The plastic is loaded with 60% metal powder and a two-part plastic system.
The primary plastic binder is then dissolved out of the part.
This creates a 20% reduction in material, and turns the part into a sponge.
Next you put it into a sintering oven.
The oven burns off the remaining 20% plastic through the sponge structure (in an environment tuned for the metal) and then brings the part very close to the melting temp.
There are four methods of mass transports, but the net effect is that the atoms diffuse together (sinter), the part shrinks by 20%, and you get a fully dense part (in casting terms, this is 98% or more).