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. 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]. Increase the injection speed. The weld line in injection molding is created when two melt streams connect, as shown in Figure 1. Weld line. Weld lines are visually undesirable and a plastic part is structurally weak at weld line positions. Edge Gates. Known also as knit lines and weld marks, weld lines are difficult to eliminate completely, especially if your injection molded part requires holes. Sink Marks Cause 2: Low Pack and Hold time. This occurs either due to injection through multiple gates or as a conse-รอยประสานของเนื้อพลาสติก Weld line ตอนที่1. When this fails to happen correctly, a weld line defect occurs. 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. It will degrade materials performance; 9. 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 lines represent the region where two separated melt fronts recombine. 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. You can expect a mold shrinkage percentage of. Weld line strength is important in blow molding as well as injection molding. To explore the formation of weld lines in injection molding, a. 9 – Over Packing. 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. In the feeding system of injection molding, the flow of plastic melt begins from the nozzle of the injection molding machine, traverses through the sprue and runners, and ultimately reaches the mold gate(you can call it the injection molding gate as well)—the entry point to the mold cavity. In other words, weld lines reduce the mechanical properties of the product. Known methods to reduce weld-line effects are the optimization of injection molding parameters, enhanced tool temperature, and using special molding tool devices. The melt stream is separated at the part’s cutout and reunited at the other end of the cutout. Raising the mold temperature will keep the material molten longer, resulting in stronger weld lines and even elimination of weld lines. 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. GlossFlow-induced fiber orientation and weld lines are com-mon features of the injection molding process. 2. Google Scholar | Crossref | ISI. [ 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. Computer simulation packages of injection molding have had success in predicting filling behavior in extremely complicated geometries, capable of predicting weld lines’ location exactly. 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. Kobayashi et al. Flow analysis of the weld line formation during injection mold filling of thermoplastics. CrossRef View in Scopus Google Scholar [17] G. The injection molding temperature and the mold temperature were fixed respectively at 230 °C and 50 °C. Figure 1a illustrates how the weld line is produced when two melt streams merge during the final filling process. Injection molding is an efficient manufacturing process for mass-producing parts with complicated geometry. (3-1) Increasing the Resin Temperature at the Weld Line Section. Unless you are manufacturing an extremely simple part with one gate and a unidirectional flow front, your part will have weld lines. 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. The hair-like weld line is the. 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. 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. 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. Quality concerns in injection-molded materials can vary fr. In an injection molding process, a weld-line forms when two flow fronts meet each other. 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. This occurs mainly in the presence of barriers that divide the melt flow in. Sink Marks Cause 3: Improper Pack or Hold Pressure. Mold Design. Threadline gap in-between part due to improper gate location in complex design parts including excess of holes (multipoint. Insufficient injection pressure and speed can cause weld lines. 025 mm (0 . th INTRODUCTION Injection molding is one of the mostWeld Lines in a plastic cover. 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. See moreEquipment. If so, look for an alternative resin. Weak weld lines are perhaps the most common and difficult injection molding defect to eliminate. 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. The holding pressure was set as approximately equal to the maximum injection pressure obtained at the time of filling the resin. + Adjust injection speed. (1998) for clear photographs. He now teaches courses in injection molding and plastic part design at the Polymers Center of Excellence and UNC-Charlotte in Charlotte, N. 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 partsí geometry. 10. Weld or Knit lines are a plane or section in an injection molding part where the flow of molten plastics meets from different directions. In this lesson we will focus on the meld and weld lines. Part design features may cause a material flow to split and rejoin as it flows around design features in a mold. 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 of weld-line, etc. In the present work, the micro specimens were investigated. The second technique, cascade injection moulding, can produce parts free from weld lines. Azuddin / Procedia Engineering 184 ( 2017 ) 663 – 672 665 2. Use a less viscous plastic. 5. injection molding. It is possible to pre. Weld line formations can be weak if they fall in areas that will bear stress or impact. A viscosity curve is developed using calculations of the fill time and plastic pressure at. Weld Lines. ac. Weld Line defect analysis, the most common of many defects in injection moulding products. When two or more strands of molten plastics meet in the mold, a weld line will be produced at the junction [1,2]. Weld lines are one of the typical quality issues of injection molded parts. distribution [4]. (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. This injection molding defect appears as a black discoloration on the surface of the molded part. Weld lines form where two or more flow fronts come together and can create a cosmetic or structural problem. The rule is, a higher pressure shall be required to push the material in the machine when there is excess mold. Voids and bubbles, 6. 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. 2. Weld line formation in the injection molding process occurs. 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. 298 µm and the weld line on the heated region was almost eliminated with the aid of induction heating. 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 of weld-line, etc. 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. 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. INJECTION MOULDING DEFECTS 1 DEFECTS 1. This customary mark is caused by differential rates of cooling in the resin where. 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. 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. They are unavoidable even for moderate complex products. In injection molding, a V-notch-shaped weld line is generated in the melt front meeting area. Thus, a right gate design raises polymer temperature to prevent the formation of weld lines and flow marks. <Materials>Injection moulding (U. 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. Wang et al. This principle has been implemented on a complex component in the case of the spring-loaded cogwheel shown in Figure 5, for instance. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. 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. Thereby creating localized pressure within the mold. 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. So today we are going to take a look at the five most common defects in injection molding and their solutions. Weld lines can be a weak spot and have therefore to be taken into consideration in part design. 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. 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-3 Ways for Detecting and Correcting. What is weld lines. 2. Weld lines have been one of the common defects puzzling the injection molding industry. A weld-line is known as a representative defect in the field of plastic injection moulding, and also as a cause of inferior mechanical propensities and poor appearance. Knit lines, weld lines, mold lines, or flow lines in injection molding are faint lines that become visible on certain plastic parts. Weld lines not. Cracks in Corners – When cracks appear in the corners of plastic parts, the cause is too much plastic shrinkage onto the mold core in that corner. As the injection process continues, the two separated melt fronts meet again along the longitudinal section of the plate forming a weld line. The plaques are prepared out of 30% glass-filled Polyamide 6 material. This paper presents the. Strength at weld the line location and warpage problem in injection molding plastic component were not new problems in injection molding industries. 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. Weld-line is a weak area which reduces the strength of the part locally. These lines can occur due to variations in wall thickness, gate location, or the geometry of the part. Weld line injection molding troubleshooting. Meld lines are formed when two separate flow fronts meets. Here are some strategies. There are several possible causes that need to be investigated to troubleshoot weld lines in injection molding. Flow lines. One test mold was used to mold three different materials using heat/cool and conventional molding techniques. Many researchers had emerged with several. The Factors that Cause Weld Line in Injection molding Pressure. Burn Marks. M. A visual mold with variotherm system for weld line study in micro injection molding. 000 0. Conclusion The formation of weld lines in micro injection molding of plastic depends on different parameters such as melt temperature, injection pressure and fabrication method. In this regard, the plasticizing ability of the plastic injection moulding machine should be checked. Reduce the amount of release agents. 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. 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. In this experiment, several ejector. The impact behavior of weld-lines in injection molding. 5. In this work, the residual strength of a fiber-reinforced injection molded part containing a hot weld line—or meld line—was evaluated. In the 1947 book, Injection Molding of Plastics, the late Dr. Lesson 6: Meld and Weld Lines (Processing) When plastic flows into an injection mold, the internal flow pattern depends on the part design, mold design, and processing factors. 010 0. Weld lines are weak regions in thermoplastic injection moldings, due to low molecular entanglement and unfavorable orientation. The following are the conclusion drawn from the present research: 1. As a result, the portion has a faint, perhaps undetectable line. He said, “Vent cavities at weld lines or junction points where two or more material streams meet and fuse into a solid mass. Known methods to reduce weld-line effects are the optimization of injection molding parameters, enhanced tool temperature, and using special molding tool devices. 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. Flow lines are lines, waves, or ring patterns that appear on the surface of a part, noted by their slightly different color from the rest of the piece. 22 shows how the weld lines are formed when the melt splits around core pins during the injection molding process. 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). 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. Instead, a line that looks like a seam forms. During injection. The weld line in injection molding is created when two melt streams connect, as shown in Figure 1. 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. Meld Lines. are very minimum. In injection molding of thermosoftening plastics, an important aspect having a bearing on the quality of articles is the appearance of weld lines that are formed when there are inserts, barriers, molding signs, reinforcements, etc. Microsyst Technol, 14 (6) (2008), pp. Redesign the mold to prevent the weld line. The weld-lines are undesirable from both aesthetic and functional points of view and their effect is different for. This paper presents the. During injection moulding, a weld-line occurs when two or more flow fronts collide in the hollow cavity. 030 0. Too much injection molding pressure lost and increase the operation difficulty of the injection molding machine; 7. For multiple gate and complex part, molding weld-lines are unavoidable, therefore mechanical behavior of the weld-line needs to be predicted. Discoloration. Investigations on WLs in micro injection molded parts were performed in Refs. Valve. 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. 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. This article will review how and why weld lines occur, how mold gates effects weld line location and strength, the importance of material selection and Injection molding weld lines refer to a line on an injection molded part that is generated where the two halves of the mold come together. enough to each other to avoid weld line issues. 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 together? Avoid knit lines when designing a molded part to improve cosmetic appearance and functionality. when two or more melt flow fronts contact each other immediately after the cooling process. Increase the melting temperature. The weld line can either be a minor cosmetic issue or potentially bad enough to negatively impact the part's strength. 2) show that the weld line is formed as expected from the middle of the cavity towards the wall. The special injection mold is designed and fabricated to produce plaques having stagnation weld-line. The hair-like weld line is the result of weak. This process makes possible to produce a wide range of products with different sizes, shapes, and for varying applications. 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. A weld line refers to a line, notch or color change that is created on the molded part caused by the convergence of two separate flows of molten plastic. 5. In injection molding, it is often important to maintain consistent color throughout the part. Weld lines have been one of the common defects puzzling the injection molding industry. 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. In injection molding weld lines are formed whenever two flow fronts are joined. (2011) and Fig. Here we are looking at most common part defect that can come from injection molding called weld line. Jiquan Li, Taidong Li, +1 author. Consequently, it causes viscous dissipation which leads to the generation of heat. 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. To elimi- nate weld lines, you can enlarge shrinkage. In this case the two flow fronts will meet and. Despite injection molding being recognized as a robust process for obtaining parts with high geometry accuracy, one last occurrence remains a challenge in micro. A meld line occurs when two moving melt fronts converge and flow parallel to each other. The results showed that the tensile strength of PE remains unchanged in the presence of weld lines. Introduction. Here are some ways to avoid. “I realized weld -line problems are a very common issue for injection molded parts,” Nicholson told Plastics Technology. A weld line is a mark left on the surface of a plastic injected molded part. Injected plates were generated using a double-gated mold under four different process conditions. Weld-line is a weak area which reduces the strength of the part locally. As compared to a weld line, the flow fronts come together at less than 135 degrees. Article Google Scholar Deng YM, Zheng D, Lu XJ (2008) Injection moulding optimization of multi-class design variables using a PSO algorithm. To respond to poor plasticization and uneven melt temperature,. 5. 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. So today we are going to take a look at the five most common defects in injection molding and their solutions. 943 µm to 0. Weld line is formed when two different melt flows converge, as illustrated in Fig. 1. 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). When the same pressure is maintained or lower than that, the drop in speed shall create the weld lines. As the unfavorable defect in micro injection molding parts, weld line brings reduced mechanical and physical properties, especially for nano filled composites. For more information on the Factors Affecting the Injection Molding Process. This leads to a better molecular chain entanglement, which in turn creates a better weld. 7 in Kobayashi et al. Effects of cavity surface temperature on mechanical properties of specimens with and without a weld line in rapid heat cycle. Weld lines appear as a result of the flow of molten plastic during the injection molding process. Prediction of the weld lines in injection molding process using neural networks. Gate location should set to avoid weld lines, keep clean of the important position of the products,. This may require higher melt temperature or lower injection speed . These lines can occur at the gate location, where the plastic enters the mold, or anywhere along the flow path. Weld-Line in an injection molded part develops when two or more melt fronts are converged together. This customary mark is caused by differential rates of cooling in the resin where. With Ixef® PARA compounds, the strength of a weld line is typically 90 MPa (13,000 psi). When two or more streams of plastic meet and fuse. Fisa et al. 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. A. Fig 13:- Detailed view of weld lines. Meld and knit lines are actually both different types of weld lines. Weld lines not only detract from an injection-molded part’s surface quality, but also significantly reduce its mechanical strength. 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. G. Increasing the temperature of the mold or plastic material. Weld-line is a weak area. John Bozzelli. (3) Adjust the temperature of the barrel and nozzle: the viscosity. 809-814. Consultant @ Effinno Technologies, BSME, MBA - Bridge between Plastics Injection Molding Knowledge and Practice. The gate vestige for injection molding is usually narrow. Weld Lines. 2017-01-0481. For multiple gate and complex part, molding weld-lines are unavoidable, therefore mechanical behavior of the weld-line needs to be predicted. 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. EasyFill Advanced provides the weld-line temperature distribution and meeting angle prediction. 1. Islyn Thomas summarized the need for a particular vent location better than I ever could. Besides, possibly it would be. [ 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. Controlling the mold’s temperature could also minimize the weld line’s effects. Weld lines. The primary cause of cracking is stress and deformation brought on by the external environment, cold mold surfaces, and low injection rates. 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. The. For multiple gate and complex part, molding weld-lines are unavoidable, therefore mechanical behavior of the weld-line needs to be predicted. 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. 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. When molten plastic flows, it forms a smooth continuous flow until it meets an obstruction. They become more critical when particulate fillers are compounded with the polymer. Here’s how to decrease the chances of weld lines: Increase the temperature of the mold or the molten plastic. Unless you are manufacturing an extremely simple part with one gate and a unidirectional flow front, your part will have weld lines. Improper fusion of two fluid flows, a state before weld line. Fig. In this paper, a weld line factor (W-L factor) was adopted to. S. In narrow fields, a limited amount of molten liquid can solidify and block the flow. Weld Lines. Wu CH, Liang WJ (2005) Effects of geometry and injection-molding parameters on weld-line strength. These lines occur when two or more flow fronts meet and fuse together, resulting in a visible seam on the molded part. To determine the effect of the weld lines on tensile strength, tensile tests were conducted with the specimens fabricated with NORYL (amorphous blends of. A weld line (weld point, flow line or a knit line) forms when separate melt fronts travelling in opposite directions meet. It is normally at the meeting points of two flow fonts. 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. Injected plates were generated using a double-gated mold under four different process conditions. 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. 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. spelling: injection molding) is a manufacturing process for producing parts by injecting molten material into a mould, or mold. Weld lines, knit lines, and meld lines are terminologies to describe the imperfections of finished parts in plastic injection molding processing. 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. The thermo-rheological findings were used to investigate the sources of weld line weakness. Sink Marks 2. It is not always easy to completely eliminate weld lines by simply adjusting the relevant injection mold design or the molding conditions. 2021年4月23日 / by admin. 13. 4. 21: Weld line failure caused by hoop stresses. Part design features may cause a material flow to split and rejoin as it flows around design features in a mold. wall thickness, gate and runner designs and re-validate the result. Another way to eliminate weld lines in plastic injection molding is to adjust the molding conditions. In a meld line, the plastic comes together at an angle greater than 135 degrees. Multiple gates are acceptable as long as they are close enough to each other to avoid weld line issues. The Solutions for Poor Welding Lines. An evaluation model based on artificial neural network method. Here are a few of the most common defects that may occur in plastic injection molding — and how product teams can avoid them. Injected plates were generated using a double-gated mold under four different process conditions. Polym Eng Sci 45(7. They decrease the. You can locate the gate at the thick part of the injection molding to prevent air pockets. Weld line. Neat polypropylene retains nearly 90% of its strength on a properly designed and molded weld line. A flow mark is commonly referred to as a flow line occurs during the process of injection molding. (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. Figure 2. The Influence of Knit-lines on the Tensile Properties of Injection Molded Parts, Polymer Engineering and Science, 23(10): 591-596. There are various types of injection molding defects that can cause such interruptions in the manufacturing cycle. Wu CY, Ku CC, Pai HY (2011) Injection molding optimization with weld line design constraint using distributed multi-population genetic algorithm. Specifically, injection-mold lines are often associated with poor mechanical strength. A weld line is a mark left on the surface of a plastic injected molded part. Injection molding weld lines where two or more flow fronts come together in the mold. 3. Kobayashi et al. Weld. The weld line’s strength is reduced when. Five injection molding process parameters, including mold temperature, melt temperature, packing pressure, injection. In this work, the residual strength of a fiber-reinforced injection molded part containing a hot weld line—or meld line—was evaluated. Injection pressures too high Reduce pressure . Plastic Injection Molding Process. Therefore, the. 010 0. Mérillou, J. Design. A new kind of weld line was. The cracking was always located in the weld/knit line and therefore caused by weakness in the bonding of the plastic at. Plastic Rapid Injection Molding is popular not only in low volume manufacturing but also in large quantities molding, it is the most common way to manufacture plastic parts. Weld lines may occur depending on the product shape and gate positions. 4 at different time steps (legend scale is volume fraction of. 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. Fig. 001 in . The gate location should enable the two flow fronts to merge easily and continue to flow beyond the weld line. An excess mold release is a major cause of weld line due to two variables: high pressure and speed. Weld lines in injection molding are frequently unavoidable.