[1] Rubber has expired and is partially gelled;
  [2] EPDM or butyl rubber is too cold;
  [3] frozen natural rubber;
  [4] Insufficient pre-plasticization of natural rubber;
  [5] Excessive water in the filler (caking);
  [6] Add viscous complexing agent at a temperature below the pour point;
  [7] Improper use of compounding agents;