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. 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. Meld and knit lines are actually both different types of weld lines. 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). Figure 2. In a weld line, reinforcement fibers. 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. Another relevant topic in injection molding is the presence of weld lines (WLs) and their properties, which could lead to part weakness through surface disturbances, optical issues and reduced bonding [21, 22]. Int J Adv Manuf Technol 52:131–141. weld lines in infill analysis of injection molding simulation was observed [5]. 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. Meld lines are formed when two separate flow fronts meets. Knit lines, weld lines, mold lines, or flow lines in injection molding are faint lines that become visible on certain plastic parts. Weld lines represent the region where two separated melt fronts recombine. Plastics manufactured by injection molding are meant to be durable and reliable, able to hold up to heavy use, high temperatures, or adverse conditions. Weld lines have been one of the common defects puzzling the injection molding industry. 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. 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 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. This failure causes the product to be damaged during use. You can locate the gate at the thick part of the injection molding to prevent air pockets. Weld or Knit lines are a plane or section in an injection molding part where the flow of molten plastics meets from different directions. 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. In some cases, a weld line can also cause a loss of strength and impact resistance. In this experiment, several ejector. S. When the two such melt flows. 025 100 120 140 160 180 200 220 240 Mold Temperature (F) LDPE PPThree terms that often get mixed up in the plastic injection molding industry are weld, meld, and knit lines. Redesign the mold to prevent the weld line. Bubbles are caused by melting flow fronts colliding in a mold cavity. 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. The gate vestige for injection molding is usually narrow. 1. 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. Thus, a right gate design raises polymer temperature to prevent the formation of weld lines and flow marks. (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. ac. The results of. It is possible to pre. Weld lines create visible marks, but this type of injection molding defect can also reduce a part’s strength and durability. Short shots can occur in this case because the air in the mold cavity has nowhere to go while the plastic is entering. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. injection molding. + Move gate (to where it is not visible). This occurs either due to injection through multiple gates or as a conse-รอยประสานของเนื้อพลาสติก Weld line ตอนที่1. Failure may result due to the assembly torque, or end-use application loads (temperature, creep). 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. 5. This principle has been implemented on a complex component in the case of the spring-loaded cogwheel shown in Figure 5, for instance. 2021年4月23日 / by admin. Here are some strategies. The performance of weld lines in talc filled polypropylene box moldings produced with a. Weld line defects – also known as knit lines – is a defect that occurs when the injected plastic meets within a mold. They found that the weld line is a zone between 2 and 8 mm wide extending throughout the thickness in whichEffects of mold temperature and pressure on shrinkage 0. distribution [4]. 2) Vents in an injection mold should let the air out, but not the liquid plastic. This line does not normally impact the overall shape or dimensions of the item. Figure 1 a illustrates how the weld line is produced when two melt streams merge during the final filling process. Consultant @ Effinno Technologies, BSME, MBA - Bridge between Plastics Injection Molding Knowledge and Practice. The welding angle can be used to differentiate weld lines and meld lines. 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. A viscosity curve is developed using calculations of the fill time and plastic pressure at. 18/06/2021. 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. Ideally, the resin flow will split into two flow fronts to pass around the obstruction and then recombine into a single flow front. This customary mark is caused by differential rates of cooling in the resin where. Molding calculation in Moldex3D: (а) melt front and weld line, (b). Int. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and. Move injection locations to make weld lines form closer to the gates. The thermo. (1983). This process makes possible to produce a wide range of products with different sizes, shapes, and for varying applications. Flow analysis of the weld line formation during injection mold filling of thermoplastics. (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. 000 0. Part design features may cause a material flow to split and rejoin as it flows around design features in a mold. Knit lines are hard to eliminate for injection molded plastic parts but. In the above diagram, the converging flow fronts (indicated by red arrows. The weld line in injection molding is created when two melt streams connect, as shown in Figure 1. Thus, it is important to understand (i). 18/06/2021. Three terms that often get mixed up in injection molding are weld, meld and knit lines. Steven. The main runner and sub-runner are too thin. * 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). There are various types of injection molding defects that can cause such interruptions in the manufacturing cycle. Weld line existence reduces the material strength; however, its demerit characteristics. According to the special features of micro injection molding process compared with normal injection molding process, the main related factors to weld line’s properties were classified as 3 group, named as processing conditions, mold design and structure, external process. Zhao, X. Redesign the mold to prevent the weld line. The need to minimize fiber breakage, plus the higher viscosity of materials with. Our injection molding company is explaining what these imperfections are, why they can compromise. The weld line’s strength is reduced when. Weld lines are formed when two or more flow fronts join, as the liquid polymer fills the. 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. Oil lines and reservoirs are typical examples for hollow composite parts which are conventionally produced using a multi-stage process. 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. Specifically, injection-mold lines are often associated with poor mechanical strength. 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. Proper venting is also required to ensure good weld-line quality. To elimi- nate weld lines, you can enlarge shrinkage. A weld line can appear during the absence of enough pressure to push the plastic flow and then meld together. 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. 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. gasket seal leakage where the weld line crosses a gasket. The default weld line result recognizes 135º as the maximum angle of collision and as that angle decreases, the severity increases as it. Flash. ). In the first, sequential injection moulding, the lines are displaced to noncritical areas of the moulding. 453-455. 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 line and warping are two critical defects for injection-moulded part. In an injection molding process, a weld-line forms when two flow fronts meet each other. Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties. Its appearance can be improved by adjusting parameters. Some of these are listed below: Plastic warpage. with a characteristic length χ 0 =0. Injection Molding Know How. Introduction. Here are a few of the most common defects that may occur in plastic injection molding — and how product teams can avoid them. Google Scholar | Crossref | ISI. injection molding is followed by the welding. Therefore, it is recommended to do a moldflow study prior to the mold being built. Depending on the resin, resin temperature, mold temperature, and filling speed, knit lines can vary. Adding vents, using overflows, or using porous metal. How Are They Formed? Molten plastic resin is injected into a mold cavity through an opening known as a gate. Weld lines are also known as knit lines. 3. Steven. The melt stream is separated at the part’s cutout and reunited at the other end of the cutout. Weld or Knit lines are a plane or section in an injection molding part where the flow of molten plastics meets from different directions. Injection molding can be performed with only one of these. 2020-01-1306. During injection moulding, a weld-line occurs when two or more flow fronts collide in the hollow cavity. 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. Cracking and/or Crazing can be defined as a fracture or surface breakage in the material of a molded part, usually found in weld line areas, but also on the surface in general. poor flow fronts that are too cold, this can also lead to a deterioration in the mechanical properties. Weld lines defects are formed when two flow fronts are merged together. Weld Lines. That’s the air escaping out of the nozzle until the hose is full of pressurized water. Knit lines, weld lines, mold lines, or flow lines in injection molding are faint lines that become visible on certain plastic parts. 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. The foundation to any robust process, in my opinion, is based on choosing the correct fill time. 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 rule is, a higher pressure shall be required to push the material in the machine when there is excess mold. 2. การแก้ไข ชิ้นงานจุดที่เกิดความอ่อนแอบริเวณ weld line คือต้องใช้ความเร็วในการฉีดต่ำในจุด. It can be determined by a visible line appearing on the surface of the plastic part where two flows of the plastic meet. 9 – Over Packing. 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. Investigations on WLs in micro injection molded parts were performed in Refs. Avoid knit lines when designing a molded part to improve cosmetic appearance and functionality. 2017-01-0481. A weld or meld line on plastic parts can cause structural problems and/or be visibly unacceptable. In this paper, a weld line computer determination method based on filling simulation with surface model is proposed, from which the positions and lengths of the weld lines can be predicted. 025 mm (0 . 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. 3: Weld Lines. spelling: injection molding) is a manufacturing process for producing parts by injecting molten material into a mould, or mold. , a post that forms a hole) and then rejoins, or when polymer melt fronts meet, from multiple injection points. The conditions are temperature, injection speed, and injection pressure. From Fig. Adjust Injection Molding Parameters: Optimizing injection speed, pressure, and temperature can help ensure better melt flow and reduce the likelihood of weld line formation. That’s the air escaping out of the nozzle until the hose is full of pressurized water. 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. Weld-line yield strength, yield strain, and failure strain of talc-filled polypropylene increase with increasing melt temperature and increasing hold pressure. Avient’s Zachary Alderman shares practical tips on designing and injection molding parts utilizing short-fiber reinforcements. Thereby creating localized pressure within the mold. 2. Weld lines, generated during injection molding, can significantly reduce the mechanical performance of structures. That helps. This is expected to achieve maximum results while the pressure holding effect will also increase. Meanwhile, Using more advanced cooling method such as cyclic cooling or conformal cooling. Weld line injection molding troubleshooting. Weld lines are a reality in the injection moulding of complex parts. Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties. The solution to an internal part cracking problem is to change the packing pressure. Therefore, it is recommended to do a moldflow study prior to the mold being built. These are preferred over weld lines as they will normally result in improved cosmetics and structure. Change the flow pattern to a single-source flow. Here are some ways to avoid. 10(a) (please see Fig. Flow marks in injection molding can be avoided if adequate measures are taken. 22 shows how the weld lines are formed when the melt splits around core pins during the injection molding process. The parting line in injection molding is which one of the following: (a) the lines formed where polymer melt meets after flowing around a core in the mold, (b) the narrow gate sections where the parts are separated from the runner, (c) where the clamping unit is joined to the injection unit in the molding machine, or (d) where the two mold halves come together? Avoid knit lines when designing a molded part to improve cosmetic appearance and functionality. In the present work, the micro specimens were investigated. 000 0. 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. In injection molding, it is often important to maintain consistent color throughout the part. in the cavity of the injection mold. Vacuum voids. 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. Weld-line strength for plastics can vary from 20% to nearly 100% of the strength of the plastic itself. The hair-like weld line is the. Raise the injection speed. In an injection molding process, a weld-line forms when two flow fronts meet each other. 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]. 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. They are unavoidable even for moderate complex products. Heating and cooling mould systems, which cost extremely high, are a typical technology to prevent the weld-line. 005 0. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. A weld line is a mark left on the surface of a plastic injected molded part. 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. Plastic Injection Molding Process. A weld line is often unavoidable. Fig. Weld lines in injection molding are a type of cosmetic defect that detracts from a plastic part’s appearance. One test mold was used to mold three different materials using heat/cool and conventional molding techniques. As the name implies, it is usually positioned along the edge of the workpiece, forming a visible mark at the demarcating line. Computer simulation packages of injection molding have had success in predicting filling behavior in extremely complicated geometries, capable of predicting weld lines’ location exactly. Figure 2. This gives a clue as to how it was formed. Weld line section presents notch or gloss difference. A flat product case was considered as an example of a typical aircraft structural element. A weld line is a mark left on the surface of a plastic injected molded part. Increase the melting temperature. Weld lines. T. Weld lines in injection molding are frequently unavoidable. Three terms that often get mixed up are weld, meld, and knit lines. Or increase the number of gates. A weld or meld line is a weakness or visible flaw created when two or more flow paths meet during the filling process. A visual mold with variotherm system for weld line study in micro injection molding. Instead of using flow leaders and deflectors to control the locations of the weld lines, this paper presents an approach to specify the locations of the weld. + Improve vent inside the mold. 2004; To study the weld line formation of colliding flow fronts the filling of a mold cavity was simulated. , 190°C mass and 20°C mold temperature, as well as. 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. technical ulletin SPECIALTY POLYME RS Design and Production of Injection Molding ToolsIncrease the injection speed in order to fill the mold more quickly and ensure the material does not prematurely cool down; in addition, when the speed is adjusted the flow fronts may meet at a different spot and move the weld line location. Molding Equipment. C. Figure 1a illustrates how the weld line is produced when. 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. Many studies were conducted to explain the weakness at the. 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. Plastic Injection Molding Process. Discoloration. The edge gate is the most popular and straightforward injection mold gate design. Emerging Materials Research. Despite injection molding being recognized as a robust process for obtaining parts with high geometry accuracy, one last occurrence remains a challenge in micro. 10. 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. 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. Causes: Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic. Unless you are manufacturing an extremely simple part with one gate and a unidirectional flow front, your part will have weld lines. 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. 1. Fig 13:- Detailed view of weld lines. The rule is, a higher pressure shall be required to push the material in the machine when there is excess mold. To determine the effect of the weld lines on tensile strength, tensile tests were conducted with the specimens fabricated with NORYL (amorphous blends of. Weld lines are one of the typical quality issues of injection molded parts. Change the gate locations so that the weld lines occur in locations less susceptible to external forces if weld strength is an issue. Increase the injection speed to reduce the amount of cooling happening in the mold. As compared to a weld line, the flow fronts come together at less than 135 degrees. Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties [3,4,5]. Definition: A seam that appears where two areas of molten plastics meet. Insufficient plasticizing ability of injection molding machine. The edge gate is the most popular and straightforward injection mold gate design. The weld line in injection molding is created when two melt streams connect, as shown in Figure 1. 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. Many researchers had emerged with several. When the polymer resin is injected into a mold, it flows through all portions of the mold cavity. Weld-line is a weak area. Flow lines are off-color lines, streaks, and other patterns that appear on the surface of a part. Effects of mold temperature and pressure on shrinkage 0. This model provides the necessary information. A weld line region is typically filled at the end of an injection stroke or during the pressure phase. Weld lines can significantly reduce ultimate tensile strength (UTS) and fracture strain of talc filled polypropylene (PP). (1998) for clear photographs. 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. Increasing the temperature of the mold or plastic material. 2. A weld or meld line is a weakness or visible flaw created when two or more flow paths meet during the filling process. Weld lines can be caused by many reasons, which covers the various aspects of mold, process, raw materials and product design, etc. Founder, Injection Molding Solutions. In narrow fields, a limited amount of molten liquid can solidify and block the flow. In this regard, the plasticizing ability of the plastic injection moulding machine should be checked. 8. Flow rate is 24 cm3/s. Injection molding, polypropylene, talc. The effects of fiber concentration has been studied in detail. Insufficient injection pressure and speed can cause weld lines. Kobayashi et al. When two or more streams of plastic meet and fuse. The weld-lines are undesirable from both aesthetic and functional points of view and their effect is different for. In weak material bonding, they will appear as hair-like weld lines. The type of resin used during injection molding plays a large role in whether or not a knit line will be present. This happens when the molten plastic is injected into the mold cavity and flows around an obstacle such as a pin, hole, or protrusion, and then comes back together. First, an injection mold with triangular inserts was built to study the influence of meeting angles on material properties at the weld line. Boost time too long Shorten boost time, lower boost pressure, or move transfer to an earlier position . In. A weld line can also be called a knit line. 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. Polym Eng Sci 45(7. Weld lines are visible to varying degrees on any molded part depending on gate location, flow length, melt/mold temperature, cavityA weld line occurs when two or more molten plastic flows meet and fuse during the injection molding process, resulting in a visible line on the surface of the molded part. 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. The main types of injection molding defects are: 1. Prediction of the weld lines in injection molding process using neural networks. but the injection pressure will loss and injection molding pressure of this gate is bigger than that of direct gate. Gate blush, 4. Premature partial solidification of the molten plastic leads to inadequate bonding — the primary cause of weld lines. 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. If the different flow fronts have cooled. Wavy lines are another feature of weld lines. This customary mark is caused by differential rates of cooling in the resin where. 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. Known methods to reduce weld-line effects are the optimization of injection molding parameters, enhanced tool temperature, and using special molding tool devices. 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]. Materials Science. Too high barrel temperature ( abnormal temperature control, deviation of material performance, and increasing cycle time); 8. 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. Weld Lines. Weak weld lines are perhaps the most common and difficult injection molding defect to eliminate. Weld-line, a common occurrence in plastic injection moulding, is a representative defect compromising mechanical qualities and product appearance [16]. ), with different mould surface finishes (polished, fine eroded, rough. 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. Complexities in the design and intricate pathways in the mold can also create flash in injection molded parts. Weld lines are one of the typical quality issues of injection molded parts. Shaofei Jiang. 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. . Microsyst Technol, 14 (6) (2008), pp. Injection molding weld lines where two or more flow fronts come together in the mold. 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. These lines are formed when two or more flowing molten plastic streams meet and fuse together, creating a visible line on the surface of the molded part. Fellahi et al. The injection molding temperature and the mold temperature were fixed respectively at 230 °C and 50 °C. The most common and one of the most challenging injection molding defects are weld and knit lines. This article will review how and why weld lines occur, how mold gates effects weld line location and strength, the importance of material selection and Injection molding weld lines refer to a line on an injection molded part that is generated where the two halves of the mold come together. 020 0. Weld lines are formed when there is an inability of two or more flow fronts to ‘knit’ or ‘weld,’ together during the molding process. The weld line degrades the. 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. ppt from SCIENCE 101 at Symbiosis International University. 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. A quantitative evaluation of the appearance quality of weld lines in RHCM parts. 030 0. 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. Thicker parts require more cooling time and can lead to air becoming trapped in the mold and weld / knit lines to form. 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. Consequently, it causes viscous dissipation which leads to the generation of heat. 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. 2. 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. It is not always easy to completely eliminate weld lines by simply adjusting the relevant injection mold design or the molding conditions. 3. Carefully consider all the relevant aspects of the mold design specification before. , 2 (3–4) (1988. But add 30% glass and weld-line strength retention drops to only 34% as high as the compound with no weld. 1. 004 m, a characteristic velocity ν 0 =0. Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties. A knit line may be unnoticeable or may look like a crack. As a result, the portion has a faint, perhaps undetectable line. 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. A weld line (weld point, flow line or a knit line) forms when separate melt fronts travelling in opposite directions meet.