A method of making a vehicle interior panel substrate with a reduced thickness portion along a boundary of an airbag door includes molding the substrate using a substrate mold that includes a movable mold member. The movable mold member is part of a mold section and moves with respect to other portions of the mold section during the molding process to form the reduced thickness portion, which may include a hinge or a tear seam. The mold member moves after plastic material is introduced into the mold cavity. The plastic material is allowed to expand when substrate mold sections are moved away from each other. Plastic material expansion may be part of a structural gas injection (SGI) process. A scrim material or other hinge reinforcement may be molded into the substrate.