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DEFECTS IN EXTRUSION PROCESS AND THEIRIMPACT ON PRODUCT QUALITY

J G Khan1, R S Dalu2 and S S Gadekar1*

*Corresponding Author: S S Gadekar, sandipgadekar77@gmail.com

In the 20th century, the number of manufacturers had established extrusion pipe manufacturingplants on the customer demand. To ensure quality extrusion pipe manufacturing, it is essentialto identify, control, and monitor all quality parameters. Some of the important parameters arecondition of equipments, operating conditions, temperatures, pressures, quality of dies, materials.Instead of the sincere efforts from the manufacturers, still there are number of obstacles in theprocess which lead to defects in the product. The purpose of this review paper is to focus on thevarious defects in the extrusion process, to identify its impact on the product quality and tosuggest the remedies for the improvement of extrusion process.

Keywords: Extrusion process, Defects in plastic pipe, Impact of defect, Product quality

INTRODUCTIONManufacturing of polyvinyl chloride (PVC) inIndia started 60 years ago with the country’sfirst PVC plant set up in Mumbai in 1951. Theplant operated by Calico had a capacity of6000 metric tonne/year. At present, the IndianPVC industry boasts of a production capacityof 1.3 million mt/year. In India, ChemplastSanmar, DCM Shriram Consolidated, DCW,Finolex and Reliance Industries have beenproducing PVC. The latest entrant into theIndian PVC market is the Vivanta group,whose 240,000 mt/year PVC plant is slated

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Int. J. Mech. Eng. & Rob. Res. 2014

1 Department of Mechanical Engineering, SSGMCE, Shegaon.2 Department of Mechanical Engineering, Government College of Engineering, Amravati, AP, India.

for start-up in 2013. PVC, one of the oldestforms of commodity polymers, is todayregarded as an infrastructure plastic and findsvarious infrastructure applications, such as inpipes, ducts, wires, cables, floorings, windowsand roofing. Furthermore, the use of PVC inother sectors such as automobiles, medicaland health care, packaging, sports and leisureis also increasing. PVC is a thermoplasticcomposed of 57% chlorine and 43% carbon.It is less dependent than other polymers oncrude oil or natural gas, which are non-renewable, and hence PVC can be regarded

Reivew Article

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as a natural resource saving plastic. AlthoughPVC can be produced from varioushydrocarbons including coal, the bulk of theworld’s PVC is currently manufactured usingethylene, which is combined with chlorine toproduce ethylene dichloride (EDC), the rawmaterial for the manufacture of Vinyl ChlorideMonomer (VCM). VCM is further polymerizedto produce PVC. PVC is also produced usingcalcium carbide, which is widely prevalent inChina. According to Chemical MarketingAssociates Inc. (CMAI), the globalconsumption of PVC in 2010 was 34.8 millionmt from an overall capacity of 46 million mt,and accounted for 18% of the totalconsumption of polymers. The global demandfor PVC is estimated to rise to 44 million mtand the global PVC capacity is expected torise to 55 million mt by 2015. According toindustry sources in India, the country’s PVCdemand is currently pegged at 2.08 million mt/year and the capacity is pegged at 1.33 millionmt/year.

The consumption of PVC in India isexpected to increase to 3.1 million mt/year by2016-17 and the nation’s PVC capacity isexpected to rise to 1.63 million mt/year. As isevident from the above-mentioned demandand supply numbers, PVC demand is expectedto exceed supply (Belofsky, 1995).

Today’s Scenario of ExtrusionPlastic in India

PVC Pipes Usage Narasimha andRejikumar (2013)

PVC Pipe SectorPipes have been the biggest end-use sectorfor PVC with a share of approximately 70%.The use of PVC in pipes results in energy

saving at all stages of the life cycle of a pipe,namely, extraction of raw materials, production,transportation, usage and recycling. Besidessaving energy, PVC pipes also help protectenvironment by reducing CO2 emissions. Withan investment of Rs. 20 lakh crore towardsinfrastructure development in India in 2011-12and a projected investment of Rs. 40 lakh croretill 2016-17, the consumption of PVC in pipesis expected to progressively increase andreach 10 million mt by 2017 from a currentestimated consumption of 6 million mt.Furthermore, energy savings from using PVCin pipes would amount to 51.6 million MWh by2016-17 from an estimated 31.4 million MWhsavings currently and 16 million MWh savingsin the period 2002 to 2007. Likewise,reduction in CO2 emissions would amount to182 million mt by 2016-17 from 57.7 millionmt in 2006-07 and an estimated 110.6 millionmt currently (Belofsky, 1995).

As per the above data, it may be concludedthat the PVC pipe sector has wide range of

Figure 1: PVC Sector-Wise Usage

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requirement in India. To fulfil such hugedemand, industries require a better extrusionProcess. It has been observed that there arelot of problems in extrusion process which leadto defective products. The common defectsare Improper System Engineering/Installation,Improper Operation, Resin Defects, ImproperMaterials Addition, Surging, Poor mixing, Melttoughness or fracture, Overheating, Moisturerelease, Trapped air, Contamination.

Extrusion ProcessThe extrusion process converts a solid plasticfeedstock material into a molten viscous fluid,and then to a finished solid or flexible filmproduct for practical use. Extrusionequipment consists of a plasticizing extruder,die assembly, a cooling assembly, and haul-off or winding equipment. Extrusion is acontinuous process, as opposed to moulding,which is a cyclic process. Extrusion is suitablefor many types of continuous plastic productsthat have a uniform outside shape and canbe coiled, cut, or wound. The transformationof a solid plastic feedstock material into amolten viscous fluid takes place in theextruder barrel, through the mechanicalshearing action of a rotating screw and theheat provided by electrical resistance heatersclamped to the outside of the extruder barreland die. The combination of the mechanical,rotating shearing action of the screw and theheat of the electrical heaters causes the solidplastic feedstock to change into a hot moltenmaterial. There are two basic types of plasticextrusion: screw extrusion and ram extrusion.This discussion is limited to screw extrusion,because ram extrusion is suitable only forspecialized plastics such as PolytetraFluoroethylene (PTFE) which cannot be

screw extruded because of its low friction. Inram extrusion, a plunger goes through a barreland pushes out the material under pressure.Screw extrusion involves a helical feed screwthat turns inside a barrel. This is often calledthe feed screw or the extruder screw. Thescrew is a single shaft with helical flights.Sometimes, when more thorough mixing isneeded, two screws are used. The screwadvances the material through the barrelwhere it is heated and compressed. Thisprocess of extrusion serves two functions: itheats the plastic material above its meltingpoint and puts the melt under pressure. Themolten plastic material can then be forcedthrough an orifice, commonly known as thedie. This process is common to all types ofextrusion. The extrusion process can producepipe and tubing, a variety of profiles, flatproducts (thin film to heavy sheet), coatingson paper or other substrates, wire insulation,cable jacketing, and monofilaments. Allsynthetic fibers are produced by some formof extrusion and plastic raw materials aremade (compounded) by extruding strandsand cutting them into pellets. Examples areblown film for bags, cast film for packagingmaterials, tubing for catheters, wire insulationfor telephone wire, rod for glue guns, profilesfor window frames, and monofilaments for fishline. In all cases, the plastic must be convertedfrom its raw material state—pellet orpowder—and melted and pressurized via theextrusion process. After that is done, thematerial is moved to the die and downstreamequipment for completing the desired product.Any discussion of the extrusion processshould begin with the extruder itselfNarasimha and Rejikumar (2013).

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Defects’ in Extrusion ProcessTo ensure successful extrusion manufacturing,every parameter must be identified, controlled,and monitored. As the extrusion processstudied and improved universally, still it ishaving some problems.

Defect is any form of deviation of theproduct’s characteristic from the specificationset up by the manufacturing process. It can becaused by a single source or the cumulativeeffect of several factors, which may arise atany stage of the processing. The commonfailure or defects which are normally occurringin plastic extrusion process are due to threemain causes: mould design, materialselection, and processing. In many cases, thefailures occur during the processing and thesefailures causes some defects that can be foundin extruded parts such as: rough surface,extruder surging, thickness variation, unevenwall thickness, diameter variation, centeringproblem. In extrusion products, defects due toprocessing include, poor understanding of theprocessing method, use of inadequate or oldmachines, lack of trained staff, machine breakdown, and inappropriate working environments(British Plastics, 1972).

The common defects in extrusion processare: Improper System Engineering/Installation,Improper Operation, Resin Defects, ImproperMaterials Addition, Surging, Poor mixing, Melttoughness or fracture, Overheating, Moisturerelease, Trapped air, Contamination.

Improper System Engineering/Installation

• Wrong Clamps used or Clamps too tight.

• Improper die setting

• Improper alignment of die and

• Incompatible fire caulk used.

• Contact of outside of pipe with incompatiblematerial (e.g., solder flux).

Remedies

• Adjust the die setting

• Check for alignment

• Check for uniformity in die heating

Improper Operation (Maddock, 1957)

• Exposure to freezing temperatures withoutfreeze protection

• Over pressurization

• Pulsating water pressure

Remedies

• Use the Digital Pressure gauge.

• Use proper/Digital temperature sensor.

Resin Defects (Maddock, 1957)

• Occlusions, char particles, voids

• Filler/pigment not well distributed

• Improper mixing of resin and additives

• Foreign contamination

• Due to over heating

Figure 2: Spider Dies

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Remedies

• Screening the resign material

• Exact addition of (Percentage of mixing)resin material

IMPROPER MATERIALADDITIONSome of the many problems that can occurduring extrusion are traceable to theprocessed raw material; that is, the rawmaterial is not up to specification. Regardlessof any quality assurances by manufacturers ofraw materials and countless programs suchas ISO 9000, reengineering, and Just In Time(JIT) manufacturing, plastic batches and evenbags or boxes within batches have theiridiosyncrasies. With all extrusion parametersthe same, the tubing or sheet will have adifferent look, bend, colour, or texture. Whilethis may sound unscientific or preposterous,these problems do occur on the floor. Despiteall the computer programs and hardwaredesigned to keep the quality constant, therewill be variations. Some materials such aspolyurethane, nylon, EVOH and others, arehygroscopic (they absorb moisture from theair). This moisture is undesirable and cancause bubbles in the melt and, in the case ofEVOH, can decrease barrier properties. Forthis reason, the polymer should be kept sealedin nitrogen-purged bags whenever possible.However, some plastics can absorb criticalamounts of moisture in the time it takes to opena bag and put it in the hopper. The materialmust be dried, preferably in a hopper dryerdesigned for this purpose. Other problemsinclude contamination with foreign materialssuch as metal chips, screws, bolts, nuts,cardboard, rodents, and rodent droppings.

Even so-called medical grade plastics areoften contaminated. Of course, thecontaminate will end up in the melt, so everyprecaution must be taken to avoidcontamination by closing bags, coveringhoppers, keeping a clean area around theextruders, and similar measures.

Remedies• Exact addition of (Percentage of mixing)

resin material.

• Check for foreign material.

• Increase rpm back pressure for bettermixing.

SURGINGSurging is a cyclical product thicknessvariation in the direction of extrusion. The surgecycle time is typically between 30 seconds and3 minutes, and the cause can be inside oroutside the extruder. Outside causes areeasier to see and correct. For example, thetake-off pull may be irregular; in this case thescrew rpm and ammeter readings remainsteady. Sometimes screw motor speed variesbecause its regulation is not working properly.This is rare and will show up as unsteady rpm.Sometimes the feed is uneven because ofparticle size, light weight, or bridging in thehopper and throat. With very small extruders,the feed channel depth is not much bigger.Than the feed particles, and they may feederratically for this reason alone.

If none of these outside causes areobserved, it is probable that the surgingoriginates inside, typically at the beginning ofthe compression zone, where the solid bed—the mass of pellets—may be locking andbreaking up irregularly. Screw rpm is steady

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but the ammeter shows variations of ± 5% ormore. Sometimes this can be cured byincreasing the temperature of the feed topromote earlier melting. Raising the rear barreltemperature may help, too. Make big changes,25-50 °F (14-28 °C), and see what happens.Sometimes raising the barrel Temperature atthe beginning of the compression zone willhelp by getting better sticking of the pellets tothe wall there.

Remedies• Try to running the extruder slower or faster

by at least 10%.

• Altering or replacing the screw.

• Installing a gear pump between the extruderand the die.

POOR MIXINGThis often sets the upper limit for output. Thescrew cannot be run any faster because thematerial is coming out with an “applesauce”surface, with streaks, parabolic ridges, andperhaps Particles of undispersed additive.Screw modifications, such as pins or separatemixing heads will help, but may raise melttemperature. Running more slowly will alwayshelp, as it provides more residence time. Ascrew with internal cooling mixes betterbecause it has the effect of a shallower channelin the metering zone. The output per rpm drops30, but may be regained by increasing thescrew speed, unless melt temperature gets toohigh or the drive system cannot safely runfaster. High pressure is good for mixing. A valvewill do this, or a gear pump pressure controlthat can serve as a valve. Cooler dies increasethe resistance and thus raise the pressure inthe system. Finer screens will raise the

pressure, but as they keep clogging, pressureis inconsistent.

Remedies• Exact addition of (Percentage of mixing)

resin material.

• Check for foreign material.

• Increase rpm back pressure for bettermixing.

MELT TOUGHNESS OR MELTFRACTUREThis refers to fine ridges or rough surface seenwhen the melt comes too fast out of a narrowdie. It is most common with polyethylene, andcan be eliminated by running the melt or thedie lips hotter, using a longer or morestreamlined die, or trying a different grade orsource of material. Additives may help greatlyin this regard.

Remedies• Use the Correct additives.

• Maintain the speed of extruder.

• Trying a different grade or source ofmaterial.

OVERHEATINGOverheating may limit the rate if the take-offcooling is l imited, or it may producedegradation or make dimensional control andsizing difficult. In such a case, stop all barrelheat except in the rear zone as needed forbite (input) control and cool the barrel ifnecessary. (In a few cases, more barrel heatwill yield a cooler melt.) There is atemperature below which the melt will not goat a given screw speed, even if all the barrelheaters are turned off.

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See if the controller around 70% down thebarrel is overriding. This means over packingof the metering zone—overbite—with muchheat generated at the entry to that zone. In thatcase, reduce the bite by changing the feedtemperature, rear barrel temperature, orparticle size.

Remedies• Stop all barrel heat except in the rear zone

as needed for bite (input) control and coolthe barrel if necessary.

MOISTURE RELEASEMoisture is absorbed by some plastics. Itpasses through the extruder and boils whenthe pressure is relieved at the die lips. Theresult is a pattern of dotted lines, long bubbles,and pits. To remove moisture, the materialmust be pre-dry, or a vent must be used in theextruder, or both. A moisture level of 0.1% isusually low enough to avoid such visualproblems.

Some plastics, such as PET, the nylons, andpolycarbonate, can degrade and weaken ifeven a tiny amount of moisture is present whenthey are melted. For these, dehumidifyingdryers are used to get moisture down to 0.01%or less.

Remedies• To remove moisture, the material must be

pre-dry.

• Vent must be used in the extruder.

• Keep the moisture level of 0.1% is usually.

TRAPPED AIRThis is not common in pelletized material usedwith long extruder barrels. However, some old

machines have short barrels, and even a longmachine can be pushed so fast that the air iscarried forward into the product. A trapped-airsurface shows bubbles and pits, but little, if any,dotted lines. Such a surface will improve if runmore slowly if moisture is not the problem. Acooler head and die may help. Vents andvacuum hoppers will eliminate trapped air andare essential for powders, where passagesbetween the particles are much smaller; theair cannot escape back through thesepassages and is carried forward instead.

Remedies• Avoid the over-speed of extrusion.

Table 1: Defects and its Impact

S.No

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

Defect

Improper SystemEngineering/Installation

ImproperOperation

Resin Defects

ImproperMaterialsAddition

Surging

Poor mixing

Melt toughnessor fracture

Overheating

Moisture release

Trapped air

Impact on Quality

Centring problems Chattermark on the Product

Uneven wall thickness

Blow holes Formation on theproduct

Material absorb themoisture from the air whichcan cause bubbles

Variation in the thickness ofthe product

This Problem create theclogging in the extrusion

This refers to fine ridges orrough surface seen whenthe melt comes too fast outof a narrow die

This create the problem inthe Cooling of product

The result is a pattern ofdotted lines, long bubbles,and pits

A trapped-air surface showsbubbles and pits, but little, ifany, dotted lines

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• Pre-Dry the material.

• Vents and vacuum hoppers will eliminatetrapped air.

CONCLUSIONFrom the Study and analysis of the variouspapers on the defect and observing their viewsof researchers by paper in extrusion processthere should be need of minimizing its causesfor the best extrusion product. These qualityproblems (Causes) are become inappropriatesetting of operational parameters as perobservation. By the application of aboveremedies the percentage of loss would beimprove, as predicted, for the products.

REFERENCES1. Belofsky H (1995), “Plastics: Product

Design and Process Engineering”,Hanser Publishers, ISBN: 1569901422.

2. Dowd L E (1962), “How to MinimizePuckering in Polyethylene Cast Films”,

Modern Plastics, Vol. 40, No. 20, pp.142,147 & 204.

3. Finolex Industries Limited, CorporationPresentation, May 2013.

4. Lekhraj Ghai (2014), Importance of PVCin Indian Petrochemical Industry ,Dissertation or Thesis.

5. Maddock B H (1957), “Factors AffectingQuality in Polyethylene Extrusion”, ModernPlastics, Vol. 34, No. 8, pp.123-136.

6. Maddock B H (1964), “Measurement andAnalysis of Extruder Stability”, SPEJournal, Vol. 20, pp. 1277-1283.

7. Narasimha M and Rejikumar R (2013),“Plastic Pipe Defects Minimmization”,International Journal of InnovativeResearch and Development, Vol. 2,No. 5, pp. 1337-1351.

8. Tadmor Z and Klein I (1970), “EngineeringPrinciples of Plasticating Extrusion”, VanNostrand Reinhold Company, New York.