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. 2-1. Weld lines in injection molding are a type of cosmetic defect that detracts from a plastic part’s appearance. Mold vents should function the same way. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. Bubbles are caused by melting flow fronts colliding in a mold cavity. The impact behavior of weld-lines in injection molding. Provide a gas vent. The. The type of resin used during injection molding plays a large role in whether or not a knit line will be present. weld lines in injection molded thermoplastic composites. The following are the conclusion drawn from the present research: 1. A weld line is often unavoidable. The governing equations for the melt and the air in the cavity are united into…. The weld line in injection molding is created when two melt streams connect, as shown in Figure 1. In this case the two flow fronts will meet and there will be very little molecular orientation across the weld. + Increase mold temp, or materials’ temp, or holding pressure. Published 5 July 2019. Weld line additionally called as knit line, is a defect that occurs when the injected plastic meets within a mold. 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. The choosing optimal gate location technique has been developed to set weld lines as far as possible from the region of maximum stresses. They decrease the. Flow analysis of the weld line formation during injection mold filling of thermoplastics. Weld line formation in the injection molding process occurs. 2. Injected plates were generated using a double-gated mold under four different process conditions. 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. This study describes how the weld line was formed on the part. th INTRODUCTION Injection molding is one of the most Weld Lines in a plastic cover. Weld lines cause significant reject rates and are a common problem that all molders face. Download : Download high-res image (856KB) Download : Download full-size image; Fig. Weld lines can compromise the strength and aesthetics of the part, so it’s important to carefully design the mold. Plastic Injection Molding Process. The melt stream is separated at the part’s cutout and reunited at the other end of the cutout. Figure 1a illustrates how the weld line is produced when two melt streams merge during the final filling process. Park et al. Weld line section presents notch or gloss difference. In an injection molding process, a weld-line forms when two flow fronts meet each other. Weld line formation in the injection molding process occurs. (3-1) Increasing the Resin Temperature at the Weld Line Section. Weld lines appear as a result of the flow of molten plastic during the injection molding process. 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. First, an injection mold with triangular inserts was built to study the influence of meeting angles on material properties at the weld line. Wu CH, Liang WJ (2005) Effects of geometry and injection-molding parameters on weld-line strength. The differences between knit and meld lines are significant, and they can dramatically impact the part’s structural integrity. Polym Eng Sci 45(7. 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. distribution [4]. 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. Causes and Remedies of Sink Marks In Injection Molding. + Adjust injection speed. Gate Placement: Properly positioning the gate can direct the melt flow to minimize weld lines or relocate the weld line to non-visible areas of the part. Carefully consider all the relevant aspects of the mold design specification before. Molding calculation in Moldex3D: (а) melt front and weld line, (b). Google Scholar | Crossref | ISI. He said, “Vent cavities at weld lines or junction points where two or more material streams meet and fuse into a solid mass. 13 shows the best gate location for which the weld lines . Weld lines typically appear in a part when an obstruction of some kind separates the melt flow and prevents it from "knitting" together. Five injection molding process parameters, including mold temperature, melt temperature, packing pressure, injection. GlossFlow-induced fiber orientation and weld lines are com-mon features of the injection molding process. The most common method for the weld-line characterization is mechanical testing by means of a tensile or bend stage [6,7]. S. Polym. Thus, it. Weld lines occur when melt flow fronts meet during the injection molding of plastic parts. In order to increase the quality of the products manufactured by injection molding, RTC technology can be used to achieve higher mold temperatures. It is normally at the meeting points of two flow fonts. 2. Specifically, injection-mold lines are often associated with poor mechanical strength. 2. 005 0. 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. 004 m, a characteristic velocity ν 0 =0. Avient’s Zachary Alderman shares practical tips on designing and injection molding parts utilizing short-fiber reinforcements. Below is one of the examples: !NOTE: if the weld lines is important to the cosmetic surface of your part, please bring this up to your mold maker during quoting phase. With tighter heat controls, you can keep the material at the proper temperature to make sure the plastic keeps flowing and fills the mold cavity completely. Which means engineers need to know how to account for this phenomenon in an efficient way. Specifically, injection-mold lines are often associated with poor mechanical strength. 020 0. – Use generous and even. In this experiment, several ejector. That helps. Weld Line Phenomenon: Surface defect in which the weld, or the part where the flows of melted plastic meet inside the mold, shows up on the surface as a groove or pattern. Weld lines have been one of the common defects puzzling the injection molding industry. The main types of injection molding defects are: 1. Three terms that often get mixed up are weld, meld, and knit lines. When a product is produced through injection molding, a high-powered tool injects melted plastic into a custom, double-sided mold. 2) show that the weld line is formed as expected from the middle of the cavity towards the wall. 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. The foundation to any robust process, in my opinion, is based on choosing the correct fill time. 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. It is important to investigate the weld line because the weld line area can induce potential failure of structural application. , 190°C mass and 20°C mold temperature, as well as. Quality concerns in injection-molded materials can vary fr. They are unavoidable even for moderate complex products. Weld or knit lines are perhaps the most common and difficult injection molding defect to eliminate. It is possible to pre. Evaluate and optimize your part design The advantages of using the moldflow analysis to avoid potential defect like deformation, shrinkage, welding line etc, part designers can change the wall thickness or enhance the ribs to improve their part design. Injection molding, polypropylene, talc. Weld lines defects are formed when two flow fronts are merged together. * Corresponding author, e-mail: fengsjpc@ku. Weld lines are also known as knit lines. 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 in injection molding occur when two flow fronts meet and create a visible line on the surface of the molded part. Weld-line is a weak area which reduces the strength of the part locally. Knit and meld lines are inherent to plastic. 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 occur when two flow fronts meet and fuse together in the injection molding process. Ejector System (Pötsch, 1995) . weld lines in infill analysis of injection molding simulation was observed [5]. Weld lines, generated during injection molding, can significantly reduce the mechanical performance of structures. Here are some ways to avoid. A meld line occurs when two moving melt fronts converge and flow parallel to each other. th INTRODUCTION Injection molding is one of the mostWeld Lines in a plastic cover. 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. Strength measurements should be made to. The weld line is an inevitable defect in the most injection molded components. Criens, R. This failure causes the product to be damaged during use. Sink Marks Cause 1: Incorrect Melt Temperature. These processing changes will increase chain. M. Let’s review what causes meld and knit line imperfections, how they’re different, and how they affect part durability. Poor mold spot Spot mold to ensure fit and vent depths at ~0 . A weld line (weld point, flow line or a knit line) forms when separate melt fronts travelling in opposite directions meet. 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. While there are many situations where they are barely perceptible, weldlines represent a potential source of weakness in molded parts. You will have to confirm them before tooling. Weld Lines. Heat/cool injection molding has a positive influence on the depth and visibility of weld lines. The impact behavior of weld-lines in injection molding. Possible causes: Low temperature or fluidity of melted plastic during injection molding. The specific method is that when the product is just filled and filled in the cavity, the insert. but the injection pressure will loss and injection molding pressure of this gate is bigger than that of direct gate. 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]. 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. Weld-Line in an injection molded part develops when two or more melt fronts are converged together. 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. This failure causes the product to be damaged during use. Better weld line quality was obtained by Huang. 010 0. Introduction. Mold Design. Flow rate is 24 cm3/s. In this lesson we will focus on the meld and weld lines. Injected plates were generated using a double-gated mold under four different process conditions. Raise the injection speed. To explore the formation of weld lines in injection molding, a. Injection molding could be applied with many polymer materials, such as. The flow of the material in the injection mould takes place through the gate. 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. Weld Lines are formed when two or more molten polymer flows meet during the injection molding process. The hair-like weld line is the result of weak. Injection molding can be performed with only one of these. Therefore, more studies covering this subject are required in order to improve the insight regarding the mechanism involved in the formation of weld lines in extrusion and,From the so-produced materials, specimens with weld lines were produced according to the first test series, but varying the injection molding parameters only twice, between the best and worst settings found in the previous study with the polypropylene types with MFR 8 g/10 min, i. 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. Expand. 2. 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 the unfavorable defect not only in normal injection molding process but in micro injection molding process. Flow lines. This paper presents the. The holding pressure was set as approximately equal to the maximum injection pressure obtained at the time of filling the resin. 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. The literature on the formation of weld lines, and their consequences, is more abundant on the area of injection molding [5-12] than on extrusion [13]. Therefore, it is recommended to do a moldflow study prior to the mold being built. Stop using mold parting agents. 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. Weld-line is a weak area. * Corresponding author, e-mail: [email protected] recently had a few pieces (2-3%) crack after assembly, while in storage (not installed, still in the package). 1. 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 this work, the residual strength of a fiber-reinforced injection molded part containing a hot weld line—or meld line—was evaluated. EasyFill Advanced provides the weld-line temperature distribution and meeting angle prediction. 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. Clamping unit berfungsi untuk memegang dan mengatur gerakan mold unit, serta gerakan ejector saat melepas benda dari molding unit, pada clamping unit lah kita bisa mengatur berapa panjang gerakan molding saat dibuka dan seberapa panjang ejector harus bergerak. A weld line region is typically filled at the end of an injection stroke or during the pressure phase. What is weld lines. You can locate the gate at the thick part of the injection molding to prevent air pockets. Wolfgang Homes, head of the applications department of Eurotool, a manufacturer of hot runner systems, clarified the two methods for IMI, noting that. The impact behavior of weld-lines in injection molding. During injection moulding, a weld-line occurs when two or more flow fronts collide in the hollow cavity. Gates should be designed larger than normal and located to minimize weld line formation. Its appearance can be improved by adjusting parameters. In spite of its cosmetic attributes, a bad knit line can significantly. Google Scholar [10] S. Plastics manufactured by injection molding are meant to be durable and reliable, able to hold up to heavy use, high temperatures, or adverse conditions. Wavy lines are another feature of weld lines. Plastic Injection Molding Process. In the plastic injection molding industry, we work with materials that are byproducts of oil refinement and molds made of metal casting, so our terminology is bound to be at least slightly confusing. [1-2] investigated the weld line zone of injection molded glass-fiber-filled polypropylene (PP) composites. 080 inches. avoid weld lines, and facilitate smooth. Weld lines in injection molding are frequently unavoidable. For example, filled resins and ABS are more likely to. . 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. 6. Weld lines are significant defects that affect injection molding quality, causing the poor appearance and low mechanical strength of injection‐molded parts. e. 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. Dischler, D. Low mold temperatures are usually employed to decrease cycle molding time and final process costs; however, resulting surface defects include weld lines, sink marks,. 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. Weld Line Factors for Thermoplastics. Knit lines, weld lines, mold lines, or flow lines in injection molding are faint lines that become visible on certain plastic parts. [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. Injected plates were generated using a double-gated mold under four different process conditions. That’s the air escaping out of the nozzle until the hose is full of pressurized water. Weld lines, knit lines, and meld lines are terminologies to describe the imperfections of finished parts in plastic injection molding processing. This is where the melt fronts divided by an insert, such as a pin or a. Weld line, 5. The plastic injection molding process, including thermoplastic injection, is nowadays used in several industrial processes, including in the automotive industry for bowden cables. Weld lines are visible lines or seams formed when molten plastic flow fronts meet and fuse together, often due to obstacles or flow interruptions in the. investigated the impact of injection molding parameters on weld line formation and strength reduction. 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. This occurs mainly in the presence of barriers that divide the melt flow in. Steven. The injection molding simulation software used offers a method for the detection of weld lines that has shown to be only reliable for the detection of stagnating weld lines. A poor knit line can cause only cosmetic blemishes or it can significantly weaken the structural integrity of a part. 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). wall thickness, gate and runner designs and re-validate the result. poor flow fronts that are too cold, this can also lead to a deterioration in the mechanical properties. 10(a) (please see Fig. When the same pressure is maintained or lower than that, the drop in speed shall create the weld lines. 025 mm (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. • Reduce runner dimensions and maintain the same flow rate. [ 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. As the unfavorable defect in micro injection molding parts, weld line brings reduced mechanical and physical properties, especially for nano filled composites. Injection-molding machine is the Negri Bossi VE210-1700 with screw diameter 60 mm. Weld lines in injection molding are frequently unavoidable. The goal is to create a uniform thickness and at the smallest (thinnest) level required. 6. One of the things that can compromise an injection molded part is a weld line, also called a knit line. Oil lines and reservoirs are typical examples for hollow composite parts which are conventionally produced using a multi-stage process. Vents in the mold allow the mold to fill. Wu CY, Ku CC, Pai HY (2011) Injection molding optimization with weld line design constraint using distributed multi-population genetic algorithm. Investigations on WLs in micro injection molded parts were performed in Refs. The results of. Here are some strategies. 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. 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 pressure. Weld lines represent the region where two separated melt fronts recombine. Mold vents should function the same way. (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 injection molding weldlines are generated when two separate melt streams join either in multigated molds or as a consequence of flow around. It will degrade materials performance; 9. Part design features may cause a material flow to split and rejoin as it flows around design features in a mold. The three parameters affecting operation were called injection pressure, injection time, and packing pressure which influence the quality of weld line. Although they often cannot be avoided it’s possible to minimize their cosmetic effects to an acceptable degree through careful product design and. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. Weld line formation in the injection molding process occurs. When two or more strands of molten plastics meet in the mold, a weld line will be produced at the junction [1,2]. gasket seal leakage where the weld line crosses a gasket. The wheel has to be gated at three places on the hub and. Weld line defects – also known as knit lines – is a defect that occurs when the injected plastic meets within a mold. Move injection locations to make weld lines form closer to the gates. Wang. Multiple gates are acceptable as long as they are close enough to each other to avoid weld line issues. They decrease the. 3. Raising the mold temperature will keep the material molten longer, resulting in stronger weld lines and even elimination of weld lines. These lines can occur at the gate location, where the plastic enters the mold, or anywhere along the flow path. T. To respond to poor plasticization and uneven melt temperature,. A visual mold with variotherm system for weld line study in micro injection molding. 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. Weld lines are visually undesirable and a plastic part is structurally weak at weld line positions. Change the gate locations. enough to each other to avoid weld line issues. The solution to an internal part cracking problem is to change the packing pressure. Weld line. 1 Prediction of weld lines with injection molding simulation softwareReduced frozen skin thickness: High injection speeds also contribute to a thinner frozen skin at the weld line. Still, it can be seen to varied degrees depending on mold. Too much injection molding pressure lost and increase the operation difficulty of the injection molding machine; 7. Article Google Scholar Deng YM, Zheng D, Lu XJ (2008) Injection moulding optimization of multi-class design variables using a PSO algorithm. Effects of cavity surface temperature on mechanical properties of specimens with and without a weld line in rapid heat cycle. (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. 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. As compared to a weld line, the flow fronts come together at less than 135 degrees. Weld lines may appear like seams or scratches, but that is not what they are. High-density PE was also moulded in the same conditions but in a lower temperature range (145 to 220°C). ac. Weld or knit lines occur when melt flow fronts collide in a mold cavity. Here is a look at the differences between them, the causes of meld and knit line imperfections, and their impact on part durability. The weld line in injection molding is created when two melt streams connect, as shown in Figure 1. This occurs wherever there is a hole, notch, or any other feature on a part which divides the flow of plastic into 2 separate “flow fronts. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and. The Influence of Knit-lines on the Tensile Properties of Injection Molded Parts, Polymer Engineering and Science, 23(10): 591-596. The micro tensile part was prepared by the double gate injection. He now teaches courses in injection molding and plastic part design at the Polymers Center of Excellence and UNC-Charlotte in Charlotte, N. Jiquan Li, Taidong Li, +1 author. Flow-induced filler orientation manifests in higher tensile prop-erties in the direction of flow compared to those normal to the direction of flow. Flash. Weld lines form where two or more flow fronts come together and can create a cosmetic or structural problem. 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. To explore the formation of weld lines in injection molding, a. Microsyst Technol, 14 (6) (2008), pp. Unless you are manufacturing an extremely simple part with one gate and a unidirectional flow front, your part will have weld lines. In order to reduce these weak points, a special ram technique for injection molding, which can be integrated into the mold, was investigated. Thus, a right gate design raises polymer temperature to prevent the formation of weld lines and flow marks. Weld lines may occur on a molded part’s surface where, after breaking off into two or more directions in a mold, the molten material has converged. They are the result of fundamental physical effects taking place inside of the mold tool cavity. For multiple gate and complex part, molding weld-lines are unavoidable, therefore mechanical behavior of the weld-line needs to be predicted. Causes: Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic. Heating and cooling mould systems, which cost extremely high, are a typical technology to prevent the weld-line. After that, the compounds were injection-molded into tensile samples with and without weld lines to. In an injection molding process, a weld-line forms when two flow fronts meet each other. 6. An excess mold release is a major cause of weld line due to two variables: high pressure and speed. A flow mark is commonly referred to as a flow line occurs during the process of injection molding. are very minimum. Melt temperature does effect the weld. Sink Marks Cause 3: Improper Pack or Hold Pressure. 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. Adjust the design for the flow pattern to be a single source flow. In some cases, a weld line can also cause a loss of strength and impact resistance. Injection Molding Know How. During injection. 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. 2. the meeting angle by optimizing variation over time in the cavity. 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. Weld lines are where two flow. With Ixef® PARA compounds, the strength of a weld line is typically 90 MPa (13,000 psi). Weld Lines-3 Ways for Detecting and Correcting. They are unavoidable even for moderate complex products. Weld lines can be caused by many reasons, which covers the various aspects of mold, process, raw materials and product design, etc. The melt stream is separated at the part’s cutout and reunited at the other end of the cutout. [ 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. Knit lines are hard to eliminate for injection molded plastic parts but. This principle has been implemented on a complex component in the case of the spring-loaded cogwheel shown in Figure 5, for instance. This is expected to achieve maximum results while the pressure holding effect will also increase. Injection molded plastic parts have comparatively less strength in the weld line section and they also have an impact on product aesthetics. Co-injection molding technique is a useful way to study weld lines, which can offer more detailed information of the. (2011) and Fig. 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]. Complexities in the design and intricate pathways in the mold can also create flash in injection molded parts. 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. It has larger cross-sectional regions that allow sufficient molten plastic flow into the cavity. , a post that forms a hole) and then rejoins, or when polymer melt fronts meet, from multiple injection points. Weld lines not. Use a less viscous plastic. 1. This customary mark is caused by differential rates of cooling in the resin where. Weld Line defect analysis, the most common of many defects in injection moulding products. Weld lines have been one of the common defects puzzling the injection molding industry. spelling: injection molding) is a manufacturing process for producing parts by injecting molten material into a mould, or mold. Strength at weld the line location and warpage problem in injection molding plastic component were not new problems in injection molding industries. Degassing – Degassing is the simple operation of opening and closing an injection mold to allow any trapped air (or gas) to escape. 5. Runner Type Venting Plastic melt and mold temperature Injection speed and pressure Learn how to troubleshoot weld lines in injection molding. 4 at different time steps (legend scale is volume fraction of. 18/06/2021. Bubbles are caused by melting flow fronts colliding in a mold cavity. The more gate points there are, the more weld lines will occur. But add 30% glass and weld-line strength retention drops to only 34% as high as the compound with no weld.