Tool Steel Grade Selection and Cavity Shrinkage Tolerances
Selecting tool steel grades for precision cavities requires balancing grain direction, heat treatment distortion, and alloy verification to hold tight part tolerances.
P20 Prehardened Steel
Meaning
Pre-hardened chromium-manganese-molybdenum alloy steels developed for plastic mold bases and cavities eliminate the need for post-machining heat treatment to avoid dimensional changes. Using P20 prehardened steel allows mold makers to mill and polish the steel directly to its final dimensions while maintaining excellent dimensional stability. This material is delivered with a uniform hardness of twenty-eight to thirty-two Rockwell C, making it suitable for medium-volume production.
Material Performance
The steel’s balanced chemical composition provides good machinability and polishing characteristics, which helps reduce tooling lead times. It also offers sufficient strength and wear resistance to withstand the pressures of injection molding without excessive deformation. However, it is not as hard as fully hardened tool steels.
This limits its use in high-stress applications.
Machining Process
Mold makers can mill, drill, and tap the steel using standard carbide tools without the risk of tool breakage or excessive wear. This ease of machining makes it the preferred choice for large mold bases and core inserts where rapid fabrication is required. The pre-hardened state also reduces the risk of warping that can occur during heat treatment.
It allows for more complex mold designs.
Application Boundary
While the steel is highly versatile, it is not suitable for molds that will run abrasive materials like glass-filled polymers or high-temperature resins. These harsh environments will cause rapid wear on the mold cavities, leading to out-of-tolerance parts and premature tool failure. For high-volume or high-stress projects, harder steels like H13 are a better choice.
Mold designers should select this steel for non-abrasive, medium-volume production runs where speed and cost are the primary drivers. This selection ensures a cost-effective tooling solution for many standard applications. It provides the optimal balance between cost and performance.