Knuckle formation from melt elasticity in plastic pipe extrusion

We develop a map to help plastics engineers predict the extrudate shape, including extrudate knuckles, and then from the mass balance over the postdie region, to predict the extrudate shape entering the cooling chamber. We find that Newtonian extrudates, or extrudates for small pipe, never knuckle. Both of these findings agree with industrial experience. We also include a worked example to help plastics engineers suppress extrudate knuckling.