 A polymer melt approaching a Newtonian fluid such as PET can have a drawdown ratio of around 20, whereas highly shear thinning and viscoelastic polymer melts such as polyethylene, polystyrene, and polypropylene may have critical drawdown ratios as low as 3.Tube forming dies allow for continuous extrusion of thick walled (relative to blown film extrusion) tubes and pipes.The dies themselves are almost identical to those used in blown film extrusion; the only major difference is the gap between the inner mandrel and outer die wall.