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What is a Parting Line?
Sep 13, 2017

A parting line is the place on a product or component where two or more molds met during the casting process. A number of considerations go into the placement of the parting line, with the goal of maintaining the overall integrity of the piece. Typically, as part of the finishing process, machinery will grind and smooth the parting line so it is no longer visible, if any extra material leaked into the space when the object was cast.

In a simple mold, there will be two halves that press together to create a cavity to fill with plastic, metal, or other materials. As an operator pours material into the mold, air can escape around the parting line, preventing bubble formation. When the material sets and people pull the molds apart, the finished object will drop out. It can be treated with finishing processes like sanding, painting, and so forth. Other molds may be more complex, with multiple components to address special shapes and design considerations.

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When people design a mold for mass production, they want to place the parting line with care. Even operating under the assumption that it will not be visible after finishing, they need to think about the best position in terms of pulling the mold apart without damaging the product, and providing support while the molded material sets. If the parting line is too close to a fragile component, for instance, that part may deform during molding or break off when the operator removes the mold. Likewise, bad placement may prevent air bubbles from escaping, causing problems with the finished product.

If molds do not meet exactly, material will leach into the space between them, creating a situation called molding flash. The operator can plane, sand, scrape, or cut off this excess material and then smooth the underlying surface to make it match the rest of the object. This problem is more common with inexpensive molds, and in some cases, manufacturers will not bother to address it; cheap plastic toys, for example, may have a visible line around the middle, showing where the molds came apart.

In specifications for products made with molding, the technical drawings will include illustrations of the molds and a discussion about the location of the parting line. The mold maker will confirm that the designs are appropriate for the application and may make suggestions for changes to address concerns. For example, there could be worries that it will be difficult to file away any molding flash because the parting line is tucked into a corner of the mold, and thus will leave the product with a rough, unfinished appearance.

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