A parting line is where two or more molds meet during casting. Placement is important for product integrity and disassembly. Molding flash can occur if molds don’t meet exactly. Technical drawings include mold illustrations and parting line location.
A parting line is the point on a product or component where two or more molds have 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, the machines grind and smooth the parting line so that it is no longer visible if any extra material has leaked into the gap when the object was cast.
In a simple mold, there will be two halves that press together to create a cavity to be filled with plastic, metal, or other materials. When an operator pours material into the mold, air can escape around the parting line, preventing bubbles from forming. When the material hardens and people take the molds apart, the finished object will fall off. It can be treated with finishing processes such as sanding, painting and so on. Other molds can be more complex, with multiple components to address special shapes and design considerations.
When people design a mold for mass production, they want to place the parting line carefully. Even assuming that it will not be visible after finishing, they have to think about the best position in terms of disassembling the mold without damaging the product and supporting it while the printed material cures. If the parting line is too close to a fragile component, for example, that part can deform during molding or break when the operator removes the mold. Similarly, poor placement can prevent air bubbles from escaping, causing problems with the finished product.
If the molds don’t meet exactly, material will leak into the space between them, creating a situation called molding flash. The operator can plane, sand, scrape or trim this excess material and then smooth the underlying surface to match the rest of the object. This problem is more common with cheap molds, and in some cases, manufacturers won’t bother to fix it; cheap plastic toys, for example, may have a visible line around the center, showing where the molds have come off.
In the specifications of the molded products, the technical drawings will include illustrations of the molds and a discussion of the location of the parting line. The moldmaker will confirm that the designs are appropriate for the application and may suggest modifications to resolve issues. For example, you may be concerned that it will be difficult to file any molding glare because the parting line is hidden in a corner of the mold and will therefore leave the product with a rough, unfinished appearance.
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