My polymer material selection courses đ Find Out About Plastics Blogđ ht. In our DFM report, we will analyze the possible weld lines. Fracture Toughness of Weld Lines in Thermoplastic Injection Molding, Polymer Engineering and Science, 33(15): 996-1001. They decrease the. CrossRef View in Scopus Google Scholar [17] G. Consultant @ Effinno Technologies, BSME, MBA - Bridge between Plastics Injection Molding Knowledge and Practice. This happens when the molten plastic is injected into the mold cavity and flows around an obstacle such as a pin, hole, or protrusion, and then comes back together. Mold Design. The Problems & Solutions course is designed to train injection molding personnel in. 1. However, research about the computer determination of weld lines in injection molding remains scarce. Weld lines occur when two flow fronts meet and fuse together in the injection molding process. Weld lines are significant defects that affect injection molding quality, causing the poor appearance and low mechanical strength of injectionâmolded parts. Weld or Knit lines are a plane or section in an injection molding part where the flow of molten plastics meets from different directions. This model provides the necessary information. In this study, we apply the technique of SAW to explore the acoustic properties near injection-weld lines by comparing the velocity of (V R) in locations close to and locations distant from the weldSince weldline in plastic injection molding (PIM) has an influence on not only the strength but also the surface appearance of a plastic product, it is preferable to reduce it as much as possible for high product quality. Weld lines are formed when two or more ïŹow fronts join, as the liquid polymer ïŹlls the. As the injection process continues, the two separated melt fronts meet again along the longitudinal section of the plate forming a weld line. There is excess air inside the mold (1) make the mold vent larger at location with weld line; (2) Check if the mold vent is blocked or not or use vacuumized forming. Blistering 4. [ 18 - 21 ] In addition, the effects of flow behavior of the polymer melt due to the optimal process parameters for the minimization of weld-line effects are an important consideration. In this study, the effects of injection temperature, injection pressure, and fiber content on the mechanical properties of weld lines were studied for carbon fiber-reinforced nylon. They found that the weld line is a zone between 2 and 8 mm wide extending throughout the thickness in whichEffects of mold temperature and pressure on shrinkage 0. Here are some ways to avoid. Weld Lines are formed when two or more molten polymer flows meet during the injection molding process. Weld line, 5. Other possible causes include: Low mold or material temperature; Insufficient holding time or. Weld line formation in the injection molding process occurs when two or more melt flow fronts contact each other immediately after the cooling process. [ 18 - 21 ] In addition, the effects. Injection molding plate for mode I testing geometry. Effect of the processing conditions on the variations of the bulk temperature, T b , at the end of mold filling at the two locations: in (left) and away (right) from the weld-line (T w , T inj. Vibration assisted injection molding. Proceedings of ANTEC (2000) Google Scholar [9] S. The special injection mold is designed and fabricated to produce plaques having stagnation weld-line. The type of resin used during injection molding plays a large role in whether or not a knit line will be present. Weld lines defects can also form when the flow fronts from two gates merge or when thick and thin sections in a part cause a flow to split and merge. Using the conventional method, weld-line depths on the surface ranged from 6 to 13 microns. Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties. The special injection mold is designed and fabricated to produce plaques having stagnation weld-line. As the polymer resin is injected into a mold, it flows through all parts of the mold. Part design features may cause a material flow to split and rejoin as it flows around design features in a mold. Weld line section presents notch or gloss difference. In a weld line, reinforcement fibers. The following are the conclusion drawn from the present research: 1. In the sense that the weld line is better formed before the resin temperature. Low mold temperatures are usually employed to decrease cycle molding time and final process costs; however, resulting surface defects include weld lines, sink marks,. Wu CH, Liang WJ (2005) Effects of geometry and injection-molding parameters on weld-line strength. The schematic diagram of a weld line formation is illustrated in Figure 7, where is the welding angle of two melt fronts. injection molding. A weld or meld line is a weakness or visible flaw created when two or more flow paths meet during the filling process. As these flow fronts come together. Unless you are manufacturing an extremely simple part with one gate and a unidirectional flow front, your part will have weld lines. In this work, the residual strength of a fiber-reinforced injection molded part containing a hot weld lineâor meld lineâwas evaluated. Problem: Weld lines are formed when plastic resin is forced around an obstruction in a mold tool and reforms on the side opposite the gate. Proper venting is also required to ensure good weld-line quality. Mold vents should function the same way. 5. When two or more strands of molten plastics meet in the mold, a weld line will be produced at the junction [1,2]. In this paper, a weld line computer determination method based on filling simulation with surface model is proposed, from which the positions and lengths of the weld lines can be predicted. The rule is, a higher pressure shall be required to push the material in the machine when there is excess mold. With unchangeable and fixed requirements for the final product, their avoidance often proves difficult. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part. 025 100 120 140 160 180 200 220 240 Mold Temperature (F) LDPE PPAccording to the weld line distribution map of six gate schemes shown in . See moreEquipment. The more gate points there are, the more weld lines will occur. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. Known also as knit lines and weld marks, weld lines are difficult to eliminate completely, especially if your injection molded part requires holes. Prediction of the weld lines in injection molding process using neural networks. The injection molding gate generates heat by dissipation. The parting line in injection molding is which one of the following: (a) the lines formed where polymer melt meets after flowing around a core in the mold, (b) the narrow gate sections where the parts are separated from the runner, (c) where the clamping unit is joined to the injection unit in the molding machine, or (d) where the two mold halves come. Weld or knit lines are perhaps the most common and difficult injection molding defect to eliminate. Many works deal with weld lines and their dependence on processing parameters for conventional injection molding, but only a few works focus on the weld line in µIM. The impact behavior of weld-lines in injection molding. This is particularly true if the wall thickness in the weld is thin â i. Effect of process parameters on the relative weld line strength From injection molding practice and literature, 1, 3,4 it is generally accepted that: if a weld line forms before the filling is. The weld-lines are undesirable from both aesthetic and functional points of view and their effect is different for varied materials, process conditions, loadings, forms. This failure causes the product to be damaged during use. Simulation of polymer molding filling process with an adaptive weld line capturing algorithm. Or increase the number of gates. In this study, we apply the technique of SAW to explore the acoustic properties near injection-weld lines by comparing the velocity of (V R) in locations close to and locations distant from the weldAmong defects of injection molding parts, weld lines are the most common defect. Avientâs Zachary Alderman shares practical tips on designing and injection molding parts utilizing short-fiber reinforcements. In this case the two flow fronts will meet and there will be very little molecular orientation across the weld. As a result this weld line will be a weak point in this part. injection molding. Weld lines are a surface defect caused when two or more flow fronts come together during the injection process. â Use generous and even. These are preferred over weld lines as they will normally result in improved cosmetics and structure. Weld lines are a common defect generated in injection molding, which apparently affects the performance of final products, but the available reports on carbon fiber-reinforced thermoplastics are. Flow analysis of the weld line formation during injection mold filling of thermoplastics. The injection molding process can be optimized to maximize weld line strength by increasing mold and melt temperatures, and by increasing pack pressure at the weld line. Thus, it is important to understand (i). The specific method is that when the product is just filled and filled in the cavity, the insert. Since it is often impossible to avoid weld lines as well as a limited possibility to increase the weld line strength by adjusting the injection molding parameters, a method to predict the weld line strength in the product development process is shown below. But add 30% glass and weld-line strength retention drops to only 34% as high as the compound with no weld. Weld Lines. A weld line is a mark left on the surface of a plastic injected molded part. The weld lines are an integral part of plastic injection molding and can seldom be prevented, even by specifically planning the mold with inserts for holes or the like. 18/06/2021. Tosello et al. Flow marks in injection molding are defects that manifest as a line or series of lines forming a pattern different in color from the other parts of the molded material. 025 mm (0 . Polym. a) Multi-fiber connector micro injection molding simulation b) 3D simulation of micro gear with 150µm diameter for weld line position prediction c) 3D micro injection molding filling simulation of micro gear with 180µm diameter Fig. 3. Weld-line is a weak area which reduces the strength of the part locally. [ 18 - 21 ] In addition, the effects of flow behavior of the polymer melt due to the optimal process parameters for the minimization of weld-line effects are an important consideration. If weld lines are unavoidable due to multiple gates in a moulding, they should be positioned in the non-critical region. Weld Line Occurrence in Plastic Injection Molded Parts Weld lines, also commonly known as knit lines, may be present in a plastic molded part depending on the partsĂ geometry. Another way to eliminate weld lines in plastic injection molding is to adjust the molding conditions. Although they often cannot be avoided itâs possible to minimize their cosmetic effects to an acceptable degree through careful product design and. When the polymer resin is injected into a mold, it flows through all portions of the mold cavity. This failure causes the product to be damaged during use. Steven. Strength at weld the line location and warpage problem in injection molding plastic component were not new problems in injection molding industries. Published 5 July 2019. Stop using mold parting agents. In this regard, the plasticizing ability of the plastic injection moulding machine should be checked. (1983). Weld lines are the most common defects in injection molded products. Save. Move injection locations to make weld lines form closer to the gates. The flow of the material in the injection mould takes place through the gate. Weld Line Factors for Thermoplastics. 8. It is a zone with reduced mechanical properties negatively influencing on the molding behavior in exploitation. Figure 2. In the first, sequential injection moulding, the lines are displaced to noncritical areas of the moulding. poor flow fronts that are too cold, this can also lead to a deterioration in the mechanical properties. Three terms that often get mixed up in injection molding are weld, meld and knit lines. Emerging Materials Research. One test mold was used to mold three different materials using heat/cool and conventional molding techniques. The structure is more complex with longer molding cycle . Weld lines not. Email him at jhenz. 6. Azieatul Azrin Dzulkipli and M. Molding Equipment. Adjust the design for the flow pattern to be a single source flow. S. Consequently, it causes viscous dissipation which leads to the generation of heat. The weld line is then created with a higher flow front temperature and is packed with more pressure. When two or more strands of molten plastics meet in the mold, a weld line will be produced at the junction [1,2]. Introduction. About Press Copyright Contact us Creators Advertise Developers Terms Privacy Policy & Safety How YouTube works Test new features NFL Sunday Ticket Press Copyright. In injection molded parts, there are 2 types of weld lines: The first type of weld mark is called cold-melt joints and is formed when the melt front enters the cavity with different gates. Weld lines often generate mechanically weak areas, because of the unfavorable perpendicular orientation of the macromolecules, fibers and/or other fillers to the direction of the main flow. Weld line location. Mold temperature controlInjection mold designers can alter the position of weld line by changing the gate locating. Weld lines are formed when two orThe experiments were performed on an Arburg Allrounder 320C 600-250 injection moulding machine using a two cavity-injection mould (Fig. The weld lines in injection moulded parts arise as a result of the collision of two fronts of the flowing material, which fills the mould cavity and are sometimes unavoidable. Weld lines occur when melt flow fronts meet during the injection molding of plastic parts. The cracking was always located in the weld/knit line and therefore caused by weakness in the bonding of the plastic at. 2. The foundation to any robust process, in my opinion, is based on choosing the correct fill time. Weld lines, generated during injection molding, can significantly reduce the mechanical performance of structures. Try to gradually raise the mold temperature and barrel temperature. Specifically, injection-mold lines are often associated with poor mechanical strength. Change the gate locations. This customary mark is caused by differential rates of cooling in the resin where. Depending on the resin, resin temperature, mold temperature, and filling speed, knit lines can vary. ). Therefore, it is recommended to do a moldflow study prior to the mold being built. The wheel has to be gated at three places on the hub and. One of the things that can compromise an injection molded part is a weld line, also called a knit line. A knit line in a plastic injection-molded part is created when two separate plastic flows meet within the mold and "resolidify" along their interface. Still, it can be seen to varied degrees depending on mold. 005 0. Weld lines can compromise the strength and aesthetics of the part, so itâs important to carefully design the mold. A simulation process was conducted to determine the recommended setting of injection moulding parameters and the range of the variable. Weld lines. consideration in gate location. Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties [3,4,5]. Mold vents should function the same way. Injection molding weld lines where two or more flow fronts come together in the mold. The melt stream is separated at the partâs cutout and reunited at the other end of the cutout. Heating and cooling mould systems, which cost extremely high, are a typical technology to prevent the weld-line. This paper presents the. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. Second, depending on the size of the overflow well, you get a little bit of flow melding together, and that helps strength so that you have some intertwining of the polymer chains. Thatâs the air escaping out of the nozzle until the hose is full of pressurized water. 7 in Kobayashi et al. Weld lines in injection molding are frequently unavoidable. but the injection pressure will loss and injection molding pressure of this gate is bigger than that of direct gate. A simulation of mold filling with multi-inlets or an insert in the cavity is pursued with an improved Level Set Method. In this study the polycarbonate (PC) and carbon fiber (CF) are used as the experimental materials to investigate the impact of injection molding process parameters on tensile strength of weld line. To prevent knit lines, injection molders can increase the temperature of the molten resin to prevent solidification from starting too soon, and increasing injection speeds and pressure are also potential solutions. If the different flow fronts have cooled. T. Ni. Weld-line strength for plastics can vary from 20% to nearly 100% of the strength of the plastic itself. Weld lines are one of the typical quality issues of injection molded parts. Known methods to reduce weld-line effects are the optimization of injection molding parameters, enhanced tool temperature, and using special molding tool devices. In this way critical areas of the interface in regard to the lack of interdiffusion and the inappropriate molecular orientation were found to be placed near. Adjust Injection Molding Parameters: Optimizing injection speed, pressure, and temperature can help ensure better melt flow and reduce the likelihood of weld line formation. It can be determined by a visible line appearing on the surface of the plastic part where two flows of the plastic meet. A weld line is a mark left on the surface of a plastic injected molded part. For the production of these weldâline specimens, a special mold was used and injection molding parameters were systematically varied, tensile (ISO 527â2) and impact (ISO 179â1/1eU. 18/06/2021. Sink Marks Cause 4: High Mold Temperature. In this paper, aiming at the problem of weld line defects in the injec- tion process of a key phone panel, the injection process is optimized by using Moldflow and drop simulation. weld lines in injection molded thermoplastic composites. Weld lines in injection molding are frequently unavoidable. Dischler, D. Numerical results show that weld lines can be predicted accurately, and the formation and evolution processes of weld line can be shown more clearly by co-injection molding technique. Flash. Although they often cannot be avoided itâs possible to minimize their cosmetic effects to an acceptable degree through careful product design and manufacturing. In this study the polycarbonate (PC) and carbon fiber (CF) are used as the experimental materials to investigate the impact of injection molding process parameters on tensile strength of weld line. To explore the formation of weld lines in injection molding, a. (2) Adjust the injection speed: high speed allows the melt to reach the confluence without cooling down, and low speed allows time for the air in the cavity to be discharged. Letâs review what causes meld and knit line imperfections, how theyâre different, and how they affect part durability. 001 in . In this study, the effects of injection temperature, injection pressure, and fiber cont. Flow marks in injection molding can be avoided if adequate measures are taken. How Are They Formed? Molten plastic resin is injected into a mold cavity through an opening known as a gate. An excess mold release is a major cause of weld line due to two variables: high pressure and speed. Weld lines have been one of the common defects puzzling the injection molding industry. As the polymer. Mérillou, J. Abstract. , less than 0. 2004; To study the weld line formation of colliding flow fronts the filling of a mold cavity was simulated. This customary mark is caused by differential rates of cooling in the resin where. For multiple gate and complex part, molding weld-lines are unavoidable, therefore mechanical behavior of the weld-line needs to be predicted. The next type of injection molding defect is âWeld Linesâ. Weld lines are among the most critical structural defects in parts made by the injection molding of fiber-reinforced thermoplastics, explained Sirmax in a press release. 3. 005 0. Heat/cool injection molding has a positive influence on the depth and visibility of weld lines. This special mould has changeable inserts to be able to inject with different gate types (standard, film, special-film, multi gates, etc. Weld lines form where two or more flow fronts come together and can create a cosmetic or structural problem. If you are familiar with any form of scientific injection molding, regardless of the name, then you know one of the first steps is performing a viscosity curve study. When it comes to injection molding, the chances you might end up making a costly mistake is too high. Weld lines are a kind of Achilles heel of plastics. In the 1947 book, Injection Molding of Plastics, the late Dr. The solution to an internal part cracking problem is to change the packing pressure. Valve. (3) Adjust the temperature of the barrel and nozzle: the viscosity. Causes: Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic. Raising the mold temperature will keep the material molten longer, resulting in stronger weld lines and even elimination of weld lines. That helps. Increase the injection speed and mold/operational temperature to prevent fracture or surface breakdown. Injection molding is an efficient manufacturing process for mass-producing parts with complicated geometry. In this study, polypropylene (PP) was compounded respectively with carbon nano fibers (CNFs) and TiO 2 nano particles at various weight fractions (10, 20, 30, 35. ac. Known methods to reduce weld-line effects are the optimization of injection molding parameters, enhanced tool temperature, and using special molding tool devices. Islyn Thomas summarized the need for a particular vent location better than I ever could. In this study, epoxy molding compounds (EMC) filled with glass beads were fabricated with a twin-screw-extruder (TSE) and tempered before further processing to generate different degrees of cross-linking (0â15%) in the compounds. This study examines the impact of injection parameters on the weld line strength of the polyamide 6 and 30% fiberglass (PA6 + 30% FG) composite samples. 000 0. Wang, G. These lines can occur at the gate location, where the plastic enters the mold, or anywhere along the flow path. In an injection molding process, a weld-line forms when two flow fronts meet each other. Definition: A seam that appears where two areas of molten plastics meet. Investigations on WLs in micro injection molded parts were performed in Refs. The formation of weld line in the injection molding method is caused by the product and mold design that is one of the common mistakes. In the case of PC, the model predicts satisfactorily the weld-line strength as a function of injection temperature. (3-1) Increasing the Resin Temperature at the Weld Line Section. Meld and knit lines are actually both different types of weld lines. 2017-01-0481. (2) Adjust the injection speed: high speed allows the melt to reach the confluence without cooling down, and low speed allows time for the air in the cavity to be discharged. Bubbles are caused by melting flow fronts colliding in a mold cavity. Injected plates were generated using a double-gated mold under four different process conditions. Sequential valve gate technology. It forces out trapped air pockets and can prevent a vacuum void from occurring. To determine the effect of the weld lines on tensile strength, tensile tests were conducted with the specimens fabricated with NORYL (amorphous blends of. Shaofei Jiang. Boost time too long Shorten boost time, lower boost pressure, or move transfer to an earlier position . In this lesson we will focus on the meld and weld lines. Insufficient plasticizing ability of injection molding machine. Abstract. The effects of filling time, packing time, packing pressure, melt temperature, and mold temperature on the ultimate tensile strength (UTS) and the elongation value of the weld. Ghazanfarpour. weld lines in infill analysis of injection molding simulation was observed [5]. 020 0. In many cases, you will also see that the part is hanging up on the core during ejection. Therefore, the. Effects of mold temperature and pressure on shrinkage 0. Below is one of the examples: !NOTE: if the weld lines is important to the cosmetic surface of your part, please bring this up to your mold maker during quoting phase. The four sets of plaques are produced by changing the magnitude of packing pressure equal to 60% of filling pressure, with increment of 20% up to 120% of filling. Depending on the resin, resin temperature, mold temperature, and filling speed, knit lines can vary. Weld lines are a common defect generated in injection molding, which apparently affects the performance of final products, but the available reports on carbon fiber-reinforced thermoplastics are still rather few. 2. Figure 1a illustrates how the weld line is produced when two melt streams merge during the final filling process. Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties. The weld lines in injection molded parts arise as a result of the collision of two fronts of the flowing material, which fills the mold cavity and are sometimes unavoidable [1][2][3] [4]. Causes: Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic. Improper fusion of two fluid flows, a state before weld line. Burn Marks. [ 18 - 21 ] In addition, the effects of flow behavior of the polymer melt due to the optimal process parameters for the minimization of weld-line effects are an important consideration. You can locate the gate at the thick part of the injection molding to prevent air pockets. An excess mold release is a major cause of weld line due to two variables: high pressure and speed. Figure 2. They are the result of fundamental physical effects taking place inside of the mold tool cavity. Conventional injection molding is widely used to produce plastic parts, mainly in the automotive industry, due to the high production ratios and quality of injection parts. The plaques are prepared out of 30% glass-filled Polyamide 6 material. In addition a weld line can be moved to an area where it is less visible. G. The Weld line result itself has a scale associated with the placement of the weld line that is directly relative to how severe the weld line will be based on the angle in which the two flow fronts meet. Place a vent in the weld line area to remove the entrapped air. when two or more melt ïŹow fronts contact each other immediately after the cooling process. 1. Many studies were conducted to explain the weakness at the. This study examines the impact of injection parameters on the weld line strength of the polyamide 6 and 30% fiberglass (PA6 + 30% FG) composite samples. Weld Line defect analysis, the most common of many defects in injection moulding products. The three parameters affecting operation were called injection pressure, injection time, and packing pressure which influence the quality of weld line. Short shots. Weld lines have been one of the common defects puzzling the injection molding industry. Weld line injection molding troubleshooting. A weld line is often unavoidable. The weld lineâs strength is reduced when. Injected plates were generated using a double-gated mold under four different process conditions. A weld line can appear during the absence of enough pressure to push the plastic flow and then meld together. Vacuum voids. This study focus on the formation of. * Corresponding author, e-mail: fengsjpc@ku. This paper presents the. 298 µm and the weld line on the heated region was almost eliminated with the aid of induction heating. Weld lines may occur depending on the product shape and gate positions. Effects of cavity surface temperature on mechanical properties of specimens with and without a weld line in rapid heat cycle. A visible weld line is observed, due to a combina-Cracking is commonly seen at the weld line area but sometimes on the entire surface. 2. Weld lines can appear on the surface of a molded part where the molten material has converged after splitting off in a mold. 1. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. Clamping unit berfungsi untuk memegang dan mengatur gerakan mold unit, serta gerakan ejector saat melepas benda dari molding unit, pada clamping unit lah kita bisa mengatur berapa panjang gerakan molding saat dibuka dan seberapa panjang ejector harus bergerak. Sink marks. (3) Adjust the temperature of the barrel and nozzle: the viscosity. In this study, polypropylene (PP) was chosen as the processing material and a micro dog-bone tensile test sample was selected as the objective part. , a post that forms a hole) and then rejoins, or when polymer melt fronts meet, from multiple injection points. â Another. Key words: weld line strength, thermoplastic composites, injection molding, wall thickness, skin and core layers Department of Materials Engineering, Faculty of Engineering, Kasetsart University, Bangkok 10900, Thailand. Weld-line, a common occurrence in plastic injection moulding, is a representative defect compromising mechanical qualities and product appearance [16]. Weld lines in design for manufacture. Flash. Runner Type Venting Plastic melt and mold temperature Injection speed and pressure Learn how to troubleshoot weld lines in injection molding. Weld Lines. Steven. Weld Lines 5. âą Reduce runner dimensions and maintain the same flow rate. of micro injection molding process simulation by technology update and progress. Weld Line Factors for Thermoplastics. Injection pressures too high Reduce pressure . Redesign the mold to. Sink Marks 2. When the same pressure is maintained or lower than that, the drop in speed shall create the weld lines. The effects of filling time, packing time, packing pressure, melt temperature, and mold temperature on the ultimate tensile strength (UTS) and the elongation value of the weld. In addition a weld line can be moved to an area where it is less visible. Fig. Plastic Injection Molding Process. The impact behavior of weld-lines in injection molding. Poor mold spot Spot mold to ensure fit and vent depths at ~0 . Kobayashi et al. Edge Gates. Figure 2. 453-455. When a product is produced through injection molding, a high-powered tool injects melted plastic into a custom, double-sided mold. In this work, the residual strength of a fiber-reinforced injection molded part containing a hot weld lineâor meld lineâwas evaluated. Weld or Knit lines are a plane or section in an injection molding part where the flow of molten plastics meets from different directions. 15 it can be found that for the opposite position of melt injection the weld lines forming under higher temperature would produce better plastics. Some of the common fixes for weld lines include: Increase the resin temperature to ensure both sides of the weld line are liquid when colliding. The resulting defects donât stop at flash only. Provide a gas vent. Sink Marks Cause 2: Low Pack and Hold time. According to the studies, the most important factor to reduce weld lines is increasing the mold temperature. Weld Lines. 010 0. Except for a few very simple injection molded parts, they occur on most injection molded parts (typically in the form of a line or V-groove), especially large complex items that require use of multi-gate molds and inserts. Optimizing the injection parameters through combination of adjusting the molding temperature, varying the gate pressure, and increasing the dwell time. also utilized the co-injection molding technique to study the flow of polypropylene at the weld line behind an obstacle in injection molding; the. Weld lines. The literature on the formation of weld lines, and their consequences, is more abundant on the area of injection molding [5-12] than on extrusion [13]. 5. Polyamide-6 (PA-66) was used as. 080 inches. There will be too many weld lines ; 6. When the same pressure is maintained or lower than that, the drop in speed shall create the weld lines.