Annular Dies for PVC/PE pipe

Dies with annular cross sections are used to extrude pipes, tubes, tubular films, and parisons for blow molding. Center-fed dies are commonly used for extruding pipes, while side-fed dies are used for tubular films and parisons. The four basic annular die designs in use at the present time are shown in below. These include (1) center-fed spider-supported mandrel dies, (2) center-fed screen pack dies, (3) side-fed mandrel dies, and (4) spiral mandrel dies. At the die exit there is usually an outer die ring that forms the die land. Plastic pipes are primarily extruded using center-fed dies. The melt stream from the extruder passes from the circular opening to the annular die by means of the mandrel support tip. The melt then passes over the spider legs which support the mandrel, and through a converging annular region, which for pipes is usually 10° to 15°. The converging region is followed by an annular region with parallel walls which imparts the final dimensions to the pipe. The outer diameter of the pipe can range from a few millimeters to approximately 1.6 m. The ratio of the mandrel radius to outer wall radius usually falls in the range of 0.8 to 0.925. The spider legs usually lead to problems in that not only are flow markings visible, but mechanically weak regions are generated. These flow lines are also referred to as weld lines and are due to a higher degree of molecular orientation caused by the high stresses imparted to the melt and the inability of the molecules to re-entangle during the time the melt spends in the die. The strength of the weld line is most likely related to the degree of re-entanglement of the molecules, which occurs as the melt passes through the die at the melt temperature. The re-entanglement time appears to be much longer than the longest relaxation time. It has been estimated from interrupted-shear experiments for polystyrene, for example, to be of the order of 300 sec and is related to self-diffusion of polymer molecules.