HOW TO SOLVEBLOWN FILM PROBLEMS

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How to Solve Blown Film Problems

This technical brochure covers some of the most common blown film problems and theirprobable solutions. It is hoped that the information contained here will be of assistance to youin your film operations.

Table of Contents Page

Blown Film Basics ........................................................................................................................ 2Prevention Checklist .................................................................................................................... 6Dies and Air Rings ....................................................................................................................... 10Maintaining and Improving Film Output ....................................................................................... 13Roll and Film Defects .................................................................................................................. 17Visible Roll Defects...................................................................................................................... 19Gauge Variations ........................................................................................................................ 22Bubble Trouble and Blemishes ................................................................................................... 26Roll Defects — Whose Fault? .................................................................................................... 30

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Blown Film Basics

The goal of this brochure is to describespecific defects that can appear in tubularblown film and to suggest probablecauses and solutions. However, a reviewof the process of blown film extrusion isworthwhile.

An operator can become so familiarwith a given film line that problems aresolved intuitively, but training newpersonnel or bringing a new line on streammay raise difficulties. Some inherent andhalf forgotten quirk of the process (andthere are many) may be of noconsequence under familiar conditions,but can become the unrecognized causeof defects when conditions are changedto accommodate new products orprocessing requirements. Reviewing theblown film extrusion process can prepareyou to handle these problems.

Blown Film Process BasicsThe process of producing film by extrud-

ing molten resin into a continuous tube is,

at first glance, extremely simple. Theelements of the process (Figure 1) in-clude the resin pellets which are fedthrough a hopper into an extruder. Here,heat and friction convert the pellets to amelt which is forced through an annularor ring-shaped die to form a tube.

The tube is inflated to increase itsdiameter and decrease the film gauge. Atthe same time, the tube is drawn awayfrom the die, also to decrease its gauge.The tube, also called a “bubble,” is thenflattened by collapsing frames and drawnthrough nip rolls and over idler rolls to awinder which produces the finished rollsof film.

However, anyone familiar with blownfilm extrusion knows this simplified expla-nation is less than half the story.Theblown film extrusion system is, in fact,one of the most complex and sensitive ofall plastics processing technologies. Thetubular blown film process is efficient andeconomical, and can produce a magnifi-cent array of products — from a lightgauge, clear converter film to heavy gaugeconstruction film, which when slit andopened, may measure 40 feet or more inwidth.

Basic Blown Film Line

Figure 1

Elements of Blown Film

Figure 2

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Main Arena of ActionMore of the problems in blown film

extrusion take place in the section of thetube illustrated in Figure 2 — from withinthe die to the far side of the nip rolls —than in any other portion of the line. Ele-ments in this section are labeled and arereferred to again in this booklet.

Even though practice does not alwaysfollow theory, theory can help explainmany of the problems encountered inextruding polyethylene into blown film. Forexample, blow-up ratio (BUR) used aloneas a film-making parameter is meaning-less. BUR must be related to draw-downratio and die gap. In Figure 3, all three ofthese parameters are used to illustrate atheory of melt orientation, an importantfactor in extruding the high quality filmrequired by customers.

Blow Up Ratio (BUR) =Bubble Diameter Die Diameter

To illustrate melt orientation, it isnecessary to separate the blow-up anddrawdown functions. In reality, however,these take place simultaneously in the meltbelow the frost line. In this area almost all

BUR = 0.637 x Layflat Width Die Diameter

of the important characteristics of the filmare fixed-orientation, shrink properties,clarity, gloss, strength, etc.

The formula to obtain the BUR anddrawdown ratios and their meanings are asfollows:

Drawdown Ratio (DDR) = Width of Die Gap

Film Thickness x BUR

BUR indicates the increase in the bubblediameter over the die diameter. The die gapdivided by the BUR indicates the theoreticalthickness of the melt after reduction byblowing. Since it is difficult to use caliperson the bubble to measure its thicknessunless you knock it down, a more practicalformula is:

The final thickness reduction in themelt after blowing is indicated by adrawdown ratio.

A third ratio, called the blow ratio (BR),is the increase of layflat width over diediameter. BR is used less frequently, butcan easily be confused in conversation withthe more common BUR.

A blow-up ratio greater than 1indicates the bubble has been blown toa diameter greater than that of the dieorifice. The film has been thinned andpossesses an orientation in thetransverse direction (TD).

A drawdown ratio greater than 1indicates that the melt has been pulledaway from the die faster than it issuedfrom the die. The film has been thinnedand possesses an orientation in themachine direction (MD).

Melt Orientation Theory

Figure 3

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In practice these numbers are onlyapproximate because the melt swells as itleaves the die gap. The above calculationsare made using the die gap dimensionbecause the degree of swell varies withthe resin used and processing conditions.

Collapsing the BubbleAlthough these ratios provide general

parameters, some incompatibility existsbetween the configuration of the bubbleand that of the film after it has beencollapsed over the various rolls. After filmis wound, its size is called the layflatwidth. Brief study of Figure 4 shows thereason for this incompatibility. Thesketches show front and side views of abubble 16 inches in diameter collapsed toa layflat width of 25 inches (somenumbers here are rounded off for ease ofcomparison).

In Figure 4, on the front, a righttriangle is formed (shaded area) with thelength of the vertical side equal to D, thedistance between the nip rolls and thebottom of collapsing frame; the length ofthe base side is equal to half the layflatwidth minus the radius of the bubble, or 4½inches.

On the side view, a right triangle is formed(shaded area) with the vertical side equal toD as before, but the base side is equal to theradius of the bubble, or 8 inches (½ of thediameter).

Since the two triangles have verticalsides of equal length, D, but different baselengths, 4½ inches vs 8 inches, the thirdsides of the two triangles (E vs C) mustalso have different lengths. In other words,the length of film that forms the edge ofthe layflat (E) is not equal to the lengththat forms the center of the layflat (C). Yetthese unequal lengths must travel from theplane of their point of contact with thecollapsing frame to the nip rolls in thesame amount of time.

Tabulated data at the bottom of Figure4 show the magnitude of this discrepancyin length. If the angle A, formed by thecenter line of the bubble and the edge ofthe collapsing frame is 22°, then distance Dmust be 20 inches for a collapsing framelong enough to accommodate the fullbubble width. By calculation, the edge E isfound to be 20½ inches long, while thecenter C is 21½ inches. The center of thissection of film is one inch, or about 5%,longer than the edge.

To bring a layflat out of the nips thatactually lays flat, the edge of the filmshould theoretically travel faster than thecenter. In other words, the velocity of thefilm should gradually increase from thecenter until, at the edge, it is 5% greaterthan that of the center. With a line speed of120 feet per minute (fpm) at the center, theedge must travel at about 126 fpm.

Theory Geometry of theCollapsing Bubble

Figure 4

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Closing down the collapsing framehowever, doubles and quadruples thesurface area of the frame in contact with thefilm. Unfortunately, films with a high surfacecoefficient of friction drag between thecollapsing frames. As the center area ofthis warm film in contact with the collapsingframes increases, the additional dragdistorts the flatness of the film, making itbaggy at the center and difficult to print andconvert.

The perfect theoretical solution to thebubble-to-layflat problem is a collapsingframe 200 feet long with a zero coefficientof friction. In this frame, the length of theedge and center of the film would not differby so much as a hair’s breadth. However,like many theoretical solutions, this one isjust not practical.

Rotation of DieRotating the die and/or air ring as

shown in Figure 5 can help mask errorsbuilt into the melt by process faults whichcause variations in the film thickness,called gauge bands.

By rotating the die and air ring, thegauge bands can be moved around thesurface of the film as the bubble isextruded. The bubble itself does not rotate.The gauge bands are thus distributedacross the face of the roll, level wound asfish line on a reel, and the result is acylindrical roll of film of perfect symmetry.

Fortunately, film made from low densitypolyethylene can stretch. The edge muststretch to permit the center to remain tautas it goes through the nips. If the edge doesnot speed up (stretch), the center will bebaggy and broad “smile” wrinkles willappear across the web.

Less extensible film — stiff overwrapfrom resin with a density of 0.935 g/cm3, ora high density, paper-like film — does nothave the ability to stretch. The broad“smile” wrinkles appear if no attempt ismade to increase the edge velocity.However, if the edge velocity is too great,edge wrinkles occur.

Normal procedure at this point is toclose down the collapsing frame. Thisprocedure decreases the angle A (seeFigure 4) and reduces the differencebetween the lengths of the center and edge.Decreasing the angle from 22° to 11°narrows the difference in length betweenthe center and edge to 1¼ %. At a 5½°angle, the difference is a mere 5/8%,essentially solving the problem, althoughnot completely.

Without rotation, these faults build upin one place on the roll of film, as fish lineon a reel without a level wind. The result isa roll of film with a surface looking like awood turning for a short thick balustrade.

Unfortunately, rotation can introduceproblems of its own in that the gaugebands now gradually move across the faceof the collapsing frames. Such actioncauses the web to move back and forth

Figure 5

Rotation

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between the frames and the layflat towander back and forth in the line down-stream from the main nips. A web guide isrequired to finally track the layflat in astraight line so the film winds up as agood roll.

Generally, broad gauge bands causedby a draft of air or a heat rise off the frontend of the extruder against the meltcannot be rotated because the melt itselfis not rotating through this fault. As aconsequence, the roll of film may betapered or have convex or concave facesas the different thicknesses of film buildup upon themselves.

Again, as the bubble or die diameter isincreased, so is the transverse speed ofgauge bands across the faces of thecollapsing frames increased for a givenrotational speed. This can cause bubbleinstability, intermittent wrinkling in the nipsand web wander downstream. Thesethree problems can be corrected byreducing rotational speed.

However, one rotation should nevertake less than the time it takes to build aroll of film. Otherwise, the gauge bandswill not have had time enough to beuniformly distributed across the entireface of the roll of film.

ConclusionMany problems occur in blown film

extrusion in the hot melt between the dieand the frost line and where the tube iscollapsed at the main nip. Other sectionsin this booklet deal more specifically withproblems such as uneven rolls, gaugebands, wrinkles, maintainng output, physicaland optical problems and solutions.

Prevention ChecklistTo prevent problems in extruding blown

film, purchase the proper equipment to dothe job. Then, make sure the equipment isinstalled properly, checked and maintainedregularly and scheduled efficiently so thathigh quality film can be produced at highoutput rates with minimum scrap.

Checklists are excellent memoryjoggers for both production andmaintenance personnel. An operator shouldcheck each film line against a checklist atleast one time per shift. Lines should alsobe checked when there is an order change,and at start-up or at shutdown, since someequipment can be inspected only at thosetimes.

All or part of the following checklistusually is incorporated in plant processdescriptions, plant operating standards orthe plant maintenance department’s sched-ule. The checklist cannot include everythingbecause each blown film extrusion shop isdifferent. However, regular use of thechecklist should minimize many potentialproblems.

Resin, Additives and Regrind1. Are the polyolefin resin and additives

the right grades for the film beingextruded?

2. Are the quantities sufficient to com-plete the run?

3. Are the resin loading system filtersclean or have they recently beenreplaced?

4. Were the additive feeders emptiedand cleaned when out of service toprevent dribble from contaminating thenext resin to be run?

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5. Were the recycle/regrind systems forsalvaging edge trim and roll scraprecently checked to make sure thatthe proportion of scrap to virgin resinis accurate?

6. Are the scrap rolls of film compatiblewith the virgin pelletized resin beingused?

7. Are all rolls of scrap clean and labeledby resin type?

8. Was the equipment cleaned betweenresin changes to preventcontamination?

9. Are all hoppers and boxes of resincovered, not only to preventcontamination of the resin, but also toprevent possible damage to theextruder by tramp metal or othermaterials?

Extruder Drives1. Most extruders are equipped with

some type of variable speed drive forconsistent output control. Are allfluctuations in revolutions per minuteor power consumption, as indicated bythe screw tachometer or driveammeter, checked along with othercauses for extruder surging?

2. Are all dirty air filters replaced?Otherwise, sensitive solid-state drivecomponents can overheat and burnout.

3. Are the extruder transmissions andthrust bearings properly lubricated?

Extruder Heaters and Controls1. Are temperature controllers for each

extruder zone checked for excessivetemperature override or insufficientheating?

2. Are the melt temperature andpressure indicators and the screwtachometer and drive ammeter —

which all indicate extruder operationstability — frequently monitored andmaintained?

3. Are all heater bands and thermo-couples around the extruder head,adapter and die checked for loosewires? Be sure the power is off when acheck is made.

4. Is the wiring for the heater bandscorrectly connected, e.g., not “inseries” if “in parallel” is required orvice versa.

5. Are thermocouple leads correctlyconnected to their correspondingextruder zones? For example, thethermocouple lead for zone 3 shouldnot be connected to the temperaturecontroller for zone 2.

6. Are all the heater bands for a specificextruder zone of the same size andwatt density? Mixing different bands inthe same zones can cause cool areasor hot spots depending upon thermo-couple location.

7. Are the die heater band terminals notaligned in a row? If they are aligned ina row, the resulting cool spot can cre-ate a gauge band in the film.

8. Is the extruder regularly checked forloose thermocouples, burned-outheater sections and loose or brokenwires? Most of the time, these prob-lems are not readily visible and findingthem requires specific maintenance.

Extruder Cooling1. Is there a specific instrument that

checks for overheating in the extruderdrives?

2. If the extruder is water-cooled, are thefeed throats cool to the touch?Inefficient plumbing that does notforce all of the air out of the water-cooled chambers of the feed throatcan cause poor circulation and hotspots on the surface of the feed throat.

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Resin can melt and stick to these hotsurfaces, bridge and interrupt the resinfeed to the extruder. The result is areduction in output, surging, or in time,the complete loss of feed andshutdown of the extruder.

3. Has the automatic barrel water coolingsystem been checked recently? Thiscan be done by lowering the set pointof the barrel zone temperaturecontroller for a moment, listening forthe pumps to start and observing atemperature drop on the instrument.

4. If air is used in automatic barrelcooling, has the blower exhaust beenchecked for hot air output?

5. If water cooling is used on the screw,are both the incoming and outgoingwater flows at the correct tempera-tures? Cooling water is normally fedthrough a siphon tube to the end ofthe screw. The outgoing water modi-fies the temperature of incoming waterbefore it reaches the end of the screw.Otherwise, if the incoming water weretoo cold, it could freeze the melt.

6. Is the water flow in both feed throatand screw cooling areas controlled onthe output side only? Control on theinput side can lead to cyclic heatingand cooling, caused by the periodicdisplacement of water with steam(water hammering effect), leading tosurging.

7. Are all the extruder water systemsfiltered and are all the filters checkedregularly?

Rotator, Die and Air Ring1. Is a record kept of screens used and

date installed?

2. Is the pressure gauge at the end ofthe barrel checked often? A highpressure indicates a filled screen. Alow pressure indicates a blow-out.

3. Are the die and air ring level andparallel to each other, with the air ringconcentric with the die orifice?

4. If the air ring is raised above the dieface, is there any dirt between the dieface and the bottom of the ring? Dirt inthis area can interrupt the air flow of meltand cause gauge bands in the film.

5. Is there any dirt or contamination onthe lips and venturi tube of the airring? Die lips should be regularlycleaned with brass shim stock orknitted copper cleaner pads.

6. If die and/or air ring rotator drives arein use, are the commutator rings andbrushes required for full rotation regu-larly cleaned?

7. Are the power and thermocouplewiring checked for freedom of move-ment and any scuffing, if rotatingsystems are in use?

8. Is the air hose used to inflate thebubble removed before die rotationbegins?

Tower and Line

1. Is the bubble symmetrical around aplumb line dropped from the centerof the main nips to the center of thedie? This symmetry is essential forwrinkle-free film.

2. Are the collapsing framessymmetrical with each other and thetops aligned with the main nip rolls?If not, the collapsing film can bedistorted as it enters the nip rolls,resulting in continuous orintermittent wrinkles on one or bothedges, depending upon themisalignment. Also, too large a gapbetween the top of the collapsingframe and the bottom of the niprolls allows the collapsed bubble topartially reinflate and cause edgeand/or full “smile” wrinkles acrossthe width of the film.

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3. Is gussetting equipment easilyadjustable? Does it have a lockingIave a matte finish to provide arelatively friction-free surface forthe film to slide upon? Film tends tocling to a brightly finished metalsurface.

4. Do the metal surfaces of equipmenthave a matte finish to provide arelatively friction-free surface for thefilm to slide upon? Film tends to clingto a brightly finished metal surface.

5. Are the collapsing frames cleanand smooth with no sticky coating,dirt or dust? If the nip rolls aresteel and rubber, it is preferable topart the film from the smooth drysurface of the steel roll rather thanfrom the rougher rubber surfacewhich can become tacky andinduce MD wrinkles. All rolls shouldbe clean and dry (not sticky) and allidler rolls should turn freely.

6. Are nip roll and other line driveschecked for speed adjustment?

7. Are the variable speed nip and linedrives individually controlled? Thissituation is difficult for an operatorto synchronize and can causeexcessive scrap when thedownstream nip is permit ted tooverdrive the primary nip.

8. On lines equipped with drives thatdepend on the main nip roll driveas a master speed reference for alldownstream nip roll (andequipment) drives, is the main niproll overdriven by excessive webtension (trim pots turned up to highon downstream drives)? Thisexcessive tension can result in theloss of line speed control,excessive speed hunting in the lineand a large amount of off-size scrapfilm.

9. Is the blown film tower stable withno vibration or sway?

10. Is the tower isolated from drafts?

11. Are any s l ight ly d iagonal MDwrinkles or web wander visible?Although it is usually beyond theability of an operator to tram (align)rolls in a tower, these defects canresult in misaligned rolls.

12. Are treater bars properly gappedand parallel with the treater rolls?The dielectric covering of the rollmust be free of punctures and theroll grounded back to the treaterthrough a carbon brush to theshaft.

13. Are in-line embossing, slip-sealing,re in f la t ing and postgusset ing,printing and bag making equipmentchecked regular ly according toprescribed procedures?

14. Are winder drive and tension controlschecked and the winder set toaccommodate the core sizes andwidths for the product to be run?

15. Finally, are roll doffing equipment,cores and packaging suppliesreadily available?

Tools and Adjustments1. Does the operator have the tools

necessary to operate and adjust aline?

2. Is the film thickness gauge calibratedregularly to maintain quality filmstandards?

For the OperatorWhen you go on shift or take over

another line, do you “stop, look, listenand feel?”

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Dies and Air Rings

Setting Up the Die and Air Ring:1. A die should always be centered

beneath the main nip roll assembly. Aplumb line dropped from the center ofthe nips should be no further out ofplumb than the edge of a dime placedon the center of the die face.

Obtaining a true plumb line can be aproblem if you use a die from anotherline or a new die which has anincorrect adapter length. The furtherthe die is off-center, the more the filmis distorted by the uneven distances itmust be drawn to the nip rolls. Thiscause of both wrinkles and poorgauge cannot be corrected by theoperator.

2. The top of the die should be leveledonly after the die and extruder are upto operating temperature, the barrelclamps tightened to the die adapterand all casters on the die cart in firmcontact with the floor. The die cannotbe leveled separately from the ex-truder for several reasons:

A. Most floors are not level, whichmeans the die must be leveled inplace.

B. The distances from the floor to thecenter-lines of the die adapter andthe extruder barrel increase atdifferent rates as they are broughtup to operating temperatures.

LOOK —Are the resin, additive and recyclesystems correctly adjusted and func-tioning properly?

Are the melt temperature pressuregauges and temperature controllersfunctioning?

Are there any temperature over- orunder-rides?

Are bubble shape and frost line nor-mal?

Are there any wrinkles in the web?

Is the trim too narrow or too wide?

How does the roll look on the winder?

LISTEN —How do the extruder drive system,resin feed system, barrel coolingpumps, relays and treater sound? Anyvariations in sound can be anindication of trouble. If you becomefamiliar with sounds of the line inoperation, many times you can stop aproblem from becoming a larger one,if not prevent it altogether.

FEEL —Is there excessive heat or vibration inthe extruder transmission and drivebearings?

Is the air chiller functioning in the airring?

Are water cooling lines cool, warm orhot?

Is the web tension too tight or tooloose?

How hard is the roll across the face?

Many elements of a properly designed,installed and adjusted blown film lineremain trouble-free for a long time. Otherparts may need repair often. Mostmalfunctions show up quickly in the form

of some defect in the film. Other sectionsof this booklet help pinpoint the causes offilm problems and explain how to correctthem with a minimum loss of time andproduct.

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C. If any of the die stand castersare not down tightly on the floor,there is a good chance the die isnot level. There is also a chancethat the die and air ring willvibrate because the extruderbarrel is supporting part of theweight of the die and air ringassembly. This vibration canmanifest itself as air ringchatter.

3. Only after the die is properly leveledcan the air ring be leveled andcentered. Because the air ring is notusually fastened to the top of the die,but is dependent upon its own weightto hold it in position, a good operatoralways checks the air ring for centerand level when the bubble is down.With a bubble up, it is not possible tosee whether the die or the air ringneeds adjustment.

4. Finally, some air rings are set on aflame retardant pad on top of the die.Other air rings are not on such a pad,but are raised above the top of the diewith the air gap providing insulation.

5. If an air gap exists between the airring and the top of the die, the air ringis usually less sensitive (moreforgiving) to blower and top openingadjustment. However, this air gapmeans there is an air flow betweenthe bottom of the air ring and the topof the die that impinges upon the hotmelt as it extrudes from the die. Anydirt, pieces of purge or silicone greasein this open space can disturb the airflow and lead to gauge bands in thefilm.

Die and Air Ring Adjustment1. Most dies currently in use are ad-

justable. Figure 6 illustrates the twobasic designs of adjustable dies andhow they are identified:

A. In the most common configura-tion (upper diagram), the adjust-ing bolts are in the die ringabove the die body.

B. In the older configuration (lowerdiagram), the adjusting bolts arein the die body below the diering.

On design A, the adjusting bolt isturned in to open the die gap, whileon design B, the adjusting bolt isturned in to close the die gap.

2. When adjusting a die,

A. You are only going to open orclose the die gap by one or twothousandths of an inch andmaybe a quarter turn of the boltat the most.

B. Except for a few very specialdies, you cannot open and closethe die gap in one place. Try it,and you strip the bolt threadsfirst. What you are trying to do iscenter the die ring around the

Film Die Gap Adjustment

Figure 6

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die mandrel so the die gap issymmetrical all the way aroundto extrude a film of constantthickness.

C. The die ring can only be adjustedby pushing on it, because there areno pull bolts. After choosing thebolt or bolts to push, loosen allother bolts until each is about one-eighth of a turn back. This preventsthe die ring from moving too far if itis suddenly released.

D. Do not try to make major adjust-ments as they may result in thebubble coming down.

E. When the adjustment is complete,bottom but don’t tighten the re-mainder of the bolts to hold thering in place.

3. In explanation of an apparentanomaly, thick film comes from anarrow die gap and thin film from awide die gap. The thick, hot melt froma wide die gap holds its heat longerand draws down to thinner film than

the hot, but thin, melt from the narrowdie gap which cools and freezes fasterinto a thicker film.

4. As shown by Figure 7, the air ringmust be clean. The air ring in Figure 7was dropped on its edge one time.Since then, this air ring’s internalperforated metal baffles have had atendency to clog in two areas 180°apart. One baffle section is shown inFigure 18.

In an experiment, a hot die wasleveled and centered to a uniform0.038 inch hot die gap. The air ringdescribed above was then leveled andcentered on the die. The two gaugeprofiles taken from film blown with thisset-up illustrate the difference clean-ing an air ring can make.

The first film profile sample inFigure 7 with the two high and twolow film gauge variations oppositeeach other also illustrates the type ofdefect that can occur when an air ringis off-center or tipped and not level.The operator, not knowing his air ringwas askew, tried to adjust the die.Figure 8 illustrates film from a die thatis out of adjustment. Note the 1.3 milfilm from the wide 0.041 inch die gapat point C and 1.65 mil film from thenarrow 0.035 inch die gap at point A. Asimilar gauge variation can be causedby an off-center air ring on a centereddie.

Figure 9 illustrates the gaugevariation that occurs in a film from acentered die and a tipped air ring.This also could just as well be from anoff-center air ring.

5. Finally, in the last three figures, notehow the bubble shape and frost lineappear in the areas of thick and thinfilm. Variations in bubble shape andfrost line are usually the first and bestindicators of changes in the quality ofthe film being extruded.

Film Gauge Variation Caused byDirty Air Ring

Figure 7

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Figures 7, 8, and 9 illustrate datadeveloped from film extruded with aneight inch die and air ring on a 3½ inchextruder.

Maintaining and ImprovingFilm Output

Causes of decreased output rates shouldbe corrected immediately to preventschedule and quali ty problems fromescalating. Several factors can cause short-term reductions in output, and in somecases, eventually result in other, moreserious problems.

Productivity can be viewed in three ways:

Maintaining Satisfactory OutputImproving on that OutputReducing Scrap

What is Satisfactory Output?Determining whether your production line

is performing at top efficiency is difficult. Tosimplify the calculation, lines can be dividedinto two types:

Those dedicated to a specific resin andproduct

Those used to run several differentresins and produce a broad range ofproducts

Resins include LDPE, HMW-LDPE,LLDPE, MLLDPE, HDPE, HMW-HDPE andblends, plus additives and reground film,such as edge strip and scrap. Productsinclude clarity sheeting, multi-wall bagliner, merchandise bags, grocery sacksand garbage bags.

Output is probably best defined as thepounds per hour (PPH) of a resin that canbe extruded through one circumferential inchof die gap (DG) or PPH/inch DG.

Figure 8

Film Gauge Variation Caused byPoor Die Adjustment

Figure 9

Film Gauge Variation Caused byTipped Air Ring

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Example:

If a 16 inch die has a 50 inchcircumference and operates at 12PPH/inch DG on high claritysheeting, the instantaneous dieoutput is 600 PPH.

If the sheeting is 48 inch wide andrequires a 1 inch trim from eachedge of a 50 inch layflat, 4% ofthe instantaneous output is beingrecycled and there is a net outputof 576 PPH of sheeting off thewinder.

To obtain the average PPH, divide thenumber of premium pounds made by thenumber of machine hours required tomake them. The closer the average PPHis to the instantaneous die output in PPH,the higher the efficiency of the line.

Dedicated lines usually operate from 10to 25 PPH/inch DG and up withrefrigerated air cooling the bubble. Higheroutput rates are obtained with internalbubble cooling and external cages to sizeand stabilize the bubble. At higher outputrates, the line versatility usually is limitedto variations of a single product extrudedfrom one type of resin.

More versatile film lines may includethe capability to wind tubing flat orgussetted, make centerfold or "j " film,produce single or double wound sheetingand, with a bag machine ahead of thewinder, generate single bags or bags in aroll.

Instantaneous output for LDPE orblends with LLDPE, plus additives andregrind, ranges from 8 to 25 PPH/inch DG.The output is largely governed by theproduct requirements of size, gauge, BURand whether the end product is woundfilm or bags.

Remember, a die can act as a valve onthe output of an extruder when you reviewthe following table of output rangesexpected from six different sizes of 24:1

L/D (length/diameter) extruders. Therange is necessarily broad to allow forage, wear, screw-design, drive, powerlevel and resin to be extruded.

Extruder Size Drive, hp Output, PPH(inches)1½ 5 - 15 20 - 601¾ 7½ - 25 30 - 1002 10 - 40 40 - 1502½ 25 - 60 100 - 2503½ 40 -125 120 - 5004½ 60 - 250 240 - 1000

By comparison, on dedicated lines withhigh horsepower extruders with 28 to 32:1L/D, the output rates can be 20 – 30%higher than those in the table.

Maintaining OutputBoth slow and rapid reductions in

normal output rates can occur for manyreasons. The true cause needs to bequickly identified and appropriatemeasures taken to correct the problem. Ifincorrect solutions are applied, theproblem can be compounded.

Some causes of reduced output areresin-related.

Fines and Fluff in the Feed StreamFines are bits of pellets that adhere to

the walls of transfer system piping andother equipment through which resinpasses, propelled by forced air. Resinfines contain a disproportionately largeamount of slip additive, as much as 8 to12%. This is because slip is intentionallyincompatible with the film resin so that itblooms to the surface as the film isextruded and performs its function ofpreventing blocking. Slip also blooms tothe surface of the pellets. Thus, becausethe surface of the pellets is the usualportion in contact with equipment walls,fines contain higher levels of slip than theresin from which they come.

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Fines that coat the upper, innersurfaces of the hopper and then fall intothe extruder cause a sudden increasedconcentration of slip, which greases theextruder screw. Melt throughput andbubble gauge both decrease for a timeuntil the extra slip clears through theextruder. Any overreaction to thisproblem, such as increasing extruderspeed, quickly results in a film with anunwanted increased gauge when the sliphas cleared.

Fluff, when recycled and added to thefeed stream, leads to an unpredictabledecrease in the bulk density in the hopper.The feed section of the screw — which isonly a conveyor — senses only bulk, notweight. The feed section does not speedup to maintain delivery of pounds of meltto the die. Thus, output and gauge godown when significant amounts of fluff gothrough the extruder. Similarly, if fluff feeddecreases, more virgin resin is fed tomaintain the bulk through the extruder,and output and gauge go up. If supplies ofscrap or fluff are small and intermittent,they can be saved and run for utility gradefilm. But if large amounts of scrap aregenerated regularly, then improved recyclesystems — proportional feeders, hopperstuffers, etc. — may be worth purchasing.

Blending of LLDPE and LDPEOn older machines with lower HP

drives, the addition of 15% to 20% ormore of LLDPE to LDPE can reduce theoutput. Raising barrel temperatures,particularly Zone 2 at the transition portionof the screw on some extruders, cansometimes eliminate the problem.

Some output problems are machine-related.

Sudden Machine Shut-DownWhen an extruder restarts after an

unforeseen shut down, it may deliver littleor no melt or it may surge. Heat from thescrew transition section has travelled backto the feed section, which in a neutralscrew, is normally cooled by the resinpellets fed to it. The feed section, nowhotter than usual, partially melts thepellets which stick to the screw, bridgingand partially or completely blocking thesection. The bridged screw, of course,does not deliver the expected meltoutput.

If the screw is not badly bridged, theadhering pellets can be cleared byinserting a flinch rod made from the resininto the feed throat while the screw isturning over slowly. The screw flight willcut sections from the rod, and these willbreak away partially melted pelletssticking to the screw. If bridging issevere, the screw may have to beremoved and cleaned. In any case, allsafety procedures must be carefullyfollowed.

For other than emergency shut-downs,the extruder and die temperatures shouldbe reduced to the point where the diedrools at a lowscrew rpm, while the massof the equipment cools before shutdown.Do not reduce the temperature to the pointwhere the melt can solidify and blow thedrive fuses or rupture a gate or die. Spe-cial heat-stabilized purge compounds areexcellent for shut-down because they flushthe system and do not decompose in theequipment as it cools.

Fluff Recycling LinesBridging is a particular problem with

lines that recycle fluff from ground trim andscrap film. Plug the front end of the bore ofthe screw and water-cool the first three tofive feed flights to partially solve this prob-lem and keep output at required levels.

16

Improper Water CoolingImproper water cooling in the extruder

can result in reduced output. For a discus-sion of water cooling, see the PreventionChecklist discussed earlier in this booklet.

Screen Pack PluggingAs screen packs gradually get plugged,

output just as gradually decreases. Inresponse, the operator often increasesscrew speed which increases melt pressureand temperature and raises the frost line.The hotter melt and subsequent higherfrost line can lead to an unstable bubble.

The operator’s solution works for only ashort time. The best way to prevent thisproblem is to keep good records of whenscreen packs were changed and haveoperators frequently refer to them. A meltpressure gauge at the end of the barrel alsoprovides a warning that screens areplugging.

Improving OutputBarring the installation of newer more

sophisticated blown film lines in their en-tirety, the output of older lines can beincreased by several means, assuming thelines are well designed and have beenmaintained. With increased output how-ever, additional tower height for coolingand increased corona surface treatmentcapacity may be necessary.

Melt Separation ScrewsNew, more efficient melt separation

screws with a barrier flight and mixing headcan be retrofitted for the extrusion ofLDPE, HMW-LDPE, LLDPE, HDPE, HMW-HDPE and blends, plus additives andreground material. These screws do nothelp an extruder that is under-powered butshould improve both film quality and outputof any extruder with an adequate drivematched to the transmission, thrust bearing

and extruder size. The addition of a newlow pressure die with a second mandrelextension to provide a wider die gap toextrude LLDPE and a dual lip air ring willimprove output rates with both LDPE andLLDPE.

Tower Modifications

To run HDPE, the tower must bemodified so the height of the main nip rolland collapsing frame assembly can bechanged. Furthermore, an adjustableheight iris must be added above the die forbubble stabilization. Also, a new, smallerdie and air ring may be necessary toaccommodate the 4:1 BUR required toobtain a high strength HMW-HDPE film.

Bubble CoolingRefrigerated air can stabilize and cool a

bubble produced by the hotter meltgenerated by increased output from an oldscrew or the additional melt output from anew mixing screw. Improved and moreconsistent cooling also results in bettergauge and less scrap. If refrigerated air isused, the air ring must be corrosion-resistant or given a protective, moisture-resistant coating, and the outside of thering should be insulated to eliminatecondensation. Condensation dropletswhich fall into the air stream strike thebubble and cause it to snap off. When theline starts up, this chilled air must beturned on to bring the ring to operatingtemperature and blow out any condensedwater droplets before the bubble is pulledup.

If all equipment operates smoothly, filmis now going through the main nips at ratesfast enough to require a line speedincrease. Many lines never operate atspeeds more than one-half to two-thirds oftheir maximums. With the commonly usedDC and eddy current variable speed drives,the slower line speeds can result in a lossof control and additional scrap.

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To illustrate, a line with a maximumspeed of 150 fpm, driven through areducing transmission by a drive with ± 1%speed control, can vary ± 1.5 fpm. If theline is run at a slow speed of 15 fpm, thissame 1.5 fpm translates to a ± 10%variation in line speed.

Four-speed transmissions that enablethe line drives to operate in the top third oftheir speed ranges with acceptable speedcontrol, regardless of line speed, are anexcellent investment for the multipurposeline. The amount of scrap produced instopping and starting the line to shift gearsshould be relatively low; actually, theamount of scrap should decrease and thefilm quality improve.

Decreasing ScrapOne way to increase output is simply to

reduce scrap. A line producing scrap is morecostly than a line sitting idle. A line producingscrap is not productive; it costs money tooperate; and it is turning out material thatrequires further handling to salvage.

Scrap can result from simple humanerror: wrong resin for the job,misinterpretation of specs for size andgauge, etc. Scrap can also be the result ofnot scheduling production orders in asequence that minimizes changeover fromsize to size, gauge to gauge, or resin toresin. These problems cannot be solved inany specific way; individual shopconditions must be considered.

Two other common sources of scrapcan, on the other hand, be easilyeliminated. Adding simple roll doffingequipment gives the operator greatercontrol in removing the roll of film from thewinder and placing it intact on a pallet orin a carton.

Having operators exchange informationabout current operations at shift change isanother way of preventing problems thatmight lead to increased scrap. It is onlyhuman for operators to feel they knowbest. When the shift changes, the firstmove an operator may make is to readjustcertain controls. The effects of thesemoves may not be apparent for severalhours. By this time the operator hasforgotten the specific changes he or shehas made — so the results areproblematic. The solution to suchoccurrences is to have some overlappingtime at shift change so operators canexchange information.

Roll and Film DefectsAll film defects ought to be detected by

the film extrusion shop manager orforeman before the roll gets out the door.But in practice, through oversight,inattention to detail or unexplainablecircumstances, defective film can goundetected until examined by thecustomer.

Overall film appearance is mostimportant. A good-looking, “machinedcylinder” roll of film is what the customershould receive. A superficially dirty orimpaired roll, which may actually convertlike a charm, can have the customerexamining the entire order for real orimaginary defects.

A machined-cylinder roll has no scuffmarks, damaged edges, fuzzy ends,protruding or buried core ends or crushedcores. Other, less obvious defects can beburied in the roll, but these apparentdefects should have been caught andeliminated by the operator.

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Wetting test kits are available, andinstructions should be followed closely ifconsistent results are to be obtained. Ink canbe applied from an aerosol can or by by ananilox roll. The film must be completely drybefore tape is applied to check adhesion.

To assist the customer, wind a one-side-treated film with the treated surface inside oroutside, as specified, and so label the box.Causes of poor treatment include:

1. Under-powered treaters

The treater must be matched to lineoutput rates and web speeds.

2. Improper grounding

The treater roll should be groundedthrough a carbon brush assembly. Thiscan improve its efficiency. Groundingthrough the treater roll bearing isinefficient because the bearinglubricant acts as an insulator.

3. Film too cold

Warm film is easier to treat. The top ofthe tower, just after the main nips, isthe best location for the treater station.

4. Wrinkled film

Film must lie smooth on the roll, andthe treater bars must be positionedparallel with the roll.

5. Improper levels of slip additive

For example, if a film resin with sliplevels designed for 1-mil film is usedto produce a 4-mil film, four times asmuch slip additive blooms to thesurface, resulting in a white coating onthe printing plates or a powdery build-up on the working and sealingsurfaces of bag-making machinery.

6. Excess silicone grease on the die andair ring surfaces

Both ink adhesion and heat sealabilityof the film can be adversely affected ifthis grease gets on the film. Excessgrease can also contaminate thesurfaces of the collapsing frames, niprolls and idler rolls.

Visible Roll DefectsVisible roll defects can usually be

classified as those caused by defective filmor poor web or roll handling. In somecases, the same defect can result fromeither cause.

1. Rolls with corrugated (uneven)ends can be caused by:

Poor tracking (web wander) inthe line ahead of the windingstation.

Variable width film resultingfrom an unstable bubble. Aweb guide located before thewinder magnifies the conditionby positioning the film on thecore so that the guided edgeof the roll winds evenly; as aresult, the the depth of thecorrugations at the oppositeend doubles.

Variable tension in the edgetrimming or slitting station. Atlow tension, the sheet iswidely slit and at high tensionthe sheet is narrowly slit.Again, web guiding into thewinder can magnify thecondition by placing thedefect at only one end of theroll.

2. Fuzzy roll ends can be caused by:

Dull cutting blades

Poorly aligned blades

Edge slit sheeting because the filmcannot be perfectly guided in orderto maintain the blade exactly in thecenter of the edge fold.

3. Symmetrically deformed rolls can becaused by:

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BlockingBlocking, or film layers sticking to one

another, can result from:

1. Too much winding tension

Tension levels can easily beoverlooked unless they are so greatthe core is crushed, making itimpossible for the operator to removethe roll from the winder shaft. If thecore is crushed only slightly, theoperator may be able to remove thecore from the shaft, but the customermay then find it impossible toremount it on a let-off stand. Just asbad a problem can arise if the windingtension is too low, blocking will notoccur, but the film may telescope outfrom one end of the roll, making thefilm impossible to convert.

2. Hot weather

The inner surfaces of insufficientlycooled film block as they pass throughthe nips. To correct:

Reduce output

Raise the main nip rolls

Use refrigerated air or other meansof increasing cooling efficiency

Reduce nip roll pressure

3. Inefficient air ring

4. Incorrect additive levels in resin

If the additive levels are designed fora 4-mil film and you are producing a 1-mil film, not enough slip and antiblockadditives are present to preventblocking from occurring.

5. Over treatment and/or high gloss

Tendency to SplitFilm with splitty tendencies can also be

overlooked unless samples are examined.The conditions below can occur singly or inconcert, leading to an increase in thefilm’s orientation in the machine direction.This makes the film easy to split.

1. Insufficient cooling2. A high frost line3. Too low a blow-up ratio4. Nips set too tightly, particularly with

an old and hardened rubber nip roll,leading to film deformation in theedge crease

5. Die lines or bad weld lines from thedie

6. Scratches from splinters in thecollapsing frame or other sharpobjects

7. Very fine weld lines caused bydegraded particles of resin or dirtlodged under or in the die lips (seeFigure 10).

Printing Problems —TreatmentIn-line treatment (or mistreatment) to

enable printing is the key to manyproblems the customer is often the first todiscover.

The two most common methods usedto check film for treatment are by asurface wetting test and by ink adhesion.

Figure 10 The weld line in the film resulted fromdirt hung up below die lands. The dieline resulted from a damaged diemandrel.

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Uniform gauge variations acrossthe width of the film. They may ormay not be immediately visible tothe customer, or to the operatorwho made them. However, if youmeasure the roll circumferences ateach end, you will find this defect.

Figure 11 illustrates asymmetrically deformed or tapered rollof film. The calculations show thatonly a very slight gauge variation wasrequired to cause this defect.Measuring 0.00002 inch is beyond thecapability of any in-plant micrometer,and measuring ten thicknesses of thefilm to determine a 0.0002-inchvariation is also difficult with mostavailable equipment.

Furthermore, the tapered roll hasan almost uniform surface hardness,although the large end may be slightlyharder than the small end. Thecustomer finds the defect quickly,particularly in a high speed printingoperation, because the one ripplededge, caused by the camber in thefilm, tracks in a straight line throughthe converting operation.

Figures 12 and 13 illustrate thesame type of roll defect. However,because the film is supported in theroll, either at the center or at bothends, by the thicker film, there isusually a noticeable difference insurface hardness across the roll face.This causes baggy film or ripplededges.

Figure 11

Cross Section of Film

Convex Roll

Figure 12

Concave Roll

Figure 13

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This defect manifests itself on alarge sheeting line where two or morerolls are cut from the web and woundon a single shaft. As soon as the thinfilm at the small diameter end of theroll is cut free from the thicker film atthe other end of the roll, the thin filmend winds with little or no tension.Then, either while winding or doffingoff the shaft, the roll will telescope,resulting in a good deal of scrap.

Often this film defect is uniformover a period of time, indicating acontinuous problem in production. Arise in the temperature or warm aircooling the barrel off the front end ofan extruder, parallel with the line andwinder, can cause a roll of tubing, orthe roll of sheeting from the back ofthe bubble above the extruder, to havea concave face. If the extruder is at aright angle to the line, the rolls cantaper. A flame retardant cover willusually solve the problem by providingsome insulation. Frequently, the intakeof an air ring blower can createenough draft to cause the problem.Hot air heaters or floor fans are othersources. If the problem occurs onlyduring the day, the cause may be anopen door that usually is closed atnight.

Tapered rolls can also result if thedie and/or air ring are tipped. In thiscase, the film from the lower side ofthe die or air ring has to travel further.Die and air ring rotation cannot correcttapered, convex or concave rolls(Figures 11, 12 & 13) because thebubble does not rotate (see BlownFilm Basics).

These defects occur in the bubblein its most sensitive area, between thedie and frost line. The finished film isformed between the die and frost lineby the simultaneous blowing anddrawing of the tube of hot melt

extruded from the die. It takes onlysmall outside forces (light air currents,heat, stretching) to deform the bubbleand cause these defects.

The same defects can be causedby something as simple as an unevenclosure across the nip rolls. In thiscase, the film is not pulled through thenips with equal force at both ends.The film may actually slip at the lesstightly closed end.

These roll defects generally do notoccur on lines with overhead rotation.In general liners with fewer layers willhave die rotation, while lines with threelayers or more will have overheadrotation. The reason for this is that theseals in a multi-layer die are too complexto allow die rotation.

Early versions of overhead rotationare very complex in nature, while newerversions are quite reliable and cause fewproblems. Older versions frequently donot rotate concentrically, which causesnon-uniformity that is directlyproportional to the time for one cycle ofthe overhead equipment.

4. Defects found more frequently on largelines

With the exception of fuzzy roll ends,the roll defects discussed in this sectionoccur more frequently, and producemuch more scrap, on large blown filmlines, particularly those producingsheet. It is easier to notice — andcorrect — an unstable bubble 16inches in diameter making a 25-inchlayflat than it is with a bubble 48 inchesin diameter making a 75-inch layflat.

The sheer size of a 48-inchdiameter bubble makes it moresusceptible to vagrant breezes anddrafts in the area than the 16-inchdiameter bubble. Similarly, rollalignment for a 75-inch layflat requiresmore attention to prevent wandering.

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Problems are further compoundedwhen the film extruded is high claritypackaging to be printed and convertedat high speeds.

To minimize some of the defects thatoccur more often on larger lines, non-contact gauges for the precisemeasurement and control of f i lmthickness may be economically justified.

Larger lines offer increased output andreduced labor needs, but operatorshandling thee lines need greater skill andtraining — and they command higherwages.

Gauge VariationsGauge variations occur both in the

length of the web in the machine direction(MD) and across the width of the web inthe transverse direction (ID).

MD Gauge VariationGauge variation in the machine

direction is generally associated withextruder or line take-up problems, ratherthan with die and air ring problems. Cyclicsurging of the extruder causes MD gaugeto run on the plus side as outputincreases, and on the minus side asoutput decreases. In severe cases, MDgauge variations can be accompanied bya change in layflat web width as the frostline rises and falls with variable output,causing the bubble diameter to change.

Each surge cycle may last from fiveminutes up to several hours. Therefore,changes in frost l ine height or bubbleinstability may not be immediately apparent— even less so on large lines because of thesize of the bubble involved (see previoussection).

Extruder SurgingExtruder surging can be caused by

common maintenance problems such asloose thermocouples, defective tempera-ture controllers, burned out barrel heaters,variable water supply for screw and/or feedthroat cooling, etc. Other causes include:

1. Overheated extruder drives

The speed of these drives should beregulated at 2% or less of base motorspeed with a 95% load change. Solid-state DC power cabinets should belocated where temperatures do notexceed 100°F or they should be fittedwith a filter and cooling blower.

2. Running high speed extruders at halfspeed

A variation of 2% of 1,800 rpm basespeed is 36 rpm; but at 900 rpm, 36 rpmis a 4% variation. It is better to changedrive belt sheaves or gears if a range ofscrew speeds is desired. It is moreefficient to operate in the upper third ofthe drive speed range.

3. Automatic on/off barrel cooling

Surging can result, particularly whentwo or three zones are cooledsimultaneously. Proportional control isneeded to eliminate shock cooling.

4. The wrong screw and/or barreltemperature profile

5. Adding fluff or scrap at variable rates

These additions alter the bulk densityof material in the extruder feed hopper.

6. Partial or intermittent bridging ofmaterial in extruder hopper, feed throator on the screw.

Cyclic SurgingCyclic surging of the line speed can

duplicate and be indistinguishable fromeffects of extruder surging. Causesinclude:

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Particularly when the line is run atlow speeds, a means of maintainingdrive speed in the high range is needed.Quick-change gear boxes, sprockets orpulley changers can all be used be-tween the drive and the nips to keepthe drive speed high.

3. Mechanical and electrical faults

Partially seized bearings, loose drivesprockets or chains, loose wiring, etc.,can all contribute to variable linespeed.

For all extruder line drives, a driveammeter can immediately indicate thearea of trouble but not whether it ismechanical or electrical. The actualcause of the surging can becomplicated, such as the combinationof an overheated DC power source withthe motor operating in mid-speedrange. The problem is intermittentbecause the DC power source coolsdown at night and warms up during theday, resulting in the system alternatelyin and out of an acceptable speedcontrol range.

Another mechanical cause of surgingcan be a loose drive chain tensioner witha sprocket loose and slipping on the shaft.The guard can be removed and the loosedrive chain fixed, but the loose andslipping sprocket can remain unnoticed.

If MD gauge variation is suspected,the die and air ring rotation should bestopped, a stub roll of film made and thencut open. At one point, the gauge shouldthen be checked down through the layersof film. Layflat web width can be checked

1. Too much tension in the web

Winder or secondary nip rolls thatoverdrive the main nip rolls can causespeed to vary and an increase in webtension.

2. Poor regulation of electronic drivespeed

at the same time. Non-contact thicknessgauges will indicate MD gauge variationas cyclic average changes.

TD Gauge BandsGauge bands in a roll of film are caused

by variable film thickness in the transversedirection (TD), or across, the web. As thefilm is rolled up and the roll increases indiameter, a build-up of thicker areas of thefilm occurs. At the same time, the thinnerareas of the film stretch down into thevalleys between the thicker bands. Thegreater the winding tension, the morepronounced the bands become and themore distorted (not flat) the film is when

Figure 14

Maximum Distortion

Minimum Distortion

Figure 15

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unwound from the roll (Figure 14). Rotatingthe die and air ring can minimize but noteliminate this distortion in the film bydistributing the gauge bands evenly acrossthe roll face (Figure 15).

Bubble shape is a fair indication of filmthickness. A symmetrically shaped bubbleindicates that there are no gross gaugedefects in the film. An unevenly shapedbubble suggests several problems. First,the die is not centered and needsadjustment, but only if the operator has setup the film line well. If the set-up is notgood, the air ring may not be concentricwith the die or, if not firmly mounted, maybe cocked and not parallel with the top ofthe die.

For unpigmented films that are not toocloudy, the evenness of the frost line isanother excellent indication of filmthickness (Figure 16). In a misshapenbubble, the frost line is cocked and notparallel with the top of the die and air ring.The high and low areas in the frost lineindicate areas of poor gauge and thelocation of problems in the equipmentbeneath the bubble. If these areas of

uneven film gauge do not rotate when thedie and air ring rotate, then the problemmay be due to drafts striking the hot meltbelow the frost line, or some defect in thenon-rotating body of the air ring.

The installation of 500-watt floodlightsin the film line area helps make the frostline fully visible to the operator. The lightsshould be mounted just above the usualfrost line height, at least 30 inches from thelargest bubble blown and aimed downthrough the frost line area towards thecenter of the bubble. The lights should beturned on during start-up for die and airring adjustment or frost line check duringproduction. The lights must be mountedcarefully; if they are too close to the frostline and bubble, their heat can result ingauge bands.

Some other causes of gauge bandsinclude:

1. Drafts of air on the bubble

Gauge bands caused by drafts areimmediately visible on a roll of filmextruded from a rotating die and airring. When stationary equipment isused, the cause is not so obvious. Oftena curtain, shroud or enclosure is placedaround the bubble to eliminate drafts.However, if there is a hole in the enclo-sure, air hits the bubble. Other causescan be a leak in a non-rotating air ringbody, a leak in a near-by air line or apiece of metal deflecting air upon aportion of the melt. The resulting gaugebands can be from less than an inch toseveral feet wide.

2. Defective or poorly adjusted dies

Older design, stationary side-fed dies withtapered mandrels, which have beensprung by a cold start, usually producefilms with two high and two low areas ofgauge opposite each other.

Figure 16: A well-shaped bubble withhorizontal frost line visuallyindicates good gauge control.

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air. Blockage of the air ring (Figure 18)causes gauge bands.

The same defect can occur when a diehas been adjusted eccentrically and the airring moved off-center to the die to align it(Figure 7). Off-center adjustment usuallyoccurs if dies have not been cleaned fora long time or if they have beenimproperly assembled after cleaning.Degraded and carbonized melt that hasleaked inside the die has made the dieimpossible to adjust. This defect isseldom caused by an out-of-round diering because, except for special flex-lipdies, it is practically impossible todeform a die ring. The threads of theadjusting bolts would strip first.

A die that has been damaged,repaired by welding, polished andreplated can still cause a gauge band inthe film from the welded area.Unfortunately, the weld metal never hasthe same heat transfer rate as theoriginal metal, thus affecting melt flowin the area and causing the gauge band(Figure 17).

3. Degraded material on the die lips

Even more noticeable than gaugebands are the lines and coarse surfaceof film extruded from a die with a build-up of degraded and carbonized materialon the die lips or in the die lands justbeneath them. Usually these depositsindicate excessive melt and dietemperatures. These deposits should becleaned from the top of the die with abrass shim stock inserted between thedie lands (Figure 17).

Other causes of die lip build-up areoffset die lips or die lips with wire edges,too large a radius or very fine nicks.These defects cause the melt tomomentarily drag across the face of thehigher die land as the melt leaves thedie.

4. Other die defects

Causes include:

Poorly designed internal flow pas-sages

Segments of a heater band burnedout

Loose heater bands

Die heater band terminals aligned onone side of the die

The last three problems cause thedie to be heated unevenly, affecting themelt flow in the die ahead of the dielands and resulting in gauge bands.

5. Defective or poorly adjusted air rings

An air ring’s first function is tostabilize the bubble; the cooling fromthe air used is a bonus. An air ringshould be machined as precisely as thedie and securely mounted parallel andconcentric to the top if the die. When adie is cleaned, the air ring should alsobe cleaned of dirt carried in the coolingFigure 17: A gauge band results from the

weld in the die lip. The build-upon the die lip drags and marksthe film surface.

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Figure 18: Dirt in screen caused 5" gaugeband.

Some air rings are mounted slightlyabove the die face with an air gap betweenthe ring and the die, while others are placedon a flame retardant gasket directly ontop of the die with no air gap. Acommon cause of gauge bands whenthe air ring is mounted above the die isan accumulation of silicon greasebetween the top of the die and thebottom of the air ring. This greasepartially blocks the uniform flow of airthrough this passage, resulting ingauge bands where the hot meltextruding from the die is not cooled.

However, despite this problem, not allair rings should be mounted on a gasket.Some types of air rings are difficult toadjust if there is no air passage betweenthe top of the die and the bottom of theair ring.

Essentially, gauge bands caused bydefective or poorly maintainedequipment cannot be corrected simplyby adjusting the die gap or centeringthe air ring. Such problems areeliminated only by proper set-up andmaintenance of the equipment.Rotation merely implants gauge bandsdeeper into the roll of film.

6. Excess silicone grease

Often used to keep hot melt fromsticking to the top of the die on startup, silicone grease, when used inexcess, is a very common cause of

gauge bands. Grease on the inner lip orin the throat of the venturi disrupts thering’s air flow. The gauge band’s widthis usually equal to about how far thegrease has been rubbed around the airring surfaces multiplied by the blow-upratio.

Bubble Trouble andBlemishes

The best film made from a die and airring can be rendered useless byinattention to the design, maintenance andset-up of the downstream bubble-collapsing and web handling equipment,including the tower and the winder.Operating conditions governing the take-off equipment all affect the film’sproperties and appearance andadjustments to one piece of equipment canlead to problems elsewhere. For example,opening the collapsing frames to reducedrag and obtain flatter film results in morespace at the top of the bubble. If air is notadded, the bubble diameter and the layflatwidth are reduced. In other words, anattempt to improve the film’s appearancecan result in decreased productivity. Allthe equipment in the film productionsystem is interconnected, a fact that mustbe recognized in adjusting operatingconditions.

Collapsing the Bubble

Conditions to check:

1. The main nips must be centered overthe die. The center of the die, bubbleand main nip rolls must be inalignment with one another. If thisequipment is not aligned, film fromone section of the die must travelfurther than film from another sectionto reach the nips. The greater the

27

3. Collapsing frames must be aligned.The best collapsing frame is free ofvibration and symmetrically mountedso it can be opened or closed aboutthe center line of the bubble by asingle control (Figure 20). Aligningthe four lower corners of the framesindividually is time consuming anddifficult to do without racking them,which would worsen rather than solvea problem.

There are two additional mountingfaults that commonly occur, eithersingly or combined, with collapsingframes:

A. If the frames are mounted too farbelow the nip rolls, the film canpartially reinflate above the frames,usually causing wrinkles into andout of nip rolls across the centerof the web.

B. Off-set or misaligned with the nipof the rolls, collapsing frames cancause continuous or intermittentwrinkling of the film edges out ofthe main nips.

Of course, collapsing frames thathave not been regularly cleaned,repaired or maintained, or rollerframes with rollers that turn unevenlyor not at all, can also contribute tofilm wrinkling. Wrinkles are also acommon occurrence after a transitionto a wider layflat film following a longrun of narrow tubing. Dirt collects onthe surfaces of the collapsing framesduring the narrow tubing run. Theedges of the wider tubing rub againstthese surfaces as the bubblecollapses. The result is wrinkles in thefilm as it moves out of the nip rolls.

However, despite perfect equip-ment alignment, determining theproper collapsing frame opening toachieve the flattest film is still, tosome extent, trial and error. Figure 4

misalignment, the more the film isdistorted and the more difficult theprocess of collapsing the bubble andprocessing the web downstreambecomes.

2. Bubble stabilizer bars must becentered. Bubble stabilizer barsshould be mounted so they can beadjusted symmetrically about thecenter line of the bubble. Off-centerstabilizer bars can lead to pulled andstretched film. The bars’ contactwith the bubble should be vibrationfree and minimal. Vibration andexcessive drag on warm, freshly-formed film can only lead to chattermarks, bagginess and wrinkles(Figure 19).

Figure 20: Collapsing frame with singlepoint adjustment. Arrowindicates crank handle.

Figure 19: Graduation on beam aidadjustment of bubble stabilizerbars. Drag causes wrinkles.

28

on page 4 suggests the closer theframes, the better the film. Unfortu-nately, the closer the frames, themore film in contact with the frame’swooden slats and the greater the dragto distort the film and make it baggy inthe center of the web.

The final frame opening is acompromise. The size of the bubble(contact area), line speed (dragacross the contact area), thickness ofthe film (remembering the amount ofslip not only varies from resin to resinbut also depending on the thickness ofthe film) and the resin used (degreeof stiffness) must all be considered.

4. Gusset blades must be aligned.

5. All other equipment must be inaligned or trammed. Not only mustthe tower be plumb and all the rollstrammed, but the film die must bevertical and aligned with the air ringand main nip rolls. Winders and otherequipment must be properly alignedand trammed with the tower if theweb of film is to track through themsmoothly.

Handling the Web Through the TowerAs the web of film moves from the main

nips to the winder, the film can bevisualized as a long rubber bandexperiencing constantly changing tension.To prevent the film from stretching as itmoves through the tower, this tensionshould be kept to a minimum. Excessivetension results in MD wrinkles that are thenfolded into the film as it passes over eachroll. The folds make the end productdefective and unacceptable. Howeverwrinkles will always appear in film on theline. Only on occasion do these wrinklesindicate a problem.

Air can carry over in the layflat from thebubble before the main nip rolls are closedat start-up. This air can reinflate the tubejust before it passes over an idler and

cause excessive MD wrinkles. The air canalso collect just ahead of a treater roll,resulting in poor treatment or treatment ofthe wrong side of the film where it hasfolded.

To prevent air collection, during set-up theweb should always be diagonally slit in anumber of places across its width right afterthe main nips to bleed off any trapped air. Ifair does appear, the nips should be checkedfor tightness and the film for size.

Wrinkles, slightly diagonal to the MD ofthe web, can indicate a roll that is eithergalloping or constricted. Again, because ofthe changing tension in the web (particularlyon older lines), film should be cut to widthonly between two pairs of nips that isolatethis station from web tension changes. If notisolated, film width can vary excessively withweb tension changes upstream. The web candrop right out of the cutting station knivesduring the momentary loss of tensiondownstream when film is cut over to a newcore on the winder.

Film AppearanceFilm appearance should first be defined

before being discussed. The term includeshaze (clarity), gloss (reflectance) andgeneral appearance (no imperfections in oron the surface). Good film appearance isthe result of the right combination of resin,equipment and process.

Resin quality is initially controlled by theresin supplier. The resin must meet thecustomer’s requirements for physicalproperties, additive levels, etc. However,even though prime resin is shipped to thecustomer, every time the resin is handled,there are additional opportunities for it to becontaminated, first by residual material inthe transfer system, and second, byresidual material in the container itself(Figure 21). Stones and dirt from transferhoses left lying on the ground alongsiderailroad sidings are often removed from thescreen packs of many extruders.

29

Figure 21: Gels were caused by moisturegetting into resin whendelivery truck was unloaded inthe rain.

Figure 22: Severe die lines indicate that adirty die is the probable sourceof gels in the film.

Resin stored in fiber drums or boxeswithout lines can pick up cellulose fibersthat can result in gels which can knockdown a 1-mil bubble faster than it can bereblown. Additional, sources ofcontamination are dirty, unchanged filtersin resin transport systems and the upperinner surfaces of extruder hoppers coatedwith layers of dust from many differentresins previously used. This material canintermittently fall into the extruder andcontaminate resin in the system.

Contamination also can come frominside the extruder by a new resingradually purging an old, degraded resinfrom stagnant areas in the system. A diethat has been overheated or that has notbeen cleaned often can cause gels in thefilm, particularly when a new resin isintroduced to the system. In both cases,the gels are usually random throughout thefilm and can be clear to amber to black incolor and look like tapioca and/orarrowheads (Figure 22).

Gels from a hole in a screen pack,from a burn in the die web or from a pointinside a side fed die where two melt flowsjoin, appear only in one area of the film.Again, one or more flood lights mountedto illuminate the frost line and the melt

below it can also help operators identifyand determine the type or origin of thegels.

Air ring chatter marks appear asclose, uniformly spaced lines around thebubble in the TD of the film. Highfrequency vibration of the hot melt justabove the die caused by improper airmovement or excessive air velocity fromthe ring are the usual causes. Normally,this vibration can be eliminated byadjusting the air ring and/or the blower.

Film can be scratched by beingdragged over any sharp or abrasivesurface. The source can be a splinter inthe collapsing frames, a sharp edge on agusset blade, dirt on the face of a“frozen” roll, etc. Scratch lines are usuallycoarse and white in color. On occasion,these lines are intermittent and arealways parallel to the MD of the film. Incontrast, die and weld lines have amelted-in appearance, and with dierotation, are slightly diagonal to the MDof the film.

“Applesauce” or “orange peel”surfaces in film can be a resin-relateddefect. These defective surfaces appearas faint to heavy aberrations in the film,distorting its clarity. Orange peel also can

30

be caused by blending resins that are notfully compatible.The extruder, die, air ringand processing conditions can have asignificant effect upon the final haze andgloss of a film from a given resin.Additives, and the proportion of scraprecycled to production all have anadditional effect.

If colorants are used, the appearanceof the film is dependent on a thoroughdispersion via a good mixing screw,assuming there is good master-batchcolor uniformly mixed with the resin priorto extrusion. Poor color dispersion showsas clouds of color in the film and canresult in heat seals with variable strengthas the amount of color and film densityvaries in the seal area.

Roll Defects — WhoseFault?

Roll and film defects discussed in thisbooklet are summarized in Table 1. Acommon converter complaint is wrinkledfilm near the core of the roll. This problemcan be due to operator error. How muchskill does the operator have in cutting thefilm from the finished roll to a new core?Was there a fixed reference mark to locatethe core or was it “eye-balled” for positionon the winder shaft with the resultantburied and protruding core ends? Or, wasthe damaged roll just too heavy for theoperator to handle?

Other, non-operator, conditions cancontribute to roll and film defects,including the location and/or theaccessibility of winder nips, slitting, andtrimming stations, treating stations,gusseting equipment, etc.

Telescoping or crushed cores, withpossible blocking, are the result of toolittle or too much winding tension. Again,

defective cores could be the result ofinattention to tension control as the rollbuilds or a need for maintenance work onthe drive.

Color in the roll ends of natural filmresults from three unrelated problems.Blue/gray coring in the first inch (or less) offilm around the core is caused by toomuch initial winding tension and canindicate crushed cores if severe enough. Ifblue/gray coring occurs, some cores maycollapse at a later time, as the film ages,shrinks and increases the stress on thecore.

Making a roll change too soon afterchanging resins can also cause a colorchange in the roll end from the core to thefinished diameter. There is nothinginherently wrong with the film, but suchtransition rolls should be scheduled tomeet orders where such a roll is accept-able. For instance, such a roll of filmmight be quite acceptable as the last rollof a general purpose clarity order ratherthan the first roll of a high clarity order.

Finally, on some equipment it is verydifficult to purge color, particularly white.Liner, rather than clarity orders, should bescheduled following orders for coloredfilm. A clarity film scheduled after a whitefilm appears clear in single thickness, buton the roll the residual color, still purgingfrom the extruder and die, whitens the endof one or several rolls of otherwise clearfilm.

Care should always be taken to knowand not exceed the number of splicesallowed in a roll and the number of splicedrolls acceptable to the customer in anorder. Splices are costly in the time theytake to check and thread throughconverting equipment. Uniform rolldiameters, consistent weights and goodappearance are important customerrequirements.

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Table 1Roll Defects Film Defects

Gauge Bands Low YieldNon-Uniform Surface Hardness Gauge Bands, TDConvex Face Variable MD GaugeConcave Face FlatnessTapered Through Length CamberWrinkles WrinklesCorrugated Ends Variable WidthFuzzy Ends Uneven GussetsTelescoping Fuzzy EndsBlocking, wound with too much tension BlockingCrushed Cores Treatment LevelRoll End Color SplittinessDamaged Edges MD Film StrengthScuff Marks Edge CreasesCore Ends Die & Weld Lines

Protruding ScratchesBuried Appearance

Splices GlossWeight ClarityDiameter ColorRoll-to Roll-Uniformity ImperfectionsIdentification—missing roll labels ApplesauceTreated Surface GelsWound Wrong Side Out ArrowheadsIncorrectly Identified Air Ring ChatterNot Identified BlockingIncorrect Additive Levels Very High Gloss

Incorrect Additive Levels

LyondellBasell's state-of-the-art research center is located in Cincinnati, Ohio. Testing, evaluation and research on new blown film polyolefin resins, process innovations and new procedures take place continuously on full-scale production equipment. Technical service engineers can provide training and advice on resin selection, trouble-shooting operations and other concerns of the blown film operation.

6483/0599

LyondellBasell IndustriesP.O. Box 3646Houston, TX 77252-3646United States

www.LYB.com

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