Parting Lines and Mold Orientation

The parting line is where mold components meet. Its position affects much more than a faint line on the finished part.

What the parting line controls

Mold orientation determines core/cavity assignment, many draft directions, natural undercuts, ejector strategy and where flash risk exists. Moving the parting line can simplify one feature while complicating another.

Put unavoidable witness lines deliberately

If a product has cosmetic surfaces, place the parting line where it is visually acceptable or can be integrated into an existing edge. Avoid running it through sealing lands, bearing surfaces or precision interfaces unless the toolmaker has a controlled shutoff strategy.

Shutoffs need steel

A shutoff is an area where tool surfaces meet to block plastic flow. Very thin or fragile shutoff steel can wear, chip or flash. Provide robust angles and enough steel around openings, slots and side features.

Flash is functional in some places

Flash is not always merely cosmetic. On sealing, medical-contact, sliding or snap surfaces, a small flash line may change function. Identify flash-sensitive regions explicitly on drawings and DFM reviews.

Draft follows the tooling concept

Do not add draft to the model before you know which tool component forms each surface. A wall may need opposite draft depending on whether it belongs to the core or cavity half.

Review method

Use a color draft analysis for each pull direction, then mark parting lines, slides, shutoffs, ejectors and gate candidates on screenshots. This turns an abstract mold concept into a geometry discussion the design team can understand.