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Perhaps the most common injection molding defect is flash which is simply when a thin layer of plastic flows outside of the mold cavity usually along the parting line where the two mold halves are pressed together. Flash can also appear along the ejector pins and really anywhere that you have metal-on-metal contact. When this thin layer of plastic cools It remains attached to the part and creates not only an undesirable aesthetic but can alter the function of that part depending on its end use. Flash CAN carefully be trimmed after the fact but this increases labor and can even cause mold damage over long periods of time which ultimately means a higher cost so the best way to eliminate flash is by adjusting the injection molding process. First, we check the amount of pressure being used to press the two halves of the mold together which is typically measured in tonnage. If not enough force is being applied to each half of the mold the melted plastic can more easily find its way outside of the mold cavity. Increasing the clamping force may be the easiest way to eliminate flash but sometimes the material is too fluid meaning the melt or mold temperature can be lowered to improve the material flow. And then of course, flash can simply be caused by a low-quality or worn-down mold that no longer fits well at the parting line. Crescent remedies this by having our in-house mold maintenance department just a few feet from our production floor allowing us to regularly clean and maintain our molds before this sort of thing becomes a problem.
The next injection molding defect is called a short shot and this simply refers to an incomplete part where the melted plastic is not fully filling the mold cavity. As you can see here short shots can range from entire sections of the mold not being filled to very subtle defects that need to be carefully looked for. In smaller parts, short shots are often the result of a material with a high viscosity meaning it does not flow easily especially into hard-to-reach areas. In this case, you may want to consider increasing the melt temperature to allow the material to flow more easily or try an entirely different resin with a lower viscosity. Short shots may also be caused by inadequate venting in the mold which can trap air pockets, a frozen flow channel that may need to be cleaned, or simply not packing enough resin into the mold cavity during injection. Much of this has to do with the injection molding process which involves many complex variables so here at Crescent we utilize a proven process called decoupled molding and have a number of certified master molders who are able to optimize the process not just to produce a good part but to do this as efficiently as possible.
Sink marks are a localized indentation or depression that appears on the surface of an injection molded part after cooling. Usually, you'll find a sink mark along thicker portions of the part where cooling rates can vary. Once again there are many potential causes and solutions but the most common are lowering the mold temperature since an overheated mold can increase uneven cooling. You can also increase the hold pressure, increase the hold time, and if process alterations don't help it may be necessary to adjust the part design and reduce the thickness of certain sections to promote more even cooling and lower the likelihood of a sink mark.
A burn mark is the result of overheated material which is especially noticeable on white or clear components. Similar to the previous defects we’ve looked at, burn marks can be addressed by altering the injection molding process and adjusting variables like the injection speed, lowering the mold temperature, improving mold venting, and properly cleaning and maintaining the mold to ensure that material does not become trapped and burn. The broader defect of part contamination can be caused by a number of variables but for medical parts specifically, we utilize our ISO 7 and ISO 8 cleanroom facilities and preventative maintenance to drastically reduce the potential for part contamination before they become a problem.