A Study Of Plastic Extrusion Process And Its Defects

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International Journal of Latest Technology in Engineering, Management & Applied Science (IJLTEMAS)Volume VII, Issue IX, September 2018 ISSN 2278-2540A Study of Plastic Extrusion Process and its DefectsPankaj M.Patil1, Prof. D.B. Sadaphale21Student of M.E(Machine Design), Mechanical Engineering Department, SSBT COET, Bambhori-Jalgao, Maharashtra, India2Faculty Member of Mechanical Engineering Department, SSBT COET, Bambhori-Jalgaon, Maharashtra, IndiaAbstract-Extrusion is by far the most important and probablythe oldest transformation and shaping process of thermoplasticpolymers. To ensure a quality through extrusion process duringmanufacturing, it is essential to discover, control, and monitor allquality parameters. Some of the important parameters are stateof equipments, working conditions, temperatures, pressures,quality of dies, materials and cooling medium. Instead of thetruthful efforts from the manufacturers, still there are number ofobstacles in the process which show the way to defects in theproduct. The purpose of this paper is to properly understandingthe extrusion process and focus on the various defects and itsimpact on product quality.Keywords-Extrusion Process, Shaping Process, Product Quality,Quality Parameters, Cooling Medium.I. INTRODUCTIONPlastic processing needs the knowledge of basics of rawmaterials, additives, process control and finally theproduct properties required to finish end products. Plasticmaterials have a useful combination of properties that can bemodified for used wide range of applications.In thermoplastics, processing techniques can beclassified into either batch or continuous process. Batchprocess includes injection moulding and roto-moulding.Extrusion of plastics is a continuous process. However, blowmoulding is available both in batch and continuous process. Inthese days, online continuous thermoforming machines areavailable along with extrusion process. [9]Various products manufactured through abovementioned processes are highlighted in the following table.Classification of plastic products as a type of processExtrusionMouldingIndustrial Injection moulding, HouseholdInjection moulding, Thermo ware/Mouldedluggage.BlowMouldimgBottles, Containers, Toys and housewares,Air Ducts,panels.RotoMouldingLarge circular tanks such as water tanks.InjectionPlasticproductsFilms and sheets, fibrous and filamentspipes, conduits and profiles, strapping.Extrusion process is the most commonly usedprocess in India and accounts for 60% of total consumptionby downstream plastic processing industries. Injectionwww.ijltemas.inmoulding is the other popular process accounting for 25% ofthe consumption. Blow moulding is used for 5% whileRotomoulding 1% while the rest of the plastic is processedthrough other processes. [2]From the above data we can see that 60 % extrusionprocess used in the industry but it has been observed that thereare lots of problems in extrusion process which lead todefective products. In extrusion products, defects due toprocessing include, poor understanding of the processingmethod, use of old machines, be short of trained staff,machine break down. Due to this it become essential theindustries have better extrusion process.II. PLASTIC EXTRUSION PROCESSPlastic extrusion is a high-volume manufacturingprocess in which raw plastic is melted and formed into acontinuous profile. This process starts by feeding plasticmaterial (pellets, granules, flakes or powders) from a hopperinto the barrel of the extruder. The material is graduallymelted by the mechanical energy generated by turning screwsand by heaters arranged along the barrel. The molten polymeris then forced into a die, which shapes the polymer into ashape that hardens during cooling.[13] There are two basictypes of plastic extrusion: screw extrusion and ram extrusion.A ram extruder is an extruder where, instead of extrusionscrew, a ram or plunger is used and a plunger goes through abarrel and pushes out the material under pressure. The ramextruder was the earliest extruder to be used in the plasticsindustry. This typical process is applied for producing profiles,sleeves, rod, block, tubing, lining sheet bars, etc. The ramextrusion process is very effective for specific materials likePTFE which are not extruded successfully using screwextruder because of its low friction. In this process plasticmaterial in powder form is gravity fed into a chamber. In theextrudating chamber the resin powder is heated on sinteringtemperature. Ultra high molecular weight polyethylenebecomes gelatinous as it melts so it can be extruded with thistype of processes. A hydraulic ram pushes the resin materialslike PTFE, UHMW, etc. from the chamber to the die. The dieactually gives the shape of the desired plastic like a rod, tubeor a profile shape with the requisite internal or outer diameter.When the material comes out of the die, it moves the length ofthe conveyor. The profiles can be manufactured endlessly andcut by the continuous extruding of each length. Screwextrusion involves a helical feed screw that turns inside abarrel. This is often called the feed screw or the extruderPage 13

International Journal of Latest Technology in Engineering, Management & Applied Science (IJLTEMAS)Volume VII, Issue IX, September 2018 ISSN 2278-2540screw. The screw is a single shaft with helical flights.Sometimes, when more thorough mixing is needed, twoscrews are used. The constantly turning screw moves the resinthrough the heated barrel where it is heated to propertemperature and blended into a homogeneous melt. Extrusionscrew design has been improving over the years, with newinnovations and ideas. Nowadays, single screws are availablethat have a secondary flights that improve speed by enablingfaster melting. This process of extrusion serves two functions:it heats the plastic material above its melting point and putsthe melt under pressure. The molten plastic material can thenbe forced through an orifice, commonly known as the die.This process is common to all types of extrusion. Most screwshave these three zones, Feed zone (also called the solidsconveying zone): this zone feeds the resin into the extruder,and the channel depth is usually the same throughout the zone.Melting zone (also called the transition or compression zone):most of the polymer is melted in this section, and the channeldepth gets progressively smaller. Metering zone (also calledthe melt conveying zone): this zone melts the last particles andmixes to a uniform temperature and composition. Like thefeed zone, the channel depth is constant throughout this zone.A great advantage of extrusion is that profiles such as pipescan be made to any length. If the material is sufficientlyflexible, pipes can be made at long lengths even coiling on areel. Another advantage is the extrusion of pipes withintegrated coupler including rubber seal.fracture, Moisture absorption, Poor Mixing, Surging, ImproperMaterial Addition, Improper system engineering andinstallation.[3] Some other defects are Sink, poor ornonuniform gloss, Weld lines , Gels and plate out , Warpage.A. Improper OperationThere is a molecular orientation change, resulting ina variation in the machine direction versus the transversedirection properties. Depending on the orientation change, theproperties in the machine direction may increase or decrease,with the corresponding properties in the transverse directionvarying in the opposite direction. In other words, if the tensileproperties in the machine direction increase, they willdecrease in the transverse direction, and vice versa. Apotential cause for the change in molecular orientation is thedraw ratio has changed between the extruder and the puller.This only occurs with a die with an adjustable gap, where thedie lip setting has been changed. As the die lips are opened, ahigher draw ratio is required to maintain the final productdimensions. In processes with nonadjustable die lip openings,the product dimensions are changed as the puller speed or theextruder screw speed is changed. With an adjustable dieopening, the product thickness exiting the die can be increasedor decreased through die adjustments. As the die is opened,the puller speed is adjusted to maintain the productdimensions, causing the product draw ratio to be increased andchanging the product orientation. [7]Gauge variations are defined as thicknessnonuniformties in the final product. This changes in productdimensions are due to the no uniform melt temperature thatresult into polymer viscosity variations and causes ofnonuniform melt temperature are poor melt mixing, anunevenly clogged screen pack due to that polymer flowsthrough one part of pack to another.Fig. 1 Plastic Extrusion Process [6]III. DEFECTSDefect is any form of deviation of the product’scharacteristic from the specification set up by themanufacturing process. It can be caused by a single source orthe cumulative effect of several factors, which may arise atany stage of the processing. The common failure or defectswhich are normally occurring in plastic extrusion process aredue to three main causes: mould design, material selection,and processing. [1]Some of the common extrusion defects are Improperoperation, Resin Defects, Trapped air, Overheating, Meltwww.ijltemas.inIf take off equipment is not functioning properly, theproduct slippage can cause thickness variations. Slippage inthe puller allows the product dimensions to rise for the reasonthat the product is not being pulled away from the extruder atthe proper rate. The caterpillar or puller pressure has to beadjusted to prevent the product from slipping without causingany distortion or damage to the final product.1) Remedies Verify puller speed correct.Verify screw speed is properly set.Check extruder temperature and adapter set points.2) Impact on Product QualityUneven wall thicknessB. Resin DefectsDue to resin contamination a chemical reaction in theextruder that may generate gases. It is easily corrected byPage 14

International Journal of Latest Technology in Engineering, Management & Applied Science (IJLTEMAS)Volume VII, Issue IX, September 2018 ISSN 2278-2540properly purging the extruder with a nonreactive resin and byavoiding or removing any contamination from the productprior to feeding it to the extruder.If moisture induced in the resin is not removed fromresin by an atmospheric or vacuum vent it exist the die assteam and causes holes in the product can result. Withpredried resins still containing a few moisture or moisture thathas condensed on the pellet surface, rather than a hole, themoisture can cause surface splay on the extruded part. Splayindicates moisture or gases are there, but their concentration islower than the critical amount required to blow bubbles in theproduct.Due to overheating resin degradation in the extruderor die occurs .It decreases product strength in all directions. Itis normally cause by the hot barrel or die temperatures, awrong screw geometry, long residence time at hightemperature.1) Remedies Purge extruder or die.Screen the resin material.Exact addition of (Percentage of mixing) resinmaterial.Increase rpm back pressure for better mixing.2) Impact On Product QualityBlow holes formation on the productC. Trapped AirTrapped air can be more problematic when startingmaterial is powder compared to pellets. Because powder bulkdensity is lesser and introduces more air into the extruder. Ahigher compression ratio screw is used with powders, ensuringthat the transition flights are full of melt, preventing air fromentering the metering section. Vents and vacuum hoppers willeliminate trapped air and are essential for powders wherepassages between particles are much smaller. The air cannotescape through this passages and it is carried forward insteadIf pelletized material used with long extruder barreltrapped air is not common. Though some old machines haveshort barrels and even long machines can be pushed so fastthat air carried forward in the in the product. Trapped airsurface shows bubbles and pits. Such surface will improve ifrun more slowly if moisture is not problem.1) Remedies Avoid the over-speed of extrusion.Increase plastic melt temperature.Increase rear barrel temperature.2) Impact on Product QualityA trapped surface shows bubbles pits, but little if any dottedlines.www.ijltemas.inD. OverheatingOverheating may produce degradation of polymermaterial that is used or make dimensional control and sizingdifficult or may limit if takeoff cooling is limited.It’s generally assumed that much of the heat formelting is generated by the barrel. That’s true in someinstances, but the heat from the barrel has to be conductedfrom the barrel wall clear through the layer of polymer in thescrew channel to do any significant melting. Since polymersare excellent insulators, conducting heat is one of their worstfeatures. In reality, melting occurs in single-screw extrudersprimarily as a result of shear heating of the polymer. Shearheating is the result of a polymer-filled screw rotating withinthe barrel. This is a lot easier to understand if you picture thescrew as a simple round shaft with no flights turning inside aheated tube filled with a highly viscous material. The amountof shear heating versus the degree of heat conducted from thebarrel for melting depends a lot on the gap between the shaftand tube. Generally speaking, the larger the screw, the deeperthe channels, and the more important the contribution of shearheating .In the case of smaller screws with shallower channeldepths, the transfer of heat into the solid occurs more quicklybecause there is less distance for the heat to travel through theinsulating layer of the polymer itself. In these cases, thetransient conductive heating of the solid polymer from thebarrel heat can add significantly to the melting or softeningtemperature suitable for extrusion processing. But that’s onlyif there is enough time available. Except in cases where thescrew is very small and turning very slowly, depending onconductive melting severely limits the potential output bywaiting for the heat to fully penetrate the solid polymer. Theinterval of time to conduct full heat transfer increasesexponentially with thickness, so that the difference in time totransfer heat through even a small increase in polymerthickness can be surprising. [12]1) Remedies Avoid overheating and make sure that thetemperature we heat the polymer is safe and causesno degradation.Very accurate temperature control is needed formaterials susceptible for degradation.2) Impact on Product QualityThis create problem in the cooling of productE. Melt Fracture or sharkskinIn the extrusion of polymer melts, below certainshear rates the emerging extrudates have smooth surface. Asthe rate increases, small amplitude periodic distortions appearon the surface (sharkskin melt fracture).As rate increasesfurther, the extrudate becomes severely distorted (gross meltfracture). [10]Page 15

International Journal of Latest Technology in Engineering, Management & Applied Science (IJLTEMAS)Volume VII, Issue IX, September 2018 ISSN 2278-2540Melt fracture is supposed to happening in the dieentry region where material is being funneled from the meltreservoir into the capillary or die. In a full-scale extruder, thiswould match up to the point where melt moves into the dieparallel portion of the die. Some further complicating effectsmay occur at the wall of the die. Melt Fracture is not surfacephenomenon as with sharkskin in its place it completely goesthrough the extrudate. The form of the melt fracture distortionvaries from one polymer type to another, but it is generallyhelical in nature. With materials such as polyethylene andpolypropylene a distortion like a screw thread may appear,while with polystyrene the extrudate may form a spiral. Otherpolymer melts may show ripples, or bamboo-like repetitivekinks.Sharkskin is believed to occur as a result of the melttearing as it extrudes from the die. This occurs because, withinthe die, melt close to the wall is moving very slowly (in thecase of the layer next to the wall this movement is zero). Asthe melt emerges, the whole extrudate moves away from thedie face at a constant speed so that the outer layers aresuddenly stretched and may tear. In the case of sharkskin, thedistortion consists of transverse ridges. [8]1) Remedies Lower the shear rate.Streamlining the dieSlowing the extruder rate.Reducing the melt viscosity.Increasing the die temperature.2) Impact on Product QualityThis refers to fine ridges or rough surface seen when the meltcomes too fast out of narrow die.F. Moisture AbsorptionMany plastic materials absorb moisture. Thismoisture passes through the extruder and boils when thepressure is relieved at the die lips. A pattern of long bubblesand pits is the result. The maximum percentage of of moisturein the granules for the production of good extrudates variesfrom polymer to polymer but in general should be kept below0.1 % as measured by water absorption in 24 hours.Thermoplastic polyster ,the nylons and polycarbonatecan degrade and weaken if even a very small amount ofmoisture present when they are melted. So they must be driedin dehumidifying dryers to get moisture turned down to 0.01% or less.1) Remedies To remove moisture, the material must be pre-dry.Vent must be used in the extruder.www.ijltemas.in2) Impact on Product QualityThe result is a pattern of dotted lines, long bubbles and pits.G. Poor MixingFirst barrel section or feed zone on the extruder isnormally water cooled with no heating present. Cooling isprovided to prevent resin or additives from prematurelymelting and sticking to the feed throat. Overtime, sticky orpremelted material can agglomerate around the feed throatopening causing bridge to block the feed opening. Screw hasdeep high pitch feed element to provide maximum open areato transport resin away from the feed opening into theextruder. At start up the screw speed is normally run low withsignificantly reduced feed rate until material exist the die.[7]Uniform heating around the whole die, transfer pipesand adapters is critical to guarantee uniform melt temperatureand viscosity, resulting in consistent polymer flow out of theeach die hole if fans or other cooling devices are blowing onone side of the die this can affect melt temperature distributionand affect polymer flow. Melt temperature kept uniform byproviding fiberglass or other insulation around the transferpipes and the die. The die or adapter linking gear pump anddie will contain both melt temperature and pressuretransducers. A pressure controller is programmed to controlgear pump suction pressure by adjusting the extruder rpm. Thegear rpm sets the extrusion rate. Finer screen pack too raisesthe pressure but they keep clogging, pressure is inconsistent.[7]1) Remedies Exact addition of (Percentage of mixing) resinmaterial.Check for foreign material.Increase rpm back pressure for better mixing.2) Impact on Product QualityThis problem create the clogging in the extrusionH. SurgingSurging is the variation in throughput (maximum rateof production at which something can be processed) resultingfrom a cyclic behavior, with excessive extruder outputfollowed by low extruder output, then excessive output, and soforth.[7] Surging changes dimensions of the extrudate andlimits extrudate tolerances in production.[11] It can be easilyidentified by die pressure, motor load, and productdimensions.If in the transition section feed or transitiontemperature is low, the complete polymer mixing may notoccur in the transition section, So, unmelted pellets enteringthe metering zone are difficult to melt and interface withscrew mixing section. It interrupts the flow of polymer intothe screw. This can be any mechanical device prior to thePage 16

International Journal of Latest Technology in Engineering, Management & Applied Science (IJLTEMAS)Volume VII, Issue IX, September 2018

obstacles in the process which show the way to defects in the product. The purpose of this paper is to properly understanding the extrusion process and focus on the various defects and its impact on product quality. Keywords-Extrusion Process, Shaping Process, Product Quality, Quality Parameters, Cooling Medium.

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