2. Causes – Remedies. In. Weld Line defect analysis, the most common of many defects in injection moulding products. Injection molding tool life is the number of production cycles that an injection mold can complete before it needs to be replaced. Cascade Moulding Avoids Weld Lines. For runny, non-viscous, crystalline resins such as nylon, PE, or PP, the recommended vent depth can range from 0. Help prevent warping with these measures: Raise the mold or material cylinder temperature. Flow Lines: Flow lines on thinner areas of molded components appear as a wavy pattern, often a different color than the surrounding area. Weld lines appear as a result of the flow of molten plastic during the injection molding process. Inadequate mold wall thickness leads to uneven cooling and hindered plastic flow, resulting in weld lines. 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. Conformal cooling channels (CCCs) are a cooling passageway which follows the profile of the mold cavity or core to perform uniform cooling process effectively in the injection molding process. A poorly designed or maintained water-cooling system can have a serious impact on production efficiency and cause many maintenance issues. Poor mold surface n Refinish a poor or worn mold surface. It is therefore important to work with reliable injection mold makers for quality services and products. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. The plant water system is a critical component of an injection molding facility. When modeling injection molding, the global approach is necessary to take into account the solid polymer transport, polymer melting and the polymer melt flow. Sources of defect: Material: Material too high viscosity; Method: Injection speed in the critical area too high (too low)Flow lines are one of the most common problems with injection molding. On thicker sections of the mold, there may be sinks caused by bosses designed into the backside of the surface. Depending on the resin, resin temperature, mold temperature, and filling. Faster injection increases pressure in front of a flow front. This slight depression is a tiny line called the weld line or knit line. The other parameters such as Mold temperature and Cooling Time also contribute in the defects appropriately. They decrease the. These lines. Drain the hopper if you were working with 100% regrind, clean if necessary. The location of the crack can tell us a lot about its specific cause. To further improve productivity, the injection mold was equipped with CCCs to shorten the cooling. 16″) will be difficult to inject, leading to flow lines, surface imperfections, and short shots. An improper flow rate and injection speed also cause flow marks. When the gate depth is much smaller than the cavity entrance depth, and the mold filling rate is very high, when the melt flow becomes unstable jet flow, the front jet has solidified and the later flowing melt fills the mold cavity, and the product will be Snake flow lines appear on the surface. When incorporating multiple gates it’s important to consider where the flows will meet as the meeting area can cause aesthetic issues and part weakness. We heat the plastic to a molten phase, inject it into the mold and cool it into a preferred shape. Machine. Injection molding could develop knit lines if one front cools before it reaches the other. 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. So much. Vacuum Voids. Knit lines, weld lines, mold lines, or flow lines in injection molding are faint lines that become visible on certain plastic parts. 6T) as thick as the nominal wall. Apart from the shape, it is important to design ribs for optimal mold performance. Flow Lines. The mold material 6 can have several effects on. Injection Molding works with molten fluid created by heating the plastic granules above their melting point. Turbulent Flow Rate Calculator. Fictiv provides you with DFM feedback so that you can minimize injection molding defects. Basic Repairs for Warping. Remedies: Raise the temperature of the mold or molten plastic. or too much shear heat burning the material prior to injection. Before injection molding, there is a large amount of air in the flow path and the cavity. One of the best ways to determine part shrinkage for complex components is with a mold flow analysis. Adjustments to injection speeds are the best way to determine which of these two are the specific cause. Image 4: Top, HDPE streamlined structure. The leading edge of the flow, or the melt. I’ve been writing about plastic flow simulation since 1984, and I. 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. Minimum turbulent flow would be 12-14 GPM, while numbers in the 20’s would be identified as an excellent flow rate. Vary injection speed, screw rpm, and backpressure. When trying to establish process control in plastic injection, watering the tool is a key variable that is often overlooked. Increase the injection speed and mold/operational temperature to prevent fracture or surface breakdown. Pushing the air out or not is the difference between a good part or burns and flow lines. 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. [1]#TROUBLESHOOTING #FLOWLINES #INJECTIONMOLDINGStudy more about troubleshooting flow lines : lines occur when two flow fronts meet and fuse together in the injection molding process. Flow lines are lines or streaks that appear on the surface of molded parts and occur due to variations in the flow of molten material during the injection molding process. Flow Lines The problem with flow lines is that they. Because you have control over exactly when and how much material enters the tool, you have tighter control of the parameters that go into making a quality part. Use high-quality, temperature-controlled materials for better mold filling. Plastic Purity. Reasons and solutions. 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. 020 0. Learn how mold flow analysis can support the development of complex and customized injection molded products by identifying defects, optimizing materials, enhancing performance, and reducing costs. Knit lines are hard to eliminate for injection molded plastic parts but designers can minimize cosmetic defects. Flow lines or flow marks are a common visual defect in injection molding. 5 to 2. As a result, the lower degree of stress when gating with valve gate systems becomes relevant. Flow lines arise from variations in the flow of molten plastic material as it fills the mold. They can be unsightly and unpleasant in high-end sunglasses. They can be off-color or discolored material compared to the intended color of the part. Clean all parting line and core vents. One exception: ribs should be about 50. Each needle valve nozzle opens only when the melt front has moved across its gating point. If necessary, the mold designer may simulate the flow of solidification of resin using an injection molding simulation program. Strengthening. Weld Lines-3 Ways for Detecting and Correcting. 3,000 ton machines can cost $700,000. Bad habits can cause many different problems with injection molding machines, and flow lines are included in that. Increasing pack or hold pressure helps to eliminate low-pressure conditions at the weld line. After an hour of running, the steel temperature stabilized at 78°F. 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. Molders should make sure the weld line is formed during the first-stage filling of the part, if possible. WeInjection molding is the most widely used form of plastics processing worldwide. Materials that have physical fillers like glass, carbon, metal flakes, etc. Symptoms: Flow lines will typically appear as discolored “bands,” or sometimes “wavy. Accept All. If possible, try to choose a little glue . 9 – Over Packing. Occasionally, the molded part has. Weld lines in injection molding are formed when two or more flow fronts of molten plastic material meet and merge during the molding process. google-site-verification: google03af4f4fcb62a9d5. While there are many situations where they are barely perceptible, weldlines represent a potential source of weakness in molded parts. Our injection molding company is explaining what these imperfections are, why they can compromise. Flow lines and flow marks. 1. Melt delivery management has a major impact on final part quality and the ultimate success of the production line. injection mould simulation at steady flow rate. 4, no. Siemens PLM Software. Cooling lines impact. Weld line defects – also known as knit lines – is a defect that occurs when the injected plastic meets within a mold. When trying to establish process control in plastic injection, watering the tool is a key variable that is often overlooked. 0005 in. Flow Line. They are also known as flow lines. Water set-up and design are every bit as important as establishing and recording a repeatable process. 015 0. With this new discovery, P&G decided to launch iMFLUX Inc. This means that the skin of the part is always formed by. 1 a. There are different levels of program available on the market. Flow lines appear as a wavy pattern often of a slightly different color than the surrounding area and generally on narrower sections of the molded component. They usually are a slightly different color than the rest of the piece, and typically occur on more narrow sections of the molded item. Available with a number of options including thermometers, pressure gauges, liquid-filled pressure gauges and quick change socket and plug. How a mold is watered is key to a processor's goal of consistency. This is a video excerpt from our Problems & Solutions Course - Jetting. Increase molten plastic temperature during injection molding. Three terms that often get mixed up are weld, meld, and knit lines. Effects of mold temperature and pressure on shrinkage 0. Bubbles are caused by melting flow fronts colliding in a mold cavity. As far as the mold builders are concerned, having good flow through each circuit and having a good pressure leak test that they can perform is an. In the injection molding process, weld line regions occur when a molten polymer flow front is first separated and then rejoined. The process involves the injection of heated, liquefied plastic into a temperature-controlled mold under high pressure. Increase the injection speed, pressure, or temperature of the material. Gate Placement: Properly positioning the gate can direct the melt flow to minimize weld lines or relocate the weld line to non-visible areas of the part. For each investigated mould temperature, a linear relationship can be observed. They are evidence of nonuniformity in flow pattern upon entry into the mold. If performing a resin change from one polymer to another a difference in viscosity can cause flow lines or gels. Thus, with an injection pressure of 1800 MPa, the flow length with 80°C is 47 mm and accordingly for. ” 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. Part walls thinner than 4mm (0. They appear as waves or lines, usually of a different color tone from the surrounding area. Micro weld lines are the common problem that occurs for plastic molded parts through injection molding process. A mark on a molded piece made by the meeting of two flow fronts during molding. Lower speeds allow gas more time to escape through vents. The mold in Figure 6 is an 8-cavity with an “H” pattern runner and a lapped edge gate. In a weld line, reinforcement fibers. This results in a much greater amount of vent flow area for the compressed air to exhaust through, and still protects the corners. Plastic Injection Molding Process. Hamstra: For the OEMs, having a good baseline of what our cooling flow and temperature through a mold is going to give us a good optimization into the next supply chain for the injection molders. The formation process of weld lines can be described as. Incorrect gate positions cause multiple flow fronts that struggle to merge seamlessly. Source: Google. The last cause of flow lines may be the operators running your daily operations. The injection speed should remain consistent as the plastic enters the mold, otherwise, it will slow down, cool, and cause flow marks. 005 0. Avoiding Voids and Sinks. The article has been structured just for you to gain an understanding of the same so that when you plan to buy plastic injection molding, you do understand what it means to do so. Increase the size of the gate and reduce the thickness of the walls of the molded part. 7 in Kobayashi et al. Weld lines are formed when there is an inability of two or more flow fronts to ‘knit’ or ‘weld,’ together during the molding process. Consequently, it causes viscous dissipation which leads to the generation of heat. Flow fronts moving too slowly can lead to pressure in front of a flow front being too low to force gas through vents. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. Increase the injection speed. Setting the proper pressure on the injectors to push the plastic into the mold with enough speed that the plastic will coat the mold and fuse with other flow lines. Edge Gates. Raise the mold temperature, maintain a high. The formation process of weld lines can be described as follows: Injection moulding (U. We restarted the mold at the same coolant flow rate and temperature. It can also aid in material selection since fill. Optimize the runner system and locate gates near the thin walls. They can also appear as ring-shaped bands on the surface of a product near the mold’s “gates. Draft – Add 0. The splay condition can completely cover a part, or it can appear inconsistently in various areas on the part. See. Look for signs of stickiness or burning. The speed of an injection moldingRepeat the two previous steps two or three times, starving the screw periodically to allow the screw to rotate without building a shot. One way to reduce the changes in speed of the flow front is to tailor the injection-speed profile. Kobayashi et al. Flow Lines. This defect compromises aesthetics. 5° draft to each side of the rib. Avoid knit lines when designing a molded part to improve cosmetic appearance and functionality. Low cushion and hold time. Add more cooling circuits. Imagine a river splitting to go around a rock, then meeting up again on the other side. The molding process is the leading cause of defects in injection molding parts. Weld lines in injection molding occur when two flow fronts meet and create a visible line on the surface of the molded part. 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. Flow lines result from uneven cooling of the plastic within the mold. By altering the part design, it may be possible to just have one flow front, and avoid multiple flow fronts having to come together. INADEQUATE INJECTION PRESSURE. ³/sec. 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. Just multiply the area of the barrel × the injection velocity. Our glossary of terms provides definitions for a wide range of injection molding terms. Listed below are some factors to consider when designing ribs for injection-molded parts. The material is kept poured into the mold for some time. The weld line and the flow pattern of co-injection molding can be seen from Fig. Introduce automation into the injection molding process for improved efficiency. . In injection molding, it is often important to maintain consistent color throughout the part. The Reynolds number takes into account the pressure, volume and viscosity of the coolant, the resistance to flow, length and diameter of the channels and the pressure loss in the cir-cuit. 008 to 0. Solution: Molding Process: This defect can be reduced by raising the material temperature and the nozzle temperature, and slowing down the injection speed, etc. There are two types of weld lines, cold and hot weld line. That’s an easy calculation to perform. These lines can occur at the gate location, where the plastic enters the mold, or anywhere along the flow. c. Depending on the resin, resin temperature, mold temperature, and filling speed, knit lines can vary. FLOW – A qualitative description of the fluidity of a plastic material during the process of molding. The part surface was observed in reflected light using optical microscope and the pictures of. 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]. What Are Flow and Knit Lines? Flow lines are defects that appear as wavy. • u. Unraveling Flow Lines. In the injection molding process, weld line regions occur when a molten polymer flow front is first separated and then rejoined. Thus, as the flow continues, the two flow fronts may gradually rejoin to. Possible Sources of Weld/Knit Lines: Temperature from the injection molding machine, runners, and/or the mold itself in order to ensure the plastic resin flows properly; Insufficient pressure resulting in the flow fronts not being merged back together with strong enough; Mold design, especially location of gates and runners impacting the resin flow 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). NX Mold Flow Analysis. Weld lines have been one of the common defects puzzling the injection molding industry. Adjust resin and cooling temperatures. The line actually indicates the parting ‘plane’ that passes through the part. Voids . The size, shape, and location of the gate have a significant impact on the finished product. 004 m, a characteristic velocity ν 0 =0. 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. This method can be helpful in. When two or more strands of molten plastics meet in the mold, a weld line will be produced at the junction [1,2]. ”. (1998) for clear photographs. If so, look for an alternative resin. 100 to 0. 5 gal/min through this circuit we will get turbulent flow—the optimum cooling situation. Aesthetics, such as injection molding gate marks, sink marks, voids and flow lines In some cases, more than one gate may be necessary to achieve the desired outcome. Each needle valve nozzle opens only when the melt front has moved across its gating point. Optimize the runner system and locate gates near the thin walls. Learn how to troubleshoot weld lines in injection molding. It is widely used in various industry sectors, and its products are ubiquitous in our daily life. Excess Mold Release Poor mold design, including inappropriate mold wall thickness and faulty gate placements, is a common cause of weld line formation in injection molding. Co-injection molding technique is a useful way to study weld lines, which can offer more detailed information of the. 1 m/s, and a zero-shear-rate viscosity η 0 (T 0)=3760 Pas. Flow lines are lines or streaks that appear on the surface of molded parts and occur due to variations in the flow of molten material during the injection molding. 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 type of gate for manufacturing often depends on the material used and the parts’ shape. Thus, as the flow continues, the two flow fronts may gradually rejoin to make one uninterrupted flow front. Plastics manufactured by injection molding are meant to be durable and reliable, able to hold up to heavy use, high temperatures, or adverse conditions. Lookalikes include surface scratches, scuffs, flow lines, delamination from contamination, and flaking. A gradual change in wall thickness is recommended to avoid any sudden change in the material flow direction. The type of resin used during injection molding plays a large role in whether or not a knit line will be present. Increase injection speeds and pressure to ensure the cavities are filled properly. There are a few adjustments that you can make to your injection molding process to prevent flow lines, including: Increase the injection speed, pressure, or material temperature: Slow-moving molten plastic is more likely to cool. Reduce the injection pressure to prevent overpacking at the gate. Mold Design. Weld lines have been one of the common defects puzzling the injection molding industry. 080" – 0. Injection molding is the most cost-competitive technology for manufacturing high volumes of identical plastic parts. Short shots refer to incomplete filling of a mold cavity. When molten plastic moves through an injection mold at different speeds, the resin solidifies at different rates, which causes this type of defect. Adjustments to injection speeds are the best way to determine which of these two are the specific cause. 3C) is to put a vent along the entire edge, but just shy of the corners. html. At some point in time, it can no. The. 010 0. Here are the top 5 injection molding defects that can occur, and how to solve them: Flow / Weld Lines. Heat Input and Cooling Calculator for Injection Molding. Mold fill usually takes 0. Taking proper care to design the mold properly, avoiding bottlenecks, and changing the number or location of gates can improve performance. Obviously, as the resin front moves through the mold, the fill material will always be behind the front. 6. 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. We stopped the machine and added glycol to the chiller to a concentration of about 40%. The Bottom Line. 2), the plastic enters the mold at a relatively fast rate and forms the skin. Shear; There are two causes of splay brought on by shear. Flow marks won. This is defined for each material by the length-to-thickness (L/T) ratio of the part. 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. Once the mold is created and the machine is set up, additional parts can be manufactured very fast and at a very low cost. 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). Non-uniform cooling of mold causes warping, creating internal stresses within the part and resulting in distortions. This is often a sign that injection speed and/or pressure are. Figure 1 presents the shrinkage of polypropylene versus wall thickness in injection molded parts. 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 co-simulation of mold flow and structure plays an important role in the design of injection molded fiber-reinforced thermoplastic (FRT) parts. From the perspective of the mold, when the molten plastic reaches the gate through a larger-sized runner, the flow resistance of the melt is greatly increased because the gate size is usually very small. Then the melt is injected into the mold and the surface of injection-molded products is. 3. In Fig. Otherwise, the heat transfer of. Different gate sizes and locations using flow simulation have been detected for defects reduction such as weld lines and air traps, air traps and warpage can be managed by varying process parameters [17]. 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. First, excessive injection pressures may be forcing the mold apart. Laminar flow in a plastic mold, described by. After exhaustive research, testing and pilot-scale demonstration, the team developed a breakthrough new technology utilizing low constant injection pressure. spelling: injection molding) is a manufacturing process for producing parts by injecting molten material into a mould, or mold. If it’s too thin or small, material flow, pressure, and temperature will. You can avoid having any weld lines with cascade injection moulding. 2. Injection molding (or injection moulding) is a widely used manufacturing process in many different industries such as aerospace, automotive, medical, and even. These benzene rings also increase the viscosity, reduce shrink rate, and increase strength with the exception of knit and/or meld lines. In the development of our calculator. Since these materials behave and respond differently, it is crucial. Avoid Drag in Molded Parts. There are several factors that can contribute to the formation of weld lines, including: Part design: The design of the part can impact the flow of the material and the likelihood of weld lines forming. With a minimum of 1. 160"). Mold flow analysis is also used to help designers locate difficult spots to fill within the mold. . However, the weld line may not continue throughout the flow. When mapping mold circuits, label each circuit with a number exclusive to the circuit and a supply or return designation (e. Phone (630) 665-8590. 0010 in. 015 0. In injection molding weld lines are formed whenever two flow fronts are joined. Prevent Flow and Knit Lines During the Injection Molding Process. (2) Adjust the injection speed: high speed allows the melt to reach the confluence without cooling down, and low speed allows time for the air in the cavity to be discharged. One of the things that can compromise an injection molded part is a weld line, also called a knit line. 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. Request a same-day quote to see how our medical injection molding experts at Aberdeen Technologies can help move your project forward. 1). (4) It can improve the strength of the fusion line. There are two halves to the injection mold that must come together and at the interface there is a chance of a parting line. Weld lines may appear like seams or scratches, but that is not what they are. Lower speeds allow gas more time to escape through vents. Mold surface is too cold. This software simulation can show how a resin will fill the mold during the injection molding process. The position, length, and angle of weld lines are dependent on the. When injection molding a part, a knit line may present no problem, or can cause a serious structural problem. 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. When two or more strands of molten plastics meet in the mold, a weld line will be produced at the junction [1,2]. We offer this Mold Cooling Calculator tool as a free service to the injection molding industry. Venting is required to allow the air in the sprue, runner and cavity to leave the tool as the melt flows into the cavity. Sprues are usually tapered, and they come with ends that are bigger at the top. 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. The flow rate should ensure the material does not lose its. and most commonly, thermoplastic polymers are used. Depending on the extent, these can become defects in the case of molded products in which their external appearance is an important aspect of quality, such as in the case of home electrical appliances, containers for. These data were obtained from many andGenerally, injection molding resins are characterized as having medium, high or very high flow. Thin wall injection molding can produce walls as thin as 0. Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties. The melting temperature is 217–237 °C, and the thermal. 5. They occur as a result of different flow rates of molten plastic in the mold cavity. 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. 4 Injection Speed. We will now look at the problem you are likely to encounter and how to fix them. 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. They are typically the same color as the part but are a different tone. Causes: Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic. The materials used in the plastic injection molding process can vary in rigidity and pliability, among other differentiating characteristics. Recently hired at plastic injection molding simulation software provider Moldex3D, Nicholson was spurred to undertake the research study following a conversation with a customer experiencing weld-line woes. (2) Improving the flow of plastic and facilitating the molding of thin-walled parts. Weld lines typically appear in a part when an obstruction of some kind separates the melt flow and prevents it from "knitting" together. 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]. By heating the solid material, it is converted into a fluid state with a low resistance to flow. Increasing injection speed. Water set-up and design are every bit as important as establishing and recording a repeatable process.