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. The specific method is that when the product is just filled and filled in the cavity, the insert. Weld lines may occur depending on the product shape and gate positions. Meddad and Fisa [3] found that the weld line is a zone between 2 and 8 mm wide extending throughout the thickness in which the fibers are oriented almost. When the two such melt flows. Adjust the position of the gate and include a venting slot. 5. 1 m/s, and a zero-shear-rate viscosity η 0 (T 0)=3760 Pas. 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. Here is a look at the differences between them, the causes of meld and knit line imperfections, and their impact on part durability. 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. He said, “Vent cavities at weld lines or junction points where two or more material streams meet and fuse into a solid mass. These lines can occur due to variations in wall thickness, gate location, or the geometry of the part. 943 µm to 0. Download : Download high-res image (856KB) Download : Download full-size image; Fig. In this case the two flow fronts will meet and there will be very little molecular orientation across the weld. To explore the formation of weld lines in injection molding, a flow model based on viscoelastic constitutive equation is presented in this article to predict and evaluate of weld lines location and properties, in which Rolie. It is possible to pre. In spite of its cosmetic attributes, a bad knit line can significantly compromise a part. The hair-like weld line is the. distribution [4]. Steven. 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. 10. 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. Weld lines can compromise the strength and aesthetics of the part, so it’s important to carefully design the mold. , 190°C mass and 20°C mold temperature, as well as. The thermo-rheological findings were used to investigate the sources of weld line weakness. Thicker parts require more cooling time and can lead to air becoming trapped in the mold and weld / knit lines to form. Low mold temperatures are usually employed to decrease cycle molding time and final process costs; however, resulting surface defects include weld lines, sink marks,. Weld line strength is important in blow molding as well as injection molding. Weld lines are a reality in the injection moulding of complex parts. But none of the current packages can predict weld lines’ properties qualitatively. 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. Weld lines form where two or more flow fronts come together and can create a cosmetic or structural problem. . Flow analysis of the weld line formation during injection mold filling of thermoplastics. GlossFlow-induced fiber orientation and weld lines are com-mon features of the injection molding process. Weld lines in injection molding occur when two flow fronts meet and create a visible line on the surface of the molded part. Instead, a line that looks like a seam forms. weld lines in injection molded thermoplastic composites. It has larger cross-sectional regions that allow sufficient molten plastic flow into the cavity. Weld lines are formed when there is an inability of two or more flow fronts to ‘knit’ or ‘weld,’ together during the molding process. A weld or meld line is a weakness or visible flaw created when two or more flow paths meet during the filling process. Figure 1 below shows the development of a weld line. Weld lines have been one of the common defects puzzling the injection molding industry. Except for a few injection molded parts with very simple geometry, they occur on most injection molding parts (the shape is usually a line or V-shaped groove), especially Large complex items requiring the use of multi-gate molds and inserts. This leads to a better molecular chain entanglement, which in turn creates a better weld. 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). Injection molded plastic parts have comparatively less strength in the weld line section and they also have an impact on product aesthetics. 6. First, an injection mold with triangular inserts was built to study the influence of meeting angles on material properties at the weld line. 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. Weld or knit lines occur when melt flow fronts collide in a mold cavity. Threadline gap in-between part due to improper gate location in complex design parts including excess of holes (multipoint. 1. EasyFill Advanced provides the weld-line temperature distribution and meeting angle prediction. Controlling the mold’s temperature could also minimize the weld line’s effects. and Mosle, H. Try to gradually raise the mold temperature and barrel temperature. 020 0. 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. They decrease the. To study the weld line formation of colliding flow fronts the filling of a mold cavity was simulated. enough to each other to avoid weld line issues. The spreading mold release agent on the part cavity surface might eventually cumulate at the V-notch groove of the weld line after each injection molding shot. 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]. One test mold was used to mold three different materials using heat/cool and conventional molding techniques. Weld Lines. The difference between a weld and meld line is simply determined by the angle at which the converging flow fronts meet. The pressurized air restricts the flow of the plastic resin. Weld lines can be a weak spot and have therefore to be taken into consideration in part design. It has larger cross-sectional regions that allow sufficient molten plastic flow into the cavity. 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. The most common and one of the most challenging injection molding defects are weld and knit lines. The weld line is then created with a higher flow front temperature and is packed with more pressure. To elimi- nate weld lines, you can enlarge shrinkage. Weld lines are common defects in injection-molded plastic parts that occur when two flow fronts of molten plastic meet and fuse together during the molding process. Mold vents should function the same way. Weld lines may appear like seams or scratches, but that is not what they are. Weld lines have been one of the common defects puzzling the injection molding industry. Mold temperature controlInjection mold designers can alter the position of weld line by changing the gate locating. Weld Lines 5. Figure 1 below shows the development of a weld line. Let’s review what causes meld and knit line imperfections, how they’re different, and how they affect part durability. Oil lines and reservoirs are typical examples for hollow composite parts which are conventionally produced using a multi-stage process. wall thickness, gate and runner designs and re-validate the result. The resulting defects don’t stop at flash only. Sink Marks Cause 1: Incorrect Melt Temperature. Weld lines, knit lines, and meld lines are terminologies to describe the imperfections of finished parts in plastic injection molding processing. During injection moulding, a weld-line occurs when two or more flow fronts collide in the hollow cavity. In this work, the residual strength of a fiber-reinforced injection molded part containing a hot weld line—or meld line—was evaluated. Materials Science. injection molding is followed by the welding. ). Plastics manufactured by injection molding are meant to be durable and reliable, able to hold up to heavy use, high temperatures, or adverse conditions. 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. Effects of cavity surface temperature on mechanical properties of specimens with and without a weld line in rapid heat cycle. Molding calculation in Moldex3D: (а) melt front and weld line, (b). 030 0. The weld line is an inevitable defect in the most injection molded components. View 08 DEFECTS & REMEDIES (61). The Injection Moulding Process is a manufacturing process used for producing parts or components by injecting molten material into the mold cavity. 1. About Press Copyright Contact us Creators Advertise Developers Terms Privacy Policy & Safety How YouTube works Test new features NFL Sunday Ticket Press Copyright. 020 0. Simulation of polymer molding filling process with an adaptive weld line capturing algorithm. Google Scholar [10] S. Weld lines can appear on the surface of a molded part where the molten material has converged after splitting off in a mold. You can expect a mold shrinkage percentage of. Despite injection molding being recognized as a robust process for obtaining parts with high geometry accuracy, one last occurrence remains a challenge in micro-injection molding, especially when junctions are. 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. The performance of weld lines in talc filled polypropylene box moldings produced with a. This study describes how the weld line was formed on the part. In other words, weld lines reduce the mechanical properties of the product. Weld lines are formed when two or more flow fronts join, as the liquid polymer fills the. As the name implies, it is usually positioned along the edge of the workpiece, forming a visible mark at the demarcating line. A new kind of weld line was. Weld Line defect analysis, the most common of many defects in injection moulding products. Weld Lines. Unless you are manufacturing an extremely simple part with one gate and a unidirectional flow front, your part will have weld lines. also utilized the co-injection molding technique to study the flow of polypropylene at the weld line behind an obstacle in injection molding; the. The 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. Plastic Injection Molding Process. Weld. Weld lines are a kind of Achilles heel of plastics. The weld-lines are undesirable from both aesthetic and functional points of view and their effect is different for. This study focus on the formation of. 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. analyzed the heat transfer during heating and cooling phases of RHCM process. There are various types of injection molding defects that can cause such interruptions in the manufacturing cycle. High-density PE was also moulded in the same conditions but in a lower temperature range (145 to 220°C). 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. 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. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. Figure 1a illustrates how the weld line is produced when two melt streams merge during the final filling process. This is expected to achieve maximum results while the pressure holding effect will also increase. Weld line. So today we are going to take a look at the five most common defects in injection molding and their solutions. Proper venting is also required to ensure good weld-line quality. If you are familiar with any form of scientific injection molding, regardless of the name, then you know one of the first steps is performing a viscosity curve study. In an injection molding process, a weld-line forms when two flow fronts meet each other. (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. 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. Quality concerns in injection-molded materials can vary fr. Here are a few of the most common defects that may occur in plastic injection molding — and how product teams can avoid them. The solution to an internal part cracking problem is to change the packing pressure. The differences between knit and meld lines are significant, and they can dramatically impact the part’s structural integrity. when two or more melt flow fronts contact each other immediately after the cooling process. Injection molding could be applied with many polymer materials, such as. , a post that forms a hole) and then rejoins, or when polymer melt fronts meet, from multiple injection points. 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 weldAmong defects of injection molding parts, weld lines are the most common defect. In a meld line, the plastic comes together at an angle greater than 135 degrees. In this study, the effects of injection temperature, injection pressure, and fiber cont. The plaques are prepared out of 30% glass-filled Polyamide 6 material. Injected plates were generated using a double-gated mold under four different process conditions. 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. Specifically, injection-mold lines are often associated with poor mechanical strength. Raising the mold temperature will keep the material molten longer, resulting in stronger weld lines and even elimination of weld lines. When the polymer resin is injected into a mold, it flows through all portions of the mold cavity. of micro injection molding process simulation by technology update and progress. Increase the size of the gate and runners to alter the mold design. Venting at Weld Lines. 18/06/2021. 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. 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. Increase the injection speed to reduce the amount of cooling happening in the mold. The Solutions for Poor Welding Lines. Remedies: Reduce packing and fill rates by adjusting until the part is properly filled with minimum stresses. 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 main types of injection molding defects are: 1. 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. Silver Streak. Weld lines can significantly reduce ultimate tensile strength (UTS) and fracture strain of talc filled polypropylene (PP). Weld lines defects are formed when two flow fronts are merged together. Weld lines, generated during injection molding, can significantly reduce the mechanical performance of structures. Weld lines are a common imperfection to be found in most plastic injected molded parts. 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, knit lines, and meld lines are terminologies to describe the imperfections of finished parts in plastic injection. The position, length, and angle of weld lines are dependent on the. The effects of fiber concentration has been studied in detail. + Increase mold temp, or materials’ temp, or holding pressure. 1. In this study the polycarbonate (PC) and carbon fiber (CF) are used as the experimental materials to investigate the impact of injection molding process parameters on tensile strength of weld line. 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. Weld Line Factors for Thermoplastics. [ 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. Weld Lines. Carefully consider all the relevant aspects of the mold design specification before. 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. Mostly, this happens when two or more flow fronts do not “knit” during the injection molding process. Or increase the number of gates. Some of these are listed below: Plastic warpage. Ideally, the resin flow will split into two flow fronts to pass around the obstruction and then recombine into a single flow front. (1) Increase the injection pressure and prolong the injection time. Boost time too long Shorten boost time, lower boost pressure, or move transfer to an earlier position . 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 molded plastic part may also appear to be rust-colored. Wang. T. A weld line can appear during the absence of enough pressure to push the plastic flow and then meld together. This weld line can act as a weak spot in the material. 4. To explore the formation of weld lines in injection molding, a. Adjust Injection Molding Parameters: Optimizing injection speed, pressure, and temperature can help ensure better melt flow and reduce the likelihood of weld line formation. The weld line’s strength is reduced when. Flow rate is 24 cm3/s. (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. 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. 025 100 120 140 160 180 200 220 240 Mold Temperature (F) LDPE PPAccording to the weld line distribution map of six gate schemes shown in . 2. As the name implies, it is usually positioned along the edge of the workpiece, forming a visible mark at the demarcating line. Here are some ways to avoid. 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. 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. 3 the path lines of selected material elements can be observed which are. Microsyst Technol, 14 (6) (2008), pp. 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. John Bozzelli. The effects of filling time, packing time, packing pressure, melt temperature, and mold temperature on the ultimate tensile strength (UTS) and the elongation value of the weld. 13. In a weld line, reinforcement fibers. (3) Adjust the temperature of the barrel and nozzle: the viscosity. Edge Gates. weld lines in infill analysis of injection molding simulation was observed [5]. 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. ac. injection molding. He now teaches courses in injection molding and plastic part design at the Polymers Center of Excellence and UNC-Charlotte in Charlotte, N. Flow-induced filler orientation and weld lines are un-avoidable facets of the injection molding process. Flashes, 3. Fig. This is expected to achieve maximum results while the pressure holding effect will also increase. The goal is to create a uniform thickness and at the smallest (thinnest) level required. (1)Injection Machine Poor plasticization and non-uniform melt temperature – appropriately extend the injection molding cycle, to ensure thorough plasticization, and change for a machine with a larger. In this paper, a kind of polypropylene (PP) foam plastic parts with special weld lines were obtained by core-back foam injection molding with chemical blowing agent. * Corresponding author, e-mail: fengsjpc@ku. Causes and Remedies of Sink Marks In Injection Molding. Known methods to reduce weld-line effects are the optimization of injection molding parameters, enhanced tool temperature, and using special molding tool devices. (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. Knit lines, weld lines, mold lines, or flow lines in injection molding are faint lines that become visible on certain plastic parts. Below is one of the examples: !NOTE: if the weld lines is important to the cosmetic surface of your part, please bring this up to your mold maker during quoting phase. A flow mark is commonly referred to as a flow line occurs during the process of 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. For more information on the Factors Affecting the Injection Molding Process. Weld line formation in the injection molding process occurs when two or more melt flow fronts contact each other immediately after the cooling process. 453-455. 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. According to the characteristics. 015 0. In injection molding, it is often important to maintain consistent color throughout the part. 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). Move injection locations to make weld lines form closer to the gates. Thus, it. The structure is more complex with longer molding cycle . Weld Line defect analysis, the most common of many defects in injection moulding products. See moreEquipment. 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. Weld-line strength for plastics can vary from 20% to nearly 100% of the strength of the plastic itself. Prediction of the weld lines in injection molding process using neural networks. Kobayashi et al. Weld-line, a common occurrence in plastic injection moulding, is a representative defect compromising mechanical qualities and product appearance [16]. Flow Lines. The more gate points there are, the more weld lines will occur. 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. 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. Root causes Many factors—including processing conditions, material, part design and mold design—can influence the visibility and strength of a weld line. As the polymer resin is injected into a mold, it flows through all parts of the mold. Mérillou, J. The relative weld-line strength is defined as the ratio of the strength of specimens with a weld line. A viscosity curve is developed using calculations of the fill time and plastic pressure at. This paper presents the. It is a zone with reduced mechanical properties negatively influencing on the molding behavior in exploitation. The weld line and the flow pattern of co-injection molding can be seen from Fig. Insufficient injection pressure and speed can cause weld lines. The results showed that the tensile strength of PE remains unchanged in the presence of weld lines. Reduce the amount of release agents. They appear on the surface of injection molded parts like lines. You can choose material of low viscosity and can increase the injection pressure. Strength measurements should be made to. This customary mark is caused by differential rates of cooling in the resin where. Injection Molding Know How. Weld line and warping are two critical defects for injection-moulded part. That’s the air escaping out of the nozzle until the hose is full of pressurized water. In. Adjust Injection Molding Parameters: Optimizing injection speed, pressure, and temperature can help ensure better melt flow and reduce the likelihood of weld line formation. A weld line (weld point, flow line or a knit line) forms when separate melt fronts travelling in opposite directions meet. Gate location should set to avoid weld lines, keep clean of the important position of the products,. Process. 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 are among the most critical structural defects in parts made by the injection molding of fiber-reinforced thermoplastics, explained Sirmax in a press release. Sink Marks Cause 4: High Mold Temperature. This special mould has changeable inserts to be able to inject with different gate types (standard, film, special-film, multi gates, etc. Sink Marks 2. investigated the impact of injection molding parameters on weld line formation and strength reduction. Here we are looking at most common part defect that can come from injection molding called weld line. A knit line may be unnoticeable or may look like a crack. 005 0. During the plastic injection molding process, one of the biggest challenges is. By the reflection method and a high speed camera, the visual system can be used to record and observe the forming process of micro weld lines during the. Recently, the injection molding of fiber reinforced thermoplastics (FRTP) containing short fibers has been widely carried out to mass-produce molded products with complex shapes. 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. They decrease the. 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. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. (1998) for clear photographs. 2. Ejector System (Pötsch, 1995) . If you adjust the ejection mechanism in the mold design, you also can reduce or eliminate your need for mold release agents. Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties. 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. Multiple gates are acceptable as long as they are close enough to each other to avoid weld line issues. Edge Gates. 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 varied materials, process conditions, loadings, forms of weld-line, etc. In order to prepare the weld line specimen by micro injection molding, a mould integrated with visual system and variotherm system was designed and constructed, shown as Fig. The Solutions for Poor Welding Lines. avoid weld lines, and facilitate smooth. In a weld line, reinforcement fibers adopt an orthogonal orientation with respect to the filling direction, which results in a significant reduction in the strength and. In this work, the residual strength of a fiber-reinforced injection molded part containing a hot weld line—or meld line—was evaluated. Degassing – Degassing is the simple operation of opening and closing an injection mold to allow any trapped air (or gas) to escape. They occur when melt flow fronts collide in a mold cavity. Insufficient plasticizing ability of injection molding machine. Sink Marks Cause 2: Low Pack and Hold time. Optimizing the injection parameters through combination of adjusting the molding temperature, varying the gate pressure, and increasing the dwell time. As the polymer. Weld lines typically appear in a part when an obstruction of some kind separates the melt flow and prevents it from "knitting" together. The weld-lines are undesirable from both aesthetic and functional points of view and their effect is different for varied materials, process conditions, loadings, forms. The most common method for the weld-line characterization is mechanical testing by means of a tensile or bend stage [6,7]. So today we are going to take a look at the five most common defects in injection molding and their solutions. Abstract. Shaofei Jiang. + Move gate (to where it is not visible). ppt from SCIENCE 101 at Symbiosis International University. 6. The rule is, a higher pressure shall be required to push the material in the machine when there is excess mold. A. Except for a few injection molded parts with very simple geometry, weld lines occur in most injection molded parts (usually with one line or V-shaped groove), especially in large-scale complex products requiring multi gate mold and inserts. Increase the injection speed. 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. Clamping Unit. In many cases, you will also see that the part is hanging up on the core during ejection. Flow lines are off-color lines, streaks, and other patterns that appear on the surface of a part. You can locate the gate at the thick part of the injection molding to prevent air pockets. Weld lines defects can also form when the flow fronts from two gates merge or when thick and thin sections in a part cause a flow to split and merge. Increase the injection speed and mold/operational temperature to prevent fracture or surface breakdown. The type of resin used during injection molding plays a large role in whether or not a knit line will be present. Injected plates were generated using a double-gated mold under four different process conditions. Our injection molding company is explaining what these imperfections are, why they can compromise. Depending on the resin, resin temperature, mold temperature, and filling speed, knit lines can vary. Thus, a right gate design raises polymer temperature to prevent the formation of weld lines and flow marks. Definition: A seam that appears where two areas of molten plastics meet. Polyamide-6 (PA-66) was used as. 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. 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. Gate blush, 4.