What Is MIM Open Porosity vs Closed Porosity?

Porosity in MIM parts exists in two forms — open and closed — and the distinction matters for corrosion resistance, pressure tightness, and measurement methods.

The difference:

Type	Description	Forms At	Effect on Properties
Open porosity	Interconnected pores that reach the part surface and connect to each other	Below ~93% density	Fluid paths — leaks pressure, traps corrosive media, falsifies Archimedes density measurement
Closed porosity	Isolated pores entirely surrounded by solid metal	Above ~93% density	Reduces strength and ductility slightly but does not create leak paths

The critical density threshold: At approximately 93% density, the pore structure transitions from interconnected (open) to isolated (closed). This is called the percolation threshold. Below 93%, parts cannot hold pressure and will trap fluids. Above 93%, pores are sealed and the part behaves more like solid metal. Why this matters for MIM parts:

Property	Below 93% Density (Open Pores)	Above 96% Density (Closed Pores)
Leak-tightness	Cannot hold gas or liquid	Can seal — suitable for valve bodies, fluid fittings
Corrosion resistance	Poor — corrosive media trapped in pores	Good — no pore access for corrosives
Archimedes density measurement	Overestimates density (water fills interconnected pores)	Accurate
Electropolishing	Acid trapped in pores, outgassing	Clean surface
Plating / coating	Plating solution trapped, blistering	Clean surface for coating

Quick Q: What is the difference between MIM open and closed porosity?

Open porosity consists of interconnected pores that reach the part surface — it creates leak paths and traps corrosive media. Closed porosity consists of isolated pores sealed within solid metal — it reduces strength slightly but does not create leak paths. The transition occurs around 93% density. MIM parts above 95-96% density effectively have only closed porosity, which is why high-density MIM is required for pressure-tight and corrosion-resistant applications.