Submarine gates are another injection molding types of gate that is located below the surface of the part, typically on the underside or back of the part. Melt temperature does. Weld line are formed whenever melt fronts separate and recombine at some downstream location. Expand. But we can’t talk about the injection molding process without discussing the injection molding machine. Near-Net-Shape Molding enables us to eliminate knit lines, add undercuts, increase options for geometry and tolerances, and reduce tooling cost for low. In injection molded parts, there are 2 types of weld lines: The first type of weld mark is called cold-melt joints and is formed when the melt front enters the cavity with different gates. Getting started; Knit Line Injection Molding; Knit Line Injection Molding - China Manufacturers, Factory, Suppliers. Injection molding is the predominant manufacturing process used for producing transparent polymer products. Increase the injection speed. Still, it can be seen to varied degrees depending on mold. 1 in. Learn how HPP maximizes these specialized processes. Dabei erreichen unsere Produkte zufriedene Käufer in mehr als 30 Ländern. Compression-molding is also suitable for ultra-large basic. Swirling can increase depending on the material you use, and the texture or polish of injection molding tools. Injection moulding is a manufacturing process that melts, injects and sets plastic into a metal mould. Here’s a rundown of helpful tricks to keep in mind when designing and choosing a material for molded parts with walls: In general, ensuring your design has uniform wall thickness between 0. But those key aspects can be viewed differently if you’re auditing mold changes before startup or troubleshooting changes for an already validated process. such as knit lines and other imperfections. Double push filling method. The knit line itself (the vertical line in the middle) is visible in this lighting, but the biggest issue is with the texture. The temperature might fall at some places like in the mold, runners leading to the mold and injection molding. In compression molding, molds are closed around the charge, and in injection molding, the charge is injected into a closed mold cavity. Trapped air in a mold manifests itself as a burn mark on the molded part. These knit lines affect the parts aesthetics. 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. This course outlines mold filling and how different gating configurations affect filling. ) Mold Analysis Tools. He is currently a tooling manager for a large, multi-plant molding and manufacturing company. Encapsulating the metal insert with plastic creates a single molded plastic piece that’s generally stronger than one created using secondary assembly. 0035"Weld lines:Witness line where 2 or more fronts of molten plastic converge, also called knit lines or flow lines, the weld around the holes, the line between the injection gate. The mold maker may think that some minor defects are acceptable, or they may not even consider them from the beginning, which can cause problems when the injection mold is being manufactured. Steven. Inadequate venting. 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 weld or knit line may develop on the opposite side of the obstruction as the two fronts of material join. 3,000 ton machines can cost $700,000. The knit lines are the result of poor molding conditions (likely molding at too low of a temperature). PVC pipes are manufactured by an extrusion process. 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. To ensure quality parts, injection molding companies must have the best injection mold design first + (91) 96067-70777 +(971) 547-161-112. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. If the load is applied in the transverse direction relative to the knit line, matrix cracking is easily initiated, and the part can fail. 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. In injection molded parts, there are 2 types of weld lines: The first type of weld mark is called cold-melt joints and is formed when the melt front enters the cavity with different gates. Gates, vents, sliding cams, pick-out inserts, and any other place where the two sides of. Whereas conventional injection molding uses. 5. These defects are not visible, like small. Comprehensive. Venting at Weld Lines. "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. There needs to be a balance between fit of the halves, function of the part, visual quality expectations, and the cost to produce the mold. Injection molding is the cost-effective method and is best suited for higher volumes. This information is intended to be used only as a guideline for designers and processors of modified thermoplastics for injection molding. For non-reinforced or non-filled nylon, the mechanical performance in the knit lines (planes) areas are approximately equal to the mechanical performance of the resin (polyamide) used. txt) or read online for free. Injection molding can be a real challenge if you don’t follow the “Five M’s”: Man, Mold, Machine, Material and Method. Gate design is another critical piece of part function in plastic injection molding. injection-molding machine (Arburg allrounder 320C, Germany). 0mm) throughout will ensure that the parts cool evenly and will prevent warping. The knit lines are the result of poor molding conditions (likely molding at too low of a temperature). Injection speed and pressure. 26 = 0. spelling: injection molding) is a process for producing parts by injecting molten material into a mould, or mold. Learn about the key characteristics of common injection molding materials, recommended applications, and how common thermoplastics compare to one another. Determine the time duration of the molten state. Knit lines. As a result, molten plastic takes the injection mold core and cavity shape. Learn more. 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. (Overpacking. A knit line is any line, visible or not, where two resin flows meet. Knit Mark – A witness mark on a molded part, often occurring at the midpoint between two transfer or injection sprue locations. Knit Line Injection Molding is a process that allows manufacturers to produce parts more quickly, accurately, and at less cost than traditional injection. The injection molding process offers a vast array of plastic resins to work with. Our Injection Molding Capabilities • Horizontal Injection Molding up to 3300 TONS • Structural Foam Molding + Structural Web • 1oz to 600oz part capacity. – Change the thickness of the product. As these flow fronts come together. 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. Additionally, it may produce sinks and voids caused by localized shrinkage during the part’s cooling process. Weld / Knit Lines; Assumptions. Designers are able to specify seamless design features with compression molding. This makes the casting process as efficient and accurate as possible. 5-3. molding run to be sure that the temperature does not exceed the recommended limits . Sink Marks Cause 5: Improper Part Geometry. Gating. After the solidification of molten plastic, the finished part is ejected out. pdf), Text File (. They again get resolidified along the surface. Mold analysis tools are used by designers of molded plastic parts and by designers of the mold tools used to manufacture those parts. As the polymer resin is injected into a mold, it flows through all parts of the mold. Notched weld lines then remain near the cavity walls. In this case it would be 0. spelling: injection molding) is a manufacturing process for producing parts by injecting molten material into a mould, or mold. There is no need to worry about flow or knit lines that may appear with injection molding. 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. 11) The part design must take into account possible knit line locations and work to direct them away from higher stress areas. Depending on the design of the mold and the material being injected, a knit line may: present no problem at all, be a cosmetic issue, or cause a potentially serious structural problem. In addition, the force of the gas holds the appearance side of the part in contact with. This article will discuss the importance of injection molding nozzles and what makes a good nozzle for a molding or injection. Mold bases, ejector plates, and some cavities (if nickel or chrome plated to prevent rust). Here is a look at the. Macromol Symp 1988;21/21: 489-500. DFM Helps Prevent Sink Marks In Plastic Injection Molding. The last cause of flow lines may be the operators running your daily operations. Weld / Knit Lines; Assumptions. RapidDirect – Your Best. 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. Such molds or tools aren’t intended to be used in operation. And like all manufacturing industries, plastic injection molding lines are more profitable the shorter the cycle time — so long as they produce quality parts. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. Let’s review what causes meld and knit line imperfections, how they’re different, and how they affect part durability. It emerges and becomes visible when the flow fronts cannot mold or knit during the molding process. Parting Lines. The comment on melt flow index is correct, but, I've molded straight PC down to . Weld lines have a significant effect on manufactured products. Final Report-Plastic Injection Molding - Free download as Word Doc (. Inadequate injection pressure can result in knit lines and a surface that. Mold Venting. Submarine Gates. In a well-designed mold, this is generally not a problem. A poor knit line can cause only cosmetic blemishes or it can significantly weaken the structural integrity of a part. Learn more about the process by reading this white paper. 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. Parts molded with ABS are susceptible to forming knit lines, which are visible lines on the surface of the part. 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. 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. 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). They again get resolidified along the surface. 080" – 0. Nozzles for Injection Molding Machines. Weld Lines-3 Ways for Detecting and Correcting. Yet a smooth finish sometimes actually highlights flaws like knit lines and blemishes in the part. While mold-filling simulation is a very common tool for predicting the fill patterns of an injection mold, in our judgment there is not yet a commercially available, satisfactory filling simulation for microcellular foam molding. 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. Runner and gates must be balanced. Bad habits can cause many different problems with injection molding machines, and flow lines are included in that. This article looks at the issues presented by a weld line in injection molding and the techniques manufacturers can use to avoid them. Industrial Knitting Machines. Using three test materials, polystyrene (PS), polycarbonate (PC) and polyoxymethylene (POM), the relationship between a design and a processing parameter. 1050 Walnut Ridge Drive. Therefore, with the cooling process, the resin in the central part shrinks and expands to the surface, resulting in insufficient filling in the central part, which is called vacuum. The uneven thickness of the product will make the melt flow resistance larger and smaller, resulting in unstable melt flow. This will helpTroubleshooting Injection Molding Weld Lines. Venting. Gate size can be increased or moved to a section with greater wall thickness to help eliminate. Download : Download full-size image; 3. directly! FCS Group, based in Taiwan, was established 40 years ago and is well known in Asia as a quality manufacturer of machinery, offering several lines of injection molding machines and other specialty and customized equipment. Plastic injection molding is the industry’s most common way to manufacture parts. In addition to the main factory and development. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and. After samples have been approved, it’s time to kickoff. Example of a gate location in a finished part Example of knit lines in a finished part. Depending on the material you use, the knit lines could be invisible or obvious—work with an injection molding company to figure out how to reduce the mold lines on your product. On the other end of the scale, a desktop injection molding machine with 5 tons of force can cost between $30,000-50,000. These knit-lines are well-known as particularly critical regions in the mold when mechanically loaded. Knit lines occur at points where resin flows converge after molding around an obstruction or protrusion within the mold, usually away from the gate area. 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. AS9100:2016. 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. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part. One common problem that occurs in injection molding is the formation of 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. Increase injection speed and pressure: Faster-moving materials will meet more quickly so one doesn’t cool too soon. Typically, injection molded parts will be between 0. The knit line is generally opposite the gate in thermoset. 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. Back to News Page. Weld line defects – also known as knit lines – is a defect that occurs when the injected plastic meets within a mold. We then. Support pillar suppliers can help determine the. One of these is knit lines, also called weld 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. This was usually with a PC/ABS material at knit lines and high fill pressures from thin walls in the part. There is high defect rate (surface defects/knit line). CAUSE: 1. Weld lines, also known as knit lines or meld lines, occur when two or more flow fronts meet during the plastic injection molding process. Crossref. Misra A. Also discussed are weld and meld lines and their effects on molded. 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. A poor design can lead to many visual defects in the final part. They occur when melt flow fronts collide in a mold cavity. 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. Reduce the temperature of the mold and plastic being injected. thick. weld line in injection molding. Mold Flow Analysis computer. Causes: Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic. 040 and 0. Parting lines placed at intricate joints. While injection molding itself is a straightforward process involving melting thermoplastic resin and injecting it into a mold cavity, knit lines can pose. Part Shorts No burn marks. Without adequate support pillars, flashing of the parting line faces and bowing of the plates may occur. 18/06/2021. Gates, vents, sliding cams, pick-out inserts, and any other place where the two sides of. PAPER ABSTRACT: Desktop injection molding machines and stereolithographic print ed molds make hands-on injection molding production possible in design studio classrooms and labs. Holes in plastic parts are normally cut with a router (or drilled if round. 3. Proper mold cooling can eliminate warp, decrease cycle times, and increase part quality. Viscosity represents a broad concept with huge effects on any efforts to standardize an injection molding process. It’s also relatively inexpensive. Shear Controlled Orientation Injection Molding of Polymeric Composites with Enhanced Properties. POROUS MOLD MATERIALS. Strength at the knit line can be as little as 20% of the. In fact, knit line strength will decrease with higher filler content and with longer fibers. Setting that Delta P is the secret to consistent fill time. PROBLEM: POOR ADHESION OF LSR MATERIAL TO. To learn more about us or to get a quote, click here or call 1. J (left) shows embossed feature and S (right) shows debossed feature in plastics through injection molding. Gating. Soft metals, such as aluminum and beryllium copper, can be used for prototype<br />. 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. Weld lines may appear like seams or scratches,. 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. 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. The point I am making here is, if you’re injecting 14. Vents for phenolic parts should be approximately ¼" (6 mm) wide and 0. 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. Parts as thin as 0. Meld and knit lines are actually both different types of weld lines. The structural stability of. 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. ABS. The fluid path has an inlet and a radially continuous, downwardly. 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. 2. Knit lines, also known as weld lines, are fine lines that resemble hairline fractures or cracks. Meld and knit lines are actually both different types of weld lines. Reduce the injection pressure. 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. shows some obvious locations for vents, a brief molding trial should be conducted to observe where the gas voids occur. It is caused by the incomplete joining of the uncured. Kaminski, in Specialized Molding Techniques,. 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. 45 in. Knit lines are formed in injection molding when two flow fronts meet in the middle of a part (a line where two sides of the. สาเหตุรองของการเกิดปัญหาการเกิดประกายเงิน (silver streak) 1. Our plastic injection molding glossary provides key terms. Islyn Thomas summarized the need for a particular vent location better than I ever could. Plastic injection molding is the industry’s most common way to manufacture parts. To avoid these defects, manufacturers can check molds for the following: Low Mold Temperature: A mold temperature that’s too low will cause the material temperature to drop fast. Weld lines, which are also referred to as knit lines, are among the most common of these blemishes. Introduction to Knit Line Injection Molding Knit Line Injection Molding is a process that allows manufacturers to produce parts more quickly, accurately, and at less cost than traditional injection molding. 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. Piccarolo, S. Excessive mould lubricant. Be wary of saving too much weight or you might end up with: cracking. It explored the use of. 3 /sec. 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. There are times when porous materials like Porcerax are used in problem or backfill areas. Three terms that often get mixed up in injection molding are weld, meld and knit lines. Mold flow analysis is a software simulation that can show how resin will fill the mold during the injection molding process. Edge gates, which can include fan and tab-shaped gates, are the most common ways to gate a tool. Firstly, molten thermoplastic with high pressure injects into an injection mold during injection molding. Here are some reasons why design is important for injection molding. turbulent flow of Injection speed set too high. Knit Lines. That’s the air escaping out of the nozzle until the hose is full of pressurized water. CURE: Run the mold in. Knit lines, holes, part configuration and the talent of your molder all come into play. New blog post is up! What Are Knit Lines in Injection Molding, and How Do You Prevent Them? #injectionmolding #moldsmart #howto #rjg…Injection moulding is a well established mass production tool for polymeric products of all shapes and sizes which is now drawing the attention of companies and researchers who seek to utilise this high throughput fabrication technique to produce devices with nanoscale dimensions []. Extreme geometries can cause knit lines. All the while, the material could be the correct material," Hill says. 2. 040-in. 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 . wall thickness, knit lines, draft. 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. Contact Us. Therefore various injection molding defects can be eliminated. doc), PDF File (. To study the weld line formation of colliding flow fronts the filling of a mold cavity was simulated. As a result, prototyping is strongly recommended for parts with. 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. The three dimensional structure is formed of a resin fiber mixture having a resin material infused with carbon fibers having a length of about 12. 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. Sink Marks Cause 2: Low Pack and Hold time. This will help the mold designer to determine the allowable gate and knit line locations. Venting at Weld Lines. 7. "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. Randy Kerkstra has been in the plastics industry for over 26 years, occupied frequently with troubleshooting injection molding. Weld or knit lines are perhaps the most common and difficult injection molding defect to eliminate. 1016/0254-0584(86)90019-2 Corpus ID: 136712450; Impact strength of knit-lines in injection moulded parts @article{Criens1986ImpactSO, title={Impact strength of knit-lines in injection moulded parts}, author={R. It tells them that you visualized how injection molded parts cool and harden. Table 2. suppressed cooling and possibly warp. They are the result of fundamental physical effects taking place inside of the mold. As. Plastic injection molding is a complex and highly intricate process. This normally occurs around holes or obstructions and causes locally weak areas in the molded parts. Insufficient injection pressure and speed can cause weld lines. Rynite® PET is usually processed with melt temperatures between 280 to 285 °C (535 to 545 °F) . RELATED CONTENT. Possible causes: Low temperature or fluidity of melted plastic during injection molding. Surface Finish: The injection molding process may result in visible knit lines or parting lines on the part’s surface. . POROUS MOLD MATERIALS. Using abrasives in a pressurized air blast to create a surface texture on the part. 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. Achieving a high-quality surface finish can be challenging, and other methods, such as polishing or coating, may be required. Each mold produces as specific part. When the mold adopts the multi-gate pouring scheme, the plastic melt. Thanks to the low air counter pressure in the cavity, this allows injection parameters to be managed in a larger window, optimizing injection time, part aesthetics and mechanical characteristics. Currently working with plastic, rubber, LSR, MIM molding companies to help them eliminate their complex molding challenges like air entrapment. Once the plastic cools, the mold is removed, and the object is ready for post-processing. Our glossary of terms provides definitions for a wide range of injection molding terms. The most common location of knit line formation is behind molded-in holes that are made by core pins in the mold. Again, you would round this value up or down based on a standard cutter size, which is 13/64 or 0. The above guidelines for initial heater band settings are for most injection molded parts. However, compression molding often provides poor product consistency and difficulty in controlling flashing, and it is not suitable for some types of parts. When venting stationary inserts or core pins in rib or part details with venting issues, I recommend no more than 0. Molding analysis study: An example of the low patterns and knit (weld) lines formation (MoldFlow® data) of injection molded round plastic part with a complex shape and with a ring. A. To prevent swirling, perform tests whenever possible and use pre-compounded resin to ensure identical color match across parts. excessive or premature wear at the parting line. 3 To get the most out of your Xometry Injection Molding experience, we offer the following part Design Guide with. Weld lines, knit lines, and meld lines are terminologies to describe the imperfections of finished parts in plastic injection molding processing. 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). 250 in. Weld Lines. Let’s stop for a second and think about the logos we usually see on a daily basis. For over four decades, our experienced team and leading edge technology have provided you with tooling solutions from start to finish. 4. 2. Rapid heat-and-cool molding technologies have attracted increasing. They have their origins in material choice, part design, tooling, and processingIn 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. Plastic injection force exceeding the clamping pressure of the press. อุณหภูมิของพลาสติกเหลวสูงเกินไปจนก๊าซหรือความชื้นแยกตัวออกมาได้. These materials have beenDetermining the correct tonnage provides a foundation that must be rock-solid to avoid flash and damage. Computer simulation packages of injection molding have had success in predicting filling behavior in extremely complicated geometries, capable of predicting weld lines’ location exactly. Understanding types of injection molding process can help to avoid some design challenges early in the process. Cold Slug: First material to enter mold during injection. Temperature is another factor that can cause weld or knit lines in injection molding. Different plastics make different types of knit lines. SPI surface finishes are the most common set of injection molding finishing standards around the world with over 12 different grades. 120 in. Mold venting is critical to the quality and consistency of the finished part. Polypropylene Consumer and Injection Molding Containers, Crates, Garden Furniture, Housewares, Luggage, TSL, TWIM lyondellbasell. Three terms that often get mixed up are weld, meld, and knit lines. 453. Left: L-shaped perform. 160"). Troubleshooting Injection Molding Weld Lines. Knit lines are hard to eliminate for injection molded plastic parts but designers can minimize cosmetic defects. Let’s review what causes meld and knit line imperfections, how they’re different, and how they affect part durability. Vibration assisted injection molding. Swirling can increase depending on the material you use, and the texture or polish of injection molding tools. 8 mm relief at the edges of the thread. Sequential valve gate technology. Holes in the part will always cause knit lines. Weld or Knit lines are a plane or section in an injection molding part where the flow of molten plastics meets from different directions. Incorrectly placed single gates may cause knit lines. Injection Molding Materials When using PolyJet molds, both tool life and part quality will be dependent on the thermoplastic material used during the injection molding process. These lines occur usually around holes or obstructions and cause locally weak areas in the molded part. Plastic injection molding has long been an affordable and efficient process for producing custom prototypes and end-use production parts. Weld lines may appear like seams or scratches, but that is not what they are. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. sinks, knit lines, and shrinkage,” Spalding says. Flow Lines. Learn about the key characteristics of common injection molding materials, recommended applications, and how common thermoplastics compare to one another. Let’s.