The term parting line is a bit of a misnomer. Perhaps the term parting surface would be better. Many plastic part designers seem to have difficulty understanding that the parting line is not simply a vector that may be randomly moved about on the plastic part. When done well, parting line indeed appears as a faint vector upon the face of the plastic part. This vector however is simply a witness of the two parting surfaces perfectly contacting each other when the mold is closed. Draft must fall away from the parting lines on ALL vertical faces in the plastic part. The widest point on the part is thus the parting line. The location of parting line and subsequent application of draft to the plastic part are design decisions that affect both the aesthetics and functionality of the part. These decisions should be made before the part is sent to the mold maker if time to market is critical.


Parting line is where it is because of the laws of physics and your part design, not because the tooling engineer wants to be ornery.

The drafting symbol for parting lines is

A close examination of the draft angles and shutoff angles sections should help you identify parting line on your plastic part.


If you need help identifying parting line and applying draft e-mail: