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What is a Parting Line?

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  • Written By: Mary McMahon
  • Edited By: Kristen Osborne
  • Last Modified Date: 23 September 2016
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    Conjecture Corporation
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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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Discuss this Article

Inaventu
Post 2

I wonder if it's even possible to create a molded piece without a parting line. To me, it's like welding two pieces of metal together. I can always use a grinding tool to minimize the appearance of a weld, but there's always going to be something visible at that joint. When two pieces of a mold come together, there's always going to be some sort of seam.

Buster29
Post 1

I once had a temporary job assignment at a local plastic injection molding company. The company produced plastic hooks for store displays. The injection molds would come together, and then some melted plastic would be injected into them. Once the molds opened, a piece containing a dozen hooks would drop into a basket. My job was to break off the individual hooks and scrape off the parting line flash with a utility knife.

A regular employee told me that I had one of the easier jobs at the factory. Other products were much more complicated, and they left a noticeable mold parting line. It would take at least five minutes with three different files to remove the parting line flash from just one piece.

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