Injection molding is a manufacturing process that involves injecting molten plastic into a mold cavity to create a specific shape or design. Influence of injection-molding parameters on the morphology and mechanical properties of glass fiber-reinforced polypropylene composite. Table 2 lists the<br /> properties of common tool steels and the typical mold components for which they are<br /> used. 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. Knit Mark – A witness mark on a molded part, often occurring at the midpoint between two transfer or injection sprue locations. Weld lines or knit lines and meld lines formed when two melt fronts run into each other or join together can be improved by. weld line in injection molding. When setting a heat profile, consider residence time, screw RPM and filler type. Edge gates, which can include fan and tab-shaped gates, are the most common ways to gate a tool. A meld line occurs when two moving melt fronts converge and flow parallel to each other. The defect may be in part appearance or mechanical strength. These knit-lines are well-known as particularly critical regions in the mold when mechanically loaded. Gating. Rynite® PET is usually processed with melt temperatures between 280 to 285 °C (535 to 545 °F) . The process involves feeding the material into a press, where it is cooled or heated as per. All injection molding ultimately comes down to the “Man, Mold, Machine, Material and Method. Misra A. As. How to Eliminate Knit Lines In Injection Molding - Xcentric Mold Want to learn how to eliminate knit lines that form during the injection molding process? Then. In addition to the main factory and development. – Change the thickness of the product. Neat polypropylene retains nearly 90% of its strength on a properly designed and molded weld line. Formation of knit line. 030- to 0. Mold Flow Analysis computer. 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. Weld-line strength for plastics can vary from 20% to nearly 100% of the strength of the plastic itself. Typical Tool Steel for Injection Mold Construction Steel Type Steel Properties Mold Component P-20 Pre-hardened, machines well, high carbon, general-purpose steel. 040-in. Knit lines. Design bosses with an outer diameter equal 2x the nominal diameter of the screw or insert. 003" - 0. The second mold portion being movable relative to the first mold portion and together defining a mold cavity. 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. The gate architecture and its location affects cycle times, tooling costs, lead times, the location of witness marks and many other potential defects that are commonplace for. 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. Polym Compos 1982; 3: 88–96. Let’s. อุณหภูมิของพลาสติกเหลวสูงเกินไปจนก๊าซหรือความชื้นแยกตัวออกมาได้. 040-in. TEFABLOC™ TESI 744 TPE compound for injection molding processing provides superior adhesion properties when processed using injection molding. SPI surface finishes are the most common set of injection molding finishing standards around the world with over 12 different grades. This course outlines mold filling and how different gating configurations affect filling. POROUS MOLD MATERIALS. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. 040 and 0. This is why the leading edge of the resin flow within a mold is always the coolest area of the resin, and, therefore, the closest to solidifying. A knit line may only be a cosmetic issue and affect. RapidDirect – Your Best. 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. While this process is largely efficient and reliable, it's not without its potential issues. Part Shorts No burn marks. ISO 9001:2015. It emerges and becomes visible when the flow fronts cannot mold or knit during the molding process. These lines occurs usually around holes or obstructions and cause locally weak areas in the molded part. wall thickness, knit lines, draft. After selecting resin material, the design and process control of injection mold directly influence the quality of formed products. mold venting is at the last filling point, whereby, while the melt flows forward in the cavity, air and gases flow out through the valve. These defects are not visible, like small. First of all, ABS is prone to knit lines — or the places where two flows of plastic resin meet during the manufacturing process. gate position or runner. Weld line defects – also known as knit lines – is a defect that occurs when the injected plastic meets within a mold. (1. 4. Islyn Thomas summarized the need for a particular vent location better than I ever could. Gating. Prototype Tool In Soon As 10 Days; Production Tool In As Soon As 3 Weeks; 10k Parts in 10 Days; GET YOUR 30 SECOND QUOTE NOW. As the melted plastic fills the mold, there are locations where the molten plastic flow stream separates and then comes back together. Insert injection molding is a process that requires an insert — typically metal — to be pre-placed in the tool for injected plastic to flow around. It can also aid in material selection since fill. POROUS MOLD MATERIALS. Edge Gates. Insufficient injection pressure and speed can cause weld lines. Swirling can increase depending on the material you use, and the texture or polish of injection molding tools. Another name given to weld lines is meld line. Product designers encounter a wide range of challenges when creating and developing molded parts, such as stress factors, warp, and those pesky knit lines. The runner and gate system used in plastic injection molding is responsible for directing molten plastic from the nozzle of the molding machine into the cavity of the tool. For injection molding of the multi-blade fans and various rotating plastic parts, the complex of multiple gating injection molding tools were used. They again get resolidified along the surface. 827 Holes and Bosses. Plastic injection molding is a complex and highly intricate process. 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. Add a 0. Avoid areas where the plastic must suddenly flow upward or downward. Using abrasives in a pressurized air blast to create a surface texture on the part. 2020-01-1306. 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. Over the years, I have developed two separate “5M” systems. Additional venting along the parting line perimeter will improve overall venting significantly. incomplete filling or shorts. Piccarolo, S. This continues until the weld lines in plastic injection molding disappear. In spite of its cosmetic attributes, a bad knit line can significantly. A poor knit line can cause cosmetic blemishes and significantly weaken the structural integrity of a part. 2. Mold Venting. Injection speed and pressure. This slight depression is a tiny line called the weld line or knit line. ABOUT THE AUTHOR. A knit line, also. The diameter of the secondary runner would be the diameter of the tertiary runner times the number of branches to the 1/3 power (cube root). Increase the mold temperature and reduce the barrel temperature for injection molding of thin-walled products. The process parameters were fixed as follows: Packing pressure of 40 MPa, injection time of 2. 250 in. Weld lines create visible marks, but this type of injection molding defect can also reduce a part’s strength and durability. He is passionate about optimizing product design, tool development, molding processes, and quality. While not always visible, weld lines (aka knit lines) are where two resin flows meet. Let the molten material cool to a solid-state. shows some obvious locations for vents, a brief molding trial should be conducted to observe where the gas voids occur. a knit line will form 180°on the opposite side from the gate location. Learn Injection Molding Glossary with Plastopia: Using actual molded parts (with images) as examples. Move the injection unit away from the mold during short breaks . Criens}, journal={Materials Chemistry and Physics}, year={1986}, volume={14}, pages={69-83} }A structural automotive sub frame component ( 10 ) that is formed from a sheet molding compound having carbon fibers. Injection molding can be a real challenge if you don’t follow the “Five M’s”: Man, Mold, Machine, Material and Method. Weld Lines. In spite of its cosmetic attributes, a bad knit line can significantly compromise a part. The chart below shows recommended wall thicknesses for common injection molding resins. Two major sources of weld lines are multiple gating and flow around pins. Weld lines, knit lines, and meld lines are terminologies to describe the imperfections of finished parts in plastic injection molding processing. com Three terms that often get mixed up in injection molding are weld, meld and knit lines. Weld lines have a significant effect on manufactured products. The part is cooled in the mold until it is solid enough to be ejected. More about the knit lines in injection molding. 26 = 0. 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. Adding vents, using overflows, or using porous metal. Expand. 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. To briefly describe, weld lines are also called knit lines. 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. Transfer molding also utilizes a press and a mold. Sink Marks Cause 3: Improper Pack or Hold Pressure. This normally occurs around holes or obstructions and causes locally weak areas in the molded parts. The most common and one of the most challenging injection molding defects are weld and knit lines. Shear Controlled Orientation Injection Molding of Polymeric Composites with Enhanced Properties. . As a result, molten plastic takes the injection mold core and cavity shape. Back to News Page. Meld and knit lines are actually both different types of weld lines. 2. 3. [email protected] 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. Knit lines are a common defect in injection molding that can affect the appearance and performance of plastic parts. suppressed cooling and possibly warp. It is important when selecting an injection molder that the right process conditions are used to reduce or eliminate the effects of. When venting stationary inserts or core pins in rib or part details with venting issues, I recommend no more than 0. For over four decades, our experienced team and leading edge technology have provided you with tooling solutions from start to finish. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. Legend: a-in. It’s also relatively inexpensive. 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. More about the knit lines in injection molding. Make sure the venting is not too large. ) Mold Analysis Tools. Knit line / Meld. Proper venting is essential to molding a defect-free part. Gates, vents, sliding cams, pick-out inserts, and any other place where the two sides of. 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. Extreme geometries can cause knit lines. Weld or knit lines are perhaps the most common and difficult injection molding defect to eliminate. Injection molding is the predominant manufacturing process used for producing transparent polymer products. Injection molded plastic begins its journey through the part tooling via an entry section called a. Injection Molding: How to Set Second. Islyn Thomas summarized the need for a particular vent location better than I ever could. 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. 1050 Walnut Ridge Drive. Refers to portion of injection molding part that has some taper to make it easier to remove from the mold. 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. Weld or knit lines are perhaps the most common and difficult injection molding defect to eliminate. Increase the melt and/or mold temperature. Increase the size of the gate and runners to alter the mold design. 5 mm) is ideal, according to injection molding engineer Jack Rulander of Protolabs in Maple Plain, MN. 0 mm) is crucial to ensuring that parts cool evenly. They are formed when two or more flow frontINJECTION MOLDING GUIDE V2. Steven. Raising the mold temperature will keep the material molten longer, resulting in stronger weld lines and even elimination of weld lines. Part Shorts and has burn marks. LOCATION: Along any parting line. It explored the use of. Whereas conventional injection molding uses. Design. 3 /sec. "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. Reduce the screw speed and adjust the backpressure. 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. Anisotropy. Near-Net-Shape Molding enables us to eliminate knit lines, add undercuts, increase options for geometry and tolerances, and reduce tooling cost for low. plastic recycling machine, blown film extruder, filament extrusion line, injection molding machine, blow molding machine, injection blow molding machine, bag making machine, flexo. 2. The injection molding design process, much like all industrial design processes, deals primarily with the creation, look, and feel of a product. 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. Knit Lines. May happen at first shots while mold is being dialed in. 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. This will helpTroubleshooting Injection Molding Weld Lines. 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. Injection molding is the process of injecting liquid thermoplastic into a mold. Understanding Knit Lines in Injection Molding: Causes and Solutions. Weld line, also known as knit line or meld line, is a visible line or mark on the surface of a plastic part where two or more flow fronts of molten plastic meet and fuse together. Inadequate injection pressure can result in knit lines and a surface that. A knit line is any line, visible or not, where two resin flows meet. Randy Kerkstra has been in the plastics industry for more than 26 years, occupied. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. 1 in. Many materials can be. CURE: Run the mold in. Plastic melt and mold temperature. Once the mold is created and the machine is set up, additional parts can be manufactured very fast and at a very low cost. . Part thickness control is through stacking of. During injection, support pillars keep closed the two halves of the plastic injection mold. The data suggest that either local sample thickening or grooves form at knit line when complete section solidification takes place first at weld zone or. Knit Lines in Injection Molding. Solution: Increase the injection pressure or speed. Bubbles are caused by melting flow fronts colliding in a mold cavity. 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. Holes in the part will always cause knit lines. Weld lines, melt lines or Knit lines could be caused by different causes: - Low temperature of molding machine barrel - Inadequate back pressure - Injection pressure or injection speed is too low - Low mold temperature - Small injection gates and/or runners - Improper location of injection gate - Excessive gate land length1. The resulting line has the. Three terms that often get mixed up are weld, meld, and knit lines. (knit) line and produces a latent, or obvious, part defect. But we can’t talk about the injection molding process without discussing the injection molding machine. Different plastics make different types of knit lines. 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. Weld lines create visible marks, but this type of injection molding defect can also reduce a part’s strength and durability. Support pillar suppliers can help determine the. If the plastic has cooled too much during the injection process it can lead to knit lines in plastic parts when they meet past an the obstruction. The recommended minimum production volume for injection molding is 500 units. This can enable the. sinks, knit lines, and shrinkage,” Spalding says. It is also where a slide mechanism might meet where it shuts off on one of the mold halves. Learn more about the process by reading this white paper. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. Product designers encounter a wide range of challenges when. , 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. The thickness of the part plays an important role in how difficult the molding process will be. Inadequate venting. Learn about the key characteristics of common injection molding materials, recommended applications, and how common thermoplastics compare to one another. Anisotropy. S. 2. The packing pins are sequentially. From: Polylactic Acid (Second Edition. 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. Swirling can increase depending on the material you use, and the texture or polish of injection molding tools. Solution: People. Overall, injection molding is a relatively simple process. Also known as a knit line, this is the point at which two flow formations meet and are unable to connect during injection molding. 17. 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. Troubleshooting Injection Molding Weld Lines. Venting. Notched weld lines then remain near the cavity walls. 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. Knit lines. Yet a smooth finish sometimes actually highlights flaws like knit lines and blemishes in the part. In the process of injection molding, when the wall thickness of the product is large, the cooling speed of the outer surface is faster than that of the central part. How weld lines form. 156 x 2 1/3 = 0. 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. 120 in. Following lists causes and solutions for weld or knit lines. 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. Know about the cause of these lines and the potential solutions. Encapsulating the metal insert with plastic creates a single molded plastic piece that’s generally stronger than one created using secondary assembly. A screw-type plunger forces the material into a heated chambe r,. Such molds or tools aren’t intended to be used in operation. Let’s review what causes meld and knit line imperfections, how they’re different, and how they affect part durability. A meld line occurs when two moving melt fronts converge and flow parallel to each other. Venting is required to allow the air in the sprue, runner and cavity to leave the tool as the melt flows into the cavity. Industrial Knitting Machines. Injection moulding is a manufacturing process that melts, injects and sets plastic into a metal mould. txt) or read online for free. Meld and knit lines are actually both different types of weld lines. Injection molding is typically a better choice for more complex parts, while compression molding is a great option for relatively simple designs, including ultra-large basic shapes that cannot be produced using extrusion techniques. This article looks at the issues presented by a weld line in injection molding and the techniques manufacturers can use to avoid them. Much of this research has focussed on the materials which. The following are the conclusion drawn from the present research: 1. Polypropylene can be processed over a wide range of mold temperatures; however, for applications where aesthetics are critical, a surface temperature of 80-95˚CSink Marks Cause 1: Incorrect Melt Temperature. Knit lines, holes, part configuration and the talent of your molder all come into play. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. Nozzles for Injection Molding Machines. suppressed cooling and possibly warp. 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). Knit lines form during the injection molding process when two resin flows meet. Knit lines form when two or more plastic flow fronts collide and solidify or “knit” during the molding process. Inappropriate temperature. They occur when melt flow fronts collide in a mold cavity. As a result, prototyping is strongly recommended for parts with. Material contamination, for example, can cause several molding issues. ”. Such molds or tools aren’t intended to be used in operation. The goal is to create a uniform thickness and at the smallest (thinnest) level required. There are times when porous materials like Porcerax are used in problem or backfill areas. 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. The Goldilocks principle applied to wall thickness. 3 To get the most out of your Xometry Injection Molding experience, we offer the following part Design Guide with. Three terms that often get mixed up are weld, meld, and knit lines. Parts molded with ABS are susceptible to forming knit lines, which are visible lines on the surface of the part. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. This range can be affected by the part wall thickness, melt temperature, mold temperature, injection speed, hold/pack pressures and also the time between molding and measuring. A. Knit lines. injection molding. These knit lines affect the parts aesthetics. Proper mold cooling can eliminate warp, decrease cycle times, and increase part quality. ABS is a good choice for handheld consumer devices. An injection mold that produces pieces while the final tool is being built. Numerical results show that weld lines can be predicted accurately, and the formation and evolution processes of weld line can be shown more clearly by co-injection molding technique. 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 Ventings Knit/Weld Lines. Final Report-Plastic Injection Molding - Free download as Word Doc (. Weak weld lines are perhaps the most common and difficult injection molding defect to eliminate. Poorly constructed or worn out mold. There are times when porous materials like Porcerax are used in problem or backfill areas. Parts with abrupt geometry changes or holes are more prone to knit lines. 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. menu. They are the result of fundamental physical effects taking place inside of the mold. 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. Redesign the mold: Redesigning the mold to. As the polymer resin is injected into a mold, it flows through all parts of the mold. Knit lines, also known as weld lines, are fine lines that resemble hairline fractures or cracks. Quotes for 25+ Manufacturing Processes. The mold settings can run flawlessly until employees consistently open the machine for inspections, interrupt cycle times, or cause temperature. By using this method, theHowever, injection molding is not without its challenges. Whenever possible, vents should be located in the movable half of the mold wherever a gas void or knit line is seen on a part. The relationship between processing conditions, morphological structure and mechanical properties of knit lines of an injection molded semi-crystalline thermoplastic. This can result in cosmetic defect or a weak point in the part. One of these is knit lines, also called weld lines. Vibration assisted injection molding. 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. 453. Highly ScalableVacuum on the mold does help and many times eliminates the weld line. Without adequate support pillars, flashing of the parting line faces and bowing of the plates may occur. On this page you will find the online, condensed version of this guide, offering quick answers to your injection molding questions. Weld lines are formed during cavity. In fact, knit line strength will decrease with higher filler content and with longer fibers. Near-Net-Shape production uses injection molding to account for 70-90% of the final geometry of a part. 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. 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. Our Injection Molding Capabilities • Horizontal Injection Molding up to 3300 TONS • Structural Foam Molding + Structural Web • 1oz to 600oz part capacity. Increase the injection speed. They occur when melt flow fronts collide in a mold cavity. Increase injection speed and pressure: Faster-moving materials will meet more quickly so one doesn’t cool too soon. occupied frequently with troubleshooting injection molding. All the while, the material could be the correct material," Hill says. Increase the injection speed. 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. Based on the resin. 1. From Plastic Materials and Processing by A. Three terms that often get mixed up in injection molding are weld, meld and knit lines. 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 injection molding process is on the surface is fairly simple, plastic resin is heated to its melting point and forced into the cavity of the mold to produce a plastic parts. 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. 08 KICKOFF PRODUCTION. SEA-LECT Plastics has 16 MILACRON injection molding machines that range from 33-ton with a 2. Parting lines placed at intricate joints. When designing for injection molding, consider these steps to prevent knit lines. It is. Kaminski, in Specialized Molding Techniques,. Surface Finish: The injection molding process may result in visible knit lines or parting lines on the part’s surface. 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. Cold Slug: First material to enter mold during injection. A good injection mold design will take all of these factors into consideration, ensuring that the final product looks great. If the parting line is located on a sidewall of the part, it is nearly impossible to construct a mold without a slight mismatch between the force and the cavity. 0mm) throughout will ensure that the parts cool evenly and will prevent warping. 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. Contact Universal Plastic Mold Today. 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. Circular Loom Machine; Raschel Knitting Machines; Shade Net Making Machinery; Inspection Machines; Non Woven Production Line. 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. While injection molding itself is a straightforward process involving melting thermoplastic resin and injecting it into a mold cavity, knit lines can pose. These flaws can affect the aesthetic and structural integrity of the. 5 to 3. See full list on fathommfg. 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. 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. 5 mm (0. Given the cosmetic requirements, there is a good chance the mold would require. 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. 040-in. Knit lines appear to be a faint line or a crack in the finished part after it comes out of the injection mold.