Flash — a thin fin of material that squeezes out at the parting line of the mold — is one of the most visible quality issues in MIM parts. It occurs when the injection pressure forces feedstock into the gap between the mold cavity and core at the parting line.
What causes flash in MIM:| Cause | Mechanism | Typical Flash Thickness | Fix |
|---|---|---|---|
| Low clamp force | Machine cannot hold mold closed against injection pressure | 0.05-0.30 mm | Increase clamp force; verify press capacity vs projected area × injection pressure |
| Worn mold shut-off | Parting line surfaces have eroded or been damaged | 0.03-0.15 mm | Resurface or repair mold; consider PVD coating on shut-off surfaces |
| Excessively high injection pressure | Pressure at cavity exceeds mold locking capacity | 0.05-0.20 mm | Reduce injection pressure if possible; optimize gate location |
| Feedstock viscosity too low | MFI above nominal range — flows into gaps too easily | 0.02-0.10 mm | Check MFI of incoming feedstock batch |
| Mold deflection | Mold plates flex under injection pressure | 0.05-0.15 mm | Increase mold plate thickness; add support pillars |
| Application | Maximum Flash Height | Removal Method |
|---|---|---|
| Non-cosmetic functional parts | 0.10-0.20 mm | Accept as-is or light tumbling |
| Cosmetic visible surfaces | 0.05 mm max | Hand trim + polish or vibratory finishing |
| Precision sealing surfaces | Zero flash | Mold must be maintained to shut-off condition |
| Medical / implant surfaces | Zero flash | Requires mold maintenance + 100% visual inspection |
Flash is a thin fin of metal that extrudes at the mold parting line during injection molding. It is caused by the injection pressure exceeding the mold's ability to remain closed. Flash thickness of 0.05-0.15 mm is common for production MIM and can be removed by tumbling, vibratory finishing, or hand trimming. Excessive flash (>0.2 mm) indicates a mold maintenance or process parameter issue that should be addressed at the source.
For critical applications, specify the maximum acceptable flash height on the part drawing. For cosmetic surfaces, gate location and parting line position should be designed to place flash on non-visible surfaces.