The parting line is the line where the two halves of the mold meet. It is visible on every injection-molded MIM part — and its location and condition affect dimensional accuracy, flash formation, and cosmetic appearance.
What the parting line is: When the mold closes, the cavity side (A-side) and core side (B-side) meet at a precisely machined surface called the parting surface. The line where these surfaces meet, transferred onto the molded part, is the parting line. How the parting line affects part quality:| Aspect | Impact | Design Consideration |
|---|---|---|
| Dimensional tolerance | Dimensions crossing the parting line are less accurate (affected by mold closure) | Place critical dimensions on the same side of the parting line |
| Flash | Flash forms at the parting line if the mold does not close perfectly | Flash thickness of 0.03-0.15 mm is normal and can be removed |
| Cosmetic appearance | The parting line is visible on the finished part | Place parting line on a non-cosmetic surface or edge |
| Part ejection | The parting line determines where ejector pins can be placed | Design with draft angles that allow the part to stay on the ejection side |
- Place the parting line on an edge or corner rather than a flat surface — it is less visible and flash is easier to remove
- For cosmetic parts, specify the parting line location on the drawing so the mold designer places it on a non-visible surface
- Avoid placing the parting line across a sealing surface — any mismatch will cause leakage
- Dimensions that cross the parting line will have ±0.05-0.15 mm additional variation due to mold closure variation
The parting line is the visible line on the molded part where the two halves of the MIM mold meet. It is unavoidable in injection molding. The parting line affects dimensional tolerance (dimensions crossing it are less precise), flash formation (flash occurs at the parting line), and cosmetic appearance. Design the parting line location early in the tooling design process to place it on non-critical surfaces.