Abstract: A method of materials processing (e.g. polymer extrusion) in which the material flows along an axially extending passageway. A pressure differential is established across the passageway transverse to the axial flow direction so as to create a flow of material through the die which has a velocity component transverse to the axial flow direction. Preferably the pressure differential transverse to the extrusion direction is continuous thereby estabilshing a continuous change of velocity component transverse to the axial flow direction. Preferably also there is at least one reversal of transverse pressure differential (and hence transverse velocity component). Apparatus for performing this method comprises for example, successive sections (3-5) of identical trapezoidal cross section with adjacent ones of these sections having their cross-sections displaced through 180.degree. relative to each other.