Abstract: Provision is made for evenly controlling the cooling of the inner die pin of an extrusion die assembly so that the hot plastic coming in contact therewith is frosted. A conduit extends through the inner die pin to a point near the tip of the inner die pin. A cap is mounted on the end of the conduit and spaced from the front face thereof to provide an annular discharge opening. At least the tip portion of the inner die pin is formed of a material of good heat conductivity, such as brass, and the cooling air is directed through the discharge opening relatively evenly over the inner portion of the inner die pin to effect uniform inside frosting of a plastic tube formed in the apparatus. The inside of the plastic tube is open to the atmosphere.