Cooling and solidification of polymers in blow moulding operations

The cooling and solidification of high density polyethylene in a blow mould ing operation has been modelled as a one-dimensional Stefan problem, which involves two solid/liquid interfaces and a variable width of the polymer. T he mathematical formulation during solidification differs from previous tre atments of blow moulding problems, since the domains occupied by the solid and liquid phases are first transformed into mathematical regions of consta nt width and then a finite difference solution is obtained using a fully im plicit scheme. However, at small times after the solidification has started at one end, convergent numerical results could only be obtained when using an additional transformation of the temperature distribution within the po lymer. To verify the accuracy of the numerical method, the numerical soluti on was compared with the small time analytical solution for a similar probl em. Numerical results of the blow moulding problem are presented for both t he temperature distribution within the polymer and the moving boundaries; t hese results were used to study the influence of the process variables on t he cooling and solidification of the polymer.