How to Select Mold Steel for TPU Materials
TPU (Thermoplastic Polyurethane) is a widely used elastomer with high melt viscosity, poor flowability, strong tendency to adhere to mold surfaces, and slight corrosiveness when exposed to high temperatures during processing. These unique material properties impose significantly higher requirements on mold steel in terms of polishability, wear resistance, rust resistance, and surface density compared to ordinary plastics. Improper steel selection can lead to a series of problems such as severe sticking, surface scratching, poor product finish, mold corrosion, and shortened service life, which directly affect production efficiency and product quality. This article systematically explains the selection principles and practical recommendations for TPU injection mold steels to provide clear guidance for industrial applications.
Core Requirements of TPU for Mold Steels
TPU materials have four key requirements for mold steels that must be prioritized during selection. First, high finish and excellent polishability are essential, as TPU products are highly sensitive to surface defects. Even minor imperfections like material lines, pinholes, or impurities in the steel can be clearly reflected on the product surface, leading to appearance defects. Second, low friction and anti-sticking performance are critical. TPU has a strong tendency to adhere to mold surfaces, so steels with high purity and dense surface structure can reduce adhesion and ensure smooth demolding. Third, good wear resistance is necessary to withstand the strong melt erosion during processing, especially for long-term production. Finally, corrosion and rust resistance are required to prevent damage from the trace acidic gases released by TPU decomposition at high temperatures, which can corrode the mold and affect its lifespan.
Graded Steel Selection by Output and Appearance
Mold steel selection should be graded based on production volume and product appearance requirements to balance performance and cost. For small-batch production, prototype molds, or non-appearance parts, domestic P20 or 718H is recommended. These are pre-hardened steels with a hardness of 28–33 HRC, offering low cost, good machinability, and no need for additional heat treatment. They are suitable for production volumes of less than 100,000 shots, but sufficient polishing is required to reduce sticking. For medium-batch production and general appearance parts, NAK80 is the most cost-effective choice. With a hardness of 38–42 HRC, it provides excellent polishability, moderate rust resistance, and good dimensional stability, making it ideal for products with matte or textured surfaces and production volumes between 100,000 and 500,000 shots. For high-volume production, high-gloss surfaces, or transparent TPU products, S136 or S136H is preferred. After quenching, S136 can reach a hardness of 50–54 HRC, while pre-hardened S136H has a hardness of 35–40 HRC. Both grades offer mirror-level polishability, superior corrosion resistance, and excellent wear resistance, ensuring long service life for high-volume production exceeding 500,000 shots. For glass-filled TPU or highly corrosive TPU grades, quenched S136 with additional surface treatments like nitriding or DLC coating is necessary to resist the increased wear and corrosion.
Material Allocation for Different Mold Parts
To optimize cost and performance, TPU molds should adopt a graded material allocation strategy for different components. Cavities and cores, which are in direct contact with the product and determine surface quality, should use NAK80 or S136 to ensure excellent finish and durability. Slides, lifters, and inserts, which experience repeated movement and friction, require materials with balanced hardness and toughness, such as NAK80 or SKD61. Gates, runners, and nozzles, which are subject to intense melt erosion, should use SKD61 or S136 to resist wear and maintain dimensional stability. Non-forming structural parts like mold bases, ejector plates, and support blocks can use S50C to reduce overall costs without compromising performance.
Necessary Surface Treatments
Regardless of the selected steel, appropriate surface treatments are essential to enhance mold performance and extend service life. High polishing is the most fundamental step, requiring at least 1000#–1500# sandpaper followed by diamond paste polishing to create a smooth surface that minimizes sticking. Nitriding can significantly improve surface hardness and wear resistance, making it suitable for medium-to-high volume production molds. DLC (Diamond-Like Carbon) coating provides ultra-low friction and excellent anti-sticking properties, making it highly effective for solving TPU sticking issues. Chrome plating is a cost-effective option that enhances surface finish, rust resistance, and anti-sticking performance, making it suitable for small-to-medium molds and standard products.
Common Mistakes in Steel Selection
There are several common mistakes to avoid in TPU mold steel selection. Using 45# steel for TPU cavities should be strictly avoided, as it leads to severe sticking, rusting, and frequent scratching, resulting in high repair costs. Standard P20 is not suitable for transparent TPU products, as its insufficient purity prevents achieving a mirror finish, leading to surface defects like haze and material lines. Skipping the polishing step before production is another critical mistake, as TPU’s material properties make it highly dependent on a smooth surface for demolding. Finally, using soft steels for high-volume production will result in short mold life, frequent repairs, and higher overall costs.
