Techniques are described for injection molding. When material inside a cavity of a tool is solidified into a molded part, the tool imparts a finished surface onto the part, including sidewalls with a zero or low-draft angle. To allow separation from the cavity without using sleeves or sliders, the cavity is widened, just prior to the part being ejected. The tool is made from metal with a high coefficient of thermal expansion, so the size of the cavity can be manipulated using temperature control. Heat applied to an outer portion of the metal surrounding the cavity pulls the metal away from the part creating an air gap within the cavity. Carefully applied cooling to an inner portion of the metal blocks the heat and keeps the surface temperature under control, which preserves the finished surface on the part. When the air gap allows, the part releases from the cavity with the finished surface intact.