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Parting Line
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While designing a plastic injection molded part, the presence of witness lines and areas of potential flash should be taken into consideration. Considering how the mold will part and along which surfaces a witness line could be allowed will ensure a cosmetically acceptable part will be molded. The optimum path for the parting line will also allow for a less complex and costly mold.
A witness line will and potential flash may be present at the parting line (see figure 1). These imperfections may not be desirable on some critical surfaces for reasons of appearance, a surface being a mating surface with another part, a surface may be a sliding surface, a feature being in tight tolerance or for other reasons from a standpoint of aesthetics or function.
Witness lines are an unavoidable result of two mating mold components. Injection molded material under pressure will be imprinted with the witness mark of two pieces of steel meeting one another.
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Figure 1. Witness Line is Present on all Molded Plastic Parts
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