Manufacture of PVC Pipe by Extrusion

FIGURE 10.3 Die designs for extrusion of a pipe. (Adapted from Miller, E. ed., Plastics Products Design Handbook, Part A, Materials and Components, Marcel Dekker, New York, NY, 1981.) 

The main advantage of the sandwich pipe is the increased bending resistance. It is left for an exercise to show that there is little increase in hoop strength (see Homework Problem 10.2). The beam bending stiffness depends on the second moment of area. The second moment of area of a thin pipe is 

FIGURE 10.4 Multimanifold die for extrusion of a foam core pipe. 

The stiffness is doubled by separating the layers. If the density of the foam cores is 50% of the solid material, then there is only a 50% weight gain for a gain in stiffness of 100%. In other words, the stiffness-to-weight gain is -2/1.5 = 1.33 for this example. 

Plastic pipe is also subject to denting. The denting resistance is dependent on the shell bending stiffness D, where for a solid wall thin pipe 

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