9 Defects prevention tips for Problem-free injection molding makenica.com/9-defects-prevention-tips-for-problem-free-injection-molding February 17, 2021
There's plenty of room for costly glitches when it comes to injection molding service. Quality concerns in injection moulded products can vary from small surface defects to more severe problems that may affect the product's safety, efficiency, and function. They can be caused by problems relating to the moulding process, the use of materials, the construction of tools or the mixture of all three. But as for most consistency issues, it's half a fight to realize how moulding flaws occur. As an importer or producer of injection-moulded goods, some knowledge of typical plastic injection molding service flaws and how to prevent them will help you reduce the costs of unsellable goods and product returns. Let us look at the most common quality defects associated with injection moulding service, what induces them and what to do to avoid those defects for problem-free plastic injection moulding service.
MOLDING DEFECTS OFTEN CAUSED BY PROCESS PROBLEMS A few of the injection moulding service flaws can be difficult or expensive to correct. Others can be avoided by changing the moulding process without upgrading the moulding tooling or replacing other manufacturing machinery. Usually, injection molding 1/7
companies can avoid these defects reasonably by changing the mould's flow rate, temperature, or pressure. 1. The Flow Lines Flow lines also occur as a wavy pattern of a subtly different colour than the surrounding area and usually on narrower areas of the moulded portion. They can also appear as ringshaped bands on the surface of the product at the mould's point of entry. Or as "gates" into which the molten material flows. Usually, the flow marks would not affect the integrity of the item. However, they can be unsightly and inappropriate if used in some consumer goods, such as high-end sunglasses. Causes and remedies for flow lines Flow lines are most likely the result of changes in the material's cooling speed as it moves through the mould in various directions. Differences in the wall thickness will also allow the fluid to cool at varying speeds, leaving the flow lines behind. E.g., molten plastic cools very rapidly during the injection process, and flow marks are visible when injection speed is too slow. The plastic becomes partly rigid and gummy while still filling the mould, which allows the wave pattern to emerge. Here are a few popular tips for plastic injection molding companies for dealing with flow lines in injection moulded products: Increase the speed of injection, the pressure and the temperature of the material. So that before cooling, the material fills the mould. Round corners of the mould where the wall's width increases to help maintain the flow rate constant and prevent flow lines. Relocation of mould gates to provide more space between them and the mould coolant to help keep the material from cooling too early during flow. Increase the diameter of the nozzle to increase the flow rate and avoid early cooling 2. Burn marks Burn marks usually occur as black or rust-coloured discoloration on the moulded plastic part's edge or surface. Injection moulding defects Burn marks typically do not harm the integrity of the part unless the plastic is too much burned to the extent of degradation. Causes and remedies for burn marks The common source of burn marks in injection moulded components is trapped air or the resin itself, which is overheated in the mould cavity during the injection. Excessive injection or heating of the material also leads to overheating, which causes burns. Find the following safety steps to be followed by injection molding companies to avoid the use of burn marks in moulded components:
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Lower the temperature of the melt and the mould to prevent overheating Reduce the speed of injection to reduce the chance of air trapping within the mould. Enlarging Gas vents and gates to allow trapped air to escape from the mould Shorten the mould cycle time to prevent any trapped air and resin from overheating. 3. Warping Warping is a deformation that can occur in injection moulded products when different parts of the component shrink unevenly. Much like wood can warp as it dries unevenly, plastics and other materials can warp throughout the cooling process when uneven shrinkage places excessive stress on various parts of the moulded component. This undue stress causes the finished portion to bend or twist as it cools. This is apparent in a component that is supposed to lie flat but leaves a void on a flat surface. Cause and remedies for warping in-moulded sections One of the critical reasons for the warping of injection-moulded plastic and similar materials is that cooling occurs too rapidly. The problem can also be exacerbated by the high temperature or low thermal conductivity of the molten material. Sometimes, the mould construction will lead to warping where the mould walls are not of uniform thickness—the shrinking increases with the wall thickness. Here are some traditional ways to be followed by injection molding companies to avoid warping of your moulded parts: Make sure that the cooling process is slow and long enough to avoid uneven stress on the material Lower material or mould temperature Try to turn to a material that shrinks less during the cooling process (such as particle-filled thermoplastics that shrink much less compared to semi-crystalline materials or unfilled grades) Redesigning of the mould with a uniform wall thickness and component symmetry to ensure greater stability during cooling. 4. Vacuum voids/air pockets Vacuum voids, or air pockets, are air bubbles stuck within the finished moulded part. Quality management experts typically deem voids to be a "minor" error. But larger or more numerous voids can, in some cases, weaken the moulded part since there is air below the surface of the part where the moulded material is to be found. Common causes and remedies to avoid vacuum voids in moulded parts One of the critical reasons for the void is a lack of moulding pressure to push trapped air out of the mould cavity. Most times, the material nearest to the mould wall cools very fast, allowing the material to harden and force the material outwards, forming an abscess. The
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material itself can be extremely susceptible to voids if its density varies dramatically from molten to hardened state. Voids are more difficult to prevent in moulded components that are more than 6 mm thick. Popular ways of preventing voids include: Boost the pressure of the injection to drive out the compressed air pockets Choose a grade of low viscosity material to minimize the chance of air bubbles developing Place the gates next to the mould's thickest areas to avoid premature cooling where the material is most susceptible to voids. 5. Sink marks Sink marks are tiny recesses or depressions on the otherwise smooth and consistent surface of the moulded component. These can arise as the moulded part's interior shrinks, dragging the material from the outside inward. Cause and remedies for the avoidance of sink marks Sink marks are identical to vacuum voids but are reversed by cause and effect. Rather than cooling the material too quickly near the component's exterior, the material cools too slowly. The resultant shrinkage forces the outer material inward until it could cool adequately, leading to depression. Compared with voids, sink marks are more likely to appear in the thicker portion of the component. Here are a few useful suggestions for plastic injection molding companies to avoid the defect: Increase the holding pressure and time to allow the material near the surface of the element to cool. Increase cooling time to minimize shrinkage Design the mould with thinner walls to allow for quicker cooling near the surface. 6. Weld lines Weld lines may appear on the moulded component's surface where the molten material has converged after separating into two or more directions in the mould. The hair-like weld line is the case of poor material bonding, which reduces the component's strength. Common causes of weld lines and remedies to avoid those Two or more polymer fronts or other liquid material must be held at a specific temperature when colliding. Otherwise, they would be partly solidified and not sufficiently bonded where they meet, resulting in weld lines. Popular treatments followed by injection molding companies for weld lines in moulded parts include: Increase material temperature to reduce partial solidification
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Boost the injection speed and the pressure to reduce the cooling until the material has filled the mould. Redesign of the mould to remove partitions Move to a low melting temperature or viscosity material to allow quicker flow and avoid early cooling. 7. Jetting Jetting refers to a type of deformation in a moulded part that can occur when an initial "jet" of molten material is inserted into a mould cavity that begins to solidify until the cavity is filled. Jetting also appears as a squiggly line on the finished part's surface, typically leading from the initial injection gate. This visible flow pattern can lead to part weakness. Causes and remedies for jetting in-moulded sections Excessive injection pressure is the primary source of jetting. When the molten polymer or other material is pumped through a narrow gate at high pressure, it frequently squirts immediately through the gate, rather than steadily filling the mould cavity. When this initial line of material cools against the mould walls and continues to harden, the residual mould material moves it, leaving impressions on the finished portion's surface. Stop jetting in-moulded sections by: Reduction of injection pressure to avoid excessive squirting of the material into the mould cavity Increased material and mould temperature to prevent the initial jet of material from being solidified early Designing mould with injection gate is positioned to guide the material through the mould rather than lengthwise. DEFECTS ASSOCIATED WITH THE MATERIAL USE OR STORAGE Injection moulding service defects may also result from the material itself or how the manufacturer stores and treats the material before the manufacturing process. These defects can vary from mild esthetic problems to the compromised strength of the finished product. Significant safety issues can also occur, depending on the intended use of the product concerned. 8. Discolouration Discoloration or "color streaking" happens when the moulded component is different from the expected colour. Often discoloration is restricted to a localized region or a few abnormally coloured stripes on a moulded portion. This flaw typically affects the appearance of the element without mitigating its strength. Causes of discoloration and how to prevent it
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Remaining pellets in the hopper or residual resin in the nozzle or mould from the previous manufacturing run is a common discoloration source. Low thermal consistency of the colouring agent or excessive mixing of the masterbatch is other possible reasons. Injection molding companies should take the below steps to reduce the chance of decolouration in plastic injection molding service: Ensure that workers adequately wipe the hopper, nozzle and mould between production runs to remove any remaining pellets or base material Consider the usage of a purging compound to eliminate unwanted colour from the machine Ensure that you or your retailer uses a colour agent with sufficient thermal stability. Ensure the masterbatch is uniformly matched for consistent colour production. 9. Delamination If you encounter thin layers on the moulded component's surface that are quickly breaking or peeling off the underlying substance, you can see a moulding flaw called delamination. It is a defect characterized by a flaking surface layer, similar to what is usually seen on flake mica. This is typically considered to be a comparatively serious defect since it decreases the strength of the part. Causes and avoidance of delamination in injection moulding Contamination of resin pellets or other base material with foreign material is the most frequent cause of delamination. Flaky separation occurs because the two materials are not adequately bound to each other. Plastic injection molding companies should take the following corrective steps to avoid recurrence if you discover delamination impacting your moulded parts: Increase the temperature of the mould or pre-dry the material adequately if excess moisture is a concern. Ensure that workers are correctly storing and treating resin pellets or base material to prevent contamination Consider redesigning the mould with an emphasis on the injection nozzle to reduce the reliance on release agents.
Conclusion Manufacturing by injection moulding service usually involves substantial initial investment in tooling. That's why it's essential to build the mould for the first time, rather than trying to start anew after discovering significant defects. Defects associated with the injection moulding service or material tend to be more straightforward and less difficult to solve. But no matter what the cause, flaws in moulded products will affect the bottom line tremendously.
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Now that you are familiar with typical injection moulding defects, you can take precautions to avoid them in the future and guarantee that your products follow your customers' quality requirements.
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