AbstractA theoretical and experimental investigation is presented for filling thin cavities of variable gap thickness. The modeling is based upon a finite‐element/finite‐difference formulation for an inelastic power‐law fluid and includes the effects of viscous heating and conduction upon the flow dynamics. Extensive results are presented for polypropylene and polystyrene melts injected into two variable‐gap‐thickness cavities, of which one has an insert. Good agreement is found between the predictions and measurements concerning the shape of the advancing melt front, the location of weld lines, and the temporal pressure trace at various positions in t