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. Plastic injection molding is a complex and highly intricate process. Weld lines in injection molding are formed when two or more flow fronts of molten plastic material meet and merge during the molding process. If necessary, the mold designer may simulate the flow of solidification of resin using an injection molding simulation program. It affects its structural integrity and exterior appearance. However, the weld line may not continue throughout the flow. Below are ways through which you can remove and prevent flow lines; a. Designers can easily set up and perform analysis with only a few clicks. 2. In other words, the long-chain polymer. 0 seconds, depending on part volume, runnergate style and size, cavity location and injection pressure. In a weld line, reinforcement fibers. 5. Several factors contribute to the formation of flow lines, including material properties, mold design, and process parameters. ”. Fictiv provides you with DFM feedback so that you can minimize injection molding defects. 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]. Taking proper care to design the mold properly, avoiding bottlenecks, and changing the number or location of gates can improve performance. For instance, the plastic used for gears must be completely smooth. 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. Reduce the injection pressure. If possible, try to choose a little glue . Flow lines are the results of slow injection rate and disparities. Contact us to learn more about SMARTFLOW flowmeters. Typically, flow lines are caused by the liquid plastic cooling at different rates. 6. This most often occurs when you have a core or shutoff. Knit lines, weld lines, mold lines, or flow lines in injection molding are faint lines that become visible on certain plastic parts. 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. They are best described as "off tone" wavy lines/streaks or patterns in the molded part around the injection ports. Where wall thickness increases, round the corners of the mold in those regions. Here are some ways manufacturers can avoid quality issues during the plastic injection molding process: Inadequate Injection/Holding Pressure: If injection and. Aesthetics, such as injection molding gate marks, sink marks, voids and flow lines In some cases, more than one gate may be necessary to achieve the desired outcome. There are important differences between an injection mold sprue and its runner system. An improper flow rate and injection speed also cause flow marks. Chart 1: Diameter versus turbulent flow. 18/06/2021. We offer this Mold Cooling Calculator tool as a free service to the injection molding industry. Flow Linesparticles, during injection molding, will produce visible flow and weld lines. Contact the material supplier for help in deciding which flow rate should be used for a specific application. 0030 in. If the cornflake pigment contains a large amount of fine particles, this will result in a higher definition of flow and weld lines. Impurity. Round the corners of the mold where wall thickness increases to help keep flow rate consistent and prevent flow lines. 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. Non-uniform cooling of mold causes warping, creating internal stresses within the part and resulting in distortions. They commonly occur when there is a large variation between cooling speeds of sections of the material as it flows through the mold. 025 inches per inch, depending on the particular resin, molding conditions, part design, part wall thickness, and direction of flow. [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. Just multiply the area of the barrel × the injection velocity. Flow lines – A common plastic injection molding defect in which discolored ring-shaped streaks, patterns, or wavy lines appear around gates or narrow sections of the finished product. The Mold. 150 in. 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. If during the design and development stage,. Plastic Injection Molding Process. Knit Line. 750 in. Conventional injection molding is widely used to produce plastic parts, mainly in the automotive industry, due to the high production ratios and quality of. 5 to 0. As the weld line can significantly lower the mechanical strength of µIM parts, it is relevant to clarify the position of weld. Flow lines are lines or streaks visible on the surface of a part. (1) Increase the injection pressure and prolong the injection time. If performing a resin change from one polymer to another a difference in viscosity can cause flow lines or gels. Minimum turbulent flow would be 12-14 GPM, while numbers in the 20’s would be identified as an excellent flow rate. (3) Improving the surface gloss of the product. Short shots refer to incomplete filling of a mold cavity. 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. As these flow fronts come together. Thus, as the flow continues, the two flow fronts may gradually rejoin to make one uninterrupted flow front. NX Mold Flow Analysis. The Analysis and countermeasures of flow lines in injection molded products Read more. This can be achieved by connecting ribs to each other. with a characteristic length χ 0 =0. Knit lines, weld lines, mold lines, or flow lines in injection molding are faint lines that become visible on certain plastic parts. Examine the flow pattern of each part to see if the plastic flow front is coming around on itself, or if there is a hesitation of the flow front in filling a thin section of the part. The main causes for short shots are inappropriate injection pressure and speed. Heat: Too much heat can have an appearance similar to moisture splay. 182-185, 2013. We stopped the machine and added glycol to the chiller to a concentration of about 40%. It can also aid in material selection since fill. There is a level of insulation from the plastic layer on the wall. Give a good distance between mold gates and. Mold Design. Flow lines are surface irregularities, wavy patterns, or a rippled effect, perhaps of a slightly different color, that can appear in injection molded parts. Mold Venting. Adjustments to injection speeds are the best way to determine which of these two are the specific cause. Investigate the mold for signs that a water leak is present, and correct the condition. Reduce the injection pressure to prevent overpacking at the gate. In injection molding weldlines are generated when two separate melt streams join either. 030 0. thick, 0. Known as thermolators, water cooling systems require a cooling source such as a tower. Figure 7 below shows the cooling channels, defined in this case using simple sketch lines. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. A flow mark is commonly referred to as a flow line occurs during the process of injection molding. Novel Nozzle Tip Prevents Stringing, Saving Molds & Cycle Time. Email Sales@Aberdeentech. Inadequate mold wall thickness leads to uneven cooling and hindered plastic flow, resulting in weld lines. • u. 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. Sink marks are like little craters formed on an injection-molded product. On the contrary, for the conformal cooling. These flow fronts occur when the molten plastic material encounters obstacles, such as inserts, ribs, or other features within the mold cavity. 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. The barrel temperature is too high. If larger gates are used, expect a longer molding cycle time. This defect is caused when plastic cools too rapidly during the injection molding process. 2. industry, to deliver intuitive injection mold-ing simulations embedded in NX. Using cooling simulation to circuit your cooling lines properly. Introduce automation into the injection molding process for improved efficiency. Available with a number of options including thermometers, pressure gauges, liquid-filled pressure gauges and quick change socket and plug. 3) Poor Preventative Maintenance. The proposed method is first applied to the simulation of HDPE melt in a classic Poiseuille flow case, and then two. Mold Design. Select a plastic that is less prone to shrinkage. This slight depression is a tiny line called the weld line or knit line. Round the corners of the mold where wall thickness increases: This. They can be caused by variations in cooling speeds of the material as it flows through the mold. 2. Flow lines or marks appear as banded discolouration on the surface. Drain the hopper if you were working with 100% regrind, clean if necessary. Vacuum on the mold does help and many times eliminates the weld line. Once the mold is created and the machine is set up, additional parts can be manufactured very fast and at a very low cost. FIG 1 Depiction of fountain flow. Unless you are manufacturing an extremely simple part with one gate and a unidirectional flow front, your part will have weld lines. Flow marks can be caused by any or all of the following factors: Slow melt injection speed. Increasing injection speed. In addition to the recommended wall thickness, it’s important to consider a material’s flow characteristics. A weld line refers to a line, notch or color change that is created on the molded part caused by the convergence of two separate flows of molten plastic. Steven. 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]. 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. Here are some of the most common scenarios for cracks in injection molded parts: Cracks in the Direction of Flow – If the crack develops in the direction of the plastic’s flow into the mold cavity, the cause is molecular orientation. Figure 1 below shows the development of a weld line. It can help you reduce defects, improve quality, and save costs by simulating how. 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. 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. Increase the injection speed and mold/operational temperature to prevent fracture or surface breakdown. This defect compromises aesthetics. Edge Gates. b. 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. Plastics manufactured by injection molding are meant to be durable and reliable, able to hold up to heavy use, high temperatures, or adverse conditions. The gate in injection molding is an essential component that directly connects to the plastic part and controls the flow of melted plastic resin into the cavity. Impurities in the resin will prevent a smooth flow through the mold. Flow Lines. Kulkarni) During the fill phase (Fig. The most important factors to consider during molding are the mold temperature, melt temperature, and injection pressure and speed. Imagine a river splitting to go around a rock, then meeting up again on the other side. 160"). FIG 1 Depiction of fountain flow. A knit line may be unnoticeable or may look like a crack. Defects like this make it harder to market the finished parts to the end user. That’s an easy calculation to perform. Flow lines. The Bottom Line. 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. Mold fill usually takes 0. 16″) will be difficult to inject, leading to flow lines, surface imperfections, and short shots. The line actually indicates the parting ‘plane’ that passes through the part. 6T) as thick as the nominal wall. Flow Lines Or Flow Marks. Fictiv provides you with DFM feedback so that you can minimize injection molding defects. Lookalikes include surface scratches, scuffs, flow lines, delamination from contamination, and flaking. Flow fronts moving too slowly can lead to pressure in front of a flow front being too low to force gas through vents. Polycarbonate Injection molding defects. 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. These are off-tone streaks or lines on the surface of the plastic, and while they don’t affect the structural integrity, they are unattractive and prevent a uniform and consistent appearance. 4) Extended Shutdowns. Acetal is a funny material in how it reacts to certain injection speeds and mold temperatures. If near the gate, check the gate seal time. 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. Increasing pack or hold pressure helps to eliminate low-pressure conditions at the weld line. Weld lines appear as a result of the flow of molten plastic during the injection molding process. Lower speeds allow gas more time to escape through vents. The melt holds enough temperature to maintain a low enough viscosity. 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. You can avoid having any weld lines with cascade injection moulding. AnalysisInjection moulding simulation with the finite element method is the most advanced technique for designing injection moulds. Thick sections take longer to cool than thin ones. Thin walls of greater length require greater pressure, which requires larger injection molding machines and. 4, no. Let’s take a look at some defects that are caused purely due to faults in the injection molding process. When two or more strands of molten plastics meet in the mold, a weld line will be produced at the junction [1,2]. Midland, MI. (1. Each gate type has specifications that significantly affect the end product’s structure and appearance. When creating the parting line, the mold design is the first place to look for the appropriate parting line for an injection molded product. Sink Marks. 5 gal/min through this circuit we will get turbulent flow—the optimum cooling situation. Weld lines may appear like seams or scratches,. 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. 750 in. A ‘parting line’ is the line of separation on the plastic part where the two halves of the plastic injection mold meet. 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. Bad habits can cause many different problems with injection molding machines, and flow lines are included in that. The edge gate is the most popular and straightforward injection mold gate design. process can be fully completed. After an hour of running, the steel temperature stabilized at 78°F. 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. 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. 2), the plastic enters the mold at a relatively fast rate and forms the skin. 2. In injection molding weld lines are formed whenever two flow fronts are joined. Fast injection gives you a glossy part, which is what we wanted in this. Pushing the air out or not is the difference between a good part or burns and flow lines. Injection molding could develop knit lines if one front cools before it reaches the other. Thus, a right gate design raises polymer temperature to prevent the formation of weld lines and flow marks. Increase molten plastic temperature during injection molding. More Control. Flow lines appear as a wavy pattern on the surface of your plastic parts. • Reduce runner dimensions and maintain the same flow rate. Within more basic plastic part design plans this plane can be a simple, flat surface, but it is often a complex form that. 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. Our glossary of terms provides definitions for a wide range of injection molding terms. The co-simulation of mold flow and structure plays an important role in the design of injection molded fiber-reinforced thermoplastic (FRT) parts. Features such as sharp corners or undercuts can. Plastic is injected into a mold at a specific rate of flow—typically in. 1 a. Weld Lines. Flow lines are linear grooves or circular ripples on the surface of a molded object caused by uneven cooling, improper residence time, or cycle time, or low mold. In the injection molding process, weld line regions occur when a molten polymer flow front is first separated and then rejoined. 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 Line defect analysis, the most common of many defects in injection moulding products. These lines can occur at the gate location, where the plastic enters the mold, or anywhere along the flow. For the measurement of the crystallinity a DSC analysis was performed. Flow marks occur as circles or lines that occur around the gate. But add 30% glass and weld-line strength retention drops to only 34% as high as the compound with no. 5. 025 100 120 140 160 180 200 220 240 Mold Temperature (F) LDPE PPWhen it comes to injection molding, the chances you might end up making a costly mistake is too high. Factors within the tooling can also contribute to flow and knit lines, calling for mold modifications. ” Flow marks do not affect component integrity. Causes: Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic. Adjustments to injection speeds are the best way to determine which of these two are the specific cause. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part. This injection molding defect appears as black or rust-colored discoloration on the molded part's surface or edge. Heat: Too much heat can have an appearance similar to moisture splay. The following are the main elements of a gating system designed for injection molding. In the injection molding process, weld line regions occur when a molten polymer flow front is first separated and then rejoined. The injection molding process involves the injection of a polymer melt into a mold where the melt cools and solidifies to form a plastic product. Lower speeds allow gas more time to escape through vents. A special mold provided with a glass window was used in order to directly evaluate the flow progression during the filling phase of the injection molding process in a thin-wall cavity and to. One of the best ways to determine part shrinkage for complex components is with a mold flow analysis. Some of the most popular brands include:Fiber reinforced thermoplastics (FRTP), reinforced with glass or carbon fibers, are used to improve the mechanical strength of injection-molded products. Effects of mold temperature and pressure on shrinkage 0. 0010 in. 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. They may also take the form of ring-shaped patterns when staying close to the mold's entry point, which is the plastic injection nozzle. mold and the blow molding process. The most common and one of the most challenging injection molding defects are weld and knit lines. It is a type of distortion caused by this premature solidification. Address 272 Commonwealth Drive, Carol Stream, IL 60188. We restarted the mold at the same coolant flow rate and temperature. 3. spelling: injection molding) is a process for producing parts by injecting molten material into a mould, or mold. Low melt temperature. Depending on the resin, resin temperature, mold temperature, and filling speed, knit lines can vary. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and. 20 mm. Basic Repairs for Warping. Flow LinesWeld lines are the def ects in. Thin walls often increase the pressure required to fill the part, and. The flow rate should ensure the material does not lose its. (4) It can improve the strength of the fusion line. These defects include the following: 1. Cooling lines have been placed in the cavity, core, and runner block following established guidelines for diameter/depth/pitch. They are evidence of nonuniformity in flow pattern upon entry into the mold. Increase molten plastic temperature during injection molding. The primary reason is insufficient plastic fluidity. 1, pp. At a theoretical flow distance of 10 mm it represented the measured values well but at a longer distance it underestimated the experimental scale. Knit and Meld lines are inherent to plastic injection. Cracking is commonly seen at the weld line area but sometimes on the entire surface. Depending on the resin, resin temperature, mold temperature, and filling speed, knit lines can vary. Flow marks won. Faster injection increases pressure in front of a flow front. A meld line occurs when two moving melt fronts converge and flow parallel to each other. This allows the melt to keep enough velocity to ensure proper filing. 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. In Fig. Impurities in the resin will prevent a smooth flow through the mold. Siemens PLM Software. Several factors can contribute to shrinkage in injection moulding, including material, part design, mould design and processing parameters. (All Illustrations: S. . In this guide, we cover evaluation and also the remedy of flow lines in injection molding. If you’re a mold designer with one of those vent-depth “rule-of. Caused by: Non-uniform cooling times and flow rates; excessively narrow part features. There are three stages of flow – laminar, transient and turbulent. PartWeld lines form where two or more flow fronts come together and can create a cosmetic or structural problem. These flow fronts occur when the molten plastic material encounters obstacles, such as inserts, ribs, or other features within the mold cavity. 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. Flow lines arise from variations in the flow of molten plastic material as it fills the mold. Avoid knit lines when designing a molded part to improve cosmetic appearance and functionality. injection molding. They can be unsightly and unpleasant in high-end sunglasses. However, surface quality is difficult to predict due to many influencing factors and their complex interactions. Lastly, you can only acquire the benefits of parting lines in injection molding during plastic production if you design the parting line correctly. Which means engineers need to know how to account for this phenomenon in an efficient way. 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. The type of gate for manufacturing often depends on the material used and the parts’ shape. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. The thermo-rheological findings were used to investigate the sources of weld line weakness. How a mold is watered is key to a processor's goal of consistency. [12] or the flow lines shown in Ref. In injection molding weld lines are formed whenever two flow fronts are joined. Molds gain streaks through molten plastic cooling and solidifying at different rates. Depending on the resin, resin temperature, mold temperature, and filling. The flow line is usually connected to injection specifications, mold and injection material, it can be defined as linear grooving, or round surges, on the surface of an injection molded part that shows the instructions of product flow within the cavity of the mold. How to recognize, prevent, and counteract flow lines defects in injection molding? Learn. The last cause of flow lines may be the operators running your daily operations. 1．Mold Temperature Too Low. Vary injection speed, screw rpm, and backpressure. ” 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. The co-simulation of mold flow and structure plays an important role in the design of injection molded fiber-reinforced thermoplastic (FRT) parts. Listed below are some factors to consider when designing ribs for injection-molded parts. Cooling lines impact. Height – Make the ribs 3 times the height of the nominal intersecting wall (3T). While there are many situations where they are barely perceptible, weldlines represent a potential source of weakness in molded parts. 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. Phone (630) 665-8590. They are also known as flow lines. A weld line (weld point, flow line or a knit line) forms when separate melt fronts travelling in opposite directions meet. In the process of injection molding, the quality of the mold plays a decisive role in the quality of the finished product. Pack (hold time) is usually 2 to 3 times the mold fill times, depending on gate size. They also can appear as ring-shaped patterns near the entry points of the mold. Flow lines result from uneven cooling of the plastic within the mold. Figure 1 presents the shrinkage of polypropylene versus wall thickness in injection molded parts. If the melt speed of each gate entering the cavity is inconsistent, it is easy to cause weld lines at the intersection. Apart from the shape, it is important to design ribs for optimal mold performance. In this regard, the gate form with less shunt should be used, and the gate position should be selected reasonably. Edge Gates. 3 the path lines of selected material elements can be observed which are. Possible defect causes. 3,000 ton machines can cost $700,000. Walls thicker than 8mm (0. Flow lines or flow marks are a common visual defect in injection molding. It is crucial to manage flow lines to ensure. Accept All. 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. Surface Delamination. A knit line in a plastic injection-molded part is created when two separate plastic flows meet within the mold and "resolidify" along their interface. In this example, the flow ratio is 80 (10 Fig 2 – The spiral flow chart provides data about maximum flow length based on several Monday, 01 December 2014 Page 1 of 2Automatic in-line process quality control plays a crucial role to enhance production efficiency in the injection molding industry. When molten plastic moves through an injection mold at different speeds, the resin solidifies at different rates, which causes this type of defect. The difference between a knit and meld line is significant, dramatically impacting the part’s structural integrity. 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. One of the things that can compromise an injection molded part is a weld line, also called a knit line. If the mold itself has cracks or defects, the finished product will also have various problems. Introduction. Before his present assignment, Bob managed a molding operation for 25. 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. Another solution may be to pull a vacuum on the mold just before injection. The CoolingCare system tests cooling channels for leaks and blockages. The linear mold shrinkage of injection molded parts will vary from 0. This can be achieved by reducing the flow. The streaks form from plastic in the mold overlapping with plastic following it. Spatter lines formed on the surface of the plastic along the direction of melt flow are called silver marks. A significant part of that is the cooling design, where there are many things to consider. Mold flow analysis is a software-enabled process that. The plant water system is a critical component of an injection molding facility. Mold flow analysis is a software simulation that can show how resin will fill the mold during the injection molding process. The injection speed should remain consistent as the plastic enters the mold, otherwise, it will slow down, cool, and cause flow marks. WeInjection molding is the most widely used form of plastics processing worldwide. Slower flow results in thicker and shorter knits. The wavy lines or patterns that form on the surface of the molded object are what we refer to as flow marks. Low mold temperatures are usually employed to decrease cycle molding time and final process costs; however, resulting surface defects include weld lines, sink 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. The steel that cored out and created the notch was very thin—roughly 0.