First, excessive injection pressures may be forcing the mold apart. 7 in Kobayashi et al. The model of polymer melting is fundamental for the development of. "Design and Mould Flow Analysis of Injection Mould for Connecting Link," International Journal on Emerging Technologies, vol. Furthermore, jetting is frequently visible as a squiggly line on the surface of the final component, typically leading from the original injection gate. More about the knit lines in injection molding. 1 Caused By The Quality of Injection Mold. Injection molding could develop knit lines if one front cools before it reaches the other. This. Several factors contribute to the formation of flow lines, including material properties, mold design, and process parameters. Known as thermolators, water cooling systems require a cooling source such as a tower. This study focus on the formation of weld lines of molten polypropylene and. This “inside-out” flow of the plastic mimics the flow in a water fountain, therefore it is called fountain flow. According to the design requirements of the back shell of mobile phone and the performance of materials, ABS is selected as the raw material for injection molding in consideration of economy [ 8 ]. Injection molding is one of the most significant material processing methods for mass production of plastic products. 5. Countermeasures to Injection. Sprues are usually tapered, and they come with ends that are bigger at the top. To study the weld line formation of colliding flow fronts the filling of a mold cavity was simulated. 10 in Nguyen-Chung et al. Injection molding is the most cost-competitive technology for manufacturing high volumes of identical plastic parts. Weld lines typically appear in a part when an obstruction of some kind separates the melt flow and prevents it from "knitting" together. Flow marks, also known as flow lines, are molding defects that can occur in the manufacturing process of injection molding. 18/06/2021. This. 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. 0mm) throughout will ensure that the parts cool evenly and will prevent warping. Errors like these manifest when processing is done incorrectly. Prevent Flow and Knit Lines Through Mold Adjustments. INADEQUATE INJECTION PRESSURE. As a result, the lower degree of stress when gating with valve gate systems becomes relevant. 015 0. Cracking is commonly seen at the weld line area but sometimes on the entire surface. After an hour of running, the steel temperature stabilized at 78°F. Mold Design. 010 0. 4) Extended Shutdowns. 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). c. 160"). When two or more strands of molten plastics meet in the mold, a weld line will be produced at the junction [1,2]. Add or increase vents as needed. 025 inches per inch, depending on the particular resin, molding conditions, part design, part wall thickness, and direction of flow. Lower speeds allow gas more time to escape through vents. Plastics manufactured by injection molding are meant to be durable and reliable, able to hold up to heavy use, high temperatures, or adverse conditions. Midland, MI. The weld-lines are undesirable from both aesthetic and functional points of view and their effect is different for varied materials, process conditions, loadings, forms. . It can also aid in material selection since fill. If the mold temperature is too low, the material temperature will drop very fast, so the injection pressure and the holding pressure will not be high enough to press the solidified layer against the mold surface, thus leaving flow lines along the melt flow direction. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and. We started the mold with 100% water circulating at 60°F at a flow rate sufficient for turbulence. At some point in time, it can no. Bad habits can cause many different problems with injection molding machines, and flow lines are included in that. These flaws might have an impact on the product’s functionality as well as its appearance. Surface Delamination. Designers can easily set up and perform analysis with only a few clicks. 005 0. Impurity. By using proven methods for cooling line design, warp and cooling time are minimized. 20 mm. (2011) and Fig. Fictiv provides you with DFM feedback so that you can minimize injection molding defects. Fig. 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. These streaks or patterns can affect a part's aesthetics, value, and. Polycarbonate injection molding is a process that is used to create polycarbonate products at high production volumes and at a low cost per part by injecting melted plastic pellets into a 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. Flow Lines and Flow Marks. Causes: Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic. A. They occur as a result of different flow rates of molten plastic in the mold cavity. Drain the hopper if you were working with 100% regrind, clean if necessary. Injection moulding can be performed with a host of materials mainly including metals (for which the process is called die-casting ), glasses, elastomers, confections, and most commonly thermoplastic and. V. Cascade Moulding Avoids Weld Lines. Help prevent warping with these measures: Raise the mold or material cylinder temperature. The most common and one of the most challenging injection molding defects are weld and knit lines. Although injection molding is considered a fairly well-established manufacturing process, several independent parameters must be properly set to achieve a high quality of the molded parts. Reduce the injection pressure to prevent overpacking at the gate. However, the weld line may not continue throughout the flow. 1. Make sure the venting is not too large. Sink Marks. Definition: Off-tone streaks, patterns or lines that appear on parts. One of the things that can compromise an injection molded part is a weld line, also called a knit line. The chart below shows recommended wall thicknesses for common injection molding resins. Plastic injection molding is a complex and highly intricate process. On-Line Solutions To Injection Molding Problems. (All Illustrations: S. 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. 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. 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. Flow lines are lines or streaks visible on the surface of a part. Inadequate cooling time. Here are some common remedies for flow lines in injection-molded products: Increase the injection speed, pressure and material temperature to ensure the material fills the mold before cooling. Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties. Plastic is injected into a mold at a specific rate of flow—typically in. Inadequate mold wall thickness leads to uneven cooling and hindered plastic flow, resulting in weld lines. Wavy lines are another feature of weld lines. Thus, a right gate design raises polymer temperature to prevent the formation of weld lines and flow marks. It is a type of distortion caused by this premature solidification. wide. Check that the machine is creating enough clamping force to keep the mold halves together. The accompanying table lists the flow rates for various barrel diameters at various injection velocities. Thickness –Make the bottom of the boss 60% (0. Investigate the mold for signs that a water leak is present, and correct the condition. Mold makers should also pay attention to sprue, runner, or gate design. The formation process of weld lines can be described as. 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. 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. injection molding. If it’s too thin or small, material flow, pressure, and temperature will. (3) Improving the surface gloss of the product. The thermo-rheological findings were used to investigate the sources of weld line weakness. 2), the plastic enters the mold at a relatively fast rate and forms the skin. 100 to 0. Flow direction is essential for mold mapping because when water flow reverses, poor turbulence and hot spots occur. On the other end of the scale, a desktop injection molding machine with 5 tons of force can cost between $30,000-50,000. Part walls thinner than 4mm (0. #TROUBLESHOOTING #FLOWLINES #INJECTIONMOLDINGStudy more about troubleshooting flow lines :. Injection molding produces defects. It is also useful for finalized prototypes that are used for consumer and/or product testing. Randy Kerkstra has been in the plastics industry for more than 26 years, occupied frequently with troubleshooting injection molding. The flow rate should ensure the material does not lose its. Prevent Flow and Knit Lines During the Injection Molding Process. NX Mold Flow Analysis. Weld lines can compromise the strength and aesthetics of the part, so it’s important to carefully design the mold and. Warping. Injection molding, in addition to extrusion, is the most important technology in the polymer processing industry. As the name implies, it is usually positioned along the edge of the workpiece, forming a visible mark at the demarcating line. More viscous amorphous materials, such as PC, TPU or PEEK, can have a recommended vent depth range from 0. It can be difficult to determine what is truly splay, as its appearance is very similar to other defects. Raise the mold temperature, maintain a high. Image 4: Top, HDPE streamlined structure. Flow lines are ripples in the plastic after the part is formed. You can also adjust both pressure and temperature and make sure the clamp is strong enough to keep the material contained. Complexities in the design and intricate pathways in the mold can also create flash in injection molded parts. In this guide, we cover evaluation and also the remedy of flow lines in injection molding. Weld Line defect analysis, the most common of many defects in injection moulding products. Effects of mold temperature and pressure on shrinkage 0. Knit and meld lines are inherent to plastic. The recommended minimum production volume for injection molding is 500 units. Adjustments to injection speeds are the best way to determine which of these two are the specific cause. Low mold temperatures are usually employed to decrease cycle molding time and final process costs; however, resulting surface defects include weld lines, sink marks,. The flexibility of rubber and its durability offers manufacturers and engineers ways to design cushioning components and seals. Defects like this make it harder to market the finished parts to the end user. Remedies: Increasing the injection pressure will force the molten plastic against the mold cavity. Chart 1: Diameter versus turbulent flow. The primary cause of cracking is stress and deformation brought on by the external environment, cold mold surfaces, and low injection rates. The difference between a knit and meld line is significant, dramatically impacting the part’s structural integrity. 1) Snake flow lines. The melting temperature is 217–237 °C, and the thermal. The main. One cause of flow lines is low injection speed, which can result in the molten plastic cooling too quickly and creating a. Edge Gates. This continues until the weld lines in plastic injection molding disappear. This carbon will hang up in areas and is difficult to remove. Increase molten plastic temperature during injection molding. Injection molding could develop knit lines if one front cools before it reaches the other. Incorrect gate positions cause multiple flow fronts that struggle to merge seamlessly. Short Shots. If you’re a mold designer with one of those vent-depth “rule-of. Circuit Identification. Features such as sharp corners or undercuts can. Using cooling simulation to circuit your cooling lines properly. Detecting Water Flow Restrictions. Vacuum Voids. On that note, we. In other words, the long-chain polymer. The wavy lines or patterns that form on the surface of the molded object are what we refer to as flow marks. While there are many situations where they are barely perceptible, weldlines represent a potential source of weakness in molded parts. Certain injection molding processes may not be effective for the production of some parts. 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. In Fig. One way to reduce the changes in speed of the flow front is to tailor the injection-speed profile. 2. Plastic Defect # 2: Flow Lines. If possible, try to choose a little glue . In this lesson we will focus on the meld and weld lines. 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. Flow Lines particles, during injection molding, will produce visible flow and weld lines. When trying to establish process control in plastic injection, watering the tool is a key variable that is often overlooked. Molds separate into two sides at a parting line, the A side , and the B side , to permit the part to be extracted. Reduce the injection pressure to prevent overpacking at the gate. Kobayashi et al. Our injection molding company is explaining what these imperfections are, why they can compromise. These lines can occur at the gate location, where the plastic enters the mold, or anywhere along the flow. Inadequate mold wall thickness leads to uneven cooling and hindered plastic flow, resulting in weld lines. Factors within the tooling can also contribute to flow and knit lines, calling for mold modifications. By heating the solid material, it is converted into a fluid state with a low resistance to flow. Sink Marks. 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. Also called “Weld 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. Peering into the Flow. Knit lines are hard to eliminate for injection molded plastic parts but designers can minimize cosmetic defects. Impurities in the resin will prevent a smooth flow through the mold. Part Design: One of the important rules in part design is to have flow lengths that will let the plastic be fluid enough not just to flow into the end of fill but also until all the packing of the part is achieved. 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. Which means engineers need to know how to account for this phenomenon in an efficient way. 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]. 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. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. Flow rates and a fluid are input to the cooling channels,. The recommended minimum production volume for injection molding is 500 units. Thin walls often increase the pressure required to fill the part, and. This production method is particularly popular when manufacturing in large quantities. In the process of injection molding, the quality of the mold plays a decisive role in the quality of the finished product. That means that for each injection pressure, the flow length for a mould temperature of 80°C and 180°C shows a constant difference of ca. Once the mold is created and the machine is set up, additional parts can be manufactured very fast and at a very low cost. Possible Causes & Remedies: Machine Cause. Weld Lines. ”Poor venting, along with the diesel effect, can also cause the mold to wear on the mold parting lines, resulting in flash (excess plastic) on the injected part. Meld and knit lines are actually both different types of weld lines. Turbulent Flow Rate Calculator. Thickness – Make the ribs 50% to 60% (0. 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]. Melt delivery management has a major impact on final part quality and the ultimate success of the production line. 0 is leading the productivity and efficiency of companies to minimize scrap rates and strive for zero-defect production, especially in the injection molding industry. Bob Hatch is manager of technical service and customer support for Prime Alliance, the Des Moines-based resin distributor. Flow marks, also known as flow lines or weld lines, are common defects that can occur in plastic molded parts. The emergence of silver marks is generally due to the rapid start of the injection which causes the air in the melt and mold cavity unable to be discharged quickly enough. with a characteristic length χ 0 =0. That’s an easy calculation to perform. [1] #TROUBLESHOOTING #FLOWLINES #INJECTIONMOLDINGStudy more about troubleshooting flow lines : Weld lines occur when two flow fronts meet and fuse together in the injection molding process. In the injection molding process, weld line regions occur when a molten polymer flow front is first separated and then rejoined. Over time polymer will degrade creating carbon (black specks). Study shows that the root cause of jetting is related to the mold design on the one hand and the viscoelasticity of the material on the other. Flow Line Phenomenon: Surface defect in which wavy streaks form like a ripple centered around the gate. May 17, 2023. The flow rate should ensure the material does not lose its. Mold is not vented properly causing slow flow at knit lines. 3. FOW Mould is one of the best companies that use injection molding. When trying to establish process control in plastic injection, watering the tool is a key variable that is often overlooked. The part surface was observed in reflected light using optical microscope and the pictures of. Plastic is injected into a mold at a specific rate of flow—typically in. The molding process is the leading cause of defects in injection molding parts. Industry 4. 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. This method can be helpful in. Haagh, in Encyclopedia of Materials: Science and Technology, 2001 3 Modeling. 020 0. Flow marks occur as circles or lines that occur around the gate. Round the corners of the mold where wall thickness increases: This. 182-185, 2013. 2. In spite of its cosmetic attributes, a bad knit line can significantly. If so, look for an alternative resin. In injection molding weld lines are formed whenever two flow fronts are joined. 5. Thereby creating localized pressure within the mold. The parting line in injection molding is which one of the following: (a) the lines formed where polymer melt meets after flowing around a core in the mold, (b) the narrow gate sections where the parts are separated from the runner, (c) where the clamping unit is joined to the injection unit in the molding machine, or (d) where the two mold halves come. If you have critical requirements for strength of knit lines, please call a Protolabs applications engineer at 877-479-3680 to discuss, or email at [email protected]. Learn about prevention methods and references. Mold Design. Contact us to learn more about SMARTFLOW flowmeters. Poor mold surface n Refinish a poor or worn mold surface. Flow Lines. Most flow lines don’t impact the functio…Flow lines are a defect in injection molding and can be a significant issue for manufacturers. Burn Marks. 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. If it is farther down the flow part, increase injection speed to decrease viscosity and allow more packing pressure. Sink marks are like little craters formed on an injection-molded product. They are caused by improper bonding and low injection pressure and speed [ 46 ]. This design is deliberate in order to help the mold. Three terms that often get mixed up are weld, meld, and knit lines. In the area of lettering, ribs, domes or sharp wall thickness changes, air which cannot escape quickly enough is swept along, overrun by the melt and embedded. When injection molding a part, a knit line may present no problem, or can cause a serious structural problem. Just multiply the area of the barrel × the injection velocity. Also called “Weld Line. 5mm (0. Flow lines are linear marks on the surface of a molded product, which indicate the flow direction of the molten plastic. They are evidence of nonuniformity in flow pattern upon entry into the mold. Here are some ways manufacturers can avoid quality issues during the plastic injection molding process: Inadequate Injection/Holding Pressure: If injection and. Consequently, it causes viscous dissipation which leads to the generation of heat. These flow fronts occur when the molten plastic material encounters obstacles, such as inserts, ribs, or other features within the mold cavity. The change in flow-front velocity can bring about the surface defects. A mark on a molded piece made by the meeting of two flow fronts during molding. 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. 2. FLOW – A qualitative description of the fluidity of a plastic material during the process of molding. Injection molding is one of the best plastic production methods. Weld lines may appear like seams or scratches, but that is not what they are. As the polymer resin is injected into a mold, it flows through all parts of the mold. 2. You must also note that non-fill can be an extension of a flow line or knit line condition. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part. We started the mold with 100% water circulating at 60°F at a flow rate sufficient for turbulence. Conventional injection molding is widely used to produce plastic parts, mainly in the automotive industry, due to the high production ratios and quality of. Part design features may cause a material flow to split and rejoin as it flows around design features in a mold. Learn how to troubleshoot weld lines in injection molding. Weld lines are formed during cavity filling whenever two separated melt flow font recombine. Energy use by thermoplastics injection molding machines is estimated to result in global CO 2 emissions in the order of 80 million metric tons annually. Splay is a cosmetic defect that occurs in plastic injection molding, when silver or white streaks appear on the surface of the part. This can be accomplished by reducing the substance’s viscosity. Unexpected flow lines, so-called tear drops, occur. The Analysis and countermeasures of flow lines in injection molded products Read more. Depending on the resin, resin temperature, mold temperature, and filling speed, knit lines can vary. This is a video excerpt from our Problems & Solutions Course - Jetting. Injection molded parts can have several different issues. Flow lines are unwanted streaks, waves, or lines on a part’s surface. 150 in. Novel Nozzle Tip Prevents Stringing, Saving Molds & Cycle Time. 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. This requires skilled labor to mold new parts that have less molding defects. Another solution may be to pull a vacuum on the mold just before injection. Polymer Injection Molding: Gas-assisted Molding. In this article, a multiscale simulation method of polymer melt injection molding filling flow is established by combining an improved smoothed particle hydrodynamics method and clustered fixed slip-link model. 3. (4) It can improve the strength of the fusion line. The best method of venting this type of part (Fig. 005 0. During the injection molding process, one of the most important requirements of the mold is to deliver melt from the barrel to the cavity. Look for signs of stickiness or burning. Machine. Strengthening. How weld lines form. They are best described as "off tone" wavy lines/streaks or patterns in the molded part around the injection ports. The mold designer cannot know your goals without your input, which is why you will be asked to approve a gate and ejector layout prior to manufacturing a mold. 1). Laminar flow in a plastic mold, described by. In this way critical areas of the interface in regard to the lack of interdiffusion and the inappropriate molecular orientation were found to be placed near the surface of the finished parts. Some of these factors could be the flow rate, the pressure, or the temperature settings of plastic injection molding. They decrease the. 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]. flow rate, in turn, causes the heat transfer effi ciency of the cooling cha nnels to vary from one. Circuiting Done Right. For injection molding grades, the MFR (MI) values for polyethylenes are generally determined at 190°C (374°F) using a static load of 2,160 g. 1 Caused By The Quality of Injection Mold. Knit Line. Flow Linesparticles, during injection molding, will produce visible flow and weld lines. This is often a sign that injection speed and/or pressure are. Flow lines arise from variations in the flow of molten plastic material as it fills the mold. 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. 0030 in. What it is:Jan 04, 1999. Cascade Moulding Avoids Weld Lines. 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. Weld line. The steel that cored out and created the notch was very thin—roughly 0. 7 of the reference. For each investigated mould temperature, a linear relationship can be observed. These lines form as a result of material flow and solidification during the injection molding process. You can avoid having any weld lines with cascade injection moulding. Do not keep the hopper full; hand-feed if necessary. It affects its structural integrity and exterior appearance. Water set-up and design are every bit as important as establishing and recording a repeatable process. The definition of weld lines: When multiple plastic flow fronts meet in the cavity after encountering inserts, holes, or areas with inconsistent flow rates or interrupted melt flows, or in the case of gate injection filling, the inability of two or more flow fronts to fully “knit” together will cause the linear weld marks. The 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. They can be unsightly and unpleasant in high-end sunglasses. Utilize advanced injection molding machines with increased injection speed and pressure. Reduce the injection pressure. Weld lines have been one of the common defects puzzling the injection molding industry. 0 seconds, depending on part volume, runnergate style and size, cavity location and injection pressure. Using a thermoplastic that stays in a. Certain injection molding processes may not be effective for the production of some parts. Ensure sufficient cooling time to avoid locking residual stresses into the part. Phone (630) 665-8590. This happens when the molten plastic is injected into the mold cavity and flows around an obstacle such as a pin, hole, or protrusion, and then comes back together. Help prevent warping with these measures: Raise the mold or material cylinder temperature. There are three stages of flow – laminar, transient and turbulent. With a minimum of 1. 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. 6 T) as thick as the nominal wall. These streaks or patterns can affect a part's aesthetics, value, and overall quality. (All Illustrations: S. Mold design is a key aspect of injection molding. Imagine a river splitting to go around a rock, then meeting up again on the other side. Figure 7 below shows the cooling channels, defined in this case using simple sketch lines. A poorly designed or maintained water-cooling system can have a serious impact on production efficiency and cause many maintenance issues. On the contrary, for the conformal cooling. Flow Lines. On average, larger injection molding machinery can cost from $50,000-$200,000 or more. They appear as visible lines or streaks on the surface of the molded part and can negatively impact its aesthetics and mechanical properties. Flow lines can be minimized in injection molding by optimizing the design of the part and the mold, as well as the processing conditions. The line where two resin flows “knit” or “weld” together during injection molding is called a weld or knit line. Apart from the shape, it is important to design ribs for optimal mold performance. process can be fully completed. Short shots refer to incomplete filling of a mold cavity. In injection molding weldlines are generated when two separate melt streams join either. Figures 3A, B and C depict different examples of “spot” vents. Allows you to accommodate a variety of part sizes. Seven Common Defects and Solutions for PC/ABS injection molding Products,Flow Lines, Pitting,Product Delamination,Stretch Marks. The type of resin used during injection molding plays a large role in whether or not a knit line will be present. Below, we’ll look at five of the most common types of injection molding defects, and how you can prevent or repair them. In the development of our calculator. 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. Mold surface is too cold.