Gates, vents, sliding cams, pick-out inserts, and any other place where the two sides of. Knit Line – A molding defect resembling a flow-line found at the point where the rubber meets between the injections. These structures are often small surfaces of about a tenth of an inch in diameter. Notched weld lines then remain near the cavity walls. Understanding Knit Lines in Injection Molding: Causes and Solutions. When designing for injection molding, consider these steps to prevent knit lines The Resin Cooling Process. Changing the. This guide to thermoplastics and injection molding material selection is aimed at an engineer who plans to quantitatively analyze a part, determine loads, stresses, strains, and environments and make an optimal material decision based on the analysis. The depth of the texture or letters is somewhat. The final molding process may be dependent upon factors. Shear Controlled Orientation Injection Molding of Polymeric Composites with Enhanced Properties. Not always a problem, but can cause serious structural issues. Bubbles are caused by melting flow fronts colliding in a mold cavity. Know about the cause of these lines and the potential solutions. Hardware components. The comment on melt flow index is correct, but, I've molded straight PC down to . An obstruction such as a core pin or holes will split the material flow and cause a weld line issue to form on the other side. Hartland, WI 53029. Remedies: Raise the temperature of the mold or molten plastic. The knit line is generally opposite the gate in thermoset. In the 1947 book, Injection Molding of Plastics, the late Dr. Called LS IMM Mexico, the subsidiary is based in Monterrey and led by plastics industry veterans Kevin Kim and Fulvio Menconi. Drying Considerations For both virgin and rework, hopper dryers sized to afford theWhen establishing the parting line, we must first define the shape and position of the parting line on the plastic molded item. Disadvantage: May rust if improperly stored. Because injection mold design and processing is complex, a set solution will not solve all problems. Weld line in injection molding is line where two or more flow molten plastics knit together. These materials have beenThe wall thicknesses of an injection molded part generally range from 2mm – 4mm (0. The temperature might fall at some places like in the mold, runners leading to the mold and injection molding. Knit lines are consideredmolding defects, and occur when the mold or/and materialtemperaturesare set too low:. BACK TAPER – Reverse draft used in mold to prevent molded articles from drawing freely. In fact, knit line strength will decrease with higher filler content and with longer fibers. molding run to be sure that the temperature does not exceed the recommended limits . Compression molding can also support large part sizes, and it has a tendency to. Injection Molding: How to Set Second. Injection molding is min-maxed for mass production. A screw-type plunger forces the material into a heated chambe r,. Thick walls are not recommended because of POM’s high shrinkage rate. Observation on a “trial and error” basis may be required. 120 in. A knit line is any line, visible or not, where two resin flows meet. Causes: Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic. They again get resolidified along the surface. The injection molding process uses a granular plastic that is gravity fed from a hopper. Mold bases, ejector plates, and some cavities (if nickel or chrome plated to prevent rust). Knit Lines: These occur when different flow fronts come together as plastic flows through the part cavities. Injection molding is the most cost-competitive technology for manufacturing high volumes of identical plastic parts. H-13 Good general purpose tool steel. H-13 Good general purpose tool steel. Be wary of saving too much weight or you might end up with: cracking. 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. Thicker parts require more cooling time and can lead to air becoming trapped in the mold and weld / knit lines to form. Complex parts can be reproduced with low cost once mold is manufactured. Weld lines, knit lines, and meld lines are terminologies to describe the imperfections of finished parts in plastic injection molding processing. Find out more by calling 573-437-4154 today or email us. Swirling can increase depending on the material you use, and the texture or polish of injection molding tools. Here are some of the most common cosmetic defects and how to avoid or fix them. 7. Still, they are almost always a cosmetic issue. As a result, prototyping is strongly recommended for parts with. Visual indicating with an finished part that are formed by the junction from two plastic flow fronts. 0 mm) is crucial to ensuring that parts cool evenly. Weld lines create visible marks, but this type of injection molding defect can also reduce a part’s strength and durability. ABOUT THE AUTHOR. Sequential valve gate technology. 1. Second, the runner leading to a tab gate takes up real estate within the. wall thickness injection molding ribs. 4. 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. What Are Cosmetic Standards for Plastic Parts?. 060 in. Understanding Knit Lines in Injection Molding: Causes and Solutions. Macromol Symp 1988;21/21: 489-500. Additionally. . 1. These lines occurs usually around holes or obstructions and cause locally weak areas in the molded part. But we can’t talk about the injection molding process without discussing the injection molding machine. 012 in. Many of these tips are geared around injection-molded parts, but some apply to machined and 3D-printed parts as well. TEFABLOC™ TOSI 431 75A is a high-flow thermoplastic elastomer (TPE) designed for large-part injection molding applications, offering exceptional flexibility and weather resistance. Insufficient injection pressure and speed can cause weld lines. Injection moulding (U. RELATED CONTENT. Draft. injection molding. opposed to two or more gates. Usually the parting line of the mold is perpendicular to the mold opening direction of the injection molding machine. Meld and knit lines are actually both different types of weld lines. Its superior flowability allows for precise and flawless filling of injection molds without the risk of flow marks and knit lines. Mold Flow Analysis computer. For example, increasing gate size can improve part filling. (1. 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. See full list on fathommfg. In. The most common and one of the most challenging injection molding defects are weld and knit lines. 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). Three terms that often get mixed up in injection molding are weld, meld and knit lines. You don't give us much to go on. The tooling engineer in the mold house will be more into venting, sprue pressure, preheating the mold, cycle times, ejector pin scheme, mold flow, getting a gate location to avoid fill issues or knit lines, excessive flash and molding pressures, and the list goes on. This radius should not be smaller than 0. Adding vents, using overflows, or using porous metal. Injection molding can form complex geometries easily, apply single part with multiple functions. 08 KICKOFF PRODUCTION. High mold temperatures usually provide less residual stress. 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. As the melted plastic fills the mold, there are locations where the molten plastic flow stream separates and then comes back together. For best results, keep thickness variations to within 15% of the nominal wall thickness. 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. reduce mold maintenance cost. When the mold adopts the multi-gate pouring scheme, the plastic melt. Learn how HPP maximizes these specialized processes. To study the weld line formation of colliding flow fronts the filling of a mold cavity was simulated. Here are some ways manufacturers can avoid quality issues during the plastic injection molding process: Inadequate Injection/Holding Pressure: If injection and holding. In this way critical areas of the interface in regard to the lack of interdiffusion and the inappropriate molecular orientation were found to be placed near. Gating. 01 inches between parts fitted together to work best. Plastic melt and mold temperature. Weld or knit lines are perhaps the most common and difficult injection molding defect to eliminate. Subscribe to Our Newsletter. In injection molding, it is often important to maintain consistent color throughout the part. Design with Plastics - for Injection Molding process. Macromol Symp 1988;21/21: 489-500. Monoform parts. 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. "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. What Are Knit Lines in Injection Molding, and How Do You Prevent Them? - RJG, Inc. It’s called sequential valve gating, or SVG. Increase the injection speed. Image credit: The Rodon Group. Reduce the injection speed to limit the risk of trapping air inside the mold. knit lines and parting lines are at the discretion of MIC-ZG. 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 point of meeting creates a weak spot because the edges of the molten plastic, already cooling, don’t mix together very well. These include: Symmetrical parts. Jabil (formerly Nypro) is uniquely qualified to offer a state-of-the-art curriculum in injection molding and related disciplines, which require highly specialized skills and continuous. 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. Reinforcing ribs play a pivotal role in the strength of a plastic part. Its superior flowability allows for precise and flawless filling of injection molds without the risk of flow marks and knit lines. In a weld line, reinforcement fibers. The Demo Mold helps those new to injection molding easily visualize the process and is a great teaching device for educators. Using three test materials, polystyrene (PS), polycarbonate (PC) and polyoxymethylene (POM), the relationship between a design and a processing parameter. ” Depending on the material you use, the knit lines could be invisible or obvious—work with an injection molding company to figure out. Nicolet Plastics LLC. This guide to thermoplastics and injection molding material selection is aimed at an engineer who plans to quantitatively analyze a part, determine loads, stresses, strains, and environments and make an optimal material decision based on the analysis. Check the diameter of the ejector pins and the pin hole tolerances, if the tolerance is too wide this could allow. The depth of the texture or letters is somewhat limited, and extra draftParts were molded at a maximum injection pressure of 235 MPa, a maximum clamping force of 800 kN, a maximum injection speed of 900 mm/s, and a maximum injection stroke of 120 mm. Also known as weld lines, knit lines. Venting at Weld Lines. Those are knit lines, formed when two opposing flows of material join together in the mold cavity. Contact Universal Plastic Mold Today. For injection molding of the multi-blade fans and various rotating plastic parts, the complex of multiple gating injection molding tools were used. More about the knit lines in injection molding. Meld and knit lines are actually both different types of weld lines. mystagingwebsite. Thin wall injection molding design tips and tricks. Knit lines may also be called stitch lines, weld lines, or meld lines. • Repair parting line • Weld up vent depth Gate blush • Melt Fracture • Adjust injection speed (fast/slow) • Increase melt/mold temperatures • Resize gateFeedstocks for injection molding show two rheological behaviors: Pseudoplastic (shear thinning) and Dilatant (shear thickening) behaviors. Reduce. Anisotropy. injection molding machines using standard industry practices . A well-designed injection-molded part will usually have draft, but the same draft that helps eject a part from the. These lines. 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. 125 inches. Depends how hard you want to push it. Weld / Knit Lines; Assumptions. Fathom injection molding services and tools offer a single-source global solution that supports your company’s low- to high-volume manufacturing process. Plasti-Co ® manufactures a wide range of injection molding nozzles from standard nozzle tips, extended nozzle tips, nozzle bodies, extended nozzles. Compression molding is one of the earliest forms of molding. 015in wall. All the while, the material could be the correct material," Hill says. As. pdf), Text File (. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part. Trapped gas is one of the most common causes of part burns, and can also lead to short shots and voids, blemishes and discernible knit lines that weaken the part. Venting. Valve gates are most efficiently. Submarine Gates. Runner Type. Dimensional stability. Even parts with metal inserts can be produced. Thick sections take longer to cool than thin ones. Thin walls often lead to material flow problems, which can cause unformed areas. Round Rock, TX 78660. Proper mold cooling can eliminate warp, decrease cycle times, and increase part quality. More about the knit lines in injection molding. They appear around holes or seams of an injection. 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. Imagine a river splitting to go around a rock, then meeting up again on the other side. They again get resolidified along the surface. 4. Venting is required to allow the air in the sprue, runner and cavity to leave the tool as the melt flows into the cavity. Weld lines: Also known as knit lines, these are molding defects that occur when two flow fronts meet without the ability to “weld. 1513 Sam Bass Rd. It happens when the molten plastic material flows around obstacles, such as inserts or cores, and rejoins on the other side. The parting lines is where the two injection mold halves (A side and B side) meet. Here are some of the most common cosmetic defects and how to avoid or fix them. 040 and 0. The Difference Between Knit and Meld Lines–and Why It Matters. Exploring common molding defects and. If you have questions about the suitability of the Protolabs process for your optical parts, contact an applications engineer at 877-479-3680. 3D printed molds in the injection molding process to lower costs and lead time. 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. In the plastic injection molding industry, we work with materials that are byproducts of oil refinement and molds made of metal casting, so our terminology is bound to be at least slightly confusing. Knit lines form when two or more plastic flow fronts collide and solidify or “knit” during the molding process. The quality of the answer is directly proportional to the information we have to work with. Observation on a “trial and error” basis may be required. The recommended minimum production volume for injection molding is 500 units. Please read on, stay posted, subscribe, and we welcome you to tell us what you think. Vertical Plastic Injection Molding. Effect of Injection Moulding Conditions on Knit-line Formation, Plastics Rubber Processing and. Typical Tool Steel for Injection Mold Construction Steel Type Steel Properties Mold Component P-20 Pre-hardened, machines well, high carbon, general-purpose steel. Very high quality and an extreme fit may. The recommended minimum production volume for injection molding is 500 units. 008 to 0. The differences between knit and meld lines are significant, and they can dramatically impact the part’s structural integrity. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. Core Geometry. Family molds. 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. Knit lines. RELATED CONTENT. Moisture in the material due to insufficient drying can form at the melt front, forming an interface between the two melt fronts. In plastic injection molding, a parting line is where two halves of a mold meet when closed. Proper venting is essential to molding a defect-free part. We look forward to offering support and advice on your next project. 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 venting7. POROUS MOLD MATERIALS. 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. 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. Other Excessive mold release Clean die and use minimal mold release. Below is a list of items that should be checked both during normal molding operations and if defects are detected. For instance, three widely used processes are insert molding, overmolding, and undercuts. Inadequate venting may cause short-shots, poor surface appearance, or weak weld-lines. 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. 080" – 0. The quality of the answer is directly proportional to the information we have to work with. Mold filling simulation software should be used to ensure that waterExpand. Injection molding is the process of injecting plastic material into a mold cavity where it cools and hardens to configuration of the cavity — is best used to mass-produce highly accurate, & often complex, 3dimensional (3D) end-use parts and products. Weld lines may appear like seams or scratches, but that is not what they are. When working with customers, our design guidelines for living hinges are 0. 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). They appear around holes or seams of an injection molded part, and some may call them “flow lines” or “weld lines”. Weld lines defects are formed when two flow fronts are merged together. 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. In spite of its cosmetic attributes, a bad knit line can significantly compromise a part. Last month, in the first part of this two-part installment we focused on the reasons viscosity can change during injection molding. Knit line Knit line Start of mold filling. wall thickness, knit lines, draft. 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. Venting at Weld Lines. They occur when melt flow fronts collide in a mold cavity. Redesign the mold: Redesigning the mold to. The leading edge of the molten material is often the coolest point and the closest to solidifying. 160"). Enlarge gas vents and gates to allow trapped air to escape the mold. The Troubleshooting Tips below offer common solutions to challenges in IML and IMD. PAPER ABSTRACT: Desktop injection molding machines and stereolithographic print ed molds make hands-on injection molding production possible in design studio classrooms and labs. 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. Knit and Meld lines are inherent to plastic injection. Product designers encounter a wide range of challenges when creating and developing molded parts, such as stress factors, warp, and those pesky knit lines. Generally all plastic components should be designed with draft where possible. The Bottom Line You might have come across the aforementioned terminology before and did not know what any of the words meant. The Difference Between Knit and Meld Lines–and Why It Matters. Steven. TEXAS. Ham Lake, MN 55304. The best we can hope for is to pressurize the knit or meld line better through the combination of melt temperature, mold temperature, flow rates, and holding pressures. The quality of the weld line can be improved through optimized processing conditions, increased mold and melt temperatures, and increased injection speed toWeld Lines. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. The mold settings can run flawlessly until employees consistently open the machine for inspections, interrupt cycle times, or cause. In places where the plastic flows around a solid piece of the mold, from a simple circle to forming complex shapes like rings or squares. Knit lines; Knit lines always occur once 2 melted materials meet with hair-like discolorations. 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. 5mm (0. Weld Line defect analysis, the most common of many defects in injection moulding products. The plastic melt that passes through the gate at high speed directly enters the cavity, then contacts the surface of the cavity to solidify, and then is pushed by the subsequent plastic melt, leaving traces of meandering. 1050 Walnut Ridge Drive. Knit lines are a common defect in injection molding that can affect the appearance and performance of plastic parts. It is. Press fits for injection-molded parts can be challenging. This is the first article in a new series that will examine the injection molding process by breaking down the path a plastic pellet takes as it is transformed from Pellet 2 Part. be impressed by improvements of bubbles (vacuum voids), short shots, unequal thickness, jetting marks, weld lines, knit lines, black streaks, strain 2. Reduce the injection pressure. 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. 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). Gating. As these flow fronts come together. [1]Weld Lines. Weld Lines-3 Ways for Detecting and Correcting. Possible causes: Low temperature or fluidity of melted plastic during injection molding. 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. Finally, knit lines occur when molten plastic flows around an object and rejoins on the far side, a phenomenon that is quite possible when inserts. When the plastic cools, it takes the shape of the mold and is then ejected and finished. These defects are not visible, like small. When assessing issues with injection molded parts, we want to make sure some core aspects of molding are checked. Weld-lines look like cracks on the appearance of plastic. Gates, vents, sliding cams, pick-out inserts, and any other place where the two sides of. The weld line can either be a minor cosmetic issue. 2. Contact Universal Plastic Mold Today. Core Geometry. This is a quick rundown on how to handle common features in injection molding designs:What are the advantages of injection molding 1. Inject the molten material into a mold. Knit Lines. We wanted to independently assess how a mold would fill and where knit lines would be, instead ofKnit Lines. 0 to 3. Plastic injection molding is a complex and highly intricate process. RJG announced that its injection molding process control system, CoPilot, will begin offering valve gate control technology for hot runners starting in September 2021. Weld lines are formed when there is an inability of two or more flow fronts to ‘knit’ or ‘weld,’ together during the molding process. As a result of weak coupling along the knit-line, the weld-line strength can be only about 60% of that of an unreinforced resin. knit line. Fortunately, there is a technique that provides almost unlimited control over which parts of a tool fill when and how fast. 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. A knit line is what happens when plastic flows coming from two different directions meet inside of the mold and solidify. 2. Discover why you should avoid knit lines when designing and manufacturing injection moulded parts. We ensure customers receive accurate, fast, on-time, sufficient support and services. Because injection mold design and processing is complex, a set solution will not solve all problems. The thermo-rheological findings were used to investigate the sources of weld line weakness. Generally, the mold structure is roughly divided into two main parts: fixed half and moving half. Injection molding a red component, for example, can pose a challenge because at time the component may be too thin for the pigment itself!. This defect is caused from excessive lube or oily stock. Strength at the knit line can be as little as 20% of the. Most failures of injection molded plastic parts are caused by either improper design of the part or improper design of the mold. • Standard (general purpose) screws with a compression ratio between 2. The bottom of your rib must also feature a fillet. Runner and gates must be balanced. The injection molding process is on the surface is fairly simple, plastic resin is heated to its melting point and forced into the cavity of the mold to produce a plastic parts. Injection molding is the most widely used method for mass-producing plastic parts. 847-952-8088. When injection molding complex parts, often knit-lines are formed during the mold filling stage. TRADITIONAL MACHINES VENTS. It is also where a slide mechanism might meet where it shuts off on one of the mold halves. 250 in. Injection molding is a popular manufacturing process used to fabricate parts in large volumes. Knit lines, weld lines, mold lines, or flow lines in injection molding are faint lines that become visible on certain plastic parts. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and. This continues until the weld lines in plastic injection molding disappear. Degassing – Degassing is the simple operation of opening and closing an injection mold to allow any trapped air (or gas) to escape. spelling: injection molding) is a process for producing parts by injecting molten material into a mould, or mold. The thermoplastic injection molding process at BEPL is a standard process involving an aluminum mold with no heating or cooling lines running through it, which means cycle times are a bit longer. Main causes of knit lines in injection molding: 1. When the molten plastic meets an obstruction. These are imperfections in the part where separated flows of cooling material meet and. Typically, the wall thickness is somewhere in the neighborhood of 50%-75% of your nominal wall thickness. Let the molten material cool to a solid-state. Self-mating parts. Gate size can be increased or moved to a section with greater wall thickness to help eliminate. Heat: Too much heat can have an appearance similar to moisture splay. Facebook. A weld or meld line is a weakness or visible. This production method is particularly popular when manufacturing in large quantities. Weld or Knit lines are a plane or section in an injection molding part where the flow of molten plastics meets from different. Weld-lines, also referred to as spider-lines or knit-lines, are regions in the wall of an extruded plastic pipe, running the whole length of the pipe, at which there is a change in dimension and a. As the melted plastic fills the mold, there are locations where the molten plastic flow stream separates and then comes back together. BLEED – (1) To give up color when in contact with water or a solvent. Cooling Lines, Injection Mold Material has a great impact on injection molded part quality. All the while, the material could be the correct material," Hill says. Fig. It is best practice to maintain a uniform thickness throughout an injection-molded part. Step 3: The injection mold undergoes rotation to align in position with injection Barrel B. See UNDERCUT. Plastic melt and mold temperature. Islyn Thomas summarized the need for a particular vent location better than I ever could. Tips for Avoiding Knit Lines in Injection Molding Keeping the proper temperature in the injection molding machine, runners, and within the mold are all key to ensuring. The melt is forced to split up during filling of the cavity. INJECTION MOLDING GUIDE V2. Thus, as the flow continues, the two flow fronts may gradually rejoin to make one uninterrupted flow front. 453. Here are a few of the most common defects that may occur in plastic injection molding — and how product teams can avoid them. Check the diameter of the ejector pins and the pin hole tolerances, if the tolerance is too wide this could allow molten plastic to flow down between the pin and hole surface. Knit lines, holes, part configuration and the talent of your molder all come into play. PTI has five injection molding presses suitable for MIM. Starting with small part size, students learned to think about detailing on a product at thisTable 2. One of the most important aspects of hot runner system design is the gate location, which is the point where the molten plastic enters the cavity. A poor knit line can cause only cosmetic blemishes or it can significantly weaken the structural integrity of a part. Without it, air and gas are trapped in the mold, which compress and heat as the cavity fills. Injection Molding Materials When using PolyJet molds, both tool life and part quality will be dependent on the thermoplastic material used during the injection molding process. Also known as string lines. Knit Lines. The knit line itself (the vertical line in the middle) is visible in this lighting, but the biggest issue is with the texture. 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. Our plastic injection molding glossary provides key terms. Look for signs of stickiness or burning. 1990-03-01 00:00:00 Knit lines in reaction injection molding (RIM) are formed when a fluid flow front passes through the mold cavity and must pass around objects in its path. Solution: People. Other common issues associated with thin walls include uneven cooling, warp, cracks, cosmetic texture inconsistencies, and weak knit lines or fracture points. Knit lines are formed in injection moulding when two flow fronts meet in the middle of a part (a line where two sides of the plastic are “knit” together). Factors were ordered. Trapped gas is one of the most common causes of part burns, and it can also lead to short shots and voids, blemishes and discernible knit lines that weaken the part. A wide variety of tool steels are available for injection mold construction. Also known as a knit line, this is the point at which two flow formations meet and are unable to connect during injection molding. Texturing may also hide surface defects such as knit lines and other imperfections. ABS.