Thin wall injection molding can produce walls as thin as 0. A weld line can also be called a knit line. Core out parts to eliminate thick walls. Contact Universal Plastic Mold Today. These lines occur usually around holes or obstructions and cause locally weak areas in the molded part. 030 and 0. 5. Gates, vents, sliding cams, pick-out inserts, and any other place where the two sides of. The knit line is generally opposite the gate in thermoset. 4. Inappropriate temperature. Learn how to troubleshoot weld lines in injection molding. We wanted to independently assess how a mold would fill and where knit lines would be, instead ofKnit Lines. Because they can be shut off independently you can control flow fronts and knit-line locations. See UNDERCUT. to prevent improperly placed knit lines or sinking in the plastic downstream from the hinge. Edge gates, which can include fan and tab-shaped gates, are the most common ways to gate a tool. Molding parts with multiple gates will almost certainly produce a weld line at right-angles to the line joining the two gates which could cause the weld lines to be exceptionally weak (see Figure 1. Starting in September, the CoPilot process control system developed by RJG will come equipped with valve gate control. Direct experience designing molds and parts for injection molding and producing the molds and parts is valuable, relevant experience for design students. The knit lines are the result of poor molding conditions (likely molding at too low of a temperature). When working with customers, our design guidelines for living hinges are 0. Steps involved in Multiple Material Molding. Contact Universal Plastic Mold Today. What Are Knit Lines in Injection Molding, and How Do You Prevent Them? - RJG, Inc. Typically, the wall thickness is somewhere in the neighborhood of 50%-75% of your nominal wall thickness. Draft. Inadequate venting may cause short-shots, poor surface appearance, or weak weld-lines. Knit lines are consideredmolding defects, and occur when the mold or/and materialtemperaturesare set too low:. sheet metal fabrication, rapid tooling, and plastic injection molding. 250 in. Lower the melt and mold temperature to prevent overheating. 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. 025 inches per inch, depending on the particular resin, molding conditions, part design, part wall thickness, and direction of flow. 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). Injection Molding: How to Set Second. Injection molding crush ribs are small, protruding features added to molding designs to support and improve the stability of press-fit connections. Release of trapped air in those cases will. Gate locations can be directly behind or on the edge of the graphic. The injection molding process uses a granular plastic that is grav ity fed from a hopper. 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. Three terms that often get mixed up in the plastic injection molding industry are weld, meld, and knit lines. Thermoplastic Elastomer Molding Guidelines. 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. Discover why you should avoid knit lines when designing and manufacturing injection moulded parts. These flaws can affect the aesthetic and structural integrity of the. Starting with small part size, students learned to think about detailing on a product at thisTable 2. 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. The linear mold shrinkage of injection molded parts will vary from 0. The process is extremely versatile, and can produce very complex shaped parts, with the use of multi-sided molds. It can also aid in material selection since fill. Image credit: The Rodon Group. and Titomanlio, G. If life safety is involved, or reliably or efficacy are absolutely required, every part should. Increasing injection speed. 5 kN/cm2 (4 T/in2 ) is required. Weld lines defects are formed when two flow fronts are merged together. MINNESOTA. Figure 1 presents the shrinkage of polypropylene versus wall thickness in injection molded parts. Our goal of 100% efficiency, zero scrap and repeatable processes can sometimes be hindered by poor setups or changes in the molding process. 6. 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. The packing pins are similar in construction to mold ejector pins and may also be mold ejector pins depending on mold configuration. Thermoplastic Elastomer Molding Guidelines. Typical Tool Steel for Injection Mold Construction Steel Type Steel Properties Mold Component P-20 Pre-hardened, machines well, high carbon, general-purpose steel. 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. This design tip offers several ways to make your next multipart design less like a root canal and more like a day on the beach. Read on to find out more! 800. Resin pellets are loaded into a barrel where. Depending on the resin, resin temperature, mold temperature, and filling. He said, “Vent cavities at weld lines or junction points where two or more material streams meet and fuse into a solid mass. This type of molding uses a single tool to create both the part's geometry and surface finish in one step. As. Runner Type. These include: Symmetrical parts. A. This article will discuss the importance of injection molding nozzles and what makes a good nozzle for a molding or injection. This summary represents a key subset of the detailed molding information found in the remainder of this molding guide . Nicolet Plastics LLC. Randy Kerkstra has been in the plastics industry for more than 26 years, occupied frequently with troubleshooting injection molding. 2. Check the diameter of the ejector pins and the pin hole tolerances, if the tolerance is too wide this could allow. The knit line itself (the vertical line in the middle) is visible in this lighting, but the biggest issue is with the texture. For over four decades, our experienced team and leading edge technology have provided you with tooling solutions from start to finish. Table 2 lists the<br /> properties of common tool steels and the typical mold components for which they are<br /> used. 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. Weld lines create visible marks, but this type of injection molding defect can also reduce a part’s strength and durability. In spite of its cosmetic attributes, a bad knit line can significantly. Injection molding machines with footprints similar to desktop FDM and SLA 3D printers are now available. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and. Know about the cause of these lines and the potential solutions. Knit lines don’t. Vacuum on the mold does help and many times eliminates the weld line. Molding is a manufacturing process in which resin in a barrel is heated to a molten state,. Wavy lines are another feature of weld lines. Here are some of the most common cosmetic defects and how to avoid or fix them. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. Table 2. Temperature is another factor that can cause weld or knit lines in injection molding. The parting lines is where the two injection mold halves (A side and B side) meet. First, they carry resin to the mold via a runner. If debossed pattern is indispensable, it should have a depth of no less than 0. RJG announced that its injection molding process control system, CoPilot, will begin offering valve gate control technology for hot runners starting in September 2021. 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. The depth of the texture or letters is somewhat. Understand the causes of knit lines in injection molding and explore effective solutions to minimize their impact. As melt temperature, viscosity and abrasiveness rise, tool life will decline. BACKING PLATE – In mold construction, a plate used as a support for the cavity blocks, guide pins, bushings, etc. Nicolet Plastics LLC. LS Mtron Injection Molding Machine (IMM) has opened a new business unit in Mexico. Depending on the design of the mold and the material being injected, a knit line may: present no problem at all, be a. 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. Short shots and knit lines are two phenomena that are caused by using highly viscousTherefore, changing the edges and corners of the mold core to make it buffer transition and keep the melt flow stable can prevent the occurrence of wavy lines. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. Gates, vents, sliding cams, pick-out inserts, and any other place where the two sides of. Let the molten material cool to a solid-state. injection molding machines using standard industry practices . In this paper, a general and efficient approach was proposed for dual-scale infiltration flow balancing based on numerical simulation. Thus, as the flow continues, the two flow fronts may gradually rejoin to make one uninterrupted flow front. Ham Lake, MN 55304. Neat polypropylene retains nearly 90% of its strength on a properly designed and molded weld line. 01 inches between parts fitted together to work best. Core Geometry. Our glossary of terms provides definitions for a wide range of injection molding terms. ITAR. The Bottom Line You might have come across the aforementioned terminology before and did not know what any of the words meant. After samples have been approved, it’s time to kickoff. Injection rate plays a role—faster fill times develop more shear, molecular orientation and residual stress. Change gate locations. 847-952-8088. So what does the 140F mold temperature give us? The molding technician thought the 140F mold water was OK because the data sheet provided by the material supplier suggested mold temperatures from 75 to 180F. These defects are not visible, like small. 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. Use the fewest number of gates possible. Weld lines are a common imperfection to be found in most plastic injected molded parts. Figure 1 presents the shrinkage of polypropylene versus wall thickness in injection molded parts. Without it, air and gas are trapped in the mold, which compress and heat as the cavity fills. Weld / knit plane forms as flow fronts recombine Weld line and failure due to flow around core. • Estimated clamp tonnage of 5. Injection molding is a popular manufacturing process used to fabricate parts in large volumes. ” Depending on the material you use, the knit lines could be invisible or obvious—work with an injection molding company to figure out. 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. They need the interference of about 0. In a weld line, reinforcement fibers. The last cause of flow lines may be the operators running your daily operations. Trapped air in a mold manifests itself as a burn mark on the molded part. When injection molding a part, a knit line may present no problem, or can cause a serious structural problem. The packing pins are sequentially. . More about the knit lines in injection molding. Injection speed and pressure. These knit-lines are well-known as particularly critical regions in the mold when mechanically loaded. 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. Two major moldability pitfalls to avoid are sink and voids. The injection molding process uses a granular plastic that is gravity fed from a hopper. Cooling Lines, Injection Mold Material has a great impact on injection molded part quality. The metal parts, made of steel grade. In the 1947 book, Injection Molding of Plastics, the late Dr. 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. Hardware components. TEXAS. Knit Mark – A witness mark on a molded part, often occurring at the midpoint between two transfer or injection sprue locations. These knit lines are formed as molten plastic in the injection mold flows around an obstruction like a feature. When needed, crush ribs can allow for draft and a tight fit for a part that requires both. It allows our molders to monitor fill pressure, cosmetic concerns, and the basic quality of the parts. 3D printed molds in the injection molding process to lower costs and lead time. This most often occurs when you have a core or shutoff feature that the plastic needs to flow around. When injection molding a part, a knit. End-of-arm tooling can make or break a program. Injection molding is a popular manufacturing process used to fabricate parts in large volumes. A screw-type plunger forces the material into a heated chamber, called a barrel, where it is melted. When venting stationary inserts or core pins in rib or part details with venting issues, I recommend no more than 0. The thermo-rheological findings were used to investigate the sources of weld line weakness. Specifically, Stokes and. CNC-Bearbeitung; Spritzgusswerkzeuge und -teile; Stereolithographie (SLA) Selektives. By altering the part design, it may be possible to just have one flow front, and avoid multiple flow fronts having to come together. Injection molding is the most cost-competitive technology for manufacturing high volumes of identical plastic parts. , 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. Submarine Gates. 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. 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. Because this allows some cooling and thickening of the resin, tab gates require a relatively large opening into the mold cavity. mystagingwebsite. 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. PTI has five injection molding presses suitable for MIM. The resin is transferred to the mold cavity under positive. By contrast, a slow injection speed will give you a matte finish and a cold mold, such as 50F, will give you a more flexible part. 012 in. Double push filling method. 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. 020"). 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. Weld / Knit Lines; Assumptions. Not always a problem, but can cause serious structural issues. Weld Lines-3 Ways for Detecting and Correcting. Causes: Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic. Rapid heat-and-cool molding technologies have attracted increasing. Not always a problem, but can cause serious structural 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. 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. PTI, a plastics and metal parts molder, is a leading developer of the MIM process. Enlarge gas vents and gates to allow trapped air to escape the mold. Three terms that often get mixed up are weld, meld, and knit lines. Meld and knit lines are actually both different types of weld lines. Increase the injection speed. This performance can ultimately impact many part properties, including surface quality, knit line strength, part ejection force (sticking), dimensions, tolerances. Injection Molding. 2. They are the result of fundamental physical effects taking place inside of the mold. This slight depression is a tiny line called the weld line or knit line. Users will be able to use in-cavity pressure or temperature sensors to control when the valves open or close. occupied frequently with troubleshooting injection molding. Imagine a river splitting to go around a rock, then meeting up again on the other side. Mold flow analysis is a software simulation that can show how resin will fill the mold during the injection molding process. With this option, users can utilize in-cavity pressure or temperature sensors to control when the valves open or close. Facebook. Observation on a “trial and error” basis may be required. Knit lines may also be called stitch lines, weld lines, or meld lines. The company provides an array of digital manufacturing expertise. Pitfall #1 – Knit Lines. Throughout the molding process the press operator visually inspects the molded parts as they come out of the mold for any imperfections such as air trap, knit lines, backrinding, or tearing. Obstructions in the part create weld lines or knit lines during injection molding. This production method is particularly popular when manufacturing in large quantities. These seams are known as “knit lines. In a weld line, reinforcement fibers. AS9100:2016. Dabei erreichen unsere Produkte zufriedene Käufer in mehr als 30 Ländern. Reduce. Guideline for Design, Material Selection and General Application of Encapsulation of Electronic Circuit Assembly by Low Pressure Molding with ThermoplasticsThis was usually with a PC/ABS material at knit lines and high fill pressures from thin walls in the part. ”. The relationship between processing conditions, morphological structure and mechanical properties of knit lines of an injection molded semi-crystalline thermoplastic. Plastic is then injected at high pressure into the mold, and it solidifies with any surface features or defects in. Knit line Knit line Start of mold filling. Inject the molten material into a mold. The recommended minimum production volume for injection molding is 500 units. More about the knit lines in injection molding. 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. 080" – 0. Effect of Injection Moulding Conditions on Knit-line Formation, Plastics Rubber Processing and. Look for signs of stickiness or burning. Weld/Meld/Knit lines are all terms describing the effect of two or more flow fronts of material joining or meeting together in the part. 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. PAPER ABSTRACT: Desktop injection molding machines and stereolithographic print ed molds make hands-on injection molding production possible in design studio classrooms and labs. As. Swirling can increase depending on the material you use, and the texture or polish of injection molding tools. 0 mm) is crucial to ensuring that parts cool evenly. In plastic part design for injection molding, it is important to design the injection mold so that the molten plastic fills the mold cavity entirely, cools as evenly and as quickly as possible without warping, and functions as intended. N174W21430 Alcan Doctor Jackson, WI 53037 262-677-9700; 16685 State Road 32 Mountain, WE 54149 715-276-4200; Prime. 7. Knit Lines: Holes in the part will always cause knit lines. Venting is required to allow the air in the sprue, runner and cavity to leave the tool as the melt flows into the cavity. Focus - Injection Molding - Free download as PDF File (. In addition, keep these in mind: Scrap rates can range from less than 1 to 5%. A medical device designer might look up a polymer's physical properties and game out. Venting at Weld Lines. Plating and painting applications usually require slower fill rates to keep orientation low. In injection molding terms, these are called “living hinges” and are made right into the design instead of adding them later. 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. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. Let’s. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. It has low shrink and high dimensional stability, and it has good resistance to acids and bases. In this, work, metal inserts were joined with polyamide 6 by using the injection-molding technique. RELATED CONTENT. They are formed when two or more flow frontKnit Line Injection Molding is a process that allows manufacturers to produce parts more quickly, accurately, and at less cost than traditional injection. Using the design cube, learn how knit lines are formed on plastic parts and tips to avoid knit lines in your plastic injection molding design. TRADITIONAL MACHINES VENTS. Mold venting is critical to the quality and consistency of the finished part. 1. Back to News Page. It happens when the molten plastic material flows around obstacles, such as inserts or cores, and rejoins on the other side. Design with Plastics - for Injection Molding process. txt) or view presentation slides online. The mold opens and closes on a vertical axis in the vertical injection molding. Depends how hard you want to push it. It’s also relatively inexpensive. If the plastic has cooled too. 1. Which affect parts aesthetics and decrease parts strength. One of the most common ways of adding venting is by manually machining it into the mold (Figs. Learn how to troubleshoot weld lines in injection molding. A meld line occurs when two moving melt fronts converge and flow parallel to each other. By contrast, a slow injection speed will give you a matte finish and a cold mold, such as 50F, will give you a more flexible part. This makes the casting process as efficient and accurate as possible. Knit lines appear to be a faint line or a crack in the finished part after it comes out of the injection mold. Thick walls are not recommended because of POM’s high shrinkage rate. Plastic Injection Molding Part Design Guidelines, Plastic part Design Guidance, Parting Line Draft Undercuts Wall Thickness & Uniformity Bosses Gussets and Ribs Radii and Corners Material Shrinkage Gating. Here are some ways manufacturers can avoid quality issues during the plastic injection molding process: Inadequate Injection/Holding Pressure: If injection and holding. Learn how material selection, part design, and optimizing process parameters can help eliminate knit lines. S. 5 mm) is ideal, according to injection molding engineer Jack Rulander of Protolabs in Maple Plain, MN. Computer simulation packages of injection molding have had success in predicting filling behavior in extremely complicated geometries, capable of predicting weld lines’ location exactly. In addition, the force of the gas holds the appearance side of the part in contact with. Vertical Plastic Injection Molding. They need the interference of about 0. 25T-0. Long shapes are produced without multiple drops or hot runner systems, eliminating knit lines and lowering tooling costs. IM: hot or warm knit lines are cause by _____ while cold knit lines are caused by _____. Specific attention to processing details will enhance quality and productivity . So what does the 140F mold temperature give us? The molding technician thought the 140F mold water was OK because the data sheet provided by the material supplier suggested mold temperatures from 75 to 180F. BACKING PLATE – In mold construction, a plate used as a support for the cavity blocks, guide pins, bushings, etc. 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). Knit lines; Knit lines always occur once 2 melted materials meet with hair-like discolorations. Knit lines form when two or more plastic flow fronts collide and solidify or “knit” during the molding process. They occur when melt flow fronts collide in a mold cavity. 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. 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. Gating. Knit lines, holes, part configuration and the talent of your molder all come into play. From the start of your project, you can count on The Toolroom to exceed your expectations for all of your plastic injection molding needs. If you want to better understand the process of injection molding, there are a few critical terms you need to know. Redesign the mold: Redesigning the mold to. A meld line occurs when two moving melt fronts converge and flow parallel to each other. placed usually in the areas last to fill, near knit lines, as well as on the runner system. be impressed by improvements of bubbles (vacuum voids), short shots, unequal thickness, jetting marks, weld lines, knit lines, black streaks, strain 2. Bad habits can cause many different problems with injection molding machines, and flow lines are included in that. The utility model relates to a mold structure capable of eliminating the appearance of weld lines of a hole-shaped injection molded product. • 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. These data were obtained from many andKnit lines in injection molding are common defects of the molded parts. However, the weld line may not continue throughout the flow. 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. Reduce the temperature of the mold and plastic being injected. Injection molded plastic parts have comparatively less strength in the weld line section and they also have an impact on product aesthetics. H-13 Good general purpose tool steel. The Goldilocks principle applied to wall thickness. Additionally. [1]Weld Lines. and 0. During the injection molding process, molten plastic is injected into a mold cavity through a gate. 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. Sequential valve gate technology. Causes: Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic. When the plastic cools, it takes the shape of the mold and is then ejected and finished. - 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. Product designers encounter a wide range of challenges when creating and developing molded parts, such as stress factors, warp, and those pesky knit lines. 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. Understand the causes of knit lines in injection molding and explore effective solutions to minimize their impact. Soft metals, such as aluminum and beryllium copper, can be used for prototype<br />. ). The difference between a knit and meld line is significant, dramatically impacting the part’s structural integrity. 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. 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. Holes in the part will always cause knit lines. A plastic product formed by the injection mold is divided into two parts, and the line separating the product is called the parting line. For best results, keep thickness variations to within 15% of the nominal wall thickness. 11) The part design must take into account possible knit line locations and work to direct them away from higher stress areas. Short molding cycle, high production efficiency, and easy automation 2. Hartland, WI 53029. These are imperfections in the part where separated flows of cooling material meet and. 5 : 1 and 3. 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. Swirling can increase depending on the material you use, and the texture or polish of injection molding tools. sinks, knit lines, and shrinkage,” Spalding says. Knit lines are consideredmolding defects, and occur when the mold or/and materialtemperaturesare set too low:. 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. Injection molding can be a real challenge if you don’t follow the “Five M’s”: Man, Mold, Machine, Material and Method. They again get resolidified along the surface. ISSUE FACED BY THE INJECTION MOLDING COMPANIES: FLOWLINES (ALSO CALLED WELD LINES OR KNIT LINES) Definition: Streaks, orange skin, or visible flow lines in parts. 1 Gate Location. Looking to Your Knit Lines with Molding. Randy Kerkstra has been in the plastics industry for over 26 years, occupied frequently with. 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. Kim, who has worked for hot runner system and auxiliary equipment supplier Yudo-Abiman for 12 years, most recently as. The leading edge of the molten material is often the coolest point and the closest to solidifying. 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. This can dramatically reduce the effects of material changes, according to the injection molding technology company. 008 to 0. Understanding Knit Lines in Injection Molding: Causes and Solutions. extend mold service life.