Abstract: Method and apparatus for extruding expandable thermoplastic material having a cellular internal body portion and formed thereon a substantially non-cellular high density skin. The extruded material is contacted by a pair of continuously traveling temperature controlled belts immediately upon leaving the extrusion die which serve to form the high density skin thereon and the spacing of the belts is selectively controlled downstream from the extrusion die with the use of selectively adjustable belt backing members. Selectively controlling the spacing of the belts while the extruded material is undergoing expansion or foaming results in a predetermined internal cellular structure of said extruded material especially with regard to the size and distribution of the cells across a cross-section of the material.