Shenzhen Alu Rapid Prototype Precision Co., Ltd.

In injection molding, the parting line (also known as a witness line) is the faint seam or line that appears on the surface of a molded plastic part where the two halves of the mold—typically the core and cavity—come together and separate during the production cycle. 

This line forms because the mold must open to eject the cooled part, and it's an inevitable feature of the process unless advanced techniques like slide molds are used.

How It Forms

1.Mold Design: Injection molds are split into at least two sections (core and cavity) to allow for part ejection. The parting line is the precise boundary where these sections align and seal under high pressure during clamping.

2.Injection Cycle: Molten plastic is injected into the closed mold. Once cooled, the mold opens along this line, leaving a subtle ridge, flash (excess material), or offset on the part's surface. 

3.Location: It can run straight, curved, or irregular depending on the part's geometry, often placed on edges, corners, or hidden features to minimize visibility. 

Implications and Best Practices

Aesthetics and Functionality: Visible parting lines can affect appearance (e.g., on consumer products) or trap debris in assemblies. Poor placement may cause weak points or uneven walls.

Design Tips: Position it away from critical surfaces, use radii or chamfers to blend it in, and collaborate with molders early to optimize. Flash can be minimized with tight tolerances and proper venting. 

Maintenance: Over time, mold wear at the parting line can lead to leaks or defects, requiring periodic repairs.