Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for interactively simulating an injection mold model. A three-dimensional CAD model is identified representing an injection mold cavity. The mold cavity includes a location of at least one gate. A potential fill pattern is determined for injection of a material into the injection mold cavity. The determined fill pattern is based at least in part on the geometry and dimensions of the modeled mold cavity and the location of the at least one gate. A strip model of the CAD model is generated based at least in part on the determined fill pattern. The strip model is used to perform a strip analysis simulating injection of the material within the injection mold cavity.