com . Figure 1 Flow Lines Knit or Weld Lines Cavity Gates Table 1 Knit line in a multi-injection-point cavity. injection molding machines using standard industry practices . Runner Type. Inadequate injection pressure can result in knit lines and a surface that appears corrugated. With fast accurate, injection-mold simulations, designing injection-molded products, developing injection molds, and shooting parts is more collaborative, accurate, and efficient, saving time and driving down costs. Texturing may also hide surface defects such as knit lines and other imperfections. ; Wood, Philip E. As a result, the portion has a faint, perhaps undetectable line. Weld / knit plane forms as flow fronts recombine Weld line and failure due to flow around core. The most common and one of the most challenging injection molding defects are weld and knit lines. The best we can hope for is to pressurize the knit or meld line better through the combination of melt temperature, mold temperature, flow rates, and holding pressures. Weld Line defect analysis, the most common of many defects in injection moulding products. The parting line location on the injection mold will have an effect on how the cavity and core (both halves of the injection mold) come together and seal. By using this method, theMolding analysis study: An example of the low patterns and knit (weld) lines formation (MoldFlow® data) of injection molded round plastic part with a complex shape and with a ring. Macromol Symp 1988;21/21: 489-500. Fictiv can help you avoid injection mold design and plastic part design for injection molding problems. Injection molding is the process of injecting plastic material into a mold cavity where it cools and hardens to configuration of the cavity — is best used to mass-produce highly accurate, & often complex, 3dimensional (3D) end-use parts and products. When injection molding complex parts, often knit-lines are formed during the mold filling stage. Weld lines defects can also form when the flow fronts from two gates merge or when thick and thin sections in a part cause a flow to split and merge. Encapsulating the metal insert with plastic creates a single. 4. Rynite® PET is usually processed with melt temperatures between 280 to 285 °C (535 to 545 °F) . 008 in. Refers to portion of injection molding part that has some taper to make it easier to remove from the mold. Its superior flowability allows for precise and flawless filling of injection molds without the risk of flow marks and knit lines. The Difference Between Knit and Meld Lines–and Why It Matters. Weld or knit lines occur when melt flow fronts collide in a mold cavity. Gate and Knit Line Locations<br />. Short shots and knit lines are two phenomena that are caused by using highly viscousTherefore, changing the edges and corners of the mold core to make it buffer transition and keep the melt flow stable can prevent the occurrence of wavy lines. To avoid these problems and create the best possible. Please read on, stay posted, subscribe, and we welcome you to tell us what you think. Generally all plastic components should be designed with draft where possible. The areas in the cavity where such buildup tends to be common are at end-of-fill areas, details that trap gas, and knit lines. Injection molded plastic parts have comparatively less strength in the weld line section and they also have an impact on product aesthetics. Weld Lines. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. In manufacturing, the Weld line or Knit line or Meld line is the line where two flow fronts meet when there is the inability of two or more flow fronts to "knit" together, or "weld", during the molding process. To learn more about us or to get a quote, click here or call 1-888-893-1587 now. wall thickness injection molding ribs. We look forward to offering support and advice on your next project. These flaws can affect the aesthetic and structural integrity of the. Exploring common molding defects and. Core out parts to eliminate thick walls. The thermoplastic injection molding process at BEPL is a standard process involving an aluminum mold with no heating or cooling lines running through it, which means cycle times are a bit longer. The injection molding process uses a granular plastic that is grav ity fed from a hopper. Knit lines are formed in injection molding when two flow fronts meet in the middle of a part (a line where two sides of the plastic are “knit” together). As the polymer resin is injected into a mold, it flows through all parts of the mold. Most failures occur at what are called “weld lines” or “knit lines”. Short molding cycle, high production efficiency, and easy automation 2. Because this allows some cooling and thickening of the resin, tab gates require a relatively large opening into the mold cavity. Users will be able to use in-cavity pressure or temperature sensors to control when the valves open or close. . Weld or knit lines are perhaps the most common and difficult injection molding defect to eliminate. 7. Adjust the design for the flow pattern to be a single source flow. Press fits for injection-molded parts can be challenging. BURN MARKS Burn Marks: burn marks can occur when the design of a plastic part, and the subsequent injection mold design, causes air to be trapped during the injection (or filling stage) of the molding cycle. Weak weld lines are perhaps the most common and difficult injection molding defect to eliminate. Commonly seen at the edge of a hole or other cored feature, knit lines are—as a. Injection molding crush ribs are small, protruding features added to molding designs to support and improve the stability of press-fit connections. Give us a call at (425) 339-0288 or email us at info@sealectplastics. Steven. Weld Lines-3 Ways for Detecting and Correcting. Operating Conditions • Fast injection fill speeds, supporting short injection times (1–3 s), especially in thin sectionsKnit lines in reaction injection moldings Knit lines in reaction injection moldings Maier, Christian; Hrymak, Andrew N. Venting is required to allow the air in the sprue, runner and cavity to leave the tool as the melt flows into the cavity. Once the mold is created and the machine is set up, additional parts can be manufactured very fast and at a very low cost. The injection molding processing conditions for filling are given in Table 1. In addition, keep these in mind: Scrap rates can range from less than 1 to 5%. Find out more by calling 573-437-4154 today or email us. Contact Universal Plastic Mold Today. As the polymer resin is injected into a mold, it flows through all parts of the mold. While mold-filling simulation is a very common tool for predicting the fill patterns of an injection mold, in our judgment there is not yet a commercially available, satisfactory filling simulation for microcellular foam molding. RELATED CONTENT. Compression molding can also support large part sizes, and it has a tendency to. Knit lines, also known as weld lines, are fine lines that resemble hairline fractures or cracks. Typically, this fillet radius should be somewhere near 0. When the mold body is closed, the contact surface of the cavity and core plates makes a parting line. Meld and knit lines are actually both different types of weld lines. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. Learn about the key characteristics of common injection molding materials, recommended applications, and how common thermoplastics compare to one another. Weld line defects – also known as knit lines – is a defect that occurs when the injected plastic meets within a mold. Using the design cube, learn how knit lines are formed on plastic parts and tips to avoid knit lines in your plastic injection molding design. S. Solution: People. In the injection molding process, knit lines are a common occurrence when two or more plastic flow fronts collide and solidify, resulting in a visible seam or line on the molded part. 5 to 3. More about the knit lines in injection molding. When injection molding a part, a knit line may present no problem, or can cause a serious structural problem. Injection molding is an excellent way to create plastic parts in bulk, but there are many problems that can cause cosmetic defects ranging from minor to major. injection molding. Injection Molding: How to Set Second. Neat polypropylene retains nearly 90% of its strength on a properly designed and molded weld line. For non-reinforced or non-filled nylon, the mechanical performance in the knit lines (planes) areas are approximately equal to the mechanical performance of the resin (polyamide) used. He is currently a tooling manager for a large, multi-plant molding and manufacturing company. Weld line. RJG announced that its injection molding process control system, CoPilot, will begin offering valve gate control technology for hot runners starting in September 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. In injection molding, a thermoplastic material is melted to a liquid state and injected into a custom, double-sided mold. 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. 3. The melt is forced to split up during filling of the cavity. Injection speed and pressure. Product designers encounter a wide range of challenges when creating and developing molded parts, such as stress factors, warp, and those pesky knit lines. Lower the melt and mold temperature to prevent overheating. 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. It can also aid in material selection since fill. Knit line Knit line Start of mold filling. Weld lines are a common imperfection to be found in most plastic injected molded parts. We have an elite team that designs for injection molding, rapid prototyping with metal and plastic, and produces world-class products under the same roof. (1. These structures are often small surfaces of about a tenth of an inch in diameter. 040-in. A weld line, or knit line, occurs when two flows of melted plastic meet and “knit” together. Moisture in the material due to insufficient drying can form at the melt front, forming an interface between the two melt fronts. The metal parts, made of steel grade. 008 to 0. Services Group. Meld and knit lines are actually both different types of weld lines. Knit lines. Our On-demand. The bottom of your rib must also feature a fillet. This guide to thermoplastics and injection molding material selection is aimed at an engineer who plans to quantitatively analyze a part, determine loads, stresses, strains, and environments and make an optimal material decision based on the analysis. However, the weld line may not continue throughout the flow. a knit line will form 180°on the opposite side from the gate location. The Bottom Line You might have come across the aforementioned terminology before and did not know what any of the words meant. 3. N174W21430 Alcan Dr Jackson, WI 53037 262-677-9700; 16685. Observation on a “trial and error” basis may be required. Knit lines. 025 inches per inch, depending on the particular resin, molding conditions, part design, part wall thickness, and direction of flow. 2 mm. While injection molding itself is a straightforward process involving melting. The gate is the entry point to the part-forming cavity, where plastic material is injected from the molding machine barrel, through the sprue and runner, into the mold. ” Depending on the material you use, the knit lines could be invisible or obvious—work with an injection molding company to figure out. Gating. It is typically used for thermoset materials, but can be used with thermoplastics. ABS is a good choice for handheld consumer devices. End-of-arm tooling can make or break a program. Weld / Knit Lines; Assumptions. These defects are not visible, like small. An undercut is any indentation or protrusion that prohibits an ejection of a part from a one-piece mold. 012 in. Such molds or tools aren’t intended to be used in operation. 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 gates. 5T, where T is your nominal wall thickness. Plastic injection molding is the industry’s most common way to manufacture parts. Shear Controlled Orientation Injection Molding of Polymeric Composites with Enhanced Properties. When designing for injection molding, consider these steps to prevent knit lines The Resin Cooling Process. As a result, prototyping is strongly recommended for parts with. One common problem that occurs in injection molding is the formation of knit lines. occupied frequently with troubleshooting injection molding. Knit lines are formed in injection moulding when two flow fronts meet in the middle of a part (a line where two sides of the plastic are “knit” together). Demo Mold. Multiple gates will always. These defects are not visible, like small. Using three test materials, polystyrene (PS), polycarbonate (PC) and polyoxymethylene (POM), the relationship between a design and a processing parameter and the effects of the knit-lines on the tensile properties of injection molded plates. 5 kN/cm2 (4 T/in2 ) is required. MINNESOTA. The thermo-rheological findings were used to investigate the sources of weld line weakness. This makes the casting process as efficient and accurate as possible. Based on the resin. In situ injection molding of continuous fiber reinforced thermoplastic composites is challenged by unbalanced dual-scale infiltration flow due to the pronounced capillary effect. See UNDERCUT. Injection Molding: How to Set Second. Knit Lines: These occur when different flow fronts come together as plastic flows through the part cavities. Figure 1 presents the shrinkage of polypropylene versus wall thickness in injection molded parts. It is caused by the incomplete joining of the uncured. If life safety is involved, or reliably or efficacy are absolutely required, every part should. PCS Company, Fraser, Michigan, has launched the Polimold Valve Gate Sequencer, providing injection molders with more precise control of the mold-filling sequence when using valve-gate hot runner systems. Enlarge gas vents and gates to allow trapped air to escape the mold. A medical device designer might look up a polymer's physical properties and game out. These data were obtained from many andKnit lines in injection molding are common defects of the molded parts. The temperature might fall at some places like in the mold, runners leading to the mold and injection molding. 2. Table 2 lists the<br /> properties of common tool steels and the typical mold components for which they are<br /> used. If you want to better understand the process of injection molding, there are a few critical terms you need to know. For best results, keep thickness variations to within 15% of the nominal wall thickness. One of the most important aspects of hot runner system design is the gate location, which is the point where the molten plastic enters the cavity. Weld lines may appear like seams or scratches,. An injection mold that produces pieces while the final tool is being built. , Rallis, A. 3. Typical Tool Steel for Injection Mold Construction Steel Type Steel Properties Mold Component P-20 Pre-hardened, machines well, high carbon, general-purpose steel. Our Injection Molding Capabilities • Horizontal Injection Molding up to 3300 TONS • Structural Foam Molding + Structural Web • 1oz to 600oz part capacity. Gate locations can be directly behind or on the edge of the graphic. Weld or Knit lines are a plane or section in an injection molding part where the flow of molten plastics meets from different. The mold settings can run flawlessly until employees consistently open the machine for inspections, interrupt cycle times, or cause. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and. And, for those who can work within its limitations, rapid injection molding may be suitable for producing some optical parts (and cost as much as 90 percent less than traditional tooling). A mold with too few gates is susceptible to cooling problems, but a mold with too many gates. 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. Flow Lines Knit or Weld Lines Cavity Gates Table 1 Knit line in a multi-injection-point cavity. Plastic is then injected at high pressure into the mold, and it solidifies with any surface features or defects in. All the while, the material could be the correct material," Hill says. The goal is to create a uniform thickness and at the smallest (thinnest) level required. knit lines and parting lines are at the discretion of MIC-ZG. Step 2: The injected molten plastic is cooled followed by the separation of the core and cavities. If the plastic has cooled too. 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. Knit/Weld Lines. Flow Lines / Knit Lines Flow/Knit Lines 11. Knit lines, also known as weld lines are areas in the. Its superior flowability allows for precise and flawless filling of injection molds without the risk of flow marks and knit lines. Valve gates are most efficiently. For over four decades, our experienced team and leading edge technology have provided you with tooling solutions from start to finish. High mold temperatures usually provide less residual stress. be impressed by improvements of bubbles (vacuum voids), short shots, unequal thickness, jetting marks, weld lines, knit lines, black streaks, strain 2. It happens when the molten plastic material flows around obstacles, such as inserts or cores, and rejoins on the other side. It is. We all know that injection moulding is great for producing both custom prototypes and end-use production parts, but if you are not careful it can cause some. Different plastics make different types of knit lines. Steps involved in Multiple Material Molding. Also known as a knit line, this is the point at which two flow formations meet and are unable to connect during injection molding. They are formed downstream through holes, and between multiple gate locations. Injection Molding Process Guidelines The following is given as a potential process starting point. In a weld line, reinforcement fibers. Visual indicating with an finished part that are formed by the junction from two plastic flow fronts. The injection molding process is fairly simple, plastic resin is heated to its melting point and forced through the machine and into your mold to produce your plastic parts. Plastic injection molding is a complex and highly intricate process. "As soon as that part or material is stressed, it can be real conducive to fracture, cracking, breaking, just because of the way it's been molded. 0 Items. 080" – 0. TRADITIONAL MACHINES VENTS. Knit lines, also known as weld lines or meld lines, are visible lines or seams that appear on a molded part where two or more flow fronts meet during the injection molding process. This slight depression is a tiny line called the weld line or knit line. The recommended minimum production volume for injection molding is 500 units. A weld line, also known as a knit line, is a visible line or mark that occurs in an injection-molded part where two or more melt fronts meet and fuse together during the molding process. Increase the injection speed. This will help the mold designer to determine the allowable gate and knit line locations. Weld or Knit lines are a plane or section in an injection molding part where the flow of molten plastics meets from different directions. 1. Nicolet Plastics LLC. These knit lines affect the parts aesthetics, but also they generally decrease the strength. In manufacturing, the Weld line or Knit line or Meld line is the line where two flow fronts meet when there is the inability of two or more flow fronts to "knit" together, or "weld", during the molding process. In an injection molded structural plastic part, a weld line (also called a knit line) will result in a region of mechanical weakness as soon as the part is submitted to an external load. 250 in. It is best practice to maintain a uniform thickness throughout an injection-molded part. This is a quick rundown on how to handle common features in injection molding designs:What are the advantages of injection molding 1. One of the most common ways of adding venting is by manually machining it into the mold (Figs. Injection molding a part with Class-A surface finish and ribs or bosses has proven an elusive goal for molders, despite their best efforts with conventional injection or internal gas-assist technology. Injection molding is a popular manufacturing process used to fabricate parts in large volumes. N174W21430 Alcan Doctor Jackson, WI 53037 262-677-9700; 16685 State Road 32 Mountain, WE 54149 715-276-4200; Prime. S. 160"). , the Weld line or Knit line or Meld line is the line where two flow fronts meet when there is the inability of two or more flow fronts to "knit" together, or "weld", during the molding process. Some are: Knit lines (discontinuities where two flows merged, creating a weak point)In some situations where a mold has parting line damage, reducing heat can actually improve flash that was a direct result of that damage. opposed to two or more gates. Plastic injection molding has long been an affordable and efficient process for producing custom prototypes and end-use production parts. 015in wall. Other materials, like LCP are great for thin wall micro molding but also notorious for having a bad knit line, potentially snapping from excessive torque in later manufacturing processes. The depth of the texture or letters is somewhat. Incorrectly placed single gates may cause knit lines. Self-mating parts. Typically, the wall thickness is somewhere in the neighborhood of 50%-75% of your nominal wall thickness. 015in wall. Proper venting is essential to molding a defect-free part. Increase injection speed and pressure: Faster-moving materials will meet more quickly so one doesn’t cool too soon. Wall Thickness: The recommended wall thickness for injection molded POM parts is between 0. Trapped gas is one of the most common causes of part burns, and can also lead to short shots and voids, blemishes and discernible knit lines that weaken the part. As the melted plastic fills the mold, there are locations where the molten plastic flow stream separates and then comes back together. Product designers encounter a wide range of challenges when creating and developing molded parts, such as stress factors, warp, and those pesky knit lines. An option to bluing agents, the long established means of gauging pressure distribution, comes via Pressurex, a thin, flexible, plastic film. The quality of the weld line can be improved through optimized processing conditions, increased mold and melt temperatures, and increased injection speed toWeld Lines. Disadvantage: May rust if improperly stored. Resin pellets are loaded into a barrel where. Injection Molding Problems and Solutions (17/17) – Weld or Knit Lines Sep 27, 2022injection-molding machine (Arburg allrounder 320C, Germany). Three terms that often get mixed up in injection molding are weld, meld and knit lines. Weld lines are among the most critical structural defects in parts made by the injection molding of fiber-reinforced thermoplastics, explained Sirmax in a press release. Obstructions in the part create weld lines or knit lines during injection molding. Remedies: Raise the temperature of the mold or molten plastic. Islyn Thomas summarized the need for a particular vent location better than I ever could. Last month, in the first part of this two-part installment we focused on the reasons viscosity can change during injection molding. . Imagine a river splitting to go around a rock, then meeting up again on the other side. Knit Lines. Meld and knit lines are actually both different types of weld lines. The only parts that count are the ones that meet quality standards, and keeping pressure evenly distributed across a mold’s parting lines is one of the many critical tasks to get those good parts molded. It’s economical, efficient, and can produce simple to complex parts with low waste. incomplete filling or shorts. So what does the 140F mold temperature give us? The molding technician thought the 140F mold water was OK because the data sheet provided by the material supplier suggested mold temperatures from 75 to 180F. Know about the cause of these lines and the potential solutions. Weld lines may appear like seams or scratches, but that is not what they are. One common problem that occurs in injection molding is the formation of knit lines. Unsymmetrical filling makes knit lines in samples, leading to the failure of part, so it must be avoided. If you have questions about the suitability of the Protolabs process for your optical parts, contact an applications engineer at 877-479-3680. Injection moulding (U. Plastic melt and mold temperature. It can be determined by a visible line appearing on the surface of the plastic part where two flows of the plastic meet. This information is intended to be used only as a guideline for designers and processors of modified thermoplastics for injection molding. Proper venting is essential to molding a defect-free part. Following lists causes and solutions for weld or knit lines. A screw-type plunger forces the material into a heated chambe r,. IM: cooling accounts for _____ % of the total cycle time. Three terms that often get mixed up in injection molding are weld, meld and knit lines. Weld-lines look like cracks on the appearance of plastic. As the melted plastic fills the mold, there are locations where the molten plastic flow stream separates and then comes back together. Figure 1: D-shaped through-hole in center of gear part accepts the drive shaft. The company provides an array of digital manufacturing expertise. 5. Change gate locations. Commonly seen at the edge of a hole or other cored feature, knit lines are—as a rule—purely cosmetic, but may create a physical failure point if present in an area of the part that receives substantial stress, such as the head of a screw. The plastic in the ‘flow front’ cannot pass through the core pin, so it flows around the core pin splitting into 2 distinct flow fronts. Here are some of the most common cosmetic defects and how to avoid or fix them. land and using maximum vent depths to get them as close to self-cleaning as possible. They are formed when two or more flow frontFor more information on our custom rubber molding services or to speak with one of our knowledgeable professionals, please feel free to Contact Us or complete our online Request For Quote form. 5 : 1 and 3. This can dramatically reduce the effects of material changes, according to the injection molding technology company. Prevent Flow and Knit Lines During the Injection Molding Process. A well-designed injection-molded part will usually have draft, but the same draft that helps eject a part from the. The recommended minimum production volume for injection molding is 500 units. Our goal of 100% efficiency, zero scrap and repeatable processes can sometimes be hindered by poor setups or changes in the molding process. One of the most common ways of adding venting is by manually machining it into the mold (Figs. Knit lines, holes, part configuration and the talent of your molder all come into play. Main causes of knit lines in injection molding: 1. RELATED CONTENT. Here are some of the most common cosmetic defects and how to avoid or fix them. In fact, knit line strength will decrease with higher filler content and with longer fibers. wall thickness, knit lines, draft. Knit / Weld Lines. Weld lines are also known as “stitch lines” or “knit lines”. What causes knit lines? Knit lines are formed when the molten plastic flowing into the mold cavity runs into an obstruction, such as a core pin that forms the through hole on a flange of an enclosure. While injection molding itself is a straightforward process involving melting. 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. Here are a few of the most common defects that may occur in plastic injection molding — and how product teams can avoid them. Knit lines are hard to eliminate for injection molded plastic parts but designers can minimize cosmetic defects. As a result of weak coupling along the knit-line, the weld-line strength can be only about 60% of that of an unreinforced resin. The knit lines are the result of poor molding conditions (likely molding at too low of a temperature). We have two unique offers to meet your needs wherever you are in the product lifecycle. With over 50 years of experience, Universal Plastic Mold can help you avoid (or at least minimize) knit lines in your molded parts. com Discover why you should avoid knit lines when designing and manufacturing injection moulded parts. Strength at the knit line can be as little as 20% of the. This information is intended to be used only as a guideline for designers and processors of modified thermoplastics for injection molding. Read on to find out more! 800. He said, “Vent cavities at weld lines or junction points where two or more material streams meet and fuse into a solid mass. In a weld line, reinforcement fibers. • Standard (general purpose) screws with a compression ratio between 2. He is currently a tooling manager for a large, multi-plant molding and manufacturing company. PTI, a plastics and metal parts molder, is a leading developer of the MIM process. The tooling engineer in the mold house will be more into venting, sprue pressure, preheating the mold, cycle times, ejector pin scheme, mold flow, getting a gate location to avoid fill issues or knit lines, excessive flash and molding pressures, and the list goes on. In plastic injection molding, a parting line is where two halves of a mold meet when closed. Computer simulation packages of injection molding have had success in predicting filling behavior in extremely complicated geometries, capable of predicting weld lines’ location exactly. TEFABLOC™ TOSI 431 75A is a high-flow thermoplastic elastomer (TPE) designed for large-part injection molding applications, offering exceptional flexibility and weather resistance. Multiple gates will always cause knit lines. sinks, knit lines, and shrinkage,” Spalding says. Knit lines, also known as weld lines are areas in the molded part where two or more flow fronts converge. Injection molding can be a real challenge if you don’t follow the “Five M’s”: Man, Mold, Machine, Material and Method. Probable Cause Suggested RemedyI am working on a thin PC/ASA injection moulded component which has a tendency to snap at the Knit-point (the final point of travel at which the…Opening and closing all the valve gates at the same time necessarily limits the degree of control over weld or knit lines, gas traps, localized packing, and unbalanced flow. Injection Pressure 0r Speed Too Low Explanation: If the injection speed or pressure is too low, the molten plastic will not be pushed into the mold cavity fast enough to fill and knit at converging flow fronts because it cools down and solidifies too soon. But add 30% glass and weld-line strength retention drops to only 34% as high as the compound with no weld line. The Troubleshooting Tips below offer common solutions to challenges in IML and IMD. Venting at Weld Lines. Knit line / Meld line / Transfer line Discoloured line where two flow fronts meet Mould or material temperatures set too low (the material is cold when they meet, so they don't bond). The cooler flow fronts of the resin aren’t forced into the mold surface texture as well as the hotter resin behind the flow front. Randy Kerkstra has been in the plastics industry for more than 26 years, occupied frequently with troubleshooting injection molding. Double push filling method. See UNDERCUT. Services. spelling: injection molding) is a process for producing parts by injecting molten material into a mould, or mold.