With over 50 years of experience, Universal Plastic Mold can help you avoid (or at least minimize) knit lines in your molded parts. Whereas conventional injection molding uses. The melt is forced to split up during filling of the cavity. Be wary of saving too much weight or you might end up with: cracking. suppressed cooling and possibly warp. 040-in. Knit and Meld lines are inherent to plastic injection. SEA-LECT Plastics has 16 MILACRON injection molding machines that range from 33-ton with a 2. Meld and knit lines are actually both different types of weld lines. plastic recycling machine, blown film extruder, filament extrusion line, injection molding machine, blow molding machine, injection blow molding machine, bag making machine, flexo. Sprue bushing too long. 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. Recent developments were oriented on the analysis of the mechanical performance at local (knit lines and welds) and bulk (molded part) areas, with the influence of molding and welding conditions. 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. 3,000 ton machines can cost $700,000. Understanding Knit Lines in Injection Molding: Causes and Solutions. Part Design Summary Bosses:Welds and/or Knit lines can be defined as the inability of two or more flow fronts to “knit” together, or "weld", during the molding process. Additional venting along the parting line perimeter will improve overall venting significantly. Vacuum. Compression molding and injection molding are very similar, but they have one major difference. This continues until the weld lines in plastic injection molding disappear. Three terms that often get mixed up are weld, meld, and knit lines. Gas trapping/ poor venting. Rynite® PET is usually processed with melt temperatures between 280 to 285 °C (535 to 545 °F) . 003" - 0. Direct experience designing molds and parts for injection molding and producing the molds and parts is valuable, relevant experience for design students. It has low shrink and high dimensional stability, and it has good resistance to acids and bases. Given the cosmetic requirements, there is a good chance the mold would require. That’s the air escaping out of the nozzle until the hose is full of pressurized water. 3. More about the knit lines in injection molding. Avoid adding threads directly on your injection molded part. The goal is to create a uniform thickness and at the smallest (thinnest) level required. The last cause of flow lines may be the operators running your daily operations. This article will discuss the importance of injection molding nozzles and what makes a good nozzle for a molding or injection. Expand. As these flow fronts come together. Contact Us. Injection Molding Problems and Solutions (17/17) – Weld or Knit Lines Sep 27, 2022 Injection Molding Problems and Solutions (16/17) – Warpage Sep 15, 2022. Long shapes are produced without multiple drops or hot runner systems, eliminating knit lines and lowering tooling costs. The injection molding machine includes a fluid path extending through the first mold portion. Specifically, by having a single, continuous injection outlet 140, molding material can be uniformly distributed throughout mold cavity 120 quickly and consistently without resulting in undesirable molding seams or knit lines. , 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. Exploring common molding defects and. ) 2. To avoid walls that are too fat or too thin, designing a wall thickness between 0. 2) Vents in an injection mold should let the air out, but not the liquid plastic. Too cool a melt. This slight depression is a tiny line called the weld line or knit line. Incompletely fused extrusion knit Line. a specific area on one side lens, a warp line, and a weft line. injection molding. Knit Lines. More about the knit lines in injection molding. The Difference Between Knit and Meld Lines–and Why It Matters. This defect is caused from excessive lube or oily stock. They occur when melt flow fronts collide in a mold cavity. A poor knit line can cause only cosmetic blemishes or it can significantly weaken the structural integrity of a part. Industrial Knitting Machines. A weld line (weld point, flow line or a knit line) forms when separate melt fronts travelling in opposite directions meet. Compression molded parts can vary in thickness without issue. 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. This production method is particularly popular when manufacturing in large quantities. Meld and knit lines are actually both different types of weld lines. Knit lines and other aesthetic characteristics of the part are strictly related to the plastic’s temperature during molding, so efficient filling time is essential for achieving proper welding lines and quality parts. In addition, the company is now providing three. There are two primary reasons for this occurrence: 1. Back to News Page. They again get resolidified along the surface. 030- to 0. All injection molding ultimately comes down to the “Man, Mold, Machine, Material and Method. . Let’s stop for a second and think about the logos we usually see on a daily basis. Bubbles are caused by melting flow fronts colliding in a mold cavity. 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. Download : Download full-size image; 3. PAPER ABSTRACT: Desktop injection molding machines and stereolithographic print ed molds make hands-on injection molding production possible in design studio classrooms and labs. 1. 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. Weld line defects – also known as knit lines – is a defect that occurs when the injected plastic meets within a mold. When injection molding complex parts, often knit-lines are formed during the mold filling stage. Weld lines are a common imperfection to be found in most plastic injected molded parts. Some of the most popular brands include:Harshal Bhogesra has 9+ years of combined experience in molding, customer & supplier development. Additionally, it may produce sinks and voids caused by localized shrinkage during the part’s cooling process. Overall, injection molding is a relatively simple process. However, compression molding often provides poor product consistency and difficulty in controlling flashing, and it is not suitable for some types of parts. (for example, a hole on the plastic part), the knit line will be unavoidable. Part Shorts No burn marks. The plastics used by injection molding processes are relatively cheap and can be used to achieve a wide variety of properties, so injection molding is popular for creating many packaging and consumer products, like LEGO! A weld line (weld point, flow line or a knit line) forms when separate melt fronts travelling in opposite directions meet. Learn about the key characteristics of common injection molding materials, recommended applications, and how common thermoplastics compare to one another. 7. Sink Marks Cause 5: Improper Part Geometry. Knit lines, weld lines, mold lines, or flow lines in injection molding are faint lines that become visible on certain plastic parts. Venting at Weld Lines. The knit line itself (the vertical line in the middle) is visible in this lighting, but the biggest issue is with the texture. What Are Knit Lines? When a product is produced through injection molding, a high-powered tool injects melted plastic into a custom, double-sided mold. matte or smooth texture on the part’s surface gives a more desirable aesthetic and hides small imperfections such as knit lines, flow lines, shadow marks, and sink marks that were caused by the mold. In fact, strength at the knit line can be as little as 20 percent of the nominal strength of the part. Adding vents, using overflows, or using porous metal. 3. When assessing issues with injection molded parts, we want to make sure some core aspects of molding are checked. Flow Lines Knit or Weld Lines Cavity Gates Table 1 Knit line in a multi-injection-point cavity. 0 mm) is crucial to ensuring that parts cool evenly. By altering the part design, it may be possible to just have one flow front, and avoid multiple flow fronts having to come together. อุณหภูมิของพลาสติกเหลวสูงเกินไปจนก๊าซหรือความชื้นแยกตัวออกมาได้. Such molds or tools aren’t intended to be used in operation. Knit lines. Depending on the design of the mold. As a result, prototyping is strongly recommended for parts with. In fact, for a 32-cavity mold, the flow rate in this example drops down to 0. Knit lines, also known as weld lines, are fine lines that resemble hairline fractures or cracks. They occur when two resin flows meet at different. I 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…Here are a few of the most common defects that may occur in plastic injection molding — and how product teams can avoid them. Mold cores can be machined from a solid piece of copper alloy or inserted to aid in construction or allow for easier replacement if a com-ponent would ever be damaged in molding. Figure 1 Flow Lines Knit or Weld Lines Cavity Gates Table 1 Knit line in a multi-injection-point cavity. Parting Lines. In working with the injection molding service offered by Protolabs, you provide. 3 To get the most out of your Xometry Injection Molding experience, we offer the following part Design Guide with. Weld 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. Gating. As melt temperature, viscosity and abrasiveness rise, tool life will decline. Knit Lines. During injection, support pillars keep closed the two halves of the plastic injection mold. The knit (weld) line is in the direction perpendicular to applied stress. To account for variations in viscosity, nylon injection molding machines need to maintain an appropriate injection pressure. injection molding. 020"). Kurt W. Obstructions in the part create weld lines or knit lines during injection molding. Proper venting is essential to molding a defect-free part. ) Mold Analysis Tools. Notched weld lines then remain near the cavity walls. Considerations: ABS-molded parts will show. Wavy lines are another feature of weld lines. 02 in. 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. Computer simulation packages of injection molding have had success in predicting filling behavior in extremely complicated geometries, capable of predicting weld lines’ location exactly. Here is a look at the. 18/06/2021. 3 /sec. 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. 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. Thermoplastic Elastomer Molding Guidelines. Potential Knit Lines • Knit lines occur where to or mold melt fronts come together-Holes, slots, multiple gates, variations in wall thickness-Meld lines occur when the melt comes together in the same flow direction • Knit lines are areas of weakness and should be located in low stress areas . 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. 5 mm (0. 120 in. Moisture in the material due to insufficient drying can form at the melt front, forming an interface between the two melt fronts. 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. As the polymer resin is injected into a mold, it flows through all parts of the mold. Knit/Weld Lines. While injection molding itself is a straightforward process involving melting thermoplastic resin and injecting it into a mold cavity, knit lines can pose. These defects are not visible, like small. Injection molding is a plastic manufacturing process to achieve a high production rate. The temperature might fall at some places like in the mold, runners leading to the mold and injection molding. It’s also relatively inexpensive. and to identify potential knit lines. This picture shows a. Mold cooling is one of the most important variables in injection molding. 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. 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. 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. Because they can be shut off independently you can control flow fronts and knit-line locations. Meld and knit lines are actually both different types of weld lines. The main causes of Weld Line or Knit lines are: Convergence of two flowing polymer melt streams during injection molding process: This is the most common cause of weld lines, as the two melt streams coming from different runners or gates in the mold meet and do not fully mix, creating a weak point in the finished product. The plastics used by injection molding processes are relatively cheap and can be used to achieve a wide variety of properties, so injection molding is popular for creating many packaging and consumer products, like LEGO!. Having uniform wall thicknesses between 0. 5. 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. The depth of the texture or letters is somewhat limited, and extra draftInjection molding tool design and build for complex parts, with precision tooling and molding capabilities for industries demanding high-quality, critical components. 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. The parting line will always be visible, but can be minimized by its location, draft allowance and part geometry. Viscosity represents a broad concept with huge effects on any efforts to standardize an injection molding process. When venting stationary inserts or core pins in rib or part details with venting issues, I recommend no more than 0. And like all manufacturing industries, plastic injection molding lines are more profitable the shorter the cycle time — so long as they produce quality parts. For over four decades, our experienced team and leading edge technology have provided you with tooling solutions from start to finish. The most common location of knit line formation is behind molded-in holes that are made by core pins in the mold. 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. The point of meeting creates a weak spot because the edges of the molten plastic, already cooling, don’t mix together very well. com support@makenica. (knit) line and produces a latent, or obvious, part defect. Minimizing or eliminating flow and knit lines is an additional benefit of using this material. Weld lines create visible marks, but this type of injection molding defect can also reduce a part’s strength and durability. 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. 4. Some are: Knit lines (discontinuities where two flows merged, creating a weak point)Those are knit lines, formed when two opposing flows of material join together in the mold cavity. Depending on the resin, resin temperature, mold temperature, and filling. Knit lines. 1. It is important when selecting an injection molder that the right process conditions are used to reduce or eliminate the effects of. When the plastic cools, it takes the shape of the mold and is then ejected and finished. Refers to portion of injection molding part that has some taper to make it easier to remove from the mold. Plastic injection molding is a process where a thermoplastic resin is heated to its melting point and injected into the cavity of an injection mold. 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. Swirling can increase depending on the material you use, and the texture or polish of injection molding tools. Runner Type. Weld lines or knit lines and meld lines formed when two melt fronts run into each other or join together can be improved by. The structural stability of. The difference between a knit and meld line is significant, dramatically impacting the part’s structural integrity. 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. The main types of injection molding defects are: 1. Cold Slug: First material to enter mold during injection. When designing for injection molding, consider these steps to prevent knit lines. In fact, knit line strength will decrease with higher filler content and with longer fibers. There are times when porous materials like Porcerax are used in problem or backfill areas. When these areas are visible, it can pose no problem at all to the piece, be nothing more than a cosmetic flaw, or can be. No matter the size of a production run, the presence of blemishes on the finished product leads to unhappy clients. Final Report-Plastic Injection Molding - Free download as Word Doc (. While not always visible, weld lines (aka knit lines) are where two resin flows meet. 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. Based on the resin. There is no need to worry about flow or knit lines that may appear with injection molding. Also known as “stitch lines” or “weld lines”. 17. 4. One of these is knit lines, also called weld lines. Possible causes: Low temperature or fluidity of melted plastic during injection molding. Injection moulding (U. Weld line in injection molding is line where two or more flow molten plastics knit together. Knit Lines: These occur when different flow fronts come together as plastic flows through the part cavities. Injection molding is an efficient, often cost-effective way to manufacture custom plastics that are strong, durable, and long-lasting. Use gradients. Snake Lines: The appearance of snake lines is caused by the formation of a jet effect as the melt travels through a gate and into the mold cavity. Injection molding can be a real challenge if you don’t follow the “Five M’s”: Man, Mold, Machine, Material and Method. . 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. material Knit Lines Weld lines Small lines on the Caused by the melt-front flowing around backside of core an object standing proud in. (1. Plastic injection molding is the industry’s most common way to manufacture parts. 0 and Table 1). Near-Net-Shape production uses injection molding to account for 70-90% of the final geometry of a part. The following are the conclusion drawn from the present research: 1. Solid or semi‐solid, this material leaves a distinct border with adjacent melted material similar in. A knit line is any line, visible or not, where two resin flows meet. Participating in DFM and mold flow analysis is a good way to achieve this. But add 30% glass and weld-line strength retention drops to only 34% as high as the compound with no weld line. 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. 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 adhesivePrevents flow and knit lines. 3 /sec of material into a mold, don’t think the material is going into each cavity of a multi-cavity mold at the same 14. In the world of injection molding, each weld. Meld and knit lines are actually both different types of weld lines. The quality of the weld line can be improved through optimized processing conditions, increased mold and melt temperatures, and increased injection speed toInjection molding, producing, and forming resin materials are three core components of injection molding technology. , Rallis, A. – Change the thickness of the product. Melt temperature does. Runner Type. 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. 0mm) throughout will ensure that the parts cool evenly and will prevent warping. 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. Know about the cause of these lines and the potential solutions. These knit lines affect the parts aesthetics, but also they generally decrease the strength of the part. Because viscosity is a variable, keeping fill time constant minimizes the influence of these variations and provides a more stable process, and therefore. 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. plastic recycling machine, blown film extruder, filament extrusion line, injection molding machine, blow molding machine, injection blow molding machine, bag making machine, flexo. Table 2 lists the<br /> properties of common tool steels and the typical mold components for which they are<br /> used. Crossref. Weld line. Disadvantage: May rust if improperly stored. land and using maximum vent depths to get them as close to self-cleaning as possible. Depending on the design of the mould and the material being injected, a knit line may: present no problem at all, be a cosmetic issue, or cause a potentially. These seams or knit lines are avoided because the molding material flow does not a meet at an interface nor must. 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. Short shots and knit lines are two phenomena that are caused by using highly viscousHowever, they can also create weld lines or knit lines in the part where the flow fronts meet. These flaws can affect the aesthetic and structural integrity of the. The Problems & Solutions course is designed to train injection molding personnel in. Recommended for the bonding of nylon (polyamide) and polypropylene materials, this thermoplastic elastomer delivers wide compatibility and thermal stability, allowing for use in demanding conditions. The injection molding process offers a vast array of plastic resins to work with. One exception: ribs should be about 50. Compression-molding is also suitable for ultra-large basic. By using this method, theHowever, injection molding is not without its challenges. Prevent Flow and Knit Lines Through Mold Adjustments. And often, the mold is the same color as the plastic polymer, or can appear that way due to a lack of contrast caused by difficulties lighting the inside of the mold. Weld lines are formed during cavity. Having a basic understanding of the injection molding process will aid in understanding how to design for injection molding as well as how to forecast production schedules. 203 in. Thin wall injection molding can produce walls as thin as 0. They are the result of fundamental physical effects taking place inside of the mold. In addition to the main factory and development. It is also where a slide mechanism might meet where it shuts off on one of the mold halves. 060 and 0. When assessing issues with injection molded parts, we want to make sure some core aspects of molding are checked. Flashing can be a common defect in injection moulding and can affect the appearance and functionality of the finished product. 08 KICKOFF PRODUCTION. In a weld line, reinforcement fibers. 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. RapidDirect – Your Best. The injection molding process uses a granular plastic that is grav ity fed from a hopper. An undercut is any indentation or protrusion that prohibits an ejection of a part from a one-piece mold. This image shows a PVC pipe after it was accidentally dropped on the ground. Such molds or tools aren’t intended to be used in operation. mixed manually and quickly stored in the tank of an injection machine just before the start of the resin injection or (2) the components are stored separately in different tanks of an injection machine with a mixing head. Transfer molding also utilizes a press and a mold. menu. 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. Bad habits can cause many different problems with injection molding machines, and flow lines are included in that. Using three test materials, polystyrene (PS), polycarbonate (PC) and polyoxymethylene (POM), the relationship between a design and a processing parameter. Also known as knit lines. 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. See full list on fathommfg. This most often occurs when you have a core or shutoff feature that the plastic needs to flow around. Part thickness control is through stacking of. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. (1987). Knit Line occurs where different flow fronts of material meet while flowing through the mold. Weld lines, which are also referred to as knit lines, are among the most common of these blemishes. 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 injection speed and pressure: Faster-moving materials will meet more quickly so one doesn’t cool too soon. Using abrasives in a pressurized air blast to create a surface texture on the part. Solution: People. In a weld line, reinforcement fibers. 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. Weld lines may appear like seams or scratches, but that is not what they are. “A knit line in a colored part will result in glaring visual-effect problems, while a knit through a glass-filled part would have a negative effect on both the aesthetics and the performance,” agrees Kurt Weiss, manager of part performance and new material introductions at GE Plastics. Gate size can be increased or moved to a section with greater wall thickness to help eliminate. Parts with abrupt geometry changes or holes are more prone to knit lines. Most failures occur at what are called “weld lines” or “knit lines”. A poor design can lead to many visual defects in the final part. Advantages Excellent moldability in thin sections Outstanding chemical resistance Heat resistance Limitations Cost Knit line strength OverviewLiquid crystal polymer (LCP) exhibits a highly ordered structure in both the melt and solid states. TWIM=Thin Wall Injection Molding; TSL=Toys,Sports and Leisure; NB=No Break; IMR=Inter Material Replacement. In the present work, the micro specimens were investigated. Flow lines are off-color lines, streaks, and other patterns that appear on the surface of a part. They again get resolidified along the surface. In the plastic injection moulding industry, we work with materials that are by-products of oil refinement and moulds made of metal casting, so our terminology is bound to be at least slightly confusing. If you have any issues getting. Let the molten material cool to a solid-state. Reduce the barrel temperature and increase the injection pressure. 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. 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. This will help the mold designer to determine the allowable gate and knit line locations. 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. They occur when melt flow fronts collide in a mold cavity. In injection molding, it is often important to maintain consistent color throughout the part. Resin cools as it is injected into a mold. 040-in. After the solidification of molten plastic, the finished part is ejected out. Setting that Delta P is the secret to consistent fill time. Because injection mold design and processing is complex, a set solution will not solve all problems. Contact Universal Plastic Mold Today. Weld or knit lines occur when melt flow fronts collide in a mold cavity. In the plastic injection moulding industry, we work with materials that are by-products of oil refinement and moulds made of metal casting, so our terminology is bound to be at least slightly confusing. Weak weld lines are perhaps the most common and difficult injection molding defect to eliminate. If the part design’s wall thickness is constant, the magnitude of the plastic’s velocity will be the same at every. Solution: Increase the injection pressure or speed. It is. Mold Venting. 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. Material contamination, for example, can cause several molding issues. placed usually in the areas last to fill, near knit lines, as well as on the runner system. The packing pins are sequentially. Weld lines often leave witness marks in the moulding surface. But we can’t talk about the injection molding process without discussing the injection molding machine. 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. One solution is to use optimal wall thickness to help slow cooling during the ABS plastic molding process. The specific method is that when the product is just filled and filled in the cavity, the insert. There. Meld and knit lines are actually both different types of weld lines. Knit Line – A molding defect resembling a flow-line found at the point where the rubber meets between the injections. As a result, the portion has a faint, perhaps undetectable line. Knit line / Meld. 2020-01-1306. N174W21430 Alcan Dr Jackson, WI 53037 262-677-9700; 16685. Knit lines are hard to eliminate for injection molded plastic parts but designers can minimize cosmetic defects. Three terms that often get mixed up are weld, meld, and knit lines. Injection molding is a popular manufacturing process used to fabricate parts in large volumes. Remove holes if possible. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and. Reducing the dimensions of the runner system.