When molding molten plastic into any mold the hope is that the plastic will cool evenly at the end of the molding cycle. If it does not, the hotter areas of the plastic, often caused by hot spots in steel molds will continue to shrink as they cool causing internal stress and that usually leads to parts that warp or are not flat or straight. Aluminum’s ability to manage temperature evenly across the surface of a mold completely eliminates hot spots.