- Mold flow analysis to control fill and knit lines. Injection Molding Injection molding is perhaps the most common and important of all plastic processing processes. 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. But add 30% glass and weld-line strength retention drops to only 34% as high as the compound with no weld line. They occur when melt flow fronts collide in a mold cavity. The quality of the weld line can be improved through optimized processing conditions, increased mold and melt temperatures, and increased injection speed toWeld Lines. Figure 1: D-shaped through-hole in center of gear part accepts the drive shaft. knit line formation, application of draft angle and design for assembly. For the metal feedstock, injection and mold-wall temperatures were 130 ºC. Injection molding, producing, and forming resin materials are three core components of injection molding technology. See full list on fathommfg. Knit lines, weld lines, mold lines, or flow lines in injection molding are faint lines that become visible on certain plastic parts. 6. As. Texturing may also hide surface defects such as knit lines and other imperfections. Gate size can be increased or moved to a section with greater wall thickness to help eliminate. The Goldilocks principle applied to wall thickness. This makes the casting process as efficient and accurate as possible. Obstructions in the part create weld lines or knit lines during injection molding. and Titomanlio, G. The mold opens and closes on a vertical axis in the vertical injection molding. 040 and 0. Molding is a manufacturing process in which resin in a barrel is. This most often occurs when you have a core or shutoff feature that the plastic needs to flow around. INJECTION MOLDING GUIDE V2. Injection molding can be a real challenge if you don’t follow the “Five M’s”: Man, Mold, Machine, Material and Method. They are the result of fundamental physical effects taking place inside of the mold. PCS Company, Fraser, Michigan, has launched the Polimold Valve Gate Sequencer, providing injection molders with more precise control of the mold-filling sequence when using valve-gate hot runner systems. The relationship between processing conditions, morphological structure and mechanical properties of knit lines of an injection molded semi-crystalline thermoplastic. TEXAS. Read on to find out more! 800. This month, we covered an array of topics including injection mold venting, business strategies and runnerless micro molds. Bubbles are caused by melting flow fronts colliding in a mold cavity. Step 1: The injection of molten plastic into the injection mold from Barrel A. Steps involved in Multiple Material Molding. 2. sheet metal fabrication, rapid tooling, and plastic injection molding. Weld line. What is copper electrode in injection mold? hashtag#plasticinjectionmold hashtag#moldmaking "Copper electrode," also known as "electrode," is a common component in mold manufacturing. Weld lines defects are formed when two flow fronts are merged together. Islyn Thomas summarized the need for a particular vent location better than I ever could. Enlarge gas vents and gates to allow trapped air to escape the mold. As the melted plastic fills the mold, there are locations where the molten plastic flow stream separates and then comes back together. 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. Advantages for your Injection Molding Machine:. It can also aid in material selection since fill. Fig. Mold venting is critical to the quality and consistency of the finished part. 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. While injection molding itself is a straightforward process involving melting. Three terms that often get mixed up in the plastic injection molding industry are weld, meld, and knit lines. The recommended minimum production volume for injection molding is 500 units. The last cause of flow lines may be the operators running your daily operations. (See Fig. Material contamination, for example, can cause several molding issues. 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. In the 1947 book, Injection Molding of Plastics, the late Dr. Change gate locations. 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. This performance can ultimately impact many part properties, including surface quality, knit line strength, part ejection force (sticking), dimensions, tolerances. 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. The weld line can either be a minor cosmetic issue. With this option, users can utilize in-cavity pressure or temperature sensors to control when the valves open or close. In the case of mold details, knits will occur on the lee side of a detail. Short molding cycle, high production efficiency, and easy automation 2. 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 or Knit lines are a plane or section in an injection molding part where the flow of molten plastics meets from different. This guide to thermoplastics and injection molding material selection is aimed at an engineer who plans to quantitatively analyze a part, determine loads, stresses, strains, and environments and make an optimal material decision based on the analysis. What you should know is that there are various failure modes for injection molding. The depth of the texture or letters is somewhat limited, and extra draftParts were molded at a maximum injection pressure of 235 MPa, a maximum clamping force of 800 kN, a maximum injection speed of 900 mm/s, and a maximum injection stroke of 120 mm. The parting line will always be visible, but can be minimized by its location, draft allowance and part geometry. Macromol Symp 1988;21/21: 489-500. Weld lines, also called knit lines, are a defect that occurs with molded parts caused by two different flows of plastic resin that do not adequately join together or weld together correctly during injection molding. [1]Weld Lines. Optimizing Mechanical Performance of Injection Molded Multiple Gated Rotating Thermoplastic Components: Part 1 - Consideration of Structural Analysis and Knit Line Effects March 2001 DOI: 10. Meld and knit lines are actually both different types of weld lines. Our plastic injection molding glossary provides key terms. Knit Line occurs where different flow fronts of material meet while flowing through the mold. ; Wood, Philip E. The point of meeting creates a weak spot because the edges of the molten plastic, already cooling, don’t mix together very well. . Knit/Weld Lines. Weld Lines. Excessive injection pressure can result in part defects such as overflow or flash. Learn about the key characteristics of common injection molding materials, recommended applications, and how common thermoplastics compare to one another. Call 800-985-6750 or email your inquiry to sales@customgasketmfg. ABS is a good choice for handheld consumer devices. It allows our molders to monitor fill pressure, cosmetic concerns, and the basic quality of the parts. Strength at the knit line can be as little as 20% of the. Additional venting along the parting line perimeter will improve overall venting significantly. The final molding process may be dependent upon factors. This slight depression is a tiny line called the weld line or knit line. 1. Knit lines appear to be a faint line or a crack in the finished part after it comes out of the injection mold. ISO 9001:2015. On the opposite side. 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. For over four decades, our experienced team and leading edge technology have provided you with tooling solutions from start to finish. 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. Mold Venting. Injection molding a part with Class-A surface finish and ribs or bosses has proven an elusive goal for molders, despite their best efforts with conventional injection or internal gas-assist technology. suppressed cooling and possibly warp. , Rallis, A. Weld lines are a common imperfection to be found in most plastic injected molded parts. Plastic injection molding is the industry’s most common way to manufacture parts. Extreme geometries can cause knit lines. You don't give us much to go on. Inadequate injection pressure can result in knit lines and a surface that appears corrugated. It is caused by the incomplete joining of the uncured. 2. This summary represents a key subset of the detailed molding information found in the remainder of this molding guide . Find out more by calling 573-437-4154 today or email us. 1-3). Learn how material selection, part design,. These knit-lines are well-known as particularly critical regions in the mold when mechanically loaded. 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. 5T, where T is your nominal wall thickness. To learn more about us or to get a quote, click here or call 1. This type of molding uses a single tool to create both the part's geometry and surface finish in one step. Mold filling simulation software should be used to ensure that waterExpand. The temperature might fall at some places like in the mold, runners leading to the mold and injection molding. Nicolet Plastics LLC. There are times when porous materials like Porcerax are used in problem or backfill areas. From Plastic Materials and Processing by A. The thickness of the part plays an important role in how difficult the molding process will be. 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. 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. Typical Tool Steel for Injection Mold Construction Steel Type Steel Properties Mold Component P-20 Pre-hardened, machines well, high carbon, general-purpose steel. The mold settings can run flawlessly until employees consistently open the machine for inspections, interrupt cycle times, or cause. ABOUT THE AUTHOR. One of the most common ways of adding venting is by manually machining it into the mold (Figs. Proper venting is essential to molding a defect-free part. Plastic injection molding is a complex and highly intricate process. ABS. 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. BACK TAPER – Reverse draft used in mold to prevent molded articles from drawing freely. Knit Lines: Holes in the part will always cause knit lines. By altering the part design, it may be possible to just have one flow front, and avoid multiple flow fronts having to come together. Reducing the dimensions of the runner system. It is best practice to maintain a uniform thickness throughout an injection-molded part. When designing for injection molding, consider these steps to prevent knit lines Resin cools as it is injected into a mold. A medical device designer might look up a polymer's physical properties and game out. An option to bluing agents, the long established means of gauging pressure distribution, comes via Pressurex, a thin, flexible, plastic film. Dimensional stability. A poor knit line can cause only cosmetic blemishes or it can significantly weaken the structural integrity of a part. Adjusting features to be suitable for molding will help resin flow well into the mold, allow it to cool evenly, and reduce the chance of cosmetic defects such as sink, knit lines, flash, and warp. Knit Lines: These occur when different flow fronts come together as plastic flows through the part cavities. From the start of your project, you can count on The Toolroom to exceed your expectations for all of your plastic injection molding needs. Fortunately, there is a technique that provides almost unlimited control over which parts of a tool fill when and how fast. 250 in. Weld lines typically appear in a part when an obstruction of some kind separates the melt flow and prevents it from "knitting" together. Meld and knit lines are actually both different types of weld lines. More about the knit lines in injection molding. Image credit: The Rodon Group. 5 : 1 and 3. Generally all plastic components should be designed with draft where possible. Learn how HPP maximizes these specialized processes. sinks, knit lines, and shrinkage,” Spalding says. Most failures of injection molded plastic parts are caused by either improper design of the part or improper design of the mold. Once the mold is created and the machine is set up, additional parts can be manufactured very fast and at a very low cost. In fact, knit line strength will decrease with higher filler content and with longer fibers. Learn how material selection, part design, and optimizing process parameters can help eliminate knit lines. In plastic injection molding, a parting line is where two halves of a mold meet when closed. (for example, a hole on the plastic part), the knit line will be unavoidable. Holes in the part will always cause knit lines. Weld lines create visible marks, but this type of injection molding defect can also reduce a part’s strength and durability. Reduction of sink can possibly be had by switching to an ABS/PC-blended material. The parting line location on the injection mold will have an effect on how the cavity and core (both halves of the injection mold) come together and seal. 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. Weld / Knit Lines; Assumptions. Weld line are formed whenever melt fronts separate and recombine at some downstream location. com . While the presence of knit lines does not always compromise the structural integrity of the plastic part, they are almost always a cosmetic issue. Monoform parts. For example, increasing gate size can improve part filling. Weld Line Phenomenon: Surface defect in which the weld, or the part where the flows of melted plastic meet inside the mold, shows up on the surface as a groove or pattern. 7584. Knit lines, also known as weld lines, are fine lines that resemble hairline fractures or cracks. If you have questions about the suitability of the Protolabs process for your optical parts, contact an applications engineer at 877-479-3680. Setting that Delta P is the secret to consistent fill time. There are several possible causes that need to be investigated to troubleshoot weld lines in injection molding. 4. In injection molded parts, there are 2 types of weld lines: The first type of weld mark is called cold-melt joints and is formed when the melt front enters the cavity with different gates. Injection molding is the most widely used method for mass-producing plastic parts. When injection molding a part, a knit. Heat: Too much heat can have an appearance similar to moisture splay. These are caused by the shot of molten plastic moving at different speeds throughout the injection. The quality of the answer is directly proportional to the information we have to work with. The cooler flow fronts of the resin aren’t forced into the mold surface texture as well as the hotter resin behind the flow front. After selecting resin material, the design and process control of injection mold directly influence the quality of formed products. Looking to Your Knit Lines with Molding. 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 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. In fact, knit line strength will decrease with higher filler content and with longer fibers. Increase the injection speed. Only then can we move on to determining the parting line itself. When assessing issues with injection molded parts, we want to make sure some core aspects of molding are checked. The knit line is generally opposite the gate in thermoset. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. Flow Lines. 040 and 0. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part. Causes 1. Compression molding is one of the earliest forms of molding. Weld Line defect analysis, the most common of many defects in injection moulding products. This radius should not be smaller than 0. This defect is caused from excessive lube or oily stock. Self-mating parts. 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. The packing pins are similar in construction to mold ejector pins and may also be mold ejector pins depending on mold configuration. 0 to 3. 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. 08 KICKOFF PRODUCTION. 008 to 0. Knit lines are a common defect in injection molding that can affect the appearance and performance of plastic parts. The following are general guidelines for molding acetal. Insufficient injection pressure and speed can cause weld lines. 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. 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. 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. Mold venting should be checked and increased if necessary, injection molding service. Operating Conditions • Fast injection fill speeds, supporting short injection times (1–3 s), especially in thin sectionsKnit lines in reaction injection moldings Knit lines in reaction injection moldings Maier, Christian; Hrymak, Andrew N. 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. These knit lines are formed as molten plastic in the injection mold flows around an obstruction like a feature. Reduce the injection speed to limit the risk of trapping air inside the mold. IM: hot or warm knit lines are cause by _____ while cold knit lines are caused by _____. knit line. Troubleshooting Injection Molding Weld Lines. ”. Weld Lines-3 Ways for Detecting and Correcting. When the mold adopts the multi-gate pouring scheme, the plastic melt. These data were obtained from many andKnit lines in injection molding are common defects of the molded parts. These lines occurs usually around holes or obstructions and cause locally weak areas in the molded part. placed usually in the areas last to fill, near knit lines, as well as on the runner system. H-13 Good general purpose tool steel. Weld line defects – also known as knit lines – is a defect that occurs when the injected plastic meets within a mold. Venting is required to allow the air in the sprue, runner and cavity to leave the tool as the melt flows into the cavity. Weld lines are formed when there is an inability of two or more flow fronts to ‘knit’ or ‘weld,’ together during the molding process. Injection molding is the process of injecting plastic material into a mold cavity where it cools and hardens to configuration of the cavity — is best used to mass-produce highly accurate, & often complex, 3dimensional (3D) end-use parts and products. Mold cooling is one of the most important variables in injection molding. 5mm (0. When injection molding complex parts, often knit-lines are formed during the mold filling stage. These include: Symmetrical parts. Fictiv can help you avoid injection mold design and plastic part design for injection molding problems. Degassing – Degassing is the simple operation of opening and closing an injection mold to allow any trapped air (or gas) to escape. For best results, keep thickness variations to within 15% of the nominal wall thickness. In fact, knit line strength will decrease with higher filler content and with longer fibers. It’s also relatively inexpensive. Meld and knit lines are actually both different types of weld lines. An edge gate introduces material at the main parting line, or where the two halves of. Three terms that often get mixed up are weld, meld, and 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. (2) Undesired movement of certain materials in a plastic. Typically, the wall thickness is somewhere in the neighborhood of 50%-75% of your nominal wall thickness. See UNDERCUT. Knit Lines and Resins. injection molding. When the plastic cools, it takes the shape of the mold and is then ejected and finished. (1987). Compression molding can also support large part sizes, and it has a tendency to. 060 in. be impressed by improvements of bubbles (vacuum voids), short shots, unequal thickness, jetting marks, weld lines, knit lines, black streaks, strain 2. It’s our responsibility to make sure your design comes out of the mold working properly. A mold with too few gates is susceptible to cooling problems, but a mold with too many gates. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and. Multiple gates will always. land and using maximum vent depths to get them as close to self-cleaning as possible. In spite of its cosmetic attributes, a bad knit line can significantly. 01 inches between parts fitted together to work best. A knit line is any line, visible or not, where two resin flows meet. holes, multi-point injection. 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. This most often occurs when you have a core or shutoff feature that the plastic needs to flow around. Adding a border around an embossed logo is a good way to protect it from abrasion. Adding vents, using overflows, or using porous metal. One common problem that occurs in injection molding is the formation of knit lines. Observation on a “trial and error” basis may be required. Injection rate plays a role—faster fill times develop more shear, molecular orientation and residual stress. As melt temperature, viscosity and abrasiveness rise, tool life will decline. The thermo-rheological findings were used to investigate the sources of weld line weakness. 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. 1 Gate Location. Remove holes if possible. RJG announced that its injection molding process control system, CoPilot, will begin offering valve gate control technology for hot runners starting in September 2021. Unsymmetrical filling makes knit lines in samples, leading to the failure of part, so it must be avoided. The Troubleshooting Tips below offer common solutions to challenges in IML and IMD. While injection molding dies are expensive to produce, each die canHowever, they can also create weld lines or knit lines in the part where the flow fronts meet. The thermoplastic injection molding process at BEPL is a standard process involving an aluminum mold with no heating or cooling lines running through it, which means cycle times are a bit longer. The splay condition can completely cover a part, or it can appear inconsistently in various areas on the part. Given the cosmetic requirements, there is a good chance the mold would require. Knit lines, also known as weld lines are areas in the. When the molten plastic meets an obstruction. ABS (Acrylonitrile Butadiene Styrene) Benefits: ABS is a tough, impact-resistant plastic that is widely used across many different industries. 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. 020"). – Change the thickness of the product. These structures are often small surfaces of about a tenth of an inch in diameter. Knit and meld lines are inherent to plastic. and 0. Here are some of the most common cosmetic defects and how to avoid or fix them. More about the knit lines in injection molding. 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). Direct experience designing molds and parts for injection molding and producing the molds and parts is valuable, relevant experience for design students. RELATED CONTENT. The example. Knit lines, holes, part configuration and the talent of your molder all come into play. Hardware components. Understanding Knit Lines in Injection Molding: Causes and Solutions. Know about the cause of these lines and the potential solutions. These defects are not visible, like small. Timing and temperature are significant contributing factors when it comes to flow and knit line defects. Plastic injection molding is the process of injecting molten plastic material into a metal tool which then cools and ejects a plastic part from the machine. To account for variations in viscosity, nylon injection molding machines need to maintain an appropriate injection pressure. Know about the cause of these lines and the potential solutions. Some are: Knit lines (discontinuities where two flows merged, creating a weak point)In some situations where a mold has parting line damage, reducing heat can actually improve flash that was a direct result of that damage. Round Rock, TX 78660. One of the most common ways of adding venting is by manually machining it into the mold (Figs. Let the molten material cool to a solid-state. Investigate the mold for signs that a water leak is present, and correct the condition. Knit Lines. Remedies: Raise the temperature of the mold or molten plastic. In. 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. A weld or meld line is a weakness or visible. The injection molding process uses a granular plastic that is gravity fed from a hopper. Knit lines are hard to eliminate for injection molded plastic parts but designers can minimize cosmetic defects. These defects are not visible, like small. Mold Flow Analysis computer. 3 To get the most out of your Xometry Injection Molding experience, we offer the following part Design Guide with. While mold-filling simulation is a very common tool for predicting the fill patterns of an injection mold, in our judgment there is not yet a commercially available, satisfactory filling simulation for microcellular foam molding. Knit lines are consideredmolding defects, and occur when the mold or/and materialtemperaturesare set too low:. Notched weld lines then remain near the cavity walls. Core Geometry. These lines occurs usually around holes or obstructions and cause locally weak areas in the molded part. Gates, vents, sliding cams, pick-out inserts, and any other place where the two sides of. Contact Universal Plastic Mold Today. 01 inches between parts fitted together to work best. These data were obtained from many andKnit lines in injection molding are common defects of the molded parts. Contact Universal Plastic Mold Today. The most common and one of the most challenging injection molding defects are weld and knit lines. Design With Plastics. Thermoplastic Elastomer Molding Guidelines. Generally, the mold structure is roughly divided into two main parts: fixed half and moving half. Learn Injection Molding Glossary with Plastopia: Using actual molded parts (with images) as examples. Macromol Symp 1988;21/21: 489-500. This is a quick rundown on how to handle common features in injection molding designs:What are the advantages of injection molding 1. 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. . Brent Strong, p. Molding Troubleshooting Guide Bond Readout Bond readout is a surface distortion similar to a hump or sink that occurs over a bond line Probable Cause Material Process • Mismatch of compliance between outer panel, inner panel and adhesive • Incompatible thermal expansion coefficients between the SM and the adhesiveWeld lines, created in the areas of collision of two flow fronts of plastic in the injection mold cavity, are the reason of lower mechanical properties and a worse surface condition of molded parts. Knit lines, weld lines, mold lines, or flow lines in injection molding are faint lines that become visible on certain plastic parts. An injection mold that produces pieces while the final tool is being built. (1. The company provides an array of digital manufacturing expertise. 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. Understanding Knit Lines in Injection Molding: Causes and Solutions. Thick sections take longer to cool than thin ones. When the temperature is not very high, there are chances of premature solidification, which can cause weld lines to form. While injection molding itself is a straightforward process involving melting. For non-reinforced or non-filled nylon, the mechanical performance in the knit lines (planes) areas are approximately equal to the mechanical performance of the resin (polyamide) used. A weld line (weld point, flow line or a knit line) forms when separate melt fronts travelling in opposite directions meet. knit lines and parting lines are at the discretion of MIC-ZG. Gates, vents, sliding cams, pick-out inserts, and any other place where the two sides of. A knit line is any line, visible or not, where two resin flows meet. Lower the melt and mold temperature to prevent overheating. Learn how to troubleshoot weld lines in injection molding. There needs to be a balance between fit of the halves, function of the part, visual quality expectations, and the cost to produce the mold. This line does not normally impact the overall shape or dimensions of the item. 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. 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. Design with Plastics - for Injection Molding process.