What is the Best Mold Steel for Thin-Wall Packaging Molds?

S136 is the premium choice for thin-wall food packaging cavities requiring mirror-finish surface quality. This martensitic stainless steel achieves hardness of 48-52 HRC after heat treatment and polishes to SPI A-1 level (Ra below 0.012 μm). Its 13% chromium content provides excellent corrosion resistance against cooling water condensation and food-grade PP processing gases. HWAMDA specifies S136 for all yogurt cup cavity inserts, IML margarine container molds, and any product where surface appearance directly affects brand perception. S136 resists the micro-pitting and corrosion that degrade surface finish over millions of cycles in humid production environments. Cost is approximately 2-3 times higher than P20 or 718H, but the investment is justified for molds requiring mirror finish and long service life. Genuine Uddeholm Stavax (the original S136) and certified Chinese equivalents (4Cr13) both deliver excellent results when properly heat-treated. Cost is approximately 2-3 times higher than P20 or 718H, but the investment is justified by mold life exceeding 5 million cycles and maintaining consistent surface quality throughout the production campaign.

2738 (modified P20) is a pre-hardened steel supplied at 28-32 HRC, eliminating the need for additional heat treatment. This reduces manufacturing cost and lead time by 1-2 weeks compared to through-hardened steels. 2738 provides adequate polishability for SPI B-2 to C-1 surface finishes, suitable for containers where surface appearance is not the primary quality criterion. HWAMDA uses 2738 for mold bases, support plates, and cavity inserts in cost-sensitive applications including disposable food containers, industrial packaging, and tableware where the textured or semi-gloss finish is acceptable. The pre-hardened condition also simplifies mold modifications and repairs. For production runs below 2 million cycles, 2738 offers excellent value. Above 2 million cycles, wear on the softer steel may require cavity refurbishment that is not needed with hardened S136 or H13. For production below 2 million cycles, 2738 offers excellent value and simplified manufacturing. Above this threshold, cavity wear may require refurbishment not needed with hardened alternatives.

718H is a versatile pre-hardened mold steel at 33-38 HRC that bridges the gap between economy 2738 and premium S136. Its nickel addition improves polishability and through-hardness uniformity compared to standard 2738. 718H achieves SPI B-1 surface finish (Ra 0.05 μm) reliably, making it suitable for standard food containers and cups that need a good but not mirror-finish surface. HWAMDA specifies 718H as the standard core and cavity steel for food container molds, milk tea cup molds, and medium-volume yogurt cup molds where S136's premium is not justified. The pre-hardened condition enables direct machining without heat treatment, keeping lead times shorter. 718H provides good corrosion resistance for normal production environments and adequate wear resistance for PP processing at standard cycle volumes. For molds that may be modified during production (wall thickness adjustments, gate changes), 718H's machinability is an advantage. For molds that may need modification during production runs, such as wall thickness adjustments or gate resizing, 718H's machinability allows in-house modifications without specialized tooling.

S136 requires vacuum heat treatment to 48-52 HRC followed by double tempering at 200-250°C. Vacuum processing prevents surface oxidation and decarburization that would ruin polishability. The heat treatment cycle takes 2-3 days and must be performed by qualified heat treatment facilities with documented processes and hardness verification. HWAMDA uses certified heat treatment partners with ISO 9001 quality management. H13 is heat treated to 48-52 HRC with higher tempering temperatures of 550-600°C for hot hardness retention. Triple tempering is recommended for maximum toughness and dimensional stability. The higher tempering temperature gives H13 its resistance to thermal fatigue during production cycling. Both S136 and H13 require stress relieving at multiple stages during machining to prevent distortion in the finished mold. Pre-hardened 2738 and 718H require no additional heat treatment, which simplifies manufacturing but limits achievable hardness and wear resistance. Both S136 and H13 require stress relieving at multiple stages during machining to prevent dimensional distortion in the finished mold components that could affect cavity accuracy and cooling performance.