The wall thicknesses of an injection molded part generally range from 2mm – 4mm (0. and Titomanlio, G. They again get resolidified along the surface. (for example, a hole on the plastic part), the knit line will be unavoidable. A knit line is any line, visible or not, where two resin flows meet. Trapped gas is one of the most common causes of part burns, and can also lead to short shots and voids, blemishes and discernible knit lines that weaken the part. An injection mold that produces pieces while the final tool is being built. The gate is the entry point to the part-forming cavity, where plastic material is injected from the molding machine barrel, through the sprue and runner, into the mold. ABS is a good choice for handheld consumer devices. 8 mm relief at the edges of the thread. Weld lines create visible marks, but this type of injection molding defect can also reduce a part’s strength and durability. A knit line is the spot where two flows meet. With the right nozzle, you can boost cycle. There is no need to worry about flow or knit lines that may appear with injection molding. Influence of injection-molding parameters on the morphology and mechanical properties of glass fiber-reinforced polypropylene composite. Knit lines are consideredmolding defects, and occur when the mold or/and materialtemperaturesare set too low:. It is. One exception: ribs should be about 50. 120 in. It is also one of the most important weapons in a molder’s arsenal for making process changes. [1] Here are a few ways to prevent knit lines in your plastic parts: Increase material temperature: Higher temperatures on both materials prevent one section from cooling too quickly. LCP can replace such materials as ceramics, metals, composites and other plastics because of its outstanding strength. And the lead-time for overmolded and insert molded parts is 15 days, a bit longer than FacFox’ single-part injection molding turnaround, but still much faster than traditional insert molding. The packing pins are sequentially. Contact us. While knit lines can be present it does not always mean the structural integrity of the plastic part is compromised. Poorly constructed or worn out mold. The melt is forced to split up during filling of the cavity. Left: L-shaped perform. A poor knit line can cause only cosmetic blemishes or it can significantly weaken the structural integrity of a part. Venting. From Plastic Materials and Processing by A. This article will discuss the importance of injection molding nozzles and what makes a good nozzle for a molding or injection. The primary rule is to apply 1 degree of draft per. In the case of mold. Mechanical performance of knit lines and welded butt joints – A key factor in designing of multiple gated injection molded and subsequent welded. The Problems & Solutions course is designed to train injection molding personnel in. These defects are not visible, like small. To prevent swirling, perform tests whenever possible and use pre-compounded resin to ensure identical color match across parts. Weld lines are also known as “stitch lines” or “knit lines”. Typical Tool Steel for Injection Mold Construction Steel Type Steel Properties Mold Component P-20 Pre-hardened, machines well, high carbon, general-purpose steel. Knit Lines. Injection Molding: How to Set Second. Parting lines placed at intricate joints. Specifically, by having a single, continuous injection outlet 140, molding material can be uniformly distributed throughout mold cavity 120 quickly and consistently without resulting in undesirable molding seams or knit lines. You can avoid knit lines and other problems by creating designs with smooth geometries. Reducing the dimensions of the runner system. Increase the injection speed. Knit/Weld Lines. The weld-lines are undesirable from both aesthetic and functional points of view and their effect is different for varied materials, process conditions, loadings, forms. Long shapes are produced without multiple drops or hot runner systems, eliminating knit lines and lowering tooling costs. Insufficient injection pressure and speed can cause weld lines. The runner and gate system used in plastic injection molding is responsible for directing molten plastic from the nozzle of the molding machine into the cavity of the tool. Part Shorts No burn marks. Highly ScalableVacuum on the mold does help and many times eliminates the weld line. Gate design is another critical piece of part function in plastic injection molding. Example of a gate location in a finished part Example of knit lines in a finished part. Notched weld lines then remain near the cavity walls. Knit lines are most common downstream of holes created in parts where the molten material meets. When assessing issues with injection molded parts, we want to make sure some core aspects of molding are checked. Mold cooling is one of the most important variables in injection molding. He is passionate about optimizing product design, tool development, molding processes, and quality. Short shots Trapped air in the mold can inhibit the flow of the material during injection, resulting in an incomplete part. These are imperfections in the part where separated flows of cooling material meet and. The fluid path has an inlet and a radially continuous, downwardly. In spite of its cosmetic attributes, a bad knit line can significantly compromise a part. Injection molded plastic begins its journey through the part tooling via an entry section called a. 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. Knit lines form during the injection molding process when two resin flows meet. Steven. When venting stationary inserts or core pins in rib or part details with venting issues, I recommend no more than 0. N174W21430 Alcan Dr Jackson, WI 53037 262-677-9700; 16685. 4. (Overpacking. There are two primary reasons for this occurrence: 1. (1. The utility model relates to a mold structure capable of eliminating the appearance of weld lines of a hole-shaped injection molded product. There are processes such as gas assist that do not require the injection unit to pressurize (pack out) the cavity, but for our discussion, we will stay with conventional injection molding. Using restrictors helps push visible knit lines into less conspicuous areas. The relationship between processing conditions, morphological structure and mechanical properties of knit lines of an injection molded semi-crystalline thermoplastic. Parts with abrupt geometry changes or holes are more prone to knit lines. Industrial Knitting Machines. Knit Line Injection Molding is a process that allows manufacturers to produce parts more quickly, accurately, and at less cost than traditional injection. Too cool a melt. If the plastic has cooled too much during the injection process it can lead to knit lines in plastic parts when they meet past an the obstruction. In a well-designed mold, this is generally not a problem. 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. When the mold adopts the multi-gate pouring scheme, the plastic melt. weld line in injection molding. The defect may be in part appearance or mechanical strength. An option to bluing agents, the long established means of gauging pressure distribution, comes via Pressurex, a thin, flexible, plastic film. The structural stability of. Insert injection molding is a process that requires an insert — typically metal — to be pre-placed in the tool for injected plastic to flow around. Obstructions in the part create weld lines or knit lines during injection molding. When injection molding a part, a knit line may present no problem, or can cause a serious structural problem. Revisiting the '5 M's' of Molding. Some are: Knit lines (discontinuities where two flows merged, creating a weak point)Those are knit lines, formed when two opposing flows of material join together in the mold cavity. In working with the injection molding service offered by Protolabs, you provide. They occur when two resin flows meet at different. Weld / Knit Lines; Assumptions. They occur when melt flow fronts collide in a mold cavity. Runner Type. Mold Venting. 160"). The main causes of Weld Line or Knit lines are: Convergence of two flowing polymer melt streams during injection molding process: This is the most common cause of weld lines, as the two melt streams coming from different runners or gates in the mold meet and do not fully mix, creating a weak point in the finished product. Weld lines can significantly reduce ultimate tensile strength (UTS) and fracture strain of talc filled polypropylene (PP). This range can be affected by the part wall thickness, melt temperature, mold temperature, injection speed, hold/pack pressures and also the time between molding and measuring. (1987). Knit lines are typically much weaker. 3. 080" – 0. spelling: injection molding) is a process for producing parts by injecting molten material into a mould, or mold. Product designers encounter a wide range of challenges when creating and developing molded parts, such as stress factors, warp, and those pesky knit lines. In injection molding, it is often important to maintain consistent color throughout the part. Knit lines, weld lines, mold lines, or flow lines in injection molding are faint lines that become visible on certain plastic parts. 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. Cold weld line or known as “knit line” isWeld Line defect analysis, the most common of many defects in injection moulding products. The effect of holding variables and of injection and mould temperature was explored. These are imperfections in the part. Rapid heat-and-cool molding technologies have attracted increasing. An injection mold that produces pieces while the final tool is being built. The part is cooled in the mold until it is solid enough to be ejected. I am working on a thin PC/ASA injection moulded component which has a tendency to snap at the Knit-point (the final point of travel at which the…Here are a few of the most common defects that may occur in plastic injection molding — and how product teams can avoid them. Download : Download full-size image; 3. Material. A poor knit line can cause only cosmetic blemishes or it can significantly weaken the structural integrity of a part. All the while, the material could be the correct material," Hill says. Injection molding flow lines typically fall into one of the categories mentioned in the following table: Category: Type of Flow Line: Appearance (What It Looks Like) 1. Sink Marks Cause 3: Improper Pack or Hold Pressure. In the plastic injection molding industry, we work with materials that are byproducts of oil refinement and molds made of metal casting, so our terminology is bound to be at least slightly confusing. 5 mm) is ideal, according to injection molding engineer Jack Rulander of Protolabs in Maple Plain, MN. 2. Misra A. When injection molding complex parts, often knit-lines are formed during the mold filling stage. One of these is knit lines, also called weld lines. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. Much of this research has focussed on the materials which. Plastic melt and mold temperature. But applying excessive force can create problems with the part, mold, and machine. Weld Lines-3 Ways for Detecting and Correcting. 2. If the part design’s wall thickness is constant, the magnitude of the plastic’s velocity will be the same at every. These seams or knit lines are avoided because the molding material flow does not a meet at an interface nor must. Knit Lines 11: 1. Solid or semi‐solid, this material leaves a distinct border with adjacent melted material similar in. 040 and 0. shows some obvious locations for vents, a brief molding trial should be conducted to observe where the gas voids occur. It can be determined by a visible line appearing on the surface of the plastic part where two flows of the plastic meet. They again get resolidified along the surface. ICOMold is a plastic injection molding company that manufactures plastic injection molded parts. The process parameters were fixed as follows: Packing pressure of 40 MPa, injection time of 2. Mold venting is critical to the quality and consistency of the finished part. Plastic injection molding has long been an affordable and efficient process for producing custom prototypes and end-use production parts. Vents for phenolic parts should be approximately ¼" (6 mm) wide and 0. Weld lines typically appear in a part when an obstruction of some kind separates the melt flow and prevents it from "knitting" together. Islyn Thomas summarized the need for a particular vent location better than I ever could. What Are Knit Lines? When a product is produced through injection molding, a high-powered tool injects melted plastic into a custom, double-sided mold. All the while, the material could be the correct material," Hill says. Vibration assisted injection molding. Conversely, texturing of part surfaces is often effective in hiding flaws and imparting a “grippy” or leather-like. matte or smooth texture on the part’s surface gives a more desirable aesthetic and hides small imperfections such as knit lines, flow lines, shadow marks, and sink marks that were caused by the mold. What causes knit lines? Knit lines are formed when the molten plastic flowing into the mold cavity runs into an obstruction, such as a core pin that forms the through hole on a flange of an enclosure. 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. Knit lines generally occur around holes or other obstructions to the melt flow. Weld lines, created in the areas of collision of two flow fronts of plastic in the injection mold cavity, are the reason of lower mechanical properties and a worse surface condition of molded parts. a specific area on one side lens, a warp line, and a weft line. The following are the conclusion drawn from the present research: 1. Injection moulding can be performed with a host of materials mainly including metals (for which the process is called die-casting ), glasses, elastomers, confections, and most commonly thermoplastic and. Flash: All plastic injection molds contain parting lines, which create the potential for small amounts of molten plastic to escape the mold cavity’s boundaries. When assessing issues with injection molded parts, we want to make sure some core aspects of molding are checked. 5mm (0. A knit line in a plastic injection-molded part is created when two separate plastic flows meet within the mold and "resolidify" along their interface. 2020-01-1306. They are the result of fundamental physical effects taking place inside of the mold. 060 and 0. The quality of the answer is directly proportional to the information we have to work with. Reduction of sink can possibly be had by switching to an ABS/PC-blended material. 040-in. Gating. It has low shrink and high dimensional stability, and it has good resistance to acids and bases. Avoid areas where the plastic must suddenly flow upward or downward. Holes in plastic parts are normally cut with a router (or drilled if round. Increasing injection speed. Reduce the screw speed and adjust the backpressure. Nozzles for Injection Molding Machines. 11) The part design must take into account possible knit line locations and work to direct them away from higher stress areas. Encapsulating the metal insert with plastic creates a single molded plastic piece that’s generally stronger than one created using secondary assembly. So what does the 140F mold temperature give us? The molding technician thought the 140F mold water was OK because the data sheet provided by the material supplier suggested mold temperatures from 75 to 180F. Injection moulding (U. As always, feel free to contact us at 877-479-3680 or [email protected] if questions or concerns emerge. There are times when porous materials like Porcerax are used in problem or backfill areas. ) or as thick as 25 mm (1 in. A weld or meld line is a weakness or visible. For the production of these weld‐line specimens, a special mold was used and injection molding parameters were systematically varied, tensile (ISO 527‐2) and impact (ISO 179‐1/1eU. And like all manufacturing industries, plastic injection molding lines are more profitable the shorter the cycle time — so long as they produce quality parts. This picture shows a. Injection mold design and custom tooling can make or break the success of a finished plastic product. Double push filling method. With over 50 years of experience, Universal Plastic Mold can help you avoid (or at least minimize) knit lines in your molded parts. SEA-LECT Plastics has 16 MILACRON injection molding machines that range from 33-ton with a 2. Weld lines may appear like seams or scratches, but that is not what they are. For injection molding of the multi-blade fans and various rotating plastic parts, the complex of multiple gating injection molding tools were used. The second mold portion being movable relative to the first mold portion and together defining a mold cavity. In an injection mold, the plastic fluid flows and spreads out in many directions as it comes out of the gate. The plastic melt that passes through the gate at high speed directly enters the cavity, then contacts the surface of the cavity to solidify, and then is pushed by the subsequent plastic melt, leaving traces of meandering. In the injection molding process, knit lines are a common occurrence when two or more plastic flow fronts collide and solidify, resulting in a visible seam or line on the molded part. Venting at Weld Lines. incomplete filling or shorts. The most common and one of the most challenging injection molding defects are weld and knit lines. 1. Polym Compos 1982; 3: 88–96. Gate size can be increased or moved to a section with greater wall thickness to help eliminate. Meld and knit lines are actually both different types of weld lines. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. They are the result of fundamental physical effects taking place inside of the mold. A structural finite element analysis (FEA) can be used to compare locations of high stress to the locations of the knit lines identified in the mold-filling analysis. As. Circular Loom Machine; Raschel Knitting Machines; Shade Net Making Machinery; Inspection Machines; Non Woven Production Line. The knit lines are the result of poor molding conditions (likely molding at too low of a temperature). Incompletely fused extrusion knit Line. [8] Bevis, MJ, Reis RL, Cunha AM. In addition, the company is now providing three. The point I am making here is, if you’re injecting 14. Direct experience designing molds and parts for injection molding and producing the molds and parts is valuable, relevant experience for design students. 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. Remedies: Increasing the barrel temperatures will allow the flow fronts to stay hotter longer and. Proper mold cooling can eliminate warp, decrease cycle times, and increase part quality. No matter the size of a production run, the presence of blemishes on the finished product leads to unhappy clients. 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). Knit lines and other aesthetic characteristics of the part are strictly related to the plastic’s temperature during molding, so efficient filling time is essential for achieving proper welding lines and quality parts. Plastic injection molding has long been an affordable and efficient process for producing custom prototypes and end-use production parts. com TYPICAL PROPERTIES MECHANICAL THERMAL OPTICAL. 18/06/2021. All injection molding ultimately comes down to the “Man, Mold, Machine, Material and Method. A poor knit line can cause only cosmetic blemishes or it can significantly weaken the structural integrity of a part. An injection molding machine having a first mold portion and a second mold portion. 4. Here are some reasons why design is important for injection molding. Gates, vents, sliding cams, pick-out inserts, and any other place where the two sides of. POROUS MOLD MATERIALS. 6 Monate. Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. Without adequate support pillars, flashing of the parting line faces and bowing of the plates may occur. Weak weld lines are perhaps the most common and difficult injection molding defect to eliminate. Swirling can increase depending on the material you use, and the texture or polish of injection molding tools. Injection Pressure 0r Speed Too Low Explanation: If the injection speed or pressure is too low, the molten plastic will not be pushed into the mold cavity fast enough to fill and knit at converging flow fronts because it cools down and solidifies too soon. 1. Knit lines. Dedicated to strict quality management and thoughtful client services, our experienced staff customers are generally available to discuss your demands and guarantee full client pleasure for Knit Line Injection Molding, Electronics. Improve venting and/or relocate to burned area. Still, they are almost always a cosmetic issue. The parting line will always be visible, but can be minimized by its location, draft allowance and part geometry. Place a vent in the weld line area to remove the entrapped air. Polym. Examples of different causes of PVC pipe failure. Polypropylene can be processed over a wide range of mold temperatures; however, for applications where aesthetics are critical, a surface temperature of 80-95˚CSink Marks Cause 1: Incorrect Melt Temperature. They appear around holes or seams of an injection molded part, and some may call them. ITAR. Submarine gates are another injection molding types of gate that is located below the surface of the part, typically on the underside or back of the part. The plastic in the ‘flow front’ cannot pass through the core pin, so it flows around the core pin splitting into 2 distinct flow fronts. Therefore various injection molding defects can be eliminated. Kaminski, in Specialized Molding Techniques,. As the polymer resin is injected into a mold, it flows through all parts of the mold. This slight depression is a tiny line called the weld line or knit line. This course outlines mold filling and how different gating configurations affect filling. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. It is one of the most common blemishes for manufactured products. Industrial Knitting Machines. Injection molding is the cost-effective method and is best suited for higher volumes. A knit line may only be a cosmetic issue and affect. One of the common defects that can occur in injection molded parts is weld line. gate position or runner. Increase the melt and/or mold temperature. Injection Molding Problems and Solutions (17/17) – Weld or Knit Lines Sep 27, 2022 Injection Molding Problems and Solutions (16/17) – Warpage Sep 15, 2022. Plastic injection force exceeding the clamping pressure of the press. Three terms that often get mixed up are weld, meld, and knit lines. Cold Slug: First material to enter mold during injection. They are formed downstream through holes, and between multiple gate locations. Injection molding is typically a better choice for more complex parts, while compression molding is a great option for relatively simple designs, including ultra-large basic shapes that cannot be produced using extrusion techniques. Getting started; Knit Line Injection Molding; Knit Line Injection Molding - China Manufacturers, Factory, Suppliers. Basic rule of thumb air in mold must be able to escape at theAbstract The knit line structure of injection moulded PA 6 samples was analysed by SEM and optical microscopy. The knit line itself (the vertical line in the middle) is visible in this lighting, but the biggest issue is with the texture. Increase injection speed. Viscosity represents a broad concept with huge effects on any efforts to standardize an injection molding process. 6. Mold cores can be machined from a solid piece of copper alloy or inserted to aid in construction or allow for easier replacement if a com-ponent would ever be damaged in molding. The mold settings can run flawlessly until employees consistently open the machine for inspections, interrupt cycle times, or cause temperature. It emerges and becomes visible when the flow fronts cannot mold or knit during the molding process. In spite of its cosmetic attributes, a bad knit line can significantly. 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. The mold maker may think that some minor defects are acceptable, or they may not even consider them from the beginning, which can cause problems when the injection mold is being manufactured. But we can’t talk about the injection molding process without discussing the injection molding machine. sinks, knit lines, and shrinkage,” Spalding says. In the 1947 book, Injection Molding of Plastics, the late Dr. Three terms that often get mixed up are weld, meld, and knit lines. In this case it would be 0. To avoid walls that are too fat or too thin, designing a wall thickness between 0. In addition to the main factory and development. Injection molding weld lines refer to a line on an injection molded part that is generated where the two halves of the mold come together. You don't give us much to go on. In layman’s terms, injection molding boils down to four simple steps: Melt material. SPI surface finishes are the most common set of injection molding finishing standards around the world with over 12 different grades. 040-in. 1050 Walnut Ridge Drive. Polypropylene Consumer and Injection Molding Containers, Crates, Garden Furniture, Housewares, Luggage, TSL, TWIM lyondellbasell. Understanding Knit Lines in Injection Molding: Causes and Solutions. Below is a list of items that should be checked both during normal molding operations and if defects are detected. Table 2. By altering the part design, it may be possible to just have one flow front, and avoid multiple flow fronts having to come together. Meld and knit lines are actually both different types of weld lines. In the world of injection molding, each weld. Figure 1: Knit lines in a cover plate. What Causes Knit and Meld Lines? Like many part inconsistencies, these unwanted features are rooted in product design. To study the weld line formation of colliding flow fronts the filling of a mold cavity was simulated. – Change the thickness of the product. Thanks to the low air counter pressure in the cavity, this allows injection parameters to be managed in a larger window, optimizing injection time, part aesthetics and mechanical characteristics. AS9100:2016. These knit lines affect the parts aesthetics, but also they generally decrease the strength of the part. Soft metals, such as aluminum and beryllium copper, can be used for prototype<br />. Venting is required to allow the air in the sprue, runner and cavity to leave the tool as the melt flows into the cavity. 1. They occur when melt flow fronts collide in a mold cavity. In this video, Chris Burda, applications engineer at Protolabs, shares what can be done to reduce knit lines and how they can impact your part's functionality. ”. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and. When injection molding complex parts, often knit-lines are formed during the mold filling stage. Flow Lines Knit or Weld Lines Cavity Gates Table 1 Knit line in a multi-injection-point cavity. What causes knit lines? Knit lines are formed when the molten plastic flowing into the mold cavity runs into an obstruction, such as a core pin that forms the through hole on a flange of an enclosure. (See Fig. A weld line can also be called a knit line. Weld and meld lines are generally caused by holes or inserts in the part, multiple gates, variable wall thickness or hesitation and. Weld or Knit lines are a plane or section in an injection molding part where the flow of molten plastics meets from different directions. Knit Line occurs where different flow fronts of material meet while flowing through the mold. The injection molding process is on the surface is fairly simple, plastic resin is heated to its melting point and forced into the cavity of the mold to produce a plastic parts. 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. Fig. Surface Finish: The injection molding process may result in visible knit lines or parting lines on the part’s surface. After selecting resin material, the design and process control of injection mold directly influence the quality of formed products. For this study, a special injection mold was designed and constructed (Figure 1) in order to produce specimens of various thicknesses (from 2 to 6 mm). Below is a list of items that should be checked both during normal molding operations and if defects are detected. Thus, as the flow continues, the two flow fronts may gradually rejoin to make one uninterrupted flow front. This is easier to accomplish on. The injection molding process uses a granular plastic that is grav ity fed from a hopper. Meld and knit lines are actually both different types of weld lines. One common problem that occurs in injection molding is the formation of knit lines. So. Gates, vents, sliding cams, pick-out inserts, and any other place where the two sides of. It is important when selecting an injection molder that the right process conditions are used to reduce or eliminate the effects of. Brent Strong, p. In the injection molding process, melting takes place via two factors that work together: shear and heat. Design bosses with an outer diameter equal 2x the nominal diameter of the screw or insert. By contrast, a slow injection speed will give you a matte finish and a cold mold, such as 50F, will give you a more flexible part. Learn how to troubleshoot weld lines in injection molding. Material contamination, for example, can cause several molding issues. Reduce the barrel temperature and increase the injection pressure. Generally all plastic components should be designed with draft where possible. Injection Molding Materials When using PolyJet molds, both tool life and part quality will be dependent on the thermoplastic material used during the injection molding process. Also known as knit lines. Neat polypropylene retains nearly 90% of its strength on a properly designed and molded weld line. 3 /sec. 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. Depending on the design of the mould and the material being injected, a knit line may: present no problem at all, be a cosmetic issue, or cause a potentially. Mold Flow Analysis computer. A component of the injection-molding machine wherein the resin pellets are melted, compressed and injected into the mold's runner system. Knit lines, holes, part configuration and the talent of your molder all come into play. Plastic injection molding has long been an affordable and efficient process for producing custom prototypes and end-use production parts. A majority of these investigations concern amorphous polymers, whereas a number of engineering polymers are semicrystalline and also contain reinforcing fillers and fibers. Plastic injection molding is a complex and highly intricate process. Last month, in the first part of this two-part installment we focused on the reasons viscosity can change during injection molding. Weld line defects – also known as knit lines – is a defect that occurs when the injected plastic meets within a mold. Weld lines, knit lines, and meld lines are terminologies to describe the imperfections of finished parts in plastic injection molding processing. They again get resolidified along the surface. The injection of molding compound into the parts through one or two gates may produce parts that have knit lines. (knit) line and produces a latent, or obvious, part defect. Sprue bushing too long. Prevent Flow and Knit Lines Through Mold Adjustments. . To learn more about us or to get a quote, click here or call 1. The goal is to create a uniform thickness and at the smallest (thinnest) level required. Fewer knit lines. If you have any issues getting. Weld-line strength for plastics can vary from 20% to nearly 100% of the strength of the plastic itself. . Mold bases, ejector plates, and some cavities (if nickel or chrome plated to prevent rust). The temperature might fall at some places like in the mold, runners leading to the mold and injection molding. Temperature is another factor that can cause weld or knit lines in injection molding. 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. Back to News Page. Piccarolo, S. 197 in. 5-3. Raising the mold temperature will keep the material molten longer, resulting in stronger weld lines and even elimination of weld lines. For over four decades, our experienced team and leading edge technology have provided you with tooling solutions from start to finish. "As soon as that part or material is stressed, it can be real conducive to fracture, cracking, breaking, just because of the way it's been molded. RapidDirect – Your Best. There.