causes

Influence on processing shrinkage

lessening

increasing

Density increase due to thermal contraction by cooling the demoulding temperature to room temperature and compression by the action of pressure
  • high effective pressure on moulding compound and contour until demoulding (holding pressure)
  • low demoulding temperature (long cooling time and/or low contour temperature)
  • low coefficient of thermal expansion (hard elastic polymers)
  • low or prematurely reduced holding pressure until demoulding
  • high demoulding temperature (short cooling time and/or high contour temperature)
  • high coefficient of thermal expansion (soft or rubber-elastic polymers)

Density increase due to thermodynamically induced structural order processes (crystallization; gelation)
  • amorphous polymers
  • ow degree of crystallinity of semi-crystalline polymers due to rapid solidification (subcooling due to low contour temperature and/or thin-walled parts)
  • high degree of gelation of polymers containing plasticizers
  • semi-crystalline polymers
  • high degree of crystallinity due to slow solidification (high contour temperature and/or thick-walled parts) and improved nucleation (nucleation additives)
  • low degree of gelation of polymers containing plasticizers

Density increase due to molecular construction and cross-linking processes (curing; vulcanization; polyreaction)
  • high degree of cross-linking and thus lower coefficient of thermal expansion (long curing or vulcanization time and/or high melt temperature)
  • Preformed or pre-crosslinked molding compounds (e.g. prepolymers)
  • low degree of cross-linking and thus higher coefficient of thermal expansion (short curing or vulcanization time and/or low melt temperature)
  • non-crosslinked precursors (oligomers) or monomers as moulding compositions

Changes in stiffness or hardness due to additives (e.g. fillers and reinforcing materials; plasticizers)
  • Additives with low coefficient of thermal expansion (e.g. inorganic fillers and reinforcing materials)
  • no or low plasticizer additives
  • Additives with high coefficient of thermal expansion (e.g. organic fillers and reinforcing materials)
  • plasticizer additives