Engineers familiar with powder metallurgy (which uses no binder and presses powder at high pressure) often ask: why can't MIM use 80% or 90% powder like PM does?
The short answer: At powder loadings above approximately 65-67 vol%, the feedstock becomes too viscous to inject into a mold cavity. The binder is not just a glue — it is the* fluid that carries the powder into the mold. What happens at different powder loadings:| Powder Loading | Behavior | Reason |
|---|---|---|
| < 55 vol% | Too much binder | Excessive shrinkage (>20%), poor dimensional accuracy, high carbon contamination risk |
| 55-65 vol% | Optimal molding range | Enough binder for flow; enough powder for good sintering |
| 65-67 vol% | High viscosity — difficult to mold | Approaching CPVC (Critical Powder Volume Concentration) |
| 67-70 vol% | Extremely high viscosity — risk of short shots | Particles are in contact; binder only fills interstitial spaces |
| > 70 vol% | Cannot be molded | No continuous binder path — material crumbles rather than flows |
CPVC is the powder loading at which the particles are packed as densely as possible while still having enough binder to fill all the spaces between them. At CPVC, the feedstock viscosity approaches infinity. For MIM powders (mixed particle sizes), CPVC is approximately 67-72 vol% depending on the PSD. This is the absolute upper limit — operating below CPVC by at least 3-5 vol% is necessary for practical molding.
Why PM can use >90% density:Conventional PM does not use injection molding — it uses mechanical pressing in a die. The powder is compressed under 400-700 MPa force, mechanically displacing the air between particles. MIM cannot use this approach because the mold cavity cannot apply such forces — the feedstock must flow into the cavity under injection pressure alone.
Quick Q: Why does MIM use only 55-65% powder?Higher powder loadings would make the feedstock too viscous to inject into the mold. The critical limit (CPVC) is approximately 67-72 vol%, above which the material behaves as a solid rather than a fluid. MIM operates at 55-65 vol% to leave enough binder for flow while maximizing powder content for sintering.