This paper presents the. The Factors that Cause Weld Line in Injection molding Pressure. 2021年4月23日 / by admin. 4 at different time steps (legend scale is volume fraction of. with a characteristic length χ 0 =0. 001 in . 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. Weld lines are a kind of Achilles heel of plastics. 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. Thus, it. Sink Marks Cause 5: Improper Part Geometry. the three-plate mold often adopt this kind of gate. Gates should be designed larger than normal and located to minimize weld line formation. Wavy lines are another feature of weld lines. This is where the melt fronts divided by an insert, such as a pin or a. The hair-like weld line is the. Adjust the position of the gate and include a venting slot. Weld line strength is important in blow molding as well as injection molding. 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]. 020 0. 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 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. , 2 (3–4) (1988. Flow-induced filler orientation manifests in higher tensile prop-erties in the direction of flow compared to those normal to the direction of flow. injection molding, (b) weld line and meld line, and (c) injection sample size and shape. Degassing – Degassing is the simple operation of opening and closing an injection mold to allow any trapped air (or gas) to escape. INJECTION MOULDING DEFECTS 1 DEFECTS 1. Article Google Scholar Deng YM, Zheng D, Lu XJ (2008) Injection moulding optimization of multi-class design variables using a PSO algorithm. Recently hired at plastic injection molding simulation software provider Moldex3D, Nicholson was spurred to undertake the research study following a conversation with a customer experiencing weld-line woes. As the plastic flows through the mold it forms a smooth and continuous flow front. 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. Increase the injection speed to reduce the amount of cooling happening in the mold. 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]. As a result this weld line will be a weak point in this part. 8. 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. Criens, R. Proceedings of ANTEC (2000) Google Scholar [9] S. 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 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. Flow front tracing simulation results for weld line developing process in micro injection molding with Comsol ® Multiphysics 3. Sink Marks Cause 1: Incorrect Melt Temperature. Introduction. The flow fronts at different times (Fig. 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. It is not always easy to completely eliminate weld lines by simply adjusting the relevant injection mold design or the molding conditions. 18/06/2021. 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 line formation in the injection molding process occurs. [1-2] investigated the weld line zone of injection molded glass-fiber-filled polypropylene (PP) composites. Weld lines are one of the typical quality issues of injection molded parts. The injection molding temperature and the mold temperature were fixed respectively at 230 °C and 50 °C. Adjust the design for the flow pattern to be a single source flow. The weld line is then created with a higher flow front temperature and is packed with more pressure. Here are a few of the most common defects that may occur in plastic injection molding — and how product teams can avoid them. 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. 1. Vents in the mold allow the mold to fill. When two or more strands of molten plastics meet in the mold, a weld line will be produced at the junction [1,2]. Increase the injection speed. Email him at jhenz. 2) Vents in an injection mold should let the air out, but not the liquid plastic. Weld-line yield strength, yield strain, and failure strain of talc-filled polypropylene increase with increasing melt temperature and increasing hold pressure. Bubbles are caused by melting flow fronts colliding in a mold cavity. Participating in DFM and mold flow analysis is a good way to achieve this. Thus, it is important to understand (i). “I realized weld -line problems are a very common issue for injection molded parts,” Nicholson told Plastics Technology. Knit and meld lines are inherent to plastic. also utilized the co-injection molding technique to study the flow of polypropylene at the weld line behind an obstacle in injection molding; the weld line can be seen clearly from the Fig. Weld lines can affect both the aesthetics and mechanical properties of the product, making them a significant concern for. When a product is produced through injection molding, a high-powered tool injects melted plastic into a custom, double-sided mold. Optimizing the Injection Parameters. (1998) for clear photographs. Meld lines are formed when two separate flow fronts meets. The weld line in injection molding is created when two melt streams connect, as shown in Figure 1. Weld line formations can be weak if they fall in areas that will bear stress or impact. The choosing optimal gate location technique has been developed to set weld lines as far as possible from the region of maximum stresses. Weld lines in injection molding are frequently unavoidable. (1983). Computer simulation packages of injection molding have had success in predicting filling behavior in extremely complicated geometries, capable of predicting weld lines’ location exactly. 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. A weld line region is typically filled at the end of an injection stroke or during the pressure phase. Discoloration. Conclusion The formation of weld lines in micro injection molding of plastic depends on different parameters such as melt temperature, injection pressure and fabrication method. In this 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 line location. Sink marks are a common type of injection molding defect. Weld lines appear as a result of the flow of molten plastic during the injection molding process. Thus, it is important to understand (i) if the melt is able to fill the cavity up to the weld line location, (ii) where the weld line forms, and (iii) if the weld line is a cold or flowing one. 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. Jiquan Li, Taidong Li, +1 author. Melt temperature does effect the weld. They are the result of fundamental physical effects taking place inside of the mold tool cavity. A weld or meld line is a weakness or visible flaw created when two or more flow paths meet during the filling process. 2020-01-1306. An excess mold release is a major cause of weld line due to two variables: high pressure and speed. 010 0. If so, look for an alternative resin. In the sense that the weld line is better formed before the resin. The schematic diagram of a weld line formation is illustrated in Figure 7, where is the welding angle of two melt fronts. Burn Marks. 6. Save. You can choose material of low viscosity and can increase the injection pressure. Figure 1a illustrates how the weld line is produced when two melt streams merge during the final filling process. Injected plates were generated using a double-gated mold under four different process conditions. Weld lines form where two or more flow fronts come together and can create a cosmetic or structural problem. Prediction of the weld lines in injection molding process using neural networks. What is weld lines. The next type of injection molding defect is ‘Weld Lines’. For multiple gate and complex part. This is expected to achieve maximum results while the pressure holding effect will also increase. (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. When it comes to injection molding, the chances you might end up making a costly mistake is too high. Increase the injection speed. 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) properties were measured, and statistical analyses were performed to gain insight on the correlation between melt flow rate and weld-line behavior. During injection. Injected plates were generated using a double-gated mold under four different process conditions. The specific method is that when the product is just filled and filled in the cavity, the insert. This paper presents the. Amongst weld line. Failure may result due to the assembly torque, or end-use application loads (temperature, creep). 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. Heating and cooling mould systems, which cost extremely high, are a typical technology to prevent the weld-line. Weld lines are a common occurrence in injection molding that affects the overall quality and strength of a finished product. The weld line degrades the. The injection molding gate generates heat by dissipation. There is excess air inside the mold (1) make the mold vent larger at location with weld line; (2) Check if the mold vent is blocked or not or use vacuumized forming. Weld lines in injection molding are frequently unavoidable. In. If the different flow fronts have cooled. Weld lines cause significant reject rates and are a common problem that all molders face. Depending on the resin, resin temperature, mold temperature, and filling speed, knit lines can vary. micro injection molded weld line’s mechanical properties. For multiple gate and complex part, molding weld-lines are unavoidable, therefore mechanical behavior of the weld-line needs to be predicted. 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. Sink Marks Cause 2: Low Pack and Hold time. They are unavoidable even for moderate complex products. It is a well-known fact that weldlines are unavoidable in most injection-molded products of even moderate complexity. 5. 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. 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. When two or more strands of molten plastics meet in the mold, a weld line will be produced at the junction [1,2]. In this work, the residual strength of a fiber-reinforced injection molded part containing a hot weld line—or meld line—was evaluated. Sink marks. In the 1947 book, Injection Molding of Plastics, the late Dr. Fracture Toughness of Weld Lines in Thermoplastic Injection Molding, Polymer Engineering and Science, 33(15): 996-1001. 010 0. Blistering 4. High-density PE was also moulded in the same conditions but in a lower temperature range (145 to 220°C). The holding pressure was set as approximately equal to the maximum injection pressure obtained at the time of filling the resin. Oil lines and reservoirs are typical examples for hollow composite parts which are conventionally produced using a multi-stage process. Weld lines can be a weak spot and have therefore to be taken into consideration in part design. Weld Lines. Neat polypropylene retains nearly 90% of its strength on a properly designed and molded weld line. Sink marks, 2. <Materials>Injection moulding (U. The three parameters affecting operation were called injection pressure, injection time, and packing pressure which influence the quality of weld line. Weak weld lines are perhaps the most common and difficult injection molding defect to eliminate. The thermo-rheological findings were used to investigate the sources of weld line weakness. 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. Using the conventional method, weld-line depths on the surface ranged from 6 to 13 microns. Weld lines are a surface defect caused when two or more flow fronts come together during the injection process. Let’s review what causes meld and knit line imperfections, how they’re different, and how they affect part durability. Insufficient injection pressure and speed can cause weld lines. Insufficient plasticizing ability of injection molding machine. In narrow fields, a limited amount of molten liquid can solidify and block the flow. Weld lines may appear like seams or scratches, but that is not what they are. Part design features may cause a material flow to split and rejoin as it flows around design features in a mold. 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. Mold temperature controlInjection mold designers can alter the position of weld line by changing the gate locating. Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties. Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties [3,4,5]. Vibration assisted injection molding. <Mold> Provide a cold slug well where weld lines occur. Controlling the mold’s temperature could also minimize the weld line’s effects. 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. (1) Increase the injection pressure and prolong the injection time. Controlling the mold’s temperature could also minimize the weld line’s effects. The weld line can either be a minor cosmetic issue or potentially bad enough to negatively impact the part's strength. Poor mold spot Spot mold to ensure fit and vent depths at ~0 . 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. Runner Type Venting Plastic melt and mold temperature Injection speed and pressure Learn how to troubleshoot weld lines in injection molding. 005 0. Flow Lines and Weld Lines. The structure is more complex with longer molding cycle . In spite of its cosmetic attributes, a bad knit line can significantly compromise a part. How Are They Formed? Molten plastic resin is injected into a mold cavity through an opening known as a gate. One solution is to prevent the weld lines from forming in regions that are. Tosello et al. But none of the current packages can predict weld lines’ properties qualitatively. Better weld line quality was obtained by Huang. In this paper, aiming at the problem of weld line defects in the injec- tion process of a key phone panel, the injection process is optimized by using Moldflow and drop simulation. 453-455. Low mold temperatures are usually employed to decrease cycle molding time and final process costs; however, resulting surface defects include weld lines, sink marks,. G. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. 10 in Nguyen-Chung et al. 2017-01-0481. 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. Founder, Injection Molding Solutions. Investigations on WLs in micro injection molded parts were performed in Refs. 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. 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. 5. (2) Adjust the injection speed: high speed allows the melt to reach the confluence without cooling down, and low speed allows time for the air in the cavity to be discharged. This principle has been implemented on a complex component in the case of the spring-loaded cogwheel shown in Figure 5, for instance. A visible weld line is observed, due to a combina-Cracking is commonly seen at the weld line area but sometimes on the entire surface. Weld or Knit lines are a plane or section in an injection molding part where the flow of molten plastics meets from different directions. These lines occur when two or more flow fronts meet and fuse together, resulting in a visible seam on the molded part. A viscosity curve is developed using calculations of the fill time and plastic pressure at. The weld lines that occur in injection mouldings are critical areas on which depends on the strength of the mouldings. The rule is, a higher pressure shall be required to push the material in the machine when there is excess mold. EasyFill Advanced provides the weld-line temperature distribution and meeting angle prediction. 13. This study focus on the formation of. 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. 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. Quality concerns in injection-molded materials can vary fr. 1. Edge Gates. 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. The plastic injection molding process, including thermoplastic injection, is nowadays used in several industrial processes, including in the automotive industry for bowden cables. Molding calculation in Moldex3D: (а) melt front and weld line, (b). Solving one problem can introduce other problems to the injection molding process. 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. A weld line is often unavoidable. Flow lines. Clamping Unit. This special mould has changeable inserts to be able to inject with different gate types (standard, film, special-film, multi gates, etc. Flow rate is 24 cm3/s. Weld lines not only detract from an injection-molded part’s surface quality, but also significantly reduce its mechanical strength. 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. 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. Still, it can be seen to varied degrees depending on mold. This customary mark is caused by differential rates of cooling in the resin where. Or increase the number of gates. The gate location should enable the two flow fronts to merge easily and continue to flow beyond the weld line. 000 0. . Published 5 July 2019. As a result, the portion has a faint, perhaps undetectable line. Weld line section presents notch or gloss difference. the meeting angle by optimizing variation over time in the cavity. Injection molding weld lines where two or more flow fronts come together in the mold. There are various types of injection molding defects that can cause such interruptions in the manufacturing cycle. when two or more melt flow fronts contact each other immediately after the cooling process. 5. Short shots can occur in this case because the air in the mold cavity has nowhere to go while the plastic is entering. Mold Design Most conventional mold designs will work with TPU’s. Weld lines can be caused by many reasons, which covers the various aspects of mold, process, raw materials and product design, etc. As compared to a weld line, the flow fronts come together at less than 135 degrees. 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. He now teaches courses in injection molding and plastic part design at the Polymers Center of Excellence and UNC-Charlotte in Charlotte, N. Voids and bubbles, 6. When the two such melt flows. They are unavoidable even for moderate complex products. Proceedings of ANTEC (2000), pp. Here is a look at the differences between them, the causes of meld and knit line imperfections, and their impact on part durability. This paper presents the. Weld lines are significant defects that affect injection molding quality, causing the poor appearance and low mechanical strength of injection‐molded parts. Int. Process. th INTRODUCTION Injection molding is one of the mostWeld Lines in a plastic cover. 3: Weld Lines. 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. 21: Weld line failure caused by hoop stresses. Thicker parts require more cooling time and can lead to air becoming trapped in the mold and weld / knit lines to form. In an injection molding process, a weld-line forms when two flow fronts meet each other. In injection molding weld lines are formed whenever two flow fronts are joined. 3. 10(a) (please see Fig. 3 the path lines of selected material elements can be observed which are. Weld lines create visible marks, but this type of injection molding defect can also reduce a part’s strength and durability. wall thickness, gate and runner designs and re-validate the result. distribution [4]. 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]. Weld lines can significantly reduce ultimate tensile strength (UTS) and fracture strain of talc filled polypropylene (PP). These are caused by the shot of molten plastic moving at different speeds throughout the injection. As the polymer. 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. 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. Weld lines form where two or more flow fronts come together and can create a cosmetic or structural problem. The hair-like weld line is the result of weak. Figure 1 below shows the development of a weld line. To study the weld line formation of colliding flow fronts the filling of a mold cavity was simulated. The goal is to create a uniform thickness and at the smallest (thinnest) level required. Weld Line Factors for Thermoplastics. The need to minimize fiber breakage, plus the higher viscosity of materials with. Weld lines may appear like seams or scratches, but that is not what they are. weld lines in injection molded thermoplastic composites. Islyn Thomas summarized the need for a particular vent location better than I ever could. In the sense that the weld line is better formed before the resin temperature. Weld Lines. While there are many situations where they are barely perceptible, weldlines represent a potential source of weakness in molded parts. It has larger cross-sectional regions that allow sufficient molten plastic flow into the cavity. These lines usually appear on the narrowest portion of the part, and appear to originate from the mold’s gates, where the molten resin flows into the mold to form. Increase the injection speed. 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. 1. Download : Download high-res image (856KB) Download : Download full-size image; Fig. Change the flow pattern to a single-source flow. Weld-line is a weak area which reduces the strength of the part locally. 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. Injection molded plastic parts have comparatively less strength in the weld line section and they also have an impact on product aesthetics. Weld lines have been one of the common defects puzzling the injection molding industry. My polymer material selection courses 👉 Find Out About Plastics Blog👉 ht. 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. The parts are molded flat and are slightly stressed once installed in an arced position, but not excessively. Weld lines, knit lines, and meld lines are terminologies to describe the imperfections of finished parts in plastic injection. Weld Lines are formed when two or more molten polymer flows meet during the injection molding process. The appearance quality of plastic parts in weld lines can be improved significantly through the rapid heating cycle molding (RHCM). The weld-lines are undesirable from both aesthetic and functional points of view and their effect is different for varied materials, process conditions, loadings, forms. In 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. Some of these are listed below: Plastic warpage. Alter the mold design. The line where two resin flows “knit” or “weld” together during injection molding is called a weld or knit line. It is possible to pre. Experimental Setup 2. การแก้ไข ชิ้นงานจุดที่เกิดความอ่อนแอบริเวณ weld line คือต้องใช้ความเร็วในการฉีดต่ำในจุด. 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. th INTRODUCTION Injection molding is one of the most Weld Lines in a plastic cover. The relative weld-line strength is defined as the ratio of the strength of specimens with a weld line. M. 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). [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. ” Another. A meld line occurs when two moving melt fronts converge and flow parallel to each other. Besides, possibly it would be. Thus, it is important to understand (i). As a result, the path of the injected melt can be extended, allowing the production of parts with more complex shapes and greater length. In this regard, the plasticizing ability of the plastic injection moulding machine should be checked. 3. Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties [3,4,5]. This month, in our FREE SimTech™ series of webinars, we will explore weld lines and how they develop. 298 µm and the weld line on the heated region was almost eliminated with the aid of induction heating. 1 Mold In order to study the formation of weld lines, the mold cavities has been. Weld line and warping are two critical defects for injection-moulded part. (3-1) Increasing the Resin Temperature at the Weld Line Section. The calculation was carried out on a mesh with solid elements obtained in ANSYS Meshing and transferred to Moldex3D by means Rhinoceros. Flow Lines. It is important to investigate the weld line because the weld line area can induce potential failure of structural application. A weld line region is typically filled at the end of an injection stroke or during the pressure phase. Specifically, injection-mold lines are often associated with poor mechanical strength. 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. Weld-Line in an injection molded part develops when two or more melt fronts are converged together. 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 quantitative evaluation of the appearance quality of weld lines in RHCM parts. 2. 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. 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. In an injection molding process, a weld-line forms when two flow fronts meet each other. 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. If you adjust the ejection mechanism in the mold design, you also can reduce or eliminate your need for mold release 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 existence reduces the material strength; however, its demerit characteristics. 10. Dischler, D. The most common and one of the most challenging injection molding defects are weld and knit lines. Sink Marks Cause 4: High Mold Temperature. [ 18 - 21 ] In addition, the effects. Weld lines typically appear in a part when an obstruction of some kind separates the melt flow and prevents it from "knitting" together. 2) Vents in an injection mold should let the air out, but not the liquid plastic.