Subsequent use in an extruder
Another frequent problem with twin screw extruder is the difficulty in maintaining uniformly dehumidified material for subsequent use in an extruder. This problem results from the non-homogeneity of moisture content in hygroscopic particulate material.
Previous combinations of a dryer and an extruder have inadequately accommodated the need for rapid color changes often encountered in production operations. The inadequacy results directly from the lead time required to dry a differently colored stock of hygroscopic material for single screw extruder.
This type of extruder results in nonuniform flow rates which are unsuitable for the extrusion of high quality finished products. Nonuniform flow rates are aggravated by pressure gradients which often occur in long vent sections. Moreover, the extrudate in such extruders frequently develops thermal nonuniformity which is induced by volatile substances undergoing a liquid-vapor phase change. Since viscosity is highly dependent on temperature, such thermal non-uniformity also has an adverse effect on extruded products.