Injection molding could develop knit lines if one front cools before it reaches the other. Flow Lines Defect in Injection Molding Process Flow lines can be defined as linear grooving, or circular ripples, on the surface of a molded part that indicate the. 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. In addition to the recommended wall thickness, it’s important to consider a material’s flow characteristics. Purpose: The examination of melt flow lines in injection moulded parts was the purpose of this research, as well as a new method of flow visualization was applied. Mold Flow - Position ribs such that the plastic has an optimal flow path. 3. In injection molding weldlines are generated when two separate melt streams join either. ” Flow marks do not affect component integrity. Where wall thickness increases, round the corners of the mold in those regions. Weld lines have been one of the common defects puzzling the injection molding industry. Molds separate into two sides at a parting line, the A side , and the B side , to permit the part to be extracted. On-Line Solutions To Injection Molding Problems. Water cooling pumps coolant through lines on the outside of the mold cavity. (3) Adjust the temperature of the barrel and nozzle: the viscosity. We will now look at the problem you are likely to encounter and how to fix them. For the production of these weld-line specimens, a special mold was used and injection molding parameters were systematically varied, tensile (ISO 527-2) and impact (ISO 179-1/1eU) properties were measured, and statistical analyses were performed to gain insight on the correlation between melt flow rate and weld-line behavior. Production was set according to an experimental design (DoE), varying injection flow rate, mold temperature, and melt temperature. A significant part of that is the cooling design, where there are many things to consider. Vary injection speed, screw rpm, and backpressure. This is often a sign that injection speed and/or pressure are. Within more basic plastic part design plans this plane can be a simple, flat surface, but it is often a complex form that. These are typically found near the narrow sections of the mould gate and can. On thicker sections of the mold, there may be sinks caused by bosses designed into the backside of the surface. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part. The gate vestige for injection molding is usually narrow. Allows you to accommodate a variety of part sizes. The flow fronts at different times (Fig. Flow lines are unwanted streaks, waves, or lines on a part’s surface. Mold Design. The plant water system is a critical component of an injection molding facility. Low-quality and poorly designed molds can also result in flow lines on injection molded parts, particularly those with uneven wall thickness, insufficient lubrication, and poor venting. These flaws might have an impact on the product’s functionality as well as its appearance. Molds gain streaks through molten plastic cooling and solidifying at different rates. If the cornflake pigment contains a large amount of fine particles, this will result in a higher definition of flow and weld lines. Typically, flow lines are caused by the liquid plastic cooling at different rates. Flow lines can be defined as linear grooving, or circular ripples, on the surface of a molded part that indicate the direction of material flow within the cavity of the mold. Flow lines. The position, length, and angle of weld lines are dependent on the. Fictiv provides you with DFM feedback so that you can minimize injection molding defects. This method can be helpful in. S. To explore the formation of weld lines in injection molding, a flow model based on viscoelastic constitutive equation is presented in this article to predict and evaluate of weld lines location. Acetal is a funny material in how it reacts to certain injection speeds and mold temperatures. Lenticular, or Silver Dollar type pigments, are thicker than cornflakes, with a narrower particle size distribution. Weld line. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. They occur as a result of different flow rates of molten plastic in the mold cavity. Weld Lines. Water set-up and design are every bit as important as establishing and recording a repeatable process. A poorly designed or maintained water-cooling system can have a serious impact on production efficiency and cause many maintenance issues. These lines form as a result of material flow and solidification during the injection molding process. Weld lines in injection molding occur when two flow fronts meet and create a visible line on the surface of the molded part. Thin wall injection molding can produce walls as thin as 0. 1). Novel Nozzle Tip Prevents Stringing, Saving Molds & Cycle Time. The melting temperature is 217–237 °C, and the thermal. 0005 in. When it comes to injection-molded products, here are several typical solutions for flow lines: To make sure the liquid reaches the mold fully before cooling, enhance the. As the material properties of. These lines form as a result of material flow and solidification during the injection molding process. Seven Common Defects and Solutions for PC/ABS injection molding Products,Flow Lines, Pitting,Product Delamination,Stretch Marks. "Design and Mould Flow Analysis of Injection Mould for Connecting Link," International Journal on Emerging Technologies, vol. Injection Molding of Multimaterial Systems. Causes – Remedies. This can be accomplished by reducing the substance’s viscosity. However, the weld line may not continue throughout the flow. The mold material 6 can have several effects on. Additionally, you can get our free Design Cube, which will provide a physical aid to help understand how knit lines form as well as other injection molding design considerations. After exhaustive research, testing and pilot-scale demonstration, the team developed a breakthrough new technology utilizing low constant injection pressure. A method of melt flow visualization was used to record the weld line location on the surface of molding prepared at the proper injection conditions (method of "record grooves effect" formation. thick, 0. Incorrect gate positions cause multiple flow fronts that struggle to merge. Adjust Injection Molding Parameters: Optimizing injection speed, pressure, and temperature can help ensure better melt flow and reduce the likelihood of weld line formation. The recommended minimum production volume for injection molding is 500 units. 750 in. As the material properties of. 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. Flow lines appear as a wavy pattern on the surface of your plastic parts. There are two types of weld lines, cold and hot weld line. If it is farther down the flow part, increase injection speed to decrease viscosity and allow more packing pressure. 6 T) as thick as the nominal wall. If you’re seeing flow lines, try extending the. Weld line defects – also known as knit lines – is a defect that occurs when the injected plastic meets within a mold. Each needle valve nozzle opens only when the melt front has moved across its gating point. Edge Gates. Low cushion and hold time. It has larger cross-sectional regions that allow sufficient molten plastic flow into the cavity. Thereby creating localized pressure within the mold. It also may be that your injection speed is too low, which will cause the molten plastic to solidify at different rates. It affects its structural integrity and exterior appearance. 0030 in. Flow Lines. Sufficient draft is an important part of any mold design, and quick-turn tooling is no exception. If you experience flash on your injection blow molding products, consider redesigning a worn out or faulty mold. The best method of venting this type of part (Fig. A look at how taking the time and data to analyze a mold problem will help you repair it right—and permanently—the first time. 2. The flow line is usually connected to injection specifications, mold and injection material, it can be defined as linear grooving, or round surges, on the surface of an injection molded part that shows the instructions of product flow within the cavity of the mold. When trying to establish process control in plastic injection, watering the tool is a key variable that is often overlooked. Machine. Weld lines can compromise the strength and aesthetics of the part, so it’s important to carefully design the mold and. Utilize hot. They usually are a slightly different color than the rest of the piece, and typically occur on more narrow sections of the molded item. 10(a) (please see Fig. Precise control of resin flow, pressure and temperature is critical. If during the design and development stage, watering is put on the back burner as an ". Impurity. Increase injection speeds and pressure to ensure the cavities are filled properly. 2. 182-185, 2013. Turbulent Flow Rate Calculator. The primary cause of cracking is stress and deformation brought on by the external environment, cold mold surfaces, and low injection rates. It is generally a three phase process comprising filling, packing and cooling phases. Plugged or inadequate vents n Clean plugged or dirty vents, particularly those along the parting line. The Mold. As a result, the lower degree of stress when gating with valve gate systems becomes relevant. Weld lines have been one of the common defects puzzling the injection molding industry. The molding process is the leading cause of defects in injection molding parts. Injection molded parts can have several different issues. The formation of weld lines and their location along the flow path is a well-studied phenomenon in conventional injection molding; however, only a few papers are devoted to micro-injection molding [11,13,14,15,16]. In the injection molding process of plastic parts, flow mark is a common defect due to the defects of parts design and injection molding process [4]. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. 005 0. Reduce the injection pressure. Minimum turbulent flow would be 12-14 GPM, while numbers in the 20’s would be identified as an excellent flow rate. com. thick, 0. 5. Flow marks can be caused by any or all of the following factors: Slow melt injection speed. Possible Causes & Remedies: Machine Cause. Faster flow results in longer, thinner knits. Flow Lines. Let’s take a look at some defects that are caused purely due to faults in the injection mould process. [22]. The gate in injection molding is an essential component that directly connects to the plastic part and controls the flow of melted plastic resin into the cavity. The best we can hope for is to pressurize the knit or meld line better through the combination of melt temperature, mold temperature, flow rate, and holding pressure. ; Burn marks – Often appears as black or rust-toned discoloration streaks on the edge or surface of a molded product. As discussed above, one of the things that contribute to plastic injection molding defects like sink marks and flow lines is the type of injection molding machine used. Rubber injection molding is defined as a process where uncured rubber is transformed into a usable product by injecting raw rubber material into a mold cavity made of metal. 025 100 120 140 160 180 200 220 240 Mold Temperature (F) LDPE PPWhen it comes to injection molding, the chances you might end up making a costly mistake is too high. The type of resin used during injection molding plays a large role in whether or not a knit line will be present. Weld line. Help prevent warping with these measures: Raise the mold or material cylinder temperature. Let’s take a look at some defects that are caused purely due to faults in the injection molding process. These are off-tone streaks or lines on the surface of the plastic, and while they don’t affect the structural integrity, they are unattractive and prevent a uniform and consistent appearance. These properties make it ideal for. At a theoretical flow distance of 10 mm it represented the measured values well but at a longer distance it underestimated the experimental scale. 1. Weld lines can be a weak spot and have therefore to be taken into consideration in part design. Melt delivery management has a major impact on final part quality and the ultimate success of the production line. This “inside-out” flow of the plastic mimics the flow in a water fountain, therefore it is called fountain flow. The process involves the injection of heated, liquefied plastic into a temperature-controlled mold under high pressure. Weld lines, also known as knit lines, are a common defect in injection molding where two or more flow fronts of molten plastic meet but don’t properly fuse together. Walls thicker than 8mm (0. Consider the following examples of cooling channel configurations in P20 mold steel with a water line diameter (D) of 11. Effects of mold temperature and pressure on shrinkage 0. Understanding the complexities of injection molding flow and solidification is a bit more challenging as. Flow lines are ripples in the plastic after the part is formed. 6. 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. In manufacturing, the Weld line or Knit line or Meld line is the line where two flow fronts meet when there is the inability of two or more flow fronts to "knit" together, or "weld", during the molding process. Factors within the tooling can also contribute to flow and knit lines, calling for mold modifications. If necessary, the mold designer may simulate the flow of solidification of resin using an injection molding simulation program. Inadequate venting may cause short-shots, poor surface appearance, or weak weld-lines. 16″) will be difficult to inject, leading to flow lines, surface imperfections, and short shots. 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. These streaks or patterns can affect a part's aesthetics, value, and overall quality. Increasing pack or hold pressure helps to eliminate low-pressure conditions at the weld line. Kobayashi et al. In injection molding weld lines are formed whenever two flow fronts are joined. Injection molding is the most cost-competitive technology for manufacturing high volumes of identical plastic parts. 1 – In plastic injection molding, the flow ratio is the distance molten plastic must travel, divided by the wall thickness of the part. Features such as sharp corners or undercuts can. If I can answer any questions you may have on optimizing the plastic part design, CNC plastic prototype, mold manufacturing, and injection molding or if I can be of any other help, please feel. How to recognize, prevent, and counteract flow lines defects in injection molding? Learn flawless products from professional manufacturers. We started the mold with 100% water circulating at 60°F at a flow rate sufficient for turbulence. wide. When trying to establish process control in plastic injection, watering the tool is a key variable that is often overlooked. Adjust resin and cooling temperatures. The line actually indicates the parting ‘plane’ that passes through the part. Strengthening. Weld lines are formed when there is an inability of two or more flow fronts to ‘knit’ or ‘weld,’ together during the molding process. Polycarbonate Injection molding defects. Learn more. The Mold. Pushing the air out or not is the difference between a good part or burns and flow lines. These lines can occur at the gate location, where the plastic enters the mold, or anywhere along the flow. SMARTFLOW ® flowmeters effectively and accurately measure flow rate. Source: Google. All of these can slow down the plastic flow, causing it to cool too quickly and lose pressure, both of which are the most likely reasons for weld lines in injection molding. 160"). These are caused by the shot of molten plastic moving at different speeds throughout the injection mold, which ultimately causes the resin to solidify at different rates. There is a level of insulation from the plastic layer on the wall. Flow lines are a typical problem for injection molding faults that appear as visible streaks or lines on the surface of the molded part. In this study, a new specimen design is introduced for the tensile testing of micro parts. This defect compromises aesthetics. They are also known as flow lines. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. The problem could be the result of machine settings made before the gate was installed. In injection molding, it is often important to maintain consistent color throughout the part. Flow velocity is very high in the area of the gate as it enters the mold, but slows down as the flow front extends away from the gate and into the cavity on a widening circle. Imagine a river splitting to go around a rock, then meeting up again on the other side. We restarted the mold at the same coolant flow rate and temperature. They can be caused by variations in cooling speeds of the material as it flows through the mold. Here are some of the most common scenarios for cracks in injection molded parts: Cracks in the Direction of Flow – If the crack develops in the direction of the plastic’s flow into the mold cavity, the cause is molecular orientation. They are evidence of nonuniformity in flow pattern upon entry into the mold. Learn about prevention methods and references. The injection speed has a great impact on the degree of molecular orientation in the component and therefore influences the shrinkage and the development of a joint line. Symptoms: Flow lines will typically appear as discolored “bands,” or sometimes “wavy. Polycarbonate is a tough, optically clear or opaque, biocompatible plastic with good thermal resistance. However, you can easily prevent such defects by simply adjusting the temperature, flow rate, or other mold parameters. Mold makers should also pay attention to sprue, runner, or gate design. A. (All Illustrations: S. #TROUBLESHOOTING #FLOWLINES #INJECTIONMOLDINGStudy more about troubleshooting flow lines :. Consequently, it causes viscous dissipation which leads to the generation of heat. By heating the solid material, it is converted into a fluid state with a low resistance to flow. It has larger cross-sectional regions that allow sufficient molten plastic flow into the cavity. The cause can vary. Before injection molding, there is a large amount of air in the flow path and the cavity. Peering into the Flow. ”. Causes: The issue might be varying flow rates as the molten plastic moves through the mold or changes in the thickness of the mold’s walls. Using cooling simulation to circuit your cooling lines properly. Once the mold is created and the machine is set up, additional parts can be manufactured very fast and at a very low cost. For the measurement of the crystallinity a DSC analysis was performed. The line where two resin flows “knit” or “weld” together during injection molding is called a weld or knit line. Mold flow analysis is a software simulation that can show how resin will fill the mold during the injection molding process. A knit line is when two flow fronts come together but, rather than merging, it’s like a head-on collision at a 4-way stop. Weld lines are injection molding defects that can be mitigated by a number of tactics that depend on your part design. ³/sec. This can be achieved by reducing the flow. injection molding. There are different levels of program available on the market. 005 0. 4 Injection Speed. The flow line is usually connected to injection specifications, mold and injection material, it can be defined as linear grooving, or round surges, on the surface of an injection molded part that shows the instructions of product flow within the cavity of the mold. Examine the flow pattern of each part to see if the plastic flow front is coming around on itself, or if there is a hesitation of the flow front in filling a thin section of the part. At some point in time, it can no. Reduce the injection pressure to prevent overpacking at the gate. A weld line (weld point, flow line or a knit line) forms when separate melt fronts travelling in opposite directions meet. flow rate, in turn, causes the heat transfer effi ciency of the cooling cha nnels to vary from one. Knit lines appear to be a faint line or a crack in the finished part after it comes out of the injection mold. Shortening the molding cycle time is a key factor in improving energy efficiency and since cooling occupies a major part of the cycle, effective design and operation of cooling systems is essential. Industry 4. 1. Results have shown that, although anisotropy is more pronounced in injection moldings, the mean of values for modulus along and at 90° to radial mold flow lines of an optimum modulus cure are the. The injection speed should remain consistent as the plastic enters the mold, otherwise, it will slow down, cool, and cause flow marks. ” What else can you do about weld lines—besides redesigning the mold or gate locations or adjust the flow pattern during fill with sequential valve gating? I recently saw a report on this issue from Moldex3D, the maker of molding simulation software. Complexities in the design and intricate pathways in the mold can also create flash in injection molded parts. (2011) and Fig. Cooling for slender cores (≤3 mm) uses air cooling instead of water. This continues until the weld lines in plastic injection molding disappear. Flow marks mainly occur in the. Flow lines – A common plastic injection molding defect in which discolored ring-shaped streaks, patterns, or wavy lines appear around gates or narrow sections of the finished product. 100 to 0. Below, we’ll look at five of the most common types of injection molding defects, and how you can prevent or repair them. They may also appear as ring-shaped bands on a product’s surface near the entry points of the mold, or “gates”, which the molten material flows through. Flow lines are linear grooving or circular ripples on the surface of an injection molded part that indicate the direction of material flow within the cavity. The goal when locating cooling channels in the mold is uniform mold surface temperature, which is determined by cooling channel depth and pitch. What are Flow Lines in Injection Molding? Flow lines, also known as weld lines or knit lines, are discontinuous lines or streaks that appear on the surface of an. The model of polymer melting is fundamental for the development of. This is a video excerpt from our Problems & Solutions Course - Jetting. Mold Venting. Phone (630) 665-8590. As the weld line can significantly lower the mechanical strength of µIM parts, it is relevant to clarify the position of weld. Knit lines, weld lines, mold lines, or flow lines in injection molding are faint lines that become visible on certain plastic parts. A method of melt flow visualization was used to record the weld line location on the surface of molding prepared at the proper injection conditions (method of "record grooves effect" formation. The following advantages exist for the use of high injection speed in the injection process. In this regard, the gate form with less shunt should be used, and the gate position should be selected reasonably. 004 m, a characteristic velocity ν 0 =0. Wavy lines are another feature of weld lines. FLOW LINE – A mark on a molded piece made by the meeting of two flow fronts during molding. 4. Flow lines are linear grooves or circular ripples on the surface of a molded object caused by uneven cooling, improper residence time, or cycle time, or low mold. When two or more strands of molten plastics meet in the mold, a weld line will be produced at the junction [1,2]. The chart below shows recommended wall thicknesses for common injection molding resins. The weld lines in injection moulded parts arise as a result of the collision of two fronts of the flowing material, which fills the mould cavity and are sometimes unavoidable. The position, length, and angle of weld lines are dependent on the gate location, injection speed, injection pressure, mold temperature, and, especially, the direction and degree of the polymer melt velocity in the. Mold flow analysis is an invaluable skill for injection molding engineers and designers, as it can help them create better products, reduce costs, and increase customer satisfaction. While the sprue is milled through the entire thickness of the injection mold, passing from the outer surface to the parting line (where the two mold halves meet), the runners are comparatively shallow channels milled into the face of one or two mold halves. Flow lines and flow marks. During the injection molding process, one of the most important requirements of the mold is to deliver melt from the barrel to the cavity. 1) Snake flow lines. to 0. In addition, when the melt is solidified in the cavity, there is a vacuum between the product and the cavity wall, also the air must enter the mold. The injection molding gate generates heat by dissipation. Unexpected flow lines, so-called tear drops, occur. Purpose: The examination of melt flow lines in injection moulded parts was the purpose of this research, as well as a new method of flow visualization was applied. Lesson 6: Meld and Weld Lines (Processing) When plastic flows into an injection mold, the internal flow pattern depends on the part design, mold design, and processing factors. The Fill system analyzes the filling of each cavity by sensing the melt presence at the end of flow with a cavity temperature sensor and compares the fill-time. The steel that cored out and created the notch was very thin—roughly 0. Its popularity is typified by the numerous products produced in this way at the present time. Sprues are usually tapered, and they come with ends that are bigger at the top. Jetting occurs in a molded component when molten material is injected into the mold cavity and starts solidifying before it fills the cavity. Increase the mold temperature. 2) show that the weld line is formed as expected from the middle of the cavity towards the wall. Defect categories. If you’re a mold designer with one of those vent-depth “rule-of. Conventional injection molding is widely used to produce plastic parts, mainly in the automotive industry, due to the high production ratios and quality of. Reducing the dimensions of the runner system. Thus, with an injection pressure of 1800 MPa, the flow length with 80°C is 47 mm and accordingly for. Figure 1 below shows the development of a weld line. In the injection molding process, weld line regions occur when a molten polymer flow front is first separated and then rejoined. Pack (hold time) is usually 2 to 3 times the mold fill times, depending on gate size. Some of the most popular brands include:Fiber reinforced thermoplastics (FRTP), reinforced with glass or carbon fibers, are used to improve the mechanical strength of injection-molded products. and most commonly, thermoplastic polymers are used. The linear mold shrinkage of injection molded parts will vary from 0. The strong molecular. Flow lines or marks appear as banded discolouration on the surface. also utilized the co-injection molding technique to study the flow of polypropylene at the weld line behind an obstacle in injection molding; the weld line can be seen clearly from the Fig. . The type of gate for manufacturing often depends on the material used and the parts’ shape. Each gate type has specifications that significantly affect the end product’s structure and appearance. The co-simulation of mold flow and structure plays an important role in the design of injection molded fiber-reinforced thermoplastic (FRT) parts. 020 0. When creating the parting line, the mold design is the first place to look for the appropriate parting line for an injection molded product. 5 to 0. In other words, the long-chain polymer. Another solution may be to pull a vacuum on the mold just before injection. sufficiently melted to a molten state in the barrel of the machine before the injection. 5 gal/min through this circuit we will get turbulent flow—the optimum cooling situation. Flow marks in injection molding can be avoided if adequate measures are taken. This most often occurs when you have a core or shutoff. The most common and one of the most challenging injection molding defects are weld and knit lines. Optimize the runner system and locate gates near the thin walls. A special mold provided with a glass window was used in order to directly evaluate the flow progression during the filling phase of the injection molding process in a thin-wall cavity and to. 015 0. To study the weld line formation of colliding flow fronts the filling of a mold cavity was simulated. Inadequate mold wall thickness leads to uneven cooling and hindered plastic flow, resulting in weld lines. The. I’ve been noodling that question around in my head for the last three years, since injection machine suppliers like Arburg, Engel, Wittmann Battenfeld and Sumitomo (SHI) Demag started showing up at trade shows with mold-filling displays on their machine controller screens. The co-simulation of mold flow and structure plays an important role in the design of injection molded fiber-reinforced thermoplastic (FRT) parts. Lastly, you can only acquire the benefits of parting lines in injection molding during plastic production if you design the parting line correctly. Design for Manufacturing (DFM), also known as pre-mold review, is a collaborative process in the injection mold industry where technical negotiations take place to meet customer requirements. Reposition gates and runners to allow resin to flow around the obstruction from a different direction. A void occurs during cooling while the part is inside or outside the mold. spelling: injection molding) is a manufacturing process for producing parts by injecting molten material into a mould, or mold. to 0. Mold surface is too cold. This may involve adjusting the temperature and pressure of the mold, using different mold materials, or adjusting the injection speed or cycle time. Flow lines appear like off-tone patterns on the surface of your finished parts. The good news is that flow lines are only an aesthetic concern and do not affect the strength or structural integrity of a part, which will perform as intended. After four years in development, an injection machine nozzle tip has been commercialized that is said to eliminate problems of stringing and all the compromises. Electronic flowmeters featuring FCI (Flow Characteristic Indicator). Defects like this make it harder to market the finished parts to the end user. More Control. Flow Lines The problem with flow lines is that they. Do not keep the hopper full; hand-feed if necessary. It is a well-known fact that weldlines are unavoidable in most injection-molded products of even moderate complexity. Flow lines. When two or more strands of molten plastics meet in the mold, a weld line will be produced at the junction [1,2]. 100 to 0. Thus, as the flow continues, the two flow fronts may gradually rejoin to. S. Spatter lines formed on the surface of the plastic along the direction of melt flow are called silver marks. 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. Features such as sharp corners or undercuts can. So what does the 140F mold temperature give us? The molding technician thought the 140F mold water was OK because the data sheet provided by the material supplier suggested mold temperatures from 75 to 180F. The production cost is closely related to productivity. Make sure the venting is not too large. Weld Lines-3 Ways for Detecting and Correcting. GPM: Measuring the amount of water passing through the mold every minute is a key measurable in determining turbulent flow. Select a plastic that is less prone to shrinkage. We offer this Mold Cooling Calculator tool as a free service to the injection molding industry. 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. Let’s review some methods to prevent flow and knit lines in plastic injection molding. Insufficient injection pressure or time.