020 0. html. The splay condition can completely cover a part, or it can appear inconsistently in various areas on the part. 6 T) as thick as the nominal wall. 1．Mold Temperature Too Low. The difference between a knit and meld line is significant, dramatically impacting the part’s structural integrity. However, you can easily prevent such defects by simply adjusting the temperature, flow rate, or other mold parameters. On the surface of a molded object, flow lines (also called flow marks) refer to the phenomenon that linear grooves or circular ripples showing the direction of material flow inside the mold cavity. 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 plant water system is a critical component of an injection molding facility. A ‘parting line’ is the line of separation on the plastic part where the two halves of the plastic injection mold meet. MFR values for polypropylenes are determined at the same load but at a higher temperature 230°C (446°F). The. Too hot and the plastic will start to degrade, but too low and flow marks increase. There are two types of weld lines, cold and hot weld line. These defects include the following: 1. mold temperature con-trol fluid flow in a mold, either laminar or turbulent, is by a dimensionless number. Flow lines arise from variations in the flow of molten plastic material as it fills the mold. Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties. Heat: Too much heat can have an appearance similar to moisture splay. At some point in time, it can no. Flow marks are a phenomenon in which a pattern of the flow tracks of the molten plastic remains on the surface of the molded product. Symptoms: Flow lines will typically appear as discolored “bands,” or sometimes “wavy. Flow marks are caused by a variety of factors in the injection molding making technology, and they happen at the gate. Sufficient draft is an important part of any mold design, and quick-turn tooling is no exception. Flow Lines. Drain the hopper if you were working with 100% regrind, clean if necessary. These lines usually occur around holes or obstructions and cause locally weak areas in the molded part. It also may be that your injection speed is too low, which will cause the molten plastic to solidify at different rates. Flow rates and a fluid are input to the cooling channels,. Flow lines are off-color lines, streaks, and other patterns that appear on the surface of a part. 2. Reposition gates and runners to allow resin to flow around the obstruction from a different direction. 025 inches per inch, depending on the particular resin, molding conditions, part design, part wall thickness, and direction of flow. How a mold is watered is key to a processor's goal of consistency. The following are the main elements of a gating system designed for injection molding. ” FLOW MARKS – Wavy surface appearance on a molded object caused by improper flow of the material into the mold. as a stand-alone, wholly owned. However, you can easily prevent such defects by simply adjusting the temperature, flow rate, or other mold parameters. This can be caused by a poor mold design, or plastic temperatures, mold temperatures, injection speeds, or. Insufficient injection pressure or time. Steven. In a weld line, reinforcement fibers. It has larger cross-sectional regions that allow sufficient molten plastic flow into the cavity. As the name implies, it is usually positioned along the edge of the workpiece, forming a visible mark at the demarcating line. This is achieved by using the needle valve nozzles to control mould filling so that only one flow front exists at a time. When the melt enters the cavity quickly, it is necessary to discharge the air quickly. What Are Flow and Knit Lines? Flow lines are defects that appear as wavy. 1 Injection molding From Wikipedia, the free encyclopedia Injection molding (British: moulding ) is a manufacturing technique for making parts from. After an hour of running, the steel temperature stabilized at 78°F. In other words, the long-chain polymer. (3) Improving the surface gloss of the product. Weld lines in injection molding occur when two flow fronts meet and create a visible line on the surface of the molded part. Faster injection increases pressure in front of a flow front. 1mm (7/16"), coolant temperature of 30ºC and cycle time of 17 seconds (see Fig. Taking proper care to design the mold properly, avoiding bottlenecks, and changing the number or location of gates can improve performance. In this study, a fully automated. First, excessive injection pressures may be forcing the mold apart. More about the knit lines in injection molding. Lenticular, or Silver Dollar type pigments, are thicker than cornflakes, with a narrower particle size distribution. When two or more strands of molten plastics meet in the mold, a weld line will be produced at the junction [1,2]. Weld lines typically appear in a part when an obstruction of some kind separates the melt flow and prevents it from "knitting" together. Avoid Drag in Molded Parts. Conformal cooling channels (CCCs) are a cooling passageway which follows the profile of the mold cavity or core to perform uniform cooling process effectively in the injection molding process. Each needle valve nozzle opens only when the melt front has moved across its gating point. It affects its structural integrity and exterior appearance. If during the design and development stage,. 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. Co-injection molding technique is a useful way to study weld lines, which can offer more detailed information of the. In injection molding weld lines are formed whenever two flow fronts are joined. The linear mold shrinkage of injection molded parts will vary from 0. Several factors contribute to the formation of flow marks in plastic molds: Mold Design:. 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. process can be fully completed. 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. 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. This new process enables thinking differently about how parts and molds can be designed. Depending on the resin, resin temperature, mold temperature, and filling speed, knit lines can vary. The part surface was observed in reflected light using optical microscope and the pictures of. Injection molding tool life is the number of production cycles that an injection mold can complete before it needs to be replaced. In the area of lettering, ribs, domes or sharp wall thickness changes, air which cannot escape quickly enough is swept along, overrun by the melt and embedded. When 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. 010 0. Midland, MI. Inadequate venting may cause short-shots, poor surface appearance, or weak weld-lines. 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. Circuiting Done Right. If possible, try to choose a little glue . Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties. Also called “Weld Line. Flow lines can be minimized in injection molding by optimizing the design of the part and the mold, as well as the processing conditions. This can enable the manufacturer to increase the molten plastic temperature at the front of each flow and avoid the occurrence of weld lines. S. However, the weld line may not continue throughout the flow. Fast injection gives you a glossy part, which is what we wanted in this. The wavy lines or patterns that form on the surface of the molded object are what we refer to as flow marks. Flow fronts moving too slowly can lead to pressure in front of a flow front being too low to force gas through vents. Furthermore, jetting is frequently visible as a squiggly line on the surface of the final component, typically leading from the original injection gate. Flow fronts moving too slowly can lead to pressure in front of a flow front being too low to force gas through vents. We heat the plastic to a molten phase, inject it into the mold and cool it into a preferred shape. 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. 5 gal/min through this circuit we will get turbulent flow—the optimum cooling situation. The goal when locating cooling channels in the mold is uniform mold surface temperature, which is determined by cooling channel depth and pitch. 005 0. Shear; There are two causes of splay brought on by shear. They appear as waves or lines, usually of a different color tone from the surrounding area. Raise the mold temperature, maintain a high. 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. 015 0. They may also appear as ring-shaped bands on a product’s surface near the entry points of the mold, or “gates”, which the molten material flows through. Plastics manufactured by injection molding are meant to be durable and reliable, able to hold up to heavy use, high temperatures, or adverse conditions. Warping. Check that the machine is creating enough clamping force to keep the mold halves together. This action could indicate that one portion of the plastic flow will move more quickly than the other. Effects of mold temperature and pressure on shrinkage 0. Flow marks mainly occur in the. Low cushion and hold time. Before his present assignment, Bob managed a molding operation for 25. 182-185, 2013. When molten plastic moves through an injection mold at different speeds, the resin solidifies at different rates, which causes this type of defect. Flow lines are a typical problem for injection molding faults that appear as visible streaks or lines on the surface of the molded part. How to recognize, prevent, and counteract flow lines defects in injection molding? Learn. This means that the skin of the part is always formed by. The weld line is then created with a higher flow front temperature and is packed with more pressure. Add or increase vents as needed. These streaks or patterns can affect a part's aesthetics, value, and overall quality. Avoid knit lines when designing a molded part to improve cosmetic appearance and functionality. 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. The best method of venting this type of part (Fig. Figure 1 presents the shrinkage of polypropylene versus wall thickness in injection molded parts. Finite differences method offers strong consent for methodological solutions. Weld‐line is a defect that arises in injection molding when two or more flow fronts collide. Sprue: This is what feeds the molten material into the runner that in tune reaches the casting by passing through the gates. 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. Bubbles are caused by melting flow fronts colliding in a 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. Weld lines are similar to flow lines, but they appear similar to a plane than a line when compared to flow lines. To study the weld line formation of colliding flow fronts the filling of a mold cavity was simulated. Flow marks won. Most flow lines don’t impact the functio…Flow lines are a defect in injection molding and can be a significant issue for manufacturers. Clean all parting line and core vents. This article explains the causes and. 4, no. 100 to 0. Increasing pack or hold pressure helps to eliminate low-pressure conditions at the weld line. Heat Input and Cooling Calculator for Injection Molding. Non-uniform cooling of mold causes warping, creating internal stresses within the part and resulting in distortions. Siemens PLM Software. The Priamus Fill system is designed for automatic closed-loop control of multi-cavity and family molds, multi-material injection, sequential valve-gated molds, as well as LSR molds. Here are some common remedies for flow lines in injection-molded products: Increase the injection speed, pressure and material temperature to ensure the material fills the mold before cooling. Weld lines are a kind of Achilles heel of plastics. Here’s a rundown of helpful tricks to keep in mind when designing and choosing a material for molded parts with walls: In general, ensuring your design has uniform wall thickness between 0. Adjust the design for the flow pattern to be a single source flow. (1998) for clear photographs. 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. One of the best ways to determine part shrinkage for complex components is with a mold flow analysis. Imperfections in manufactured plastic goods, such as a flow line defect in injection molding, are unacceptable. Precise control of resin flow, pressure and temperature is critical. This method can be helpful in. Detecting Water Flow Restrictions. 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]. 750 in. Below, we’ll look at five of the most common types of injection molding defects, and how you can prevent or repair them. Also called “Weld Line. Round the corners of the mold where wall thickness increases to help keep flow rate consistent and prevent flow lines. Co-injection molding technique is a useful way to study weld lines, which can offer more detailed information of the. Reduce the injection pressure to prevent overpacking at the gate. Round the corners of the mold where wall thickness increases to help keep flow rate consistent and prevent flow lines. Otherwise, the heat transfer of. The differences between knit and meld lines are significant, and they can dramatically impact the part’s structural integrity. 160"). Select a plastic that is less prone to shrinkage. Kulkarni) During the fill phase (Fig. injection molding. (2011) and Fig. There are several factors that can contribute to the formation of weld lines, including: Part design: The design of the part can impact the flow of the material and the likelihood of weld lines forming. This defect compromises aesthetics. Jetting occurs in a molded component when molten material is injected into the mold cavity and starts solidifying before it fills the cavity. The speed of an injection moldingRepeat the two previous steps two or three times, starving the screw periodically to allow the screw to rotate without building a shot. Purpose: The examination of melt flow lines in injection moulded parts was the purpose of this research, as well as a new method of flow visualization was applied. Surface Delamination. ”. They can be caused by variations in cooling speeds of the material as it flows through the mold. Seven Common Defects and Solutions for PC/ABS injection molding Products,Flow Lines, Pitting,Product Delamination,Stretch Marks. For each investigated mould temperature, a linear relationship can be observed. During plastic injection molding, molten material flows through an opening called a gate and fills a mold cavity. 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. Design for Manufacturing (DFM), also known as pre-mold review, is a collaborative process in the injection mold industry where technical negotiations take place to meet customer requirements. Timing and temperature are significant contributing factors when it comes to flow and knit line defects. You can avoid having any weld lines with cascade injection moulding. 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. Mold venting is critical to the quality and consistency of the finished part. The Reynolds number takes into account the pressure, volume and viscosity of the coolant, the resistance to flow, length and diameter of the channels and the pressure loss in the cir-cuit. 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. 10 in Nguyen-Chung et al. Unraveling Flow Lines. Meld and knit lines are actually both different types of weld lines. mold and the blow molding process. Poor mold surface n Refinish a poor or worn mold surface. 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. Flow lines are lines or streaks that appear on the surface of molded parts and occur due to variations in the flow of molten material during the injection molding process. Solution: Utilize a material that has the stiffest flow possible without causing non-fill. Weld lines in injection molding are formed when two or more flow fronts of molten plastic material meet and merge during the molding process. Solution: Molding Process: This defect can be reduced by raising the material temperature and the nozzle temperature, and slowing down the injection speed, etc. 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. Flow Lines: Flow lines on thinner areas of molded components appear as a wavy pattern, often a different color than the surrounding area. FIG 1 Depiction of fountain flow. 150 in. If during the design and development stage, watering is put on the back burner as an ". Weld line. If necessary, the mold designer may simulate the flow of solidification of resin using an injection molding simulation program. ³/sec. So when two. The most important factors to consider during molding are the mold temperature, melt temperature, and injection pressure and speed. Flow Line Phenomenon: Surface defect in which wavy streaks form like a ripple centered around the gate. Although injection molding is considered a fairly well-established manufacturing process, several independent parameters must be properly set to achieve a high quality of the molded parts. Just multiply the area of the barrel × the injection velocity. This injection molding defect appears as black or rust-colored discoloration on the molded part's surface or edge. Neat polypropylene retains nearly 90% of its strength on a properly designed and molded weld line. SMARTFLOW ® flowmeters effectively and accurately measure flow rate. 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. Low mold temperatures are usually employed to decrease cycle molding time and final process costs; however, resulting surface defects include weld lines, sink marks,. But add 30% glass and weld-line strength retention drops to only 34% as high as the compound with no. to 0. Investigate the mold for signs that a water leak is present, and correct the condition. The weld lines are formed when there is an obstacle in the cavity of the injection mold. Inadequate mold wall thickness leads to uneven cooling and hindered plastic flow, resulting in weld lines. Thin wall injection molding can produce walls as thin as 0. Weld lines can be a weak spot and have therefore to be taken into consideration in part design. Water set-up and design are every bit as important as establishing and recording a repeatable process. Acetal is a funny material in how it reacts to certain injection speeds and mold temperatures. Once the mold is created and the machine is set up, additional parts can be manufactured very fast and at a very low cost. Flow lines and weld lines are lines or streaks in the final molded part. They are also known as flow lines. Raising the temperature of the mold slightly can help to prevent flow lines, as it prevents the molten material from cooling immediately upon impact with the cavity walls. If a mold cavity has a significant flow length-to-thickness ratio, the material should have a low enough viscosity to enable a continuous flow. 750 in. Defects like this make it harder to market the finished parts to the end user. Within more basic plastic part design plans this plane can be a simple, flat surface, but it is often a complex form that. Flow marks, also known as flow lines, are molding defects that occur in the manufacturing process of injection molding. One way to reduce the changes in speed of the flow front is to tailor the injection-speed profile. 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. 080" – 0. The Fill system analyzes the filling of each cavity by sensing the melt presence at the end of flow with a cavity temperature sensor and compares the fill-time. 1, pp. Possible defect causes. Flow lines are ripples in the plastic after the part is formed. In this guide, we cover evaluation and also the remedy of flow lines in. The line where two resin flows “knit” or “weld” together during injection molding is called a weld or knit line. The last cause of flow lines may be the operators running your daily operations. Faster injection increases pressure in front of a flow front. Once you have a cool shell formed the rest of the mold entering absorbs heat from the layer next to the wall. Plastic is injected into a mold at a specific rate of flow—typically in. 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. Too hot and the plastic will start to degrade, but too low and flow marks increase. 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]. Injection molding could develop knit lines if one front cools before it reaches the other. These defects include the following: 1. Weld lines are injection molding defects that can be mitigated by a number of tactics that depend on your part design. We restarted the mold at the same coolant flow rate and temperature. Injection Molding of Multimaterial Systems. Weld lines not only affect the appearance and quality of products, but also cause negative influences on the mechanical properties. 1). The Bottom Line. Lenticular, or Silver Dollar type pigments, are thicker than cornflakes, with a narrower particle size distribution. Flow Rate. They are off tone wavy lines/streaks or patterns in the molded part around the injection ports. The Mold. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and. There are a few adjustments that you can make to your injection molding process to prevent flow lines, including: Increase the injection speed, pressure, or material temperature: Slow-moving molten plastic is more likely to cool. Certain injection molding processes may not be effective for the production of some parts. 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. Effects of mold temperature and pressure on shrinkage 0. Flow lines are the results of slow injection rate and disparities. Imagine a river splitting to go around a rock, then meeting up again on the other side. 1 m/s, and a zero-shear-rate viscosity η 0 (T 0)=3760 Pas. The gate vestige for injection molding is usually narrow. The injection speed has a great impact on the degree of molecular orientation in the component and therefore influences the shrinkage and the development of a joint line. 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. 0010 in. After exhaustive research, testing and pilot-scale demonstration, the team developed a breakthrough new technology utilizing low constant injection pressure. 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. The mold in Figure 6 is an 8-cavity with an “H” pattern runner and a lapped edge gate. Purpose: The examination of melt flow lines in injection moulded parts was the purpose of this research, as well as a new method of flow visualization was applied. With a minimum of 1. Lesson 6: Meld and Weld Lines (Processing) When plastic flows into an injection mold, the internal flow pattern depends on the part design, mold design, and processing factors. Flow lines or marks appear as banded discolouration on the surface. 1 a. When two or more strands of molten plastics meet in the mold, a weld line will be produced at the junction [1,2]. PROBLEM: FLASH ON PARTING LINE There are a few key reasons why flash appears on the parting line of a part. In narrow fields, a limited amount of molten liquid can solidify and block the flow. The Mold. A knit line may be unnoticeable or may look like a crack. 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. As the name implies, it is usually positioned along the edge of the workpiece, forming a visible mark at the demarcating line. In the development of our calculator. 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. Injection molding could develop knit lines if one front cools before it reaches the other. When molten plastic moves through an injection mold at different speeds, the resin solidifies at different rates, which causes this type of defect. Flow lines are linear marks on the surface of a molded product, which indicate the flow direction of the molten plastic. The co-simulation of mold flow and structure plays an important role in the design of injection molded fiber-reinforced thermoplastic (FRT) parts. Injection molding is the most cost-competitive technology for manufacturing high volumes of identical plastic parts. Knit and meld lines are inherent to plastic. Apart from the shape, it is important to design ribs for optimal mold performance. Mold Design. Edge Gates. reduce the ability to positively influence the strength. In addition to accurate temperature control, optimizing flow through a mold is critical and, quite often, results in decreased cycle times and increased product quality and repeatability, even after increasing chilled water temperature. The plastic pellets must be. Voids . Molders should make sure the weld line is formed during the first-stage filling of the part, if possible. Learn how to determine it, and the role of SPI Mold Classes. Mold fill usually takes 0. Spatter lines formed on the surface of the plastic along the direction of melt flow are called silver marks. The accompanying table lists the flow rates for various barrel diameters at various injection velocities. One of the things that can compromise an injection molded part is a weld line, also called a knit line. Study shows that the root cause of jetting is related to the mold design on the one hand and the viscoelasticity of the material on the other. 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. The formation process of weld lines can be described as follows: Injection moulding (U. Thus, as the flow continues, the two flow fronts may gradually rejoin to make one uninterrupted flow front. Weld lines may appear like seams or scratches,. It can also aid in material selection since fill. How to solve flow lines in injection molded products?Injection molding is a heat-transfer process. (2) Adjust the injection speed: high speed allows the melt to reach the confluence without cooling down, and low speed allows time for the air in the cavity to be discharged. 3,000 ton machines can cost $700,000. Complexities in the design and intricate pathways in the mold can also create flash in injection molded parts. 060 and 0. spelling: injection molding) is a process for producing parts by injecting molten material into a mould, or mold. Several factors can contribute to shrinkage in injection moulding, including material, part design, mould design and processing parameters. As the material properties of. That’s an easy calculation to perform. 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. 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. At a theoretical flow distance of 10 mm it represented the measured values well but at a longer distance it underestimated the experimental scale. Sink Marks. Round the corners of the mold where wall thickness increases: This. A flow mark is commonly referred to as a flow line occurs during the process of injection molding. #TROUBLESHOOTING #FLOWLINES #INJECTIONMOLDINGStudy more about troubleshooting flow lines :. Draft – Add 0. Turbulent Flow IS Always Better. If the melt speed of each gate entering the cavity is inconsistent, it is easy to cause weld lines at the intersection. It is a well-known fact that weldlines are unavoidable in most injection-molded products of even moderate complexity. Increase injection speeds and pressure to ensure the cavities are filled properly. AnalysisInjection moulding simulation with the finite element method is the most advanced technique for designing injection moulds. 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. Molds separate into two sides at a parting line, the A side , and the B side , to permit the part to be extracted. Higher numbers assure that water flow is adequate, and that turbulent flow is at an acceptable level. These flow fronts occur when the molten plastic material encounters obstacles, such as inserts, ribs, or other features within the mold cavity. Reduce the injection pressure to prevent overpacking at the gate. Possible Sources of Weld/Knit Lines: Temperature from the injection molding machine, runners, and/or the mold itself in order to ensure the plastic resin flows properly; Insufficient pressure resulting in the flow fronts not being merged back together with strong enough; Mold design, especially location of gates and runners impacting the resin flow Knit lines are formed in injection molding when two flow fronts meet in the middle of a part (a line where two sides of the plastic are “knit” together). From the perspective of the mold, when the molten plastic reaches the gate through a larger-sized runner, the flow resistance of the melt is greatly increased because the gate size is usually very small. We started the mold with 100% water circulating at 60°F at a flow rate sufficient for turbulence. Polymer Injection Molding: Gas-assisted Molding. Our glossary of terms provides definitions for a wide range of injection molding terms. Known as thermolators,. 2. To further improve productivity, the injection mold was equipped with CCCs to shorten the cooling. The recommended minimum production volume for injection molding is 500 units. Figures 3A, B and C depict different examples of “spot” vents. How a mold is watered is key to a processor's goal of consistency. Caused by: Non-uniform cooling times and flow rates; excessively narrow part features. 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. Expand. Secondly, it is easy to form obvious. Mold Flow - Position ribs such that the plastic has an optimal flow path. Possible causes: Low temperature of injected melted plastic or slow injection speed during injection molding resulting in low fluidity of plastic inside the mold. This is defined for each material by the length-to-thickness (L/T) ratio of the part. How weld lines form. For injection molding grades, the MFR (MI) values for polyethylenes are generally determined at 190°C (374°F) using a static load of 2,160 g. Weld line. Mold surface is too cold.