It is therefore important to work with reliable injection mold makers for quality services and products. Flow Lines. Plastic Injection Molding Process. This article explains the causes and. Faster injection increases pressure in front of a flow front. 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. Water cooling pumps coolant through lines on the outside of the mold cavity. The following are the main elements of a gating system designed for injection molding. Flow marks mainly occur in the. ” FLOW MARKS – Wavy surface appearance on a molded object caused by improper flow of the material into the mold. Electronic flowmeters featuring FCI (Flow Characteristic Indicator). Prior to this late stage in production, however, 3D printing is much more affordable and flexible for products in the early stages of design. Select a plastic that is less prone to shrinkage. 0030 in. The wall thicknesses of an injection molded part generally range from 2mm – 4mm (0. "Design and Mould Flow Analysis of Injection Mould for Connecting Link," International Journal on Emerging Technologies, vol. Causes: Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic. personnel to assess cooling lines in the mold design. How to solve flow lines in injection molded products?Injection molding is a heat-transfer process. An on-line compounding injection molding machine, incorporating the co-rotating, intermeshing twin-screw extruder and a reciprocating screw injection molding machine fitted with the oscillating pressure valve, has been developed. Weld-line is a defect that arises in injection molding when two or more flow fronts collide. Available with a number of options including thermometers, pressure gauges, liquid-filled pressure gauges and quick change socket and plug. Sink Marks. S. Low cushion and hold time. Mold Design. The materials used in the plastic injection molding process can vary in rigidity and pliability, among other differentiating characteristics. Manifesting as wavy lines or streaks on the surface of the molding, flow lines generally occur when molten material flows. The weld line and the flow pattern of co-injection molding can be seen from Fig. The leading edge of the flow, or the melt. Acetal is a funny material in how it reacts to certain injection speeds and mold temperatures. 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. Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties. Vary injection speed, screw rpm, and backpressure. 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. Flow lines are unwanted streaks, waves, or lines on a part’s surface. Using cooling simulation to circuit your cooling lines properly. Flow lines or marks appear as banded discolouration on the surface. Flow LinesWeld lines are the def ects in. Then the melt is injected into the mold and the surface of injection-molded products is. 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. Mold flow analysis is also used to help designers locate difficult spots to fill within the mold. 4. The line where two resin flows “knit” or “weld” together during injection molding is called a weld or knit line. Three terms that often get mixed up are weld, meld, and knit 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. Injection moulding (U. Weld Line defect analysis, the most common of many defects in injection moulding products. There are two halves to the injection mold that must come together and at the interface there is a chance of a parting line. 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. If the cornflake pigment contains a large amount of fine particles, this will result in a higher definition of flow and weld lines. 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. Imagine a river splitting to go around a rock, then meeting up again on the other side. [1]#TROUBLESHOOTING #FLOWLINES #INJECTIONMOLDINGStudy more about troubleshooting flow lines : lines occur when two flow fronts meet and fuse together in the injection molding process. Faster injection increases pressure in front of a flow front. Molders should make sure the weld line is formed during the first-stage filling of the part, if possible. By heating the solid material, it is converted into a fluid state with a low resistance to flow. 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. 020 0. If possible, try to choose a little glue . 18/06/2021. This article continues our series of troubleshooting reports from one of the leading on-the-spot problem solvers in the molding industry. Short Shots. When a poor-quality plastic that has impurities in it is used for injection molding, it is less likely to flow smoothly through the mold. If near the gate, check the gate seal time. 120 in. Thin walls of greater length require greater pressure, which requires larger injection molding machines and. 5 to 0. The formation process of weld lines can be described as. Flow Lines: Flow lines on thinner areas of molded components appear as a wavy pattern, often a different color than the surrounding area. Steven. A gradual change in wall thickness is recommended to avoid any sudden change in the material flow direction. The model of polymer melting is fundamental for the development of. They commonly occur when there is a large variation between cooling speeds of sections of the material as it flows through the mold. However, surface quality is difficult to predict due to many influencing factors and their complex interactions. As the material properties of. The mold should have a fine matte finish to allow air to vent quickly and the parison to conform to the mold surface while it is still hot. Vertical walls, meaning those part surfaces parallel to the direction of mold operation, should have a minimum draft angle of a 1/2 degree, and 2 degrees is even better; heavily textured surfaces may require 5 degrees or. Flow marks: Flow lines Directionally "off tone" wavy lines or patterns Injection speeds too slow (the plastic has cooled down. Weld line. Impurities in the resin will prevent a smooth flow through the mold. If the mold itself has cracks or defects, the finished product will also have various problems. wide. More Control. 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. Cooling lines have been placed in the cavity, core, and runner block following established guidelines for diameter/depth/pitch. 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 resistance. Vents have three components. Lower speeds allow gas more time to escape through vents. This defect compromises aesthetics. Weld lines are formed during cavity filling whenever two separated melt flow font recombine. The mold in Figure 6 is an 8-cavity with an “H” pattern runner and a lapped edge gate. Mold is not vented properly causing slow flow at knit lines. Use high-quality, temperature-controlled materials for better mold filling. Thickness – Make the ribs 50% to 60% (0. Vacuum Voids. More about the knit lines in injection molding. Water set-up and design are every bit as important as establishing and recording a repeatable process. Flow marks, also known as flow lines, are molding defects that can occur in the manufacturing process of injection molding. A. . When it comes to injection-molded products, here are several typical solutions for flow lines: To make sure the liquid reaches the mold fully before cooling, enhance the. Add or increase vents as needed. Select a plastic that is less prone to shrinkage. The splay condition can completely cover a part, or it can appear inconsistently in various areas on the part. 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. Where wall thickness increases, round the corners of the mold in those regions. (2011) and Fig. Mold flow analysis is a software simulation that can show how resin will fill the mold during the injection molding process. Kulkarni) During the fill phase (Fig. During plastic injection molding, molten material flows through an opening called a gate and fills a mold cavity. Mold design is a key aspect of injection molding. In the process of injection molding, the quality of the mold plays a decisive role in the quality of the finished product. 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]. They are best described as "off tone" wavy lines/streaks or patterns in the molded part around the injection ports. They are off tone wavy lines/streaks or patterns in the molded part around the injection ports. Reducing the dimensions of the runner system. For the measurement of the crystallinity a DSC analysis was performed. They usually are a slightly different color than the rest of the piece, and typically occur on more narrow sections of the molded item. Injection molding, in addition to extrusion, is the most important technology in the polymer processing industry. Fig. Weld lines in injection molding are formed when two or more flow fronts of molten plastic material meet and merge during the molding process. 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. Each needle valve nozzle opens only when the melt front has moved across its gating point. A knit line is when two flow fronts come together but, rather than merging, it’s like a head-on collision at a 4-way stop. 5. Knit lines are hard to eliminate for injection molded plastic parts but designers can minimize cosmetic defects. Vacuum on the mold does help and many times eliminates the weld line. In the injection molding process of plastic parts, flow mark is a common defect due to the defects of parts design and injection molding process [4]. In injection molding weld lines are formed whenever two flow fronts are joined. 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. One of the things that can compromise an injection molded part is a weld line, also called a knit line. Injection molding machine . These data were obtained from many andGenerally, injection molding resins are characterized as having medium, high or very high flow. Knit and Meld lines are inherent to plastic injection. Flow lines arise from variations in the flow of molten plastic material as it fills the mold. Flow marks, also known as flow lines, are molding defects that occur in the manufacturing process of injection molding. These flow fronts occur when the molten plastic material encounters obstacles, such as inserts, ribs, or other features within the mold cavity. Injection molding produces defects. Plugged or inadequate vents n Clean plugged or dirty vents, particularly those along the parting line. 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. Take these steps to avoid knit lines. 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. Finite differences method offers strong consent for methodological solutions. Source: Google. Like flow lines, burn marks do not usually compromise the item's integrity, but they may become a problem once the product is burnt to the point of degradation. Lastly, you can only acquire the benefits of parting lines in injection molding during plastic production if you design the parting line correctly. Mold design is a key aspect of injection molding. Weld lines are a common imperfection to be found in most plastic injected molded parts. How a mold is watered is key to a processor's goal of consistency. This slight depression is a tiny line called the weld line or knit line. As the weld line can significantly lower the mechanical strength of µIM parts, it is relevant to clarify the position of weld. 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. 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. 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. Basic Repairs for Warping. The gate vestige for injection molding is usually narrow. In this guide, we cover evaluation and also the remedy of flow lines in. The Problems & Solutions course is designed to train in the recognition and analysis. On a small four-cavity tool, the part had a U-shaped notch in the center. The CoolingCare system tests cooling channels for leaks and blockages. Weld lines may appear like seams or scratches, but that is not what they are. Also, heating the material in the cylinder. This occurs wherever there is a hole, notch, or any other feature on a part which divides the flow of plastic into 2 separate “flow fronts. The part surface was observed in reflected light using optical microscope and the pictures of. Weld lines represent the region where two separated melt fronts recombine. The edge gate is the most popular and straightforward injection mold gate design. 5° draft to each side of the rib. Machine. WeInjection molding is the most widely used form of plastics processing worldwide. However, the weld line may not continue throughout the flow. 3,000 ton machines can cost $700,000. Sufficient draft is an important part of any mold design, and quick-turn tooling is no exception. Flow rates and a fluid are input to the cooling channels,. 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. Micro weld lines are the common problem that occurs for plastic molded parts through injection molding process. Knit and meld lines are inherent to plastic. Basic Repairs for Warping. Accept All. Several factors contribute to the formation of flow marks in plastic molds: Mold Design:. As a result this weld line will be a weak point in this part. INADEQUATE INJECTION PRESSURE. The flow rate should ensure the material does not lose its. Let’s take a look at some defects that are caused purely due to faults in the injection mould process. In. The resulting defects don’t stop at flash only. Weld lines are formed when there is an inability of two or more flow fronts to ‘knit’ or ‘weld,’ together during the molding process. When the melt enters the cavity quickly, it is necessary to discharge the air quickly. New design is said to eliminate mold damage from nozzle stringing or work-arounds that slow cycles. This depends on a. wide. Reduce the temperature of the mold and plastic being injected. Kobayashi et al. The position, length, and angle of weld lines are dependent on the. Flow marks can be caused by any or all of the following factors: Slow melt injection speed. The main. This steel section retained heat, not allowing the plastic to cool and requiring an excessively long cycle time of 40 sec. Each material has a maximum flow length for a given injection pressure and wall thickness — this is known as the flow ratio. Secondly, it is easy to form obvious. Increase injection speeds and pressure to ensure the cavities are filled properly. The thermo-rheological findings were used to investigate the sources of weld line weakness. 020"). The plant water system is a critical component of an injection molding facility. 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 injection molded parts, there are 2 types of weld lines: The first type of weld mark is called cold-melt joints and is formed when the melt front enters the cavity with different gates. Precise control of resin flow, pressure and temperature is critical. 5. 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. 9 – Over Packing. Increasing injection speed. 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]. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part. Pushing the air out or not is the difference between a good part or burns and flow lines. A knit line in a plastic injection-molded part is created when two separate plastic flows meet within the mold and "resolidify" along their interface. 1) Snake flow lines. Increase the mold temperature. 008 to 0. Injection Molding (IM) Solutions 1019 Balfour St. It affects its structural integrity and exterior appearance. Help prevent warping with these measures: Raise the mold or material cylinder temperature. 750 in. Countermeasures to Injection. Solution: If possible, operate the machine on. c. Incorrect gate positions cause multiple flow fronts that struggle to merge. Adjustments to injection speeds are the best way to determine which of these two are the specific cause. Flow lines, also referred to as flow marks or weld lines, are visible lines or patterns on the surface of a plastic component. With a minimum of 1. Before injection molding, there is a large amount of air in the flow path and the cavity. Knit lines appear to be a faint line or a crack in the finished part after it comes out of the injection mold. Figures 3A, B and C depict different examples of “spot” vents. Too hot and the plastic will start to degrade, but too low and flow marks increase. After an hour of running, the steel temperature stabilized at 78°F. SMARTFLOW ® flowmeters effectively and accurately measure flow rate. During the injection molding process, one of the most important requirements of the mold is to deliver melt from the barrel to the cavity. Listed below are some factors to consider when designing ribs for injection-molded parts. Prevent Flow and Knit Lines During the Injection Molding Process. FIG 1 Depiction of fountain flow. This production method is particularly popular when manufacturing in large quantities. If larger gates are used, expect a longer molding cycle time. Contact the material supplier for help in deciding which flow rate should be used for a specific application. A. Thereby creating localized pressure within the mold. ” Flow marks do not affect component integrity. It has larger cross-sectional regions that allow sufficient molten plastic flow into the cavity. In this regard, the gate form with less shunt should be used, and the gate position should be selected reasonably. Injection molding could develop knit lines if one front cools before it reaches the other. 0mm) throughout will ensure that the parts cool evenly and will prevent warping. Apart from the shape, it is important to design ribs for optimal mold performance. Add more cooling circuits. This action could indicate that one portion of the plastic flow will move more quickly than the other. The material is kept poured into the mold for some time. Introduce automation into the injection molding process for improved efficiency. Heat: Too much heat can have an appearance similar to moisture splay. 1. Using a thermoplastic that stays in a. Neat polypropylene retains nearly 90% of its strength on a properly designed and molded weld line. Check the diameter of the ejector pins and the pin hole tolerances, if the tolerance is too wide this could allow molten plastic to flow down between the pin and hole surface. The location of the crack can tell us a lot about its specific cause. 5mm (0. 2). 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. If packing around a detail is causing cracks/ shorts/burns on the knit line, reduce the heat to improve knit line seal and strength. Weld lines may appear like seams or scratches,. In injection molding, it is often important to maintain consistent color throughout the part. This is achieved by using the needle valve nozzles to control mould filling so that only one flow front exists at a time. to 0. Consider the following examples of cooling channel configurations in P20 mold steel with a water line diameter (D) of 11. This carbon will hang up in areas and is difficult to remove. However, FRTP has problems such as the formation of weld lines, the deterioration of appearance due to the exposure of fibers on the molded product surface, and the deterioration of the strength of. I’ve been noodling that question around in my head for the last three years, since injection machine suppliers like Arburg, Engel, Wittmann Battenfeld and Sumitomo (SHI) Demag started showing up at trade shows with mold-filling displays on their machine controller screens. Knit Line. One exception: ribs should be about 50. Flow Lines. More viscous amorphous materials, such as PC, TPU or PEEK, can have a recommended vent depth range from 0. 182-185, 2013. Flow lines can be minimized in injection molding by optimizing the design of the part and the mold, as well as the processing conditions. Errors like these manifest when processing is done incorrectly. The importance of understanding the causes of flow lines is critical for preventing them from occurring during the injection molding process. You must also note that non-fill can be an extension of a flow line or knit line condition. Investigate the mold for signs that a water leak is present, and correct the condition. ” 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. They are visible on one side of the parts. Midland, MI. 750 in. 0 seconds, depending on part volume, runnergate style and size, cavity location and injection pressure. S. In addition to the recommended wall thickness, it’s important to consider a material’s flow characteristics. Flow lines or marks appear as banded discolouration on the surface. 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. Detecting Water Flow Restrictions. The weld‐lines are undesirable from both aesthetic and functional points of view and their effect. If necessary, the mold designer may simulate the flow of solidification of resin using an injection molding simulation program. On a small four-cavity tool, the part had a U-shaped notch in the center. Figure 1 presents the shrinkage of polypropylene versus wall thickness in injection molded parts. 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. The primary reason is insufficient plastic fluidity. Plastic Defect # 2: Flow Lines. 5. process can be fully completed. Weld lines have been one of the common defects puzzling the injection molding industry. To study the weld line formation of colliding flow fronts the filling of a mold cavity was simulated. Pack (hold time) is usually 2 to 3 times the mold fill times, depending on gate size. This requires skilled labor to mold new parts that have less molding defects. Also called “Weld Line. 010 0. The moldability validation and flow analysis tools enable you to iden-tify and correct potential injection molding problems as you design the product. 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. 6T) as thick as the nominal wall. Reduce the injection pressure to prevent overpacking at the gate. Turbulent Flow IS Always Better. Flow speed too fast or too slow. Weld lines in injection molding are formed when two or more flow fronts of molten plastic material meet and merge during the molding process. The co-simulation of mold flow and structure plays an important role in the design of injection molded fiber-reinforced thermoplastic (FRT) parts. , 1 IN, 1 OUT, 2 IN, 2 OUT). Weld lines can compromise the strength and aesthetics of the part, so it’s important to carefully design the mold and. As the name implies, it is usually positioned along the edge of the workpiece, forming a visible mark at the demarcating line. • Reduce runner dimensions and maintain the same flow rate. 6 T) as thick as the nominal wall. Injection molding is a great technology for finished production on a massive scale. Mold Flow Analysis. Kobayashi et al. Flow marks occur as circles or lines that occur around the gate. The. Flow marks won. 000 0. What it is:Jan 04, 1999. The Smartflow Reynolds Number Calculator is provided as a service to the injection molding community as an easy-to-use online resource. Furthermore, jetting is frequently visible as a squiggly line on the surface of the final component, typically leading from the original injection gate. The material is poured into a mold, which shapes the size and looks of the article. They appear as waves or lines, usually of a different color tone from the surrounding area. The most common and one of the most challenging injection molding defects are weld and knit lines. Flow Lines Defect in Injection Molding Process Flow lines can be defined as linear grooving, or circular ripples, on the surface of a molded part that indicate the. spelling: injection molding) is a process for producing parts by injecting molten material into a mould, or mold. What Are Flow and Knit Lines? Flow lines are defects that appear as wavy. Except for a few very simple injection molded parts, they occur on most injection molded parts (typically in the form of a line or V-groove), especially large complex items that require use of multi-gate molds and inserts. These flow fronts occur when the molten plastic material encounters obstacles, such as inserts, ribs, or other features within the mold cavity. Flow lines appear like off-tone patterns on the surface of your finished parts. 5 gal/min through this circuit we will get turbulent flow—the optimum cooling situation. 005 0. The last cause of flow lines may be the operators running your daily operations. Siemens PLM Software. spelling: injection molding) is a manufacturing process for producing parts by injecting molten material into a mould, or 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. 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. The Bottom Line. Low melt temperature. When trying to establish process control in plastic injection, watering the tool is a key variable that is often overlooked. injection mould simulation at steady flow rate. 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. When combined with low injection pressure, this problem leads to decreasing melt speeds and can cause flow lines. There are two types of weld lines, cold and hot weld line. Learn about prevention methods and references. While there are many situations where they are barely perceptible, weldlines represent a potential source of weakness in molded parts. 015 0. Fixing Process Variables That Can Cause Flash. Novel Nozzle Tip Prevents Stringing, Saving Molds & Cycle Time. 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. 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. 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. They are caused by improper bonding and low injection pressure and speed [ 46 ]. These streaks or patterns can affect a part's aesthetics, value, and. However, you can easily prevent such defects by simply adjusting the temperature, flow rate, or other mold parameters. Well, what this simply implies is that injection polycarbonate molding makes it possible for you to create a wide range of part sizes. The most important factors to consider during molding are the mold temperature, melt temperature, and injection pressure and speed. Plastic is injected into a mold at a specific rate of flow—typically in. Water set-up and design are every bit as important as establishing and recording a repeatable process. This can be achieved by connecting ribs to each other. RapidDirect – Your Best Injection Molding Partner. Voids . Investigate the mold for signs that a water leak is present, and correct the condition. Flow Lines. 7 in Kobayashi et al. 000 0. In the development of our calculator. This can enable the manufacturer to increase the molten plastic temperature at the front of each flow and avoid the occurrence of weld lines. Increase molten plastic temperature during injection molding. Flow Lines Or Flow Marks. Several factors can contribute to shrinkage in injection moulding, including material, part design, mould design and processing parameters.