160"). A weld or meld line is a weakness or visible. You don't give us much to go on. Last month, in the first part of this two-part installment we focused on the reasons viscosity can change during injection molding. 08 KICKOFF PRODUCTION. Depending on the resin, resin temperature, mold temperature, and filling speed, knit lines can vary from virtually invisible to something that looks like cracks in the plastic. Multiple gates will always cause knit lines. Facebook. 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. Solution: Increase the injection pressure or speed. As a result, prototyping is strongly recommended for parts with. Depending on the design of the mold and the material being injected, a knit line may: present no problem at all, be a. Our goal of 100% efficiency, zero scrap and repeatable processes can sometimes be hindered by poor setups or changes in the molding process. They again get resolidified along the surface. An injection mold that produces pieces while the final tool is being built. Core out parts to eliminate thick walls. As. All that considered, for a specific simple part with specific quality requirements. Trapped gas is one of the most common causes of part burns, and it can also lead to short shots and voids, blemishes and discernible knit lines that weaken the part. Look for signs of stickiness or burning. Fortunately, there is a technique that provides almost unlimited control over which parts of a tool fill when and how fast. Flow Lines Knit or Weld Lines Cavity Gates Table 1 Knit line in a multi-injection-point cavity. Subscribe to Our Newsletter. Here are some ways manufacturers can avoid quality issues during the plastic injection molding process: Inadequate Injection/Holding Pressure: If injection and holding. Injection moulding (U. To prevent swirling, perform tests whenever possible and use pre-compounded resin to ensure identical color match across parts. 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. Molding Troubleshooting Guide Bond Readout Bond readout is a surface distortion similar to a hump or sink that occurs over a bond line Probable Cause Material Process • Mismatch of compliance between outer panel, inner panel and adhesive • Incompatible thermal expansion coefficients between the SM and the adhesiveWeld lines, created in the areas of collision of two flow fronts of plastic in the injection mold cavity, are the reason of lower mechanical properties and a worse surface condition of molded parts. (If parts are to be molded by Protomold’s rapid injection molding process, our Design Guidelines should be used. Complex parts can be reproduced with low cost once mold is manufactured. land and using maximum vent depths to get them as close to self-cleaning as possible. Focus - Injection Molding - Free download as PDF File (. Without it, air and gas are trapped in the mold, which compress and heat as the cavity fills. Injection molding a red component, for example, can pose a challenge because at time the component may be too thin for the pigment itself!. 250 in. Example of a gate location in a finished part Example of knit lines in a finished part. ”. Learn more about the process by reading this white paper. On the opposite side. Weld lines, also called knit lines, are a defect that occurs with molded parts caused by two different flows of plastic resin that do not adequately join together or weld together correctly during injection molding. Meld and knit lines are actually both different types of weld lines. Macromol Symp 1988;21/21: 489-500. Fig. Family molds. 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. reduce mold maintenance cost. 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. When the mold body is closed, the contact surface of the cavity and core plates makes a parting line. In a weld line, reinforcement fibers. 6 Monate. The mold’s sprue bushing and locating ring is placed in a way that makes sure the injection molding nozzle is properly aligned and centered. Knit Lines: These occur when different flow fronts come together as plastic flows through the part cavities. weld lines may form during the injection molding process and work closely with the plastic injection molder and tool builder to choose the best possible gate location. Proper venting is essential to molding a defect-free part. Self-mating parts. Behind the obstruction, the flows try to merge again, but this is made more difficult by a low temperature of the mold surface and a low melt pressure. Depends how hard you want to push it. Enlarge gas vents and gates to allow trapped air to escape the mold. While injection molding itself is a straightforward process involving melting. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. Table 2. Three terms that often get mixed up in injection molding are weld, meld and knit lines. H-13 Good general purpose tool steel. End-of-arm tooling can make or break a program. mystagingwebsite. 7. Here are some of the most common cosmetic defects and how to avoid or fix them. Randy Kerkstra has been in the plastics industry for more than 26 years, occupied frequently with troubleshooting injection molding. Notched weld lines then remain near the cavity walls. 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. Throughout the molding process the press operator visually inspects the molded parts as they come out of the mold for any imperfections such as air trap, knit lines, backrinding, or tearing. The uneven thickness of the product will make the melt flow resistance larger and smaller, resulting in unstable melt flow. In this paper, a general and efficient approach was proposed for dual-scale infiltration flow balancing based on numerical simulation. It explored the use of. A poor knit line can cause only cosmetic blemishes or it can significantly weaken the structural integrity of a part. Injection molding can be a real challenge if you don’t follow the “Five M’s”: Man, Mold, Machine, Material and Method. The final molding process may be dependent upon factors. Second, the runner leading to a tab gate takes up real estate within the. Injection Mold Tooling Options and Quality Reporting. The melt is forced to split up during filling of the cavity. Nicolet Plastics LLC. Als einer der führenden deutschen Hersteller von Masterbatches stellen wir nicht nur Farb-Masterbatches, Additiv-Masterbatches und funktionelle Kunststoff-Compounds her – wir leben deutsches Forscher-, Ingenieurs- und Unternehmertum. Knit / Weld Lines. Bociaga and Jaruga (2007) investigated flowing weld line in injection process by injection molding machine, which the part surface was observed in reflected light using the optimal microscope and. Mold venting is critical to the quality and consistency of the finished part. 1-3). S. In injection molding terms, these are called “living hinges” and are made right into the design instead of adding them later. This article will look at two common venting options—using traditional machined vents and venting through porous materials—to help you decide which is right for your application. Learn about the key characteristics of common injection molding materials, recommended applications, and how common thermoplastics compare to one another. To learn more about us or to get a quote, click here or call 1. The temperature might fall at some places like in the mold, runners leading to the mold and injection molding. Back to News Page. Flash: All plastic injection molds contain parting lines, which create the potential for small amounts of molten plastic to escape the mold cavity’s boundaries. Which affect parts aesthetics and decrease parts strength. An undercut is any indentation or protrusion that prohibits an ejection of a part from a one-piece mold. 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. Flow Lines. – Change the thickness of the product. They need the interference of about 0. 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. 3. Finally, knit lines occur when molten plastic flows around an object and rejoins on the far side, a phenomenon that is quite possible when inserts. 125 inches. spelling: injection molding) is a manufacturing process for producing parts by injecting molten material into a mould, or mold. This most often occurs when you have a core or shutoff feature that the plastic needs to flow around. Three terms that often get mixed up in injection molding are weld, meld and knit lines. A weld or meld line is a weakness or visible. Without it, air and gas are trapped in the mold, which compress and heat as the cavity fills. 060 in. These structures are often small surfaces of about a tenth of an inch in diameter. injection molding machines using standard industry practices . 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. More about the knit lines in injection molding. Meld and knit lines are actually both different types of. . Weld-lines look like cracks on the appearance of plastic. 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. They are the result of fundamental physical effects taking place inside of the mold. injection molding. It happens when the molten plastic material flows around obstacles, such as inserts or cores, and rejoins on the other side. Knit lines; Knit lines always occur once 2 melted materials meet with hair-like discolorations. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part. On a hydraulic machine you can program hold pressure: For example, if you set 850 psi (59 bar), you will get 850 psi hold pressure for the time you set on. We look forward to offering support and advice on your next project. Knit lines, also known as weld lines are areas in the. Exploring common molding defects and. Here are a few ways to prevent knit lines in your plastic parts: Increase material temperature: Higher temperatures on both materials prevent one section from cooling too quickly. Not always a problem, but can cause serious structural issues. Thin wall injection molding can produce walls as thin as 0. Slower injection provides less orientation and stress. Weld-lines, also referred to as spider-lines or knit-lines, are regions in the wall of an extruded plastic pipe, running the whole length of the pipe, at which there is a change in dimension and a. 010 inches. All the while, the material could be the correct material," Hill says. Quick Guide to Injection Molding Amodel® polyphthalamide (PPA) equipment Amodel® PPA resins can be processed on conventional injection molding equipment. 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. Find out more by calling 573-437-4154 today or email us. To account for variations in viscosity, nylon injection molding machines need to maintain an appropriate injection pressure. 1990-03-01 00:00:00 Knit lines in reaction injection molding (RIM) are formed when a fluid flow front passes through the mold cavity and must pass around objects in its path. Specific attention to processing details will enhance quality and productivity . wall thickness injection molding ribs. The relationship between processing conditions, morphological structure and mechanical properties of knit lines of an injection molded semi-crystalline thermoplastic. sheet metal fabrication, rapid tooling, and plastic injection molding. When injection molding a part, a knit line may present no problem, or can cause a serious structural problem. From Plastic Materials and Processing by A. Solution: People. The Difference Between Knit and Meld Lines–and Why It Matters. Shorten the mold cycle time so that any trapped air and resin don’t have a chance to overheat. 040 and 0. Direct experience designing molds and parts for injection molding and producing the molds and parts is valuable, relevant experience for design students. 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. - You hit on this a little, but dealing with the parting line and ejector pin marks is pretty important, and isn't as simple as it seems at first look. 0 Items. Learn more. Also, since this part will likely be painted or chrome plated, knit lines are a concern. Basic rule of thumb air in mold must be able to escape at theInjection molding is the predominant manufacturing process used for producing transparent polymer products. Also known as string lines. They appear around holes or seams of an injection molded part, and some may call them “flow lines” or “weld lines”. Weld lines defects are formed when two flow fronts are merged together. Imagine a river splitting to go around a rock, then meeting up again on the other side. Weld or knit lines are perhaps the most common and difficult injection molding defect to eliminate. Gate locations can be directly behind or on the edge of the graphic. Usually the parting line of the mold is perpendicular to the mold opening direction of the injection molding machine. 25T-0. 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. Knit Line – A molding defect resembling a flow-line found at the point where the rubber meets between the injections. Observation on a “trial and error” basis may be required. Our plastic injection molding glossary provides key terms. The most common and one of the most challenging injection molding defects are weld and knit lines. With over 50 years of experience, Universal Plastic Mold can help you avoid (or at least minimize) knit lines in your molded parts. Injection Molding: How to Set Second. TRADITIONAL MACHINES VENTS. Weld / Knit Lines; Assumptions. The specimens with a weld line at the center were obtained by double gate molding and the specimens without a weld line wereInjection runner too small Increase runner as needed. It can also aid in material selection since fill. Those are knit lines, formed when two opposing flows of material join together in the mold cavity. [8] Bevis, MJ, Reis RL, Cunha AM. Injection molding crush ribs are small, protruding features added to molding designs to support and improve the stability of press-fit connections. Swirling can increase depending on the material you use, and the texture or polish of injection molding tools. Runner Type. Because they can be shut off independently you can control flow fronts and knit-line locations. Redesign the mold: Redesigning the mold to. 030 and 0. 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. In fact, knit line strength will decrease with higher filler content and with longer fibers. 1. Knit lines, also known as weld lines, are fine lines that resemble hairline fractures or cracks. One common problem that occurs in injection molding is the formation of knit lines. [8] Bevis, MJ, Reis RL, Cunha AM. Monoform parts. Still, it can be seen to varied degrees depending on mold. ). In fact, knit line strength will decrease with higher filler content and with longer fibers. Gates that are too small can also cause the uneven flow of material or result in knit lines, blemishes that occur where two resin flows meet. Injection mold design and custom tooling can make or break the success of a finished plastic product. Once the plastic sets, the mold opens to eject the product. 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. 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). Injection molding is an efficient, often cost-effective way to manufacture custom plastics that are strong, durable, and long-lasting. Most failures occur at what are called “weld lines” or “knit lines”. Knit Lines and Resins. This makes the casting process as efficient and accurate as possible. In plastic part design for injection molding, it is important to design the injection mold so that the molten plastic fills the mold cavity entirely, cools as evenly and as quickly as possible without warping, and functions as intended. When the temperature is not very high, there are chances of premature solidification, which can cause weld lines to form. 763-755-7575. Weld-line strength for plastics can vary from 20% to nearly 100% of the strength of the plastic itself. 344. It’s called sequential valve gating, or SVG. These knit-lines are well-known as particularly critical regions in the mold when mechanically loaded. 025 inches per inch, depending on the particular resin, molding conditions, part design, part wall thickness, and direction of flow. Design with Plastics - for Injection Molding process. Main causes of knit lines in injection molding: 1. These defects are not visible, like small. A poor knit line can cause only cosmetic blemishes or it can significantly weaken the structural integrity of a part. The melt is forced to split up during filling of the cavity. Plastic injection molding is the industry’s most common way to manufacture parts. Plasti-Co® Nozzles for Injection Molding Machines. We recommend walls to be no less than 40 to 60 percent of adjacent wall thickness, and all should fit within recommended thickness ranges for the selected resin. These knit lines affect the parts aesthetics, but also they generally decrease the strength. Weld Line Phenomenon: Surface defect in which the weld, or the part where the flows of melted plastic meet inside the mold, shows up on the surface as a groove or pattern. 0 and Table 1). Weld / Knit Lines; Assumptions. Remedies: Raise the temperature of the mold or molten plastic. In places where the plastic flows around a solid piece of the mold, from a simple circle to forming complex shapes like rings or squares. The areas in the cavity where such buildup tends to be common are at end-of-fill areas, details that trap gas, and knit lines. 1050 Walnut Ridge Drive. Call 800-985-6750 or email your inquiry to sales@customgasketmfg. Increasing injection speed. Different plastics make different types of knit lines. a knit line will form 180°on the opposite side from the gate location. 1. Plastic injection molding is the process of injecting molten plastic material into a metal tool which then cools and ejects a plastic part from the machine. Disadvantage: May rust if improperly stored. Knit Lines. Weld lines are formed when there is an inability of two or more flow fronts to ‘knit’ or ‘weld,’ together during the molding process. Change gate locations. When injection molding complex parts, often knit-lines are formed during the mold filling stage. 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. Knit lines. Three terms that often get mixed up in the plastic injection molding industry are weld, meld, and knit lines. txt) or view presentation slides online. While this process is largely efficient and reliable, it's not without its potential issues. Check the diameter of the ejector pins and the pin hole tolerances, if the tolerance is too wide this could allow molten plastic to flow down between the pin and hole surface. Insufficient injection pressure and speed can cause weld lines. Inadequate venting may cause short-shots, poor surface appearance, or weak weld-lines. Disadvantage: May rust if improperly stored. For the metal feedstock, injection and mold-wall temperatures were 130 ºC. Core Geometry. Ham Lake, MN 55304. It is typically used for thermoset materials, but can be used with thermoplastics. • Holes formed by core pins are, in essence, 360° inside corners and should follow. The injection molding processing conditions for filling are given in Table 1. . 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. These are imperfections in the part where separated flows of cooling material meet and. TEFABLOC™ TOSI 431 75A is a high-flow thermoplastic elastomer (TPE) designed for large-part injection molding applications, offering exceptional flexibility and weather resistance. A plastic product formed by the injection mold is divided into two parts, and the line separating the product is called the parting line. 01 inches between parts fitted together to work best. See UNDERCUT. The knit line strength and other packing related properties both aesthetic and functional of injection molded polymeric products is improved by equipping the mold with packing pins. Those are knit lines, formed when two opposing flows of material join together in the mold cavity. 3. 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. 11) The part design must take into account possible knit line locations and work to direct them away from higher stress areas. What Are Cosmetic Standards for Plastic Parts?. By understanding material fill, injection mold designers can better position gates, anticipate where knit lines will appear, and locate difficult to fill spots in the mold. This information is intended to be used only as a guideline for designers and processors of modified thermoplastics for injection molding. 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. 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. INJECTION MOLDING GUIDE V2. , Rallis, A. Because injection mold design and processing is complex, a set solution will not solve all problems. 5 : 1 and 3. Fathom injection molding services and tools offer a single-source global solution that supports your company’s low- to high-volume manufacturing process. . Having uniform wall thicknesses between 0. Three terms that often get mixed up in injection molding are weld, meld and knit lines. Degassing – Degassing is the simple operation of opening and closing an injection mold to allow any trapped air (or gas) to escape. Trapped air in a mold manifests itself as a burn mark on the molded part. Get a free des. By contrast, a slow injection speed will give you a matte finish and a cold mold, such as 50F, will give you a more flexible part. Knit Lines and Resins. "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. 2. Table 2 lists the<br /> properties of common tool steels and the typical mold components for which they are<br /> used. Knit and meld lines are inherent to plastic. Image credit: The Rodon Group. Strength at the knit line can be as little as 20% of the. It is best practice to maintain a uniform thickness throughout an injection-molded part. 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. A well-designed injection-molded part will usually have draft, but the same draft that helps eject a part from the. Runner and gates must be balanced. injection molding. Imagine a river splitting to go around a rock, then meeting up again on the other side. There are processes such as gas assist that do not require the injection unit to pressurize (pack out) the cavity, but for our discussion, we will stay with conventional injection molding. Weld or Knit lines are a plane or section in an injection molding part where the flow of molten plastics meets from different directions. For this study, a special injection mold was designed and constructed (Figure 1) in order to produce specimens of various thicknesses (from 2 to 6 mm). To avoid these problems and create the best possible. Plastic Injection Molding Part Design Guidelines, Plastic part Design Guidance, Parting Line Draft Undercuts Wall Thickness & Uniformity Bosses Gussets and Ribs Radii and Corners Material Shrinkage Gating. Weak weld lines are perhaps the most common and difficult injection molding defect to eliminate. Islyn Thomas summarized the need for a particular vent location better than I ever could. Weld lines typically appear in a part when an obstruction of some kind separates the melt flow and prevents it from "knitting" together. 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. ABS is a good choice for handheld consumer devices. Knit lines are consideredmolding defects, and occur when the mold or/and materialtemperaturesare set too low:. Knit lines don’t. Valve gates are most efficiently. A machine heats and mixes the plastic until it is in a pliable molten shape, then a screw device turns to force the plastic into an outlet that leads to the mold. In layman’s terms, injection molding boils down to four simple steps: Melt material. TRADITIONAL MACHINES VENTS. Also known as weld lines, knit lines. Understanding Knit Lines in Injection Molding: Causes and Solutions. MINNESOTA. One of the most common ways of adding venting is by manually machining it into the mold (Figs. When venting stationary inserts or core pins in rib or part details with venting issues, I recommend no more than 0. Weld-lines or knit-lines are formed during the mold filling process when the split melt flow fronts meet at the same downstream location. Weld line. (2) Undesired movement of certain materials in a plastic. Material contamination, for example, can cause several molding issues. Hartland, WI 53029. Depending on the resin, resin temperature, mold temperature, and filling. The bottom of your rib must also feature a fillet. This line does not normally impact the overall shape or dimensions of the item. When working with customers, our design guidelines for living hinges are 0. To prevent swirling, perform tests whenever possible and use pre-compounded resin to ensure identical color match across parts. This production method is particularly popular when manufacturing in large quantities. Learn how HPP maximizes these specialized processes. 3D printed molds in the injection molding process to lower costs and lead time. Please read on, stay posted, subscribe, and we welcome you to tell us what you think. There are several possible causes that need to be investigated to troubleshoot weld lines in injection molding. The linear mold shrinkage of injection molded parts will vary from 0. Submarine Gates. Heat resistance. Based on the resin. Meld and knit lines are actually both different types of weld lines. Vertical Plastic Injection Molding. Figure 1 presents the shrinkage of polypropylene versus wall thickness in injection molded parts. Injection moulding (U. With over 50 years of experience, Universal Plastic Mold can help you avoid (or at least minimize) knit lines in your molded parts. H-13 Good general purpose tool steel. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and. Causes: Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic. Meld and knit lines are actually both different types of weld lines. A special mold provided with a glass window was used in order to directly evaluate the flow progression during the filling phase of the injection molding process in a thin-wall cavity and to. Adjusting features to be suitable for molding will help resin flow well into the mold, allow it to cool evenly, and reduce the chance of cosmetic defects such as sink, knit lines, flash, and warp. If debossed pattern is indispensable, it should have a depth of no less than 0. 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. Weld lines typically appear in a part when an obstruction of some kind separates the melt flow and prevents it from "knitting" together. Knit lines are a common defect in injection molding that can affect the appearance and performance of plastic parts. Shear Controlled Orientation Injection Molding of Polymeric Composites with Enhanced Properties. For instance, three widely used processes are insert molding, overmolding, and undercuts. Flow lines are off-color lines, streaks, and other patterns that appear on the surface of a part. The quality of the weld line can be improved through optimized processing conditions, increased mold and melt temperatures, and increased injection speed toWeld Lines. (1. A knit line is any line, visible or not, where two resin flows meet. 4. . 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. Weld lines: Also known as knit lines, these are molding defects that occur when two flow fronts meet without the ability to “weld. As these flow fronts come together. 01 inches between parts fitted together to work best. Weld or knit lines occur when melt flow fronts collide in a mold cavity. In the case of mold details, knits will occur on the lee side of a detail. 3. This defect is caused from excessive lube or oily stock. After selecting resin material, the design and process control of injection mold directly influence the quality of formed products. 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). 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. After selecting the direction in which the mold hole will be cut, it is much simpler to locate the separating line. A meld line occurs when two moving melt fronts converge and flow parallel to each other. Our glossary of terms provides definitions for a wide range of injection molding terms. Steven. When injection molding a part, a knit. 18/06/2021. Additional venting along the parting line perimeter will improve overall venting significantly. Advantages for your Injection Molding Machine:. Three terms that often get mixed up are weld, meld, and knit lines. Multiple parts with complex design and differing wall. Knit Lines. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and. Behind the obstruction, the flows try to merge again, but this is made more difficult by a low temperature of the mold surface and a low melt pressure. (for example, a hole on the plastic part), the knit line will be unavoidable. When needed, crush ribs can allow for draft and a tight fit for a part that requires both.