Figure 1: Knit lines in a cover plate. In fact, knit line strength will decrease with higher filler content and with longer fibers. land and using maximum vent depths to get them as close to self-cleaning as possible. 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. Surface Finish: The injection molding process may result in visible knit lines or parting lines on the part’s surface. Insufficient injection pressure and speed can cause weld lines. The effect of holding variables and of injection and mould temperature was explored. These lines can manifest as visible marks on the component’s surface and potentially affect its structural integrity. POROUS MOLD MATERIALS. Weld line in injection molding is line where two or more flow molten plastics knit together. Mold filling simulation software should be used to ensure that waterInjection Molding Problems and Solutions (17/17) – Weld or Knit Lines Sep 27, 2022Main causes of knit lines in injection molding: 1. 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 knit line itself (the vertical line in the middle) is visible in this lighting, but the biggest issue is with the texture. Excessive mould lubricant. สาเหตุรองของการเกิดปัญหาการเกิดประกายเงิน (silver streak) 1. Troubleshooting Injection Molding Weld Lines. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and. Injection. More about the knit lines in injection molding. 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. Remedies: Raise the temperature of the mold or molten plastic. Observation on a “trial and error” basis may be required. A thermoplastic resin is heated to its melting point and injected into the cavity of an injection mold to produce a specific part geometry. Knit Lines and Resins A knit line is any line, visible or not, where two resin flows meet (see Figure 2). However, compression molding often provides poor product consistency and difficulty in controlling flashing, and it is not suitable for some types of parts. Knit Lines in Injection Molding. weld line in injection molding. Learn how to troubleshoot weld lines in injection molding. Compression molding produces solid parts that are free of flow and knit lines. 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. Holes in the part will always cause knit lines. The temperature might fall at some places like in the mold, runners leading to the mold and injection molding. Example of a gate location in a finished part Example of knit lines in a finished part. Knit lines are most common downstream of holes created in parts where the molten material meets. turbulent flow of Injection speed set too high. 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. 0 mm) is crucial to ensuring that parts cool evenly. Gating. Using abrasives in a pressurized air blast to create a surface texture on the part. Multiple gates will always cause knit lines. spelling: injection molding) is a manufacturing process for producing parts by injecting molten material into a mould, or mold. Avoid adding threads directly on your injection molded part. Also known as knit lines. A knit line is what happens when plastic flows coming from two different directions meet inside of the mold and solidify. Check/add vacuum; Check mold lube; Check/increase venting. Anisotropy. Product designers encounter a wide range of challenges when. Plastics Injection Molding Expert John Bozzelli covers. Mold Flow Analysis computer. Injection molding is capable of producing a lot of aesthetic options as well. Weld or knit lines occur when melt flow fronts collide in a mold cavity. Some of the most popular brands include:Harshal Bhogesra has 9+ years of combined experience in molding, customer & supplier development. Contact us. 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. Influence of injection-molding parameters on the morphology and mechanical properties of glass fiber-reinforced polypropylene composite. . A knit line may only be a cosmetic issue and affect. Three terms that often get mixed up are weld, meld, and knit lines. land and using maximum vent depths to get them as close to self-cleaning as possible. Weld lines are among the most critical structural defects in parts made by the injection molding of fiber-reinforced thermoplastics, explained Sirmax in a press release. a specific area on one side lens, a warp line, and a weft line. AS9100:2016. More about the knit lines in injection molding. 0mm) throughout will ensure that the parts cool evenly and will prevent warping. Learn how to troubleshoot weld lines in injection molding. RapidDirect – Your Best. Edge gates, which can include fan and tab-shaped gates, are the most common ways to gate a tool. Weld / Knit Lines; Assumptions. com Three terms that often get mixed up in injection molding are weld, meld and knit lines. We originally thought it may be caused by a silicone release agent that was occasionally sprayed on the injection mold, but testing with minimal agent and later built-in slip agent showed no change. An undercut is any indentation or protrusion that prohibits an ejection of a part from a one-piece mold. Knit lines form when two or more plastic flow fronts collide and solidify or “knit” during the molding process. 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. Possible causes: Low temperature or fluidity of melted plastic during injection molding. Such molds or tools aren’t intended to be used in operation. After samples have been approved, it’s time to kickoff. The main types of injection molding defects are: 1. He said, “Vent cavities at weld lines or junction points where two or more material streams meet and fuse into a solid mass. There are times when porous materials like Porcerax are used in problem or backfill areas. 1. Venting is required to allow the air in the sprue, runner and cavity to leave the tool as the melt flows into the cavity. 4. Plastic injection molding has long been an affordable and efficient process for producing custom prototypes and end-use production parts. Knit lines, also called parting lines, are any lines where two resin flows meet and may or may not be visible. ”. Product designers encounter a wide range of challenges when. 3 /sec. . 2. Near-Net-Shape Molding enables us to eliminate knit lines, add undercuts, increase options for geometry and tolerances, and reduce tooling cost for low. 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. Resin cools as it is injected into a mold. As the polymer resin is injected into a mold, it flows through all parts of the mold. In manufacturing, the Weld line or Knit line or Meld line is the line where two flow fronts meet when there is the inability of two or more flow fronts to "knit" together, or. Injection moulding (U. Knit lines. Quotes for 25+ Manufacturing Processes. Weld lines may appear like seams or scratches,. A poor knit line can cause only cosmetic blemishes or it can significantly weaken the structural integrity of a part. Avoid areas where the plastic must suddenly flow upward or downward. S. 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. Operating Conditions • Fast injection fill speeds, supporting short injection times (1–3 s), especially in thin sectionsGate and Knit Line Locations<br />. Kaminski, in Specialized Molding Techniques,. 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. ABOUT THE AUTHOR. 4. For example, increasing gate size can improve part filling. This picture shows a. As always, feel free to contact us at 877-479-3680 or [email protected] if questions or concerns emerge. Parts molded with ABS are susceptible to forming knit lines, which are visible lines on the surface of the part. For instance, three widely used processes are insert molding, overmolding, and undercuts. 2. Viscosity represents a broad concept with huge effects on any efforts to standardize an injection molding process. Orientation of these knit (weld) lines, are perpendicular to the tensile stresses, applied to the ring (Figure 6-8). LCP can replace such materials as ceramics, metals, composites and other plastics because of its outstanding strength. This line does not normally impact the overall shape or dimensions of the item. Injection molding is a popular manufacturing process used to fabricate parts in large volumes. How weld lines form. Flow rate and flow speed are cut in half at every runner branch. Contact Universal Plastic Mold Today. Injection moulding (U. Very high quality and an extreme fit may. Rapid heat-and-cool molding technologies have attracted increasing. Direct experience designing molds and parts for injection molding and producing the molds and parts is valuable, relevant experience for design students. The knit (weld) line is in the direction perpendicular to applied stress. To avoid walls that are too fat or too thin, designing a wall thickness between 0. Meld and knit lines are actually both different types of weld lines. Find out more by calling 573-437-4154 today or email us. Knit and Meld lines are inherent to plastic injection. Knit lines appear to be a faint line or a crack in the finished part after it comes out of the injection mold. With over 50 years of experience, Universal Plastic Mold can help you avoid (or at least minimize) knit lines in your molded parts. 5. When the mold adopts the multi-gate pouring scheme, the plastic melt. 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. suppressed cooling and possibly warp. The process parameters were fixed as follows: Packing pressure of 40 MPa, injection time of 2. Remedies: Increasing the barrel temperatures will allow the flow fronts to stay hotter longer and. 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. 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. 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. Achieving a high-quality surface finish can be challenging, and other methods, such as polishing or coating, may be required. J (left) shows embossed feature and S (right) shows debossed feature in plastics through injection molding. To account for variations in viscosity, nylon injection molding machines need to maintain an appropriate injection pressure. 5-3. The comment on melt flow index is correct, but, I've molded straight PC down to . Rynite® PET is usually processed with melt temperatures between 280 to 285 °C (535 to 545 °F) . Overall, injection molding is a relatively simple process. The parting lines is where the two injection mold halves (A side and B side) meet. This production method is particularly popular when manufacturing in large quantities. Conversely, texturing of part surfaces is often effective in hiding flaws and imparting a “grippy” or leather-like. Knit line / Meld. Mechanical performance of knit lines and welded butt joints – A key factor in designing of multiple gated injection molded and subsequent welded. Again, you would round this value up or down based on a standard cutter size, which is 13/64 or 0. Weld line defects – also known as knit lines – is a defect that occurs when the injected plastic meets within a mold. It can also aid in material selection since fill. A knit line is any line, visible or not, where two resin flows meet. It is also where a slide mechanism might meet where it shuts off on one of the mold halves. Randy Kerkstra has been in the plastics industry for more than 26 years, occupied. “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. 08 KICKOFF PRODUCTION. As the polymer resin is injected into a mold, it flows through all parts of the mold. DFM Helps Prevent Sink Marks In Plastic Injection Molding. Design bosses with an outer diameter equal 2x the nominal diameter of the screw or insert. 18/06/2021. A knit line is the spot where two flows meet. อุณหภูมิของพลาสติกเหลวสูงเกินไปจนก๊าซหรือความชื้นแยกตัวออกมาได้. Increase the size of the gate and runners to alter the mold design. But we can’t talk about the injection molding process without discussing the injection molding machine. There are times when porous materials like Porcerax are used in problem or backfill areas. One of these is knit lines, also called weld lines. These knit lines affect the parts aesthetics. The pipe underwent brittle fracture into many pieces because of the high residual stresses in the wall of the pipe. When venting stationary inserts or core pins in rib or part details with venting issues, I recommend no more than 0. The only parts that count are the ones that meet quality standards, and keeping pressure evenly distributed across a mold’s parting lines is one of the many critical tasks to get those good parts molded. H-13 Good general purpose tool steel. Submarine Gates. 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. An edge gate introduces material at the main parting line, or where the two halves of the mold meet, with a perpendicular runner. Typical Tool Steel for Injection Mold Construction Steel Type Steel Properties Mold Component P-20 Pre-hardened, machines well, high carbon, general-purpose steel. pdf), Text File (. The process involves feeding the material into a press, where it is cooled or heated as per. Most failures occur at what are called “weld lines” or “knit lines”. Thus, as the flow continues, the two flow fronts may gradually rejoin to make one uninterrupted flow front. shows some obvious locations for vents, a brief molding trial should be conducted to observe where the gas voids occur. Neat polypropylene retains nearly 90% of its strength on a properly designed and molded weld line. To prevent swirling, perform tests whenever possible and use pre-compounded resin to ensure identical color match across parts. Runner and gates must be balanced. PROBLEM: POOR ADHESION OF LSR MATERIAL TO. Flashing can be a common defect in injection moulding and can affect the appearance and functionality of the finished product. Plastic injection molding has long been an affordable and efficient process for producing custom prototypes and end-use production parts. Obstructions in the part create weld lines or knit lines during injection molding. When the plastic cools, it takes the shape of the mold and is then ejected and finished. They are formed downstream through holes, and between multiple gate locations. 7. The knit line strength and other packing related properties both aesthetic and functional of injection molded polymeric products is improved by equipping the mold with packing pins. Weld / Knit Lines; Assumptions. Because they can be shut off independently you can control flow fronts and knit-line locations. The structural stability of. Shear Controlled Orientation Injection Molding of Polymeric Composites with Enhanced Properties. Additional venting along the parting line perimeter will improve overall venting significantly. Using three test materials, polystyrene (PS), polycarbonate (PC) and polyoxymethylene (POM), the relationship between a design and a processing parameter and the effects of the knit-lines on the tensile properties of injection molded plates. When venting stationary inserts or core pins in rib or part details with venting issues, I recommend no more than 0. In the plastic injection molding industry, we work with materials that are byproducts of oil refinement and molds made of metal casting, so our terminology is bound to be at least slightly confusing. An injection mold that produces pieces while the final tool is being built. Our plastic injection molding glossary provides key terms. A weld line (weld point, flow line or a knit line) forms when separate melt fronts travelling in opposite directions meet. Melt temperature does. 5 mm) is ideal, according to injection molding engineer Jack Rulander of Protolabs in Maple Plain, MN. When injection molding a part, a knit line may present no problem, or can cause a serious structural problem. To learn more about us or to get a quote, click here or call 1. By altering the part design, it may be possible to just have one flow front, and avoid multiple flow fronts having to come together. Nicolet Plastics LLC. Learn about the key characteristics of common injection molding materials, recommended applications, and how common thermoplastics compare to one another. Weld lines are formed during cavity. 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. In fact, knit line strength will decrease with higher filler content and with longer fibers. 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. These defects are not visible, like small. While not always visible, weld lines (aka knit lines) are where two resin flows meet. A component of the injection-molding machine wherein the resin pellets are melted, compressed and injected into the mold's runner system. By altering the part design, it may be possible to just have one flow front, and avoid multiple flow fronts having to come together. Weld lines create visible marks, but this type of injection molding defect can also reduce a part’s strength and durability. Figure 1 Flow Lines Knit or Weld Lines Cavity Gates Table 1 Knit line in a multi-injection-point cavity. On this page you will find the online, condensed version of this guide, offering quick answers to your injection molding questions. Proper mold cooling can eliminate warp, decrease cycle times, and increase part quality. M. Reducing the dimensions of the runner system. 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. Currently working with plastic, rubber, LSR, MIM molding companies to help them eliminate their complex molding challenges like air entrapment. 080" – 0. 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. Based on the resin. Weld or knit lines are perhaps the most common and difficult injection molding defect to eliminate. When injection molding complex parts, often knit-lines are formed during the mold filling stage. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. A knit line is any line, visible or not, where two resin flows meet. Venting. 1. Change gate locations. ICOMold is a plastic injection molding company that manufactures plastic injection molded parts. Polym Compos 1982; 3: 88–96. Weld lines often leave witness marks in the moulding surface. A meld line occurs when two moving melt fronts converge and flow parallel to each other. Reducing the dimensions of the runner system. 197 in. Causes: Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic. The knit lines are the result of poor molding conditions (likely molding at too low of a temperature). The cracking was always located in the weld/knit line and therefore caused by weakness in the bonding of the plastic at that line. This makes the casting process as efficient and accurate as possible. They again get resolidified along the surface. Injection molding is an efficient, often cost-effective way to manufacture custom plastics that are strong, durable, and long-lasting. As. As melt temperature, viscosity and abrasiveness rise, tool life will decline. Considerations: ABS-molded parts will show. Injection molding is a simple and efficient process that produces high-quality plastic parts. This article looks at the issues presented by a weld line in injection molding and the techniques manufacturers can use to avoid them. When designing for injection molding, consider these steps to prevent knit lines. gate position or runner. However, the weld line may not continue throughout the flow. Flow Lines Knit or Weld Lines Cavity Gates Table 1 Knit line in a multi-injection-point cavity. Exploring common molding defects and. 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 . CURE: Run the mold in. 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. Knit lines are hard to eliminate for injection molded plastic parts but designers can minimize cosmetic defects. Degassing – Degassing is the simple operation of opening and closing an injection mold to allow any trapped air (or gas) to escape. Knit lines, also known as weld lines, are fine lines that resemble hairline fractures or cracks. In injection molding, a thermoplastic material is melted to a liquid state and injected into a custom, double-sided mold. Macromol Symp 1988;21/21: 489-500. This type of molding uses a single tool to create both the part's geometry and surface finish in one step. Material. Compression molding and injection molding are very similar, but they have one major difference. Typically, injection molded parts will be between 0. ”. 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. Gas trapping/ poor venting. Weld lines may appear like seams or scratches, but that is not what they are. 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. Knit lines are formed in injection molding when two flow fronts meet in the middle of a part (a line where two sides of the plastic are “knit” together). A knit line, also. Proper venting is essential to molding a defect-free part. This image shows a PVC pipe after it was accidentally dropped on the ground. Adjust the design for the flow pattern to be a single source flow. In the world of injection molding, each weld. While this process is largely efficient and reliable, it's not without its potential issues. 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. It’s also relatively inexpensive. Place vents at last place to fill and at converging flow fronts. cold and hot weld line. 060 and 0. The injection of molding compound into the parts through one or two gates may produce parts that have knit lines. 1. They again get resolidified along the surface. 7. Injection molding flow lines typically fall into one of the categories mentioned in the following table: Category: Type of Flow Line: Appearance (What It Looks Like) 1. What Causes Knit and Meld Lines? Like many part inconsistencies, these unwanted features are rooted in product design. S. No matter the size of a production run, the presence of blemishes on the finished product leads to unhappy clients. Notched weld lines then remain near the cavity walls. A weld line, or knit line, occurs when two flows of melted plastic meet and “knit” together. It is. With over 50 years of experience, Universal Plastic Mold can help you avoid (or at least minimize) knit lines in your molded parts. Misra A. 160"). Figures courtesy of CAE Services. Inadequate injection pressure can result in knit lines and a surface that. Because viscosity is a variable, keeping fill time constant minimizes the influence of these variations and provides a more stable process, and therefore. First of all, ABS is prone to knit lines — or the places where two flows of plastic resin meet during the manufacturing process. wall thickness, knit lines, draft. This production method is particularly popular when manufacturing in large quantities. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. Effect of Injection Moulding Conditions on Knit-line Formation, Plastics Rubber Processing and. Koelling, Ronald C. If the part design’s wall thickness is constant, the magnitude of the plastic’s velocity will be the same at every. Weld or knit lines are perhaps the most common and difficult injection molding defect to eliminate. Examples of different causes of PVC pipe failure. ) 2. 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. Undercuts add cost to tooling and molding, which is recommended to eliminate or minimize as possible. Yet a smooth finish sometimes actually highlights flaws like knit lines and blemishes in the part. Alter the mold design. 1050 Walnut Ridge Drive. Also known as a knit line, this is the point at which two flow formations meet and are unable to. If you turn on the spigot to a garden hose, you will hear a hissing sound at the nozzle end for the first few seconds. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part. When 2 flows meet, small hair-like discolorations may develop. Industrial Knitting Machines. Solution: Increase the injection pressure or speed. These lines occurs usually around holes or obstructions and cause locally weak areas in the molded part. Weld lines have a significant effect on manufactured products. More about the knit lines in injection molding. The second mold portion being movable relative to the first mold portion and together defining a mold cavity. Over the years, I have developed two separate “5M” systems. 020"). 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. The Fusion of Weld Lines. This will help the mold designer to determine the allowable gate and knit line locations. Without it, air and gas are trapped in the mold, which compress and heat as the cavity fills. 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. However, product designers relying on this molding method encounter various challenges during tool design and development. A thermoset injection molding process may save as much as 50% of machine time per shot with a faster cycle time compared to a thermoset compression molding process, allowing molders to produce high quantities of thermoset components per day in multi-cavity tools. These knit-lines are well-known as particularly critical regions in the mold when mechanically loaded. These are caused by the shot of molten plastic moving at different speeds throughout the injection. In a weld line, reinforcement fibers. Incorrectly placed single gates may cause knit lines. In spite of its cosmetic attributes, a bad knit line can significantly compromise a part. 2. thick. Venting at Weld Lines. LOCATION: Along any parting line. Also discussed are weld and meld lines and their effects on molded. 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. The plastic melt that passes through the gate at high speed directly enters the cavity, then contacts the surface of the cavity to solidify, and then is pushed by the subsequent plastic melt, leaving traces of meandering. 156 x 2 1/3 = 0. 827 Holes and Bosses. However, with any practice, when the proper care and planning aren’t taken, problems can occur, and with injection molding, knit lines are one of the most common problems. As. Increase the injection speed. RELATED CONTENT. and Titomanlio, G. This month, we covered an array of topics including injection mold venting, business strategies and runnerless micro molds. During injection, support pillars keep closed the two halves of the plastic injection mold. Injection moulding is a manufacturing process that melts, injects and sets plastic into a metal mould. Misra A. Still, it can be seen to varied degrees depending on mold.