John Bozzelli. Sink Marks 2. 010 0. Known methods to reduce weld-line effects are the optimization of injection molding parameters, enhanced tool temperature, and using special molding tool devices. 18/06/2021. To explore the formation of weld lines in injection molding, a. A visual mold with variotherm system for weld line study in micro injection molding. Meld Lines. 10(a) (please see Fig. Heat/cool injection molding has a positive influence on the depth and visibility of weld lines. The relative weld-line strength is defined as the ratio of the strength of specimens with a weld line. Weld lines are the most common defects in injection molded products. It forces out trapped air pockets and can prevent a vacuum void from occurring. Weld lines are a reality in the injection moulding of complex parts. Stop using mold parting agents. A viscosity curve is developed using calculations of the fill time and plastic pressure at. technical ulletin SPECIALTY POLYME RS Design and Production of Injection Molding ToolsIncrease the injection speed in order to fill the mold more quickly and ensure the material does not prematurely cool down; in addition, when the speed is adjusted the flow fronts may meet at a different spot and move the weld line location. If the large molded product is. The weld line degrades the. + Adjust injection speed. Weld line defects in injection molding is a line or lines on the part surface that caused the part looking ugly or cuased part lifetime, or even broken easily. Silver Streak. [ 18 - 21 ] In addition, the effects. The melt stream is separated at the part’s cutout and reunited at the other end of the cutout. During injection. In this work, the residual strength of a fiber-reinforced injection molded part containing a hot weld line—or meld line—was evaluated. Proper venting is also required to ensure good weld-line quality. This failure causes the product to be damaged during use. Weld Lines. 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. ”. 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. 001 in . Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and. Polymers 2023, 15, 2440 3 of 21. When two or more streams of plastic meet and fuse together, a visible line or weakness is formed on the surface of the finished product. Gava and co-workers implemented an investigation devoted to the de-Injection-mold weld lines may or may not be easily identified by naked eye or straightforward optical microscopy. 6. , less than 0. They are unavoidable even for moderate complex products. Some of these are listed below: Plastic warpage. For more information on the Factors Affecting the Injection Molding Process. 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. Three terms that often get mixed up are weld, meld, and knit lines. For multiple gate and complex part, molding weld-lines are unavoidable, therefore mechanical behavior of the weld-line needs to be predicted. Injection molding can be performed with only one of these. One solution is to prevent the weld lines from forming in regions that are. 1. Specifically, injection-mold lines are often associated with poor mechanical strength. See moreEquipment. Knit and meld lines are inherent to plastic. 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 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. Or increase the number of gates. It is a zone with reduced mechanical properties negatively influencing on the molding behavior in exploitation. The weld line’s strength is reduced when. As the name implies, it is usually positioned along the edge of the workpiece, forming a visible mark at the demarcating line. The differences between knit and meld lines are significant, and they can dramatically impact the part’s structural integrity. This paper presents the. 030 0. Injection-molding machine is the Negri Bossi VE210-1700 with screw diameter 60 mm. Other possible causes include: Low mold or material temperature; Insufficient holding time or. The impact behavior of weld-lines in injection molding. One method is the local te mpering of the affected areas to prevent weld lines. Weld Line Factors for Thermoplastics. In the sense that the weld line is better formed before the resin. 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. Optimizing the injection parameters through combination of adjusting the molding temperature, varying the gate pressure, and increasing the dwell time. Weld lines are a common occurrence in injection molding that affects the overall quality and strength of a finished product. 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 the cavity of the injection mold. Kobayashi et al. ac. Weld line existence reduces the material strength; however, its demerit characteristics. 3: Weld Lines. 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. Increase the size of the gate and runners to alter the mold design. High-density PE was also moulded in the same conditions but in a lower temperature range (145 to 220°C). In injection molding, a V-notch-shaped weld line is generated in the melt front meeting area. Mold vents should function the same way. A weld line region is typically filled at the end of an injection stroke or during the pressure phase. Effects of mold temperature and pressure on shrinkage 0. Insufficient plasticizing ability of injection molding machine. Criens, R. Introduction. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. Flow marks in injection molding can be avoided if adequate measures are taken. avoid weld lines, and facilitate smooth. 020 0. 10. Weld lines appear as a result of the flow of molten plastic during the injection molding process. The weld line in injection molding is created when two melt streams connect, as shown in Figure 1. According to the special features of micro injection molding process compared with normal injection molding process, the main related factors to weld line’s properties were classified as 3 group, named as processing conditions, mold design and structure, external process. Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties. The default weld line result recognizes 135º as the maximum angle of collision and as that angle decreases, the severity increases as it. The edge gate is the most popular and straightforward injection mold gate design. 5. Here are some ways to avoid. 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. In the injection molding process, weld line regions occur when a molten polymer flow front is first separated and then rejoined. 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. 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. Plastic Injection Molding Process. Gate location should set to avoid weld lines, keep clean of the important position of the products,. This can allow the. Weld lines represent the region where two separated melt fronts recombine. The plastic injection molding process, including thermoplastic injection, is nowadays used in several industrial processes, including in the automotive industry for bowden cables. Abstract. The weld lines are an integral part of plastic injection molding and can seldom be prevented, even by specifically planning the mold with inserts for holes or the like. 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. In an injection molding process, a weld-line forms when two flow fronts meet each other. Short shots. (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. 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. 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. They occur when melt flow fronts collide in a mold cavity. Causes: Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic. 6. Mold flow marks, also known as flow lines or weld lines, are often caused by variations in the flow of molten plastic material during the injection molding process. In this lesson we will focus on the meld and weld lines. A weld line region is typically filled at the end of an injection stroke or during the pressure phase. Weld lines in injection molding occur when two flow fronts meet and create a visible line on the surface of the molded part. There are several possible causes that need to be investigated to troubleshoot weld lines in injection molding. 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. Increase the injection speed. Figure 1a illustrates how the weld line is produced when. Mérillou, J. Vacuum voids. Weld Lines 5. When the same pressure is maintained or lower than that, the drop in speed shall create the weld lines. Multiple gating, splitting of the melt flow due to inserts in the cavity or through holes, as well as changes of thickness give rise to points within the structure where the flowing fronts will recombine and weld. 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. Adjust Injection Molding Parameters: Optimizing injection speed, pressure, and temperature can help ensure better melt flow and reduce the likelihood of weld line formation. Threadline gap in-between part due to improper gate location in complex design parts including excess of holes (multipoint. Weld Lines. 809-814. Ideally, the resin flow will split into two flow fronts to pass around the obstruction and then recombine into a single flow front. It is possible to pre. Despite injection molding being recognized as a robust process for obtaining parts with high geometry accuracy, one last occurrence remains a challenge in micro. Shaofei Jiang. Sink Marks Cause 4: High Mold Temperature. A weld line is often unavoidable. Weld Lines are formed when two or more molten polymer flows meet during the injection molding process. Weld line. Conventional injection molding is widely used to produce plastic parts, mainly in the automotive industry, due to the high production ratios and quality of injection parts. Nguyen-Chung. Google Scholar | Crossref | ISI. Flow analysis of the weld line formation during injection mold filling of thermoplastics. 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. This occurs either due to injection through multiple gates or as a conse-รอยประสานของเนื้อพลาสติก Weld line ตอนที่1. 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. 7 in Kobayashi et al. 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. Carefully consider all the relevant aspects of the mold design specification before. 2017-01-0481. The resulting defects don’t stop at flash only. Mold Design Most conventional mold designs will work with TPU’s. 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. Here is a look at the differences between them, the causes of meld and knit line imperfections, and their impact on part durability. Sink marks, 2. Weld or Knit lines are a plane or section in an injection molding part where the flow of molten plastics meets from different directions. Therefore, it is recommended to do a moldflow study prior to the mold being built. Injection Molding Know How. A flat product case was considered as an example of a typical aircraft structural element. 2. 025 mm (0 . These are caused by the shot of molten plastic moving at different speeds throughout the injection. The Injection Moulding Process is a manufacturing process used for producing parts or components by injecting molten material into the mold cavity. [ 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 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. According to the characteristics. Features such as sharp corners or undercuts can. Let’s review what causes meld and knit line imperfections, how they’re different, and how they affect part durability. 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 injection molding gate generates heat by dissipation. The weld line is an inevitable defect in the most injection molded components. 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. In the present work, the micro specimens were investigated. carried out to mass-produce molded products with complex shapes. The structure is more complex with longer molding cycle . 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. Abstract. In this paper, two different injection molding tests were completed. These processing changes will increase chain. 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. 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. The most common and one of the most challenging injection molding defects are weld and knit lines. Weld Lines-3 Ways for Detecting and Correcting. Preventing sink marks of injection molded parts using CAE analysis. Weak weld lines are perhaps the most common and difficult injection molding defect to eliminate. Strength at weld the line location and warpage problem in injection molding plastic component were not new problems in injection molding industries. Flow-induced filler orientation and weld lines are un-avoidable facets of the injection molding process. A weld line is often unavoidable. 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. 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. For multiple gate and complex part. with a characteristic length χ 0 =0. Fig 13:- Detailed view of weld lines. Unless you are manufacturing an extremely simple part with one gate and a unidirectional flow front, your part will have weld lines. You will have to confirm them before tooling. If you turn on the spigot to a garden hose, you will hear a hissing sound at the nozzle end for the first few seconds. Weld lines form where two or more flow fronts come together and can create a cosmetic or structural problem. 13 shows the best gate location for which the weld lines . That helps. Increase the injection speed and mold/operational temperature to prevent fracture or surface breakdown. Weld lines can be a weak spot and have therefore to be taken into consideration in part design. Article Google Scholar Deng YM, Zheng D, Lu XJ (2008) Injection moulding optimization of multi-class design variables using a PSO algorithm. The results showed that the tensile strength of PE remains unchanged in the presence of weld lines. 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. Bubbles are caused by melting flow fronts colliding in a mold cavity. The flow of the material in the injection mould takes place through the gate. 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. In injection molding, it is often important to maintain consistent color throughout the part. Park et al. • Optimize the design of the runner system. Weld or Knit lines are a plane or section in an injection molding part where the flow of molten plastics meets from different directions. Figure 1a illustrates how the weld line is produced when two melt streams merge during the final filling process. 1 Mold In order to study the formation of weld lines, the mold cavities has been. Injected plates were generated using a double-gated mold under four different process conditions. 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. Voids and bubbles, 6. Weld lines. About Press Copyright Contact us Creators Advertise Developers Terms Privacy Policy & Safety How YouTube works Test new features NFL Sunday Ticket Press Copyright. The melt stream is separated at the part’s cutout and reunited at the other end of the cutout. Three terms that often get mixed up in injection molding are weld, meld and knit lines. Wu CH, Liang WJ (2005) Effects of geometry and injection-molding parameters on weld-line strength. Alter the mold design. (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 study, polypropylene (PP) was chosen as the processing material and a micro dog-bone tensile test sample was selected as the objective part. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. (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. They have their origins in material choice, part design, tooling, and processingIncrease the mold temperature and smooth out any sharp corners or turns in the mold to facilitate even melt flow. Optimizing the Injection Parameters. The more gate points there are, the more weld lines will occur. Figure 2. Injection molding of large/complicated products are usually prepared by multiple compounds, which produces weldlines once the melt fronts are joined by impingement flow. In the first stage, the two part halves are injection molded and then, in a second process step outside of the molding machine, they are welded together. They decrease the. Another relevant topic in injection molding is the presence of weld lines (WLs) and their properties, which could lead to part weakness through surface disturbances, optical issues and reduced bonding [21, 22]. 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). Use a less viscous plastic. Weld lines occur when two flow fronts meet and fuse together in the injection molding process. Edge Gates. Venting at Weld Lines. 10 in Nguyen-Chung et al. (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. ), with different mould surface finishes (polished, fine eroded, rough. Injected plates were generated using a double-gated mold under four different process conditions. Raising the mold temperature will keep the material molten longer, resulting in stronger weld lines and even elimination of weld lines. In injection molding, a V-notch-shaped weld line [1] is generated in the melt front meeting area. 15 shows the frozen layer and weld lines at the end of mold filling. With unchangeable and fixed requirements for the final product, their avoidance often proves difficult. Meld and knit lines are actually both different types of weld lines. If you adjust the ejection mechanism in the mold design, you also can reduce or eliminate your need for mold release agents. The Difference Between Knit and Meld Lines–and Why It Matters. * Corresponding author, e-mail: fengsjpc@ku. 080 inches. Weld lines have been one of the common defects puzzling the injection molding industry. 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. Double push filling method. The hair-like weld line is the result of weak. 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. Save. A simulation process was conducted to determine the recommended setting of injection moulding parameters and the range of the variable. Five injection molding process parameters, including mold temperature, melt temperature, packing pressure, injection. The next type of injection molding defect is ‘Weld Lines’. Polym Eng Sci 45(7. 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. Definition: A seam that appears where two areas of molten plastics meet. Injection molded plastic parts have comparatively less strength in the weld line section and they also have an impact on product aesthetics. 1. Redesign the mold to prevent the weld line. G. 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]. wall thickness, gate and runner designs and re-validate the result. 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. 005 0. Part design features may cause a material flow to split and rejoin as it flows around design features in a mold. 2004; To study the weld line formation of colliding flow fronts the filling of a mold cavity was simulated. In addition a weld line can be moved to an area where it is less visible. The results of. Increase the injection speed to reduce the amount of cooling happening in the mold. That’s the air escaping out of the nozzle until the hose is full of pressurized water. When an external load is applied to a weld line region, the part’s potential for mechanical failure is increased. The parting line in injection molding is which one of the following: (a) the lines formed where polymer melt meets after flowing around a core in the mold, (b) the narrow gate sections where the parts are separated from the runner, (c) where the clamping unit is joined to the injection unit in the molding machine, or (d) where the two mold halves come. Weld lines typically appear in a part when an obstruction of some kind separates the melt flow and prevents it from "knitting" together. Introduction Weld lines are formed during mold filling whenever two separated melt streams recombine. 1. Depending on the resin, resin temperature, mold temperature, and filling speed, knit lines can vary. In the sense that the weld line is better formed before the resin temperature. Sink marks are a common type of injection molding defect. Injection molded plastic parts have comparatively less strength in the weld line section and they also have an impact on product aesthetics. In this paper, a weld line factor (W-L factor) was adopted to describe the. Valve. + Improve vent inside the mold. 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. Complexities in the design and intricate pathways in the mold can also create flash in injection molded parts. 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. [1] The best we can hope for is to pressurize the knit or meld line better through the combination of melt temperature, mold temperature, flow rate, and holding pressure. A weld or meld line is a weakness or visible flaw created when two or more flow paths meet during the filling process. 1. 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. He now teaches courses in injection molding and plastic part design at the Polymers Center of Excellence and UNC-Charlotte in Charlotte, N. The conditions are temperature, injection speed, and injection pressure. Weld line location. 2-1. weld lines in injection molded thermoplastic composites. 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. How Are They Formed? Molten plastic resin is injected into a mold cavity through an opening known as a gate. That helps. The most common method for the weld-line characterization is mechanical testing by means of a tensile or bend stage [6,7]. The Influence of Knit-lines on the Tensile Properties of Injection Molded Parts, Polymer Engineering and Science, 23(10): 591-596. Weld lines are where two flow. Mold Design. If the different flow fronts have cooled. Weld lines occur when melt flow fronts meet during the injection molding of plastic parts. The schematic diagram of a weld line formation is illustrated in Figure 7, where is the welding angle of two melt fronts. 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. The pressurized air restricts the flow of the plastic resin. First, an injection mold with triangular inserts was built to study the influence of meeting angles on material properties at the weld line. When a product is produced through injection molding, a high-powered tool injects melted plastic into a custom, double-sided mold. Weld-line strength for plastics can vary from 20% to nearly 100% of the strength of the plastic itself. 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. Known also as knit lines and weld marks, weld lines are difficult to eliminate completely, especially if your injection molded part requires holes. 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 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. 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. e. Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties. 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. Weld lines may appear like seams or scratches, but that is not what they are. Weld lines are a common imperfection to be found in most plastic injected molded parts. These lines are formed when two or more flowing molten plastic streams meet and fuse together, creating a visible line on the surface of the molded part. 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. Thicker parts require more cooling time and can lead to air becoming trapped in the mold and weld / knit lines to form. when two or more melt flow fronts contact each other immediately after the cooling process. The rule is, a higher pressure shall be required to push the material in the machine when there is excess mold. 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. Weld lines, knit lines, and meld lines are terminologies to describe the imperfections of finished parts in plastic injection. Melt temperature does effect the weld. But add 30% glass and weld-line strength retention drops to only 34% as high as the compound with no weld. Thus, it is important to understand (i). Multiple gates are acceptable as long as they are close enough to each other to avoid weld line issues. injection molding, (b) weld line and meld line, and (c) injection sample size and shape. poor flow fronts that are too cold, this can also lead to a deterioration in the mechanical properties. 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]. Injected plates were generated using a double-gated mold under four different process conditions. analyzed the heat transfer during heating and cooling phases of RHCM process. As the name implies, it is usually positioned along the edge of the workpiece, forming a visible mark at the demarcating line. Here we are looking at most common part defect that can come from injection molding called weld line. A weld line is a mark left on the surface of a plastic injected molded part. Islyn Thomas summarized the need for a particular vent location better than I ever could. Specifically, injection-mold lines are often associated with poor mechanical strength. A poor knit line can cause only cosmetic blemishes or it can significantly weaken the structural integrity of a part. Runner Type Venting Plastic melt and mold temperature Injection speed and pressure Learn how to troubleshoot weld lines in injection molding. Weld Lines. This is a video excerpt from our Problems & Solutions Course - Weldlines. M. If weld lines are unavoidable due to multiple gates in a moulding, they should be positioned in the non-critical region. A weld or meld line on plastic parts can cause structural problems and/or be visibly unacceptable. 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. Streaks 3. 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.