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. A weld or meld line is a weakness or visible. This article will discuss the importance of injection molding nozzles and what makes a good nozzle for a molding or injection. Support pillar suppliers can help determine the. 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. Gas-assisted Injection Molding: Influence of Processing Conditions and Material Properties. Meld and knit lines are actually both different types of weld lines. The fluid path has an inlet and a radially continuous, downwardly. 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-line is a defect that arises in injection molding when two or more flow fronts collide. Weld lines create visible marks, but this type of injection molding defect can also reduce a part’s strength and durability. Submarine gates are another injection molding types of gate that is located below the surface of the part, typically on the underside or back of the part. (1. 060 and 0. Depending on the design of the mold and the material being injected, a knit line may: present no problem at all,. 5-3. Two major sources of weld lines are multiple gating and flow around pins. injection-molding machine (Arburg allrounder 320C, Germany). 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. Gating. 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. land and using maximum vent depths to get them as close to self-cleaning as possible. 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. Multiple parts with complex design and differing wall. Weld line. Piccarolo, S. matte or smooth texture on the part’s surface gives a more desirable aesthetic and hides small imperfections such as knit lines, flow lines, shadow marks, and sink marks that were caused by the mold. Don’t forget to eliminate entrapped air in the weld line injection molding; otherwise, it’ll create unfilled portions in your part design, causing more weakening. Weld line defects – also known as knit lines – is a defect that occurs when the injected plastic meets within a mold. Learn how to troubleshoot weld lines in injection molding. Venting. An injection mold that produces pieces while the final tool is being built. weak knit lines and fracture points. While this process is largely efficient and reliable, it's not without its potential issues. 197 in. 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. Weld lines also can be visible after a part is painted and can cause wrinkles on in-mold-label injection-molded products. Knit Lines and Resins A knit line is any line, visible or not, where two resin flows meet (see Figure 2). Injection molding is the predominant manufacturing process used for producing transparent polymer products. Rapid heat-and-cool molding technologies have attracted increasing. plastic recycling machine, blown film extruder, filament extrusion line, injection molding machine, blow molding machine, injection blow molding machine, bag making machine, flexo. Moldflow is computer software that is widely used across the plastic injection molding industry to predict and optimize the injection molding process and assist with mold design. opposed to two or more gates. In this paper, two different injection molding tests were completed. But we can’t talk about the injection molding process without discussing the injection molding machine. As the melted plastic fills the mold, there are locations where the molten plastic flow stream separates and then comes back together. Gates, vents, sliding cams, pick-out inserts, and any other place where the two sides of. 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. Rynite® PET is usually processed with melt temperatures between 280 to 285 °C (535 to 545 °F) . Moisture in the material due to insufficient drying can form at the melt front, forming an interface between the two melt fronts. Bad habits can cause many different problems with injection molding machines, and flow lines are included in that. For fiber-glass reinforced thermoplastics, the strength in these local areas (knit/weld and meld lines) is less than in bulk areas, dueMolding 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 adhesiveThe best option once you have the approximate tonnage required is to find a molding partner that has machines in that range. 2) Vents in an injection mold should let the air out, but not the liquid plastic. 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. The point I am making here is, if you’re injecting 14. occupied frequently with troubleshooting injection molding. Notched weld lines then remain near the cavity walls. Mold Venting. 7. Encapsulating the metal insert with plastic creates a single molded plastic piece that’s generally stronger than one created using secondary assembly. Depends how hard you want to push it. 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 Ventings Knit/Weld Lines. Different plastics make different types of knit lines. This defect is caused when the polymer resin is injected into a mold and flows around a solid piece and forms when the resin collides back together without completely fusing back together. Weld 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. The main causes of Weld Line or Knit lines are: Convergence of two flowing polymer melt streams during injection molding process: This is the most common cause of weld lines, as the two melt streams coming from different runners or gates in the mold meet and do not fully mix, creating a weak point in the finished product. The process involves feeding the material into a press, where it is cooled or heated as per. They are the result of fundamental physical effects taking place inside of the mold. Misra A. Mold cores can be machined from a solid piece of copper alloy or inserted to aid in construction or allow for easier replacement if a com-ponent would ever be damaged in molding. The mold settings can run flawlessly until employees consistently open the machine for inspections, interrupt cycle times, or cause. Use gradients. Weld lines, also known as knit lines or meld lines, occur when two or more flow fronts meet during the plastic injection molding process. Mold venting should be checked and increased if necessary, injection molding service. Understanding Knit Lines in Injection Molding: Causes and Solutions. [1] Here are a few ways to prevent knit lines in your plastic parts: Increase material temperature: Higher temperatures on both materials prevent one section from cooling too quickly. An injection molding machine having a first mold portion and a second mold portion. A meld line occurs when two moving melt fronts converge and flow parallel to each other. This makes the casting process as efficient and accurate as possible. There is high defect rate (surface defects/knit line). Orientation of these knit (weld) lines, are perpendicular to the tensile stresses, applied to the ring (Figure 6-8). Part Shorts and has burn marks. 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. Also known as “stitch lines” or “weld lines”. Reduce the screw speed and adjust the backpressure. 3 /sec. Three terms that often get mixed up in injection molding are weld, meld and knit lines. Draft. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. Sink Marks Cause 2: Low Pack and Hold time. Last month, in the first part of this two-part installment we focused on the reasons viscosity can change during injection molding. Knit lines in injection molding are common defects of the molded parts. Knit Line – A molding defect resembling a flow-line found at the point where the rubber meets between the injections. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. Knit / Weld Lines. Flow Lines. In fact, strength at the knit line can be as little as 20 percent of the nominal strength of the part. Let’s stop for a second and think about the logos we usually see on a daily basis. Undercuts add cost to tooling and molding, which is recommended to eliminate or minimize as possible. Conversely, texturing of part surfaces is often effective in hiding flaws and imparting a “grippy” or leather-like. Shear Controlled Orientation Injection Molding of Polymeric Composites with Enhanced Properties. Remove the hardened material from the mold. In this case it would be 0. They again get resolidified along the surface. Specifically, by having a single, continuous injection outlet 140, molding material can be uniformly distributed throughout mold cavity 120 quickly and consistently without resulting in undesirable molding seams or knit lines. Runner Type. In this video, Chris Burda, applications engineer at Protolabs, shares what can be done to reduce knit lines and how they can impact your part's functionality. It has low shrink and high dimensional stability, and it has good resistance to acids and bases. A meld line occurs when two moving melt fronts converge and flow parallel to each other. Plastic injection molding has long been an affordable and efficient process for producing custom prototypes and end-use production parts. By altering the part design, it may be possible to just have one flow front, and avoid multiple flow fronts having to come together. 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. Weld lines are a common imperfection to be found in most plastic injected molded parts. That’s the air escaping out of the nozzle until the hose is full of pressurized water. Let’s review what causes meld and knit line imperfections, how they’re different, and how they affect part durability. Three terms that often get mixed up are weld, meld, and knit lines. Expand. Meld and knit lines are actually both different types of weld lines. Obstructions in the part create weld lines or knit lines during injection molding. Expand. Direct experience designing molds and parts for injection molding and producing the molds and parts is valuable, relevant experience for design students. How weld lines form. Because edge gates are easy to machine, they can be widened or modified to help material flow. They again get resolidified along the surface. , LTD. When assessing issues with injection molded parts, we want to make sure some core aspects of molding are checked. The runner and gate system used in plastic injection molding is responsible for directing molten plastic from the nozzle of the molding machine into the cavity of the tool. Most failures of injection molded plastic parts are caused by either improper design of the part or improper design of the mold. (Overpacking. Compression molding produces solid parts that are free of flow and knit lines. Therefore various injection molding defects can be eliminated. Meld and knit lines are actually both different types of weld lines. To avoid these defects, manufacturers can check molds for the following: Low Mold Temperature: A mold temperature that’s too low will cause the material temperature to drop fast. Weld lines, knit lines, and meld lines are terminologies to describe the imperfections of finished parts in plastic injection molding processing. Islyn Thomas summarized the need for a particular vent location better than I ever could. These knit lines affect the parts aesthetics, but also they generally decrease the strength. 3,000 ton machines can cost $700,000. These lines can manifest as visible marks on the component’s surface and potentially affect its structural integrity. LOCATION: Along any parting line. Weak weld lines are perhaps the most common and difficult injection molding defect to eliminate. The most common location of knit line formation is behind molded-in holes that are made by core pins in the mold. Typically, injection molded parts will be between 0. What Causes Knit and Meld Lines? Like many part inconsistencies, these unwanted features are rooted in product design. Holes in plastic parts are normally cut with a router (or drilled if round. 45 in. Insufficient injection pressure and speed can cause weld lines. Effect of Injection Moulding Conditions on Knit-line Formation, Plastics Rubber Processing and. The data suggest that either local sample thickening or grooves form at knit line when complete section solidification takes place first at weld zone or. In fact, knit line strength will decrease with higher filler content and with longer fibers. Macromol Symp 1988;21/21: 489-500. Avoid areas where the plastic must suddenly flow upward or downward. Fig. Release of trapped air in those cases will. Multiple gates will always cause knit lines. Weld or knit lines are perhaps the most common and difficult injection molding defect to eliminate. Such molds or tools aren’t intended to be used in operation. The mold’s sprue bushing and locating ring is placed in a way that makes sure the injection molding nozzle is properly aligned and centered. Fewer knit lines. Many materials can be. As. 2. 080" – 0. 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 is a video excerpt from our Problems & Solutions Course - Weldlines. (knit) line and produces a latent, or obvious, part defect. 5. Weld lines or knit lines and meld lines formed when two melt fronts run into each other or join together can be improved by. What Are Knit Lines? When a product is produced through injection molding, a high-powered tool injects melted plastic into a custom, double-sided mold. sinks, knit lines, and shrinkage,” Spalding says. Knit lines, also called parting lines, are any lines where two resin flows meet and may or may not be visible. Swirling can increase depending on the material you use, and the texture or polish of injection molding tools. Knit lines, also known as weld lines, are fine lines that resemble hairline fractures or cracks. The specific method is that when the product is just filled and filled in the cavity, the insert. These are imperfections in the part where separated flows of cooling material meet and. ) Mold Analysis Tools. excessive or premature wear at the parting line. Material contamination, for example, can cause several molding issues. The defect may be in part appearance or mechanical strength. The three dimensional structure is formed of a resin fiber mixture having a resin material infused with carbon fibers having a length of about 12. Weld lines defects are formed when two flow fronts are merged together. The company provides an array of digital manufacturing expertise. Product designers encounter a wide range of challenges when. The thickness of the part plays an important role in how difficult the molding process will be. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. How to Eliminate Knit Lines In Injection Molding - Xcentric Mold Want to learn how to eliminate knit lines that form during the injection molding process? Then. Causes 1. Weld line. Plastic injection molding is the industry’s most common way to manufacture parts. Knit Lines. Reducing the dimensions of the runner system. TWIM=Thin Wall Injection Molding; TSL=Toys,Sports and Leisure; NB=No Break; IMR=Inter Material Replacement. 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 adhesivePrevents flow and knit lines. Let the molten material cool to a solid-state. He said, “Vent cavities at weld lines or junction points where two or more material streams meet and fuse into a solid mass. To account for variations in viscosity, nylon injection molding machines need to maintain an appropriate injection pressure. When injection molding a part, a knit line may present no problem, or can cause a serious structural problem. In layman’s terms, injection molding boils down to four simple steps: Melt material. A screw-type plunger forces the material into a heated chambe r,. Knit lines and other aesthetic characteristics of the part are strictly related to the plastic’s temperature during molding, so efficient filling time is essential for achieving proper welding lines and quality parts. Weld-line strength for plastics can vary from 20% to nearly 100% of the strength of the plastic itself. lines are formed when there is an obstacle in the cavity of the injection mold. This production method is particularly popular when manufacturing in large quantities. We then. injection molding. The cracking was always located in the weld/knit line and therefore caused by weakness in the bonding of the plastic at that line. It is also where a slide mechanism might meet where it shuts off on one of the mold halves. Design bosses with an outer diameter equal 2x the nominal diameter of the screw or insert. Using restrictors helps push visible knit lines into less conspicuous areas. Meld and knit lines are actually both different types of weld lines. Plastic injection molding is a process where a thermoplastic resin is heated to its melting point and injected into the cavity of an injection mold. Each mold produces as specific part. Example of a gate location in a finished part Example of knit lines in a finished part. plastic recycling machine, blown film extruder, filament extrusion line, injection molding machine, blow molding machine, injection blow molding machine, bag making machine, flexo. 4. Once the mold is created and the machine is set up, additional parts can be manufactured very fast and at a very low cost. More about the knit lines in injection molding. 27-ounce shot size to 600-ton with a 105-ounce shot size. The Difference Between Knit and Meld Lines–and Why It Matters. We originally thought it may be caused by a silicone release agent that was occasionally sprayed on the injection mold, but testing with minimal agent and later built-in slip agent showed no change. Inadequate venting. By using this method, theHowever, injection molding is not without its challenges. Most failures occur at what are called “weld lines” or “knit lines”. But add 30% glass and weld-line strength retention drops to only 34% as high as the compound with no weld line. 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. There are two primary reasons for this occurrence: 1. 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). 120 in. The injection molding machine includes a fluid path extending through the first mold portion. 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. Very high quality and an extreme fit may. First of all, ABS is prone to knit lines — or the places where two flows of plastic resin meet during the manufacturing process. Surface Finish: The injection molding process may result in visible knit lines or parting lines on the part’s surface. Plastic injection molding is a complex and highly intricate process. Mold Flow Analysis computer. In the 1947 book, Injection Molding of Plastics, the late Dr. As melt temperature, viscosity and abrasiveness rise, tool life will decline. Increasing injection speed. It can also aid in material selection since fill. Flow Lines. Neat polypropylene retains nearly 90% of its strength on a properly designed and molded weld line. Gas trapping/ poor venting. Potential Knit Lines • Knit lines occur where to or mold melt fronts come together-Holes, slots, multiple gates, variations in wall thickness-Meld lines occur when the melt comes together in the same flow direction • Knit lines are areas of weakness and should be located in low stress areas . Sequential valve gate technology. A thermoset injection molding process may save as much as 50% of machine time per shot with a faster cycle time compared to a thermoset compression molding process, allowing molders to produce high quantities of thermoset components per day in multi-cavity tools. Much of this research has focussed on the materials which. Add a 0. Injection moulding (U. Weld Lines. Improve venting and/or relocate to burned area. "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. Parts molded with ABS are susceptible to forming knit lines, which are visible lines on the surface of the part. Based on the resin. incomplete filling or shorts. Weld line in injection molding is line where two or more flow molten plastics knit together. Also known as knit lines. A good injection mold design will take all of these factors into consideration, ensuring that the final product looks great. There are times when porous materials like Porcerax are used in problem or backfill areas. Reduction of sink can possibly be had by switching to an ABS/PC-blended material. They appear around holes or seams of an injection molded part, and some may call them. Our goal of 100% efficiency, zero scrap and repeatable processes can sometimes be hindered by poor setups or changes in the molding process. 0 to 3. 3 /sec. When venting stationary inserts or core pins in rib or part details with venting issues, I recommend no more than 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). land and using maximum vent depths to get them as close to self-cleaning as possible. Near-Net-Shape Molding enables us to eliminate knit lines, add undercuts, increase options for geometry and tolerances, and reduce tooling cost for low. Meld and knit lines are actually both different types of weld lines. Increase the melt and/or mold temperature. Edge Gates. For example, warping, sink marks, drag marks, and knit lines can all impact the visual appeal of the manufactured part. The thermo-rheological findings were used to investigate the sources of weld line weakness. 3 To get the most out of your Xometry Injection Molding experience, we offer the following part Design Guide with. As a result, molten plastic takes the injection mold core and cavity shape. Get a free des. 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. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. This slight depression is a tiny line called the weld line or knit line. Also, since this part will likely be painted or chrome plated, knit lines are a concern. Depending on the resin, resin temperature, mold temperature, and filling speed, knit lines can vary from virtually invisible to something that looks like cracks in the plastic. Our glossary of terms provides definitions for a wide range of injection molding terms. ) 2. After samples have been approved, it’s time to kickoff. The Injection Mold Core A mold core is any member that forms the interior of a plastic part, usually on the "B" side of the mold parting line. Flow lines are off-color lines, streaks, and other patterns that appear on the surface of a part. Material contamination, for example, can cause several molding issues. The plastics used by injection molding processes are relatively cheap and can be used to achieve a wide variety of properties, so injection molding is popular for creating many packaging and consumer products, like LEGO!. More about the knit lines in injection molding. Contact Us. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. The quality of the weld line can be improved through optimized processing conditions, increased mold and melt temperatures, and increased injection speed toInjection molding, producing, and forming resin materials are three core components of injection molding technology. Three terms that often get mixed up are weld, meld, and knit lines. Take these steps to avoid knit lines. Firstly, molten thermoplastic with high pressure injects into an injection mold during injection molding. Quotes for 25+ Manufacturing Processes. RELATED CONTENT. Injection Molding: How to Set Second. Venting. The weld line can either be a minor cosmetic issue. a weld or knit line may develop on the opposite side of the obstruction as the two fronts of material join. 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. Influence of injection-molding parameters on the morphology and mechanical properties of glass fiber-reinforced polypropylene composite. 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. Poor venting. 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. Learn more. Weld or knit lines occur when melt flow fronts collide in a mold cavity. 2. Injection moulding (U. Increase the injection speed. Be wary of saving too much weight or you might end up with: cracking. Strength at the knit line can be as little as 20% of the. 3. I am working on a thin PC/ASA injection moulded component which has a tendency to snap at the Knit-point (the final point of travel at which the…Here are a few of the most common defects that may occur in plastic injection molding — and how product teams can avoid them. 5 mm) is ideal, according to injection molding engineer Jack Rulander of Protolabs in Maple Plain, MN. Alter the mold design. Soft metals, such as aluminum and beryllium copper, can be used for prototype<br />. A poor knit line can cause only cosmetic blemishes or it can significantly weaken the structural integrity of a part. While not always visible, weld lines (aka knit lines) are where two resin flows meet. (1. This occurs wherever there is a hole, notch, or any other feature on a part which divides the flow of plastic into 2 separate “flow fronts. But those key aspects can be viewed differently if you’re auditing mold changes before startup or troubleshooting changes for an already validated process. Plastics Injection Molding Expert John Bozzelli covers. Cold weld line or known as “knit line” isWeld Line defect analysis, the most common of many defects in injection moulding products. In the case of mold. 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. Using three test materials, polystyrene (PS), polycarbonate (PC) and polyoxymethylene (POM), the relationship between a design and a processing parameter. 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. Obstructions in the part create weld lines or knit lines during injection molding. Bead Blasting. Adjust the design for the flow pattern to be a single source flow. H-13 Good general purpose tool steel. The comment on melt flow index is correct, but, I've molded straight PC down to . A majority of these investigations concern amorphous polymers, whereas a number of engineering polymers are semicrystalline and also contain reinforcing fillers and fibers. Cold Slug: First material to enter mold during injection. PAPER ABSTRACT: Desktop injection molding machines and stereolithographic print ed molds make hands-on injection molding production possible in design studio classrooms and labs. 040 and 0. Als einer der führenden deutschen Hersteller von Masterbatches stellen wir nicht nur Farb-Masterbatches, Additiv-Masterbatches und funktionelle Kunststoff-Compounds her – wir leben deutsches Forscher-, Ingenieurs- und Unternehmertum. These materials have beenDetermining the correct tonnage provides a foundation that must be rock-solid to avoid flash and damage. Knit lines are consideredmolding defects, and occur when the mold or/and materialtemperaturesare set too low:. POROUS MOLD MATERIALS. 5mm (0. Still, it can be seen to varied degrees depending on mold. Here’s a rundown of helpful tricks to keep in mind when designing and choosing a material for molded parts with walls: In general, ensuring your design has uniform wall thickness between 0. There are times when porous materials like Porcerax are used in problem or backfill areas. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. Place vents at last place to fill and at converging flow fronts. Make sure the venting is not too large. 156 x 2 1/3 = 0. Injection Molding Guide. The Problems & Solutions course is designed to train injection molding personnel in. If you need injection molding expertise, we’re here to help. Short shots and knit lines are two phenomena that are caused by using highly viscousHowever, they can also create weld lines or knit lines in the part where the flow fronts meet. Inadequate injection pressure can result in knit lines and a surface that. Thick sections take longer to cool than thin ones. These are imperfections in the part. Know about the cause of these lines and the potential solutions. Flow Lines Knit or Weld Lines Cavity Gates Table 1 Knit line in a multi-injection-point cavity. Knit Lines. The part is cooled in the mold until it is solid enough to be ejected. Polym. 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. 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. pdf), Text File (. Inject the molten material into a mold. In the injection molding process, melting takes place via two factors that work together: shear and heat. This continues until the weld lines in plastic injection molding disappear.