Certain injection molding processes may not be effective for the production of some parts. For optimal cooling, these waterlines are located as close as possible to the surface of the molded part. 10 in Nguyen-Chung et al. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part. This. We restarted the mold at the same coolant flow rate and temperature. 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. 1．Mold Temperature Too Low. A weld line (weld point, flow line or a knit line) forms when separate melt fronts travelling in opposite directions meet. Flow Lines and Flow Marks. 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. Introduce automation into the injection molding process for improved efficiency. Polymer Injection Molding: Gas-assisted Molding. Investigate the mold for signs that a water leak is present, and correct the condition. We started the mold with 100% water circulating at 60°F at a flow rate sufficient for turbulence. Part design features may cause a material flow to split and rejoin as it flows around design features in a mold. Height – Make the ribs 3 times the height of the nominal intersecting wall (3T). See. It affects its structural integrity and exterior appearance. 3. Knit Line. Flow lines are lines or streaks visible on the surface of a part. Peering into the Flow. Flow fronts moving too slowly can lead to pressure in front of a flow front being too low to force gas through vents. Although they typically do not impact the functionality of a product, flow lines—off-color streaks or patterns across a product’s surface—may look unprofessional when visible to consumers. Three terms that often get mixed up are weld, meld, and knit lines. Flow lines, also referred to as flow marks or weld lines, are visible lines or patterns on the surface of a plastic component. Flow Rate. Mold Flow - Position ribs such that the plastic has an optimal flow path. Flow lines and weld lines are lines or streaks in the final molded part. Flow Lines The problem with flow lines is that they. The Mold. 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. 020 0. Prior to this late stage in production, however, 3D printing is much more affordable and flexible for products in the early stages of design. The injection speed should remain consistent as the plastic enters the mold, otherwise, it will slow down, cool, and cause flow marks. 182-185, 2013. 0010 in. From a technical point of view, valve gate technology enables the production of low-stress injection molding parts, which almost always meet the requirements of a very low vestige. A. This software simulation can show how a resin will fill the mold during the injection molding process. 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. Facebook. 1mm (7/16"), coolant temperature of 30ºC and cycle time of 17 seconds (see Fig. 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. They again get resolidified along the surface. On larger parts where knit lines or flow lines are a concern, you can use sequential valve gates to control the flow and move the knit lines. Flow lines appear as a wavy pattern on the surface of your plastic parts. The mold settings can run flawlessly until employees consistently open the machine for inspections, interrupt cycle times, or cause temperature. Learn how to troubleshoot weld lines in injection molding. 10(a) (please see Fig. ”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. Weld-line strength for plastics can vary from 20% to nearly 100% of the strength of the plastic itself. Electronic flowmeters featuring FCI (Flow Characteristic Indicator). Increase the injection speed. 5 to 2. The location of the crack can tell us a lot about its specific cause. We will now look at the problem you are likely to encounter and how to fix them. The size, shape, and location of the gate have a significant impact on the finished product. These streaks or patterns can affect a part's aesthetics, value, and. Most flow lines don’t impact the functio…Flow lines are a defect in injection molding and can be a significant issue for manufacturers. Mold design is a key aspect of injection molding. The materials used in the plastic injection molding process can vary in rigidity and pliability, among other differentiating characteristics. Injection Molding (IM) Solutions 1019 Balfour St. Listed below are some factors to consider when designing ribs for injection-molded parts. It is also useful for finalized prototypes that are used for consumer and/or product testing. Description: Flow lines are streaks, patterns, or lines - commonly off-toned in color - that show up on the prototype part as a consequence of the physical path and cooling profile. Pack (hold time) is usually 2 to 3 times the mold fill times, depending on gate size. ”. These flow fronts occur when the molten plastic material encounters obstacles, such as inserts, ribs, or other features within the mold cavity. ” FLOW MARKS – Wavy surface appearance on a molded object caused by improper flow of the material into the mold. Features such as sharp corners or undercuts can. 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. Round the corners of the mold where wall thickness increases to help keep flow rate consistent and prevent flow lines. In the development of our calculator. 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. Also called “Weld Line. This action could indicate that one portion of the plastic flow will move more quickly than the other. 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. The formation process of weld lines can be described as follows: Injection moulding (U. Cascade Moulding Avoids Weld Lines. Short shots refer to incomplete filling of a mold cavity. Machine. Weld lines have been one of the common defects puzzling the injection molding industry. 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. Thereby creating localized pressure within the mold. Flow Lines. RapidDirect – Your Best Injection Molding Partner. On thicker sections of the mold, there may be sinks caused by bosses designed into the backside of the surface. 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. or too much shear heat burning the material prior to injection. Molds separate into two sides at a parting line, the A side , and the B side , to permit the part to be extracted. When trying to establish process control in plastic injection, watering the tool is a key variable that is often overlooked. 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. b. This most often occurs when you have a core or shutoff feature that the plastic needs to flow around. The main causes for short shots are inappropriate injection pressure and speed. Shear; There are two causes of splay brought on by shear. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. One way to reduce the changes in speed of the flow front is to tailor the injection-speed profile. When the mold is full, the machine continues its injection pressure and compresses the plastic. 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. Look for signs of stickiness or burning. Weld line defects – also known as knit lines – is a defect that occurs when the injected plastic meets within a mold. Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties. The differences between knit and meld lines are significant, and they can dramatically impact the part’s structural integrity. We started the mold with 100% water circulating at 60°F at a flow rate sufficient for turbulence. These lines can occur at the gate location, where the plastic enters the mold, or anywhere along the flow. (All Illustrations: S. Flow marks, also known as flow lines or weld lines, are common defects that can occur in plastic molded parts. The recommended minimum production volume for injection molding is 500 units. Using a thermoplastic that stays in a. How to recognize, prevent, and counteract flow lines defects in injection molding? Learn flawless products from professional manufacturers. 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. Listed below are some factors to consider when designing ribs for injection-molded parts. SMARTFLOW ® flowmeters effectively and accurately measure flow rate. Flow lines are a defect in injection molding and can be a significant issue for manufacturers. Production was set according to an experimental design (DoE), varying injection flow rate, mold temperature, and melt temperature. When the melt enters the cavity quickly, it is necessary to discharge the air quickly. It is therefore important to work with reliable injection mold makers for quality services and products. Defects like this make it harder to market the finished parts to the end user. Increase the injection speed and mold/operational temperature to prevent fracture or surface breakdown. However, the weld line may not continue throughout the flow. The accompanying table lists the flow rates for various barrel diameters at various injection velocities. #TROUBLESHOOTING #FLOWLINES #INJECTIONMOLDINGStudy more about troubleshooting flow lines :. 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. In the process of injection molding, the quality of the mold plays a decisive role in the quality of the finished product. Too hot and the plastic will start to degrade, but too low and flow marks increase. Mold fill usually takes 0. Each material has a maximum flow length for a given injection pressure and wall thickness — this is known as the flow ratio. Reduce the injection pressure to prevent overpacking at the gate. Spatter lines formed on the surface of the plastic along the direction of melt flow are called silver marks. process can be fully completed. This carbon will hang up in areas and is difficult to remove. 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. The melting temperature is 217–237 °C, and the thermal. The primary cause of cracking is stress and deformation brought on by the external environment, cold mold surfaces, and low injection rates. diverse parameters of the injection-molding process, needs a lot of experience in order to achieve optimum results, especially if. Leadtime Leader 2024. For example, baffles require water to flow in a. 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. When molten plastic moves through an injection mold at different speeds, the resin solidifies at different rates, which causes this type of defect. 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. A. 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. This method can be helpful in. They are the result of fundamental physical effects taking place inside of the mold. Its popularity is typified by the numerous products produced in this way at the present time. 3). • u. Weld lines are a common imperfection to be found in most plastic injected molded parts. Do not keep the hopper full; hand-feed if necessary. Flow lines, also referred to as flow marks or weld lines, are visible lines or patterns on the surface of a plastic component. Materials that have physical fillers like glass, carbon, metal flakes, etc. Lenticular, or Silver Dollar type pigments, are thicker than cornflakes, with a narrower particle size distribution. Weld lines in injection molding are formed when two or more flow fronts of molten plastic material meet and merge during the molding process. In injection molding weld lines are formed whenever two flow fronts are joined. Thus, as the flow continues, the two flow fronts may gradually rejoin to. Avoid knit lines when designing a molded part to improve cosmetic appearance and functionality. Flow marks occur as circles or lines that occur around the gate. The part surface was observed in reflected light using optical microscope and the pictures of. 004 m, a characteristic velocity ν 0 =0. html. Minimum turbulent flow would be 12-14 GPM, while numbers in the 20’s would be identified as an excellent flow rate. Note: It is applicable to thermoplastic materials only. The material is poured into a mold, which shapes the size and looks of the article. The following are the main elements of a gating system designed for injection molding. 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. This method can be helpful in. 7. Injection molding is the most cost-competitive technology for manufacturing high volumes of identical plastic parts. Utilize advanced injection molding machines with increased injection speed and pressure. Flow lines were created during injection moulding process. How a mold is watered is key to a processor's goal of consistency. Plastic Defect # 2: Flow Lines. Basic Repairs for Warping. PROBLEM: FLASH ON PARTING LINE There are a few key reasons why flash appears on the parting line of a part. 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. Certain injection molding processes may not be effective for the production of some parts. The type of gate for manufacturing often depends on the material used and the parts’ shape. Knit lines appear to be a faint line or a crack in the finished part after it comes out of the injection mold. 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. Steven. There are three stages of flow – laminar, transient and turbulent. Effects of mold temperature and pressure on shrinkage 0. Make sure the venting is not too large. 025 inches per inch, depending on the particular resin, molding conditions, part design, part wall thickness, and direction of flow. Detecting Water Flow Restrictions. Flow lines result from uneven cooling of the plastic within the mold. 010 0. 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. Once you have a cool shell formed the rest of the mold entering absorbs heat from the layer next to the wall. Weld lines are a kind of Achilles heel of plastics. In. Vacuum Voids. Injection molding, in addition to extrusion, is the most important technology in the polymer processing industry. [12] or the flow lines shown in Ref. Understanding the complexities of injection molding flow and solidification is a bit more challenging as. As the polymer resin is injected into a mold, it flows through all parts of the mold. 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. This continues until the weld lines in plastic injection molding disappear. (4) It can improve the strength of the fusion line. Laminar flow in a plastic mold, described by. Knit and meld lines are inherent to plastic. If the mold itself has cracks or defects, the finished product will also have various problems. Flow marks won. Weld lines may appear like seams or scratches, but that is not what they are. 5. 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. g. . (All Illustrations: S. Here are the top 5 injection molding defects that can occur, and how to solve them: Flow / Weld Lines. Injection molding is one of the most significant material processing methods for mass production of plastic products. In this guide, we cover evaluation and also the remedy of flow lines in. This new process enables thinking differently about how parts and molds can be designed. Bottom, PC with random structure. They commonly occur when there is a large variation between cooling speeds of sections of the material as it flows through the mold. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. Mold flow analysis is a powerful tool for optimizing the design and performance of injection molding processes. Injection molding is the most cost-competitive technology for manufacturing high volumes of identical plastic parts. The Plastic Troubleshooter Defect - Flow Lines. This means that the skin of the part is always formed by. It can be difficult to determine what is truly splay, as its appearance is very similar to other defects. 5-3. I’ve been writing about plastic flow simulation since 1984, and I. Depending on the resin, resin temperature, mold temperature, and filling. Possible defect causes. Conventional injection molding is widely used to produce plastic parts, mainly in the automotive industry, due to the high production ratios and quality of injection parts. Mold Flow Analysis. These defects are not visible, like small. 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. 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. Avoid Drag in Molded Parts. mold temperature con-trol fluid flow in a mold, either laminar or turbulent, is by a dimensionless number. Flow lines can occur if any of the following affect the mold: A runner, gate, or sprue that is too small for purpose creates more flow. This is achieved by using the needle valve nozzles to control mould filling so that only one flow front exists at a time. Thickness – Make the ribs 50% to 60% (0. They appear around holes or seams of an injection molded part, and some may call them “flow lines” or “weld lines”. The polymer material is melted by the heat from the heaters and viscous heating due to the screw rotation in the plasticization stage as shown in Fig. This means that the skin of the part is always formed by. Part walls thinner than 4mm (0. Injection molding can be performed with only one of these materials like glass, plastics, etc. This article continues our series of troubleshooting reports from one of the leading on-the-spot problem solvers in the molding industry. (2) Improving the flow of plastic and facilitating the molding of thin-walled parts. The Smartflow Reynolds Number Calculator is provided as a service to the injection molding community as an easy-to-use online resource. Sink marks are like little craters formed on an injection-molded product. Thin wall injection molding can produce walls as thin as 0. In other words, the long-chain polymer. The gate vestige for injection molding is usually narrow. The Table 2 categorically reports Melt Temperature, Injection Pressure and Injection Speed are the main contributors in the defects such as the short-shot, weld-lines, flow marks, burn-marks etc. It can also aid in material selection since fill. This article explains the causes and. Surface Delamination. This may involve adjusting the temperature and pressure of the mold, using different mold materials, or adjusting the injection speed or cycle 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. The melt holds enough temperature to maintain a low enough viscosity. Flow lines are a typical problem for injection molding faults that appear as visible streaks or lines on the surface of the molded part. 1 m/s, and a zero-shear-rate viscosity η 0 (T 0)=3760 Pas. This. 16″) will be difficult to inject, leading to flow lines, surface imperfections, and short shots. The splay condition can completely cover a part, or it can appear inconsistently in various areas on the part. 1 a. Let’s take a look at some defects that are caused purely due to faults in the injection molding process. 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. ” Flow marks do not affect component integrity. Otherwise, the heat transfer of. Allows you to accommodate a variety of part sizes. Flow lines. To further improve productivity, the injection mold was equipped with CCCs to shorten the cooling. 31″) will require more. WeInjection molding is the most widely used form of plastics processing worldwide. Caused by: Non-uniform cooling times and flow rates; excessively narrow part features. Knit lines, weld lines, mold lines, or flow lines in injection molding are faint lines that become visible on certain plastic parts. Where wall thickness increases, round the corners of the mold in those regions. 0mm) throughout will ensure that the parts cool evenly and will prevent warping. Injection molding gates occur in different forms, taking distinct sizes and shapes – appearing narrow, wide, tapered, etc. (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. • Reduce runner dimensions and maintain the same flow rate. Flow lines are linear marks on the surface of a molded product, which indicate the flow direction of the molten plastic. 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. Prevent Flow and Knit Lines During the Injection Molding Process. Plastic is injected into a mold at a specific rate of flow—typically in. The mold flow model is usually a single-part model, that is, the FRT part itself, but the mechanics model is often a multi-part model, an assembly of the FRT part and others. In injection molding weld lines are formed whenever two flow fronts are joined. Bubbles are caused by melting flow fronts colliding in a mold cavity. Consider the following examples of cooling channel configurations in P20 mold steel with a water line diameter (D) of 11. ³/sec. Depending on the resin, resin temperature, mold temperature, and filling speed, knit lines can vary. Injection Molding of Multimaterial Systems. Over time polymer will degrade creating carbon (black specks). The formation process of weld lines can be described as. 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]. 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]. In spite of its cosmetic attributes, a bad knit line can significantly. Increase the size of the gate and reduce the thickness of the walls of the molded part. When trying to establish process control in plastic injection, watering the tool is a key variable that is often overlooked. personnel to assess cooling lines in the mold design. These knit lines are formed as molten plastic in the injection mold flows around an obstruction like a feature. If necessary, the mold designer may simulate the flow of solidification of resin using an injection molding simulation program. It is widely used in various industry sectors, and its products are ubiquitous in our daily life. Here are some ways manufacturers can avoid quality issues during the plastic injection molding process: Inadequate Injection/Holding Pressure: If injection and. He is currently a tooling manager for a large, multi. So when two. 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]. 100 to 0. Midland, MI. . 1 Caused By The Quality of Injection Mold. Injection molding is a great technology for finished production on a massive scale. Causes: Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic. Another tactic to promote more chain entangling at the weld line is to raise the mold temperature 10° C (50° F). Flow lines are ripples in the plastic after the part is formed. In injection molding weldlines are generated when two separate melt streams join either. 5. 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. Timing and temperature are significant contributing factors when it comes to flow and knit line defects. Figure 7 below shows the cooling channels, defined in this case using simple sketch lines. 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. Low mold temperatures are usually employed to decrease cycle molding time and final process costs; however, resulting surface defects include weld lines, sink marks,. Source: Google. Flow lines are unwanted streaks, waves, or lines on a part’s surface. Molds gain streaks through molten plastic cooling and solidifying at different rates. 3. A significant part of that is the cooling design, where there are many things to consider. Flow lines. 5mm (0. Sprue: This is what feeds the molten material into the runner that in tune reaches the casting by passing through the gates. Lower speeds allow gas more time to escape through vents. Non-uniform cooling of mold causes warping, creating internal stresses within the part and resulting in distortions. 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. Mold Design. 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. Injection Molding works with molten fluid created by heating the plastic granules above their melting point. 4 Injection Speed. When two or more streams of plastic meet and fuse together, a visible line or weakness is formed on the surface of the finished product. Several factors contribute to the formation of flow marks in plastic molds: Mold Design:. Excess Mold ReleasePoor mold design, including inappropriate mold wall thickness and faulty gate placements, is a common cause of weld line formation in injection molding. Complexities in the design and intricate pathways in the mold can also create flash in injection molded parts. 080" – 0. 0 seconds, depending on part volume, runnergate style and size, cavity location and injection pressure. There are different levels of program available on the market. Add more cooling circuits. Errors like these manifest when processing is done incorrectly. When molten plastic moves through an injection mold at different speeds, the resin solidifies at different rates, which causes this type of defect. By heating the solid material, it is converted into a fluid state with a low resistance to flow. 1, pp. 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 ]. The steel that cored out and created the notch was very thin—roughly 0. Locally black discolorations of the molded part occur in the area of the weld lines, at the end of the flow path or of individual areas of the molded part (for example, a rib). In this study, a new specimen design is introduced for the tensile testing of micro parts. 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. Flow lines are surface irregularities, wavy patterns, or a rippled effect, perhaps of a slightly different color, that can appear in injection molded parts. S. Once the mold is created and the machine is set up, additional parts can be manufactured very fast and at a very low cost. Depending on the resin, resin temperature, mold temperature, and filling speed, knit lines can vary. Introduction.