock temperature X x x x Decrease stock temperature x x X X

x x x x x x x x x x

Increase holding pressure and time X X X

Decrease holding pressure X x x x X X

X and time

Increase nozzle temperature X X X X X Clear nozzle X Clear shutoff valve x x Increase screw rpm X Decrease screw rpm x x

valve X X Inject with rotating screw X x x x x x x Increase clamping pressure X Start injection later X Decrease injection speed x x x x x x x x x x x x x x Increase injection speed X Increase back pressure X x x x x x x x x

x x x x x Tighten nozzle or shutoff

x x Decrease back pressure x x X Enlarge nozzle orifice x x x x x x x x x Increase mold temperature X x x x x x x x x X Decrease mold temperature x x x x x x x x x x x Polish mold and break

comers x x x Rework mold X X Polish sprue, runners, and

gates X X Increase size of gates x x x x x x x x x x X Provide vents in mold X X X Enlarge cold slug well x x x Use dry material X x x x x x x x Use uncontaminated

Fill hopper or remove

Increase feed x x x x X Use mold release Adjust nozzle pressure X Check radius of nozzle and

sprue bushing X Reduce nozzle temperature,

break sprue later X Reduce temperature, rear

Balance mold filling; rework

Lengthen cooling

Shorten cooling and

material x x obstruction X

zonea X

runners Provide air for ejection X

and mold-open time x x X X

mold-open time X

Exception: Increase temperature for nylon.

X

X

x x

974 11


X

X

X

xx

x x

xx

x

xx

X

xx

X

xx

x

xx

xx

x

x x

X

X

x x

x

xx

x

xx

xx

xx

x

xx

x

x

x X

xx

x

x x

X

xx

X

X

x x

x

X

xx

xx

x

x

xx

x

x

X

x x

xx

x

x

xx

x

x x

xx

x

xx

x

xx

x

x x

xx

xx

x

xx

X

XX

x

xx

X

X

X

xx

x

xx

x

x

X

xx

x

x

X

X

x x

x

x X

x

xx

X

xx

x

xx

x

xx

x

x x

X

x x

x

xx

x

x

xx

x


Table 11-3 Troubleshooting guide for clear plastic moldings

Likely Solutions

Problem Other Possible Remedies ~~

Splay and splay marks, silver

Low gloss, dull or rough surface + + + * * + + Clean and polish cavity surfaces. Surface lamination, peeling * + + Possibly caused by contamination

Sinks - + + + + * - * + Reduce cooling time in mold by

streaking i ' * - * + + * * Reducecycle time.

by other resins.

using water bath. Even out part cross sections, if possible.

Blisters * + + * + Decrease screw speed.

Cloudiness, haze * * + * + + + Bubbles, shrinkage voids * - + - + + * + - Cool more slowly: Use hot water bath.

Weld lines, flow marks + + + + + * * + * Equalize filling rate between cavities. Reduce clamp pressure. Vent at parting line or weld point.

Jetting Black spots or streaks

Check core positioning. + - * * + ChecknozzIeopening.

* + * + * * Look for hot spots. Check screw clearance.

"+ = increase; - = decrease; * = check.

1. Fill time 2. Packing time and rate 3. Cooling time 4. Ejection time 5. Open time 6. Mold temperature 7. Sprue and runner design 8. Gate size and location 9. Section thickness 10. Length of flow path

This differs somewhat from the molding machine operating cycle, which is commonly divided into (1) plunger forward time, (2) mold closed time, and (3) mold open time,

a division that is convenient for setting machine controls.

Molding cycle problem analysis is concerned with the three major elements in the molding operation, as follows:

1. Injection molding machine. Is it adequate in clamping capacity, in pounds per hour capacity, in shot capacity, etc.?

2. Mold. Does it function properly? Is there an engineering design deficiency?

3. Material. Is the polymer formulation correct for the part specification, molding cycle adjustment limitations, etc.?

The performance of these three operating elements is influenced by three major


Table 11-4 Troubleshooting: simplified approach for “cause due to plastic material”

Possible Causes

Problems

Y C

0 E

s W 0 C 0 .I c)

G d -5

C a, > B 3

a, N m a, 1 C

.e - 8 8 5 3

Sink marks Flow marks Birttleness Discoloration Surface blemishes Varying shrinkage Varying dimensional

Sticking to the mold Varying strength

stability

~ ~~

x x x X X X X x x X x x x

X x x x x X X X

X X X

X X

x x

variables: (1) time, (2) pressure, and (3) temperature. Most of the difficulties occurring during the molding cycle are corrected by the adjustment of these three variables, each of which may be adjusted to a varying degree in each of the operating elements. As they are all interrelated, attention must be directed to each during the analysis of molding problems.

Finding the Fault

Before correcting a fault, one must find it. To find a fault, good quality control is necessary. Quality control should not start when a customer returns rejects. It should be a continuing process that starts when the raw material is ordered and follows each operation until the product is shipped. Also, unless the equipment is adequate and subjected to a continual, effective maintenance program, consistent-quality injection molding is not possible. Molds must be kept in good operating condition. Auxiliary equipment, such as mold heating and refrigeration units,

grinders, finishing tools, and gauges, must be readily available.

If the cause of a problem is obvious, the problem can be corrected by an adjustment in the three major variables. If the area of difficulty is not apparent, however, then each set of adjustment variables must be examined and corrections made when necessary. When a molder is starting up a new mold using a material for which certain data are available, he or she uses past experience on similar molds and materials to set up an approximate cycle. If the moldings are not perfect on this cycle, he or she will vary the pressure, temperature, and time sequences by adjusting the machine conditions until good pieces are obtained. Adjustments are always made in the machine variables first (use the “mold-area- diagram” approach (Fig. 4-1).

If acceptable pieces are not produced after machine conditions have been changed, then the design of the mold should be examined. Any changes in the mold design can affect the temperature, pressure, and time sequences, but these interrelations are difficult to calcu- late and predict.


Table 11-5 Errors in mold and product design with possible consequences for process andlor molded producta

Faults Possible Problems

Wrong location of gate

Gates and/or runners too narrow

Runners too large

Unbalanced cavity layout in multiple-cavity molds

Nonuniform mold cooling

Poor or no venting

Poor or no air injection

Poor ejector system or bad location of

Sprue insufficiently tapered

Sprue too long

No round edge at end of sprue

Bad alignment and locking of cores and

ejectors

other mold components

Mold movement due to insufficient mold

Radius of sprue bushing too small

Mold and injection cylinder out of alignment

Draft of molded part too small

Sharp transitions in part wall thickness and

support

sharp corners

Cold weld lines, flow lines, jetting, air entrapment, venting problems, warping, stress concentrations, voids, and/or sink marks.

Short shots, plastics overheated, premature freezing of runners, sink marks, andlor voids and other marks.

Longer molding cycle, waste of plastics, and pressure losses.

Unbalanced pressure buildup in mold, mold distortion, dimensional variation between products (shrinkage control poor), poor mold release, flash, and stresses.

Longer molding cycle, high aftershrinkage, stresses (warping), poor mold release, irregular surface finish, and distortion of part during ejection.

Need for higher injection pressure, burned plastic (brown streaks), poor mold release, short shots, and flow lines.

Poor mold release for large parts, part distortion, and higher ejection force.

Poor mold release, distortion or damage in molding, and upsets in molding cycle.

Poor mold release, higher injection pressure, and mold wear.

Poor mold release, pressure losses, longer molding cycle, and premature freezing of sprue.

Notch sensitivity (cracks, bubbles, etc.) and stress concentrations.

Distortion of components, air entrapment, dimensional variations, uneven stresses, and poor mold release.

Part flashes, dimensional variations, poor mold release, and pressure losses.

Plastic leakage, poor mold release, and pressure losses.

Poor mold release, plastic leakage, cylinder pushed back, and pressure losses.

Poor mold release, distortion of molded part, and dimensional variations.

Parts unevenly stressed, dimensional variations, air entrapment, notch sensitivity, and mold wear.

For details on this subject see Chapter 8.

Most molding problems are solved by varying machine conditions, and a few more are solved by additional changes of mold conditions; if, however, problems persist after try- ing both of these approaches, their cause and possible solution may be found by examining

polymer variables such as:

1. Flow characteristics. Melt viscosity at molding temperature and change in viscosity at different flow rates (shear dependence of viscosity)

978 I1 Troubleshooting and Maintenance

Table 11-6 Processing temperature ranges for general purpose grades of thermoplastics

Processing Temperature

Range

Material "C "F

ABS Acetal Acrylic Nylon Polycarbonate Polyethylene

Low-density High-density

Polypropylene Polystyrene Polyvinyl chloride,

180-240 185-225 180-250 260-290 280-3 10

160-240 200-280

180-260 rigid 160-180

200-300

356464 365-437 356482 500-554 536-590

320464 392-536 392-572 356-500 320-356

2. Thermal properties. Heat distortion (setup temperature), specific heat, heat of fu- sion, thermal conductivity, and crystallization induction time (the delay before crystallites start to form)

3. Granulations. Granulation size and shape and granulation lubrication

A guide on how to identify molded parts from a too-cold molding cycle is as follows:

1. Parts have no flash; thin sections are

2. Parts exhibit poor gloss or dull finish. 3. There are no shrink marks evident on

4. Part dimensions are at the high-

5. Packing rings (blush) are visible at the

6. Warping of parts is reduced. 7. Parts are cloudy in appearance or show

8. Parts craze when they make contact with

9. There is a visible weld line opposite the

10. The part cracks when it is flexed or

barely filled out.

the parts.

tolerance limit or are oversized.

gate.

a loss of transparency.

solvent.

gate.

bent.

11. Parts are heavier than standard. 12. Parts stick in cavity but are free on

13. Parts distort when heated, releasing

14. Durometer readings are higher

cores.

mold stress. Parts are impossible to anneal.

(harder) than standard.

Shrinkages and Warpages

An assortment of techniques are used to troubleshoot shrinkage and warpage. For example use is made of an integrated CAE tool that is based on modeling the plastic material transformation throughout the entire process (i.e., from the beginning of the filling to the end of postfilling including packing and cooling). The data generated by an integrated filling-postfilling-residual stress analysis of the plastic and by a mold cooling analysis are coupled with a structural analysis program.

This analysis approach enables the present CAE tool to capture the nonlinear interac- tion of different process and design parameters (such as injection temperature and time, postfilling time, holding time, gate and cooling channel design, and coolant temperature). It predicts the effect of any changes made in these parameters on shrinkage and warpage. In addition this CAE tool can provide hints as to the leading mechanism for shrinkage and warpage such as uneven mold cooling, nonuniform volumetric shrinkage, and orientation-induced anisotropic mechanical behavior (152).

Weld Lines

As reviewed in Chapters 6, 9, and 12, a number of approaches can be used to eliminate or significantly reduce weld line problems. There are various melt flow oscilla- tion techniques such as the Scorim Process (Cinpres-Scorim), Rheomolding Process (Thermold's RP), and the Press Alpha


Process (Sumitomo Heavy Industries and Sankyo Chemical Engineering of Japan’s PAP). Another example is the SP multi-line feed molding process where two packing pis- tons oscillate 180” out of phase to eliminate weld lines, etc.

The RP system provides 3-D orientation based on the concept of melt rheology as a function of vibration frequency and ampli- tude as well as temperature and pressure. The equipment utilizes piston-type melt accumu- lators set up adjacent to the melt stream of the plasticator. The piston oscillates back and forth. The PAP system uses compression pins that are actuated when the cavity fills. These pins protrude into the cavity and begin oscil- lating to create localized compressions. This eliminates weld lines, sinks, and warpage; reduces filling pressures and localized thin wall molding; and allows for gate positioning flexibility.

Co un terflo w

To eliminate or reduce weld lines, two sep- arate injection units are used to move melt in and out of the cavity. Melt from one unit flows through the cavity and into the secondary unit. This action is repeated and is programmed to maximize melt flow patterns (Chap. 15, Melt Counterflow Moldings).

Troubleshooting Guides

Equipment and material suppliers provide many different guides. However, the “problems-to-solutions” guidelines are usually developed when setting up a fabricating line. A simplified approach to troubleshooting is to develop a checklist that incorporates the rules of a problem-to-solution procedure:

1. Have a plan and keep updating it based on experience gained in operating the equipment.

2. Watch the processing conditions. 3. Change one condition at a time.

4. Allow sufficient time for each change. 5. Keep an accurate log of each change. 6. Check housekeeping, storage areas,

granulators, and personnel behavior. 7. Narrow down the problem to a partic-

ular area-that is, machine, mold, operating conditions, material, part design, or management (352,528). Some tips are:

0 Change the material. If the problem remains the same, it probably is not the material.

0 Changing the type of material may pin- point the problem.

0 If the trouble occurs at random, it is probably a function of the machine, temperature control system or heating bands. Changing the mold from one press to another permits a determination of whether the problem is in the machine, mold, and/or powder. If the problem appears, disappears, or changes with the operator, look for differences in the action of the operators.

0 If the problem appears in about the same position of a single-cavity mold, it is probably a function of the flow pattern and system from the front of the plunger through the nozzle, sprue, runner, and gate. It might also indicate a scored cylinder or some hang-up there. If the problem appears in the same cavity or cavities of a multicavity mold, it is in the cavity or gate and runner system. If the machine operation malfunctions, check hydraulic or electric circuits. For example, a pump makes oil flow, but there must be resistance to flow to generate pressure. Determine where fluid is going. If actuators fail to move or move slowly, the fluid must be bypassing them or going somewhere else. Trace it by disconnecting lines if necessary. No flow (or less than normal flow) in a system will indicate that the pump or pump drive is at fault. Machine instruction manuals will provide details con- cerning correcting malfunctions.

8. Set up a procedure to “break in” a new mold:


a Obtain samples and molding cycle information if the mold is new to the shop but has been run before.

shows a simplified troubleshooting guide for the operator.

The first section of Table 11-7 concerns the basic molding machine operation, listing causes and remedies of problems related to

Clean the mold. Visually inspect the mold. Obvious cor-

a

a e

a a

a

a

e

a a

a e

rections, such as improving the polish or removing undercuts, should be done before the mold is put in service. Check out actions of the mold: Try cams, slides, locks, unscrewing devices, and other

the machine, mold, material, and/or molding cycle.

Flashes

devices on the bench. Install safety devices. Operate the mold in the press and move it very slowly under low pressure. Open the mold and inspect it. Dry cycle the mold without injecting material. Check the knockout stroke, speeds, cushions, and low-pressure closing. After the mold is at operating temperature, dry cycle it again. Expansion or contraction of the mold parts may affect the fits. Take a shot using maximum mold lubrication and under conditions least likely to cause mold damage. These are usually low material feed and pressure. Build up slowly to operating conditions. Run until stabilized, at least 1 to 2 h. Record operating information. Take the part to quality control for ap- proval. Make required changes. Repeat the process until it is approved by quality control and/or the customer.

Although the cause of flash may seem el- ementary, its cure is not. An understanding of temperature, cavity pressure, and timing forms a solid basis for being able to make a long-term fix. Basically, flash is caused when the pressure of the plastic is greater than that of the clamp hold. While that sounds apparent, the cause and cure may be less than obvious. The basic problem can lie with the plastic, machine, controls, or mold; chances are good that they are all tied together. Most flash is really a temperature, pressure, or timing problem.

The viscosity of the plastic can have a lot to do with flash. Less viscous plastics will seep into the slightest crack at the parting line and act as a wedge to force apart the mold halves. For example, highly fluid nylon can overcome a clamp force of 500 tons or more. However, a melt that is too viscous will exhibit a high resistance to flow, resulting in a backup of pressure that can also flash the mold.

An important factor is temperature, since What follows is a review of the problems it plays a direct role in the viscosity of the

that are encountered during injection molding (Tables 11-7 to 11.16 and Figs. 11-2 to 11-4). The probable cause and/or possible remedy for each problem is also given. Note that there may be several causes for each difficulty, as well as several possible remedies for each cause. Any one remedy may solve the problem, but it may be necessary to try several remedies. The goal is to determine specifically what action should be taken to correct the problem. If the upfront time and expense were spent to properly evaluate how to operate the machine with the mold and material, then some troubleshooting guidelines already exist since they would have been developed during the setup time. Table 11-4

plastic melt. The higher it goes, the more fluid the melt; the lower the temperature, the more viscous. If we assume that viscosity is properly controlled, the pressure of the melt in the cavity will determine whether or not parts flash.

The pressure must be sufficient to fill the cavity, compress the plastic, and compensate for a volumetric phase change of as much as 25% as it goes from a liquid melt to solid form. During injection molding pressure, an extra 15% of plastic can be forced into the cavity. With too little pressure the result could be short shots, voids and/or sinks, or weld lines. Too much pressure and there may be flashing, burn marks, sticking in the cavity, or warpage.


Table 11-7 Machine operation basics: causes and remedies

Cause Possible Remedy

Black Specks (also see black streaks) Flaking off of burned plastic on cylinder walls

Airborne dirt. (especially polyethylene).

Black Spots Air trapped in mold causing burning.

Black Streaks Frictional burning of cold granules against one

another andlor the cylinder walls.

Plunger off center; frictional burning of

Burning in nozzle that is too hot. Wide cycling of nozzle temperature.

material between plunger and cylinder wall.

Brittleness Degradation of the material during molding. Accentuated by a part designed at the low

limits of mechanical strength.

Contamination from degradation of other

Thermal degradation of material on cylinder

Purge heating cylinder. Purge through a stiffer molding compound to

scour cylinder walls. Avoid holding plastic for long periods at high

temperatures. Cover hopper. Keep cover on virgin material.

resins previously in cylinder: Clean cylinder.

wall: Clean cylinder wall.

Vent mold properly. Redesign part. Relocate gate. Reduce injection pressure or speed. Alter flow pattern in mold by raising or

reducing cylinder and mold temperature.

Relocate plunger and allow sufficient tolerance to permit air to escape back around the plunger.

Avoid finely ground material that can come between the plunger and wall.

Use externally lubricated plastic. Lubricate regrind. Raise rear cylinder temperature. Reduce nozzle temperature. Avoid "on-off" controller. Use variable

transformer.

Materials Contaminated material: Clean. Wet material: Dry. Volatiles in material: Use material with lower

volatile content. Too much regrind: Reduce the amount of

regrind. Low-strength materials: Increase strength of

material (e.g., add more rubber to high-impact polystyrene).

Mold Part design too thin: Redesign. Gate too small: Change. Rubber too small: Change. Add reinforcement (ribs, fillets).

Low cylinder temperature: Increase cylinder

Low nozzle temperature: Increase nozzle

Molding

temperature.

temoerature.


Table 11-7 (Continued)


Brown Streaks (also see black streaks) Hang-up in cylinder or nozzle causing burning.

Either general or local overheated cylinder.

Bubbles (also see sinks) Nonuniform mold temperature.

Moisture on granules. Short shot (insufficient) plastic in the mold to

prevent excessive shrinkage caused by: Heavy sections, bosses, and ribs. Injection pressure too low. Plunger forward time too short.

Insufficient feed.

If material is thermally degrading, lower

Increase injection speed. Increase injection pressure. Increase injection forward time. Increase injection boost. Low mold temperature: Increase mold

Part stressed: Mold so that part has minimum

Weld lines: Mold to minimize weld line. Screw speed too high, degrading the material:

cylinder and nozzle temperature.

temperature.

stress.

Adjust speed. Machine

Machine plasticizing capacity too low for

Cylinder obstruction degrading the material: the machine: Change as needed.

Change as needed

Purge cylinder, remove nozzle and clean; or if necessary, remove cylinder and clean.

needed.

needed.

Nozzle temperature too high: Adjust as

Cylinder temperature too high: Adjust as

Dry granules before molding. Avoid drastic temperature changes before molding.

Rearrange water lines to obtain good mold temperature uniformity.

Short shot: Change as needed. Increase feed. Insufficient injection pressure: Adjust as

needed. Insufficient injection time: Adjust as needed. Excessive feed buildup in cylinder (cushion):

Stock temperature too high Adjust. Excessive restriction in plastic flow due to

Change as needed.

undersized gates, sprues, runners, or part design; Correct design.

Improper gate location: Change as needed. Gate land length too long: Change as needed. Machine undersized for shot size: Make

changes needed.

Charred Area Insufficient mold venting. Increase venting.



Cause

CrackingKrazing Mold temperature too low. Improper mold draft or undercuts. Ejector pins or ring poorly located. Packing excess plastic into mold.

Delamination Resin contaminated. Dimensional Variation Inconsistent machine control. Incorrect molding conditions.

Poor part design. Variations in materials.

Discoloration Burning of plastic Degradation of plastic.

Possible Remedy

Raise mold temperature. Rework mold. Locate for balanced removal force. It is better

to push off than pull off. Reduce feed. Reduce injection pressure.

Clean resin.

Machine: Make corrections for: Malfunctioning feed system in a plunger

Inconsistent screw stop action. Inconsistent screw speed. Malfunctioning nonreturn valve. Worn nonreturn valve. Uneven back pressure adjustment. Malfunctioning thermocouple. Malfunctioning temperature control system. Malfunctioning heater band. Insufficient plasticizing capacity. Inconsistent cycle, machine-caused.

Uneven mold temperature: Make changes

Low injection pressure: Increase injection

Insufficient fill or hold time: Increase

Too-high barrel temperature: Lower barrel

Too-high nozzle temperature: Lower nozzle

Inconsistent cycle: Eliminate. Mold: Make corrections for:

Incorrect mold dimensions causing parts to appear out of tolerance.

Distortion during ejection. Uneven mold filling. Interrupted mold filling. Incorrect gate dimensions. Incorrect runner dimensions. Inconsistent cycle, mold-caused.

Materials: Make corrections for: Batch-to-batch variation. Irregular particle size. Wet material.

machine.

Molding

required.

pressure.

injection forward time or injection boost time.

temperature.

temperature.

Temperature Cylinder temperature too high: Decrease

temperature.




Material contamination. Nozzle temperature too high: Decrease temperature.

Machine Clean nozzle. Inspect nozzle and sprue bushing for burrs. Purge cylinder. Reseat nozzle. Clean cylinder and check for burrs. Check for cracked cylinder. Dirty machine: Clean. Dirty hopper dryer: Clean. Dirty atmosphere; colorants can float in the air

and settle in the hopper and grinder. Take necessary precautions.

Injection end of machine too large: Adjust as needed.

Thermocouple not functioning: Adjust as needed. Temperature control system not functioning:

Heater band not functioning: Adjust as needed. Cylinder obstruction degrading the material:

Adjust as needed.

Change as needed.

Decrease screw speed. Decrease back pressure. Reduce clamp pressure. Decrease injection pressure. Decrease injection forward time. Decrease injection boost time. Slow down injection rate. Decrease cycle.

Vent mold. Increase gate size. Increase runner-sprue-nozzle system. Change gating pattern. Remove lubricant and oil from mold. Investigate mold lubricant.

Materials: Make corrections for: Contamination. Material that is not dry. Too many volatiles in the material. Material degrading. Colorant degrading. Additives degrading.

Molding

Mold

Drooling Overheated material. (The objection to nozzle

drooling is that it introduces solidified material into the part, which causes surface defects. It may also interfere with the flow and mechanical properties.)

Nozzle Use positive-seal-type nozzle. Use reverse taper nozzle. Reduce nozzle bore diameter.



Possible Remedy Cause

Ejection Poor Mold part remains in mold.

Flashing Material too hot. Pressure too high. Excessive feed. Erratic feed. Poor parting line or mating surfaces.

Molding Reduce nozzle temperature. Increase suckback. Use sprue break. Decrease material temperature. Reduce injection pressure. Reduce injection forward time. Reduce injection boost time.

Increase cold slug well. Increase runoff.

Check for contamination. Dry the material.

Mold

Materials

Make adjustments for roughness or undercuts

Eliminate excessive mold packing. Change inadequate knockout system. Correct insufficient taper or draft.

in mold.

Erratic Cycle Holding mold open various lengths of time.

Erratic pressures.

Erratic feed. Nonuniform mold temperature. Nonuniform cylinder temperature (cycling).

Maintain an overall constant cycle time by the

Ensure sufficient pressure to fill the mold use of mold-open timers.

consistently.

Check pressure system for leaks, etc. Check feeding mechanism. Mold. Use mold temperature control.

Provide proper waterlines in the mold. Allow proper venting of the mold. Provide proper hookup for water through the

mold. Cylinder

Check temperature controls to ensure proper

Use the best temperature controls available. Check line voltage to the machines for

Ensure that heater bands are working properly. Have material temperature constant from one

drum to another before placing material in the hopper.

operation.

consistency.

Adjust material flow that is too soft for parts. Temperature: Make correction for:

Cylinder temperature too high. Nozzle temperature too high. Mold temperature too high.




Mold deficiency. Erratic cycle time. Insufficient clamp.

Flow Lines and Folds Material temperature too low. Mold temperature too low. Gates too small, causing jetting. Nonuniform section thickness.

Gate (splay, blush, lamination, dull spots) Melt fracture as material expands entering mold. Material too cold. Mold too cold. Slow injection speed. Insufficient pressure. Plunger dwell too long. Contamination of material. Excessive mold lubricant.

Molding: Adjust for: Clamp pressure too low. Injection pressure too high. Injection time too long. Boost time too long. Injection feed too fast. Unequalized filling rate in cavities. Interrupted flow into cavities. Feed setting too high. Inconsistent cycle, operator-caused.

Projected area of the molding parts too large

Machine set incorrectly. Mold put in incorrectly. Clamp pressure not maintained. Machine platens not parallel. Tie-bars unequally strained. Inconsistent cycle, machine-caused.

Cavities and cores not sealing. Cavities and cores out of line. Mold plates not parallel. Insufficient support for cavities and cores. Mold not sealing off because of foreign

material (flash) between surfaces. Something other than flash keeping the mold

open (e.g., foreign material in leader pin bushing so that leader pin is obstructed when entering the bushing, keeping the mold open).

Insufficient venting. Vents too large. Land area around the cavities too large,

reducing the sealing pressure. Inconsistent cycle, mold-caused.

Machine: Adjust for:

for clamping capacity of machine.

Mold: Make correction for:

Increase plastic temperature. Increase mold temperature. Enlarge gates and reduce injection speed. Redesign part to obtain greater uniformity of

Eliminate heavy bosses and ribs. section thickness.

Molding Mold temperature too low: Adjust. Nozzle temperature too low: Adjust. Injection speed too fast: Adjust. Increase injection pressure. Change injection forward time. Use minimum lubricant. Change lubricant.




Mold Runners and gates too large or small. Excessive mold heat, particularly at sprue or Lower mold temperature.

Increase gate size. Change gate shape (tab or flare gate). Increase cold slug well. Increase runner size. Change gate location. Increase venting. Radius gate at cavity.

Dry material. Remove contaminants from the material.

center gates.

Material

Increase plastic temperature. Lengthen cycle. Use restricted nozzle.

Granules Unmelted Too low plastic temperature. Too fast a cycle for cylinder capacity. Insufficient restriction to flow. Insert Cracking Insufficient material around insert.

Jetting Resin too cold.

Long Cycles High material temperatures. Mold temperature excessive. Erratic cycle time.

Insufficient heating capacity.

Inadequate cooling of local heavy section. Excessive flash requiring operator trimming

Excessive delay in machine operation. (see flashing).

Slow setup in mold.

Low Heat Distortion Temperature Variations in section thickness.

Too-low mold temperature. Incorrect cylinder temperature relative to

Excessive feed. mold temperature.

Excessive pressure.

Excessive plunger dwell. Excessive mold temperature variation

between front and back.

Poor part design: Change as needed. Contamination: Remove.

Injection too fast: Adjust. Gate too small: Change. Gate land too long: Change.

Lower temperatures. Reduce mold temperature. Maintain a constant overall cycle time by the

Change mold to larger press and/or preheat use of a mold-open timer.

the material.

Locate bubbles to cool area. Use quenching

Reduce machine dead time as much as

Reduce mold temperature. Try more heat-resistant grade of material.

bath.

possible.

Maintain as uniform a section thickness as possible.

Increase the mold temperature.

Select proper cylinder temperature and mold

Reduce feed and starve feed if

Reduce pressure. Use a minimum plunger forward time and

temperature.

possible.

dwell.




Gate slow freezing.

Short Shot Cold material. Cold mold. Insufficient pressure. Nonuniform mold temperature. Insufficient feed. Entrapped air. Insufficient external lubricant. Insufficient plunger forward time. Improper balance of plastic flow in

multiple-cavity molds. Insufficient injection speed. Small gates. Shot size larger than machine capacity.

Keep both front and rear mold temperatures

Reduce size of gate. as nearly the same as possible.

Molding condition causes: Correct for: Injection pressure too low. Loss of injection pressure during cycle. Injection forward time too short. Injection boost time too short. Injection speed too low. Unequalized filling rate in cavities. Interrupted flow in cavities.

Inconsistent cycle, operator-caused.

Temperature-related causes Raise cylinder temperature. Raise nozzle temperature. Check pyrometer, thermocouple, heating

Raise mold temperature. Check mold temperature equipment.

Runners too small. Gate too small. Nozzle opening too small. Improper gate location. Insufficient number of gates. Cold slug well too small. Insufficient venting. Inconsistent cycles, mold-caused.

Machine causes: Correct for: No material in the hopper. Hopper throat partially or completely

Feed control set too low. Feed control set too high, which can cause

bands system.

Mold-related causes: Correct for:

blocked.

lowering of injection pressure in a plunger machine.

Feed system operating incorrectly. Plasticizing capacity of machine too small for

the shot. Inconsistent cycles: machine-caused,

operator-caused, or mold-caused. Malfunctioning of return valve on tip of screw.

This is usually indicated by screw turning during injection.

Shrinkage (also see warpage) Excessive shrinkage and warpage are

usually caused by design of the part, gate location, and molding conditions. Orientation and high stress levels are also factors.




Silver Streaks Excessive nozzle, torpedo, or cylinder

Exceeding plasticizing capacity of machine in

Variation in temperature of material being

Plastic temperature too high.

temperatures.

pounds per hour.

placed in hopper.

Injection pressure too high. Air trapped between granules in cold end of

Mold temperature too low. machine.

Injection speed too fast. Intermittent flow in the cavity.

Moisture on granules. Lack of external lubrication. Excessive external lubrication. Nonuniform external lubrication. Mixture of coarse and fine granules (as with

regrind).

Sink Marks (also see bubbles) Insufficient plastic in mold to allow for

shrinkage due to:

Thick sections, bosses, ribs, etc. Not enough feed. Injection pressure too low.

Plunger forward time too short.

Unbalanced gates. Injection speed too slow.

Plastic too hot.

Machine

Reduce nozzle temperature first, then cylinder

Lengthen cycle or operate mold in machine

Preheat material or install hopper dryers to

temperature.

with larger heating capacity.

maintain material temperature.

Reduce injection pressure.

Reduce rear cylinder temperature and avoid use of regrind.

Operate with no cushion of material ahead of plunger.

Mold Raise mold temperature. Vent mold.

Balance gates.

Relocate gates. Maintain uniform mold temperature. Obtain as uniform a section thickness as

possible. Material

Dry material prior to use or use hopper dryer. Avoid exposing material to drastic temperature changes prior to molding.

Add zinc stearate, often necessary with regrind.

Avoid use of nonuniform material, or screen to give uniform granule size.

Blend with nonlubricated or regrind material. Allow longer blending time, or add a little

more lubricant and blend.

Material

Dry material. Add lubricant. Reduce volatiles in material.

Piece cooled too long in mold, preventing shrinking from the outside in: Make needed changes.

Changes in cooling conditions

Shorten mold cooling time. Cool part in hot water.




Piece ejected too hot. Molding Variation in mold open time. No cushion in front of injection ram with

Insufficient feed Adjust. Increase the injection pressure.

Increase the injection forward time. Increase the boost time. Increase the injection speed. Increase the overall cycle. Change method of molding (intrusion). Inconsistent cycles, operator-caused: Correct.

Increase plasticizing capacity of the machine. Make cycle consistent.

Temperature: Adjust for: Material too hot, causing excessive shrinkage. Material too cold, causing incomplete filling

and packing. Mold temperature too high so the material on

the wall does not set up quickly enough. Mold temperature too low, preventing

complete filling. Local hot spots on the mold. Mold temperature control system

volumetric feed.

Too much cushion in front of ram.

Machine changes

malfunctioning. Mold

Increase the gate size. Increase the runner size. Increase the sprue size. Increase the nozzle size. Vent mold. Equalize filling rate of cavity. Prevent interrupted flow into the cavities. Put gate in thick sections. Reduce uneven wall thickness when possible. Use cores, ribs, and fillets. Inconsistent cycle, mold-caused: Correct.

Sprue Sticking Undercuts in mold. Mold rough surface.

Excessive pressure. Hot material. Excessive size of sprue. Insufficient draft. Improper fit between sprue bushing and

Too much feed. Long plunger dwell. Vacuum under deep draw part. Variation of mold open time. Core shifting.

nozzle.

Molding Use sprue break (machine moves back

slightly, breaking contact between nozzle and sprue).

Increase suckback. Reduce feed. Reduce injection pressure.

Reduce ram forward time.

Reduce injection boost time. Reduce material temperature. Reduce cylinder temperature. Reduce nozzle temperature. Use more mold-release agent.




Use proper mold-release agent.

Reduce material feed. Reduce injection pressure. Reduce injection forward time. Reduce injection boost time. Reduce mold temperature. Increase overall cycles. This lowers

Unbalanced gates in multicavity or single-cavity molds with two or more gates.

temperature, making the part more rigid, and increases the amount of shrinkage.

Make inconsistent cycles (operator-caused) consistent.

Materials Remove contamination in material. Add lubricant to the material. Dry the material.

Repair any malfunctioning of the knockout

Lengthen insufficient knockout travel

Make inconsistent cycles (machine-caused)

Check to see if platens are parallel. Check the tie-rod bushings.

Increase the pull-out force of the sprue-puller

Reduce mold temperature. See if sticking is caused by short shot not

Remove undercuts. Remove burrs, nicks, and similar irregularities. Remove scratches and pits. Improve the mold surface. Restone and polish using movement only in

the direction of ejection. Increase the taper. Increase the effective knockout area. Decrease the gate size. Add additional gates. Relocate the gates. Equalize the mold-filling rate. Prevent interrupted filling. Determine whether the part is strong enough

Radius and reinforce parts, giving greater

Machine

system.

distance.

consistent.

Mold

system.

engaging knockout system.

for ejection.

rigidity. Surface Defects Slow injection. Unbalanced flow in gates and runners. Poor flow within mold cavity.

Molding Reduce screw speed. Reduce back pressure.




Cold material. Mold too cold. Injection pressure too low. Water on mold face. Excess mold lubricant on mold. Not enough plastic into mold to contact mold

metal at all points.

Excessive internal or external lubricants. Poor surface on mold.

Tearing Mold part tears.

Voids (also see bubbles) Trapped gases.

Warpage (also see shrinkage) Part ejected too hot. Plastic too cold. Variation of section thickness or contour of

part.

Alter injection speed. Increase injection pressure. Increase injection forward time. Increase booster time. Increase cycle.

Temperature

Too low or too high cylinder temperature, depending on problem: Change temperature profile of cylinder.

Too low mold temperature: Raise mold temperature.

Material Nonuniform mold temperature: Check.

Use uniform-size particles. Reduce the amount of fines. Use minimum amount of lubricant. Change type of lubricant.

Increase runner extension. Increase runner. Polish sprue runner and gate. Open gate or change gate to tab. Change gate location. If jetting, flare gate or

Increase venting. Improve mold surface. Clean mold surface. Water caused by leaks and condensation:

Remove. Flow over depressions and raised section:

Change the part design. Try localized gate heating.

Check nozzle for partial obstruction. Check sprue-nozzlexylinder system for

Mold

use tab or flared gate.

Machine

restrictions and burrs.

Inadequate core cooling: Correct. Hot core pins: Make needed changes.

Vent cavities. Provide for thin to thick transition. Mold surface too cold Correct. Resin too hot: Correct. Lack of pressure: Open gate, runner and

decrease gate land length.

Molding Increase cycle time. Increase injection pressure without excessive

packing.



Possible Remedy Cause

Too much feed.

Unbalanced gates on parts having more than

Poorly designed or operated ejection system. Mold temperature nonuniform. Excessive material discharged from or packed

into the area around the gate.

Increase injection forward time without

Increase injection boost time without

Increase the feed without excessive packing. Lower the material temperature. Keep packing at a minimum.

Increase injection speed. Slow down ejection mechanism. Anneal parts after molding to reduce

warping. Cool in shrink fixture. Cool in water. Make cycle consistent.

Use quicker-curing material.

Change gate size. Change gate location. Add additional gates. Increase knockout area. Keep knockouts even. Have sufficient venting, especially for deep

Strengthen part by increasing wall thickness. Strengthen part by adding ribs and fillets. If differential shrinking and warping caused

by irregular wall section, core, if possible, or change the part design.

To reduce warpage, reduce mold temperature to stiffen the outer surface.

To decrease shrinkage, raise mold temperature to increase packing.

Check mold dimensions. Wrong mold dimensions may cause parts to appear to have shrunk excessively.

excessive packing.

one gate. excessive packing.

Material

Mold

parts.

Weak Parts Part “breaks.”

Weld Lines/Flow Marks Plastic too cold. Excess mold lubricant on mold. Weld line too far from gate. Air unable to escape from mold fast enough. Section thickness variation within part. Mold too cold. Insufficient pressure. Slow injection speed.

Excessive moisture in resin: Remove. Stock temperature too high: Reduce. Contamination: Remove. Poor welds: Correct.

Molding: Correct for: Injection pressure too low. Injection feed too slow.

Temperature: Correct for: Too low cylinder temperature. Too low nozzle temperature. Too low mold temperature. Too low mold temperature at spot of weld.

994 I 1 Troubleshooting and Maintenance



Gas trap. Uneven melt temperature.

Insufficient venting of the piece: Add runoff at weld. Runner system too small: Correct. Gate system too small: Correct. Sprue opening too small: Correct. Mold shifting, causing one wall to be too thin:

Make necessary adjustments. Part too thin at weld: Thicken it. Unequal filling rate: Equalize it. Interrupted filling: Correct. Nozzle opening too small: Correct. Gate too far from the weld: Add additional

Mold

gates (these might add additional welds but place them in a less objectionable location).

freezing: Make needed changes.

Make necessary adjustments.

Wall section too thin, causing premature

Core shifting, causing one wall to be too thin:

Machine Plasticizing capacity too small for the shot:

Excessive loss of pressure in the cylinder Make needed changes.

(plunger machine): Correct. Materials

Contaminated material, which can prevent

Poor material flow: Lubricate material for knitting properly: Purify.

better flow.

Although the physical mechanisms that cause flash may be relatively well understood, the operating conditions leading to flash are often not. Troubleshooting skills may reside solely in the heads of seasoned machine operators. Table 11-8 provides an approach to solutions. Curing flash as well as other injection molding problems requires a systematic ex- amination of the plastic, machinery, and processing parameters.

Hot-Runners

Details on hot runners are given in Chap. 4. Troubleshooting information on hot runners appears in Table 11-10.

Hot-Stamp Decorating

Details on hot-stamp decorating are given in Chap. 10. Troubleshooting information on hot-stamp decorating appears in Table 11-11.

Injection Structural Foams Paint-Lines

The information given in Table 11-9 per- tains to troubleshooting the injection molding of structural plastic foam. Details on this process are given in Chap. 15.

Details on painting are given in Chap. 10. Troubleshooting information on painting appears in Table 11-12.


Table 11-8 Guide to flash: causes and remedies Symptom Source Cause Where to Look

Flash part Resin Too wet. Contamination.

Regrind.

Additives.

Improper processing.

Equipment Clamp tonnage too low. Improperly sized.

Hydraulic pressure too low.

Injection pressure: High fill can result in cavity pressure greater than clamp force.

Injection pressure: High hold pressure can result in overpacking or flash.

Transfer too late. Long fill phase can result in overpacking or flash.

Injection speed too fast. Temperature too high. Temperature too low. Too much material.

Platens not parallel. Platen rigidity.

Mold rigidity.

Plunger too far forward. Wrong controller. Controller malfunction. Screw worn.

Mold Vent depth too shallow. Vents clogged. Mold worn.

Corekavity slippage. Imbalance in runner. Clamp pressure too high.

Check drier, resin storage, and handling. Check material for type of contamination.

If metal pieces, check for source and repair. If other, check storage and . handling. Interim fix, filter resin, melt.

Check allowable % regrind with resin supplier. Is it contaminated?

Check operating specs with resin supplier.

Compare machine settings with operating parameters for resin and machine.

Check pressure reading and adjust. Runners, gates, cores, material, part size

and/or configuration not considered. Recalculate. Move mold to proper size machine. Adjust die height.

Check oil pressure, also for leaks. Adjust valve(s).

Compare settings with specs.

Check and adjust.

Check transfer point; set for earlier switchover.

Back off by adjusting flow-control valve. Check heating elements, screw speed. Check heating elements, screw speed. Check for nozzle drool; reduce cushion.

Check for screw or barrel wear. Adjust positioning. Usually stationary platen problem. Check

clamp tonnage. Check for slippage at parting lines. Can

use Prussian blue to check fit. Parts overpacked or flashing. Back off. Check resin and machine specs. Check controller troubleshooting guide. Inspect and repair or replace; readjust

Regrind vents. Inspect and clean vents. Reached design limit for parts. Clamp

Prussian blue to check fit; realign. Poor design; modify or replace. Repair mold and adjust tonnage or run in

speed-pressure.

tonnage too high? Rework or replace.

smaller machine.



Symptom Source Cause Where to Look

Runner flash Resin

Equipment

Flash and burn Resin

Equipment

Mold

Flash and short Equipment

Mold

Gate, runner, sprue.

Parting line edges not sharp, clean.

Manifolds, pockets not supported properly.

Parting lines not matched.

Viscosity too high.

Fill pressure too high, causing deflection in runner.

Injection speed too fast.

Injection speed too slow.

Clamp force too low. Mold temperature too

low. Causes higher viscosity and greater cavity pressure.

Nozzle temperature too low. Causes higher melt viscosity.

Contamination. Resin specs not

followed. Screw speed too fast.

Temperature too high.

Vents clogged. Damaged. Vent holes too shallow. Cushion incorrect. Nonreturn malfunction. Screw or barrel worn.

Clamp force uneven. Platens not parallel. Imbalance in runner. Core/cavity shift.

Uneven mold heating.

Considered in sizing? Runner vented?

Regrind and repair. Adequately sized, position?

Repair mold.

Check for core or cavity slippage. Prussian blue, short shoot, and measure walls.

Platen and mold base rigidity. Check resin-processing specs against

Reduce pressure. machine settings.

Adjust flow-control valve and/or

Can use accumulator-assisted

Check clamp pressure and sizing. Check heater elements.

increase temperature.

injection rate.

Place insulator between nozzle and sprue or use sprue break if possible.

Identify source, eliminate cause, filter melt. Compare machine settings with

material specs; follow directions. Reduce speed and/or adjust transfer

point. Check screw speed and pressure; check

heaters. Inspect and clean. Repair or replace. Regrind to increase depth. Adjust cushion. Check valve and repair or replace. Slippage occurring: Check settings and

repair or replace. Check machine and resin settings.

Check tie-bar loading. Inspect and adjust. Check design; repair or replace. Shoot short and measure opposing

Check heating elements. wall thicknesses; realign.


Table 11-9 Structural foam basics: causes and remedies

Mold Nor Completely FilIed Cause

1. Shot size too small. 2. Insufficient blowing agent or inefficient

3. Insufficient venting of mold. use of blowing agent.

Rough Surface Cause

1. Mold temperature too low. 2. Injection rate too low. 3. Injection pressure too low. 4. Poor resin flow.

Postmold Swelling of Parts Cause

1. Cooling cycle too short. 2. Mold temperature too high. 3. Shot size too large.

Density Too High Cause

1. Shot size too large. 2. Insufficient blowing agent or inefficient

3. Blowing agent decomposing too early. use of blowing agent.

4. Intrusion molding during plastication.

Cycle Too Long Cause

1. Mold temperature too high. 2. Stock temperature too high.

3. Insufficient cooling of mold.

4. Blowing agent level too high. Cell-Size Too Large: Nonuniform

Cause 1. Injection speed too slow. 2. Melt viscosity too low. 3. Density of part too low. 4. Blowing agent decomposed too early

5. Expansion taking place in cylinder in cylinder.

or nozzle.

Corrective action 1. Increase shot size. 2. Use additional 0.5% blowing agent

or increase stock temperature. 3. Increase size and number of vents

if 1 and 2 do not correct fault.

Corrective action 1. Increase mold temperature. 2. Increase injection rate. 3. Increase injection pressure. 4. Increase stock temperature or

use higher-melt-flow-resin.

Corrective action 1. Increase cooling cycle time. 2. Reduce mold temperature. 3. Reduce shot size. 4. If all of the above fail, use postmold

quenching of the part in water.

Corrective action 1. Reduce shot size. 2. Use additional 0.5% blowing agent

or increase stock temperature. 3. Lower temperature in rear zones or use

higher-temperature blowing agent. 4. Install shutoff nozzle; increase screw

forward time,

Corrective action 1. Reduce mold temperature. 2. Reduce stock temperature.

(Do not go below decomposition temperature of the blowing agent).

3. Increase flow of cooling medium through mold. Use postmold quenching.

4. Reduce level of blowing agent.

Corrective action 1. Increase injection speed. 2. Lower temperature profile. 3. Increase shot size. 4. Use nucleating agent.

5. Use lower-melt-index resin.

6. Install shutoff nozzle on machine.


Table 11-10 Hot runner basics: causes and remedies

Problem Problem Identifiers Possible Solutions

Gate vestige and quality stringing at the gate,

Visual part inspection reveals

drooling, tip freeze-offs, large gate vestige protrusions, poor appearance (blush or haze) in gate area.

Temperature control

Inconsistent quality. Temperature alarms. No load response. Inability to reach molding temperature. Excessive startup times. Hot spots on parts, leader pin bindings, or excessive cycle times. Nozzle gets hot, but controller does not shut off. Temperature keeps rising. Inadequate cooling in mold plates and cavities can be identified by extended cycles or gate-area distortions on parts.

Solutions are case by case, but often involve gate cooling adjustments or modifications of gate geometry. Check gate detail to ensure it is built to system manufacturer’s specs. Check for proper probe location. Check gate diameter and probe for erosion or wear. Investigate purchasing systems with a thermocouple at the tip to sense temperature conditions at the orifice. Ensure all heater thermocouples and temperature controllers are functioning and properly calibrated. Check for excessive manifold heat. Check back pressure, shear heat from improper gate location, and excessive injection pressure. Check ohm reading of each zone and compare it against the reading of the system when it is first received. Install special filters. Inspect parts for damage under a microscope. Check water supply and temperature to gate areas. Verify nozzle set-back.

installing it in a machine. Use systems that protect the heating circuit (such as conductive systems that remove generated heat, systems with no air gaps between the heater and components, systems thermally balanced to avoid hot spots, and systems with no exposed heaters that can oxidize). Ensure all heaters are receiving proper voltage supply. Check incoming voltage with a voltmeter. Check heater resistance (overall wattage) at rated voltage and compare it against line voltage: Replace heaters with proper wattage. Install power correction capacitors if needed. Retune controller or use autotuning control system. Run at faster cycle times. Regarding mold temperature control, recircuit water lines, isolate mold plates, and keep up-to-date with cooling technology.

Benchtest mold with controller prior to

I1 Troubleshooting and Maintenance 999


Problem Problem Identifiers Possible Solutions

Material leakage Visual inspection. Screw bottoms out on fill cycle. Short shots. Heaters may burn out or heat load will substantially increase if leakage in the manifold is excessive.

Color changes

Wiring

Plugged gates

Original color bleeds out of the gate after purging. Original color blemishes new production. Too much time required to make color change.

Controller reads fault (no power output for a particular zone, loss of set-point temperature). Bad parts.

Short shots or nothing at all.

Check hot-runner stack-height dimensions for proper system preload and system components for obvious mechanical failures. Verify proper mold startup and processing parameters. Disassemble tool and check all seals and sealing surfaces. Check all system integration dimensions against general assembly drawings.

materials flow with minimal dead spots. Follow color change procedures set by the hot-runner manufacturer. Open mold and remove insulating (frozen) material layer at least in. back from the tip. Check to ensure that material flow exiting the manifold is smaller in diameter than the drop diameter. Clean manifold; ensure that plugs are flush and flow-channel sidewalls are smooth. Check the flow path after the next color change to ensure that the flow channel does not have laminar flow, allowing an annular ring of the original color to form. Fill system with neutral color first.

visually inspect suspected defective heaters for wire pinching or slices. Check electrical wiring with a good digital multimeter. Check for proper ohm readings for the heater and from the heater to ground. Hire experienced personnel or better train existing employees. Replace failed components or purchase systems with manifold wires sheathed in stainless steel and heat lead protection.

Open the mold. Purge the entire system (the gate and manifold). Clean purging from the drops and remove at least 4 to in. of material from the end of the probes. Purchase nozzle filters. (Warning: Filters may reduce gate blockage but will create a pressure drop during injection and decompression, thereby causing high gates and stringing.)

Buy a system offering unrestricted

Disassemble manifold plates and

1000


11 Troubleshooting and Maintenance

Problem Problem Identifiers Possible Solutions ~~~ ~~

Material Silver streaks (splay), brown streak

in parts. Part mechanical integrity also may be affected.

Ensure that all control zones are at degradation discolorations, and black specks correct processing temperature.

Downsize machine barrel, lower temperature, reduce shot size. Check for tooling marks and mismatches with hot-runner system. Use a system that is thermally balanced. Better training.

Molders should purchase balanced systems, especially when using hot runners in multicavity and family molds. Perform mold flow analysis.

Flow balance Short shots, flash, part dimensional variations from cavity to cavity.

Table 11-11 Decorating basics: causes and remedies

Problem Cause Solution

Flattened characters when tipping raised letters or beads

Distorted imprint on plastic part

Blurred image or imprint Weak impression or no

Inconsistent transfer of imprint

decoration to the parts

Decoration fails to adhere to plastic

Die too hot, too much pressure on die, or excessive dwell time.

Skidding of die on contact with foil due to fixture deflection.

Excessive die heat. Insufficient air pressure.

Variation in parts (thickness, warpage, sink marks).

Heat variations at die face.

Insufficient cure time or strip delay.

Air trapped under foil.

Contamination on part.

Wrong foil used.

Special coating on plastic.

Lower heater setting or head pressure. Reduce dwell timer setting.

Realign die on head slide so that it is directly under the press ram. Modify or redesign fixture.

Reduce heater temperature. Check for obstruction in air line or

need for larger air line. Modify heat, pressure, dwell time

settings to optimize for parts from all mold cavities.

Check that heat control is holding to preset tolerances. Look for air gaps between heater block and die dove-tail due to die shim, or heat loss due to riser block.

problem, manually lay a section of foil on the part, cycle the press, and peel off the foil. If the imprint is good, stripping must be adjusted, either by reducing head upstroke speed, or adjusting stripper bar springs.

Check foil feed and die contact with foil to determine cause of entrapment.

Determine what the contaminant is and its source and eliminate it.

Check compatibility of foil; replace with correct foil.

Determine what coating is and change to a foil that is formulated to be compatible.

To determine if stripping time is the



Problem Cause Solution

Imprint deeper on one end of part than the other

Flaking in decoration, feathery edges, or fill in

Loss of gloss in foil Inconsistent imprints in

multiple-part setups Inconsistent transfer of

decoration to the parts

Uneven imprint

Repeated void in decoration at same location

Machine is not level.

Dwell time too long.

Dwell time too long. Part irregularities, that is, sinks,

Die temperature too low, or thickness variations.

inadequate pressure.

Dwell time too short. Off-spec foil.

Unevenly heated die. Die head cocked off center. Fixture may have shifted.

Molded-in part feature, that is, rib or boss, unsupported by fixture.

Level machine and mount die directly under arm.

Shorten dwell time by adjusting

Shorten dwell time. Shim the fixtures to compensate for

part irregularities. If die impresses the plastic, increase

die temperature. If imprint does not impress the plastic, boost pressure on die.

Lengthen dwell time. Manually place a section of foil from a

roll that has run well on a part and cycle the press. If it prints well, replace the roll on the machine.

Check die temperature, look for cartridge heater outage. Determine why die head is cocked and realign as necessary. Reset fixture as needed.

airflow control valves.

Shim or modify fixture.

Granulator Rotors

Details on granulators are given in Chap. 10. Troubleshooting information on rotors appears in Table 11.13.

Auxiliary Equipment

Details on auxiliary equipment are given in Chap. 10. Troubleshooting information on different equipment appears in Table 11.14.

Screw Wear Guide

This section concerns how screw wear affects the performance of molded parts. Al- though injection molding machines have several parts that wear, the barrel and screw contact surfaces receive most of the attention. Corrosive, abrasive, and erosive wear are primarily caused by agents or additives in the

machine and plastic. To combat these situa- tions, the barrel and screw are made of metals selected for their resistance to damaging agents (as reviewed in Chaps. 2 and 3).

When rubbing contact does occur, noises are usually emitted and these provide a means of locating and analyzing the type of wear. If locations exist where contact cannot be avoided, such as at the feed section, and melt film strength is inadequate, then plasticator design modifications should be considered (Fig. 11-1).

Wear created when the screw flight rubs against the barrel wall, however, is usually due to mechanical conditions that can be pre- vented or controlled. Also known as adhesive wear, the problem is caused by raised points on one surface contacting the raised points on the mating surface as they slide past each other. With enough force applied, a protective oxide layer is removed and the contact points yield and form a series of molecular bonds. As the surfaces continue to slide past each other, shearing and tearing occur, but

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Table 11-13 Granulator basics: causes and remedies

~~

Problems Solutions

Melted or burnt material to open rotor.

Diminished Check rotor drive throughput components. Replace

Airflow restricted. Change

belts. If direct drive, replace coupling. Check blades for sharpness. Resharpen or replace.

Restrict infeed.

worn bearings. Change accordingly.

Coat with impact-resistant

Rotor stalls Overload condition.

Rotor vibrating Rotor journals bent or

Rotor pitting or excessive wear C02 weld wire.

not necessarily at the same location. Even- tually, particles break loose, smearing and galling other sections.

Wear severity is a function of the strength of the momentary bonds formed by the metal-to-metal contact. Metals have different affinities for each other, with like metals forming the strongest bonds. Hardening of the surfaces reduces deformation of contact points under load and therefore lessens the bonding between surfaces. Various techniques are used by suppliers of barrel and screws (and other machine parts) to reduce wear. These include heat treating, flame hardening, nitriding, plating, and intrinsically hard coating with cobalt and nickel alloys inlayed to the barrel bore.

Metal-to-metal contact is expected and the construction materials are selected to minimize this and other forms of wear that the process introduces. This is certainly true at start-up, when the screw rests on the bottom of the barrel and then rotates up and around the side of the bore until the plastic acts to center it. Although it is difficult to prevent start-up contact, steps should be taken to minimize it and prevent rubbing during molding operation. The sliding surfaces must either be separated with a minimum clearance or have a lubrication film between them.

The viscous plastic melt pumped through the plasticator should make an excellent lubricant. Sufficient film pressure must be

developed between the flight outside diameter and the barrel wall to center the screw and keep the surfaces separated from each other. This pressure can in part be hydrostatic, which is developed in the melt at certain points by screw geometry. However, mostly it is hydrodynamic, developed by the surface speed of the screw flight when eccentric to the barrel wall and the pumping of the viscous melt into a converging wedge. Pressure builds as the clearance decreases and thus forces the screw back toward the center.

Contact occurs if the melt is not present or is inadequate in film strength to prevent contact. Owing to design, construction, installation, or operation there are certain conditions that can overcome the film-bearing support. The screw centerline is not coincident with the barrel bore and, at some point depending on clearance, contact will be made. Design and installation include aligning the barrel assembly to its driving force. Different problems can occur during installation and particularly operation, such as developing a bent screw. Depending on the amount and where they are bowed, screws will be constrained inside the barrel and form a wavelike series of contacts.

When the barrel assembly is aligned at installation, the foundation must maintain this position under all operating conditions. If the barrel sags or moves radially during screw rotation, contact is likely. Bolting hardware to the barrel without a provision for flexing can force the barrels out of alignment.

The cumulative thermal expansion of the barrels will cause significant growth in the ax- ial direction. Also, the screw must be free to grow radially outward without loss of clearance, if it is hotter than the barrel or made of a more thermally expansive material such as stainless steel.

One of the less commonly discussed as- pects of maintaining injection molding machines is the inspection of screws and barrels. From time to time, they should be examined to determine whether they are in condition to render the services expected. Screws do not have a continuous outside diameter. This requires special techniques in manufacturing and inspection. The following methods give reliable results and save time (158).


Table 11-14 Auxiliary equipment basics: causes and remedies

Problem Possible Causes Suggested Solutions

Process air temperature too low

Process air temperature

Material not drying too high

Stalled machine

Material overheating

Too many fines in material

Bearing overheating

Excessive knife wear Knife breakage

Dryer Incorrect temperature selected

Controller malfunction. on control panel.

Process heating elements.

Hose connections at wrong location.

Supply voltage different from that of dryer.

Thermocouple not located properly at inlet of hopper.

Process andlor auxiliary filter(s) clogged.

Incorrect blower rotation. Regeneration heating elements

Desiccant assembly not rotating. in-operative.

Material residence time in hopper too short.

Moist room air leaking into dry process air.

Desiccant contaminated. Granulators

1. Overloading. 2. Worn, damaged, or improperly

set knives. 3. Screen andlor blower chute

blockage. 4. Drive-belt-slippage. 5. Loss of power.

6. Motor running in reverse.

See items 1,2,3, and 6. Screen too small.

Worn, damaged, or improperly

Failure to lubricate properly. set knives.

Too much tension on drive

Highly abrasive material. Tramp metal in scrap.

belts.

Dial in correct temperature.

Check electrical connections, replace controller if necessary.

Check electrical connections, replace elements if necessary.

Check to make sure delivery hose is entering bottom of hopper.

Check supply voltage against nameplate voltage.

Secure thermocouple probe into coupling at inlet of dryer.

Clean filter(s).

Check rotation. Check electrical connections; replace

Check motor electrical connections. elements if necessary.

Replace motor if necessary. Check drive assembly for slippage; adjust.

Drying hopper too small for material being processed; replace with larger model.

tighten if required. Check hoses for cracks; replace as necessary. Check filter covers for tightness, secure.

Check all hose connections and

Replace desiccant cartridge.

Feed material slower. Readjust or replace as required.

Check to see if line is clogged and check rotation of blower.

Check tensioning. Check power supply, electrical

hookup, and safety switches. Check direction of rotation and

rewire per diagram if necessary. Same as above. Change to screen with larger-

Readjust or replace as required. diameter holes.

Check frequency of lubrication and

Adjust tensioning.

Change to higher-alloy knives. Check scrap material for foreign

type of grease used.

matter.




Screen breakage Motor will not start

Spiral Conveyors Excessive wear

Excessive spiral wear 01 breakage

Low delivery rate

Belt Conveyors

Slipping clutch Improper belt tracking

Electrical malfunctions

Parts jam up or fall off

Loose or stretched bolts.

Uneven knife seats. Improperly seated. Power supply failure. Overheated motor.

Starter failure.

Inoperative safety switches.

Conveying Equipment System may have kinks, sags,

sharp or compound bends, or contact with sharp surfaces.

vibration. System may exhibit excessive

Are abrasive materials being conveyed?

There may not be sufficient clearance at inlet end of conveyor for spiral to expand, if we take into account length and inclination of conveyor, and bulk density of material.

Material may not be flowing properly into inlet.

Oil contamination or excessive lubrication.

Belt not properly tightened when changed or installed.

System damaged in delivery. Motor may be exposed to

excessive heat under molding machine.

Transition points not long enough.

Check bolts and retorque per

Inspect and clean seat surfaces. Check that screen is fitted correctly. Check main power supply and fuses. Allow motor to cool; reset starter

overloads. Check for burned-out contacts;

replace if necessary. Check that hand guard is secure

and all contact points closed. Replace if necessary.

specification sheet.

Reposition drive-end tube supports or feed hopper.

Inspect for proper feeding into inlet (consult supplier if material is bridging).

Consult supplier.

Shorten spiral.

Rotate outer tube so that inlet opening is aligned with hopper feed.

Install bin vibrator and/or agitator. Adjust spiral length as per manual

Reverse spiral direction if incorrect. Seal openings in system if material is

instructions.

hygroscopic and system is installed in high-humidity environment.

Clean system, adjust clutch properly.

Check alignment and tighten.

Install overload-protection temperature limit switches.

Contact supplier.




Parts too wide for machine.

Belt speed too slow Improper adjustment of Change sprockets.

Identify to supplier what is being conveyed before purchase.

or fast variable-speed drive motor. Install variable-speed options or

speed-adjusting kit. Specify desired speed range to

supplier before purchase. Damaged in delivery.

Pneumatic Parts Conveyors Parts will not move Air orifices blinded. Inspect regularly for

contamination and clean. Replace fan belt.

Pneumatic Materials Conveyors Material will not move Filters clogged.

Filters improperly sized. Blower blinded. System improperly sized to

suit plant layout.

Blower overheating

Blower too noisy

Dry-Solids Metering Weight distortion

Mechanical and electrical component failure: improper weight signals

Belts stretching and mistracking

Liquids Metering Materials will not flow

Inspect filters regularly and frequently. Specify to supplier exactly what types of materials are to be conveyed. Check vacuum-pressure gauges. Replace filters. Indicate to supplier anticipated future growth plans if possible. Check vacuum seals.

Blower blinded. Excessive ambient-temperature Check filters. Make sure they

exposure. are properly sized. Install temperature-limit switches.

Install muffler. Enclose in a well-ventilated sound enclosure. Place blower in a sound-proofed room.

Metering/Proportioning/Feeding Equipment

Dust or adhesive dry solids Install a dust-exhaust system or

accumulation in critical areas. accumulation. hardware to remove dust

Clean belts, trays, augers periodically. Evaluate several systems in

production trials before purchase.

Dust, environmental conditions, materials adhering to underside of belt or other system components

Clean system components periodically.

Inspect regularly. Consult supplier. Install corrective recalibration devices.

Material buildup or proximity of system to moisture.

Improperly sized system, cannot Install drum pump. Specify to supplier what type of material to be conveyed. Make sure tubing is properly sized.

handle highly viscous materials.




Proportioning Loaders Sluggish loading,

excessive loading time needed to fill hopper

Excessive dust carryover into the dust collector

Cooling water lines frozen

System shuts down or cools slowly or poorly while refrigerant pressure is low or dropping; bubbles in refrigerant-level sight glass; oily-looking moisture on coolant- circulating tubes or floor nearby

System shuts down while refrigerant pressure is high

pressure Low cooling-water

Slow or inadequate cooling of molds, high water pressure

Clogged filter in either dust collector or pump.

Material line clogged. Vacuum leak in either material or

Hopper lid or receiver not sealed. vacuum line.

Valves not sealed.

Air-to-material ratio not correct for feed tubes (too much air).

Valves not sealing.

Improper feed tube setting. Too much air, not enough material creates high velocities.

Excessive fines and dust in material or improper blending.

Chillers Thermostat set too low (Le., below

40°F without antifreeze). Insufficient antifreeze in process

cooling water. Refrigerant leak.

Condenser not getting enough cooling water because it is constricted or blocked by dirt.

Leak in process-water circulating lines.

Empty water-storage tank. Broken pump motor.

Broken pump seal. Water flow constricted by dirt or

mineral scale.

Clean filter (replace if necessary).

Clean line. Seal lines.

Clamp lid or replace hopper

Clear obstruction. Check for

Adjust feed tubes.

seals if necessary.

proper air pressure.

Check for obstruction and

Adjust feed tube to give highest proper air pressure.

obtainable vacuum and smoothest flow.

Consult material supplier.

Check thermostat and reset if necessary to 40°F or higher.

Add antifreeze.

Replace refrigerant, plug leak source. clean condenser.

Clean condenser.

Plug leak.

Fill tank. Repair or replace (hermetically

sealed motor/pump assemblies must usually be replaced).

Replace. Clean water-circulating system.

Treat water. Install intermediate heat

exchanger for mold-cooling water (optional).




Slow or inadequate mold cooling, low pressure

Water-circulating line leak. Repair leak.

Process water heats slowly or insufficiently

Cannot attain desired air inlet temperature

Dewpoint as measured at air inlet to the hopper is unacceptable

Pump seal failure.

Heating element encrusted with dirt, mineral scale, or in oil-circulating systems, carbonized oil.

Replace seal or tighten packing

Clean or replace heating element, gland.

treat water, replace oil.

Dehumidifying Dryer Performance

Heater failure.

Hose leakages and excessive length air inlet side.

Line, hopper, filter blockage.

Loss of regeneration heaters in one or both beds or line fuses.

Loss of timer or clock motor switching ability from one head to the other, that is, continuous operation on only one desiccant bed.

Check process air or after heaters, regeneration heaters play no part in this aspect of operation.

Locate and repair: If the hose is old and brittle, replace. Shorten all hose to minimum lengths.

Check for collapsed or pinched lines, valves that are closed (some makes have airflow valves located on the air inlet side of the hopper). Filters should be changed or cleaned frequently-a good trial period is every four weeks until experience dictates a shorter or longer period.

These can be checked with a volt meter at the control panel.

Desiccant has deteriorated or been contaminated.

Loss of power to one or both desiccant beds.

Low or nonexistent airflow Fan motor burnout. Loose fan on motor shaft. Clogged filter(s). I

Restricted or collapsed air lines. Correct and relieve restrictions. Blower motor is reversed. Use of a pressure gauge or Row meter

is suggested. Proper rotation is that for which the highest flow is indicated.

Check clock motor for movement by observing either function indicators or valve-shifting mechanisms. Note that loss of regeneration heaters may occur if the clock motor or shifting mechanism malfunctions.

checking the desiccant annually and replacing it when it does not meet test criteria. Typically, two to three years is a reasonable interval, depending on the severity of service.

During the regeneration cycle, the exterior of the desiccant bed should be hot to touch. Check contacts on relays or printed circuit board for flaws, also line fuses if so equipped.

Most manufacturers suggest

Replace. Tighten. Change.


Fig. 11-2 Inspection rollers make it easy to inspect screws.

Inspection Rollers

Proper visual inspection of a screw or barrel requires that it be turned many times in order to see all sides. Screws and barrels are often heavy and difficult to turn when supported by the usual means. A few years ago, Welex Corp. showed how roller supports make the job much easier. Spirex has modified and standardized this simple but useful device as shown in Fig. 11-2. The device uses two sets of double-conveyor rollers supported by multislotted angle irons. These angle irons are mounted on plain wood blocks that can be spaced to accept the screw.

Diameters

The shank and many other diameters are easy to measure by the usual methods. Other diameters, such as the root diameter or outside flighted diameter, require special methods. Measuring the root diameter is not always a reliable way to obtain channel depths. Another problem exists: If the micrometer sits on the radius on both sides, it can give a false reading. If the outside diameter is severely worn, this is still the best method to determine the correct channel depths. Pitches

diameter is to find the outside diameter and then subtract the channel depths.

Root-diameter measurement The outside diameter is measured with the assistance of a “mike” bar spanning two flights (Fig. 11-3). The thickness of the bar is subtracted from the measurement obtained. The usual technique is to place the bar on top and hold the anvil of the micrometer against the flight at the bottom with the left hand. The right hand adjusts the micrometer while making rocking motions along the screw. The bar will

less than square or very deep channels aggra- MEASURING OUTSIDE DIAMETER

vate the problem. The best way to obtain root Fig. 11-3 Measuring outside diameter.


rock with the micrometer as the final setting is reached. Of course, it is essential that the bar be straight and of uniform thickness. It is best to check the outside diameter at 90 deg from the original set of measurements because screws can be manufactured egg- shaped or become worn that way.

Depths

The channel depth of small screws can be easily checked with a standard-depth micrometer. With larger screws, the depth micrometer will not span from flight to flight. If you intend to check many screws, it is best to make a screw-depth indicator or buy one. The screw-depth indicator consists of a wide angle “V” block with a dial indicator mounted on top and the probe extending down through the center of the V. The indicator is placed with the V resting on top of the screw and the probe on top of the flight. The gauge is then adjusted to zero. When the tip moves down into the root, the dial gives an accurate indi- cation of channel depth. It is also very fast, allowing many measurements to be made very rapidly, and it can give continuous readings as the screw is rotated.

This last feature is also helpful in locating the starting and ending points of the feed, transition, and metering sections of a screw. This is not easy to do without inspection rollers. The original channel depths of a severely worn screw are difficult to determine by this method. In the case of a severely worn screw, it is best to use the root-diameter method. Sometimes deep-jawed calipers can help if micrometers are running on the radii. A Spirex channel-depth gauge is shown in Fig. 11-4.

Concentricity and Straightness

Checking for straightness is difficult for the average plastics processor. If a good, long granite inspection table can be found, it will be helpful in checking concentricity and straightness. A preliminary check for

Fig. 11-4 Channel depth gauge.

straightness is possible just by rolling the screw on the table. If the screw is not straight, it will roll unevenly and show light under the flights in the low areas. This technique is appropriate only if the screw is not worn. The approximate amount that the screw is bent can be determined by feeler gauges. This technique is not completely accurate because the weight of the screw will tend to straighten it against the table. Most injection screws can be mounted between centers on a lathe and checked with an indicator while they are rotated. This requires an accurate center on both ends.

Extrusion screws usually do not have a center on the discharge end, requiring that a center be installed and then rewelded after testing and possible straightening. Checking the runout on the flighted portion is done with a “T” bar that spans at least three flights. The bar leans against the flights and an indicator measures the movement of the bar.

Straightness and concentricity can be determined on screws that do not have centers by rotating them in V blocks on an accurate inspection table. This is done with the help of a height gauge. This method is particularly useful in inspecting the pilot at the discharge end of injection screws.

Hardness

The hardness of most portions of a screw is difficult to measure, because the screw is usually too large to test in a Rockwell-type tester. Also, the curved surfaces of the screw present a problem, and it is undesirable to make penetration marks on the surface. A


satisfactory method for checking screw hardness is to use the impact or falling-ball type of tester such as a Shore scleroscope. It is portable, works on curved surfaces, and can be reliable if checked against calibrated reference samples.

Finish and Coating Thickness

Finishes can be verified by a number of pro- filometers. A portable thickness tester can be used to test for chrome-plating thickness, or with experience, nickel and other nonmag- netic coatings.

Screw Manufacturing Tolerances

All machined items are manufactured to predetermined dimensional tolerances. For reference, standard screw manufacturing tolerances are published by suppliers and made available on request. These tolerances have been established according to practical application requirements and reasonable ease of machining. Some tolerances can be held closer than indicated on the published list if necessary for a specific application.

Barrel Inspection Guide

Inside Diameters

The very front of a barrel can be measured with inside micrometers, but this is a limited measurement and will not show far enough into the barrel to be of much use. An inexpensive cylinder gauge can be rigged up with a long handle to slide the full length. Such a device is accurate enough to determine the need for replacement or repairs. It is not accurate enough for machining purposes. There is a problem with light and the return glare off the gauge from a flashlight. The Starrett cylinder gauge is an excellent choice. There are also a number of good bore gauges available. These gauges measure accurately at great depths and have the indicator outside the barrel for easy reading (Chap. 2).

Straightness and Concentricity

Barrel straightness is difficult to determine by conventional methods. The inner diameter and outer diameter are not always exactly concentric. The first method to measure straightness is to set the barrel on precision rollers at each end. The inner diameter is then indicated for runout with a dial indicator. This is limited in depth. Tolerances allow straightness deviations to accumulate to a total allowable indicated runout.

The second method is the use of a test bar, usually about 70% of the barrel length. The test bar is a slotted and chrome-plated bar that is precision-ground to approximately midrange of the normal screw size tolerances. In theory, if the test bar slides easily through the barrel, the screw will also. In addition, this procedure will catch rapid changes or kinks that would otherwise be allowed under the accumulation of tolerances. A different-size bar is needed to test each inner diameter. Be- cause these bars are quite expensive, their use is impractical for most organizations except for the barrel manufacturers.

Barrel Hardness

Obviously, the standard hardness testers are not able to get inside a barrel to measure hardness. The instrument most commonly used for this purpose is the internal mobile hardness tester. When testing and comparing the hardness of bimetallic lines, you must remember that absolute hardness, as measured, is not necessarily in direct proportion to wear resistance. Sometimes, you may be measuring the softer matrix rather than the wear- resistant carbides. The degree of lubricity or how well the screw and barrel slide against each other is very important. This is not always related to hardness but is very critical to barrel and screw wear.

Barrel Specifications

Chemistry and hardness information for various types of domestic barrels will be supplied for the processor’s reference on


request. Chemical information supplied is for the “as cast” condition. The actual chemistry may vary widely after final machining is complete. It is important to note that the chemistry and hardness are not necessarily indica- tive of wear resistance. Other considerations include how materials (such as tungsten and carbide) are combined and how near to the bore they are located.

Putting into practice these inspection tips to detect problems and taking steps to solve those problems will enhance the operations of any processor using barrels and screws (Chap. 3).

Preventive Maintenance

Even though preventive maintenance is not the most glamorous operation in a plant, it certainly is one of the more important. The most common maintenance failure is not doing something incorrectly, it is not doing anything at all. A well-planned maintenance schedule that is carefully adhered to always boost’s plant productivity and profitability (Table 11-15). It is also part of meeting IS0 9000 quality certification.

Table 11-15 Relate errors to processing

Surveys indicate that small machines (300 tons and less) require the most service. They often run the most complex parts. Their cycle times are usually shorter and mold changes are more frequent. To do the same amount of work as the larger machines, smaller machines must operate at higher hydraulic pressures than the usual IMM. With faster operating speeds and higher pressures accelerated degradation of the hydraulic fluid occurs. The smaller fluid reser- voirs cause contaminant concentrations to increase. Fluid contamination can cause 70 to 90% of machine failures.

Maintaining the proper oil level and oil quality can be as important as maintaining the rest of the machine. The more sensitive and sophisticated machines with microprocessor control and those with servovalves are particularly vulnerable to hydraulic fluid contamination. Microprocessor controlled machines require very accurate, instantaneous feedback to maintain part quality and consistent cycles. A sluggish response caused by poor oil quality affects everything from fill rate repeatability, to cushion size, to pressure switching, to platen movement. Owing to their very tight internal clearances and

Faults Possible Problems

Wrong location of gate

Gates andlor runners too narrow

Runners too large Unbalanced cavity layout

in multiple cavity mold

Nonuniform cooling; not properly applied

Poor or no venting

Poor ejection system or bad locations of ejector(s)

Insufficient sprue to nozzle contact

Sprue too long

Draft of molded part too small

Weld line(s), flow line(s), melt jetting, air entrapment, voids,

Short shot, overheating plastic, premature melt freezing,

Increased cycle time, plastic waste, pressure loss, etc. Unbalanced cavity pressure buildup, mold distortion,

warping, stress concentrations, sink mark(s), etc.

voids, etc.

dimensional variation due to poor shrinkage control, stresses, flash, etc.

Increased cycle time, distortion during ejection, high after shrinkage, stresses or warpage, poor part release, etc.

Use higher injection pressure, plastic burns or streaks, short shot, etc.

Poor mold release, part distortion or damage, changes or upsets in molding cycle, etc.

Melt leakage occurs, mold wear, higher injection pressure required, poor cycle repeatability, etc.

Pressure loss develops, longer molding cycle, requires increase in heat requirement, premature freezing of sprue, etc.

Poor mold release, part distortion, dimensional variation, etc.

1014 I1 Troubleshooting and Maintenance

openings, servovalves are extremely suscep- tible to contamination. For this reason some machines use fine (3-, 5, or 10-pm) filters in their hydraulic system. The filter, like other machine components, needs regular inspection and replacement. The problem with the oil is that suspended particles cause abrasive wear and suspended sludge increases oil viscosity, making machine process control more difficult. Dissolved water and metals acceler- ate hydraulic oxidation of the oil. Part of the routine maintenance procedure is to check not only oil levels but also viscosity, pH, and particulate level.

A pump in a hydraulic system does not in itself develop pressure. It is only when the free flow from the pump is restricted that an elevated pressure can develop in the system.

Vane-type pumps are generally not recommended for operation against system pressures in excess of 3,OOOpsi (21 MPa), whereas piston pumps can be used with operating pressures over 3,000 psi.

Hydraulic actuators may have almost the same construction as the hydraulic pump. The speed at which the actuator works is governed by the oil supply (volume) forced through the rotor. This type of actuator or motor is commonly used to drive the plasticating screw.

The piston or ram provides a linear movement. Again, this can be explained as a piston-type pump in reverse. This system is widely used to close presses either by direct movement or through toggle linkages. The piston is also used to reciprocate the extruder screw, move the extruder, actuate auxiliary parts on the mold, etc. Often, use is made of a double acting piston, that is, one in which the ram can be moved hydraulically in either direction. Small-diameter cylinders and rams are used for high-speed movement, whereas larger-diameter cylinders and rams are used for clamping force when large tonnage may be necessary.

Pressure-control valves are used to divert all or some of the oil back into the oil reservoir. There are many variations of relief valves that serve specialized functions, but all tend to release or reduce the oil pressure being delivered for mechanical usage.

Flow-control valves are used to control oil flow when a constant flow is only feasi-

ble at pressures lower than the lowest pressure available in a circuit during its pressure drop. For example, a flow-control valve rated for settings between 0 and 100 gpm (0 to 0.038 m3/min) can be set to deliver any flow between these values and will pass that amount of oil, regardless of whether the valve is used in a circuit with a pressure drop from 200 to 500 psi (1.4 to 3.5 MPa).

Directional control valves are used to direct flow coming from the pump to various parts of the system as desired during different parts of the cycle. They allow the oil to flow in one direction only and, in this sense, act as check valves. These valves may be two- way to five-way and be of the two- or three- position type. The two-way has two parts, one in and one out; the three-way has an inlet and usually two outlet parts in which one may be the return oil line. The four-way has an inlet, oil return, and two outlet parts, etc. Two-position valves have two possible sets of interconnections between the parts of the valve, whereas the three-position valves have three sets of possible interconnections. Val- ves of these types may be operated manually, semiautomatically, or automatically by a solenoid or pilot hydraulic system.

When analyzing a hydraulic circuit diagram, try to reduce each individual stage in a cycle to a simple equivalent circuit.

Cleaning the Plasticator Screw

To clean the plasticator, the following steps are recommended: Back it away from the mold so the nozzle is clear. Let it run with resin at operating temperatures [300"F (149°C) for low- and medium-density resins and 380°F (193°C) for high-densityresins, but these may be lower if desired] without further feeding until the screw can be seen or the shot stops accumulating.

Turn off all electricity and cooling systems to the plasticator. Disconnect electrical lines whenever necessary. Use the required safety equipment, such as asbestos gloves for handling any hot parts during cleaning with a brass knife and/or brush. Note: Do not use steel knives or brushes for cleaning, as these can scratch or mar the finishes, which may


cause excess wear or faulty extrusion later on. Push or pull the screw forward: It may be necessary to jog the drive system to get it started.

To clcsn the screw, use a copper- or brass- bladed scraper (putty knife) to remove most of the molten polymer adhering to the screw. After having scraped off the bulk of the polymer melt, clean the screw with copper or brass wool (not steel). After the screw has been thoroughly cleaned, a light coating of silicone grease may be applied to help protect the surfaces from moisture while it is out of the barrel.

Always clean the barrel whenever you clean the screw, if possible. Run a brass brush at the end of a long handle through the barrel to remove the remaining polymer melt. While the screw and barrel are cleaned, they should be examined for possible damage and measured for excessive wear.

suspect a burned-out band heater to be the cause of an observed drop in temperature, he or she must call for an electrician to replace the heater.

Thermocouples in the barrel, adapter, and mold must also be checked periodically for tight contact. A loose thermocouple is unre- liable. The same stands for a worn-out one. Finding loose-fitting or worn-out thermocouples requires regular maintenance checks. Such thermocouples must be replaced, pre- ferably with ones with bayonet-type lock mounts. (It is, however, advisable to replace a thermocouple with one of the same type and length.)

At regular intervals, check whether the points at which you set your various zone heater temperatures are accurately shown on the indicator dials on the temperature-control board. Procedures for checking thermocouples, lead wires, and instruments for cali- bration can be found in instrument manuals.

Oil Changes and Oil Leaks Alignment, Level, and Parallelism

The oil should be changed in the gearbox and transmission (if oil-driven) about every six months if the machine is operated on one shift daily, and every three to four months if it is operated on two or more shifts daily. (See the manufacturer’s recommendations.) In between, the lubrication of all bearings, gears, shaft seals, etc. must be checked regularly. Oil leaks might require replacement of a gas- ket or seal. Or they might indicate a clogged part or too much oil. In such cases, the part must be cleaned or the oil drained. A lack of oil in any part that requires good lubrication can do great damage. Such external parts as hinges, clamps, and swing bolts should be lubricated periodically with heat-resistant lubricants.

Checking Band Heaters, Thermocouples, and Instruments

Electric band heaters should be regularly examined for tightness. A loose heater will not serve its purpose and moreover will have a short life. It must fit around the barrel, adapter, or die. If the operator has reason to

The alignment, level, and whenever necessary, parallelism of the various main parts of the molds, mold press, and plasticator of the injection molding equipment should be checked regularly. These must be corrected when required.

Hydraulic, Pneumatic, and Cooling- Water Systems

An adequate level of oil must be kept in the hydraulic system. The oil used in the system must be clean and should be of the type recommended by the equipment manufacturer.

The air in the pneumatic system that op- erates valves, automatic stripping, and other parts of the equipment should be lightly oiled if the same pneumatic system does not provide the air for part ejection. This requirement provides that the system contains an oil and a water strainer to protect the oil from becoming water-contaminated,

The operator must constantly watch both the hydraulic and pneumatic systems for pressure changes caused by faulty valves and


gauges or by leaks, as indicated on dial instruments.

A closed water circuit is frequently prefer- able to the use of city water in the cooling system. City water requires a cleaning tower in the system. Well water could be hard, and mineral deposits might lessen the effective- ness of the heat exchanger for the oil in the hydraulic system, which should be kept cool.

The operator must watch the mold-cooling water temperature either by means of instruments such as thermocouples or pyrome- ters or manually by occasionally touching the water-line surfaces.

Hydraulic Hose

The life span of the usual hydraulic hose used with injection molding machines and auxiliary equipment depends on many service conditions. These include pressure, temperature (internal and external), vibration, shock, and abrasion. A hose used in a rugged, high-pressure application operating twenty- four hours a day, seven days a week is more likely to fail before a hose used in a mild environment at ambient temperature and only pressurized a few times a day.

In applications where the hose is subjected to extreme service conditions, periodic hose replacement should be part of normal maintenance to avoid unscheduled downtime.

Care must be taken when examining hose failures; snap diagnoses and quick conclu- sions should be avoided. A trained eye can look for certain clues in hose failure that can pin down a possible cause. There are equally important points to look for on the equipment itself.

Recurrence of the same hose failure on the same equipment should generally be examined systematically. Plant engineers can follow a number of guidelines in this situation:

Try to isolate the portion of the circuit where failure occurred. Check to see if other pieces of the same equipment in the plant are experiencing the same failures. Check pressure, fluid flow rate, and fluid temperature at a point as close to the hose

assembly as possible and determine if they are within the specified range. The use of transducers will capture pressure spikes more accurately than gauges. Contact the manufacturer of the hose; most manufacturers are willing to evaluate hose failure to determine the mode of failure.

If all the foregoing steps have been checked and are acceptable, then the hose failure itself needs to be examined. Some common and frequently occurring hose failures are listed in Table 11.16.

Keep the Shop Clean

Cleanliness in the molding shop is a very important aspect of the maintenance job. Contamination of the resin by dust, dirt, and especially small metal parts will make the production of good pieces impossible; it may also damage the screw, the die, and other parts of the machine.

Keep Spare Parts in Stock

It always pays to keep in stock small sets of comparatively inexpensive spare parts, such as thermocouples, heaters, fuses, etc. The operator must be prepared for emergencies occurring when least expected. Spare parts in stock will mean less downtime.

Return on Investment

Industry has left few stones unturned to improve productivity and the bottom line in recent years. Virtually everything has been analyzed, and in may cases, hard decisions have been made that have broken with long- standing procedures and traditions. There has been an overpowering focus on robotic production, computer-controlled factories, quality circles, just-in-time inventories, and other high-profile techniques to enhance productivity and quality.

1 I Troubleshooting and Maintenance 1017

Table 11-16 Hydraulic hose basics: causes and remedies

Failure Mode Cause Solution

Worn hose cover A worn hose cover with exposed rusty wire reinforcement indicates inadequate protection from abrasive environment. Moisture or chemicals reached the wire reinforcement, causing it to corrode, weaken, and fail. The hose may have been too close to a moving part, causing a severed cover. Even slight contact with a sharp edge can sever a hose cover quickly.

Cracked and stiff hose Cover may be exposed to excessive cover internal or external

temperatures, or both. The specified hose temperature rating may have been exceeded, causing the hose to prematurely “dry out.”

Spongy, soft, or Compatibility problem between the hose and system fluid. Bubbles may form under the cover with fluid under the bubbles. In the worst case, pieces of the inner tube may be breaking down and clogging up filtration systems or directional valving.

swollen hose cover

Burst hose at the fitting Commonly excessive bending of the hose at the fitting or insufficient slack in the hose run. When a hose is pressurized, it shortens in length and increases in diameter. A hose must have sufficient slack to compensate for these dimensional changes to preclude a burst at the fitting.

Best to reroute the hose run. Proper use of clamps and adapters can help keep the hose away from moving parts or sharp objects. If rerouting the hose run is not a feasible option, a protective sleeve or guard can be placed over the hose to prevent damage to the cover. Sometimes, the hose is too short.

Special high- and low-temperature hoses are available for applications where system temperatures exceed the rating of standard hose. For high- external-temperature conditions, such as near an exhaust manifold, silicon rubber-coated fiberglass sleeves can be slipped over the hose.

The type of fluid (brand name) used in the system should be examined. The fluid and hose manufacturers should be contacted to ensure the chemical compatibility of the fluid with the elastomeric inner-tube material. For example, a phosphate ester fire-resistant fluid is not compatible with a hose designed for use with petroleum-based fluids. Hose manufacturers publish chemical compatibility tables in their catalogs.

Typical rule of thumb in routing a hose is to allow a straight length of hose, twice the outside diameter, between the fitting and bend. This reduces stresses on the hose near the fitting. To compensate for the dimensional changes in a pressurized hose, sufficient slack should be provided in the hose run. For the same reason, slack should also be provided between the clamps in a hose run.



Failure Mode Cause Solution

Burst hose at the bend Typically, minimum bend-radius Most hose manufacturers specify the minimum bend-radius criteria in their catalogs. The specified bend radius must be designed into all new installations. The bend radius should also accommodate the flexing and additional bending of the hose that occur during normal operation.

criterion was ignored. Bending the hose more than the specified minimum radius overstresses the wire reinforcement, as well as the inner tube.

Equally important to production efficiency and the bottom line as high-tech equipment investment is plain, old, low-tech maintenance. It is generally believed to be one of the least considered of all the potential return- on-investment activities. Some even classify maintenance operations to be the last fron- tiers for cost-conscious companies.

Maintenance, or more specifically preventive maintenance, is much more extensive than torquing, descaling, and lubricating. Ide- ally, it encompasses not only the job of keeping machines running efficiently and safely but also upgrading procedures and equipment. A million-dollar machine tool that pro- duces 1 YO more or 1 YO less production per day because of good or poor maintenance can be the source of profit or loss. So can batches of $400 assembly line handheld power tools that are frequently taken for granted and not rebuilt or replaced. And then there is the inadequate spare parts inventory, an arch- fiend contributing to unnecessary and extensive downtime.

Obviously, and rightfully so, unless the tools used by workers are as productive as possible, the great emphasis that has been placed on workers’ attitude adjusting for higher productivity is illusionary. Much can be done by industry to improve maintenance, but to achieve and retain an effective program in this area, companies need to recog- nize how profitability can be directly equated to the maintenance function. Corporate or- ganizational structures must be reviewed to determine if maintenance supervisors have a direct link to top management.

Maintenance

Most injection molding machines will run reliably if they are properly serviced and maintained. It is important that a scheduled maintenance program be established. Com- mon problems on machines arise from lack of lubrication, insufficient cooling water, failure to change filters and strainers, and sloppy house keeping.

However, as molding becomes more sophisticated, particularly with regard to control systems, troubleshooting requires more logical understanding. It is a very important function to understand failures arising from dirt and contamination. These can be classified into three categories:

8. Catastrophic failure occurs when a large particle enters a pump or valve. The result may well be complete seizure of the pump or motor. In a spool valve, a large particle trapped at the right place can completely stop a spool from closing.

9. Intermittent failure is caused by contaminant on the seat of a poppet valve, which prevents it from reseating properly.

10. Degradation failure follows wear, corrosion, and cavitation erosion (4). They cause increased internal leakage in the system components, but this condition is often difficult to detect.

Sometimes, too little attention is paid to the cleanliness required in the oil and the better maintenance care needed. Dirt is re- sponsible for a majority of malfunctions,


unsatisfactory component performance, and machine degradation. This factor has become even more important with the increased use of electrohydraulic servosystems. Injection pressure, holding pressure, plasticating pressure, boost pressure, boost cutoff, and other controls all affect the finished part performance. All these parameters are adversely affected by increased contamination levels in the fluid.

Dirt can be introduced into hydraulic systems sometimes at the time of fabrication of the components and during manufacture of the machine. Contamination found, particularly in the past, in oil samples taken from a system after a short run-in period of a new machine includes metal chips from tubing burrs, pipe threads, and/or particles generated during component manufacture and tank fabrication.

Although oil is refined and blended under relatively clean conditions, it is usually stored in drums or a bulk tank at the user’s factory. At this point, it is no longer clean because the filling lines contribute metal and rubber particles, and the drum can add flakes of metal or scale. Storage tanks could be a real problem, because water could easily condense in them to cause rusting, and contamination from the atmosphere finds its way in, unless satisfactory air-breather filters are fitted.

If the oil is being stored under reasonable conditions, the principal contamination introduced to the machine will be metal, silica, and fibers. With a portable transfer unit or some other filtration arrangement, it is possible to remove much of the contamination present in new oil before it enters the system and is ground down to finer particles.

Dirt is continually introduced into operating hydraulic systems because of wear and degradation of the working components. The wearing action of working parts such as pumps, fluid motors, valves, and cylinders generates contamination. Rust scale from the reservoir caused by condensation above the oil level is also a source of dirt. Burrs on tubing and piping break loose during service, and flexing of components releases particles not removed during initial cleaning of the hydraulic system.

It is well known that contamination particles come in all shapes and sizes and that the finer they are, the more difficult it is to count them and determine the material of which they are composed. However, we can say that the majority are abrasive and when they interact with surfaces, they plough and cut little pieces from them. This wear accounts for about 90% of the failures attributed to contamination or dirt. The effect of these contaminant particles on various system components reflects itself differently, depending on the mechanism of operation.

The dirt level in a fluid is controlled with filters. To satisfy the performance requirements, a number of factors must be considered in the selection of hydraulic filters. These include degree of filtration, flow rate, pressure drop, dirt capacity, compatibility, element cleanability or replacement, system pressure, and temperature.

It is generally recommended that filtration to at least a 25-pm range be provided for a hydraulic system. Some dirt particles in a system are magnetic. They are built into the system while the machinery is being fabricated. They are also generated within the system from the action of moving parts and fluid erosion. In addition, they can enter the system through the reservoir openings and air breather.

These particles are normally abrasive and can react chemically with hydraulic oil to de- compose the oil. They should be removed from the system. However, most of this type of dirt consists of very small particles. A fine filter would have to be used, which means an increase in cost and probably maintenance. Both these factors can be avoided by using a magnet. If many magnetic particles are present in a system, a relatively coarse element used in conjunction with a magnet can be as effective as a finer filter. The magnet will catch the small metal particles. The element will catch the larger dirt particles and not become clogged as quickly as a finer element. Cost and maintenance are reduced.

It is especially recommended that magnets be used with fire-resistant fluids. Petroleum oil allows many of the metal particles to settle to the bottom of the reservoir. Fire-resistant fluids are more detergent-like and tend to


keep these particles in suspension. Conse- quently, there can be more magnetic particles in a stream of fire-resistant fluid than petroleum oil.

The degree to which hydraulic fluid should be filtered is another important consideration in the process of providing means for controlling dirt. The component supplier may provide data in the catalog relative to particle- size sensitivity.

In addition to the degree of filtration, the placement of the filters within a system is an important consideration. Hydraulic filters can be installed in the intake, pressure, or return lines of the system or in the reservoir.

A hydraulic system may be equipped with the best filters available, and they may be po- sitioned in the system where they do the most good. However, if the filters are not taken care of and maintained or cleaned when dirty, the money spent for the filters and their installation has been wasted. The whole key to good filtration is filter maintenance.

Hydraulic Fluid Maintenance Procedures

Select the viscosity and type of hydraulic fluid recommended by the component and hydraulic equipment manufacturer and include fluids that are not necessarily petroleum based. Be sure that the fluid is clean to the degree required by the component or equipment manufacturer. On certain machines with servosystems, the filtration requirements are generally 10 Fm absolute or less. This means that when adding fresh hydraulic fluid in any quantity, the fluid must be filtered by some auxiliary means to the degree recommended for the equipment. This same procedure for extra clean or ultrafine filtration follows for systems with electrically modulated hydraulic valves. The same filtration applies for fluid being transferred from holding tanks, lubrication carts, and partially opened barrels of fluids.

Temperature control can be a very effective way of increasing fluid life. The operating temperature is generally held from 212 to 248°F (100 to 120°C) for best results. Heat exchangers should be periodically

cleaned to make sure they are functioning properly. Check the fluid condition for both foreign particle contamination and a chemical condition every 90 to 120 days. On systems requiring ultraclean filtration, checks should be made every 60 days or less. Periodic inspection and testing of hydraulic fluid on a regular schedule are vital parts of any effective fluid conservation program.

Hydraulic fluid can be contaminated before it is even added to a hydraulic processing system, causing problems from the start. Improper storage of fresh fluid either outside or inside the plant, where the contaminants can collect on the exterior of drums, can result in harmful dirt and other contaminants being introduced. Use the proper and fluid suppliers’ recommended methods of storage to prevent moisture and other contaminants from developing.

Problems and Solutions

To better understand potential maintenance problems or the variables involved, it is helpful to consider the relationships of machine capabilities, plastics processing variables, and product performance requirements. It may be impossible to meet the product requirement because the equipment does not have the capability and/or the plastic does not have the capability.

One can only obtain so much out of one’s equipment. Companies that want to stay ahead of competition must consider purchasing new equipment to obtain better molding performance or for processing better per- forming plastics. This is nothing new since this approach has been used for centuries even before plastic products were developed. To avoid mistakes in using cause-effect relationships to their best advantage a distinction between machine conditions and processing variables must be made (Chap. 7).

To resolve variables or problems, a logical and systematic method of dealing with them is required. The method should use a language that everyone understands. Terms and phrases should not be ambiguous; they should not be prepared like a document


where all kinds of definitions could exist just for one term or phrase. Unfortunately, certain terms and phrases may have different meanings to different people. Resolve this situation by clearly identifying each term by a complete definition. It may be important to include what it does not mean to eliminate any misunderstandings. If it is necessary to include engineering equations or chemical for- mulas, explain them in terms understood by the nontechnical person (or even by the tech- nical person who may misunderstand them).

When the specific problem has been identified, record how to eliminate it. This type of information should be documented in the operating manual used in the production line. If applicable, include the information directly into the line’s process control system. Unfor- tunately, at times, someone, particularly the operator, might be informed that Chisolm’s Law is occurring. This law, which has been around a long time (and would be good rule for many a government), states that if at time things appear to be going better, you have overlooked something. It is always important to analyze failures. Carefully studying one’s failures and mishaps can be a route to even- tual success. Putting failures under the microscope of an objective critique, in fact, is far better than playing Pollyanna. You may not want or need to schedule a full-scale inquest every time, but even a quick postmortem on a project that has foundered may keep you from botching another one.

Downtime Maintenance

The approach of uniting the processor with primary and secondary equipment suppliers has reduced production downtime and cut costs. Processors can have their routine maintenance and problem solving done in a matter of minutes. The use of a modem for diagnostic purposes comes as a natural progression of two actions in process control technology, namely increased connectivity among com- puters and improved diagnostic software.

A past survey of European injection molding plants by Phillips GmbH of Germany, one of the world’s largest suppliers of industrial

control systems, showed that 60% of all machine downtime resulted from operator errors, 30% could be attributed to mechanical failures, 9% was caused by faulty electrical systems, and just 1 YO resulted from faulty process control. Since that time it has been re- ported that with the use of modems, downtime has been significantly reduced in all the above categories.

Preventative Maintenance

It is in your best interest to practice preventative maintenance (PM). Equipment can be built to last a long time, but proper maintenance will allow it to perform at its maximum output for the longest length of time. Ultimately it is less expensive to maintain equipment than it is to replace it. Schedule periodic checkups based on equipment suppliers and/or your experience, so that they become habit forming. Have the proper people available for specific tasks in addition to the machine operator performance requirements in the schedule.

Maintenance of IMM and specifically preventative maintenance procedures should be used to decrease or eliminate downtime and optimize performance. The following is a guide that can be performed by the operator and/or maintenance personnel. As a safety practice, the IMM must be shut down when any maintenance or inspection is made.

Daily procedures

Inspect hydraulic and electrical safety devices at each shift. Check oil-level gauge; maintain oil at proper tank level. Inspect for external oil leaks; tighten all loose joints. Check lubricator reservoir for clean grease and proper operation. Check operating oil temperature. Remind operators of the importance of keeping oil temperature under control. Do not allow oil temperature to exceed 120°F. If oil temperature exceeds 120”F, stop the machine and determine the cause. Install safety alarm system if necessary.

1022 I 1 Troubleshooting and Maintenance

Check hydraulic pressures on the machine; do not permit pressure to exceed the maximum range. Inspect all electrical-component enclo- sures. Keep cabinet door tightly closed. Check material hopper for foreign objects before filling. Clean and lubricate strain rods and path- ways.

Weekly procedures

Inspect all valve solenoids; tighten if necessary. Inspect ram packing; take up or replace when they are leaking excessively. Test pressure gauges for accuracy.

0 Install pressure gauge in test port of master relief valves and test valve: do not allow pressure to exceed recommended settings. Check and tighten all screws and nuts. Inspect all electrical components, relays, timers, and heating bands. Keep contacts clean. Replace worn or burned contacts.

0 Check transmission-oil level of injection unit; add oil if necessary.

Monthly procedures

filters.

other contamination.

Remove, disassemble, and clean all suction

Have hydraulic oil analyzed for water and

Test oil thermometer for accuracy. Inspect and clean air cleaners in hydraulic- oil tank. Remove drain plug located under each end of each electric motor. Pump grease into fittings located on top of each end of the motor until clean grease emerges from the drain hole. Replace drain plug. After drain plug has been replaced, do not p u m p grease. Check couplings between motors and pumps for leaking seals. Replace seals if necessary. Couplings will require little lubrication if seals are intact. If additional lubrication is required, remove the pipe plug located in the center of the sleeve and insert a grease fitting. Remove the opposite pipe plug. Pump grease into fitting until clean

grease emerges from hole on opposite side of sleeve. Remove grease fitting. Replace both pipe plugs.

Semiannual procedures

Remove hydraulic oil, clean tank, and refill with clean oil. Remove and clean heat exchanger. Drain, flush, and refill the extruder-unit transmission. Remove grease from reservoir, clean reservoir, and pump new grease to purge system.

Services

Servicing involves different requirements based on equipment and molds to be maintained. Equipment manufacturers and per- sonal experience provide appropriate information. A preventive maintenance schedule must be set up for all equipment and molds both during their operations and during downtime. Computer maintenance software, such as the Spirex Corp. (Youngstown, Ohio) Program on Maintenance Professional for Injection Molding, can keep ongoing spare parts inventory and provide master lists of replacement part numbers with a list of quali- fied venders. It tracks, via graphics, significant changes including screw wear, maintenance schedules and histories, sets-up preventative requirements, and preparation for ISO-9000 quality certification for each operating machine and piece of auxiliary equipment.

Injection molding machines should be subject to preventative maintenance procedures to decrease or eliminate downtime and optimize performance. The machine operator and/or the maintenance personnel can follow the maintenance guidelines supplied by the IMM manufacturers. For example, leakage (drooling) from or around the nozzle area during injection is an undesirable situation. The problem is usually caused by plastic being trapped between the nozzle and the mold bushing.

Molds usually represent an important and very costly part of the production line. Thus,


they require very careful handling and storage. Any protruding parts should be pro- tected against damage in transfer. The mold surfaces, especially cavities and cores, should be covered with a protective, easy to remove, coating to protect against surface corrosion when the mold is not operating. For special protection, vacuum containers are used after the mold is properly dried. Records should be kept to ensure required maintenance is ac- complished on a regular time schedule (587).

Safety

With troubleshooting and maintenance the vital subject of safety naturally arises. Chapter 2’s section on Safety offers different explanations and procedures when operating or being around equipment (Figs. 2-55 to 2-64). Other chapters provide additional information that relates to hazardous conditions. There are numerous safety procedures to be followed during troubleshooting. These continue to be updated by injection molding and auxiliary machinery manufacturers (granulators, materials-handling systems, blenders, etc.), material suppliers, plant materials-handling systems, plant safety offi- cers or departments, and by the research com- munity (1,7,18,43).

Maintenance Software

As previously mentioned the windows- based program Maintenance Professional for Injection Molding by Spirex Corp. tracks maintenance schedules and histories for each injection molding machine. Machines and components are depicted graphically on the screen. This program keeps an ongoing spare parts inventory and provides master lists of replacement part numbers, with a list of qual- ified vendors for all components. Thus you will know exactly what parts are needed and where to get them. The program not only helps you remember when to perform maintenance but also tracks significant changes such as screw wear. Spirex also has auxiliary

equipment maintenance module software and a mold base module (Chap. 9).

Summary

The key to understanding troubleshooting is to gain as complete as possible a knowledge of what the machine and auxiliary equipment are doing to the plastic, what the plastic is doing to the mold, etc. This book describes the complete process so that you can obtain an in-depth understanding of all the parameters involved. Figure 1-1 summarizes the complete FALL0 (follow all opportunities) process, including troubleshooting as a major parameter.

Terminology

Air entrapment A phenomenon wherein air gets trapped in a plastic giving rise to undesired blisters, bubbles, and/or voids. Air entrapment can occur during fabrication. The bubbles could result from air alone, from moisture due to improper plastic material drying, from compounding agent volatiles, from plastic degradation, or from the use of contaminated regrind. The first step in resolv- ing this problem is to be sure what problem exists. A logical troubleshooting approach can be used.

Angel hair Long strings of plastic created when soft plastic is cut. Some products, such as those for medical and electronic application, may require machining. These require an absolutely clean cut without burrs, dust, or so-called angel hair. Lubricating the knife with water, alcohol, or mineral oil can often help to provide a smooth clean cut. Knives coated with PTFE or of high polished chrome are used to reduce friction, resulting in clean cuts.

Barrel alignment The alignment at installation and routine maintenance checks of the screw, mold, and any auxiliary equipment at- tached to the barrel (Chap. 2).


Barrel and feed unit heat control A feed- throat casting, generally water-cooled, used to prevent an early temperature rise of the plastic. A good starting point is to have the temperature about 110 to 120°F (43 to 49°C) or “warm to the touch” to help ensure the development of a stable feed. If the temperature becomes too high, it may cause the plastic to adhere to the surface of the feed opening, causing a material-conveying problem to the screw. The overheated plastic solidifies at the base of the hopper or above the barrel bore causing bridging, which prevents material from entering the screw.

The problem can also develop on the screw, with plastic sticking to it, restricting forward movement of material. Overcooling the hopper can also have a negative effect on performance, because the screw’s heat sink effect would pull heat from the feed zone of the barrel. Hopper block cooling is primarily used to prevent sticking or bridging in that area. Thus, the hopper should not be run colder than necessary. Always control water flow in the throat cooling systems from the outlet side to prevent steam flashing and to minimize air pockets.

Barrel inspection To ensure proper performance, different parts of the barrel can be checked to meet tolerance requirements (usually set up by the manufacturer) and determine if any wear has occurred. The barrel’s inside diameter, straightness and concentricity, and surface condition should all be checked.

Barrel wear Most barrels are made with nitrided steel or one of several types of bimetallic construction. Nitriding is a surface- hardening technique. The maximum effective depth achieved is less than 0.4 mm (0.016 in.). Wearing away of this thin surface layer de- grades the barrel’s abrasive wear resistance because only the steel substrate remains. Bimetallic barrels combine a structural steel exterior with an alloy inlay of a tool steel or alloy lining to improve resistance to abrasion and corrosion. In contrast to nitrided steel, bimetallic linings are uniformly hard throughout their depth. Depths are typically

about 1.5 mm (0.060 in.) for centrifugal cast linings and about 6.3 mm (0.250 in.) for tool steel or alloy linings. Bimetallics are far more durable than nitrided ones. The main types of bimetallic barrels are tungsten carbide com- posites, chromium-modified iron-boron alloys, and nickel alloys.

Blister A cavity or sac that deforms the surface of a material. It is usually a raised area on the part’s surface caused by the pressure of gases or air trapped inside the part that surfaces during fabrication.

Bloom The result of ingredients coming out of “solution” in the fabricated plastic product and migrating to its surface.

Chisolm’s law Anytime things appear to be going better, you have overlooked something.

Clamping platen, troubleshooting With platens not operating properly and/or molding operation not properly controlled typical problems that can develop include: mold wear or damage, mold flashing, out of-tolerance parts, tie-bar stress, and unbalanced mold filling. In addition to various text- books, material suppliers and machine manufacturers generally provide guidelines relat- ing these type problems with causes and solutions.

Clean-area, fabricating Technology provides a milieu of artificial purity to protect sensitive products from air-laden particle contamination. Required measures include: (1) a workplace correctly designed for clean- air technology and suitable conduct by employees, (2) effective filtration of the air supply and carefully planned air ducting, (3) easy to clean surfaces throughout the clean area, (4) a high degree of automation of all work operations, and ( 5 ) regular monitoring with the aid of suitable particle measuring technology.

Cleaning equipment Equipment requires cleaning on a periodical maintenance time schedule to ensure its proper operation.


Economically operated cleaning devices for molds, extruder dies and screen changers, molded flash, etc. are available for safely removing contaminated plastics. The routine techniques used include blow torches, hot plates, hand working, scraping, burn-off ovens, vacuum pyrolysis, hot sand, molten salt, dry crystals, high pressure water, ultra- sonic chemical baths, heated oil, and lasers.

Personnel have to be careful not to damage expensive tooling by spot annealing, mechanical abuse, etc. Commercial cleaning systems use aluminum oxide beds (fluidized beds), salt baths, hot air ovens, and vacuum pyrolysis. For example, the vacuum pyrolysis cleaner utilizes heat and vacuum to remove the plastic. Most of the plastic is melted and trapped. The remaining plastic is vaporized and appropriately collected in a trap.

Cleaning plastic To clean fabricated products different techniques are used: solvents, ultrasonics, blasting with dry ice (car- bon dioxide) pellets, toxic chemicals, and even PCFC-based solvents, particularly for medical devices (Chap. 10).

Clean room A room for the manufacture of objects that is maintained at a high level of cleanliness by special means. In the past clean rooms were found only in some of the larger plants or those involved in specialized operations concerned with medical or phar- maceutical products. But processors have not been able to isolate themselves from the trend toward clean room production in order to achieve the necessary quality levels for the electronics and microelectronics industries, and lately, even production for the automotive and entertainment industries might require clean rooms. With careful plan- ning, considerable savings can be made in in- vestments and operating costs. The required degree of cleanliness, in particular, deter- mines costs to a large extent and is directly influenced by a number factors such as the size of the room and contaminants.

The worst enemy is dust, which must be eliminated, with the greatest producer being human beings. The smallest dust particles are less than 0 . 5 ~ m . Moreover, the number

of particles depends on the type and speed of any motions. Since the continued production of dust is unavoidable, measures must be taken to reduce the total particle count. The lower the permissible amount of dust in a planned production area, the greater the resultant costs.

Clean room standard The U.S. Federal Standard 209E, Airborne Particulate Clean- liness Classes in Clean-rooms and Clean Zones, is required for manufacturers who want to conform to quality system regulation. Via the industrial I S 0 European Commu- nity, it has been integrated with ISO. Among the more important recent changes are met- rication, revision of upper confidence level (UCL) requirement, provisions for sequen- tial sampling, and an alternative verification procedure based on determination of the concept of ultra-fine particles known as U descriptors.

Contamination Any unwanted or foreign body in a material or the processing area, including air, that affects or detracts from part quality.

Crack growth Crack growth behavior can be analyzed using fracture mechanics, which can provide fracture toughness to prevent fracture. Fracture is a crack-dominated failure mode. For fracture to occur, a crack must somehow be created, then initiate, and fi- nally propagate. The prevention of any of these events will prevent fracture. Cracks can be considered elastic discontinuities that can come from a variety of sources such as internal voids or dirt, and/or surface scratch, em- brittlement, or weld line. Cracks can be consequences of faulty design, poor processing, and/or poor handling of raw material, assuming material arrived clean.

Crazing See Stress whitening.

Definition It is important to define words or terms, as well as abbreviations, to ensure that proper communication exists. Many times there can be more than one


definition to meet different requirements as set up by different organizations, industries, legal documents, etc. In fact the definitions could have opposite or completely different meanings.

Finagle’s law Once a job is fouled up, anything done to improve it makes it worst.

Fines Finely crushed or powdered material.

Flow mark Excessive linear surface texture. Molding can cause product surface melt flow marks. The major contributor to the markings is the injection speed (13,181-183).

Machine alignment Without proper machine installation the precision alignment built into equipment is lost when not properly supported on all its mounting points. Installation involves factors such as ground support stability, precise alignment of equipment, uniform support, and effective control of vibration. Installation and alignment has to be done with extreme accuracy. Assuming proper alignment occurs at room temperature and significant movement occurs during heat up or during operation, the causes of movement must be reconciled to prevent excessive wear or even failure of components. With plasticators the prime objective is to keep the screw and barrel centerlines coincident, meeting the production line height requirement. Installation is a multistep procedure that consists of building a foundation, setting and leveling the machine supports, and aligning the machine components to each other.

Machines not alike Just like people, not all machines may be created equal. Identical machine models, including auxiliary equipment, built and delivered with consecutive se- rial numbers to the same site can perform so differently as to make some completely unacceptable by the customer, assuming they were installed properly.

Material impurity Presence of one or more substances in another, often in such low

concentrations that it cannot be measured quantitatively by ordinary analytical methods. To avoid forming microscopic cavities in a molded part, when processing TP materials it is important to maintain a minimum pressure (rather than maximum) during injection of the melt. As the melt cools, the bubbles grow, which in turn can decrease mechanical and other properties of the part. The majority of the cavities formed result from water vapor present on the surface as well as imbedded in the plastic particles themselves. When these bubbles form on the surface, they are called splay.

Material received, checking All types of incoming materials (plastics, steel, etc.) must always conform and be checked against specifications. Unfortunately, with time after processing materials, specifications may have to be changed to meet an unforeseen important test.

Optical sheet Black specs, bubbles or voids, die lines, surging, surface imperfec- tions, etc. are among the major problems encountered by processors of optical sheets (film, etc.). The majority of problems can be traced to the way the plastic was dried and handled.

Outgassing During processing certain thermoplastic and thermoset plastic com- pounds, particularly TSs, gas forms and has to be removed so that it does not damage the part by forming voids or thin sections or altering mechanical performance. Pro- cedures to prevent outgassing include providing vents, bumping, etc. When applying coatings on plastic, such as with metallizing, gas release after coating can cause the coating to be stripped, blistered, etc.

Stress whitening Also called crazing. It is the appearance of white regions in a material when it is stressed. Stress whitening or crazing is damage that can occur when a TP is stretched near its yield point. The surface takes on a whitish appearance in regions that are under high stress. It is usually associated with yielding.


For practical purposes, stress whiting is the result of the formation of microcracks or crazes. Crazes are not true fractures because they contain strings of highly oriented plastic that connect the two flat surfaces of the crack. These fibrils are surrounded by air voids. Because they are filled with highly oriented fibrils, crazes are capable of carry- ing stress, unlike true fractures. As a result, a heavily crazed part can carry significant stress even though the part may appear frac- tured.

It is important to note that crazes, microc- racking, and stress whitening represent irre- versible first damage to a material that could ultimately cause failure. This damage usually lowers the impact strength and other properties. In the total design evaluation, the

formation of stress cracking or crazing damage should be a criterion for failure based on the stress applied.

Striation A longitudinal line in a part caused by disturbance in the melt path during fabrication. Striation also identifies the separation of color resulting from incomplete mixing and/or melting of the plastic.

Warpage The dimensional distortion in a plastic part after processing. The most common cause is variation in shrinkage of the part. The major processing factors involved are flow orientation, area shrinkage, and differential cooling.

Whitening See Stress whitening.

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