Extrusion apparatus is employed in a co-extrusion process to produce a triangular-shaped barrel for a writing instrument. An inner layer of material is extruded in tubular form having a substantially circular cross-section. A coating layer of three strips of material, equally spaced one from the other,...http://www.google.es/patents/US6017477?utm_source=gb-gplus-sharePatente US6017477 - Extrusion apparatus and process
