Enhancing slit die performance by optimization of restrictor profiles

A gradient-based method is applied to determine optimum profiles of the restrictor (choker bar) required to obtain the best possible flow distribution uniformity in a 1.2 m wide slit die of given design, in a range of applications involving three materials of varying degrees of shear thinning, each at a high and a low flow rate. The kernel of the optimization is a simulation of die performance that takes into account the effects of die body deflection (‘clam shelling’) in a fully coupled flow/deflection analysis. For the most sharply varying restrictor profile, a linear elastic analysis is carried out, which shows that the optimum profile cannot be obtained by bending the bar without exceeding the tool steel yield point. The use of inter-changeable bars, machined to the optimum profiles computed for various applications, is proposed to extend the operating range of dies. Alternatively, the highly adjustable membrane restrictor technology may be used. The developed software makes it possible to predict whether restrictor adjustment can produce acceptably uniform flow distribution in a given application, using a given die, and shows what restrictor profile should be set up, thereby eliminating or reducing the need for online trial and error adjustment.