Knit / Weld Lines. Vacuum on the mold does help and many times eliminates the weld line. Let’s review what causes meld and knit line imperfections, how they’re different, and how they affect part durability. The knit lines are the result of poor molding conditions (likely molding at too low of a temperature). Weld-line strength for plastics can vary from 20% to nearly 100% of the strength of the plastic itself. [8] Bevis, MJ, Reis RL, Cunha AM. , Rallis, A. 7. Temperature is another factor that can cause weld or knit lines in injection molding. These defects are not visible, like small. 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. TEXAS. Vacuum. Reducing the dimensions of the runner system. The cooler flow fronts of the resin aren’t forced into the mold surface texture as well as the hotter resin behind the flow front. Multiple gates will always cause knit lines. 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. • 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. The Demo Mold helps those new to injection molding easily visualize the process and is a great teaching device for educators. A weld or meld line is a weakness or visible. Except for a few very simple injection molded parts, they occur on most injection molded parts (typically in the form of a line or V-groove), especially large complex items that require use of multi-gate molds and inserts. Also discussed are weld and meld lines and their effects on molded. Because injection mold design and processing is complex, a set solution will not solve all problems. 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. Islyn Thomas summarized the need for a particular vent location better than I ever could. 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. Looking to Your Knit Lines with Molding. In. The parting line location on the injection mold will have an effect on how the cavity and core (both halves of the injection mold) come together and seal. Most failures occur at what are called “weld lines” or “knit lines”. Slower injection provides less orientation and stress. Injection moulding (U. 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. Based on the resin. Knit lines don’t always compromise the structural integrity of plastic parts. 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. Injection speed and pressure. Wavy lines are another feature of weld lines. On a hydraulic machine you can program hold pressure: For example, if you set 850 psi (59 bar), you will get 850 psi hold pressure for the time you set on. 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. Figure 1: D-shaped through-hole in center of gear part accepts the drive shaft. - You hit on this a little, but dealing with the parting line and ejector pin marks is pretty important, and isn't as simple as it seems at first look. 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 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. RELATED CONTENT. Because injection mold design and processing is complex, a set solution will not solve all problems. 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. 010 inches. 18/06/2021. Family molds. Observation on a “trial and error” basis may be required. (1. When designing for injection molding, consider these steps to prevent knit lines Resin cools as it is injected into a mold. By altering the part design, it may be possible to just have one flow front, and avoid multiple flow fronts having to come together. Weld line in injection molding is line where two or more flow molten plastics knit together. Knit line Knit line Start of mold filling. The injection molding process offers a vast array of plastic resins to work with. 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. Reduce clamping force. The packing pins are similar in construction to mold ejector pins and may also be mold ejector pins depending on mold configuration. 1. They occur when melt flow fronts collide in a mold cavity. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. So. Mold venting is critical to the quality and consistency of the finished 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. Setting that Delta P is the secret to consistent fill time. Extreme geometries can cause knit lines. Knit Lines. Find out more by calling 573-437-4154 today or email us. We ensure customers receive accurate, fast, on-time, sufficient support and services. The Problems & Solutions course is designed to train injection molding personnel in. Injection molding can use thermosets or thermoplastics. One of the most common ways of adding venting is by manually machining it into the mold (Figs. Plastic injection molding is a complex and highly intricate process. These flaws can affect the aesthetic and structural integrity of the. Timing and temperature are significant contributing factors when it comes to flow and knit line defects. In fact, knit line strength will decrease with higher filler content and with longer fibers. Injection molding is a popular manufacturing process used to fabricate parts in large volumes. 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. Over the years, I have developed two separate “5M” systems. Knit lines form when two or more plastic flow fronts collide and solidify or “knit” during the molding process. The result is a consistent flow of molten plastic during injection and a quicker filling time. A mold with too few gates is susceptible to cooling problems, but a mold with too many gates. Three terms that often get mixed up are weld, meld, and knit lines. Inject the molten material into a mold. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. 5T, where T is your nominal 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. A weld line (weld point, flow line or a knit line) forms when separate melt fronts travelling in opposite directions meet. These knit-lines are well-known as particularly critical regions in the mold when mechanically loaded. 763-755-7575. (2) Undesired movement of certain materials in a plastic. In injection molding, a thermoplastic material is melted to a liquid state and injected into a custom, double-sided mold. For example, increasing gate size can improve part filling. Services. Injection molding is an efficient, often cost-effective way to manufacture custom plastics that are strong, durable, and long-lasting. 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. Commonly seen at the edge of a hole or other cored feature, knit lines are—as a. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and. Changing the. Bubbles are caused by melting flow fronts colliding in a mold cavity. With over 50 years of experience, Universal Plastic Mold can help you avoid (or at least minimize) knit lines in your molded parts. Core Geometry. 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. S. They are the result of fundamental physical effects taking place inside of the mold. 1-3). knit line. The definition of weld lines: When multiple plastic flow fronts meet in the cavity after encountering inserts, holes, or areas with inconsistent flow rates or interrupted melt flows, or in the case of gate injection filling, the inability of two or more flow fronts to fully “knit” together will cause the linear weld marks. Inadequate injection pressure can result in knit lines and a surface that appears corrugated. Focus - Injection Molding - Free download as PDF File (. 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. 060 in. 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. They again get resolidified along the surface. It’s also relatively inexpensive. Shorten the mold cycle time so that any trapped air and resin don’t have a chance to overheat. If you have questions about the suitability of the Protolabs process for your optical parts, contact an applications engineer at 877-479-3680. . Which affect parts aesthetics and decrease parts strength. Knit lines. Moisture in the material due to insufficient drying can form at the melt front, forming an interface between the two melt fronts. Knit Line occurs where different flow fronts of material meet while flowing through the mold. . CNC-Bearbeitung; Spritzgusswerkzeuge und -teile; Stereolithographie (SLA) Selektives. Basic rule of thumb air in mold must be able to escape at theInjection molding is the predominant manufacturing process used for producing transparent polymer products. thickness with a spacing of about 0. "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. Remove holes if possible. Contact Universal Plastic Mold Today. This article will discuss the importance of injection molding nozzles and what makes a good nozzle for a molding or injection. Plastic injection molding is the industry’s most common way to manufacture parts. sinks, knit lines, and shrinkage,” Spalding says. 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. • Standard (general purpose) screws with a compression ratio between 2. Venting is required to allow the air in the sprue, runner and cavity to leave the tool as the melt flows into the cavity. 140 in. You don't give us much to go on. In the 1947 book, Injection Molding of Plastics, the late Dr. 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. 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. Vertical Plastic Injection Molding. Our plastic injection molding glossary provides key terms. Gate size can be increased or moved to a section with greater wall thickness to help eliminate. [1] Plastic injection molding is the industry’s most common way to manufacture parts. The advantages of compression molding include more isotropic fiber distribution, dimensional stability, and surface finish. This can dramatically reduce the effects of material changes, according to the injection molding technology company. The knit line itself (the vertical line in the middle) is visible in this lighting, but the biggest issue is with the texture. An edge gate introduces material at the main parting line, or where the two halves of. 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. Reduce the injection speed to limit the risk of trapping air inside the mold. Venting at Weld Lines. 015in wall. The Jabil Molding Institute empowers employees, customers and suppliers to utilize the most advanced plastics and related manufacturing techniques in the world. 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. PTI, a plastics and metal parts molder, is a leading developer of the MIM process. Wall Thickness: The recommended wall thickness for injection molded POM parts is between 0. As a result, the portion has a faint, perhaps undetectable line. Flow Lines. 5. Weld or knit lines are perhaps the most common and difficult injection molding defect to eliminate. ”. reduce mold maintenance cost. What Are Cosmetic Standards for Plastic Parts?. More about the knit lines in injection molding. These lines are the result of the molten plastic material recombining after splitting and flowing around an obstacle in the mold cavity. Mold filling simulation software should be used to ensure that waterExpand. INJECTION MOLDING GUIDE V2. The temperature might fall at some places like in the mold, runners leading to the mold and injection molding. Piccarolo, S. 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. Flow Lines / Knit Lines Flow/Knit Lines 11. The area generally has lower strength than the other areas of the part. Depends how hard you want to push it. 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). molding run to be sure that the temperature does not exceed the recommended limits . Weld / knit plane forms as flow fronts recombine Weld line and failure due to flow around core. There is high defect rate (surface defects/knit line). 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. Gate and Knit Line Locations<br />. ABS. S. BLEED – (1) To give up color when in contact with water or a solvent. Knit and meld lines are inherent to plastic. Weld lines defects can also form when the flow fronts from two gates merge or when thick and thin sections in a part cause a flow to split and merge. 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. 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. Weld Line defect analysis, the most common of many defects in injection moulding products. Additionally. A knit line is any line, visible or not, where two resin flows meet. Mold Flow Analysis computer. 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. In layman’s terms, injection molding boils down to four simple steps: Melt material. Adjust the design for the flow pattern to be a single source flow. 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. Visual indicating with an finished part that are formed by the junction from two plastic flow fronts. N174W21430 Alcan Doctor Jackson, WI 53037 262-677-9700; 16685 State Road 32 Mountain, WE 54149 715-276-4200; Prime. 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. Usually the parting line of the mold is perpendicular to the mold opening direction of the injection molding machine. The melt is forced to split up during filling of the cavity. Plastic is then injected at high pressure into the mold, and it solidifies with any surface features or defects in. Multiple gates will always. Here are some of the most common cosmetic defects and how to avoid or fix them. Corrosion resistance. When the mold body is closed, the contact surface of the cavity and core plates makes a parting line. Fictiv can help you avoid injection mold design and plastic part design for injection molding problems. 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. In injection molding terms, these are called “living hinges” and are made right into the design instead of adding them later. wall thickness injection molding ribs. These lines occurs usually around holes or obstructions and cause locally weak areas in the molded part. Mold flow analysis is a software simulation that can show how resin will fill the mold during the injection molding process. 3. Advantages for your Injection Molding Machine:. 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. 5 to 3. 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. Additional venting along the parting line perimeter will improve overall venting significantly. BACK TAPER – Reverse draft used in mold to prevent molded articles from drawing freely. Knit Lines 11: 1. Kim, who has worked for hot runner system and auxiliary equipment supplier Yudo-Abiman for 12 years, most recently as. Those are knit lines, formed when two opposing flows of material join together in the mold cavity. A parting-line vent. It can also aid in material selection since fill. From the start of your project, you can count on The Toolroom to exceed your expectations for all of your plastic injection molding needs. Contact Universal Plastic Mold Today. A knit line is what happens when plastic flows coming from two different directions meet inside of the mold and solidify. pdf), Text File (. Dabei erreichen unsere Produkte zufriedene Käufer in mehr als 30 Ländern. (2) Undesired movement of certain materials in a plastic. Step 3: The injection mold undergoes rotation to align in position with injection Barrel B. injection molding. These defects are not visible, like small. 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. Self-mating parts. 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. Causes: Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic. 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. He is currently a tooling manager for a large, multi-plant molding and manufacturing company. Main causes of knit lines in injection molding: 1. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and. Injection rate plays a role—faster fill times develop more shear, molecular orientation and residual stress. 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. The Bottom Line You might have come across the aforementioned terminology before and did not know what any of the words meant. 847-952-8088. Hardware components. 015in wall. Encapsulating the metal insert with plastic creates a single. 3. Refers to portion of injection molding part that has some taper to make it easier to remove from the mold. Inadequate venting may cause short-shots, poor surface appearance, or weak weld-lines. These are caused by the shot of molten plastic moving at different speeds throughout the injection. There are several possible causes that need to be investigated to troubleshoot weld lines in injection molding. Material contamination, for example, can cause several molding issues. Troubleshooting Injection Molding Weld Lines. A meld line occurs when two moving melt fronts converge and flow parallel to each other. Knit lines don’t. See UNDERCUT. Thin wall injection molding design tips and tricks. The bottom of your rib must also feature a fillet. Brent Strong, p. Very high quality and an extreme fit may. Knit Lines: These occur when different flow fronts come together as plastic flows through the part cavities. 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. 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. Still, they are almost always a cosmetic issue. 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. 2. extend mold service life. A knit line is any line, visible or not, where two resin flows meet. Knit lines, also known as weld lines are areas in the. Weld Line Phenomenon: Surface defect in which the weld, or the part where the flows of melted plastic meet inside the mold, shows up on the surface as a groove or pattern. Reduce the injection pressure. Here are a few of the most common defects that may occur in plastic injection molding — and how product teams can avoid them. When assessing issues with injection molded parts, we want to make sure some core aspects of molding are checked. 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. Meld and knit lines are actually both different types of weld lines. 3 To get the most out of your Xometry Injection Molding experience, we offer the following part Design Guide with. After selecting resin material, the design and process control of injection mold directly influence the quality of formed products. The example. 4. Heat resistance. Most issues occur before entry into the 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. This continues until the weld lines in plastic injection molding disappear. Weld lines typically appear in a part when an obstruction of some kind separates the melt flow and prevents it from "knitting" together. Prevent Flow and Knit Lines During the Injection Molding Process. And the lead-time for overmolded and insert molded parts is 15 days, a bit longer than FacFox’ single-part injection molding turnaround, but still much faster than traditional insert molding. This course outlines mold filling and how different gating configurations affect filling. Venting is required to allow the air in the sprue, runner and cavity to leave the tool as the melt flows into the cavity. Thick walls are not recommended because of POM’s high shrinkage rate. As the melted plastic fills the mold, there are locations where the molten plastic flow stream separates and then comes back together. With this option, users can utilize in-cavity pressure or temperature sensors to control when the valves open or close. ). This line does not normally impact the overall shape or dimensions of the item. 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. Weld lines, knit lines, and meld lines are terminologies to describe the imperfections of finished parts in plastic injection molding processing. The company provides an array of digital manufacturing expertise. He said, “Vent cavities at weld lines or junction points where two or more material streams meet and fuse into a solid mass. The gate location affects the. Injection molding, producing, and forming resin materials are three core components of injection molding technology. 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). 7584. The parting line will always be visible, but can be minimized by its location, draft allowance and part geometry. ICOMold is a plastic injection molding company that manufactures plastic injection molded parts. This, in turn, leaves a large tab to be trimmed, a process that can mar the finished surface. Venting at Weld Lines. 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. Proper venting is essential to molding a defect-free part. The uneven thickness of the product will make the melt flow resistance larger and smaller, resulting in unstable melt flow. Look for signs of stickiness or burning. As these flow fronts come together. Thermoplastic Elastomer Molding Guidelines. RE: Knit Line Affect On Material Strength lorelei5607 (Automotive). Reduction of sink can possibly be had by switching to an ABS/PC-blended material. The mold-opening force varies widely depending on the type and size of the molding machine. Injection Molding Press. Plastic melt and mold temperature. The quality of the answer is directly proportional to the information we have to work with. These lines. Injection moulding (U. Let’s. Neat polypropylene retains nearly 90% of its strength on a properly designed and molded weld line. Design with Plastics - for Injection Molding process. (called also weld marks or knit lines), and meld lines. 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. This is why the leading edge of the resin flow within a. A knit line in a plastic injection-molded part is created when two separate plastic flows meet within the mold and "resolidify" along their interface. The Difference Between Knit and Meld Lines–and Why It Matters. Reduce. spelling: injection molding) is a process for producing parts by injecting molten material into a mould, or mold. Call 800-985-6750 or email your inquiry to sales@customgasketmfg. Reduce the injection pressure. injection molding. Monoform parts. Mold Venting. Weld or knit lines occur when melt flow fronts collide in a mold cavity. Understanding Knit Lines in Injection Molding: Causes and Solutions. wall thickness, knit lines, draft. Macromol Symp 1988;21/21: 489-500. This production method is particularly popular when manufacturing in large quantities. Learn how material selection, part design, and optimizing process parameters can help eliminate knit lines. Injection molding crush ribs are small, protruding features added to molding designs to support and improve the stability of press-fit connections. These lines occurs usually around holes or obstructions and cause locally weak areas in the molded part. These data were obtained from many andKnit lines in injection molding are common defects of the molded parts. 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. A medical device designer might look up a polymer's physical properties and game out. 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. 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. Facebook. In working with the injection molding service offered by Protolabs, you provide. The leading edge of the molten material is often the coolest point and the closest to solidifying. 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. Called LS IMM Mexico, the subsidiary is based in Monterrey and led by plastics industry veterans Kevin Kim and Fulvio Menconi. Given the cosmetic requirements, there is a good chance the mold would require. 008 in. To account for variations in viscosity, nylon injection molding machines need to maintain an appropriate injection pressure. The thermo-rheological findings were used to investigate the sources of weld line weakness. It’s called sequential valve gating, or SVG. 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 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. Understanding Knit Lines in Injection Molding: Causes and Solutions. Even parts with metal inserts can be produced. Weld lines may appear like seams or scratches, but that is not what they are. weld lines may form during the injection molding process and work closely with the plastic injection molder and tool builder to choose the best possible gate location.