Abstract: The pumpability and extrudability of a mixture of linear polymer and filler can be controlled by using a mixture of filler particle sizes. For 100 parts by weight of polymer there is mixed in from 50 to 150 parts by weight of filler mixture, the filler mixture comprising from 0.5 to 50 percent by weight of a large particle size filler having an average particle diameter of from 0.5 to 10 micrometers and from 50 to 99.5 percent by weight of a small particle size filler having an average particle diameter of less than 0.1 micrometers, the large particle size filler having an average particle diameter of at least 10 times the average particle diameter of the small particle size filler. By adjusting the ratio of large particle size filler to small particle size filler, the change in viscosity of the mixture with change in temperature can be controlled.