Mold Steel Selection Guide: P20 vs H13 vs S136

P20 (AISI P20, DIN 1.2311) is a pre-hardened chromium-molybdenum alloy steel supplied at 30-36 HRC, eliminating the need for post-machining heat treatment. Its key advantage is machinability -- P20 machines approximately 30 percent faster than H13, reducing mold manufacturing lead time by 2-3 weeks on a typical 8-cavity yogurt cup mold. Thermal conductivity is 29-34 W/m-K, adequate for moderate cycle times. However, P20's limitations become apparent in high-volume thin-wall production. At injection pressures above 160 MPa, P20 cores running 0.5mm-wall yogurt cups show measurable surface wear after 400,000-500,000 shots, manifesting as dimensional drift on wall thickness. P20 is best suited for prototype molds, low-volume production runs under 500,000 shots, or non-critical mold components like support plates and spacer blocks. For food packaging molds running on HWAMDA SPV5 machines at 4.0-4.5s cycle times, P20 should only be specified for outer mold plates and ejector plates, not for core or cavity inserts that contact the melt.

S136 (AISI 420 modified, DIN 1.2083) is a high-chromium (13.5 percent) stainless tool steel that combines corrosion resistance with hardness of 48-52 HRC after heat treatment. Its primary application in thin-wall molding is for food packaging molds that must resist corrosion from condensation in chilled mold environments, or when processing hygroscopic materials like nylon. Thermal conductivity is 20-23 W/m-K -- the lowest of the three grades -- requiring beryllium copper inserts in thermally critical areas to maintain cycle times. S136 mold life reaches 5-10 million shots for PP containers, the longest of any standard mold steel. The premium cost is significant: S136 raw material is approximately 3-4 times the cost of P20 and 1.5-2 times the cost of H13. For an 8-cavity yogurt cup mold, the steel cost difference between H13 and S136 cores is approximately USD 8,000-12,000. S136 is justified when running molds in humid environments (Southeast Asia, tropical regions), when FDA or EU 10/2011 compliance requires documentation of corrosion-free mold surfaces, or when target mold life exceeds 5 million shots.

The thermal conductivity difference between P20 (29-34 W/m-K), H13 (24-28 W/m-K), and S136 (20-23 W/m-K) directly affects achievable cycle times and cooling uniformity. On an 8-cavity yogurt cup mold running 0.5mm-wall PP at a mold temperature of 30 degrees C, switching from H13 to S136 cores without modifying the cooling circuit adds approximately 0.3-0.5s to the cycle time -- a 7-12 percent productivity loss. Compensating requires additional cooling measures: conformal cooling channels manufactured by direct metal laser sintering reduce the cooling gap by providing uniform cooling within 3-5mm of the cavity surface versus 8-12mm for conventional drilled channels. Alternatively, beryllium copper (BeCu) alloy inserts with thermal conductivity of 105-130 W/m-K can be press-fitted into thermally critical areas such as core tips and thin ribs. On HWAMDA SPV5 production lines, the optimal approach for S136 molds is a hybrid design: S136 cavity blocks with BeCu core tips, reducing the cycle time penalty to under 0.2s while maintaining full corrosion protection on melt-contact surfaces.

When ordering molds for HWAMDA SPV5 thin-wall machines, specifying mold steel correctly prevents costly mistakes. The mold specification should include: steel grade with hardness range (e.g., H13, 48-50 HRC), heat treatment standard (NADCA 207-2003 or equivalent), surface finish requirement per SPI classification, and material certification with mill certificate. For multi-cavity molds, all cavity inserts must be sourced from the same steel heat to ensure uniform hardness and thermal behavior across cavities -- hardness variation between cavities should not exceed 1 HRC. The mold maker should provide Charpy impact test results (minimum 15 J for H13 at room temperature) and a metallographic report confirming tempered martensite microstructure with no retained austenite above 5 percent. For HWAMDA's standard mold interface specifications, the mold locating ring should be 160mm diameter for the HMD 380M8-SPV and 200mm for the HMD 600M8-SPV. Sprue bushing radius must match the HWAMDA nozzle tip radius of 15mm (standard) or 20mm (large-bore option).