K Value is an indicator of molecular weight : K-57 is medium for injection molding,K-67 is medium-high for rigid extrusion, K-70 is higher for plasticized extrusion or calendering. K value relates to other ASTM indicators of molecular weight like Intrinsic Viscosity or Inherent Viscosity.
Producers add no special additives---it's a matter of reaction time in the polymerization kettle to acheive desired molecular weight.
Gelation time and temperature (or Fusion time/temp.) is when the PVC FORMULATION (with all the stabilizers, lubricants,fillers,pigments, process aids) melts----best seen in a Brabender fusion curve.
Porosity in a product is due to poor gelation or fusion---extruded or molded too cold, not fully melted.