Weld lines may appear like seams or scratches, but that is not what they are. Let’s review what causes meld and knit line imperfections, how they’re different, and how they affect part durability. In our DFM report, we will analyze the possible weld lines. This line does not normally impact the overall shape or dimensions of the item. Here is a look at the differences between them, the causes of meld and knit line imperfections, and their impact on part durability. There are several factors that can contribute to the formation of weld lines, including: Part design: The design of the part can impact the flow of the material and the likelihood of weld lines forming. Adjust Injection Molding Parameters: Optimizing injection speed, pressure, and temperature can help ensure better melt flow and reduce the likelihood of weld line formation. 809-814. This failure causes the product to be damaged during use. 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. * 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). Except for a few very simple injection molded parts, they occur on most injection molded parts (typically in the form of a line or V-groove), especially large complex items that require use of multi-gate molds and inserts. 4. When injection molding large products, molding is usually performed with multi-point gates, but weld lines can occur depending on the product shape and gate position. In an injection molding process, a weld-line forms when two flow fronts meet each other. As the plastic flows through the mold it forms a smooth and continuous flow front. Conventional injection molding is widely used to produce plastic parts, mainly in the automotive industry, due to the high production ratios and quality of. Injection molded plastic parts have comparatively less strength in the weld line section and they also have an impact on product aesthetics. Flow marks in injection molding can be avoided if adequate measures are taken. When two or more strands of molten plastics meet in the mold, a weld line will be produced at the junction [1,2]. 10(a) (please see Fig. Although they often cannot be avoided it’s possible to minimize their cosmetic effects to an acceptable degree through careful product design and. Mold vents should function the same way. Plastic Injection Molding Process. Weld lines typically appear in a part when an obstruction of some kind separates the melt flow and prevents it from "knitting" together. 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. Low mold temperatures are usually employed to decrease cycle molding time and final process costs; however, resulting surface defects include weld lines, sink marks,. + Improve vent inside the mold. 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. The vents must be deep enough to allow the trapped air and gasses to escape but not deep enough to allow the plastic to flash. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part. The plaques are prepared out of 30% glass-filled Polyamide 6 material. When two or more strands of molten plastics meet in the mold, a weld line will be produced at the junction [1,2]. Sink Marks 2. Causes: Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic. Ideally, the resin flow will split into two flow fronts to pass around the obstruction and then recombine into a single flow front. 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. * Corresponding author, e-mail: fengsjpc@ku. Increasing the temperature of the mold or plastic material. For more information on the Factors Affecting the Injection Molding Process. Sink marks, 2. [ 18 - 21 ] In addition, the effects. Avient’s Zachary Alderman shares practical tips on designing and injection molding parts utilizing short-fiber reinforcements. Our injection molding company is explaining what these imperfections are, why they can compromise. Injection molds: (a) mold type A, (b) fixed plate of the mold type B, and (c) moving plate of mold type B The second mold (mo ld type B) was used for the ISO D2 specimen, as shown in Figure 2b,c . High-density PE was also moulded in the same conditions but in a lower temperature range (145 to 220°C). ”. This is expected to achieve maximum results while the pressure holding effect will also increase. This paper presents the. The appearance quality of plastic parts in weld lines can be improved significantly through the rapid heating cycle molding (RHCM). – Use generous and even. Weld lines are significant defects that affect injection molding quality, causing the poor appearance and low mechanical strength of injection‐molded parts. Weld lines appear on the molded part where molten plastic meets. of micro injection molding process simulation by technology update and progress. Known methods to reduce weld-line effects are the optimization of injection molding parameters, enhanced tool temperature, and using special molding tool devices. The weld line and the flow pattern of co-injection molding can be seen from Fig. 15 shows the frozen layer and weld lines at the end of mold filling. 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. Weld lines are formed when two or more flow fronts join, as the liquid polymer fills the. Adjust the design for the flow pattern to be a single source flow. 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. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. Place a vent in the weld line area to remove the entrapped air. 48 applied high-frequency induction heating to eliminate weld lines in an injection-molded plastic part, and experimental observation indicated that weld depth was reduced from 2. You can expect a mold shrinkage percentage of. with a characteristic length χ 0 =0. It has larger cross-sectional regions that allow sufficient molten plastic flow into the cavity. If the different flow fronts have cooled. ), with different mould surface finishes (polished, fine eroded, rough. Mold vents should function the same way. In this study, epoxy molding compounds (EMC) filled with glass beads were fabricated with a twin-screw-extruder (TSE) and tempered before further processing to generate different degrees of cross-linking (0–15%) in the compounds. 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. These lines can occur at the gate location, where the plastic enters the mold, or anywhere along the flow path. Fellahi et al. If you have critical requirements for strength of knit lines, please call a Protolabs applications engineer at 877-479-3680 to discuss, or email at [email protected]. The hair-like weld line is the. The schematic diagram of a weld line formation is illustrated in Figure 7, where is the welding angle of two melt fronts. 030 0. Meanwhile, Using more advanced cooling method such as cyclic cooling or conformal cooling. 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. When a product is produced through injection molding, a high-powered tool injects melted plastic into a custom, double-sided mold. 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. A weld line is often unavoidable. • Optimize the design of the runner system. Flashes, 3. The hair-like weld line is the result of weak. The melt stream is separated at the part’s cutout and reunited at the other end of the cutout. Expand. That’s the air escaping out of the nozzle until the hose is full of pressurized water. Adjust the position of the gate and include a venting slot. Weld lines in injection molding occur when two flow fronts meet and create a visible line on the surface of the molded part. These lines. The Injection Moulding Process is a manufacturing process used for producing parts or components by injecting molten material into the mold cavity. This leads to a better molecular chain entanglement, which in turn creates a better weld. 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. Thus, it. Simulation of polymer molding filling process with an adaptive weld line capturing algorithm. For example, filled resins and ABS are more likely to. A knit line in a plastic injection-molded part is created when two separate plastic flows meet within the mold and "resolidify" along their interface. In a weld line, reinforcement fibers. . The. The Factors that Cause Weld Line in Injection molding Pressure. Weld Lines. There are various types of injection molding defects that can cause such interruptions in the manufacturing cycle. Flow analysis of the weld line formation during injection mold filling of thermoplastics. A poor knit line can cause only cosmetic blemishes or it can significantly weaken the structural integrity of a part. The impact behavior of weld-lines in injection molding. Weld-line is a weak area which reduces the strength of the part locally. Kobayashi et al. 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. This study presents an innovative approach to enhancing weld line strength in advanced polymer injection molding through applying gas-assisted mold. Here are some ways to avoid. Proper venting is also required to ensure good weld-line quality. • Move injection locations to make flow fronts meet more obliquely, turning the weld line into a meld line. 8. It is normally at the meeting points of two flow fonts. The effects of fiber concentration has been studied in detail. (a) (b) Figure 1. According to the studies, the most important factor to reduce weld lines is increasing the mold temperature. Runner Type Venting Plastic melt and mold temperature Injection speed and pressure Learn how to troubleshoot weld lines in injection molding. John Bozzelli. 4. The special injection mold is designed and fabricated to produce plaques having stagnation weld-line. When two or more strands of molten plastics meet in the mold, a weld line will be produced at the junction [1,2]. The injection molding temperature and the mold temperature were fixed respectively at 230 °C and 50 °C. In injection molding, it is often important to maintain consistent color throughout the part. 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 lines have been one of the common defects puzzling the injection molding industry. Flash. 025 mm (0 . In this paper, a weld line factor (W-L factor) was adopted to. Weld lines are a kind of Achilles heel of plastics. 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. To explore the formation of weld lines in injection molding, a. EasyFill Advanced provides the weld-line temperature distribution and meeting angle prediction. 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. Injection Molding Know How. 2-1. The main types of injection molding defects are: 1. Steven. Polyamide-6 (PA-66) was used as. consideration in gate location. As the name implies, it is usually positioned along the edge of the workpiece, forming a visible mark at the demarcating line. Too much injection molding pressure lost and increase the operation difficulty of the injection molding machine; 7. Weld Lines. Weld line location. the meeting angle by optimizing variation over time in the cavity. 1. Weld lines can compromise the strength and aesthetics of the part, so it’s important to carefully design the mold. One test mold was used to mold three different materials using heat/cool and conventional molding techniques. The melt stream is separated at the part’s cutout and reunited at the other end of the cutout. These lines can occur due to variations in wall thickness, gate location, or the geometry of the part. 3. It can be determined by a visible line appearing on the surface of the plastic part where two flows of the plastic meet. “I realized weld -line problems are a very common issue for injection molded parts,” Nicholson told Plastics Technology. Weld-line, a common occurrence in plastic injection moulding, is a representative defect compromising mechanical qualities and product appearance [16]. Depending on the resin, resin temperature, mold temperature, and filling speed, knit lines can vary. They appear on the surface of injection molded parts like lines. The rule is, a higher pressure shall be required to push the material in the machine when there is excess mold. 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. 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. In injection molding, a V-notch-shaped weld line is generated in the melt front meeting area. Sink marks. Melt temperature does effect the weld. They are unavoidable even for moderate complex products. the three-plate mold often adopt this kind of gate. Still, it can be seen to varied degrees depending on mold. Weld Line defect analysis, the most common of many defects in injection moulding products. Thus, it is important to understand (i). gasket seal leakage where the weld line crosses a gasket. 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. 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. Weld lines are visually undesirable and a plastic part is structurally weak at weld line positions. In this lesson we will focus on the meld and weld lines. Gate blush, 4. ppt from SCIENCE 101 at Symbiosis International University. The second technique, cascade injection moulding, can produce parts free from weld lines. (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. Proceedings of ANTEC (2000) Google Scholar [9] S. Weld lines cause significant reject rates and are a common problem that all molders face. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. 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. When the same pressure is maintained or lower than that, the drop in speed shall create the weld lines. The parts are molded flat and are slightly stressed once installed in an arced position, but not excessively. Vacuum voids. Weld lines are weak regions in thermoplastic injection moldings, due to low molecular entanglement and unfavorable orientation. Figure 1 below shows the development of a weld line. Boost time too long Shorten boost time, lower boost pressure, or move transfer to an earlier position . M. It forces out trapped air pockets and can prevent a vacuum void from occurring. The authors [5, 6] attempted to reduce the weld lines and clamping force of plastic injection moulding through numerical and experimental investigation, and discovered that the 3D cooling channel. This occurs mainly in the presence of barriers that divide the melt flow in. Ghazanfarpour. 2017-01-0481. 3. 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). Insufficient plasticizing ability of injection molding machine. e. In this paper, a weld line factor (W-L factor) was adopted to describe the. wall thickness, gate and runner designs and re-validate the result. a) Multi-fiber connector micro injection molding simulation b) 3D simulation of micro gear with 150µm diameter for weld line position prediction c) 3D micro injection molding filling simulation of micro gear with 180µm diameter Fig. The gate location should enable the two flow fronts to merge easily and continue to flow beyond the weld line. 2021年4月23日 / by admin. The thin weld line is the result of weak material bonding, which lowers the strength of the part. 15 it can be found that for the opposite position of melt injection the weld lines forming under higher temperature would produce better plastics. Proceedings of ANTEC (2000), pp. The need to minimize fiber breakage, plus the higher viscosity of materials with. This reduced thickness of the frozen skin further enhances the visual appeal of the. 22 shows how the weld lines are formed when the melt splits around core pins during the injection molding process. การแก้ไข ชิ้นงานจุดที่เกิดความอ่อนแอบริเวณ weld line คือต้องใช้ความเร็วในการฉีดต่ำในจุด. Weld Line defect analysis, the most common of many defects in injection moulding products. It is a well-known fact that weldlines are unavoidable in most injection-molded products of even moderate complexity. 9 – Over Packing. Weld lines create visible marks, but this type of injection molding defect can also reduce a part’s strength and durability. He now teaches courses in injection molding and plastic part design at the Polymers Center of Excellence and UNC-Charlotte in Charlotte, N. Weld-Line in an injection molded part develops when two or more melt fronts are converged together. Dischler, D. Optimizing the Injection Parameters. Complexities in the design and intricate pathways in the mold can also create flash in injection molded parts. 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. When an external load is applied to a weld line region, the part’s potential for mechanical failure is increased. [1-2] investigated the weld line zone of injection molded glass-fiber-filled polypropylene (PP) composites. GlossFlow-induced fiber orientation and weld lines are com-mon features of the injection molding process. In this experiment, several ejector. That helps. What are Weld Line Defects? Weld line defects – also known as knit lines – is a defect that occurs when the injected plastic meets within a mold. Lesson 6: Meld and Weld Lines (Processing) When plastic flows into an injection mold, the internal flow pattern depends on the part design, mold design, and processing factors. Weld line formation in the injection molding process occurs when two or more melt flow fronts contact each other immediately after the cooling process. Gates should be designed larger than normal and located to minimize weld line formation. That helps. G. This is where the melt fronts divided by an insert, such as a pin or a. 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. The specific method is that when the product is just filled and filled in the cavity, the insert. 453-455. If the different flow fronts have cooled. 943 µm to 0. As a result, the path of the injected melt can be extended, allowing the production of parts with more complex shapes and greater length. 2(a, b). For multiple gate and complex part, molding weld-lines are unavoidable, therefore mechanical behavior of the weld-line needs to be predicted. 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. 6. ) . Weld-line is a defect that arises in injection molding when two or more flow fronts collide. Published 5 July 2019. A weld line can also be called a knit line. Double push filling method. Weld lines are a common occurrence in injection molding that affects the overall quality and strength of a finished product. Sink Marks Cause 3: Improper Pack or Hold Pressure. Consequently, it causes viscous dissipation which leads to the generation of heat. Edge Gates. 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. Weld-line is a weak area which reduces the strength of the part locally. Which means engineers need to know how to account for this phenomenon in an efficient way. The more gate points there are, the more weld lines will occur. Weld lines create visible marks, but this type of injection molding defect can also reduce a part’s strength and durability. weld lines in injection molded thermoplastic composites. These are preferred over weld lines as they will normally result in improved cosmetics and structure. Poor mold spot Spot mold to ensure fit and vent depths at ~0 . If you have critical requirements for strength of knit lines, please call a Protolabs applications engineer at 877-479-3680 to discuss, or email at [email protected]. To explore the formation of weld lines in injection molding, a. Weld Lines. in the cavity of the injection mold. What is weld lines. The wheel has to be gated at three places on the hub and. 10. Injected plates were generated using a double-gated mold under four different process conditions. The Solutions for Poor Welding Lines. 1. Degassing – Degassing is the simple operation of opening and closing an injection mold to allow any trapped air (or gas) to escape. Emerging Materials Research. 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. 2) Vents in an injection mold should let the air out, but not the liquid plastic. Effects of mold temperature and pressure on shrinkage 0. Weld lines have been one of the common defects puzzling the injection molding industry. It is a zone with reduced mechanical properties negatively influencing on the molding behavior in exploitation. Weld line. Weld lines occur when melt flow fronts meet during the injection molding of plastic parts. A meld line occurs when two moving melt fronts converge and flow parallel to each other. Quality concerns in injection-molded materials can vary fr. The edge gate is the most popular and straightforward injection mold gate design. Weld Lines are formed when two or more molten polymer flows meet during the injection molding process. As these flow fronts come together. Key words: weld line strength, thermoplastic composites, injection molding, wall thickness, skin and core layers Department of Materials Engineering, Faculty of Engineering, Kasetsart University, Bangkok 10900, Thailand. Wang et al. Weld lines are formed when two orThe experiments were performed on an Arburg Allrounder 320C 600-250 injection moulding machine using a two cavity-injection mould (Fig. 6. Weld line problem which occurs in polymer parts produced by plastic injection method and affects appearance and strength on a large scale was researched in this paper. Many researchers had emerged with several. 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. 1. 015 0. Weld line (also known as a knit line) is the line where two flow fronts meet when there is the inability of two or more flow fronts to “knit” together, or “weld,” during the molding process. Co-injection molding technique is a useful way to study weld lines, which can offer more detailed information of the. Specifically, injection-mold lines are often associated with poor mechanical strength. (1998) for clear photographs. Tosello et al. Thicker parts require more cooling time and can lead to air becoming trapped in the mold and weld / knit lines to form. 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. A flow mark is commonly referred to as a flow line occurs during the process of injection molding. Weld lines form where two or more flow fronts come together and can create a cosmetic or structural problem. 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. Sink marks are a common type of injection molding defect. 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. (3-1) Increasing the Resin Temperature at the Weld Line Section. Weld Lines-3 Ways for Detecting and Correcting. Molding Equipment. For multiple gate and complex part, molding weld-lines are unavoidable, therefore mechanical behavior of the weld-line needs to be predicted. If the injection gate is not properly designed, it can cause the formation of defects like sink-mark, warpage, short shot, weld-line, air traps, and other faults [15]. Sink MarksWeld lines, also known as knit lines, are a common defect in injection molding where two or more flow fronts of molten plastic meet but don’t properly fuse together. Weld lines. Silver Streak. In an injection molding process, a weld-line forms when two flow fronts meet each other. Weld or Knit lines are a plane or section in an injection molding part where the flow of molten plastics meets from different directions. Weld Lines. Knit and meld lines are inherent to plastic. Increase the injection speed and mold/operational temperature to prevent fracture or surface breakdown. Injection molding plate for mode I testing geometry. 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 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. 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. Short shots can occur in this case because the air in the mold cavity has nowhere to go while the plastic is entering. investigated the impact of injection molding parameters on weld line formation and strength reduction. 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. Its appearance can be improved by adjusting parameters. 1. Thus, a right gate design raises polymer temperature to prevent the formation of weld lines and flow marks. A. 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. Figure 2. In the first, sequential injection moulding, the lines are displaced to noncritical areas of the moulding. , a post that forms a hole) and then rejoins, or when polymer melt fronts meet, from multiple injection points. 8. The position, length, and angle of weld lines are dependent on the. Google Scholar [10] S. They decrease the. The relative weld-line strength is defined as the ratio of the strength of specimens with a weld line. Causes: Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic. Knit lines, weld lines, mold lines, or flow lines in injection molding are faint lines that become visible on certain plastic parts. In. 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. Wang, G. In addition a weld line can be moved to an area where it is less visible. Typical injection molding processing temperatures of TPU materials are as follows: IM ZONE 60A TO 75A 80A TO 90A 95A TO 75D REAR 315 – 340 °F 345 – 370 °F 375 – 400 °FTo study the weld line formation of colliding flow fronts the filling of a mold cavity was simulated. Controlling the mold’s temperature could also minimize the weld line’s effects. Although they often cannot be avoided it’s possible to minimize their cosmetic effects to an acceptable degree through careful product design and manufacturing. 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. injection molding. The weld line’s strength is reduced when. Islyn Thomas summarized the need for a particular vent location better than I ever could. Wu CY, Ku CC, Pai HY (2011) Injection molding optimization with weld line design constraint using distributed multi-population genetic algorithm. S. 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. When molten plastic flows, it forms a smooth continuous flow until it meets an obstruction. So today we are going to take a look at the five most common defects in injection molding and their solutions. 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. The solution to an internal part cracking problem is to change the packing pressure. A flat product case was considered as an example of a typical aircraft structural element. (3-1) Increasing the Resin Temperature at the Weld Line Section. Heat/cool injection molding has a positive influence on the depth and visibility of weld lines. The calculation was carried out on a mesh with solid elements obtained in ANSYS Meshing and transferred to Moldex3D by means Rhinoceros. Park et al. Figure 1a illustrates how the weld line is produced when two melt streams merge during the final filling process. Some of these are listed below: Plastic warpage. Therefore, the. Knit lines, weld lines, mold lines, or flow lines in injection molding are faint lines that become visible on certain plastic parts. g. ” Another. Burn Marks.