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. These knit lines are formed as molten plastic in the injection mold flows around an obstruction like a feature. Injection molding machines with footprints similar to desktop FDM and SLA 3D printers are now available. Injection Molding Process Guidelines The following is given as a potential process starting point. As the polymer resin is injected into a mold, it flows through all parts of the mold. injection molding machines using standard industry practices . The most common and one of the most challenging injection molding defects are weld and knit lines. Weld line defects – also known as knit lines – is a defect that occurs when the injected plastic meets within a mold. Plastic injection molding is the industry’s most common way to manufacture parts. Heat: Too much heat can have an appearance similar to moisture splay. - Mold flow analysis to control fill and knit lines. Injection Molding Press. Bad habits can cause many different problems with injection molding machines, and flow lines are included in that. 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. Runner Type. This most often occurs when you have a core or shutoff feature that the plastic needs to flow around. Flow lines, also known as weld lines or knit lines, are discontinuous lines or streaks that appear on the surface of an injection-molded part. These are imperfections in the part where separated flows of cooling material meet and. In this, work, metal inserts were joined with polyamide 6 by using the injection-molding technique. Insert injection molding is a process that requires an insert — typically metal — to be pre-placed in the tool for injected plastic to flow around. Injection molding is an efficient, often cost-effective way to manufacture custom plastics that are strong, durable, and long-lasting. Mold Flow Analysis computer. 030 and 0. Weld Lines-3 Ways for Detecting and Correcting. Weld lines create visible marks, but this type of injection molding defect can also reduce a part’s strength and durability. This continues until the weld lines in plastic injection molding disappear. Runner Type. Weld or knit lines are perhaps the most common and difficult injection molding defect to eliminate. Islyn Thomas summarized the need for a particular vent location better than I ever could. They need the interference of about 0. This is a video excerpt from our Problems & Solutions Course - Weldlines. It happens when the molten plastic material flows around obstacles, such as inserts or cores, and rejoins on the other side. For over four decades, our experienced team and leading edge technology have provided you with tooling solutions from start to finish. The final molding process may be dependent upon factors. Weld Lines. This slight depression is a tiny line called the weld line or knit line. Knit Line – A molding defect resembling a flow-line found at the point where the rubber meets between the injections. Image credit: The Rodon Group. They again get resolidified along the surface. For this study, a special injection mold was designed and constructed (Figure 1) in order to produce specimens of various thicknesses (from 2 to 6 mm). Knit lines, also known as weld lines or meld lines, are visible lines or seams that appear on a molded part where two or more flow fronts meet during the injection molding process. Monoform parts. Gate locations can be directly behind or on the edge of the graphic. A medical device designer might look up a polymer's physical properties and game out. The recommended minimum production volume for injection molding is 500 units. Gates that are too small can also cause the uneven flow of material or result in knit lines, blemishes that occur where two resin flows meet. Demo Mold. Injection molding crush ribs are small, protruding features added to molding designs to support and improve the stability of press-fit connections. Services Group. Knit lines, also known as weld lines, are fine lines that resemble hairline fractures or cracks. Fictiv can help you avoid injection mold design and plastic part design for injection molding problems. Understanding types of injection molding process can help to avoid some design challenges early in the process. INJECTION MOLDING GUIDE V2. 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 adhesiveMolding 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 adhesiveMost failures of injection molded plastic parts are caused by either improper design of the part or improper design of the mold. RELATED CONTENT. Three terms that often get mixed up in the plastic injection molding industry are weld, meld, and knit lines. Three terms that often get mixed up are weld, meld, and knit lines. They need the interference of about 0. Submarine Gates. Shorten the mold cycle time so that any trapped air and resin don’t have a chance to overheat. Gates, vents, sliding cams, pick-out inserts, and any other place where the two sides of. It has low shrink and high dimensional stability, and it has good resistance to acids and bases. It explored the use of. (for example, a hole on the plastic part), the knit line will be unavoidable. 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. This type of molding uses a single tool to create both the part's geometry and surface finish in one step. Our glossary of terms provides definitions for a wide range of injection molding terms. There are times when porous materials like Porcerax are used in problem or backfill areas. Plastics Technology article (June 2018): Technical capabilities of the process have advanced to erase former cycle-time penalties, and ongoing developments address creation of a materials database and new applications in hybrid composite injection overmolding. Piccarolo, S. In fact, knit line strength will decrease with higher filler content and with longer fibers. Guideline for Design, Material Selection and General Application of Encapsulation of Electronic Circuit Assembly by Low Pressure Molding with ThermoplasticsThis was usually with a PC/ABS material at knit lines and high fill pressures from thin walls in the part. As. Knit lines, holes, part configuration and the talent of your molder all come into play. Injection molding can use thermosets or thermoplastics. 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. 2. 5 kN/cm2 (4 T/in2 ) is required. Remove the hardened material from the mold. 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. Weld lines are also known as “stitch lines” or “knit lines”. Let’s review what causes meld and knit line imperfections, how they’re different, and how they affect part durability. The advantages of compression molding include more isotropic fiber distribution, dimensional stability, and surface finish. placed usually in the areas last to fill, near knit lines, as well as on the runner system. Knit lines don’t. Weld lines, also known as knit lines, are lines that appear on the surface of an injection-molded part where two or more flow fronts meet and fuse together. Reduce the temperature of the mold and plastic being injected. The thermo-rheological findings were used to investigate the sources of weld line weakness. Additional venting along the parting line perimeter will improve overall venting significantly. Using the design cube, learn how knit lines are formed on plastic parts and tips to avoid knit lines in your plastic injection molding design. The company provides an array of digital manufacturing expertise. Because injection mold design and processing is complex, a set solution will not solve all problems. Molding is a manufacturing process in which resin in a barrel is heated to a molten state,. The specimens with a weld line at the center were obtained by double gate molding and the specimens without a weld line wereInjection runner too small Increase runner as needed. spelling: injection molding) is a manufacturing process for producing parts by injecting molten material into a mould, or mold. One common problem that occurs in injection molding is the formation of knit lines. Weld lines defects are formed when two flow fronts are merged together. Generally all plastic components should be designed with draft where possible. Solution: Increase the injection pressure or speed. 13758 Johnson Street NE. These flaws can affect the aesthetic and structural integrity of the. Please read on, stay posted, subscribe, and we welcome you to tell us what you think. Knit lines, weld lines, mold lines, or flow lines in injection molding are faint lines that become visible on certain plastic parts. To avoid these problems and create the best possible. All that considered, for a specific simple part with specific quality requirements. Thin walls often lead to material flow problems, which can cause unformed areas. What Are Cosmetic Standards for Plastic Parts?. 5 to 3. The mold’s sprue bushing and locating ring is placed in a way that makes sure the injection molding nozzle is properly aligned and centered. Unsymmetrical filling makes knit lines in samples, leading to the failure of part, so it must be avoided. Specifically, Stokes and. occupied frequently with troubleshooting injection molding. The recommended minimum production volume for injection molding is 500 units. Most failures of injection molded plastic parts are caused by either improper design of the part or improper design of the mold. . But any. Gating. The mold settings can run flawlessly until employees consistently open the machine for inspections, interrupt cycle times, or cause. For example, increasing gate size can improve part filling. Rynite® PET is usually processed with melt temperatures between 280 to 285 °C (535 to 545 °F) . Weld-line strength for plastics can vary from 20% to nearly 100% of the strength of the plastic itself. Also known as string lines. The injection molding processing conditions for filling are given in Table 1. A poor knit line can cause only cosmetic blemishes or it can significantly weaken the structural integrity of a part. The point of meeting creates a weak spot because the edges of the molten plastic, already cooling, don’t mix together very well. reduce mold maintenance cost. Shear Controlled Orientation Injection Molding of Polymeric Composites with Enhanced Properties. Learn how material selection, part design, and optimizing process parameters can help eliminate knit lines. We have two unique offers to meet your needs wherever you are in the product lifecycle. Facebook. A poor knit line can cause only cosmetic blemishes or it can significantly weaken the structural integrity of a part. 5 : 1 and a L/D (length toMold flow analysis is a software simulation which show how the resin fill in the mold in injection molding process. They are the result of fundamental physical effects taking place inside of the mold. Know about the cause of these lines and the potential solutions. Injection molding can be a real challenge if you don’t follow the “Five M’s”: Man, Mold, Machine, Material and Method. PCS Company, Fraser, Michigan, has launched the Polimold Valve Gate Sequencer, providing injection molders with more precise control of the mold-filling sequence when using valve-gate hot runner systems. He is currently a tooling manager for a large, multi-plant molding and manufacturing company. Reduction of sink can possibly be had by switching to an ABS/PC-blended material. Also known as a knit line, this is the point at which two flow formations meet and are unable to connect during injection molding. 1. Mold bases, ejector plates, and some cavities (if nickel or chrome plated to prevent rust). Weld lines may appear like seams or scratches, but that is not what they are. It’s called sequential valve gating, or SVG. 0 and Table 1). [8] Bevis, MJ, Reis RL, Cunha AM. 3. The temperature might fall at some places like in the mold, runners leading to the mold and injection molding. knit lines and parting lines are at the discretion of MIC-ZG. While this process is largely efficient and reliable, it's not without its potential issues. As the polymer resin is injected into a mold, it flows through all parts of the mold. These lines occurs usually around holes or obstructions and cause locally weak areas in the molded part. The mold-opening force varies widely depending on the type and size of the molding machine. They have their origins in material choice, part design, tooling, and processingInjection molding is generally restrictive (not including special processes) in the cross-sectional size of the part, while blow molding allows for channels in the part that increases strength. There are processes such as gas assist that do not require the injection unit to pressurize (pack out) the cavity, but for our discussion, we will stay with conventional injection molding. and Titomanlio, G. Prevent Flow and Knit Lines During the Injection Molding Process. Depends how hard you want to push it. Imagine a river splitting to go around a rock, then meeting up again on the other side. 008 to 0. Mold filling simulation software should be used to ensure that waterExpand. When the mold adopts the multi-gate pouring scheme, the plastic melt. Draft. Reinforcing ribs play a pivotal role in the strength of a plastic part. The Demo Mold helps those new to injection molding easily visualize the process and is a great teaching device for educators. sheet metal fabrication, rapid tooling, and plastic injection molding. Shorten the mold cycle time so that any trapped air and resin don’t have a chance to overheat. Core Geometry. "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. Knit lines, also known as weld lines or meld lines, are visible lines or seams that appear on a molded part where two or more flow fronts meet during the injection molding process. 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 adhesiveIt tells them that you visualized how injection molded parts cool and harden. be impressed by improvements of bubbles (vacuum voids), short shots, unequal thickness, jetting marks, weld lines, knit lines, black streaks, strain 2. Proper venting is essential to molding a defect-free part. Reduce clamping force. Mold venting is critical to the quality and consistency of the finished part. It’s economical, efficient, and can produce simple to complex parts with low waste. 4271. After selecting the direction in which the mold hole will be cut, it is much simpler to locate the separating line. You don't give us much to go on. 1. Fathom injection molding services and tools offer a single-source global solution that supports your company’s low- to high-volume manufacturing process. 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. 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. Compression molding can also support large part sizes, and it has a tendency to. Thin wall injection molding can produce walls as thin as 0. To account for variations in viscosity, nylon injection molding machines need to maintain an appropriate injection pressure. Short molding cycle, high production efficiency, and easy automation 2. A knit line is any line, visible or not, where two resin flows meet. The difference between a knit and meld line is significant, dramatically impacting the part’s structural integrity. A weld line, also known as a knit line, is a visible line or mark that occurs in an injection-molded part where two or more melt fronts meet and fuse together during the molding process. Those are knit lines, formed when two opposing flows of material join together in the mold cavity. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part. Weld lines are a common imperfection to be found in most plastic injected molded parts. The best we can hope for is to pressurize the knit or meld line better through the combination of melt temperature, mold temperature, flow rate, and holding pressure. (If parts are to be molded by Protomold’s rapid injection molding process, our Design Guidelines should be used. Complex parts can be reproduced with low cost once mold is manufactured. Once the mold is created and the machine is set up, additional parts can be manufactured very fast and at a very low cost. such as knit lines and other imperfections. Here are some ways manufacturers can avoid quality issues during the plastic injection molding process: Inadequate Injection/Holding Pressure: If injection and holding. pdf), Text File (. Most failures occur at what are called “weld lines” or “knit lines”. 4. They occur when melt flow fronts collide in a mold cavity. Usually the parting line of the mold is perpendicular to the mold opening direction of the injection molding machine. Contact Universal Plastic Mold Today. Vacuum on the mold does help and many times eliminates the weld line. Plating and painting applications usually require slower fill rates to keep orientation low. wall thickness, knit lines, draft. Our prototype injection molding process helps you cut product development cycles by weeks—sometimes months—and bridge to production with injection-molded parts within days. 453. Imagine a river splitting to go around a rock, then meeting up again on the other side. One common problem that occurs in injection molding is the formation of knit lines. Increase the injection speed. They are the result of fundamental physical effects taking place inside of the mold. Texturing may also hide surface defects such as knit lines and other imperfections. The uneven thickness of the product will make the melt flow resistance larger and smaller, resulting in unstable melt flow. Also known as a knit line, this is the point at which two flow formations meet and are unable to. Molding parts with multiple gates will almost certainly produce a weld line at right-angles to the line joining the two gates which could cause the weld lines to be exceptionally weak (see Figure 1. Injection Molding Injection molding is perhaps the most common and important of all plastic processing processes. 040 and 0. Injection moulding (U. To learn more about us or to get a quote, click here or call 1. As the melted plastic fills the mold, there are locations where the molten plastic flow stream separates and then comes back together. Exploring common molding defects and. Three terms that often get mixed up in injection molding are weld, meld and knit lines. Bubbles are caused by melting flow fronts colliding in a mold cavity. Figure 1 Flow Lines Knit or Weld Lines Cavity Gates Table 1 Knit line in a multi-injection-point cavity. 1513 Sam Bass Rd. As these flow fronts come together. Solution: Increase the injection pressure or speed. When injection molding a part, a knit. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. Which affect parts aesthetics and decrease parts strength. Typically, injection molded parts will be between 0. 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. The specific method is that when the product is just filled and filled in the cavity, the insert is used to cut the soft material melt in. Because this allows some cooling and thickening of the resin, tab gates require a relatively large opening into the mold cavity. 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. Change gate locations. RJG announced that its injection molding process control system, CoPilot, will begin offering valve gate control technology for hot runners starting in September 2021. Wavy lines are another feature of weld lines. occupied frequently with troubleshooting injection molding. A plastic product formed by the injection mold is divided into two parts, and the line separating the product is called the parting line. We all know that injection moulding is great for producing both custom prototypes and end-use production parts, but if you are not careful it can cause some. Advantages for your Injection Molding Machine:. Services. [1]Weld Lines. 6. Weld lines, knit lines, and meld lines are terminologies to describe the imperfections of finished parts in plastic injection molding processing. Wax Cold wax Increase wax temperature. Observation on a “trial and error” basis may be required. Obstructions in the part create weld lines or knit lines during injection molding. Remedies: Raise the temperature of the mold or molten plastic. Effect of Injection Moulding Conditions on Knit-line Formation, Plastics Rubber Processing and. This performance can ultimately impact many part properties, including surface quality, knit line strength, part ejection force (sticking), dimensions, tolerances. Knit lines, holes, part configuration and the talent of your molder all come into play. Learn how material selection, part design,. Find out more by calling 573-437-4154 today or email us. Injection moulding can be performed with a host of materials mainly including metals (for which the process is called die-casting ), glasses, elastomers, confections, and most commonly thermoplastic and. In addition, keep these in mind: Scrap rates can range from less than 1 to 5%. A mold with too few gates is susceptible to cooling problems, but a mold with too many gates. Starting with small part size, students learned to think about detailing on a product at thisTable 2. Knit and meld lines are inherent to plastic. The area generally has lower strength than the other areas of the part. These materials have beenThe wall thicknesses of an injection molded part generally range from 2mm – 4mm (0. Not always a problem, but can cause serious structural issues. RELATED CONTENT. 030- to 0. With the help of mold flow analysis and mold flow service, mold designers understand how the material fill to position gates better, anticipate where the knit line will appear and locate difficult to fill spots in the injection mold. Plastic melt and mold temperature. Different plastics make different types of knit lines. Learn Injection Molding Glossary with Plastopia: Using actual molded parts (with images) as examples. 3. The areas in the cavity where such buildup tends to be common are at end-of-fill areas, details that trap gas, and knit lines. Look for signs of stickiness or burning. 008 to 0. These are caused by the shot of molten plastic moving at different speeds throughout the injection. Once the mold is created and the machine is set up, additional parts can be manufactured very fast and at a very low cost. Troubleshooting Injection Molding Weld Lines. But add 30% glass and weld-line strength retention drops to only 34% as high as the compound with no weld line. An injection mold that produces pieces while the final tool is being built. Check the diameter of the ejector pins and the pin hole tolerances, if the tolerance is too wide this could allow. Also known as knit lines. Knit lines and other aesthetic characteristics of the part are strictly related to the plastic’s temperature during molding, so efficient filling time is essential for achieving proper welding lines and quality parts. TRADITIONAL MACHINES VENTS. By altering the part design, it may be possible to just have one flow front, and avoid multiple flow fronts having to come together. These flaws can affect the aesthetic and structural integrity of the. Ductile Injection Molding 101 - Injection Molding Varying. Meld and knit lines are actually both different types of weld lines. We have an elite team that designs for injection molding, rapid prototyping with metal and plastic, and produces world-class products under the same roof. , Rallis, A. Weld lines are a common imperfection to be found in most plastic injected molded parts. If the plastic has cooled too. Quick Guide to Injection Molding Amodel® polyphthalamide (PPA) equipment Amodel® PPA resins can be processed on conventional injection molding equipment. 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. Strength at the knit line can be as little as 20% of the. Knit / Weld Lines. The injection molding process uses a granular plastic that is grav ity fed from a hopper. These lines. Inject the molten material into a mold. Weld-lines look like cracks on the appearance of plastic. While not always visible, weld lines (aka knit lines) are where two resin flows meet. In an injection molded structural plastic part, a weld line (also called a knit line) will result in a region of mechanical weakness as soon as the part is submitted to an external load. 1-3). This course outlines mold filling and how different gating configurations affect filling. Shear Controlled Orientation Injection Molding of Polymeric Composites with Enhanced Properties. BACK TAPER – Reverse draft used in mold to prevent molded articles from drawing freely. These structures are often small surfaces of about a tenth of an inch in diameter. A screw-type plunger forces the material into a heated chambe r,. This production method is particularly popular when manufacturing in large quantities. 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. 847-952-8088. ISO 9001:2015. Soft metals, such as aluminum and beryllium copper, can be used for prototype<br />. Main causes of knit lines in injection molding: 1. A weld line (weld point, flow line or a knit line) forms when separate melt fronts travelling in opposite directions meet. Such molds or tools aren’t intended to be used in operation. The parting line will always be visible, but can be minimized by its location, draft allowance and part geometry. 5. These lines. When injection molding complex parts, often knit-lines are formed during the mold filling stage. Macromol Symp 1988;21/21: 489-500. Thicker parts require more cooling time and can lead to air becoming trapped in the mold and weld / knit lines to form. H-13 Good general purpose tool steel. Any of the above can result in “knit lines” on final products, but mold flow analysis prior to mold machining can keep these issues from cropping up at all in iteration and production. The Bottom Line You might have come across the aforementioned terminology before and did not know what any of the words meant. (1987). 01 inches between parts fitted together to work best. 060 and 0. Over the years, I have developed two separate “5M” systems. The leading edge of the molten material is often the coolest point and the closest to solidifying. One of the most common ways of adding venting is by manually machining it into the mold (Figs. Our Injection Molding Capabilities • Horizontal Injection Molding up to 3300 TONS • Structural Foam Molding + Structural Web • 1oz to 600oz part capacity. Product designers encounter a wide range of challenges when creating and developing molded parts, such as stress factors, warp, and those pesky knit 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. 2 mm. If life safety is involved, or reliably or efficacy are absolutely required, every part should. What causes knit lines? Knit lines are formed when the molten plastic flowing into the mold cavity runs into an obstruction, such as a core pin that forms the through hole on a flange of an enclosure. The weld line can either be a minor cosmetic issue. 01 inches between parts fitted together to work best. Injection molding is a popular manufacturing process used to fabricate parts in large volumes. Without it, air and gas are trapped in the mold, which compress and heat as the cavity fills. The relationship between processing conditions, morphological structure and mechanical properties of knit lines of an injection molded semi-crystalline thermoplastic. Understand the causes of knit lines in injection molding and explore effective solutions to minimize their impact. Our plastic injection molding glossary provides key terms. 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. Thick walls are not recommended because of POM’s high shrinkage rate. Proper venting is essential to molding a defect-free part. N174W21430 Alcan Doctor Jackson, WI 53037 262-677-9700; 16685 State Road 32 Mountain, WE 54149 715-276-4200; Prime. Knit Lines: Holes in the part will always cause knit lines. When the molten plastic meets an obstruction. Gate design is another critical piece of part function in plastic injection molding. The mold opens and closes on a vertical axis in the vertical injection molding. We recommend walls to be no less than 40 to 60 percent of adjacent wall thickness, and all should fit within recommended thickness ranges for the selected resin. So what does the 140F mold temperature give us? The molding technician thought the 140F mold water was OK because the data sheet provided by the material supplier suggested mold temperatures from 75 to 180F. 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. Computer simulation packages of injection molding have had success in predicting filling behavior in extremely complicated geometries, capable of predicting weld lines’ location exactly. 020"). Only then can we move on to determining the parting line itself. However, the weld line may not continue throughout the flow. Learn more about the process by reading this white paper. Injection Mold Tooling Options and Quality Reporting. Encapsulating the metal insert with plastic creates a single. Learn how to troubleshoot weld lines in injection molding. In this paper, a general and efficient approach was proposed for dual-scale infiltration flow balancing based on numerical simulation. Hardware components. Flow lines are off-color lines, streaks, and other patterns that appear on the surface of a part. The injection molding process uses a granular plastic that is gravity fed from a hopper. Weld lines, also called knit lines, are a defect that occurs with molded parts caused by two different flows of plastic resin that do not adequately join together or weld together correctly during injection molding. During the injection molding process, molten plastic is injected into a mold cavity through a gate. Multiple parts with complex design and differing wall. Also known as weld lines, knit lines. Islyn Thomas summarized the need for a particular vent location better than I ever could. 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. Swirling can increase depending on the material you use, and the texture or polish of injection molding tools. When injection molding a part, a knit line may present no problem, or can cause a serious structural problem. End-of-arm tooling can make or break a program. Let’s review what causes meld and knit line imperfections, how they’re different, and how they affect part durability. The last cause of flow lines may be the operators running your daily operations. 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. Thermoplastic Elastomer Molding Guidelines. From Plastic Materials and Processing by A.