​Extrusion of Pipe and Tubing

Dies for pipe and tubing production consist essentially of a female die ring that shapes the pipe outside diameter and a male mandrel that shapes the inside diameter. The difficulty is to support the mandrel in rigid and accurate alignment with the die ring without compromising the product. The spider type (Fig. 4.45) uses three or four spider legs (ribs) to support the mandrel, but these legs cause axial weld lines as the melt flows around them [56]. An alternative is the breaker plate design in which the mandrel is supported by a disc pierced by a multitude of small holes. A better solution, particularly for larger pipe dies, is the spiral mandrel. Here the mandrel is rigidly supported, and the melt flow is directed around it in an initially helical pattern, which is converted to axial flow by the geometry of the flow channels and lands. Weld lines are eliminated, but the difficulty is to ensure an even flow rate at all points of the die outlet.