Extrusion dies

Abstract

Despite its apparent geometrical simplicity, the extrusion head plays a fundamental role in the extrusion line. It is aimed at shaping the melt pumped by the extruder into the desired cross-section at the specified rate, but it also contributes decisively to the final physical and aesthetical characteristics of the extrudate. Since the various post-extrusion phenomena taking place induce variations in the extrudate cross-section along the extrusion line, the die-lip dimensions are generally different from those of the final product. Not only is this difference extremely difficult to predict theoretically, but it also varies with the operating conditions and with the rheological properties of the material being processed. Thus, in practice, extrusion dies are usually provided with adjusting systems that, directly or indirectly, change the local thickness of the flow channel, thus providing the possibility of producing extrudates with the adequate dimensional tolerances. The operator’s skill is also determinant for this purpose.