Selecting Steel for Molds
The steel you choose determines how long the mold will run and how much maintenance it requires. Here is how we choose between pre-hardened steel and hardened steel.
Introduction
Almost every mold-making project starts with the part's geometry, but the very first question we ask is about production volume. A mold designed to produce 50,000 parts is completely different from one built for 2 million parts—and that difference comes down to the choice of steel.
Content
Pre-hardened steels like P20 and 718 are our shop's workhorses. They machine cleanly, can be welded and modified when designs change, and for low-to-medium volume runs, they hold a good enough surface finish without the cost and lead time of through-hardening. Most housings, enclosures, and general packaging molds start here. When parts use glass-filled plastics, abrasive additives, or run at high volumes, we move to hardened steels like H13 or S136, machining the cavity after heat treatment. It costs more and takes longer, but the cavity edges hold up significantly longer, keeping parts within tolerance rather than drifting over time. Corrosion matters more than most people expect. PVC and certain flame retardants will eat through standard tool steel from the inside out. Choosing stainless for the cavity and core is much cheaper than replacing rusted inserts a year later. We also mix steel grades within the same mold. Bases and clamping plates might use lower grades, while inserts, slides, and wear points get the harder steel. This keeps the cost sensible while putting the good steel where the mold actually wears. If you tell us your resin, annual volume, and required surface finish, we will tell you which steel makes sense and why—before you commit to the order.