Numerical Study of Residual Stresses Due to External Cooling in Extruded Polymer Profiles

Profile extrusion is a production technique to make polymer profiles and hollow tubes. At a certain distance after the die exit the polymer extrudate is guided into a cooling chamber, where the polymer is cooled extensively to set the part dimensions. Uniform cooling in the cooling chamber is required to avoid warpage and twisting of the extrudate due to differential shrinkage. The effect of extrudate cooling on the shape of the extrudate and the residual stresses in the final extrudate product are numerically studied. Two different cases are studied: a 2D axisymmetric problem of a viscoelastic fluid exiting an annular die and a 3D problem of a hollow square shaped die. The extrudate is extensively cooled from a distance (Formula presented.) from the die exit. Extrudate cooling is modeled by subjecting the extrudate to heat loss via convection. For (Formula presented.) the Nusselt number Nu is fixed to 1 to represent the part of the extrudate in between the die exit and the entrance of the cooling chamber. For (Formula presented.) the Nusselt number is varied to mimic different cooling speeds. The Weissenberg number Wi, the Nusselt number and (Formula presented.) are varied in this work. Results show that for high values of Wi a significant skin-effect of high residual stresses appear near the extrudate edges for high Nusselt numbers and small values of (Formula presented.). It is also shown that for high Weissenberg numbers the shape of the extrudate is significantly altered due to extrudate swell, when the extrudate is cooled slower. For fast cooling and small values of (Formula presented.), the polymer does not have time to relax and the elastic stresses are “frozen in.” This leads to less extrudate swell but large residual stresses near the extrudate edges.