An extruder is the machine that melts plastic granules and feeds them to the die under pressure. Its main parts are the feed hopper into which the granules are poured, a heated barrel and the screw that turns inside the barrel. As the screw turns, it carries the granules forward; the heat from the barrel and the heat generated by friction melt and mix the material.
At the end of the screw, the melt passes through screens and reaches the die. On a blown film line, this die is an annular die. On co-extrusion lines there is a separate extruder for each layer, and the melts come together in the die. The extruder’s temperature settings and screw speed directly affect the homogeneity of the melt and therefore the quality of the film.