) . <Materials>Injection moulding (U. Knit and meld lines are inherent to plastic. Molding Equipment. 13 shows the best gate location for which the weld lines . In this lesson we will focus on the meld and weld lines. Improper fusion of two fluid flows, a state before weld line. Controlling the mold’s temperature could also minimize the weld line’s effects. 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. 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. Stop using mold parting agents. 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. In this case the two flow fronts will meet and. View 08 DEFECTS & REMEDIES (61). 2) show that the weld line is formed as expected from the middle of the cavity towards the wall. Effects of mold temperature and pressure on shrinkage 0. Five injection molding process parameters, including mold temperature, melt temperature, packing pressure, injection. [ 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. investigated the impact of injection molding parameters on weld line formation and strength reduction. As a result, the portion has a faint, perhaps undetectable line. So today we are going to take a look at the five most common defects in injection molding and their solutions. Thus, a right gate design raises polymer temperature to prevent the formation of weld lines and flow marks. 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. Proceedings of ANTEC (2000), pp. Fracture Toughness of Weld Lines in Thermoplastic Injection Molding, Polymer Engineering and Science, 33(15): 996-1001. 4 at different time steps (legend scale is volume fraction of. Part design features may cause a material flow to split and rejoin as it flows around design features in a mold. 030 0. GlossFlow-induced fiber orientation and weld lines are com-mon features of the injection molding process. 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. A meld line occurs when two moving melt fronts converge and flow parallel to each other. Boost time too long Shorten boost time, lower boost pressure, or move transfer to an earlier position . There are several possible causes that need to be investigated to troubleshoot weld lines in injection molding. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. 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. Expand. Ideally, the resin flow will split into two flow fronts to pass around the obstruction and then recombine into a single flow front. ppt from SCIENCE 101 at Symbiosis International University. High-density PE was also moulded in the same conditions but in a lower temperature range (145 to 220°C). There will be too many weld lines ; 6. Here’s how to decrease the chances of weld lines: Increase the temperature of the mold or the molten plastic. Weld Line Factors for Thermoplastics. 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. In manufacturing, the Weld line or Knit line or Meld 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. When the same pressure is maintained or lower than that, the drop in speed shall create the weld lines. He now teaches courses in injection molding and plastic part design at the Polymers Center of Excellence and UNC-Charlotte in Charlotte, N. This study focus on the formation of. Short shots can occur in this case because the air in the mold cavity has nowhere to go while the plastic is entering. 2004; To study the weld line formation of colliding flow fronts the filling of a mold cavity was simulated. Download : Download high-res image (856KB) Download : Download full-size image; Fig. Too much injection molding pressure lost and increase the operation difficulty of the injection molding machine; 7. 005 0. 010 0. 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. Redesign the mold to. In this regard, the plasticizing ability of the plastic injection moulding machine should be checked. The spreading mold release agent on the part cavity surface might eventually cumulate at the V-notch groove of the weld line after each injection molding shot. The appearance quality of plastic parts in weld lines can be improved significantly through the rapid heating cycle molding (RHCM). 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. Zhao, X. Abstract. If you have critical requirements for strength of knit lines, please call a Protolabs applications engineer at 877-479-3680 to discuss, or email at [email protected]. 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. 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. Try to gradually raise the mold temperature and barrel temperature. In this work, the residual strength of a fiber-reinforced injection molded part containing a hot weld line—or meld line—was evaluated. Ni. 809-814. 2. Weld lines in design for manufacture. Founder, Injection Molding Solutions. Usually it’s a straight line, and often to be found radiating. Yes, it does help, for two reasons: Often the weld line traps air and the ‘dump’ acts as a vent to allow the air to escape. If the different flow fronts have cooled. 020 0. Flow Lines and Weld Lines. In injection molding, it is often important to maintain consistent color throughout the part. 6. Flow-in-duced fiber orientation manifests in higher tensile properties in the direction of flow compared to normal to the direction of flow [13–15]. Weld-line, a common occurrence in plastic injection moulding, is a representative defect compromising mechanical qualities and product appearance [16]. This paper presents the. You can expect a mold shrinkage percentage of. Controlling the mold’s temperature could also minimize the weld line’s effects. 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). Weld-line is a defect that arises in injection molding when two or more flow fronts collide. 5. Flow marks in injection molding can be avoided if adequate measures are taken. Increase the injection speed to reduce the amount of cooling happening in the mold. As a result this weld line will be a weak point in this part. In this experiment, several ejector. Therefore, the. enough to each other to avoid weld line issues. Weld Lines. 025 mm (0 . The effects of these two injection molding process parameters are explained in terms of their influence on the skin-core morphology observed on the fracture surfaces in the weld zone. It is a well-known fact that weldlines are unavoidable in most injection-molded products of even moderate complexity. 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. The thin weld line is the result of weak material bonding, which lowers the strength of the part. Weld line existence reduces the material strength; however, its demerit characteristics. You can locate the gate at the thick part of the injection molding to prevent air pockets. What is weld lines. Figure 2. This is expected to achieve maximum results while the pressure holding effect will also increase. 001 in . 8. 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. As the plastic flows through the mold it forms a smooth and continuous flow front. Injection molding weld lines where two or more flow fronts come together in the mold. The injection molding temperature and the mold temperature were fixed respectively at 230 °C and 50 °C. Fig. Root causes Many factors—including processing conditions, material, part design and mold design—can influence the visibility and strength of a weld line. A flat product case was considered as an example of a typical aircraft structural element. Meld and knit lines are actually both different types of weld lines. 3. Meld Lines. 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. 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. Silver Streak. 8. The weld line in injection molding is created when two melt streams connect, as shown in Figure 1. A weld line is often unavoidable. This paper presents the. In order to prepare the weld line specimen by micro injection molding, a mould integrated with visual system and variotherm system was designed and constructed, shown as Fig. and Mosle, H. Injection Molding Know How. These weld lines occur usually around holes or at the material flow joint area in the injeciton molded part, some welding lins is very small, specially if. Injection molding of large/complicated products are usually prepared by multiple compounds, which produces weldlines once the melt fronts are joined by impingement flow. When two or more strands of molten plastics meet in the mold, a weld line will be produced at the junction [1,2]. 943 µm to 0. The Influence of Knit-lines on the Tensile Properties of Injection Molded Parts, Polymer Engineering and Science, 23(10): 591-596. C. Cracking and/or Crazing can be defined as a fracture or surface breakage in the material of a molded part, usually found in weld line areas, but also on the surface in general. Yes, it does help, for two reasons: Often the weld line traps air and the ‘dump’ acts as a vent to allow the air to escape. 298 µm and the weld line on the heated region was almost eliminated with the aid of induction heating. Insufficient plasticizing ability of injection molding machine. For example, filled resins and ABS are more likely to. 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. You can choose material of low viscosity and can increase the injection pressure. In injection molding weldlines are generated when two separate melt streams join either in multigated molds or as a consequence of flow around. + Optimize holding pressure time. Carefully consider all the relevant aspects of the mold design specification before. Gate location should set to avoid weld lines, keep clean of the important position of the products,. It can be determined by a visible line appearing on the surface of the plastic part where two flows of the plastic meet. The results showed that the tensile strength of PE remains unchanged in the presence of weld lines. [1-2] investigated the weld line zone of injection molded glass-fiber-filled polypropylene (PP) composites. Weld lines, knit lines, and meld lines are terminologies to describe the imperfections of finished parts in plastic injection. Materials Science. An excess mold release is a major cause of weld line due to two variables: high pressure and speed. Weld line is formed when two different melt flows converge, as illustrated in Fig. Injection molds: (a) mold type A, (b) fixed plate of the mold type B, and (c) moving plate of mold type B The second mold (mo ld type B) was used for the ISO D2 specimen, as shown in Figure 2b,c . S. Weld lines are one of the typical quality issues of injection molded parts. 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. Mold vents should function the same way. Weld-line, a common occurrence in plastic injection moulding, is a representative defect compromising mechanical qualities and product appearance [16]. The part becomes partially solidified and won’t sufficiently bond where the two halves meet, resulting in. Optimizing the Injection Parameters. but the injection pressure will loss and injection molding pressure of this gate is bigger than that of direct gate. The most common and one of the most challenging injection molding defects are weld and knit lines. Weld lines are a common issue in injection molding, and occur when two flow fronts meet and fuse together as the material cools and solidifies. Weld lines have been one of the common defects puzzling the injection molding industry. 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. Tosello et al. Failure may result due to the assembly torque, or end-use application loads (temperature, creep). Nguyen-Chung. + Adjust injection speed. Increase the injection speed to reduce the amount of cooling happening in the mold. Avoid knit lines when designing a molded part to improve cosmetic appearance and functionality. Weld lines are where two flow. 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 create visible marks, but this type of injection molding defect can also reduce a part’s strength and durability. 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. Micro weld lines are the common problem that occurs for plastic molded parts through injection molding process. In Fig. Change the gate locations so that the weld lines occur in locations less susceptible to external forces if weld strength is an issue. 15 it can be found that for the opposite position of melt injection the weld lines forming under higher temperature would produce better plastics. Instead, a line that looks like a seam forms. Published 5 July 2019. wall thickness, gate and runner designs and re-validate the result. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. injection molding, (b) weld line and meld line, and (c) injection sample size and shape. 010 0. 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. poor flow fronts that are too cold, this can also lead to a deterioration in the mechanical properties. Weld line formation in the injection molding process occurs. Weld lines can compromise the strength and aesthetics of the part, so it’s important to carefully design the mold. [ 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. 1. Weld line formations can be weak if they fall in areas that will bear stress or impact. Weld lines may appear like seams or scratches, but that is not what they are. Weld lines appear as a result of the flow of molten plastic during the injection molding process. The weld lines that occur in injection mouldings are critical areas on which depends on the strength of the mouldings. In a weld line, reinforcement fibers. A weld line (weld point, flow line or a knit line) forms when separate melt fronts travelling in opposite directions meet. Weld-line is a weak area which reduces the strength of the part locally. 2) Vents in an injection mold should let the air out, but not the liquid plastic. This occurs either due to injection through multiple gates or as a conse-รอยประสานของเนื้อพลาสติก Weld line ตอนที่1. Weld line location. 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. They are unavoidable even for moderate complex products. 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]. Multiple gates are acceptable as long as they are close enough to each other to avoid weld line issues. 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. This principle has been implemented on a complex component in the case of the spring-loaded cogwheel shown in Figure 5, for instance. These lines can occur due to variations in wall thickness, gate location, or the geometry of the part. Shaofei Jiang. The edge gate is the most popular and straightforward injection mold gate design. T. Plastic Injection Molding Process. Google Scholar [10] S. 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. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part. 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. Increase the injection speed and mold/operational temperature to prevent fracture or surface breakdown. weld lines in injection molded thermoplastic composites. 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. You will have to confirm them before tooling. This gives a clue as to how it was formed. 18/06/2021. Adjust the design for the flow pattern to be a single source flow. Weld lines can appear on the surface of a molded part where the molten material has converged after splitting off in a mold. gas, which can cause blistering, as well as poor weld lines, warpage and the formation of cold slugs that affect filling and part mechanical properties. Solving one problem can introduce other problems to the injection molding process. Weld lines can be caused by many reasons, which covers the various aspects of mold, process, raw materials and product design, etc. Which means engineers need to know how to account for this phenomenon in an efficient way. Weld or knit lines occur when melt flow fronts collide in a mold cavity. If the melt temperature is low or the injection speed and pressure are low, you may end up with a more pronounced weld line. Weld lines appear as a result of the flow of molten plastic during the injection molding process. In weak material bonding, they will appear as hair-like weld lines. Figure 1a illustrates how the weld line is produced when two melt streams merge during the final filling process. This study presents an innovative approach to enhancing weld line strength in advanced polymer injection molding through applying gas-assisted mold. 004 m, a characteristic velocity ν 0 =0. are very minimum. Injection molding is an efficient manufacturing process for mass-producing parts with complicated geometry. The relative weld-line strength is defined as the ratio of the strength of specimens with a weld line. When molten plastic flows, it forms a smooth continuous flow until it meets an obstruction. This paper evaluates the shrinkage and strength of weld line using Design of Experiment and Response Surface Methodology in multi-objective optimisation utilising the injection moulding parameters. Optimizing the injection parameters through combination of adjusting the molding temperature, varying the gate pressure, and increasing the dwell time. Voids and bubbles, 6. Adding vents, using overflows, or using porous metal. The resulting defects don’t stop at flash only. Fig. (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. Injection molded plastic parts have comparatively less strength in the weld line section and they also have an impact on product aesthetics. (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. Polymers 2023, 15, 2440 3 of 21. Increase the injection speed. Weld-line is a weak area which reduces the strength of the part locally. e. The Injection Moulding Process is a manufacturing process used for producing parts or components by injecting molten material into the mold cavity. Knit lines, weld lines, mold lines, or flow lines in injection molding are faint lines that become visible on certain plastic parts. Injection molding plate for mode I testing geometry. 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. One solution is to prevent the weld lines from forming in regions that are. Weld lines may occur depending on the product shape and gate positions. Preventing sink marks of injection molded parts using CAE analysis. (3-1) Increasing the Resin Temperature at the Weld Line Section. Adjust Injection Molding Parameters: Optimizing injection speed, pressure, and temperature can help ensure better melt flow and reduce the likelihood of weld line formation. When this fails to happen correctly, a weld line defect occurs. Reduce the amount of release agents. This line does not normally impact the overall shape or dimensions of the item. The second technique, cascade injection moulding, can produce parts free from weld lines. Typical injection molding processing temperatures of TPU materials are as follows: IM Zone 60A to 75A 80A to 90A 95A to 75D Rear 315 – 40 ºF45 -370 75 400The weld line in an injection molding product can significantly impact the strength and durability of the last part [12] [13][14][15]. Consequently, it causes viscous dissipation which leads to the generation of heat. The parts are molded flat and are slightly stressed once installed in an arced position, but not excessively. Email him at jhenz. Use a less viscous plastic. If weld lines are unavoidable due to multiple gates in a moulding, they should be positioned in the non-critical region. How Are They Formed? Molten plastic resin is injected into a mold cavity through an opening known as a gate. In our DFM report, we will analyze the possible weld lines. 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. 015 0. Weld lines can affect both the aesthetics and mechanical properties of the product, making them a significant concern for. According to the studies, the most important factor to reduce weld lines is increasing the mold temperature. Venting at Weld Lines. The weld line and the flow pattern of co-injection molding can be seen from Fig. Insufficient injection pressure and speed can cause weld lines. Increase the injection speed. The schematic diagram of a weld line formation is illustrated in Figure 7, where is the welding angle of two melt fronts. Adjust the position of the gate and include a venting slot. In this lesson we will focus on the meld and weld lines. INJECTION MOULDING DEFECTS 1 DEFECTS 1. In the present work, the micro specimens were investigated. An obstruction such as a core pin or holes will split the material flow and cause a weld line issue to form on the other side. 2. + Improve vent inside the mold. Runner Type Venting Plastic melt and mold temperature Injection speed and pressure Learn how to troubleshoot weld lines in injection molding. Design. This paper presents the. When it comes to injection molding, the chances you might end up making a costly mistake is too high. Knit lines are hard to eliminate for injection molded plastic parts but. The flow of the material in the injection mould takes place through the gate. The next type of injection molding defect is ‘Weld Lines’. Weld lines are visually undesirable and a plastic part is structurally weak at weld line positions. Weld lines: Cold feedstock in the die: Increase injection speed, mold temperature, and feedstock temperature, enlarge gate opening, add venting channels or overflow wells near weld line locations, move gate location, redesign parts to avoid stream partition: Flow mark: Cold feedstock in the dieWeld lines are a common imperfection to be found in most plastic injected molded parts. Save. consideration in gate location. The weld lines in blow molding occur wherever the pari son knits together in the head, mold tacking areas and mold parting pinch-off region. Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties [3,4,5]. when two or more melt flow fronts contact each other immediately after the cooling process. , 2 (3–4) (1988. 2-1. Weld lines. In an injection molding process, a weld-line forms when two flow fronts meet each other. Flow front tracing simulation results for weld line developing process in micro injection molding with Comsol ® Multiphysics 3. 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. As the unfavorable defect in micro injection molding parts, weld line brings reduced mechanical and physical properties, especially for nano filled composites. Discoloration. Wolfgang Homes, head of the applications department of Eurotool, a manufacturer of hot runner systems, clarified the two methods for IMI, noting that. 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. Wu CY, Ku CC, Pai HY (2011) Injection molding optimization with weld line design constraint using distributed multi-population genetic algorithm. Mold Design. 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. Introduction. They are the result of fundamental physical effects taking place inside of the mold tool cavity. 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. Sink Marks 2. Ejector System (Pötsch, 1995) . 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. This is where the melt fronts divided by an insert, such as a pin or a block, in a mold cavity rejoin each other downstream of the insert. These lines. (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. A new kind of weld line was. One test mold was used to mold three different materials using heat/cool and conventional molding techniques. The thermo. Weld lines are one of the typical quality issues of injection molded parts. 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. 030 0. Vacuum voids. A weld line region is typically filled at the end of an injection stroke or during the pressure phase. The holding pressure was set as approximately equal to the maximum injection pressure obtained at the time of filling the resin. 015 0. 3. A viscosity curve is developed using calculations of the fill time and plastic pressure at. The plastic injection molding process, including thermoplastic injection, is nowadays used in several industrial processes, including in the automotive industry for bowden cables. Wang, G. Weld lines are weak regions in thermoplastic injection moldings, due to low molecular entanglement and unfavorable orientation. injection molding. 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. Weld Line defect analysis, the most common of many defects in injection moulding products. Or increase the number of gates. An excess mold release is a major cause of weld line due to two variables: high pressure and speed. Micro-injection molding (µIM) is a widespread process for the production of plastic parts with at least one dimension, or feature, in the microscale (conventionally below 500 µm). If the different flow fronts have cooled. 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. 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. 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. In order to reduce these weak points, a special ram technique for injection molding, which can be integrated into the mold, was investigated. 000 0. when two or more melt flow fronts contact each other immediately after the cooling process. 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. Injected plates were generated using a double-gated mold under four different process conditions. Sink MarksWeld lines, also known as knit lines, are a common defect in injection molding where two or more flow fronts of molten plastic meet but don’t properly fuse together. The hair-like weld line is the. Flashes, 3. 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. In the first, sequential injection moulding, the lines are displaced to noncritical areas of the moulding. In this work, the weldline strength of injection molded short-glass-fiber reinforced polycarbonate was derived by an analytical method of modified rule-of-mixtures as a function of the area fraction between skin and core layers. What are Weld Line Defects? Weld line defects – also known as knit lines – is a defect that occurs when the injected plastic meets within a mold. ac. ac. This month, in our FREE SimTech™ series of webinars, we will explore weld lines and how they develop. also utilized the co-injection molding technique to study the flow of polypropylene at the weld line behind an obstacle in injection molding; the. The primary cause of cracking is stress and deformation brought on by the external environment, cold mold surfaces, and low injection rates. Int. Weld lines are a common occurrence in injection molding that affects the overall quality and strength of a finished product.