Not always a problem, but can cause serious structural issues. Commonly seen at the edge of a hole or other cored feature, knit lines are—as a. The mold-opening force varies widely depending on the type and size of the molding machine. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part. Molding is a manufacturing process in which resin in a barrel is heated to a molten state,. 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. Knit lines, weld lines, mold lines, or flow lines in injection molding are faint lines that become visible on certain plastic parts. 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. BACK TAPER – Reverse draft used in mold to prevent molded articles from drawing freely. 4271. Imagine a river splitting to go around a rock, then meeting up again on the other side. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. This can enable the manufacturer to increase the molten plastic temperature at the front of each flow and avoid the occurrence of weld lines. Knit and Meld lines are inherent to plastic injection. Below is a list of items that should be checked both during normal molding operations and if defects are detected. While injection molding itself is a straightforward process involving melting. Its superior flowability allows for precise and flawless filling of injection molds without the risk of flow marks and knit lines. Weld lines create visible marks, but this type of injection molding defect can also reduce a part’s strength and durability. The injection molding process uses a granular plastic that is gravity fed from a hopper. When injection molding a part, a knit line may present no problem, or can cause a serious structural problem. This makes the casting process as efficient and accurate as possible. Also known as knit lines. This summary represents a key subset of the detailed molding information found in the remainder of this molding guide . Reinforcing ribs play a pivotal role in the strength of a plastic part. Computer simulation packages of injection molding have had success in predicting filling behavior in extremely complicated geometries, capable of predicting weld lines’ location exactly. 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. 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. POROUS MOLD MATERIALS. Example of a gate location in a finished part Example of knit lines in a finished part. They appear around holes or seams of an injection. If debossed pattern is indispensable, it should have a depth of no less than 0. Ideally, a safety strap should be strong enough to prevent the mold from opening under various unforeseen circumstances, but always less than the holding power of the mold clamps and the machine’s mold-opening force. It’s called sequential valve gating, or SVG. 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). ; Wood, Philip E. The last cause of flow lines may be the operators running your daily operations. This line does not normally impact the overall shape or dimensions of the item. Swirling can increase depending on the material you use, and the texture or polish of injection molding tools. The thickness of the part plays an important role in how difficult the molding process will be. ). Material contamination, for example, can cause several molding issues. Understanding Knit Lines in Injection Molding: Causes and Solutions. In the 1947 book, Injection Molding of Plastics, the late Dr. The splay condition can completely cover a part, or it can appear inconsistently in various areas on the part. Knit line / Meld line / Transfer line Discoloured line where two flow fronts meet Mould or material temperatures set too low (the material is cold when they meet, so they don't bond). This is a quick rundown on how to handle common features in injection molding designs:What are the advantages of injection molding 1. knit line. The Troubleshooting Tips below offer common solutions to challenges in IML and IMD. In the injection molding process, knit lines are a common occurrence when two or more plastic flow fronts collide and solidify, resulting in a visible seam or line on the molded part. Knit / Weld Lines. 3. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. To prevent swirling, perform tests whenever possible and use pre-compounded resin to ensure identical color match across parts. A weld line, or knit line, occurs when two flows of melted plastic meet and “knit” together. What is copper electrode in injection mold? hashtag#plasticinjectionmold hashtag#moldmaking "Copper electrode," also known as "electrode," is a common component in mold manufacturing. This course outlines mold filling and how different gating configurations affect filling. The Goldilocks principle applied to wall thickness. The injection molding process is fairly simple, plastic resin is heated to its melting point and forced through the machine and into your mold to produce your plastic parts. Obstructions in the part create weld lines or knit lines during injection molding. H-13 Good general purpose tool steel. A meld line occurs when two moving melt fronts converge and flow parallel to each other. Prevent Flow and Knit Lines During the Injection Molding Process. BACKING PLATE – In mold construction, a plate used as a support for the cavity blocks, guide pins, bushings, etc. The recommended minimum production volume for injection molding is 500 units. Islyn Thomas summarized the need for a particular vent location better than I ever could. When working with customers, our design guidelines for living hinges are 0. Proper venting is essential to molding a defect-free part. The Jabil Molding Institute empowers employees, customers and suppliers to utilize the most advanced plastics and related manufacturing techniques in the world. 5 mm) is ideal, according to injection molding engineer Jack Rulander of Protolabs in Maple Plain, MN. 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. In the case of mold details, knits will occur on the lee side of a detail. As. Injection molding can form complex geometries easily, apply single part with multiple functions. 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. Injection molding is an excellent way to create plastic parts in bulk, but there are many problems that can cause cosmetic defects ranging from minor to major. Last month, in the first part of this two-part installment we focused on the reasons viscosity can change during injection molding. The linear mold shrinkage of injection molded parts will vary from 0. Ductile Injection Molding 101 - Injection Molding Varying. The metal parts, made of steel grade. Weld line defects – also known as knit lines – is a defect that occurs when the injected plastic meets within a mold. Injection Molding. Injection molding can be a real challenge if you don’t follow the “Five M’s”: Man, Mold, Machine, Material and Method. ISO 9001:2015. Trapped air in a mold manifests itself as a burn mark on the molded part. Enlarge gas vents and gates to allow trapped air to escape the mold. In addition, the force of the gas holds the appearance side of the part in contact with. The knit lines are the result of poor molding conditions (likely molding at too low of a temperature). When needed, crush ribs can allow for draft and a tight fit for a part that requires both. Inadequate venting may cause short-shots, poor surface appearance, or weak weld-lines. "As soon as that part or material is stressed, it can be real conducive to fracture, cracking, breaking, just because of the way it's been molded. Material contamination, for example, can cause several molding issues. This article will look at two common venting options—using traditional machined vents and venting through porous materials—to help you decide which is right for your application. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. IM: hot or warm knit lines are cause by _____ while cold knit lines are caused by _____. Once the mold is created and the machine is set up, additional parts can be manufactured very fast and at a very low cost. It is caused by the incomplete joining of the uncured. From the start of your project, you can count on The Toolroom to exceed your expectations for all of your plastic injection molding needs. 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. Dabei erreichen unsere Produkte zufriedene Käufer in mehr als 30 Ländern. One common problem that occurs in injection molding is the formation of knit lines. 18/06/2021. Ham Lake, MN 55304. Which affect parts aesthetics and decrease parts strength. The most common and one of the most challenging injection molding defects are weld and knit lines. Venting at Weld Lines. Injection molding is an excellent way to create plastic parts in bulk, but there are many problems that can cause cosmetic defects ranging from minor to major. These lines. 453. Troubleshooting Injection Molding Weld Lines. More about the knit lines in injection molding. Injection speed and pressure. extend the time between mold maintenance shutdowns. To avoid walls that are too fat or too thin, designing a wall thickness between 0. Many of these tips are geared around injection-molded parts, but some apply to machined and 3D-printed parts as well. A special mold provided with a glass window was used in order to directly evaluate the flow progression during the filling phase of the injection molding process in a thin-wall cavity and to. It can be determined by a visible line appearing on the surface of the plastic part where two flows of the plastic meet. While injection molding itself is a straightforward process involving melting. 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. 030 and 0. Still, they are almost always a cosmetic issue. The packing pins are sequentially. Two major moldability pitfalls to avoid are sink and voids. Gating. Mold venting should be checked and increased if necessary, injection molding service. Let’s. These defects are not visible, like small. A poor knit line can cause only cosmetic blemishes or it can significantly weaken the structural integrity of a part. Very high quality and an extreme fit may. txt) or view presentation slides online. 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. This can dramatically reduce the effects of material changes, according to the injection molding technology company. Flow lines typically manifest as visible marks, often slightly. Weld lines: Also known as knit lines, these are molding defects that occur when two flow fronts meet without the ability to “weld. Weld lines are formed when there is an inability of two or more flow fronts to ‘knit’ or ‘weld,’ together during the molding process. Let’s review what causes meld and knit line imperfections, how they’re different, and how they affect part durability. 25T-0. Here are some of the most common cosmetic defects and how to avoid or fix them. 008 in. Randy Kerkstra has been in the plastics industry for more than 26 years, occupied frequently with troubleshooting injection molding. injection molding machines using standard industry practices . Mold cooling is one of the most important variables in injection molding. Inappropriate temperature. 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. 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. Injection molding is the most widely used method for mass-producing plastic parts. Call 800-985-6750 or email your inquiry to sales@customgasketmfg. TRADITIONAL MACHINES VENTS. Molding is a manufacturing process in which resin in a barrel is. Texturing may also hide surface defects such as knit lines and other imperfections. With fast accurate, injection-mold simulations, designing injection-molded products, developing injection molds, and shooting parts is more collaborative, accurate, and efficient, saving time and driving down costs. Proper mold cooling can eliminate warp, decrease cycle times, and increase part quality. Plating and painting applications usually require slower fill rates to keep orientation low. Gate locations can be directly behind or on the edge of the graphic. Plastic is then injected at high pressure into the mold, and it solidifies with any surface features or defects in. But we can’t talk about the injection molding process without discussing the injection molding machine. injection molding. Weld or knit lines are perhaps the most common and difficult injection molding defect to eliminate. When assessing issues with injection molded parts, we want to make sure some core aspects of molding are checked. Knit lines are hard to eliminate for injection molded plastic parts but designers can minimize cosmetic defects. Mold Venting. To avoid these problems and create the best possible. Gates, vents, sliding cams, pick-out inserts, and any other place where the two sides of. Thermoplastic Elastomer Molding Guidelines. Dimensional stability. 025 inches per inch, depending on the particular resin, molding conditions, part design, part wall thickness, and direction of flow. A knit line is any line, visible or not, where two resin flows meet. 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. On the opposite side. – Change the thickness of the product. This defect is caused from excessive lube or oily stock. Randy Kerkstra has been in the plastics industry for over 26 years, occupied frequently with. Three terms that often get mixed up are weld, meld, and knit lines. Mold venting is critical to the quality and consistency of the finished part. Weld or knit lines are perhaps the most common and difficult injection molding defect to eliminate. Weld lines typically appear in a part when an obstruction of some kind separates the melt flow and prevents it from "knitting" together. Knit Lines and Resins. These lines occurs usually around holes or obstructions and cause locally weak areas in the molded part. When designing for injection molding, consider these steps to prevent knit lines Resin cools as it is injected into a mold. Reduce. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. A. Complex parts can be reproduced with low cost once mold is manufactured. occupied frequently with troubleshooting injection molding. The Problems & Solutions course is designed to train injection molding personnel in. Solution: Increase the injection pressure or speed. Causes: Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic. 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. So. 125 inches. Other Excessive mold release Clean die and use minimal mold release. 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. 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. (If parts are to be molded by Protomold’s rapid injection molding process, our Design Guidelines should be used. What Are Cosmetic Standards for Plastic Parts?. Knit lines, weld lines, mold lines, or flow lines in injection molding are faint lines that become visible on certain plastic parts. But any. Injection molding crush ribs are small, protruding features added to molding designs to support and improve the stability of press-fit connections. Weld line defects – also known as knit lines – is a defect that occurs when the injected plastic meets within a mold. 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 adhesiveWeld lines, created in the areas of collision of two flow fronts of plastic in the injection mold cavity, are the reason of lower mechanical properties and a worse surface condition of molded parts. . Knit Lines. 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. Solution: People. Mold Flow Analysis computer. a specific area on one side lens, a warp line, and a weft line. injection molding. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and. A medical device designer might look up a polymer's physical properties and game out. 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. 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. Weld / knit plane forms as flow fronts recombine Weld line and failure due to flow around core. ” 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. 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. 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. Plastic injection molding has long been an affordable and efficient process for producing custom prototypes and end-use production parts. As the polymer resin is injected into a mold, it flows through all parts of the mold. Below is a list of items that should be checked both during normal molding operations and if defects are detected. , under its Advanced Technology Program (ATP). Depending on the resin, resin temperature, mold temperature, and filling. And, for those who can work within its limitations, rapid injection molding may be suitable for producing some optical parts (and cost as much as 90 percent less than traditional tooling). . The gate location affects the. The company provides an array of digital manufacturing expertise. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and. Injection molding is a popular manufacturing process used to fabricate parts in large volumes. 6 Monate. Three terms that often get mixed up are weld, meld, and knit lines. Three terms that often get mixed up are weld, meld, and knit lines. Knit lines form when two or more plastic flow fronts collide and solidify or “knit” during the molding process. They appear around holes or seams of an injection molded part, and some may call them “flow lines” or “weld lines”. This design aid shows the mechanics of how the A-side and B-side of a tool work together during the injection molding process to create plastic parts. Raising the mold temperature will keep the material molten longer, resulting in stronger weld lines and even elimination of weld lines. Services. These data were obtained from many andKnit lines in injection molding are common defects of the molded parts. Weld or Knit lines are a plane or section in an injection molding part where the flow of molten plastics meets from different. 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. 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. Strength at the knit line can be as little as 20% of the. The mold settings can run flawlessly until employees consistently open the machine for inspections, interrupt cycle times, or cause. 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. Core out parts to eliminate thick walls. There are several possible causes that need to be investigated to troubleshoot weld lines in injection molding. Knit lines are a common defect in injection molding that can affect the appearance and performance of plastic parts. suppressed cooling and possibly warp. 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. If you have questions about the suitability of the Protolabs process for your optical parts, contact an applications engineer at 877-479-3680. Knit lines, also known as weld lines, are fine lines that resemble hairline fractures or cracks. Also known as weld lines, knit lines. They need the interference of about 0. The parting lines is where the two injection mold halves (A side and B side) meet. This information is intended to be used only as a guideline for designers and processors of modified thermoplastics for injection molding. spelling: injection molding) is a manufacturing process for producing parts by injecting molten material into a mould, or mold. An injection mold that produces pieces while the final tool is being built. INJECTION MOLDING GUIDE V2. Weld / Knit Lines; Assumptions. Depends how hard you want to push it. Weld lines, knit lines, and meld lines are terminologies to describe the imperfections of finished parts in plastic injection molding processing. 5 to 3. Bociaga and Jaruga (2007) investigated flowing weld line in injection process by injection molding machine, which the part surface was observed in reflected light using the optimal microscope and. 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. They occur when melt flow fronts collide in a mold cavity. be impressed by improvements of bubbles (vacuum voids), short shots, unequal thickness, jetting marks, weld lines, knit lines, black streaks, strain 2. Read on to find out more! 800. Reducing the dimensions of the runner system. More about the knit lines in 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. 040-in. The injection molding process uses a granular plastic that is grav ity fed from a hopper. Flow lines are off-color lines, streaks, and other patterns that appear on the surface of a part. Edge Gates. Three terms that often get mixed up in injection molding are weld, meld and knit lines. Reduce the temperature of the mold and plastic being injected. Learn how to troubleshoot weld lines in injection molding. Knit Line occurs where different flow fronts of material meet while flowing through the mold. Injection Mold Tooling Options and Quality Reporting. Gate design is another critical piece of part function in plastic injection molding. Probable Cause Suggested RemedyI am working on a thin PC/ASA injection moulded component which has a tendency to snap at the Knit-point (the final point of travel at which the…Opening and closing all the valve gates at the same time necessarily limits the degree of control over weld or knit lines, gas traps, localized packing, and unbalanced flow. The packing pins are similar in construction to mold ejector pins and may also be mold ejector pins depending on mold configuration. Injection Molding: How to Set Second. Contact Universal Plastic Mold Today. When the mold adopts the multi-gate pouring scheme, the plastic melt. Knit lines are consideredmolding defects, and occur when the mold or/and materialtemperaturesare set too low:. holes, multi-point injection. The comment on melt flow index is correct, but, I've molded straight PC down to . In this, work, metal inserts were joined with polyamide 6 by using the injection-molding technique. Draft. Knit and meld lines are inherent to plastic. 5 : 1 and 3. Typically, injection molded parts will be between 0. 5. Three terms that often get mixed up in injection molding are weld, meld and knit lines. opposed to two or more gates. Injection molding a red component, for example, can pose a challenge because at time the component may be too thin for the pigment itself!. 3D printed molds in the injection molding process to lower costs and lead time. They are very much related but independent disciplines. You don't give us much to go on. The parting line will always be visible, but can be minimized by its location, draft allowance and part geometry. [1]Weld Lines. to prevent improperly placed knit lines or sinking in the plastic downstream from the hinge. Because injection mold design and processing is complex, a set solution will not solve all problems. 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. Factors were ordered. These lines. 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. 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). The utility model relates to a mold structure capable of eliminating the appearance of weld lines of a hole-shaped injection molded product. Our Injection Molding Capabilities • Horizontal Injection Molding up to 3300 TONS • Structural Foam Molding + Structural Web • 1oz to 600oz part capacity. This article will look at two common venting options—using traditional machined vents and venting through porous materials—to help you decide which is right for your application. When designing for injection molding, consider these steps to prevent knit lines The Resin Cooling Process. Figure 1 presents the shrinkage of polypropylene versus wall thickness in injection molded parts. Round Rock, TX 78660. Plastic injection molding has long been an affordable and efficient process for producing custom prototypes and end-use production parts. This most often occurs when you have a core or shutoff feature that the plastic needs to flow around. While injection molding dies are expensive to produce, each die canHowever, they can also create weld lines or knit lines in the part where the flow fronts meet. 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. Because this allows some cooling and thickening of the resin, tab gates require a relatively large opening into the mold cavity. 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. 140 in. Steps involved in Multiple Material Molding. In the plastic injection moulding industry, we work with materials that are by-products of oil refinement and moulds made of metal casting, so our terminology is bound to be at least slightly confusing. They occur when melt flow fronts collide in a mold cavity. . S. Such molds or tools aren’t intended to be used in operation. Remove holes if possible. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. 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. Specifically, Stokes and. They again get resolidified along the surface. Proper venting is essential to molding a defect-free part. Step 3: The injection mold undergoes rotation to align in position with injection Barrel B. 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. Thin wall injection molding can produce walls as thin as 0. One of the most common ways of adding venting is by manually machining it into the mold (Figs. Most failures occur at what are called “weld lines” or “knit lines”. Make sure the venting is not too large. 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. weld line in injection molding. The thermo-rheological findings were used to investigate the sources of weld line weakness. For over four decades, our experienced team and leading edge technology have provided you with tooling solutions from start to finish. In layman’s terms, injection molding boils down to four simple steps: Melt material. Lower the melt and mold temperature to prevent overheating. Take these steps to avoid knit lines. They occur when melt flow fronts collide in a mold cavity. S. . Understanding Knit Lines in Injection Molding: Causes and Solutions. Enlarge gas vents and gates to allow trapped air to escape the mold. This slight depression is a tiny line called the weld line or knit line. Figure 1 Flow Lines Knit or Weld Lines Cavity Gates Table 1 Knit line in a multi-injection-point cavity. ITAR. Gates, vents, sliding cams, pick-out inserts, and any other place where the two sides of. • Estimated clamp tonnage of 5. Looking to Your Knit Lines with Molding. Starting with small part size, students learned to think about detailing on a product at thisTable 2. Injection molding is a popular manufacturing process used to fabricate parts in large volumes. Step 2: The injected molten plastic is cooled followed by the separation of the core and cavities. Those are knit lines, formed when two opposing flows of material join together in the mold cavity. While not always visible, weld lines (aka knit lines) are where two resin flows meet. Mold venting is critical to the quality and consistency of the finished part. After selecting resin material, the design and process control of injection mold directly influence the quality of formed products. Thin wall injection molding design tips and tricks. Also known as a knit line, this is the point at which two flow formations meet and are unable to connect during injection molding. 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. Injection molding is min-maxed for mass production. Weld lines may appear like seams or scratches, but that is not what they are. Monoform parts. Multiple parts with complex design and differing wall. Depends how hard you want to push it. Typically, the wall thickness is somewhere in the neighborhood of 50%-75% of your nominal wall thickness. 6. In injection molding terms, these are called “living hinges” and are made right into the design instead of adding them later. Holes in the part will always cause knit lines. Learn about the key characteristics of common injection molding materials, recommended applications, and how common thermoplastics compare to one another.