Remedies: Raise the temperature of the mold or molten plastic. (See Fig. Meld and knit lines are actually both different types of weld lines. As the melted plastic fills the mold, there are locations where the molten plastic flow stream separates and then comes back together. The advantages of compression molding include more isotropic fiber distribution, dimensional stability, and surface finish. 1050 Walnut Ridge Drive. Additional venting along the parting line perimeter will improve overall venting significantly. In addition, the force of the gas holds the appearance side of the part in contact with. 5 : 1 and a L/D (length toMold flow analysis is a software simulation which show how the resin fill in the mold in injection molding process. Once the mold is created and the machine is set up, additional parts can be manufactured very fast and at a very low cost. Operating Conditions • Fast injection fill speeds, supporting short injection times (1–3 s), especially in thin sectionsKnit lines in reaction injection moldings Knit lines in reaction injection moldings Maier, Christian; Hrymak, Andrew N. BLEED – (1) To give up color when in contact with water or a solvent. Users will be able to use in-cavity pressure or temperature sensors to control when the valves open or close. Molding is a manufacturing process in which resin in a barrel is heated to a molten state,. 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. Knit lines. Gates, vents, sliding cams, pick-out inserts, and any other place where the two sides of. injection molding machines using standard industry practices . The mold opens and closes on a vertical axis in the vertical injection molding. 0 to 3. The knit line is generally opposite the gate in thermoset. 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. Core out parts to eliminate thick walls. Thermoplastic Elastomer Molding Guidelines. This article will look at two common venting options—using traditional machined vents and venting through porous materials—to help you decide which is right for your application. The depth of the texture or letters is somewhat. The mold’s sprue bushing and locating ring is placed in a way that makes sure the injection molding nozzle is properly aligned and centered. Please read on, stay posted, subscribe, and we welcome you to tell us what you think. Call 800-985-6750 or email your inquiry to sales@customgasketmfg. placed usually in the areas last to fill, near knit lines, as well as on the runner system. 3. Inadequate venting may cause short-shots, poor surface appearance, or weak weld-lines. Contact us. Observation on a “trial and error” basis may be required. The Problems & Solutions course is designed to train injection molding personnel in. 763-755-7575. Vacuum. 2. These knit-lines are well-known as particularly critical regions in the mold when mechanically loaded. In a plastic injection-molded component, a knit line occurs when 2 different plastic flows emerge in a mold. Use the fewest number of gates possible. A weld line can also be called a knit line. This is a quick rundown on how to handle common features in injection molding designs:What are the advantages of injection molding 1. 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. Knit/Weld Lines. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic. Proper venting is essential to molding a defect-free part. Reduce the temperature of the mold and plastic being injected. The goal is to create a uniform thickness and at the smallest (thinnest) level required. Swirling can increase depending on the material you use, and the texture or polish of injection molding tools. Our goal of 100% efficiency, zero scrap and repeatable processes can sometimes be hindered by poor setups or changes in the molding process. The injection molding process uses a granular plastic that is grav ity fed from a hopper. Our glossary of terms provides definitions for a wide range of injection molding terms. Plastics Technology article (June 2018): Technical capabilities of the process have advanced to erase former cycle-time penalties, and ongoing developments address creation of a materials database and new applications in hybrid composite injection overmolding. 025 inches per inch, depending on the particular resin, molding conditions, part design, part wall thickness, and direction of flow. The knit lines are the result of poor molding conditions (likely molding at too low of a temperature). knit line formation, application of draft angle and design for assembly. Reduce. The Goldilocks principle applied to wall thickness. thickness with a spacing of about 0. AS9100:2016. (2) Undesired movement of certain materials in a plastic. Round Rock, TX 78660. Weld line. Wall Thickness: The recommended wall thickness for injection molded POM parts is between 0. 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. What you should know is that there are various failure modes for injection molding. For non-reinforced or non-filled nylon, the mechanical performance in the knit lines (planes) areas are approximately equal to the mechanical performance of the resin (polyamide) used. Hartland, WI 53029. Wax or ceramic core size or location See Section 3. The metal parts, made of steel grade. A knit line is any line, visible or not, where two resin flows meet. Incorrectly placed single gates may cause knit lines. H-13 Good general purpose tool steel. Our Injection Molding Capabilities • Horizontal Injection Molding up to 3300 TONS • Structural Foam Molding + Structural Web • 1oz to 600oz part capacity. Main causes of knit lines in injection molding: 1. There are several possible causes that need to be investigated to troubleshoot weld lines in injection molding. The linear mold shrinkage of injection molded parts will vary from 0. The melt is forced to split up during filling of the cavity. 1990-03-01 00:00:00 Knit lines in reaction injection molding (RIM) are formed when a fluid flow front passes through the mold cavity and must pass around objects in its path. RELATED CONTENT. 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. Focus - Injection Molding - Free download as PDF File (. The parting line will always be visible, but can be minimized by its location, draft allowance and part geometry. PCS says each module of the Polimold Valve Gate Sequencer controls two zones, with the option to control up to 14. BACK TAPER – Reverse draft used in mold to prevent molded articles from drawing freely. All the while, the material could be the correct material," Hill says. Weld lines may appear like seams or scratches,. Know about the cause of these lines and the potential solutions. An edge gate introduces material at the main parting line, or where the two halves of. When injection molding complex parts, often knit-lines are formed during the mold filling stage. You don't give us much to go on. 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. Knit lines. Plasti-Co ® manufactures a wide range of injection molding nozzles from standard nozzle tips, extended nozzle tips, nozzle bodies, extended nozzles. After selecting the direction in which the mold hole will be cut, it is much simpler to locate the separating line. • Holes formed by core pins are, in essence, 360° inside corners and should follow. Design with Plastics - for Injection Molding process. Those are knit lines, formed when two opposing flows of material join together in the mold cavity. In injection molding, it is often important to maintain consistent color throughout the part. Many of these tips are geared around injection-molded parts, but some apply to machined and 3D-printed parts as well. 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. to prevent improperly placed knit lines or sinking in the plastic downstream from the hinge. When the temperature is not very high, there are chances of premature solidification, which can cause weld lines to form. Also discussed are weld and meld lines and their effects on molded. Three terms that often get mixed up in the plastic injection molding industry are weld, meld, and knit lines. Three terms that often get mixed up in injection molding are weld, meld and knit lines. TEXAS. knit line. Molding Troubleshooting Guide Bond Readout Bond readout is a surface distortion similar to a hump or sink that occurs over a bond line Probable Cause Material Process • Mismatch of compliance between outer panel, inner panel and adhesive • Incompatible thermal expansion coefficients between the SM and the adhesiveMolding Troubleshooting Guide Bond Readout Bond readout is a surface distortion similar to a hump or sink that occurs over a bond line Probable Cause Material Process • Mismatch of compliance between outer panel, inner panel and adhesive • Incompatible thermal expansion coefficients between the SM and the adhesiveMost failures of injection molded plastic parts are caused by either improper design of the part or improper design of the mold. As a result, the portion has a faint, perhaps undetectable line. 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. Learn more about the process by reading this white paper. Knit lines don’t. We recommend walls to be no less than 40 to 60 percent of adjacent wall thickness, and all should fit within recommended thickness ranges for the selected resin. An undercut is any indentation or protrusion that prohibits an ejection of a part from a one-piece mold. The resin is transferred to the mold cavity under positive. Understanding Knit Lines in Injection Molding: Causes and Solutions. 020"). When designing for injection molding, consider these steps to prevent knit lines Resin cools as it is injected into a mold. Knit lines, also known as weld lines, are fine lines that resemble hairline fractures or cracks. The weld line can either be a minor cosmetic issue. 3. weld lines may form during the injection molding process and work closely with the plastic injection molder and tool builder to choose the best possible gate location. mixed manually and quickly stored in the tank of an injection machine just before the start of the resin injection or (2) the components are stored separately in different tanks of an injection machine with a mixing head. This summary represents a key subset of the detailed molding information found in the remainder of this molding guide . Brent Strong, p. Commonly seen at the edge of a hole or other cored feature, knit lines are—as a. Knit lines, also known as weld lines are areas in the. Weld lines typically appear in a part when an obstruction of some kind separates the melt flow and prevents it from "knitting" together. The company provides an array of digital manufacturing expertise. Lower the melt and mold temperature to prevent overheating. 3—Another issue with the four-gate design was that knit lines ended on or near mounting holes, causing a. The knit line itself (the vertical line in the middle) is visible in this lighting, but the biggest issue is with the texture. Rynite® PET is usually processed with melt temperatures between 280 to 285 °C (535 to 545 °F) . Venting at Weld Lines. Thermoplastic Elastomer Molding Guidelines. Be wary of saving too much weight or you might end up with: cracking. 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. The Difference Between Knit and Meld Lines–and Why It Matters. While injection molding dies are expensive to produce, each die canHowever, they can also create weld lines or knit lines in the part where the flow fronts meet. It is best practice to maintain a uniform thickness throughout an injection-molded part. 344. A plastic product formed by the injection mold is divided into two parts, and the line separating the product is called the parting line. occupied frequently with troubleshooting injection molding. Services. The comment on melt flow index is correct, but, I've molded straight PC down to . 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. Most failures occur at what are called “weld lines” or “knit lines”. They are formed when two or more flow frontFor more information on our custom rubber molding services or to speak with one of our knowledgeable professionals, please feel free to Contact Us or complete our online Request For Quote form. Knit lines are formed in injection moulding when two flow fronts meet in the middle of a part (a line where two sides of the plastic are “knit” together). Based on the resin temperature, filling speed, and resin, there is a variation in the knit lines. Notched weld lines then remain near the cavity walls. Islyn Thomas summarized the need for a particular vent location better than I ever could. When needed, crush ribs can allow for draft and a tight fit for a part that requires both. They have their origins in material choice, part design, tooling, and processingInjection molding is generally restrictive (not including special processes) in the cross-sectional size of the part, while blow molding allows for channels in the part that increases strength. Shorten the mold cycle time so that any trapped air and resin don’t have a chance to overheat. Heat: Too much heat can have an appearance similar to moisture splay. An injection mold that produces pieces while the final tool is being built. For over four decades, our experienced team and leading edge technology have provided you with tooling solutions from start to finish. reduction on the side the part is folding towards. Imagine a river splitting to go around a rock, then meeting up again on the other side. Here are some ways manufacturers can avoid quality issues during the plastic injection molding process: Inadequate Injection/Holding Pressure: If injection and holding. Here are some of the most common cosmetic defects and how to avoid or fix them. These knit lines are formed as molten plastic in the injection mold flows around an obstruction like a feature. 01 inches between parts fitted together to work best. Facebook. Once the plastic sets, the mold opens to eject the product. Behind the obstruction, the flows try to merge again, but this is made more difficult by a low temperature of the mold surface and a low melt pressure. Only then can we move on to determining the parting line itself. ”. Causes: Weld lines are caused by the inadequate bonding of two or more flow fronts when there is partial solidification of the molten plastic. 4. Reduce the temperature of the mold and plastic being injected. For instance, three widely used processes are insert molding, overmolding, and undercuts. Thick walls are not recommended because of POM’s high shrinkage rate. What Are Cosmetic Standards for Plastic Parts?. ICOMold is a plastic injection molding company that manufactures plastic injection molded parts. Piccarolo, S. The injection molding process offers a vast array of plastic resins to work with. N174W21430 Alcan Dr Jackson, WI 53037 262-677-9700; 16685. Computer simulation packages of injection molding have had success in predicting filling behavior in extremely complicated geometries, capable of predicting weld lines’ location exactly. In the 1947 book, Injection Molding of Plastics, the late Dr. This performance can ultimately impact many part properties, including surface quality, knit line strength, part ejection force (sticking), dimensions, tolerances. To learn more about us or to get a quote, click here or call 1. A poor knit line can cause only cosmetic blemishes or it can significantly weaken the structural integrity of a part. While not always visible, weld lines (aka knit lines) are where two resin flows meet. Make sure the venting is not too large. and 0. Knit line / Meld line / Transfer line Discoloured line where two flow fronts meet Mould or material temperatures set too low (the material is cold when they meet, so they don't bond). Unsymmetrical filling makes knit lines in samples, leading to the failure of part, so it must be avoided. Weld or knit lines occur when melt flow fronts collide in a mold cavity. They again get resolidified along the surface. High mold temperatures usually provide less residual stress. To avoid walls that are too fat or too thin, designing a wall thickness between 0. The difference between a knit and meld line is significant, dramatically impacting the part’s structural integrity. 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. It’s called sequential valve gating, or SVG. Vertical Plastic Injection Molding. Gating. Still, it can be seen to varied degrees depending on mold. com Discover why you should avoid knit lines when designing and manufacturing injection moulded parts. He is currently a tooling manager for a large, multi-plant molding and manufacturing company. A meld line occurs when two moving melt fronts converge and flow parallel to each other. Extreme geometries can cause knit lines. As a result, prototyping is strongly recommended for parts with. Flow lines typically manifest as visible marks, often slightly. Prevent Flow and Knit Lines During the Injection Molding Process. As a result, a rib at the weld line junction can strengthen the bosses at the mold line. Understanding types of injection molding process can help to avoid some design challenges early in the process. Excessive injection pressure can result in part defects such as overflow or flash. It happens when the molten plastic material flows around obstacles, such as inserts or cores, and rejoins on the other side. 250 in. Depends how hard you want to push it. Disadvantage: May rust if improperly stored. Plastic injection molding has long been an affordable and efficient process for producing custom prototypes and end-use production parts. They appear around holes or seams of an injection. The knit line strength and other packing related properties both aesthetic and functional of injection molded polymeric products is improved by equipping the mold with packing pins. 1-3). Read on to find out more! 800. Hardware components. Knit lines, also known as weld lines or meld lines, are visible lines or seams that appear on a molded part where two or more flow fronts meet during the injection molding process. This course outlines mold filling and how different gating configurations affect filling. As the melted plastic fills the mold, there are locations where the molten plastic flow stream separates and then comes back together. Knit Lines and Resins. There are times when porous materials like Porcerax are used in problem or backfill areas. To study the weld line formation of colliding flow fronts the filling of a mold cavity was simulated. 012 in. 11) The part design must take into account possible knit line locations and work to direct them away from higher stress areas. Injection molding, producing, and forming resin materials are three core components of injection molding technology. While injection molding itself is a straightforward process involving melting. Vacuum on the mold does help and many times eliminates the weld line. The comment on melt flow index is correct, but, I've molded straight PC down to . molding run to be sure that the temperature does not exceed the recommended limits . This defect is caused from excessive lube or oily stock. 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. Imagine a river splitting to go around a rock, then meeting up again on the other side. Thus, as the flow continues, the two flow fronts may gradually rejoin to make one uninterrupted flow front. By understanding material fill, injection mold designers can better position gates, anticipate where knit lines will appear, and locate difficult to fill spots in the mold. Refers to portion of injection molding part that has some taper to make it easier to remove from the mold. A knit line is any line, visible or not, where two resin flows meet. Also known as weld lines, knit lines. When designing for injection molding, consider these steps to prevent knit lines The Resin Cooling Process. 060 in. ”. Based on the resin. TRADITIONAL MACHINES VENTS. ; Wood, Philip E. Introduction to Knit Line Injection Molding Knit Line Injection Molding is a process that allows manufacturers to produce parts more quickly, accurately, and at less cost than traditional injection molding. sinks, knit lines, and shrinkage,” Spalding says. Plasti-Co® Nozzles for Injection Molding Machines. Injection speed and pressure. Weld or Knit lines are a plane or section in an injection molding part where the flow of molten plastics meets from different directions. 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. 3D printed molds in the injection molding process to lower costs and lead time. These flaws can affect the aesthetic and structural integrity of the. The leading edge of the molten material is often the coolest point and the closest to solidifying. Double push filling method. Image credit: The Rodon Group. This, in turn, leaves a large tab to be trimmed, a process that can mar the finished surface. Find out more by calling 573-437-4154 today or email us. Injection Molding Press. Given the cosmetic requirements, there is a good chance the mold would require. How weld lines form. Let the molten material cool to a solid-state. Typical Tool Steel for Injection Mold Construction Steel Type Steel Properties Mold Component P-20 Pre-hardened, machines well, high carbon, general-purpose steel. 025 inches per inch, depending on the particular resin, molding conditions, part design, part wall thickness, and direction of flow. Increasing injection speed. In injection molding terms, these are called “living hinges” and are made right into the design instead of adding them later. 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. Rapid heat-and-cool molding technologies have attracted increasing. 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. These defects are not visible, like small. . Weld lines, also known as knit lines, are lines that appear on the surface of an injection-molded part where two or more flow fronts meet and fuse together. Using three test materials, polystyrene (PS), polycarbonate (PC) and polyoxymethylene (POM), the relationship between a design and a processing parameter. A weld line, or knit line, occurs when two flows of melted plastic meet and “knit” together. The packing pins are sequentially. Fortunately, there is a technique that provides almost unlimited control over which parts of a tool fill when and how fast. Change gate locations. Compression molding is one of the earliest forms of molding. 5. They occur when melt flow fronts collide in a mold cavity. Because viscosity is a. They are formed downstream through holes, and between multiple gate locations. Injection moulding (U. Soft metals, such as aluminum and beryllium copper, can be used for prototype<br />. Knit Lines. The area generally has lower strength than the other areas of the part. For the metal feedstock, injection and mold-wall temperatures were 130 ºC. Injection molding can use thermosets or thermoplastics. Product designers encounter a wide range of challenges when creating and developing molded parts, such as stress factors, warp, and those pesky knit lines. Three terms that often get mixed up are weld, meld, and knit lines. The linear mold shrinkage of injection molded parts will vary from 0. 6. Cooling Lines, Injection Mold Material has a great impact on injection molded part quality. This design tip offers several ways to make your next multipart design less like a root canal and more like a day on the beach. Depending on the design of the mold and the material being injected, a knit line may: present no problem at all, be a. Wax Cold wax Increase wax temperature. Behind the obstruction, the flows try to merge again, but this is made more difficult by a low temperature of the mold surface and a low melt pressure. Weld lines are formed when there is an inability of two or more flow fronts to ‘knit’ or ‘weld,’ together during the molding process. Gate locations can be directly behind or on the edge of the graphic. Step 3: The injection mold undergoes rotation to align in position with injection Barrel B. Weld / Knit Lines; Assumptions. 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. By using this method, theMolding analysis study: An example of the low patterns and knit (weld) lines formation (MoldFlow® data) of injection molded round plastic part with a complex shape and with a ring. This guide to thermoplastics and injection molding material selection is aimed at an engineer who plans to quantitatively analyze a part, determine loads, stresses, strains, and environments and make an optimal material decision based on the analysis. , under its Advanced Technology Program (ATP). Observation on a “trial and error” basis may be required. Injection molding is the most cost-competitive technology for manufacturing high volumes of identical plastic parts. As melt temperature, viscosity and abrasiveness rise, tool life will decline. More about the knit lines in injection molding. Slower injection provides less orientation and stress. Any of the above can result in “knit lines” on final products, but mold flow analysis prior to mold machining can keep these issues from cropping up at all in iteration and production. thick. Because injection mold design and processing is complex, a set solution will not solve all problems. Injection molding is a popular manufacturing process used to fabricate parts in large volumes. 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. Bad habits can cause many different problems with injection molding machines, and flow lines are included in that. These flaws can affect the aesthetic and structural integrity of the. On the opposite side. Knit lines, also known as weld lines, are fine lines that resemble hairline fractures or cracks. We ensure customers receive accurate, fast, on-time, sufficient support and services. 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. 140 in. S. Shorten the mold cycle time so that any trapped air and resin don’t have a chance to overheat. Remedies: Raise the temperature of the mold or molten plastic. Contact Universal Plastic Mold Today. The last cause of flow lines may be the operators running your daily operations. Drying Considerations For both virgin and rework, hopper dryers sized to afford theWhen establishing the parting line, we must first define the shape and position of the parting line on the plastic molded item. Venting is required to allow the air in the sprue, runner and cavity to leave the tool as the melt flows into the cavity. They occur when melt flow fronts collide in a mold cavity. A weld or meld line is a weakness or visible. Steven. Increase injection speed and pressure: Faster-moving materials will meet more quickly so one doesn’t cool too soon. The Demo Mold helps those new to injection molding easily visualize the process and is a great teaching device for educators. Gates that are too small can also cause the uneven flow of material or result in knit lines, blemishes that occur where two resin flows meet. RELATED CONTENT. 0 Items. 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. , Rallis, A. This is a video excerpt from our Problems & Solutions Course - Weldlines. Factors were ordered. Reduce the injection pressure. 2. Venting. The example. Weld lines: Also known as knit lines, these are molding defects that occur when two flow fronts meet without the ability to “weld. 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 parting lines is where the two injection mold halves (A side and B side) meet. Also known as string lines. There needs to be a balance between fit of the halves, function of the part, visual quality expectations, and the cost to produce the mold. We have an elite team that designs for injection molding, rapid prototyping with metal and plastic, and produces world-class products under the same roof. Bociaga and Jaruga (2007) investigated flowing weld line in injection process by injection molding machine, which the part surface was observed in reflected light using the optimal microscope and. Having a basic understanding of the injection molding process will aid in understanding how to design for injection molding as well as how to forecast production schedules. In a weld line, reinforcement fibers. In an injection molded structural plastic part, a weld line (also called a knit line) will result in a region of mechanical weakness as soon as the part is submitted to an external load. • Repair parting line • Weld up vent depth Gate blush • Melt Fracture • Adjust injection speed (fast/slow) • Increase melt/mold temperatures • Resize gateFeedstocks for injection molding show two rheological behaviors: Pseudoplastic (shear thinning) and Dilatant (shear thickening) behaviors. Weld lines are a common imperfection to be found in most plastic injected molded parts. Learn how to troubleshoot weld lines in injection molding. 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. They again get resolidified along the surface. Inappropriate temperature. 5. In plastic part design for injection molding, it is important to design the injection mold so that the molten plastic fills the mold cavity entirely, cools as evenly and as quickly as possible without warping, and functions as intended. Proper mold cooling can eliminate warp, decrease cycle times, and increase part quality. This article will discuss the importance of injection molding nozzles and what makes a good nozzle for a molding or injection. Increase the injection speed. Obstructions in the part create weld lines or knit lines during injection molding. Knit Lines and Resins. There is high defect rate (surface defects/knit line). Injection moulding (U.