Abstract of the Disclosure It is known to graft silane compounds to polymers during an ex-trusion process, in the presence of peroxides. In the known process there tends to be a non-homogenous mixture of the silane compound, the polymer, and the peroxide in the extruder barrel. As extrusion is a relatively high temperature process from about 180°C to 220°C there results localized cross-linking of the polymer and consequently the formation of a gel in the extruded product. The presence of a gel in the product tends to cause a rough surface making the product unsuitable for some applications such as electrical in-sulation. The present invention overcomes this drawback by providing in a method for cross-linking thermoplastic or elastomeric material wherein cross-linking is preceded by grafting of silane and is carried out in the presence of moistures, the combination of steps comprising: providing the thermo-plastic or elastomeric material as dry, fluid powder; agitating the powder to obtain fluidization thereof and raising the temperature by operation of the agitating; adding a silane compound to the material prior to completion of the agitation, the silane compound being of the variety that permits grafting on the molecules of said powder; providing for a temperature below the cry-stalite melting range so that the silane or silane compound as agitated to-gether with the said material fluid is caused to diffuse into the fluid particles and providing for grafting of the silane or silane compound mole-cules to the molecules of the thermoplastic or elastomeric material.