Knit lines, weld lines, mold lines, or flow lines in injection molding are faint lines that become visible on certain plastic parts. Injection Molding: How to Set Second. occupied frequently with troubleshooting injection molding. Below is a list of items that should be checked both during normal molding operations and if defects are detected. 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. Venting is required to allow the air in the sprue, runner and cavity to leave the tool as the melt flows into the cavity. Look for signs of stickiness or burning. 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. May 24th, 2019 Posted in Injection Molding Flaw Repair Preventing Flow Lines, Knit Lines, & Blush Injection molding isn’t an exact science, and there is a lot that can go. Adjust the design for the flow pattern to be a single source flow. Our On-demand. 4. and 0. 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. CNC-Bearbeitung; Spritzgusswerkzeuge und -teile; Stereolithographie (SLA) Selektives. 040-in. Troubleshooting Injection Molding Weld Lines. They occur when melt flow fronts collide in a mold cavity. These lines. Weld lines: Also known as knit lines, these are molding defects that occur when two flow fronts meet without the ability to “weld. This information is intended to be used only as a guideline for designers and processors of modified thermoplastics for injection molding. Figure 1 Flow Lines Knit or Weld Lines Cavity Gates Table 1 Knit line in a multi-injection-point cavity. Knit lines are a common defect in injection molding that can affect the appearance and performance of plastic parts. In situ injection molding of continuous fiber reinforced thermoplastic composites is challenged by unbalanced dual-scale infiltration flow due to the pronounced capillary effect. Injection moulding (U. Services. Plastic injection molding has long been an affordable and efficient process for producing custom prototypes and end-use production parts. Other Excessive mold release Clean die and use minimal mold release. 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. Steven. He said, “Vent cavities at weld lines or junction points where two or more material streams meet and fuse into a solid mass. Usually the parting line of the mold is perpendicular to the mold opening direction of the injection molding machine. Know about the cause of these lines and the potential solutions. Reduce the temperature of the mold and plastic being injected. 3. Resin pellets are loaded into a barrel where. wall thickness injection molding ribs. The result is a consistent flow of molten plastic during injection and a quicker filling time. An edge gate introduces material at the main parting line, or where the two halves of. PAPER ABSTRACT: Desktop injection molding machines and stereolithographic print ed molds make hands-on injection molding production possible in design studio classrooms and labs. Meld and knit lines are actually both different types of. Its superior flowability allows for precise and flawless filling of injection molds without the risk of flow marks and knit lines. 3—Another issue with the four-gate design was that knit lines ended on or near mounting holes, causing a. Tips for Avoiding Knit Lines in Injection Molding Keeping the proper temperature in the injection molding machine, runners, and within the mold are all key to ensuring. The difference between a knit and meld line is significant, dramatically impacting the part’s structural integrity. Contact Universal Plastic Mold Today. Figure 1 presents the shrinkage of polypropylene versus wall thickness in injection molded parts. Heat: Too much heat can have an appearance similar to moisture splay. We recommend walls to be no less than 40 to 60 percent of adjacent wall thickness, and all should fit within recommended thickness ranges for the selected resin. 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. 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. ” Depending on the material you use, the knit lines could be invisible or obvious—work with an injection molding company to figure out. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and. Knit Lines. End-of-arm tooling can make or break a program. Short molding cycle, high production efficiency, and easy automation 2. More about the knit lines in injection molding. In the 1947 book, Injection Molding of Plastics, the late Dr. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. Other common issues associated with thin walls include uneven cooling, warp, cracks, cosmetic texture inconsistencies, and weak knit lines or fracture points. . Design with Plastics - for Injection Molding process. They again get resolidified along the surface. 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. As melt temperature, viscosity and abrasiveness rise, tool life will decline. Plasti-Co® Nozzles for Injection Molding Machines. When assessing issues with injection molded parts, we want to make sure some core aspects of molding are checked. Shear Controlled Orientation Injection Molding of Polymeric Composites with Enhanced Properties. • Holes formed by core pins are, in essence, 360° inside corners and should follow. to prevent improperly placed knit lines or sinking in the plastic downstream from the hinge. As these flow fronts come together. (2) Undesired movement of certain materials in a plastic. Here are some of the most common cosmetic defects and how to avoid or fix them. S. These lines occurs usually around holes or obstructions and cause locally weak areas in the molded part. Release of trapped air in those cases will. Without it, air and gas are trapped in the mold, which compress and heat as the cavity fills. Injection Molding Injection molding is perhaps the most common and important of all plastic processing processes. Compression molding is one of the earliest forms of molding. extend mold service life. What you should know is that there are various failure modes for injection molding. 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. 1-3). Also known as a knit line, this is the point at which two flow formations meet and are unable to connect during injection molding. The linear mold shrinkage of injection molded parts will vary from 0. Imagine a river splitting to go around a rock, then meeting up again on the other side. Pitfall #1 – Knit Lines. The depth of the texture or letters is somewhat. Injection molding weld lines refer to a line on an injection molded part that is generated where the two halves of the mold come together. 18/06/2021. Three terms that often get mixed up are weld, meld, and knit lines. 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. Three terms that often get mixed up are weld, meld, and knit lines. . Most failures occur at what are called “weld lines” or “knit lines”. Mold Venting. The quality of the weld line can be improved through optimized processing conditions, increased mold and melt temperatures, and increased injection speed toWeld Lines. Heat resistance. Press fits for injection-molded parts can be challenging. Let’s review what causes meld and knit line imperfections, how they’re different, and how they affect part durability. Mold filling simulation software should be used to ensure that waterExpand. Knit and Meld lines are inherent to plastic injection. Disadvantage: May rust if improperly stored. When designing for injection molding, consider these steps to prevent knit lines Resin cools as it is injected into a mold. opposed to two or more gates. 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. An option to bluing agents, the long established means of gauging pressure distribution, comes via Pressurex, a thin, flexible, plastic film. Obstructions in the part create weld lines or knit lines during injection molding. Injection Mold Tooling Options and Quality Reporting. Many of these tips are geared around injection-molded parts, but some apply to machined and 3D-printed parts as well. 0 Items. Brent Strong, p. 3 To get the most out of your Xometry Injection Molding experience, we offer the following part Design Guide with. (1987). In working with the injection molding service offered by Protolabs, you provide. (called also weld marks or knit lines), and meld lines. Soft metals, such as aluminum and beryllium copper, can be used for prototype<br />. be impressed by improvements of bubbles (vacuum voids), short shots, unequal thickness, jetting marks, weld lines, knit lines, black streaks, strain 2. Reduce the injection speed to limit the risk of trapping air inside the mold. 140 in. Focus - Injection Molding - Free download as PDF File (. In plastic injection molding, a parting line is where two halves of a mold meet when closed. A knit line is what happens when plastic flows coming from two different directions meet inside of the mold and solidify. 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. After selecting resin material, the design and process control of injection mold directly influence the quality of formed products. Product designers encounter a wide range of challenges when creating and developing molded parts, such as stress factors, warp, and those pesky knit lines. From the start of your project, you can count on The Toolroom to exceed your expectations for all of your plastic injection molding needs. INJECTION MOLDING GUIDE V2. Learn how material selection, part design,. Using three test materials, polystyrene (PS), polycarbonate (PC) and polyoxymethylene (POM), the relationship between a design and a processing parameter. Imagine a river splitting to go around a rock, then meeting up again on the other side. Gating. Even parts with metal inserts can be produced. 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. Hartland, WI 53029. While this process is largely efficient and reliable, it's not without its potential issues. This information is intended to be used only as a guideline for designers and processors of modified thermoplastics for injection molding. The Bottom Line You might have come across the aforementioned terminology before and did not know what any of the words meant. When designing for injection molding, consider these steps to prevent knit lines The Resin Cooling Process. As. To study the weld line formation of colliding flow fronts the filling of a mold cavity was simulated. These lines occurs usually around holes or obstructions and cause locally weak areas in the molded part. Different plastics make different types of knit lines. Injection Molding. 01 inches between parts fitted together to work best. 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. Such molds or tools aren’t intended to be used in operation. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic. One common problem that occurs in injection molding is the formation of knit lines. Injection molding, producing, and forming resin materials are three core components of injection molding technology. Weld / knit plane forms as flow fronts recombine Weld line and failure due to flow around core. Anisotropy. Fig. Our plastic injection molding glossary provides key terms. Double push filling method. The packing pins are similar in construction to mold ejector pins and may also be mold ejector pins depending on mold configuration. 008 to 0. . This slight depression is a tiny line called the weld line or knit line. The parting lines is where the two injection mold halves (A side and B side) meet. Weld-lines look like cracks on the appearance of plastic. Plastic injection molding is a complex and highly intricate process. 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). RELATED CONTENT. Depends how hard you want to push it. We have two unique offers to meet your needs wherever you are in the product lifecycle. There are times when porous materials like Porcerax are used in problem or backfill areas. 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. 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. Knit lines, holes, part configuration and the talent of your molder all come into play. 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. 1. The resin is transferred to the mold cavity under positive. This defect is caused from excessive lube or oily stock. ”. ABS (Acrylonitrile Butadiene Styrene) Benefits: ABS is a tough, impact-resistant plastic that is widely used across many different industries. Ideally, a safety strap should be strong enough to prevent the mold from opening under various unforeseen circumstances, but always less than the holding power of the mold clamps and the machine’s mold-opening force. It is also where a slide mechanism might meet where it shuts off on one of the mold halves. 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. This makes the casting process as efficient and accurate as possible. Knit lines, also known as weld lines, are fine lines that resemble hairline fractures or cracks. The relationship between processing conditions, morphological structure and mechanical properties of knit lines of an injection molded semi-crystalline thermoplastic. 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. When the temperature is not very high, there are chances of premature solidification, which can cause weld lines to form. In fact, knit line strength will decrease with higher filler content and with longer fibers. 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. H-13 Good general purpose tool steel. Weld / Knit Lines; Assumptions. In addition, the force of the gas holds the appearance side of the part in contact with. Reducing the size and depth of depressions helps minimize the impact of knit lines. (If parts are to be molded by Protomold’s rapid injection molding process, our Design Guidelines should be used. From Plastic Materials and Processing by A. He is currently a tooling manager for a large, multi-plant molding and manufacturing company. 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. - 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. These data were obtained from many andKnit lines in injection molding are common defects of the molded parts. Venting at Weld Lines. Depending on the resin, resin temperature, mold temperature, and filling. Temperature is another factor that can cause weld or knit lines in injection molding. Back to News Page. Vacuum on the mold does help and many times eliminates the weld line. 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. TEFABLOC™ TOSI 431 75A is a high-flow thermoplastic elastomer (TPE) designed for large-part injection molding applications, offering exceptional flexibility and weather resistance. 6. Weld lines typically appear in a part when an obstruction of some kind separates the melt flow and prevents it from "knitting" together. The last cause of flow lines may be the operators running your daily operations. Swirling can increase depending on the material you use, and the texture or polish of injection molding tools. TRADITIONAL MACHINES VENTS. So. This can enable the manufacturer to increase the molten plastic temperature at the front of each flow and avoid the occurrence of weld lines. Gates, vents, sliding cams, pick-out inserts, and any other place where the two sides of. All the while, the material could be the correct material," Hill says. 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. These structures are often small surfaces of about a tenth of an inch in diameter. 5 to 3. [8] Bevis, MJ, Reis RL, Cunha AM. 4. 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. ). This month, we covered an array of topics including injection mold venting, business strategies and runnerless micro molds. 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. 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. 7584. txt) or view presentation slides online. 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. Be wary of saving too much weight or you might end up with: cracking. Factors were ordered. The goal is to create a uniform thickness and at the smallest (thinnest) level required. Observation on a “trial and error” basis may be required. Our goal of 100% efficiency, zero scrap and repeatable processes can sometimes be hindered by poor setups or changes in the molding process. Randy Kerkstra has been in the plastics industry for more than 26 years, occupied frequently with troubleshooting injection molding. The tooling engineer in the mold house will be more into venting, sprue pressure, preheating the mold, cycle times, ejector pin scheme, mold flow, getting a gate location to avoid fill issues or knit lines, excessive flash and molding pressures, and the list goes on. Compression molding can also support large part sizes, and it has a tendency to. Reduction of sink can possibly be had by switching to an ABS/PC-blended material. Knit Mark – A witness mark on a molded part, often occurring at the midpoint between two transfer or injection sprue locations. Insufficient injection pressure and speed can cause weld lines. The best we can hope for is to pressurize the knit or meld line better through the combination of melt temperature, mold temperature, flow rates, and holding pressures. The knit lines are the result of poor molding conditions (likely molding at too low of a temperature). Our Injection Molding Capabilities • Horizontal Injection Molding up to 3300 TONS • Structural Foam Molding + Structural Web • 1oz to 600oz part capacity. 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. Solution: Increase the injection pressure or speed. Islyn Thomas summarized the need for a particular vent location better than I ever could. The metal parts, made of steel grade. When assessing issues with injection molded parts, we want to make sure some core aspects of molding are checked. To prevent swirling, perform tests whenever possible and use pre-compounded resin to ensure identical color match across parts. Remedies: Raise the temperature of the mold or molten plastic. If life safety is involved, or reliably or efficacy are absolutely required, every part should. These knit-lines are well-known as particularly critical regions in the mold when mechanically loaded. Only then can we move on to determining the parting line itself. BACK TAPER – Reverse draft used in mold to prevent molded articles from drawing freely. 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. The injection molding design process, much like all industrial design processes, deals primarily with the creation, look, and feel of a product. Three terms that often get mixed up are weld, meld, and knit lines. 012 in. TEXAS. If life safety is involved, or reliably or efficacy are absolutely required, every part should. Injection molding is an efficient, often cost-effective way to manufacture custom plastics that are strong, durable, and long-lasting. Typically, injection molded parts will be between 0. 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. Troubleshooting Injection Molding Weld Lines. com. These flaws can affect the aesthetic and structural integrity of the. – Change the thickness of the product. When the mold body is closed, the contact surface of the cavity and core plates makes a parting line. Understanding Knit Lines in Injection Molding: Causes and Solutions. This radius should not be smaller than 0. Nicolet Plastics LLC. We ensure customers receive accurate, fast, on-time, sufficient support and services. The quality of the answer is directly proportional to the information we have to work with. 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. Fictiv can help you avoid injection mold design and plastic part design for injection molding problems. A plastic product formed by the injection mold is divided into two parts, and the line separating the product is called the parting line. Most failures of injection molded plastic parts are caused by either improper design of the part or improper design of the mold. For injection molding of the multi-blade fans and various rotating plastic parts, the complex of multiple gating injection molding tools were used. ABS. Typical Tool Steel for Injection Mold Construction Steel Type Steel Properties Mold Component P-20 Pre-hardened, machines well, high carbon, general-purpose steel. A screw-type plunger forces the material into a heated chamber, called a barrel, where it is melted. Gates, vents, sliding cams, pick-out inserts, and any other place where the two sides of. Weld lines are formed when there is an inability of two or more flow fronts to ‘knit’ or ‘weld,’ together during the molding process. He is currently a tooling manager for a large, multi-plant molding and manufacturing company. Knit Lines: These occur when different flow fronts come together as plastic flows through the part cavities. 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. Plastic melt and mold temperature. A parting-line vent. By understanding material fill, injection mold designers can better position gates, anticipate where knit lines will appear, and locate difficult to fill spots in the mold. Imagine a river splitting to go around a rock, then meeting up again on the other side. These are imperfections in the part where separated flows of cooling material meet and. Mold bases, ejector plates, and some cavities (if nickel or chrome plated to prevent rust). Sequential valve gate technology. Additionally. The mold-opening force varies widely depending on the type and size of the molding machine. For over four decades, our experienced team and leading edge technology have provided you with tooling solutions from start to finish. Find out more by calling 573-437-4154 today or email us. H-13 Good general purpose tool steel. - Mold flow analysis to control fill and knit lines. It is best practice to maintain a uniform thickness throughout an injection-molded part. To prevent knit lines, injection molders can increase the temperature of the molten resin to prevent solidification from starting too soon, and increasing injection speeds and pressure are also potential solutions. The advantages of compression molding include more isotropic fiber distribution, dimensional stability, and surface finish. 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. 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 or knit lines occur when melt flow fronts collide in a mold cavity. From the start of your project, you can count on The Toolroom to exceed your expectations for all of your plastic injection molding needs. To avoid walls that are too fat or too thin, designing a wall thickness between 0. 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. This is a quick rundown on how to handle common features in injection molding designs:What are the advantages of injection molding 1. It’s our responsibility to make sure your design comes out of the mold working properly. This most often occurs when you have a core or shutoff feature that the plastic needs to flow around. When working with customers, our design guidelines for living hinges are 0. We have an elite team that designs for injection molding, rapid prototyping with metal and plastic, and produces world-class products under the same roof. Learn how to troubleshoot weld lines in injection molding. The injection molding process offers a vast array of plastic resins to work with. These defects are not visible, like small. In this paper, a general and efficient approach was proposed for dual-scale infiltration flow balancing based on numerical simulation. Reduce the injection speed to limit the risk of trapping air inside the mold. Solution: Increase the injection pressure or speed. Flow Lines Knit or Weld Lines Cavity Gates Table 1 Knit line in a multi-injection-point cavity. Material contamination, for example, can cause several molding issues. Ham Lake, MN 55304. Exploring common molding defects and. The injection molding process uses a granular plastic that is gravity fed from a hopper. First, they carry resin to the mold via a runner. Other materials, like LCP are great for thin wall micro molding but also notorious for having a bad knit line, potentially snapping from excessive torque in later manufacturing processes. In layman’s terms, injection molding boils down to four simple steps: Melt material. 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. 1513 Sam Bass Rd. 1 Gate Location. The Troubleshooting Tips below offer common solutions to challenges in IML and IMD. More about the knit lines in injection molding. Check the diameter of the ejector pins and the pin hole tolerances, if the tolerance is too wide this could allow. To learn more about us or to get a quote, click here or call 1-888-893-1587 now. BLEED – (1) To give up color when in contact with water or a solvent. These seams are known as “knit lines. In fact, knit line strength will decrease with higher filler content and with longer fibers. The Problems & Solutions course is designed to train injection molding personnel in. LS Mtron Injection Molding Machine (IMM) has opened a new business unit in Mexico. Weld line defects – also known as knit lines – is a defect that occurs when the injected plastic meets within a mold. To prevent knit lines, injection molders can increase the temperature of the molten resin to prevent solidification from starting too soon, and increasing injection speeds and pressure are also potential solutions. This production method is particularly popular when manufacturing in large quantities. Swirling can increase depending on the material you use, and the texture or polish of injection molding tools. A knit line is any line, visible or not, where two resin flows meet. Investigate the mold for signs that a water leak is present, and correct the condition. In this, work, metal inserts were joined with polyamide 6 by using the injection-molding technique. A parting-line vent. Considerations: ABS-molded parts will show knit lines and can have sink and voids in thick areas. Submarine Gates. These data were obtained from many andKnit lines in injection molding are common defects of the molded parts. Self-mating parts. Injection moulding (U. 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. Those are knit lines, formed when two opposing flows of material join together in the mold cavity. Injection Molding Process Guidelines The following is given as a potential process starting point. Injection molding can be a real challenge if you don’t follow the “Five M’s”: Man, Mold, Machine, Material and Method. Knit Lines. Excessive injection pressure can result in part defects such as overflow or flash. injection molding.