You can expect a mold shrinkage percentage of. If weld lines are unavoidable due to multiple gates in a moulding, they should be positioned in the non-critical region. Polyamide-6 (PA-66) was used as. Int. (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. Amongst weld line. , a post that forms a hole) and then rejoins, or when polymer melt fronts meet, from multiple injection points. 2004; To study the weld line formation of colliding flow fronts the filling of a mold cavity was simulated. The. Melt temperature does effect the weld. Its properties make it possible to create thin-walled parts having close tolerances in fast cycles over long. Thus, it is important to understand (i). Introduction. The edge gate is the most popular and straightforward injection mold gate design. Fig 13:- Detailed view of weld lines. One test mold was used to mold three different materials using heat/cool and conventional molding techniques. Figure 1 below shows the development of a weld line. Causes: Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic. Weld lines can significantly reduce ultimate tensile strength (UTS) and fracture strain of talc filled polypropylene (PP). There are various types of injection molding defects that can cause such interruptions in the manufacturing cycle. Injection molded plastic parts have comparatively less strength in the weld line section and they also have an impact on product aesthetics. According to the characteristics. Here are a few of the most common defects that may occur in plastic injection molding — and how product teams can avoid them. Flash. The weld line can either be a minor cosmetic issue or potentially bad enough to negatively impact the part's strength. 5. distribution [4]. Let’s review what causes meld and knit line imperfections, how they’re different, and how they affect part durability. Weld Line Phenomenon: Surface defect in which the weld, or the part where the flows of melted plastic meet inside the mold, shows up on the surface as a groove or pattern. Weld lines are the most common defects in injection molded products. You will have to confirm them before tooling. Raise the injection speed. 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. In this case the two flow fronts will meet and there will be very little molecular orientation across the weld. Three terms that often get mixed up are weld, meld, and knit lines. The three parameters affecting operation were called injection pressure, injection time, and packing pressure which influence the quality of weld line. 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. Weld lines can appear on the surface of a molded part where the molten material has converged after splitting off into two or more directions in a mold. (1)Injection Machine Poor plasticization and non-uniform melt temperature – appropriately extend the injection molding cycle, to ensure thorough plasticization, and change for a machine with a larger. In this paper, a weld line factor (W-L factor) was adopted to. During injection. A weld line is often unavoidable. This model provides the necessary information. Weld Lines 5. In an injection molding process, a weld-line forms when two flow fronts meet each other. Strength at weld the line location and warpage problem in injection molding plastic component were not new problems in injection molding industries. Fellahi et al. Effects of cavity surface temperature on mechanical properties of specimens with and without a weld line in rapid heat cycle. Wang. Weld lines are where two flow. 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. Part design features may cause a material flow to split and rejoin as it flows around design features in a mold. 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. CrossRef View in Scopus Google Scholar [17] G. However, research about the computer determination of weld lines in injection molding remains scarce. The differences between knit and meld lines are significant, and they can dramatically impact the part’s structural integrity. Threadline gap in-between part due to improper gate location in complex design parts including excess of holes (multipoint. See moreEquipment. These are caused by the shot of molten plastic moving at different speeds throughout the injection. It is important to investigate the weld line because the weld line area can induce potential failure of structural application. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part. Weld line (also known as a knit line) is the line where two flow fronts meet when there is the inability of two or more flow fronts to “knit” together, or “weld,” during the molding process. A simulation of mold filling with multi-inlets or an insert in the cavity is pursued with an improved Level Set Method. (3-1) Increasing the Resin Temperature at the Weld Line Section. , 2 (3–4) (1988. Weld lines may appear like seams or scratches, but that is not what they are. Weld lines, generated during injection molding, can significantly reduce the mechanical performance of structures. Weld lines create visible marks, but this type of injection molding defect can also reduce a part’s strength and durability. Recently, the injection molding of fiber reinforced thermoplastics (FRTP) containing short fibers has been widely carried out to mass-produce molded products with complex shapes. Injection molding weld lines where two or more flow fronts come together in the mold. It will degrade materials performance; 9. 21: Weld line failure caused by hoop stresses. Blistering 4. Weld lines are a kind of Achilles heel of plastics. This is particularly true if the wall thickness in the weld is thin – i. It has larger cross-sectional regions that allow sufficient molten plastic flow into the cavity. Flow Lines and Weld Lines. You can choose material of low viscosity and can increase the injection pressure. . Introduction. Gate location should set to avoid weld lines, keep clean of the important position of the products,. Insufficient plasticizing ability of injection molding machine. Lesson 7: Meld and Weld Lines (Part Design) When plastic flows into an injection mold, the internal flow pattern depends on the part design, mold design, and processing factors. 453-455. These lines occur when two or more flow fronts meet and fuse together, resulting in a visible seam on the molded part. 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. One of the defects observed in injection molding parts is weld line strength, which forms when two or more separate melt fronts traveling from different directions meet and join as the mold cavity is filled. Short shots can occur in this case because the air in the mold cavity has nowhere to go while the plastic is entering. 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. 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. 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. Introduction Weld lines are formed during mold filling whenever two separated melt streams recombine. 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. 2. Kobayashi et al. Recently hired at plastic injection molding simulation software provider Moldex3D, Nicholson was spurred to undertake the research study following a conversation with a customer experiencing weld-line woes. (3) Adjust the temperature of the barrel and nozzle: the viscosity. Weld line. 2021年4月23日 / by admin. Weld lines are a reality in the injection moulding of complex parts. Weld-line, a common occurrence in plastic injection moulding, is a representative defect compromising mechanical qualities and product appearance [16]. The impact behavior of weld-lines in injection molding. In the 1947 book, Injection Molding of Plastics, the late Dr. This study focus on the formation of. In a weld line, reinforcement fibers adopt an orthogonal orientation with respect to the filling direction, which results in a significant reduction in the strength and. In injection molding, a V-notch-shaped weld line is generated in the melt front meeting area. A simulation process was conducted to determine the recommended setting of injection moulding parameters and the range of the variable. That helps. Sink marks, 2. injection molding. They are the result of fundamental physical effects taking place inside of the mold tool cavity. when two or more melt flow fronts contact each other immediately after the cooling process. Figure 1a illustrates how the weld line is produced when. 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. The molded plastic part may also appear to be rust-colored. Recently hired at plastic injection molding simulation software provider Moldex3D, Nicholson was spurred to undertake the research study following a conversation with a customer experiencing weld-line woes. The calculation was carried out on a mesh with solid elements obtained in ANSYS Meshing and transferred to Moldex3D by means Rhinoceros. Here’s how to decrease the chances of weld lines: Increase the temperature of the mold or the molten plastic. Where the weld lines are, where the sink marks or voids will be, and how the part warps—all of these are determined by where the molten plastic is injected into the part cavity. EasyFill Advanced provides the weld-line temperature distribution and meeting angle prediction. This occurs either due to injection through multiple gates or as a conse-รอยประสานของเนื้อพลาสติก Weld line ตอนที่1. Fisa et al. The injection molding simulation software used offers a method for the detection of weld lines that has shown to be only reliable for the detection of stagnating weld lines. Weld lines in injection molding are frequently unavoidable. Another way to eliminate weld lines in plastic injection molding is to adjust the molding conditions. Weld lines cause significant reject rates and are a common problem that all molders face. A viscosity curve is developed using calculations of the fill time and plastic pressure at. The weld line’s strength is reduced when. Expand. Clamping Unit. avoid weld lines, and facilitate smooth. In conclusion, reducing weld lines in injection molding requires a combination of factors, including adjusting the melt temperature, injection speed, pressure, and gate size, as well as using a hot runner system, modifying the part design, using a different material, reducing the wall thickness, using a mold release agent, and working. Weld lines appear on the surface of a molded part where the molten material converges after splitting off into two or more directions in a mold. (1) Flash: Extra plastic material in the shape of films or burrs, which appears on the parting surface, around the runner or in the insert crevice. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and. This line does not normally impact the overall shape or dimensions of the item. In this work, the residual strength of a fiber-reinforced injection molded part containing a hot weld line—or meld line—was evaluated. The foundation to any robust process, in my opinion, is based on choosing the correct fill time. An excess mold release is a major cause of weld line due to two variables: high pressure and speed. Burn Marks. The melt stream is separated at the part’s cutout and reunited at the other end of the cutout. Known methods to reduce weld-line effects are the optimization of injection molding parameters, enhanced tool temperature, and using special molding tool devices. The most common and one of the most challenging injection molding defects are weld and knit lines. Adjust Injection Molding Parameters: Optimizing injection speed, pressure, and temperature can help ensure better melt flow and reduce the likelihood of weld line formation. 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. Weld lines are the unfavorable defect not only in normal injection molding process but in micro injection molding process. , less than 0. Polym. For the production of these weld‐line specimens, a special mold was used and injection molding parameters were systematically varied, tensile (ISO 527‐2) and impact (ISO 179‐1/1eU. Which means engineers need to know how to account for this phenomenon in an efficient way. Weld lines are the result of fundamental physical effects taking place inside of the mold tool cavity, they are common imperfection to be found in most plastic injection molded parts. The thin weld line is the result of weak material bonding, which lowers the strength of the part. 025 mm (0 . 1. Thus, a right gate design raises polymer temperature to prevent the formation of weld lines and flow marks. Wu CY, Ku CC, Pai HY (2011) Injection molding optimization with weld line design constraint using distributed multi-population genetic algorithm. Possible Sources of Weld/Knit Lines: Temperature from the injection molding machine, runners, and/or the mold itself in order to ensure the plastic resin flows properly;Weld line is one of the plastic injection molding defects. Tosello et al. 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 (0–15%) in the compounds. Weld line formation in the injection molding process occurs. Injection molded plastic parts have comparatively less strength in the weld line section and they also have an impact on product aesthetics. 2) Vents in an injection mold should let the air out, but not the liquid plastic. Injected plates were generated using a double-gated mold under four different process conditions. Shaofei Jiang. The more gate points there are, the more weld lines will occur. Weld lines are formed when two or more flow fronts join, as the liquid polymer fills the. Weld line, 5. A weld or meld line on plastic parts can cause structural problems and/or be visibly unacceptable. The impact behavior of weld-lines in injection molding. Flow lines. 2. Weld lines can be caused by many reasons, which covers the various aspects of mold, process, raw materials and product design, etc. 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. It is a well-known fact that weldlines are unavoidable in most injection-molded products of even moderate complexity. สาเหตุหลักชิ้นงานพลาสติกเกิดรอยเชื่อมประสาน (weld line) คือ การประสานหรือการรวมตัวกันของพลาสติกเหลวไม่ดีเท่าที่ควรThis insertion ensures the separation of the melt flow. When molten plastic flows, it forms a smooth continuous flow until it meets an obstruction. A new kind of weld line was. The Solutions for Poor Welding Lines. Sink Marks Cause 2: Low Pack and Hold time. e. Sink Marks Cause 5: Improper Part Geometry. Mold Design Most conventional mold designs will work with TPU’s. This can occur when a polymer flow splits to go around an interruption (e. In this paper, two different injection molding tests were completed. In the plastic injection molding industry, we work with materials that are byproducts of oil refinement and molds made of metal casting, so our terminology is bound to be at least slightly confusing. This study describes how the weld line was formed on the part. Additionally, you can get our free Design Cube, which will provide a physical aid to help understand how knit lines form as well as other injection molding design considerations. Remedies: Raise the temperature of the mold or molten plastic. injection molding, (b) weld line and meld line, and (c) injection sample size and shape. + Move gate (to where it is not visible). Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties. Maintain a holding pressure that is no more than 1/2 the. Meld Lines. Fracture Toughness of Weld Lines in Thermoplastic Injection Molding, Polymer Engineering and Science, 33(15): 996-1001. Weld lines can be caused by material flowing around holes or inserts in the part, multiple injection gates or variable wall thickness where hesitation or "race tracking" can occur. The holding pressure was set as approximately equal to the maximum injection pressure obtained at the time of filling the resin. 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. Flow Lines. 1 m/s, and a zero-shear-rate viscosity η 0 (T 0)=3760 Pas. also utilized the co-injection molding technique to study the flow of polypropylene at the weld line behind an obstacle in injection molding; the. In a weld line, reinforcement fibers. This principle has been implemented on a complex component in the case of the spring-loaded cogwheel shown in Figure 5, for instance. In other words, weld lines reduce the mechanical properties of the product. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part. In this paper, a kind of polypropylene (PP) foam plastic parts with special weld lines were obtained by core-back foam injection molding with chemical blowing agent. 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. + Improve vent inside the mold. 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. Still, it can be seen to varied degrees depending on mold. Weld line existence reduces the material strength; however, its demerit characteristics. Features such as sharp corners or undercuts can. 48 applied high-frequency induction heating to eliminate weld lines in an injection-molded plastic part, and experimental observation indicated that weld depth was reduced from 2. A knit line, also called a weld line, occurs when the liquid plastic moves through the mold and hits more melted plastic and they don’t flow or fuse together. In spite of its cosmetic attributes, a bad knit line can significantly. The Difference Between Knit and Meld Lines–and Why It Matters. In this work, the residual strength of a fiber-reinforced injection molded part containing a hot weld line—or meld line—was evaluated. 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. Sink Marks Cause 3: Improper Pack or Hold Pressure. 2. The resulting defects don’t stop at flash only. This paper presents the. [ 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. These lines. Flow LinesThe most simple design modification for eliminating the weakening effect of a weld line is to increase the wall thickness in the weld line zone (Figure 4). Here we are looking at most common part defect that can come from injection molding called weld line. The appearance quality of plastic parts in weld lines can be improved significantly through the rapid heating cycle molding (RHCM). Founder, Injection Molding Solutions. Published 5 July 2019. Figure 1 a illustrates how the weld line is produced when two melt streams merge during the final filling process. These lines can occur due to variations in wall thickness, gate location, or the geometry of the part. e. The parts are molded flat and are slightly stressed once installed in an arced position, but not excessively. A quantitative evaluation of the appearance quality of weld lines in RHCM parts. Adjust the position of the gate and include a venting slot. 5. Instead of using flow leaders and deflectors to control the locations of the weld lines, this paper presents an approach to specify the locations of the weld. Adjust Injection Molding Parameters: Optimizing injection speed, pressure, and temperature can help ensure better melt flow and reduce the likelihood of weld line formation. (1) Increase the injection pressure and prolong the injection time. Jiquan Li, Taidong Li, +1 author. 3. Weld lines are also known as knit lines. One solution is to prevent the weld lines from forming in regions that are. John Bozzelli. It is normally at the meeting points of two flow fonts. Flash. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. The structure is more complex with longer molding cycle . weld lines in infill analysis of injection molding simulation was observed [5]. 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. Insufficient injection pressure and speed can cause weld lines. The quality of the weld line can be improved through optimized processing conditions, increased mold and melt temperatures, and increased injection speed to reduce the viscosity of the polymer. Meanwhile, Using more advanced cooling method such as cyclic cooling or conformal cooling. Plastic Injection Molding Process. Remedies: Reduce packing and fill rates by adjusting until the part is properly filled with minimum stresses. So today we are going to take a look at the five most common defects in injection molding and their solutions. In places where the plastic flows around a solid piece of the mold, from a simple circle to forming complex shapes like rings or squares. The position, length, and angle of weld lines are dependent on the. In injection molding weld lines are formed whenever two flow fronts are joined. Proceedings of ANTEC (2000) Google Scholar [9] S. * Corresponding author, e-mail: [email protected] recently had a few pieces (2-3%) crack after assembly, while in storage (not installed, still in the package). In an injection molding process, a weld-line forms when two flow fronts meet each other. If the plasticizing ability of the plastics injection molding machine is insufficient, the plastic cannot be fully plasticized, resulting in weld lines when filling the mold. Weld lines are significant defects that affect injection molding quality, causing the poor appearance and low mechanical strength of injection‐molded parts. Reduce the amount of release agents. The weld lines that occur in injection mouldings are critical areas on which depends on the strength of the mouldings. Many researchers had emerged with several. When injection molding large products, molding is usually performed with multi-point gates, but weld lines can occur depending on the product shape and gate position. 020 0. Unless you are manufacturing an extremely simple part with one gate and a unidirectional flow front, your part will have weld lines. For more information on the Factors Affecting the Injection Molding Process. Weld lines may occur on a molded part’s surface where, after breaking off into two or more directions in a mold, the molten material has converged. Depending on the resin, resin temperature, mold temperature, and filling speed, knit lines can vary. 6. With unchangeable and fixed requirements for the final product, their avoidance often proves difficult. wall thickness, gate and runner designs and re-validate the result. The melt stream is separated at the part’s cutout and reunited at the other end of the cutout. This month, in our FREE SimTech™ series of webinars, we will explore weld lines and how they develop. examined the relationship between the structural and mechanical qualities in HDPE/PA6 mixture which is molded with weld line and without weld line in plastic injections [6]. consideration in gate location. – Use generous and even. That’s the air escaping out of the nozzle until the hose is full of pressurized water. (2) Causes & Countermeasures (See the table below) *Note: Flash also tends to occur when the injection pressure/speed are too high, but molding time is too long and mold temperature is too low. 1. A. 001 in . They are the result of fundamental physical effects taking place inside of the mold tool cavity. Effects of mold temperature and pressure on shrinkage 0. when two or more melt flow fronts contact each other immediately after the cooling process. 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. The relative weld-line strength is defined as the ratio of the strength of specimens with a weld line. Meld and knit lines are actually both different types of weld lines. In this regard, the plasticizing ability of the plastic injection moulding machine should be checked. They appear on the surface of injection molded parts like lines. 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 is a weak area. Abstract. the meeting angle by optimizing variation over time in the cavity. [ 18 - 21 ] In addition, the effects. Move injection locations to make weld lines form closer to the gates. Abstract. 2(a, b). The most common and one of the most challenging injection molding defects are weld and knit lines. When two or more strands of molten plastics meet in the mold, a weld line will be produced at the junction [1,2]. Alter the mold design. C. Computer simulation packages of injection molding have had success in predicting filling behavior in extremely complicated geometries, capable of predicting weld lines’ location exactly. Weld line formation in the injection molding process occurs. 015 0. Weld lines appear as a result of the flow of molten plastic during the injection molding process. The terahertz time-domain spectroscopy (THz-TDS) imaging technique was used to detect the weld line defect of an injection-moulded short glass fiber-reinforced composite. The weld line degrades the. An excess mold release is a major cause of weld line due to two variables: high pressure and speed. Injection molding is an efficient manufacturing process for mass-producing parts with complicated geometry. Adding vents, using overflows, or using porous metal. Weld lines occur when two flow fronts meet and fuse together in the injection molding process. + Optimize holding pressure time. The plaques are prepared out of 30% glass-filled Polyamide 6 material. Use a less viscous plastic. 1. Preventing sink marks of injection molded parts using CAE analysis. In order to increase the quality of the products manufactured by injection molding, RTC technology can be used to achieve higher mold temperatures. Except for a few injection molded parts with very simple geometry, weld lines occur in most injection molded parts (usually with one line or V-shaped groove), especially in large-scale complex products requiring multi gate mold and inserts. Sink Marks Cause 4: High Mold Temperature. In this lesson we will focus on the meld and weld lines. If the plastic encounters a pin or other obstruction, the plastic flow front must split to form two flow fronts to go around the pin. Weld lines can be a weak spot and have therefore to be taken into consideration in part design. Weld lines in injection molding are a type of cosmetic defect that detracts from a plastic part’s appearance. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. with a characteristic length χ 0 =0. Typical injection molding processing temperatures of TPU materials are as follows: IM ZONE 60A TO 75A 80A TO 90A 95A TO 75D REAR 315 – 340 °F 345 – 370 °F 375 – 400 °FTo study the weld line formation of colliding flow fronts the filling of a mold cavity was simulated. ” Another. Adjust the design for the flow pattern to be a single source flow. 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. When the same pressure is maintained or lower than that, the drop in speed shall create the weld lines. If the injection gate is not properly designed, it can cause the formation of defects like sink-mark, warpage, short shot, weld-line, air traps, and other faults [15]. • Reduce runner dimensions and maintain the same flow rate. 005 0. These lines can occur at the gate location, where the plastic enters the mold, or anywhere along the flow path. Don’t forget to eliminate entrapped air in the weld line injection molding; otherwise, it’ll create unfilled portions in your part design, causing more weakening. Bubbles are caused by melting flow fronts colliding in a mold cavity. Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties. 9 – Over Packing. Strength measurements should be made to. Meddad and Fisa [3] found that the weld line is a zone between 2 and 8 mm wide extending throughout the thickness in which the fibers are oriented almost. Low mold temperatures are usually employed to decrease cycle molding time and final process costs; however, resulting surface defects include weld lines, sink marks,. In this experiment, several ejector. For the production of these weld‐line specimens, a special mold was used and injection molding parameters were systematically varied, tensile (ISO 527‐2) and impact (ISO 179‐1/1eU. These potential causes include:Weld lines Weld lines are weak areas formed by converging flows of plastic that can lead to mechanical part failure if not properly accounted for in the part design. There are several factors that can contribute to the formation of weld lines, including: Part design: The design of the part can impact the flow of the material and the likelihood of weld lines forming. Weld lines create visible marks, but this type of injection molding defect can also reduce a part’s strength and durability. 1. 6. For example, filled resins and ABS are more likely to.