The manufacture of polyvinyl (PVC) fascia boards involves the extrusion of a pressurized mixture of molten PVC and foaming agents through a die that is shaped in the exterior of a board; subsequent pressure release triggers foaming, so that the interior of the board is filled, simultaneously as the board is cooled. Sometimes, however, the foam does not fill the interior, leading to undesired voids, especially at corners. We consider a mathematical model for the foaming process that attempts to account for the formation of undesired voids. Preliminary model results suggest that severe temperature gradients through the cross-section are most likely responsible for void formation; two mechanisms are suggested.