The choice between hot runner and cold runner systems in MIM molds affects tooling cost, cycle time, material utilization, and maintenance. Unlike plastic injection molding where hot runners are common, MIM adds extra considerations because the abrasive feedstock accelerates wear.
Comparison:| Factor | Cold Runner | Hot Runner |
|---|---|---|
| Tooling cost | Baseline (1.0x) | 1.4-2.0x (more complex, more components) |
| Cycle time | Baseline | 15-25% faster (no sprue/runner cooling time) |
| Material waste | 15-30% (regrind available but limited) | 2-5% (only unsteady-state shots) |
| Gate type options | Edge, tunnel, tab, fan | Pinpoint, valve gate |
| Mold maintenance | Lower (only cavity and core maintenance) | Higher (heater bands, thermocouples, manifold seals) |
| Color/grade change | Simple (run the material through) | Complex (purge manifold, can take hours) |
| Best for | Low-medium volume, multiple materials, simple parts | High-volume, single-material, automated production |
| Consideration | Cold Runner Advantage | Hot Runner Advantage |
|---|---|---|
| Abrasive feedstock wear | No hot runner valves to wear | Valve pins in hot nozzles wear faster with MIM feedstock |
| Binder degradation | Material in cold runner is only heated once | Material in hot runner manifold sees prolonged heat exposure |
| Gate vestige | Tunnel gate self-trims to 0.03-0.10 mm | Valve gate produces very low vestige (0.01-0.05 mm) |
| Regrind quality | Clean, unmolded regrind from cold runner | No regrind from hot runner (but different waste stream) |
For volumes below 200,000 parts/year: cold runner is more cost-effective — lower tooling cost, simpler maintenance, and MIM's limited regrind capability (10-20%) is manageable. For volumes above 500,000 parts/year: hot runner's faster cycle and zero runner waste typically justifies the higher tooling investment. Many MIM molds run cold runner because the feedstock abrasiveness shortens hot runner component life.