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International Journal of Latest Technology in Engineering, Management & Applied Science (IJLTEMAS) Volume VII, Issue IX, September 2018 | ISSN 2278-2540

A Study of Process and its Defects

Pankaj M.Patil1, Prof. D.B. Sadaphale2 1Student of M.E(Machine Design), Mechanical Engineering Department, SSBT COET, Bambhori-Jalgao, Maharashtra, India 2Faculty Member of Mechanical Engineering Department, SSBT COET, Bambhori-Jalgaon, Maharashtra, India

Abstract-Extrusion is by far the most important and probably moulding is the other popular process accounting for ~25% of the oldest transformation and shaping process of the consumption. Blow moulding is used for ~5% while polymers. To ensure a quality through extrusion process during Rotomoulding 1% while the rest of the plastic is processed manufacturing, it is essential to discover, control, and monitor all through other processes. [2] quality parameters. Some of the important parameters are state of equipments, working conditions, temperatures, pressures, From the above data we can see that ~60 % extrusion quality of dies, materials and cooling medium. Instead of the process used in the industry but it has been observed that there truthful efforts from the manufacturers, still there are number of are lots of problems in extrusion process which lead to obstacles in the process which show the way to defects in the defective products. In extrusion products, defects due to product. The purpose of this is to properly understanding processing include, poor understanding of the processing the extrusion process and focus on the various defects and its method, use of old machines, be short of trained staff, impact on product quality. machine break down. Due to this it become essential the Keywords-Extrusion Process, Shaping Process, Product Quality, industries have better extrusion process. Quality Parameters, Cooling Medium. II. PROCESS I. INTRODUCTION Plastic extrusion is a high-volume manufacturing process in which raw plastic is melted and formed into a lastic processing needs the knowledge of basics of raw continuous profile. This process starts by feeding plastic materials, additives, process control and finally the P material (pellets, granules, flakes or powders) from a hopper product properties required to finish end products. Plastic into the of the extruder. The material is gradually materials have a useful combination of properties that can be melted by the mechanical energy generated by turning screws modified for used wide range of applications. and by heaters arranged along the barrel. The molten polymer In , processing techniques can be is then forced into a die, which shapes the polymer into a classified into either batch or continuous process. Batch shape that hardens during cooling.[13] There are two basic process includes and roto-moulding. types of plastic extrusion: screw extrusion and ram extrusion. Extrusion of is a continuous process. However, blow A ram extruder is an extruder where, instead of extrusion moulding is available both in batch and continuous process. In screw, a ram or plunger is used and a plunger goes through a these days, online continuous machines are barrel and pushes out the material under pressure. The ram available along with extrusion process. [9] extruder was the earliest extruder to be used in the . This typical process is applied for producing profiles, Various products manufactured through above sleeves, rod, block, tubing, lining sheet bars, etc. The ram mentioned processes are highlighted in the following table. extrusion process is very effective for specific materials like Classification of plastic products as a type of process PTFE which are not extruded successfully using screw Films and sheets, fibrous and filaments extruder because of its low friction. In this process plastic Extrusion pipes, conduits and profiles, . material in powder form is gravity fed into a chamber. In the extrudating chamber the resin powder is heated on sintering Injection Industrial Injection moulding, Household Injection moulding, Thermo ware/Moulded temperature. Ultra high molecular weight Plastic Moulding luggage. becomes gelatinous as it melts so it can be extruded with this products Blow , , Toys and housewares, type of processes. A hydraulic ram pushes the resin materials Mouldimg Air Ducts,panels. like PTFE, UHMW, etc. from the chamber to the die. The die actually gives the shape of the desired plastic like a rod, Roto Large circular tanks such as water tanks. Moulding or a profile shape with the requisite internal or outer diameter. When the material comes out of the die, it moves the length of the conveyor. The profiles can be manufactured endlessly and Extrusion process is the most commonly used cut by the continuous extruding of each length. Screw process in India and accounts for ~60% of total consumption extrusion involves a helical feed screw that turns inside a by downstream plastic processing industries. Injection barrel. This is often called the feed screw or the extruder www.ijltemas.in Page 13

International Journal of Latest Technology in Engineering, Management & Applied Science (IJLTEMAS) Volume VII, Issue IX, September 2018 | ISSN 2278-2540 screw. The screw is a single shaft with helical flights. fracture, Moisture absorption, Poor Mixing, Surging, Improper Sometimes, when more thorough mixing is needed, two Material Addition, Improper system engineering and screws are used. The constantly turning screw moves the resin installation.[3] Some other defects are Sink, poor or through the heated barrel where it is heated to proper nonuniform gloss, Weld lines , Gels and plate out , Warpage. temperature and blended into a homogeneous melt. Extrusion A. Improper Operation screw design has been improving over the years, with new innovations and ideas. Nowadays, single screws are available There is a molecular orientation change, resulting in that have a secondary flights that improve speed by enabling a variation in the machine direction versus the transverse faster melting. This process of extrusion serves two functions: direction properties. Depending on the orientation change, the it heats the plastic material above its melting point and puts properties in the machine direction may increase or decrease, the melt under pressure. The molten plastic material can then with the corresponding properties in the transverse direction be forced through an orifice, commonly known as the die. varying in the opposite direction. In other words, if the tensile This process is common to all types of extrusion. Most screws properties in the machine direction increase, they will have these three zones, Feed zone (also called the solids decrease in the transverse direction, and vice versa. A conveying zone): this zone feeds the resin into the extruder, potential cause for the change in molecular orientation is the and the channel depth is usually the same throughout the zone. draw ratio has changed between the extruder and the puller. Melting zone (also called the transition or compression zone): This only occurs with a die with an adjustable gap, where the most of the polymer is melted in this section, and the channel die lip setting has been changed. As the die lips are opened, a depth gets progressively smaller. Metering zone (also called higher draw ratio is required to maintain the final product the melt conveying zone): this zone melts the last particles and dimensions. In processes with nonadjustable die lip openings, mixes to a uniform temperature and composition. Like the the product dimensions are changed as the puller speed or the feed zone, the channel depth is constant throughout this zone. extruder screw speed is changed. With an adjustable die A great advantage of extrusion is that profiles such as pipes opening, the product thickness exiting the die can be increased can be made to any length. If the material is sufficiently or decreased through die adjustments. As the die is opened, flexible, pipes can be made at long lengths even coiling on a the puller speed is adjusted to maintain the product reel. Another advantage is the extrusion of pipes with dimensions, causing the product draw ratio to be increased and integrated coupler including rubber seal. changing the product orientation. [7] Gauge variations are defined as thickness nonuniformties in the final product. This changes in product dimensions are due to the no uniform melt temperature that result into polymer viscosity variations and causes of nonuniform melt temperature are poor melt mixing, an unevenly clogged screen pack due to that polymer flows through one part of pack to another. If take off equipment is not functioning properly, the product slippage can cause thickness variations. Slippage in the puller allows the product dimensions to rise for the reason that the product is not being pulled away from the extruder at the proper rate. The caterpillar or puller pressure has to be adjusted to prevent the product from slipping without causing any distortion or damage to the final product. Fig. 1 Plastic Extrusion Process [6] 1) Remedies III. DEFECTS  Verify puller speed correct. Defect is any form of deviation of the product’s  Verify screw speed is properly set. characteristic from the specification set up by the manufacturing process. It can be caused by a single source or  Check extruder temperature and adapter set points. the cumulative effect of several factors, which may arise at 2) Impact on Product Quality any stage of the processing. The common failure or defects which are normally occurring in plastic extrusion process are Uneven wall thickness due to three main causes: mould design, material selection, B. Resin Defects and processing. [1] Due to resin contamination a chemical reaction in the Some of the common extrusion defects are Improper extruder that may generate gases. It is easily corrected by operation, Resin Defects, Trapped air, Overheating, Melt www.ijltemas.in Page 14

International Journal of Latest Technology in Engineering, Management & Applied Science (IJLTEMAS) Volume VII, Issue IX, September 2018 | ISSN 2278-2540 properly purging the extruder with a nonreactive resin and by D. Overheating avoiding or removing any contamination from the product Overheating may produce degradation of polymer prior to feeding it to the extruder. material that is used or make dimensional control and sizing If moisture induced in the resin is not removed from difficult or may limit if takeoff cooling is limited. resin by an atmospheric or vacuum vent it exist the die as It’s generally assumed that much of the heat for steam and causes holes in the product can result. With melting is generated by the barrel. That’s true in some predried resins still containing a few moisture or moisture that instances, but the heat from the barrel has to be conducted has condensed on the pellet surface, rather than a hole, the from the barrel wall clear through the layer of polymer in the moisture can cause surface splay on the extruded part. Splay screw channel to do any significant melting. Since polymers indicates moisture or gases are there, but their concentration is are excellent insulators, conducting heat is one of their worst lower than the critical amount required to blow bubbles in the features. In reality, melting occurs in single-screw extruders product. primarily as a result of shear heating of the polymer. Shear Due to overheating resin degradation in the extruder heating is the result of a polymer-filled screw rotating within or die occurs .It decreases product strength in all directions. It the barrel. This is a lot easier to understand if you picture the is normally cause by the hot barrel or die temperatures, a screw as a simple round shaft with no flights turning inside a wrong screw geometry, long residence time at high heated tube filled with a highly viscous material. The amount temperature. of shear heating versus the degree of heat conducted from the barrel for melting depends a lot on the gap between the shaft 1) Remedies and tube. Generally speaking, the larger the screw, the deeper  Purge extruder or die. the channels, and the more important the contribution of shear  Screen the resin material. heating .In the of smaller screws with shallower channel  Exact addition of (Percentage of mixing) resin depths, the transfer of heat into the solid occurs more quickly material. because there is less distance for the heat to travel through the  Increase rpm back pressure for better mixing. insulating layer of the polymer itself. In these cases, the transient conductive heating of the solid polymer from the 2) Impact On Product Quality barrel heat can add significantly to the melting or softening Blow holes formation on the product temperature suitable for extrusion processing. But that’s only if there is enough time available. Except in cases where the C. Trapped Air screw is very small and turning very slowly, depending on Trapped air can be more problematic when starting conductive melting severely limits the potential output by material is powder compared to pellets. Because powder bulk waiting for the heat to fully penetrate the solid polymer. The density is lesser and introduces more air into the extruder. A interval of time to conduct full heat transfer increases higher compression ratio screw is used with powders, ensuring exponentially with thickness, so that the difference in time to that the transition flights are full of melt, preventing air from transfer heat through even a small increase in polymer entering the metering section. Vents and vacuum hoppers will thickness can be surprising. [12] eliminate trapped air and are essential for powders where 1) Remedies passages between particles are much smaller. The air cannot escape through this passages and it is carried forward instead  Avoid overheating and make sure that the temperature we heat the polymer is safe and causes If pelletized material used with long extruder barrel no degradation. trapped air is not common. Though some old machines have  Very accurate temperature control is needed for short and even long machines can be pushed so fast materials susceptible for degradation. that air carried forward in the in the product. Trapped air surface shows bubbles and pits. Such surface will improve if 2) Impact on Product Quality run more slowly if moisture is not problem. This create problem in the cooling of product 1) Remedies E. Melt Fracture or sharkskin  Avoid the over-speed of extrusion. In the extrusion of polymer melts, below certain  Increase plastic melt temperature. shear rates the emerging extrudates have smooth surface. As  Increase rear barrel temperature. the rate increases, small amplitude periodic distortions appear 2) Impact on Product Quality on the surface (sharkskin melt fracture).As rate increases further, the extrudate becomes severely distorted (gross melt A trapped surface shows bubbles pits, but little if any dotted fracture). [10] lines. www.ijltemas.in Page 15

International Journal of Latest Technology in Engineering, Management & Applied Science (IJLTEMAS) Volume VII, Issue IX, September 2018 | ISSN 2278-2540

Melt fracture is supposed to happening in the die 2) Impact on Product Quality entry region where material is being funneled from the melt The result is a pattern of dotted lines, long bubbles and pits. reservoir into the capillary or die. In a full-scale extruder, this would match up to the point where melt moves into the die G. Poor Mixing parallel portion of the die. Some further complicating effects First barrel section or feed zone on the extruder is may occur at the wall of the die. Melt Fracture is not surface normally water cooled with no heating present. Cooling is phenomenon as with sharkskin in its place it completely goes provided to prevent resin or additives from prematurely through the extrudate. The form of the melt fracture distortion varies from one polymer type to another, but it is generally melting and sticking to the feed throat. Overtime, sticky or helical in nature. With materials such as polyethylene and premelted material can agglomerate around the feed throat opening causing bridge to block the feed opening. Screw has a distortion like a screw thread may appear, deep high pitch feed element to provide maximum open area while with the extrudate may form a spiral. Other to transport resin away from the feed opening into the polymer melts may show ripples, or bamboo-like repetitive kinks. extruder. At start up the screw speed is normally run low with significantly reduced feed rate until material exist the die.[7] Sharkskin is believed to occur as a result of the melt Uniform heating around the whole die, transfer pipes tearing as it extrudes from the die. This occurs because, within and adapters is critical to guarantee uniform melt temperature the die, melt close to the wall is moving very slowly (in the and viscosity, resulting in consistent polymer flow out of the case of the layer next to the wall this movement is zero). As each die hole if fans or other cooling devices are blowing on the melt emerges, the whole extrudate moves away from the die face at a constant speed so that the outer layers are one side of the die this can affect melt temperature distribution suddenly stretched and may tear. In the case of sharkskin, the and affect polymer flow. Melt temperature kept uniform by providing fiberglass or other insulation around the transfer distortion consists of transverse ridges. [8] pipes and the die. The die or adapter linking gear pump and 1) Remedies die will contain both melt temperature and pressure transducers. A pressure controller is programmed to control  Lower the shear rate. gear pump suction pressure by adjusting the extruder rpm. The  Streamlining the die gear rpm sets the extrusion rate. Finer screen pack too raises  Slowing the extruder rate. the pressure but they keep clogging, pressure is inconsistent.  Reducing the melt viscosity. [7]  Increasing the die temperature. 1) Remedies 2) Impact on Product Quality  Exact addition of (Percentage of mixing) resin This refers to fine ridges or rough surface seen when the melt material. comes too fast out of narrow die.  Check for foreign material. F. Moisture Absorption  Increase rpm back pressure for better mixing. Many plastic materials absorb moisture. This 2) Impact on Product Quality moisture passes through the extruder and boils when the This problem create the clogging in the extrusion pressure is relieved at the die lips. A pattern of long bubbles and pits is the result. The maximum percentage of of moisture H. Surging in the granules for the production of good extrudates varies Surging is the variation in throughput (maximum rate from polymer to polymer but in general should be kept below of production at which something can be processed) resulting 0.1 % as measured by water absorption in 24 hours. from a cyclic behavior, with excessive extruder output Thermoplastic polyster ,the and followed by low extruder output, then excessive output, and so can degrade and weaken if even a very small amount of forth.[7] Surging changes dimensions of the extrudate and moisture present when they are melted. So they must be dried limits extrudate tolerances in production.[11] It can be easily in dehumidifying dryers to get moisture turned down to 0.01 identified by die pressure, motor load, and product % or less. dimensions. 1) Remedies If in the transition section feed or transition temperature is low, the complete polymer mixing may not  To remove moisture, the material must be pre-dry. occur in the transition section, So, unmelted pellets entering  Vent must be used in the extruder. the metering zone are difficult to melt and interface with screw mixing section. It interrupts the flow of polymer into the screw. This can be any mechanical device prior to the www.ijltemas.in Page 16

International Journal of Latest Technology in Engineering, Management & Applied Science (IJLTEMAS) Volume VII, Issue IX, September 2018 | ISSN 2278-2540 screw such as a feeder or the geometry of the flow passage the droppings. Even so-called medical grade plastics are often polymer follows to get to the screw. Also, the polymer bulk- contaminated. Of course, the contaminate will end up in the flow characteristics can cause bridging or tunneling that result melt, so every precaution must be taken to avoid in erratic flow to the screw. If the motor speed is pulsating, the contamination by closing , covering hoppers, keeping a screw speed is changing, with more and less material being clean area around the extruders, and similar measures.[7] pumped out the extruder as the screw speed changes. Some of the many problems that can occur during If none of these outside causes are observed, it is extrusion are traceable to the processed raw material; that is, probable that the surging originates inside, typically at the the raw material is not up to specification. Regardless of any beginning of the compression zone, where the solid bed the quality assurances by manufacturers of raw materials and mass of pellets may be locking and breaking up irregularly. countless programs such as ISO 9000, reengineering, and Just Screw rpm is steady but the ammeter shows variations of ± In Time (JIT) manufacturing, plastic batches and even bags or 5% or more. Sometimes this can be cured by increasing the within batches have their idiosyncrasies. With all temperature of the feed to promote earlier melting. Raising the extrusion parameters the same, the tubing or sheet will have a rear barrel temperature may help, too. Make big changes, 25- different look, bend, colour, or texture. While this may sound 50 °F (14-28 °C), and see what happens. Sometimes raising unscientific or preposterous, these problems do occur on the the barrel Temperature at the beginning of the compression floor. Despite all the computer programs and hardware zone will help by getting better sticking of the pellets to the designed to keep the quality constant, there will be wall there. [3] variations.[7] 1) Remedies 1) Remedies  Replace the screw.  Exact addition of (Percentage of mixing) resin  Installing a gear pump between the extruder and the material. die.  Check for foreign material.  Try to running the extruder slower or faster by at  Increase rpm back pressure for better mixing. least 10%. 2) Impact on Product Quality 2) Impact on Product Quality Material absorb the moisture from the air which can cause Variation in the thickness of the product bubbles I. Improper Material Addition J. Improper System Engineering And Installation The Resin materials such as , , and polycarbonate and polycarbonate are very hygroscopic, absorbing moisture rapidly from the air. At extrusion temperatures, moisture degrades these materials to lower molecular weight polymers, resulting in poorer property performance. Proper drying to eliminate moisture is critical to obtain the optimum property performance in the final product. Other materials, e.g., acrylics, Ultem , Noryl and acrylonitrile butadiene styrene (ABS), also absorb moisture from the air and must be dried prior to processing. Any moisture in the polymer is converted to steam in the extruder and, depending on the quantity present, can cause surface imperfections such as splay, holes in the product, or a foamy product. Some polymers, e.g., nylon, are shipped dry in moisture-proof containers. With proper handling, these resins Fig.2 Spider Die [4] do not normally require additional drying prior to processing. However, if the seal is broken on the or the is The extrudate enters directly from the extruder not completely resealed after opening, the product will absorb through the breaker plate into the die. Large dies require a die moisture and have to be dried prior to extrusion. are stand or cart to support the die weight and prevent distorting particularly sensitive to moisture and must be dried in the extruder barrel. An adapter may be present between the dehumidifying dryers, transported with dry air, and blanketed extruder and the or tubing die, versus the direct with dry air or nitrogen in the extruder feed hopper. Other connection shown in Figure. The entrance cone distributes the problems include contamination with foreign materials such as melt uniformly around the mandrel. The mandrel or center metal chips, screws, bolts, nuts, cardboard, rodents, and rodent section is held in place by spokes radiating out from the www.ijltemas.in Page 17

International Journal of Latest Technology in Engineering, Management & Applied Science (IJLTEMAS) Volume VII, Issue IX, September 2018 | ISSN 2278-2540 mandrel, called a spider ring with the individual spokes 2) Impact on product quality referred to as spider legs. The mandrel centered in the die and Sink mark. bushing centered on mandrel. To make adjustments, the bushing clamping ring loosened and the die adjustments bolts L. Poor or Nonuniform gloss are used .If the bushing improperly centered the pipe or tube Gloss is a measure of the ability of a material to concentricity will be incorrect with one side having thicker reflect light. It is an optical feature of all materials whether wall than other side. Through one spider leg there is air inlet transparent or opaque. When a beam of light strikes a surface, tube that supplies air to the centre of the pipe. The air supply is at atmospheric pressure. The die gap area where the melt the amount reflected will depend not only on the type of channel is a constant diameter is called the die land area and it surface finish, and refractive index of the material, but also on the angle of incidence and the angle of viewing of the is used to shape the molten polymer into a pipe or tube before reflected beam. [8] exist the die. [7] Improperly adjusted rolls can lead to poor gloss and Problems can be created by the pull rolls or winder if they are not properly synchronized with the three-roll stack, rough sheet. To obtain the best gloss, highly polished roll either generating too much tension or pulling the product surfaces at high temperature with the correct pressure are required. A guideline for setting roll temperature is to raise the across the chill rolls. Potential problem associated with the roll temperatures after the process is running until the sheet or winder or pull rolls overpowering the three roll stack or chill film just starts to stick to the roll surface, and then lower the roll are nonuniform thickness is created in the transverse temperature approximately 10˚F (5.5˚C). The roll surfaces direction by excessive pulling of the winder or pull rolls, causing the sheet to slip and stick in the roll stack. When the have to be clean with uniform pressure applied across the sheet is slipping on the rolls, the resultant product necking is rolls.[7] increased, and the product is more likely to be thin on the Nonuniform or inconsistent gloss across the sheet can edges and thicker in the middle than when it is adhering to the be caused by nonuniform roll temperatures, inconsistent gauge three-roll stack and moving with those rolls. In some or thickness, volatiles, or high melt temperatures. Inconsistent situations the product is slipping constantly, and lines are gauge can originate in the extruder through surging, in the die formed in the transverse direction by the apparent action of by improperly setting the die lip opening and/or improperly starting and stopping in the nip or chatter as it goes through setting the choker bar, and/or inappropriate roll gap the nip.[7] adjustment. Sources for volatiles include resin moisture caused by improper drying, excessive melt temperature 1) Remedies leading to polymer or additive degradation, plugged extruder  Adjust the die setting vent preventing gas removal, and a plugged or insufficient  Check for alignment vacuum to successfully remove all the volatiles.[7] 2) Impact on Product Quality 1) Remedies Centering problems and chatter marks on the product  Set roll temperature high as possible.  Assure there no extruder surging. Some other defects are  Check resin for moisture K. Sink  Adjust die lips. Sink is defined as an indentation in the surface 2) Impact on product quality opposite a rib or boss. This is generally a problem only in profile extrusion. Sink is caused by shrinkage. Where a flat Volatile defects show up as splay or other surface defects. area intersects with a rib, there is more material present in the M. Weld Lines cross section. As the material cools, more shrinkage occurs in the thicker section, causing a surface indentation opposite the In polymer processing, weld lines can be formed by rib. Indentation is minimized by decreasing the rib width and joining two different fluid fronts or when a fluid breaks up due profile extrusion. Sink is caused by shrinkage by decreasing to the presence of an obstacle, then rejoins downstream.[4] the rib width and the radius going into the rib. However, a Dies containing mandrels normally have a spider to center the very narrow rib compared to the part’s main thickness can mandrel in the middle of the die. Polymer flows around each lead to warpage due to the rib cooling and shrinking faster pin or spider arm holding the mandrel in place and then versus the rest of the part. [7] recombines just prior to exiting the die. Contamination or a foreign object such as a wire from the screen pack trapped in 1) Remedies the die prior to or in the die lip area can cause a weld line to  Change the part design form in any extruded shape. [7] www.ijltemas.in Page 18

International Journal of Latest Technology in Engineering, Management & Applied Science (IJLTEMAS) Volume VII, Issue IX, September 2018 | ISSN 2278-2540

2) Remedies 2) Impact on product qualities  Raise barrel temperature. Due to gels- Defects in extrudate (usually film) that looks like  Remove any foreign material or contaminants from eyes of fish and are the result of small unmelted resin die. particles.  Increase pressure on molecules forcing them O. Warpage together. Warpage is a type of distortion where the surface of part 3) Impact on product quality do not follow intended shape of design.[5] Warpage is caused It generates weld lines in pipe or tubing. by the differential or nonuniform shrinkage in the extrudate cross section. This is often referred to as internal stresses in N. Gels and Plate out the part resulting from the molecular orientation, molecular Gels are high-molecular-weight polymer particles relaxation outside the die, or molecular orientation induced in that do not melt during processing. Gels may soften and the final product by drawing.[7] elongate as they pass through a screen pack designed to filter Proper part cooling is critical to produce warpage free or trap contaminants. However, they never really melt. After parts with the acceptable dimensions and performance. Part passing through the screens, they snap back or return to their warpage is caused by differential shrinkage. To minimize original shape, forming small, hard particles in the melt. differential shrinkage, the part must be cooled uniformly on all Others define gels as any contamination (dirt, other polymer, sides. If one side or area of the extrudate solidifies before cross-linked polymer, and so forth) that passes through the another, the part will warp, bending toward the side that die. In the final part, gels are a speck (may be clear) that forms solidified last. If one side of the extrudate is dragged over an a defect. In polyethylene, gels are occasionally formed in the object in the cooling operation, molecular orientation is process as material hangs up or stagnates in the system. induced on that side, causing it to shrink differently from the Polyethylene held at processing temperatures over time can other side, leading to warpage.[7] cross-link and form high molecular-weight particles that do not melt in the process. If gels are formed in the die, the die Pressure exerted by the puller must be sufficient to needs to be streamlined to prevent stagnation. To remove gels prevent product slippage in the puller, but low enough to that are already present in the polymer, raise the melt prevent part distortion or marks on the product surface. temperature and/or use a finer screen pack to filter them out. Extreme puller pressure can crush the final part, rendering it [7] useless. The puller may be a long distance from the extruder; however, it must be properly aligned with the extruder to Plate-out or die lip build-up is from low- prevent the part from being pulled in one direction or another, molecularweight polymer or additives that migrate to the inducing molecular orientation that may lead to warpage.[7] extrudate surface and are deposited on the die lips. Die build- up needs to be chemically analyzed to determine the source. Crystalline materials need to be cooled uniformly to Low-molecular-weight polymer is in the low-end tail on the generate the same crystallinity and crystal size. If the polymer molecular weight distribution curve. To eliminate crystallinity is higher in one section compared to another due plate-out, a different resin can be purchased with a narrower to extrudate quenching or cooling differences, the section with molecular weight band. If the deposits are additives that higher crystallinity will shrink more. This shrinkage difference volatilize and recondense on the die surface, alternative induces warpage in the final part.[7] additives can be used or processing conditions changed (lower 1) Remedies temperatures) to minimize the deposits. Regardless of the cause, solutions are required to minimize plate-out in order to  Puller must be properly aligned with the extruder. maximize product yields and process efficiencies. [7]  Uniform Melt temperature. 1) Remedies 2) Impact on product quality For gels- The part will twist or deform.  Raise melt temperature IV.CONCLUSION  Streamline die to prevent stagnation  Increase screen pack mesh to filter out. By studying various and books on the extrusion process and its defects it is observed these defects For plate out are due to the improper setting of operational parameter. By  Lowering the die head temperature. applying the above remedies it is possible to reduce the losses  Reducing extrusion speed. that occur due to defects in the product.  Change the formulation. www.ijltemas.in Page 19

International Journal of Latest Technology in Engineering, Management & Applied Science (IJLTEMAS) Volume VII, Issue IX, September 2018 | ISSN 2278-2540

REFERENCES [6]. S.Siva Subramanian, S.Durga, K R Loshni, V Dinesh Kumar (2016), “A Review on Control of Plastic Extrusion Process” Vol.5, [1]. Narasimha M and Rejikumar R (2013), “Plastic Pipe Defects Special Issue 1,Page No.167-171. Minimization”, International Journal of Innovative Research and [7]. Harold F. Giles, Jr. John R. Wagner, Jr. Eldridge M. Mount, III , Development, Vol. 2, No. 5, Page No- 1337-1351. “Extrusion: The Definitive Processing Guide and Handbook” [2]. FICCI, “Potential of plastic industry in northern India with special William Andrew publication, ISBN: 0-8155-1473-5. focus on and food processing”, 2014. [8]. John Goff and Tony Whelan, “The Dynisco Extrusion Processors [3]. J G Khan, R S Dalu, S S Gadekar (2014), “Defects In Extrusion Handbook”, Dynisco company. Process And Their Impact On Product Quality” Vol. 3, No. 3, [9]. Muralisrinivasan Natamal Subramanian, “The Basics of Page No-187-194. Troubleshooting in Plastics Processing” Wiley Publication, ISBN: [4]. Olga Sousa Carneiro, Luis lima ferras, Paulo Teixeira, Celio Pinto 978-0-470-62606-1. Fernandes, and Joao Miguel Nobrega(2015), “Weld Lines in [10]. Igor B. Kazatchkov(1994), “Melt Fracture Behaviour Of Molten extrusion:Understanding the role of the flow conditions” AIP ”,Thesis. Conference proceedings 1662, 030012-1-030012-10;doi: [11]. Keith Luker, Randcastle Extrusion Systems, Cedar Grove, “Surge 10.1063/1.4918887. Suppression-A New means to limit surging”. [5]. N.Keleshian, R.Kyser , J.Rodriguez, C. Cueva, V.Vega, E.W.Lee, [12]. https://www.ptonline.com/columns/extrusion-barrel-heat-and- J.Ogren, O.S.Es-said(2011), “On the Distortion and Warpage of melting 7249 Aluminum Alloy After Quenching and Machining” [13]. https://en.wikipedia.org/wiki/Plastics_extrusion Vol.20(7), Page No-1230-1234

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