a moldable unsaturated polyester resin composition having improved antishrinking and shape - retaining properties contains an unsaturated polyester resin / styrene mixture , fillers , glass fibers , an antishrinkage agent and a demolding agent , wherein the demolding agent is calcium montanate and wherein the antishrinkage agent is preferably a methyl methacrylate polymer . the composition is particularly useful for the manufacture of lamp reflectors having superior shape and weight retention integrity and reduced reflector surface fogging .