Abstract: A mold for making bathtubs includes a mold receptor and a deck insert attached to the mold receptor face. The preferred deck insert has at least one projection designed to reduce the cross-sectional area of a portion of a void between the shell's deck and the deck forming face of the insert. Preferably, the distance from the outer edge of the shell's deck to the outer edge of the projection is about 16% of the length of the deck of the sanitary fixture. In an alternative embodiment, the deck insert has at least one outermost projection and at least one non-outermost projection. The outermost projection is preferably about half the height of the non-outermost projection. In another preferred embodiment, the deck insert includes at least one outermost projection and at least one non-outermost projection. The height of the outermost projection varies from a maximum height near the middle of the deck, to a minimum height near the end of the deck.