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How to SolveBlow Molding Problems

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How to Solve Blow Molding ProblemsThis booklet reviews some problems that may occur during blow molding operations. Althoughmillions of objects are blow molded each year, blow molding is not a simple processing technique.Possible defects are many and various. This booklet primarily deals with these problems as theyappear in lightweight, thin-walled parts, such as bottles and containers. However, many of therecommended solutions also apply to heavier, thicker objects.

Table of Contents Page

Rocker Bottoms and Oval Necks ............................................................................................ 2Defects Within the Blown Wall ................................................................................................ 2Poor Weld or Pinch-off and Indented Parting Lines ................................................................ 3Poor Bottle Surface ................................................................................................................. 5Curtaining and Webbing ......................................................................................................... 6Blow-outs ................................................................................................................................ 7Parison Curl, Stringing, Hooking, Sag and Length Inconsistency ............................................ 7Foreign Matter in the Melt ........................................................................................................ 9Die Lines or Streaking in the Parison ...................................................................................... 10Shrinkage ............................................................................................................................................... 10

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Rocker Bottoms and Oval NecksThese two defects in blow molded objects arethe result of warpage. The cause is, in general,the same that results in warping in parts madefrom other plastics molding processes:inadequate or uneven cooling of the part priorto its removal from the mold. Possible causesand solutions follow:

1. Insufficient cooling water to the moldn Increase flow and determine if this

eliminates warpage.2. Stock temperature too highn Try dropping the temperature by small

amounts to see if this will solve the prob-lem without resulting in new concerns suchas cold spots in the container wall.

3. Blocked cooling channelsn Check the cool out throughput. If it is

significantly less than when the mold wasnew, a thorough cleaning of the channelsis needed.

4. Cycles too shortn If 1, 2 and 3 have not solved the problem,

determine whether the cycle times areunrealistically short to keep productivityhigh. If cycles are reasonable for adequatecooling, proceed to the next group ofpossibilities.

5. Poorly designed cooling channelsn The mold may have to be reworked to

increase cooling capability or theuniformity of cooling.

6. Poorly designed partn Too great a variation in the distribution of

material in the part, yieldingunnecessarily thick and thin sections,result in warpage unless the part isthoroughly cooled. This may demand anuneconomically long cycle. Parisonprogramming or a redesign of the partitself may be necessary.

Defects Within the Blown WallBubbles in the wall and cold spots both spoilthe appearance of the blow molded object.While these two defects are unrelated, theymay have similar causes.

Bubbles are generally symmetrical areas on thewall of the object, usually clearer than thesurrounding area. Moisture in the melt is the usualcause. Following are some methods foreliminating this problem.

Rocker Bottoms

Bubbles

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1. Moisture condenses on the surface of coldresin that is exposed to the warmth of the blowmolding shop.

n Allow the resin to warm up and themoisture to evaporate before it enters theextruder.

n Let the resin warm up by keeping a fewunopened gaylords in a warm room.

n Discharge direct draws from hopper cars ortrucks into indoor surge bins where the resincan warm up and moisture evaporate beforemoving to the extruder hopper.

n Increase barrel temperature in the feedand/or transition sections of the screw toremove any further moisture.

2. Overdone water cooling on the throat of theextruder, resulting in water condensing in thebarrel

n Decrease water cooling on the throat.3. Non-moisture causes of bubblesn Check material leaking from mismatched

head sections and rectify, as this also canbe a source of degraded resin.

n Make sure back pressure is sufficient. Ifnot, adjust the back pressure gauge onreciprocating blow molding machines andchange to a finer mesh screen pack oncontinuous extruders.

n Check for screw wear. If severe, replacethe screw.

Cold Spots show up as non-homogeneous areasin the wall of the blown object. This can also becaused by insufficient back pressure or non-uniformity in the melt. Solutions are the same asfor bubbles.

Cold Spots also can result from a resin mixture inwhich some material has a lower melt index. Theresin does not melt completely and extrusionconditions are inadequate for the lower melt indexmaterial, resulting in defects on the blow moldedpart.

Poor Weld or Pinch-off andIndented Parting LinesThere are three distinct weld or pinch-offdefects:

n Thinning of the weldn Tearing of the flash during trimmingn Cutting at the pinch-off

Causes include the resin used, moldingconditions, mold design or a combination ofthese factors.

Thinning

1. Stock temperature too highn As the molds close and the pinch-off is

made, the pinch-off lands fail to forceenough material into the weld line to makea strong weld. First, gradually drop thestock temperature to see if this solves theproblem. If it does not, continue below.

2. Too short or too sharp a pinch-off land length

n Adjust the pinch-off land length.

n Refinish pinch-off lands.3. Excessive pre-blow or high pressure air

coming on too earlyn Gradually back off the pre-blow air to a

point that does not affect the other areasof the container.

n If the above does not solve the problem,increase the blow delay time. Care mustbe taken when doing this to avoidaffecting other areas of the container.

Cold Spots

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Tearing

1. Pinch-off land length too longn Long pinch-off land length forces more

material into the weld line, resulting in astrong weld. If overdone, however, a longpinch-off land length can prevent themolds from closing completely, leaving athick pinch-off line that is apt to tear duringtrimming. The weld is then ragged, roughand possibly torn open.

n Have the molds reworked to reduce theland length to 0.010 to 0.015 inch, thecommon range in polyethylene blowmolding. This land length results in a goodbalance between a strong weld and atrimmable pinch-off.

2. Pinch-offs wornn Correct temporarily by rolling them

back; refurbishing is necessary.

3. Molds mismatchedn Caused by worn locating pins which must

be replaced.4. Molds damaged

n Nicks and other damage in the pinch-offareas indicate a need to fill and regrind themolds.

5. Mold clamp pressure uneven

n Molds that appear to have torn pinch-offsin one area and not another are probablynot closing and clamping evenly. Adjust theclamp. If the clamp cannot be adjusted,the molds can be shimmed out at theproblem area.

Cutting can be the problem if a hole or slit isfound in a weld in a part with flash attached.

1. Stock temperature too lown If the weld and pinch-off both seem

satisfactory but there is a hole or slit alongthe weld, the parison may be tearing asthe mold closes because of cold stock.Raise the stock temperature in 5-10°Fincrements until the cutting ceases.

2. Molds close too fast and slam or snap shut —Ease off the closing cycle to make betterwelds and preserve the molds.

Indented Parting Lines appear as a “V” shapewhere they pull into the blown object.1. Insufficient blow pressure or air entrapment

due to poor ventingn Raise the blow pressure.

n Clean the mold vents.n If the above do not work, sandblast the

molds to improve venting.

Thinning of the Weld

Cutting at the Pinch-Off

Tearing of the Flash

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Poor Bottle Surface

Roughness, pits and “orange peel” are onlya few of the terms used to describe the lessthan perfect surfaces that can be found onblown bottles. One cause is an imperfectparison. Other causes are related to themold and the blow molding process.

1. Poor mold surfacen Refinish a poor or worn mold surface. The

mold should have a fine matte finish toallow air to vent quickly and the parison toconform to the mold surface while it is stillhot.

2. Plugged or inadequate ventsn Clean plugged or dirty vents, particularly

those along the parting line.n Rework the mold to enlarge vents that are

too small or increase the number of ventsto enable air to escape rapidly. When aircannot escape, surface problems result.

3. Low blow pressure or blow rate

n Increase the blow pressure.n Determine whether the blow pin is large

enough to handle the required amount ofair to fully and rapidly blow mold the part.

n Check for restrictions or partial plugging ofthe air lines.

4. Air leak around the blow pinn Repair the leak. Air leakage around the

blow pin means that there is not sufficientair pressure to hold the part tightly againstthe mold.

5. Low stock temperature

n Gradually increase the stock temperature.A cold parison will not reproduce thesurface of the mold well, particularly iflettering or designs are involved. However,be careful when raising temperatures.Raising temperatures too fast or tooextensively can lead to the problemsassociated with too high a stocktemperature.

6. Condensed water in the moldn Gradually raise the mold temperature, if

possible.n If raising the mold temperature cannot be

done without increasing cycle time, it may benecessary to air condition some localizedareas of the plant during periods of highhumidity. High humidity combined with lowmold temperatures can result in watercondensing in the mold between the ejectionof a part and the blow molding of the next.

7. Rough parisonn A rough parison usually results from

operating the blow molding process at ashear rate that produces flow instability inthe melt. Melt instability is more likely tooccur when continuous extruders are runnear or at the highest rates of which theyare capable. With reciprocating oraccumulator types of blow moldingmachines, melt instability is more likely tooccur when the extruders are run near orat the slowest rates of which they arecapable.

Condensed Water in Mold

Rough Parison

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With continuous extruders:n decrease the extrusion rate until rough

parisons no longer occur.n change extrusion tooling to a set with

longer land length.

With reciprocating or accumulator typeextruders:

n decrease extrusion rates (increase droptime)—but this is generally unsatisfactorybecause of the decrease in productivity.

n increase extrusion rates, and thus moveup out of the region of melt instability(decrease drop time)—a more satisfactorysolution.

n increase stock temperature or change to ahigher melt index resin, if extrusion ratescannot be increased.

n change extrusion tooling to a set withlonger land length.

Curtaining and Webbing

Curtaining or folding of the parison as itextrudes from the die can cause webbing inbottle necks and handles. If the effect issevere, the handle can be partially orcompletely blocked. Folds in the neck can leadto problems in filling operations or in reamingand facing, thus affecting the fit of the cap.

1. Poor tooling designn Too short a land length between bushing

and mandrel results in poor control of flowthrough the die gap, leading to areas ofhigh and low flow. The result is parisonfolding.

2. Die misalignment

n Bushing and mandrel misalignment alsoproduces differential flow rates in differentareas, resulting in folding.

3. High stock temperature

n Very high stock temperatures result inparisons with low melt strengths. Suchparisons collapse and fold before they canbe blown.

4. Resin swelln If the resin used has too high a weight

swell, an attempt is often made to reducepart weight by decreasing the die gap.However, decreasing the die gap can yielda very thin parison that quickly collapsesand folds. A resin with low diameter swellcharacteristics may cause lower handlewebbing and in severe cases, even blow-outs. In this case, the parison is caught intwo places by the inner handle pinch-off.When blown, these areas meet to form aheavy area known as a web.

5. Low parison diameter swell

n High stock temperature and/or low shotpressure will yield a parison with lowdiameter swell, resulting in lower handlewebbing.

Curtaining of the Parison

Webbing in the Handle

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6. Mold positionn Improperly positioned molds that are

shifted away from the parison will result incontainers with lower handle webbing.

7. Hooking parisonn If the parison hooks away from the handle,

webbing problems similar to thosedescribed above will also occur.

8. Ovalized toolingn If oval tooling is improperly designed, the

problems described above can occur.9. Pre-blow air pressuren If pre-blow air pressure is low or is lacking

entirely, curtaining and webbing can occur.Increase the pre-blow air pressure tominimize this effect.

Blow-OutsThe blow-outs can have many causes. Severalare listed on the following pages, along withsuggestions for correcting the conditions thatcaused the problems. Start with the cause thatseems most probable and check it first; thencontinue down the list of possibilities.

1. Pinch-off too sharpn This can cut the parison and cause a blow-

out. Increase the width of the pinch-offland.

2. Pinch-off too widen This causes mold separation and the

chance of a blow-out.3. Clamp pressure too low

n This also can cause mold separation andblow-outs.

4. Pinch-off too hotn Poor or uneven mold cooling can prevent

the pinch-off from cooling sufficiently andcause it to cut the parison just as it would ifit were too sharp. Refer to the section on“Warping” for solutions.

5. Blow pressure too bign Stretching the parison at too high a rate

can lead to blow-outs. Back off graduallyon the pressure to solve this problem.

6. Parison too shortn A short parison is not caught at the bottom

pinch-off and result in a blow-out.Lengthen the parison.

7. Blow-up ratio too highn If there is a large difference between the

diameters of the parison and the finalblown part, the parison may be stretchedtoo much, resulting in a thin area on theend product. A blow-out can occur. Changethe head tooling to a larger diameter die.

8. Resin swell too lown A low swell results in a smaller than

expected parison, yielding a blown partwith thin spots and blow-outs.Additionally, the bottle handle can bemissed. Switch to a higher swell resin.

Parison Curl, Stringing, Hooking, Sagand Length InconsistencyParison curl occurs during the extrusion ofthe parison, usually as a result of the melttemperature being too cold. Parison curl issometimes called “doughnutting,” and usuallyresults from one of the three causes describedbelow:

Blow-out of the Part

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1. Mandrel or bushing too coldn If the curling occurs when the blow

molding machine is started, then graduallydisappears as the machine approachesoperating temperature, check the mandrelor bushing. Allow a longer warm-up periodbefore starting production.

n Check whether the die bushing heater,with which most blow molding machinesare equipped, is working. If the heater isnot operating properly, a very long warm-up time will be needed.

2. Die or mandrel misalignmentn If the machine is fully warmed up and

parison curl occurs, check the die andmandrel alignment. Usually, the mandreledge is recessed within the die. As aresult, the parison contacts the die andtends to hang up on one side and curl.Some-times this misalignment has beenpurposeful, in an attempt to blow a bottlelighter in weight than that for which thetooling was designed.

n Obtain new tooling or machine the diebushing to bring the mandrel back to a flushor lower position with the die face.

n Parison curl also can result from airleakage around the tooling thus coolingthe mandrel or die.

3. Foreign matter or degraded resin in the diebushing

n Uneven build-up of foreign material on thedie bushing can distort the parison as it isextruded. Thoroughly clean the die toprevent foreign matter from accumulatingin the bushing.

Parison stringing also occurs when theparison is extruded, but is caused by too high amelt temperature. Thin, feathery strings of meltare attached to the top and bottom flash of theblown part. If stringing is severe, bottles remainattached to each other and the lower bottle’sweight may thin out the next parison to thepoint that blow-outs occur, particularly in ahandled bottle. Causes follow:

1. Stock temperature too highn Gradually lower the temperature.

However, if cold spots occur when thetemperature is lowered, it may benecessary to change to a different resin.

2. Fill pressure on reciprocating screw-typemachines too highn Excessive “weeping” of polymer from the

die occurs, causing the parison to stringduring part removal.

Parison hooking occurs when a parison doesnot drop straight, but “hooks” to one side. Thisresults in poor distribution of resin to the walls,webbing and/or blow-outs in the handle. Causesfollow:

1. Non-uniform die temperature2. Warped die or mandrel3. Die gap out of adjustment4. Air flowing on parison

Parison sag or drawdown occurs when a parisonthins out near the die during extrusion andappears to do so from its own weight. Causesfollow:1. Stock temperature too high2. Extrusion rate too slow3. Mold open time too long4. High die heats5. Resin melt index too highParison length inconsistency occurs when theparisons vary from too short, resulting in blow-outs, to too long, resulting in wall distributionproblems and tails sticking to the blown partduring unloading. Parison length can vary for thefollowing reasons:1. Insufficient back pressure in the extruder2. Changing the regrind/virgin resin mixture3. Malfunctioning temperature controllers4. Bridged or colored screw5. Worn barrel of screw

Melt Stringing

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Parison length inconsistency from head tohead is self-explanatory. It can be caused by:1. Non-uniform head heats2. Non-uniform bottle weights3. Manifold chokes out of adjustment4. Head chokes out of adjustment

Foreign Matter in the MeltForeign matter in the melt, whether fromoutside contamination of the resin ordegraded resin, manifests itself in severalways, such as die lines or streaking in theparison, off-colored particles in the wall ofthe molded part or holes or “windows” inthe blown part. The only solution to thisproblem is locating and eliminating thesource of the foreign material. Possiblesources are described below.

Foreign Matter (Close-up)

1. The easiest potential source to check ismaterial handling procedures and generalhousekeeping in the blow molding shop.

n Keep the resin clean and free of con-tamination. Close all gaylords to preventdust accumulation. Carefully introducebagged resin into the feed in a way thatprevents dirt and lint from entering at thesame time.

n Keep regrind free from contamination aswell. Keep parts with foreign matter inthem out of the grinder.

Note: If parison streaking is occurring,before checking for foreign matter,check heater bands. A burned-outband will cause a cold spot in thebarrel, resulting in uneven melt flowand parison streaking. Replace theinoperative band.

2. The source of degraded resin is moredifficult to pinpoint. To locate it, follow theprocedures outlined below:n If 1 to 3 streaks suddenly appear in the

parison, it is likely that foreign matter hasbecome lodged between the die and themandrel. Increase the die gap briefly topurge the particles; then reset to theoriginal gap.

n If parison streaking or defective bottles(foreign particles or holes) are occurringregularly, and materials handling proce-dures have been eliminated as a sourceof contamination, the degraded resin maybe generated from within the head,adaptor or in the extruder. There isprobably a blind spot or non-streamlinedarea where the melt is hanging up,degrading and then breaking awayperiodically. A poor fit between head andadaptor, or adaptor and extruder barrelare two possible causes of this. Reworkthe parts for a smoother fit.

Hole or "Window"

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n The gradual appearance of many die linesor streaks after extended periods of opera-tion indicate a build-up of carbon or de-graded resin in the head. Usually thisoccurs on the die and mandrel which areeasily accessible. Clean these with acopper or brass tool which does not marthe surfaces or damage these parts of themachine.

n Not all the foreign matter will have lodgedin the die. If the dead spots in the headhave been eliminated as a source, it will benecessary to check out the extruder.Check the extruder head for mismatchedor non-streamlined head sections orinadequate seals that are allowing polymerto leak.

n Degraded resin more often results fromoperating error, such as:

– Poor purge when changing fromone resin to another.

– Not allowing the extruder to cooldown on purge before shutting itoff can result in specks in bottlesupon start-up. This results fromresin in the die lips which hasbeen oxidized by exposure to air.Shorten both shut-down and start-up times by installing an air-lockplant to seal off the die lips fromthe air during these periods.

– Running at too high a temperatureat some point or points along thebarrel may cause resindegradation back in the extruderbarrel. The high temperature maybe intentional in an effort to run adifficult resin and still maintainfast cycle times. If slowing thescrew speed is not an acceptablesolution, switch to a resin that iseasier to extrude.

– Too high a temperature in themetering zone of the screw alsocan yield degraded resin. Try areverse temperature profile:a. increase the temperature in

the transition zoneb. decrease the temperature in

the metering zone

c. smoothly increase the temperatureto the desired final melt temperaturethrough the adaptor and head

Purge the extruder once a workabletemperature profile has been found.However, if the degraded resin situationhas existed for a long time, the extrudermay require a thorough cleaning.

Die Lines or Streaking in theParisonStreaking is usually the result of foreignmatter or degraded resin lodged between thedie and mandrel. See the section on degradedresin under “Foreign Matter in the Melt,” onthe preceding page.

ShrinkageSome shrinkage in a polyethylene container aftermolding is normal. Excessive shrinkage can becaused by a variety of problems. Causes aredescribed below and the solutions involve adjust-ments in various machine settings. If additionalshrinkage is required for your application, thereverse of these solutions helps.1. Wall thickness or weight too high2. Mold temperature too high3. Stock temperature too high4. Low blow pressure5. Blow time too short6. Mold volume incorrect7. Bottle storage area too hot8. Density of resin too high

Die Lines or “Streaking”

Lyondell Chemical Company1221 McKinney Street, Suite 1600P.O. Box 2483Houston, TX 77252-2583

Cincinnati Technology Center11530 Northlake DriveCincinnati, OH 45249(513) 530-4000

6088/0504

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