IMM 11/97

Does anyone have information on such types of tool steels as H-13, P-20 and S-7, and the lives of tools based on SPI 101?

S-7 is a good all-around steel for SPI 101 molds, offering a good combination of toughness and durability. But it is very sensitive to cracking above a Rockwell C of 56. H-13 offers good high-temperature hardness and is used for high-temperature engineering plastics. It's a good choice where high-temperature materials are processed. P-20 is generally used in its pre-hardened condition (Rockwell C of 32 to 35). It is not a good choice for SPI 101 molds and is generally used for pre-production or prototype molds. The life of all tools is dependent on the abrasiveness of the plastic being molded, proper preventive maintenance, and good mold design. A good-quality mold steel that is properly heat treated can easily meet SPI 101 requirements with the above considerations.
-J. Petti, Delaney Engineering Inc., Milford, CT, (203) 878-6766.

We use S-7 in the annealed state. We're considering switching to P-20 at a Rockwell C hardness of 32 to achieve a better polish. I'd choose P-20 or S-7 over H-13, as H-13 tends to work harden excessively, making rework more difficult. As for mold life, I can't tell much difference between P-20 and S-7.
-B. Thompson, Houston Ceophysical Products, Houston, TX, (713) 472-4355.