A common question from engineers designing for MIM: can the process mold internal threads directly, saving a secondary tapping operation?
The short answer: No. MIM cannot produce internal threads directly. The thread profile cannot be ejected from a rigid mold core, and even collapsible cores are impractical for MIM due to powder particle entrapment. What MIM can do: Mold a cored hole (smooth bore) that is sized for post-sintering tapping. This eliminates the drilling step — only the tapping operation remains. Thread design rules for MIM parts:| Thread Size | Min. Cored Hole Dia (pre-tap) | Tap Drill Size Equivalent | Practical for MIM? |
|---|---|---|---|
| M1.0 × 0.25 | 0.75 mm | #60 (1.0 mm) | Yes — micro tapping possible |
| M1.6 × 0.35 | 1.25 mm | #55 (1.3 mm) | Yes |
| M2.0 × 0.40 | 1.60 mm | #52 (1.6 mm) | Yes — most common MIM thread |
| M2.5 × 0.45 | 2.05 mm | #46 (2.1 mm) | Yes |
| M3.0 × 0.50 | 2.50 mm | #39 (2.6 mm) | Yes |
| M4.0 × 0.70 | 3.30 mm | #30 (3.2 mm) | Yes |
| M5.0 × 0.80 | 4.20 mm | #18 (4.2 mm) | Yes |
| M6.0 × 1.00 | 5.00 mm | #5 (5.0 mm) | Yes — near MIM max practical wall thickness |
- Mold the cored hole 0.05-0.15 mm undersized relative to the standard tap drill to account for sintering shrinkage variation and provide enough stock for the tap
- Minimum thread engagement: 2x thread diameter for standard applications; 3x for high-strength fasteners
- Avoid threads closer than 1.5x the thread diameter to any part edge (risk of wall breakthrough during tapping)
- MIM cored holes have a slight taper (0.5-1.0° draft) — the tapping operation cuts through this to produce straight threads
Internal threads cannot be molded directly — they must be tapped after sintering. External threads can be molded to near-net shape but typically require a chasing operation for precision class threads. Always design cored holes for tapping rather than expecting threads to be molded.
For external threads on MIM parts, the thread profile can be molded to approximately 80-90% completion, then finished with a thread roll or die chasing operation.