A parting line is the line on a molded part where the two halves of the mold — the cavity and the core — meet. It marks the plane along which the mold opens, and it is visible on the finished part as a faint seam, often with a trace of flash.
In short: it is the seam left where the mold splits, and where the mold designer chose to split it decides how the whole part is tooled.
What Is a Parting Line in Injection Molding?
Every two-plate mold consists of a cavity plate (the "A" side, usually forming the cosmetic outer surface) and a core plate (the "B" side, forming the inside). The surface where those two plates close against each other is the parting surface, and its trace around the part is the parting line. It can be flat, stepped or follow a curve, depending on the part's shape.
The parting line fixes the direction of draw. Once it is chosen, every wall must be drafted away from it, any feature that cannot be pulled straight off in that direction becomes an undercut needing a side action, and the ejector pins sit on the core side of it.
It also does a job during molding. The tiny gap at the parting surface is where air escapes as the cavity fills, so vents are cut into it. If that gap opens further — from worn steel or too little clamping force — molten plastic squeezes into it and forms flash.
Complex parts may have more than one. A three-plate mold, for example, has a second parting line that separates the runner from the parts.
Parting Line vs Flash
The two are often confused because they appear in the same place. The parting line is a design feature; flash is a defect.
| Parting line | Flash | |
|---|---|---|
| What it is | The seam where the mold halves meet | Thin excess plastic that leaked into the gap between the halves |
| Is it avoidable? | No — every molded part has one | Yes — with good mold fit and correct clamping force |
| How it looks | A faint witness line, sometimes a slight step | A ragged fin or skin along the seam |
| What controls it | Where the mold designer places it | Mold condition, clamping force, injection pressure |
Where Should the Parting Line Go?
- Put it on a flat or simply curved surface. A parting surface that has to follow complex geometry is harder to machine, harder to seal and more likely to flash.
- Keep it off cosmetic faces. Along an edge or at a natural change of surface, a parting line disappears; across a show surface it needs post-finishing or a texture to hide it.
- Choose it to minimize undercuts, so the mold needs as few side actions as possible.
- Note any surfaces where a witness line or mismatch is unacceptable on the drawing, so the mold maker can plan for it.
Parting-line placement is decided in mold design, before any steel is cut, and it is one of the items reviewed during the DFM stage of a plastic injection molding project.
Frequently Asked Questions
What is a parting line in injection molding?
It is the line on a molded part where the two halves of the mold close together. The mold opens along that plane to release the part, and the line stays visible on the part as a faint seam.
Can a parting line be removed?
Not entirely — every part molded in a rigid two-piece mold has one. It can be moved to an edge or a non-cosmetic surface, minimized with a well-fitted mold, or hidden with texture or post-finishing.
Why does flash form at the parting line?
Because that is where the two mold halves meet, and any gap between them fills with molten plastic under injection pressure. Worn or poorly fitted mold faces, too little clamping force or too much injection pressure all open the gap enough for flash to form.
Related Services at NICE Rapid
- Plastic Injection Molding — parting-line placement is reviewed during DFM, before tooling starts.
- Rapid Tooling — CNC-machined aluminum and steel molds with cavity tolerances to ±0.05 mm, which is what keeps the parting surface tight and flash-free.
- Request a Quote — share your model and note any faces where a parting line is not acceptable.
Sources
- MIT OpenCourseWare, 2.008 Design and Manufacturing II, Lecture 03: Injection Molding II (Spring 2025) — parting line defined as "a dividing line between a cavity plate and a core plate of a mold"; place it on a flat or simple-curved surface so flash cannot be generated; used for venting; flash develops at the parting line when clamping force is too weak or injection pressure too high — ocw.mit.edu
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