A weld line (also called knit line) forms when two flow fronts of molten MIM feedstock meet inside the mold cavity — typically around a core pin (for a hole) or at the end of fill in a multi-gated part. Unlike plastic injection molding where weld lines can often be designed away, MIM weld lines require extra attention because the sintered part has reduced strength at the weld line if the two flow fronts do not fully bond.
Effect of weld lines on MIM part strength:| Condition | Strength at Weld Line (% of full strength) | Visible on Surface? |
|---|---|---|
| Optimal (high temp, high pressure, good venting) | 80-95% | Faint line, may be invisible |
| Average (standard process parameters) | 60-80% | Visible line, slightly recessed |
| Poor (cold mold, low pressure, trapped gas) | 40-60% | Clear line, potential crack initiation |
| Strategy | Effectiveness | Implementation |
|---|---|---|
| Relocate gate to move weld line to low-stress area | High | Move gate so flow fronts meet at non-structural surface |
| Increase wall thickness at weld line region | Medium | Thicker section traps heat longer, improves bonding |
| Add overflow tab (weld line trap) | High | Small tab at weld line location captures the end of flow |
| Eliminate or reshape core pins | Medium | Round holes create weld lines behind the pin; consider D-shaped holes |
| Increase mold temperature | Medium | Hotter mold keeps flow fronts from freezing before bonding |
Yes — a weld line can reduce local strength to 60-80% of the surrounding material if process parameters are not optimized. For structural parts, relocate gates to place weld lines in low-stress regions, or add an overflow tab. For cosmetic parts, a visible weld line may be a rejection criterion regardless of strength.
The most effective fix: use mold flow simulation (such as Moldex3D or Moldflow) during tooling design to predict weld line locations before steel is cut.