It depends on the type of extrusion process, but for rigid plastic extrusion PVC compound is usually used in pellet form.
At MKM Extrusions, the rigid stock range on our online shop is manufactured from an external grade of rigid PVC. The process starts with PVC granules, which are heated and pushed through a shaped die to form continuous profile lengths which are cut to size. This material is used to produce a range of rigid (and in some cases flexible) profiles, including plastic angle trims, plastic flat strips, plastic channels and plastic edging trims.