The knit (weld) line is in the direction perpendicular to applied stress. Vacuum. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. Add a 0. Examples of different causes of PVC pipe failure. Parting Lines. Following lists causes and solutions for weld or knit lines. Conversely, texturing of part surfaces is often effective in hiding flaws and imparting a “grippy” or leather-like. weak knit lines and fracture points. 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. CAUSE: 1. Degassing – Degassing is the simple operation of opening and closing an injection mold to allow any trapped air (or gas) to escape. Meld and knit lines are actually both different types of weld lines. The main causes of Weld Line or Knit lines are: Convergence of two flowing polymer melt streams during injection molding process: This is the most common cause of weld lines, as the two melt streams coming from different runners or gates in the mold meet and do not fully mix, creating a weak point in the finished product. doc), PDF File (. placed usually in the areas last to fill, near knit lines, as well as on the runner system. Solution: People. When venting stationary inserts or core pins in rib or part details with venting issues, I recommend no more than 0. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. Knit lines are hard to eliminate for injection molded plastic parts but designers can minimize cosmetic defects. Take these steps to avoid knit lines. This article looks at the issues presented by a weld line in injection molding and the techniques manufacturers can use to avoid them. 2. 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. Vibration assisted injection molding. Weld lines can significantly reduce ultimate tensile strength (UTS) and fracture strain of talc filled polypropylene (PP). 1 in. They are the result of fundamental physical effects taking place inside of the mold. Based on the resin. Knit Line Injection Molding is a process that allows manufacturers to produce parts more quickly, accurately, and at less cost than traditional injection. By contrast, a slow injection speed will give you a matte finish and a cold mold, such as 50F, will give you a more flexible part. The influence of knit-lines on the tensile properties of injection molded parts. Reduce the injection pressure. Multiple parts with complex design and differing wall. For injection molding of the multi-blade fans and various rotating plastic parts, the complex of multiple gating injection molding tools were used. Given the cosmetic requirements, there is a good chance the mold would require. ) are easy to produce. Achieving a high-quality surface finish can be challenging, and other methods, such as polishing or coating, may be required. Gate design is another critical piece of part function in plastic injection molding. J (left) shows embossed feature and S (right) shows debossed feature in plastics through injection molding. 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. This continues until the weld lines in plastic injection molding disappear. This will help the mold designer to determine the allowable gate and knit line locations. 060 and 0. 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. Reduce the temperature of the mold and plastic being injected. The resulting line has the. Incorrectly placed single gates may cause knit lines. Inject the molten material into a mold. Because injection mold design and processing is complex, a set solution will not solve all problems. Knit lines, also known as weld lines, are fine lines that resemble hairline fractures or cracks. Weld or knit lines are perhaps the most common and difficult injection molding defect to eliminate. Bad habits can cause many different problems with injection molding machines, and flow lines are included in that. land and using maximum vent depths to get them as close to self-cleaning as possible. A screw-type plunger forces the material into a heated chambe r,. One exception: ribs should be about 50. 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. Notched weld lines then remain near the cavity walls. Bad habits can cause many different problems with injection molding machines, and flow lines are included in that. 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. Sink. SEA-LECT Plastics has 16 MILACRON injection molding machines that range from 33-ton with a 2. Injection molding is a manufacturing process that involves injecting molten plastic into a mold cavity to create a specific shape or design. 040 and 0. 5 to 3. No matter the size of a production run, the presence of blemishes on the finished product leads to unhappy clients. Sequential valve gate technology. Knit Lines. Use a thread with a pitch greater than 0. “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. A. 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. Three terms that often get mixed up are weld, meld, and knit lines. pdf), Text File (. The mold settings can run flawlessly until employees consistently open the machine for inspections, interrupt cycle times, or cause. Remedies: Raise the temperature of the mold or molten plastic. Parts as thin as 0. An option to bluing agents, the long established means of gauging pressure distribution, comes via Pressurex, a thin, flexible, plastic film. 3. Injection molded plastic parts have comparatively less strength in the weld line section and they also have an impact on product aesthetics. It can be determined by a visible line appearing on the surface of the plastic part where two flows of the plastic meet. It happens when the molten plastic material flows around obstacles, such as inserts or cores, and rejoins on the other side. Flashing can be a common defect in injection moulding and can affect the appearance and functionality of the finished product. 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. Learn more. 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. To study the weld line formation of colliding flow fronts the filling of a mold cavity was simulated. 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. SPI surface finishes are the most common set of injection molding finishing standards around the world with over 12 different grades. com Three terms that often get mixed up in injection molding are weld, meld and knit lines. More about the knit lines in injection molding. To account for variations in viscosity, nylon injection molding machines need to maintain an appropriate injection pressure. Long shapes are produced without multiple drops or hot runner systems, eliminating knit lines and lowering tooling costs. Near-Net-Shape production uses injection molding to account for 70-90% of the final geometry of a part. They again get resolidified along the surface. Anisotropy. 5. The knit line itself (the vertical line in the middle) is visible in this lighting, but the biggest issue is with the texture. Two major sources of weld lines are multiple gating and flow around pins. Injection molding weld lines refer to a line on an injection molded part that is generated where the two halves of the mold come together. In fact, strength at the knit line can be as little as 20 percent of the nominal strength of the part. The quality of the answer is directly proportional to the information we have to work with. The structural stability of. Example of a gate location in a finished part Example of knit lines in a finished part. Injection molding is the process of injecting liquid thermoplastic into a mold. The GLS™ Injection Molding Guide provides in-depth information on part design, mold design and processing for styrenic block copolymer (SBC) thermoplastic elastomers (TPEs). The temperature might fall at some places like in the mold, runners leading to the mold and injection molding. Sink Marks Cause 2: Low Pack and Hold time. Thin wall injection molding can produce walls as thin as 0. Inadequate venting may cause short-shots, poor surface appearance, or weak weld-lines. Knit lines, also known as weld lines are areas in the. Meld and knit lines are actually both different types of weld lines. This normally occurs around holes or obstructions and causes locally weak areas in the molded parts. By altering the part design, it may be possible to just have one flow front, and avoid multiple flow fronts having to come together. injection-molding machine (Arburg allrounder 320C, Germany). 040-in. We complete the part to your precise specifications with CNC Milling operations. Knit lines are hard to eliminate for injection molded plastic parts but designers can minimize cosmetic defects. Expand. Bubbles are caused by melting flow fronts colliding in a mold cavity. Weld lines, also known as knit lines or meld lines, occur when two or more flow fronts meet during the plastic injection molding process. Too cool a melt. This line does not normally impact the overall shape or dimensions of the item. Understanding Knit Lines in Injection Molding: Causes and Solutions. Macromol Symp 1988;21/21: 489-500. On this page you will find the online, condensed version of this guide, offering quick answers to your injection molding questions. Remove the hardened material from the mold. 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. Once the plastic cools, the mold is removed, and the object is ready for post-processing. M. Reduce the screw speed and adjust the backpressure. com. They again get resolidified along the surface. Having uniform wall thicknesses between 0. In injection molding, it is often important to maintain consistent color throughout the part. Injection molding is the most cost-competitive technology for manufacturing high volumes of identical plastic parts. 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. Below is a list of items that should be checked both during normal molding operations and if defects are detected. Thicker parts require more cooling time and can lead to air becoming trapped in the mold and weld / knit lines to form. Let’s. Thermoplastic Elastomer Molding Guidelines. 030- to 0. menu. 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. Weld lines may appear like seams or scratches,. Be wary of saving too much weight or you might end up with: cracking. As these flow fronts come 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. While knit lines can be present it does not always mean the structural integrity of the plastic part is compromised. The most common and one of the most challenging injection molding defects are weld and knit lines. Bubbles are caused by melting flow fronts colliding in a mold cavity. The resin is transferred to the mold cavity under positive. Considerations: ABS-molded parts will show. 11) The part design must take into account possible knit line locations and work to direct them away from higher stress areas. 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. These knit lines are usually the weakest areas of the part. Overall, injection molding is a relatively simple process. The part weight x number of cavities + runner weight should. Some of the most popular brands include:Harshal Bhogesra has 9+ years of combined experience in molding, customer & supplier development. Gas-assisted Injection Molding: Influence of Processing Conditions and Material Properties. Weld lines are formed when there is an inability of two or more flow fronts to ‘knit’ or ‘weld,’ together during the molding process. There are times when porous materials like Porcerax are used in problem or backfill areas. A knit line is any line, visible or not, where two resin flows meet. Download : Download full-size image; 3. The mold settings can run flawlessly until employees consistently open the machine for inspections, interrupt cycle times, or cause temperature. Swirling can increase depending on the material you use, and the texture or polish of injection molding tools. As the polymer resin is injected into a mold, it flows through all parts of the mold. They are formed downstream through holes, and between multiple gate locations. Multiple gates will always cause knit lines. Misra A. It’s also relatively inexpensive. 160"). Influence of injection-molding parameters on the morphology and mechanical properties of glass fiber-reinforced polypropylene composite. . gate position or runner. 250 in. On the other end of the scale, a desktop injection molding machine with 5 tons of force can cost between $30,000-50,000. Notched weld lines then remain near the cavity walls. POROUS MOLD MATERIALS. thick. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. a weld or knit line may develop on the opposite side of the obstruction as the two fronts of material join. This defect is caused when the polymer resin is injected into a mold and flows around a solid piece and forms when the resin collides back together without completely fusing back together. Our glossary of terms provides definitions for a wide range of injection molding terms. Inadequate injection pressure can result in knit lines and a surface that. 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. and Titomanlio, G. 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. These flaws can affect the aesthetic and structural integrity of the. Increase size of the vents. 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. Encapsulating the metal insert with plastic creates a single molded plastic piece that’s generally stronger than one created using secondary assembly. material Knit Lines Weld lines Small lines on the Caused by the melt-front flowing around backside of core an object standing proud in. 120 in. First, an injection mold with triangular inserts was built to study the influence of meeting angles on material properties at the weld line. Material. Solution: Increase the injection pressure or speed. 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. 5-3. As the melted plastic fills the mold, there are locations where the molten plastic flow stream separates and then comes back together. Industrial Knitting Machines. Sprue bushing too long. Compression molding produces solid parts that are free of flow and knit lines. There is high defect rate (surface defects/knit line). LOCATION: Along any parting line. 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. These seams or knit lines are avoided because the molding material flow does not a meet at an interface nor must. Knit Lines. Move the injection unit away from the mold during short breaks . This can result in cosmetic defect or a weak point in the part. Weld or knit lines are perhaps the most common and difficult injection molding defect to eliminate. Plastic injection molding is a complex and highly intricate process. 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. Knit lines. Gate size can be increased or moved to a section with greater wall thickness to help eliminate. Circular Loom Machine; Raschel Knitting Machines; Shade Net Making Machinery; Inspection Machines; Non Woven Production Line. Circular Loom Machine; Raschel Knitting Machines; Shade Net Making Machinery; Inspection Machines; Non Woven Production Line. When the mold adopts the multi-gate pouring scheme, the plastic melt. 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. Mechanical performance of knit lines and welded butt joints – A key factor in designing of multiple gated injection molded and subsequent welded. 0 and Table 1). 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. Having a basic understanding of the injection molding process will aid in understanding how to design for injection molding as well as how to forecast production schedules. (1. There are two primary reasons for this occurrence: 1. 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. Weld lines are formed during cavity. The parting line location on the injection mold will have an effect on how the cavity and core (both halves of the injection mold) come together and seal. suppressed cooling and possibly warp. 2) Vents in an injection mold should let the air out, but not the liquid plastic. A knit line is the spot where two flows meet. spelling: injection molding) is a manufacturing process for producing parts by injecting molten material into a mould, or mold. Anisotropy. Different plastics make different types of knit lines. A meld line occurs when two moving melt fronts converge and flow parallel to each other. 2. Injection Molding Guide. It is caused by the incomplete joining of the uncured. Weld lines typically appear in a part when an obstruction of some kind separates the melt flow and prevents it from "knitting" together. Plastic injection molding has long been an affordable and efficient process for producing custom prototypes and end-use production parts. 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. 040-in. Moisture in the material due to insufficient drying can form at the melt front, forming an interface between the two melt fronts. 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. Reduction of sink can possibly be had by switching to an ABS/PC-blended material. Nicolet Plastics LLC. Most failures occur at what are called “weld lines” or “knit lines”. Consider adding mold release (can be added to the polymer itself by the polymer vendor) or at machine side Knit lines Weak knit lines Increase melt temperature Mold temperature too cold Increase injection speed Avoid nucleated products if possible Inadequate ventingWhat you should know is that there are various failure modes for injection molding. Use gradients. The point of meeting creates a weak spot because the edges of the molten plastic, already cooling, don’t mix together very well. 8 mm relief at the edges of the thread. Rapid heat-and-cool molding technologies have attracted increasing. Islyn Thomas summarized the need for a particular vent location better than I ever could. wall thickness, knit lines, draft. What Are Knit Lines in Injection Molding, and How Do You Prevent Them? - RJG, Inc. injection molding. Weld lines have a significant effect on manufactured products. Place vents at last place to fill and at converging flow fronts. 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. These seams or knit lines are avoided because the molding material flow does not a meet at an interface nor must. Be wary of saving too much weight or you might end up with: cracking. Increase the injection speed. sinks, knit lines, and shrinkage,” Spalding says. This most often occurs when you have a core or shutoff feature that the plastic needs to flow around. Compression molding and injection molding are very similar, but they have one major difference. Injection speed and pressure. Near-Net-Shape Molding enables us to eliminate knit lines, add undercuts, increase options for geometry and tolerances, and reduce tooling cost for low. Resin cools as it is injected into a mold. 12515 NE 95th Street , Vancouver, WA 98682 CALL TODAY: 1-800-839-0366Contact FU CHUN SHIN MACHINERY MANUFACTURE CO. Make sure the venting is not too large. The injection of molding compound into the parts through one or two gates may produce parts that have knit lines. 0 mm) is crucial to ensuring that parts cool evenly. 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). 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. Flow Lines. Because edge gates are easy to machine, they can be widened or modified to help material flow. TWIM=Thin Wall Injection Molding; TSL=Toys,Sports and Leisure; NB=No Break; IMR=Inter Material Replacement. 02 in. There. 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. The Mold Tools toolbar provides tools for creating mold tooling (cores, cavities, etc. 197 in. Knit lines. Swirling can increase depending on the material you use, and the texture or polish of injection molding tools. Knit lines are most common downstream of holes created in parts where the molten material meets. N174W21430 Alcan Dr Jackson, WI 53037 262-677-9700; 16685. 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. Let’s stop for a second and think about the logos we usually see on a daily basis. He is passionate about optimizing product design, tool development, molding processes, and quality. Weld Lines. From Plastic Materials and Processing by A. When injection molding a part, a knit line may present no problem, or can cause a serious structural problem. Why. The specific method is that when the product is just filled and filled in the cavity, the insert. Avoid areas where the plastic must suddenly flow upward or downward. [1] Here are a few ways to prevent knit lines in your plastic parts: Increase material temperature: Higher temperatures on both materials prevent one section from cooling too quickly. Figures courtesy of CAE Services. Also known as “stitch lines” or “weld lines”. 18/06/2021. 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. This image shows a PVC pipe after it was accidentally dropped on the ground. Adding draft or slope to vertical walls of a part makes it easier to eject or remove the part from the mold. Many materials can be. In this video, Chris Burda, applications engineer at Protolabs, shares what can be done to reduce knit lines and how they can impact your part's functionality. 17. (for example, a hole on the plastic part), the knit line will be unavoidable. 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. As. A meld line occurs when two moving melt fronts converge and flow parallel to each other. The primary rule is to apply 1 degree of draft per. 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. a knit line will form 180°on the opposite side from the gate location. The part is cooled in the mold until it is solid enough to be ejected. 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. 6. In fact, knit line strength will decrease with higher filler content and with longer fibers. Knit Mark – A witness mark on a molded part, often occurring at the midpoint between two transfer or injection sprue locations. 003" - 0. ISSUE FACED BY THE INJECTION MOLDING COMPANIES: FLOWLINES (ALSO CALLED WELD LINES OR KNIT LINES) Definition: Streaks, orange skin, or visible flow lines in parts. Typically, injection molded parts will be between 0. Meld and knit lines are actually both different types of weld lines. With over 50 years of experience, Universal Plastic Mold can help you avoid (or at least minimize) knit lines in your molded parts. The area generally has lower strength than the other areas of the part. Still, they are almost always a cosmetic issue. First of all, ABS is prone to knit lines — or the places where two flows of plastic resin meet during the manufacturing process. Knit Lines in Injection Molding. . Weld line defects – also known as knit lines – is a defect that occurs when the injected plastic meets within a mold. Formation of knit line. ” What else can you do about weld lines—besides redesigning the mold or gate locations or adjust the flow pattern during fill with sequential valve gating? I recently saw a report on this issue from Moldex3D, the maker of molding simulation software. ) or as thick as 25 mm (1 in. Knit lines. Therefore various injection molding defects can be eliminated. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. Weld lines create visible marks, but this type of injection molding defect can also reduce a part’s strength and durability. The above guidelines for initial heater band settings are for most injection molded parts. 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 Lines 11: 1. These knit-lines are well-known as particularly critical regions in the mold when mechanically loaded. 0mm) throughout will ensure that the parts cool evenly and will prevent warping. Sink Marks Cause 5: Improper Part Geometry. From the start of your project, you can count on The Toolroom to exceed your expectations for all of your plastic injection molding needs. The present. Learn Injection Molding Glossary with Plastopia: Using actual molded parts (with images) as examples. Weld Lines-3 Ways for Detecting and Correcting. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part. Back to News Page. Mold analysis tools are used by designers of molded plastic parts and by designers of the mold tools used to manufacture those parts. Draft. Plasti-Co. Three terms that often get mixed up in injection molding are weld, meld and knit lines. Improve venting and/or relocate to burned area. 4. LCP can replace such materials as ceramics, metals, composites and other plastics because of its outstanding strength. RapidDirect – Your Best. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. 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. A poor knit line can cause cosmetic blemishes and significantly weaken the structural integrity of a part. The uneven thickness of the product will make the melt flow resistance larger and smaller, resulting in unstable melt flow. All injection molding ultimately comes down to the “Man, Mold, Machine, Material and Method. Trapped air in a mold manifests itself as a burn mark on the molded part. Contact Universal Plastic Mold Today. Knit Lines. The structural stability of. 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. Whereas conventional injection molding uses. 3 /sec. TEFABLOC™ TESI 744 TPE compound for injection molding processing provides superior adhesion properties when processed using injection molding. Depends how hard you want to push it.