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P20 Mold Steel
Meaning
A pre-hardened chrome-molybdenum alloy tool steel is widely used for the construction of injection molds and extrusion dies. This material, designated as P20 mold steel, governs the tooling choice for medium-volume production runs due to its balance of machinability and wear resistance. The alloy applies to molds processing non-corrosive polymers such as polyolefins and polystyrenes but is unsuitable for resins that release corrosive acids during processing, such as polyvinyl chloride.
Material Composition
The steel is supplied in a pre-hardened state of approximately 30 to 32 Rockwell C hardness, which eliminates the need for heat treatment after machining. P20 mold steel contains chromium, manganese, and silicon to improve hardenability and grain structure. This pre-hardened condition ensures dimensional stability since the completed mold does not undergo thermal distortion from heat treatment.
Machining Advantage
Tool makers can cut, drill, and polish the steel directly to finished dimensions without risking the stress cracking or warping that occurs during post-machining hardening. This direct path accelerates the tool build schedule and reduces manufacturing costs. The steel can also be nitrided or chrome-plated to improve surface hardness and wear resistance when needed.
Production Capability
The material provides sufficient strength to resist the clamping and injection forces of typical molding runs of up to five hundred thousand cycles. However, abrasive glass-filled resins will cause rapid gate and runner wear on raw steel surfaces. Molders must select higher hardness steels or apply surface coatings when processing these filled compounds.