Mold Steel Selection for PP Injection Molding
PP (polypropylene) has good fluidity, low corrosion and moderate shrinkage, and is widely used in daily necessities, home appliances, automobiles and other fields. The selection of PP injection mold steel focuses on wear resistance, polishability, machinability and cost balance, which shall be based on product precision, surface requirements, production scale and mold lifespan.
Selection by Product Surface Requirements
Ordinary Appearance Products
For PP products with no high requirements on surface gloss and texture (such as daily necessities, industrial structural parts and ordinary toys), cost-effective pre-hardened mold steels are preferred, which do not require additional heat treatment and can shorten the mold manufacturing cycle.
P20 (718S): Pre-hardened to 28–32HRC, with good machinability, can be polished to Ra≤0.8μm, suitable for mass production of ordinary PP products such as turnover boxes and home appliance shells, with low cost and strong versatility.
NAK80: Pre-hardened to 35–40HRC, with better polishability than P20 (Ra≤0.4μm), no sand holes and air bubbles on the surface, suitable for PP products with slightly higher surface requirements such as home appliance panels and high-quality daily necessities shells, easy to maintain and longer service life.
High-Gloss/Mirror Products
For PP products with high requirements on surface gloss and transparency (such as cosmetic boxes, food containers and high-end home appliance panels), high-purity and high-polishability mold steels must be selected to avoid surface defects.
S136 (S136H): Stainless steel with high purity and strong corrosion resistance, can achieve mirror polishing (Ra≤0.05μm), meets food and medical safety standards, suitable for transparent PP products and food-contact parts, with good rust resistance. S136H is pre-hardened, while S136 can reach 50–55HRC after quenching, suitable for long-life mass production molds.
STAVAX: Imported high-end mirror mold steel, with higher purity and polishability than S136, stable surface finish, suitable for ultra-high-gloss and precision PP parts such as medical accessories, with long service life but high cost.
Textured/Etched Products
For PP products with surface textures (such as frosted home appliance shells and leather-textured toys), mold steels with good etching resistance and wear resistance are required to ensure clear and uniform textures.
718H: Pre-hardened to 33–37HRC, with uniform structure and fine grains, excellent etching effect, suitable for PP products with medium textures such as matte home appliance shells, with good machinability and moderate cost.
NAK80: In addition to superior polishability, it also has good etching performance, and the textured surface after etching is smooth and uniform, suitable for PP products with fine textures such as high-end toys.
Selection by Production Scale
Small-Batch Production (<100,000 shots): 45# steel (low cost, easy to process) or P20 is selected, suitable for prototype molds and small-batch trial production.
Medium-Batch Production (100,000–500,000 shots): 718H or NAK80 is preferred, which balances wear resistance, machinability and cost, suitable for continuous production of home appliance accessories and daily necessities.
Mass Production (>500,000 shots): Quenched high-hardness mold steels such as H13 (48–52HRC) and S136 (50–55HRC) are selected, with strong wear resistance and fatigue resistance, ensuring long-term stable production and reducing mold maintenance costs.
Selection for Special Working Conditions
Glass Fiber Reinforced PP Products: The glass fiber in the melt will accelerate mold wear, so high-hardness wear-resistant steels such as H13 (SKD61) and DC53 are selected. DC53 (60–62HRC after quenching) has better wear resistance than H13, suitable for PP products with high glass fiber content (>20%).
Food-Grade/Medical-Grade PP Products: Stainless steel with no harmful substance migration is required. S136 (S136H) meets FDA and LFGB certification, while 1.2316 (German stainless steel) has similar performance to S136 and lower cost, which is a cost-effective choice.
Humid/Corrosive Environments: S136H (pre-hardened stainless steel) has good rust and corrosion resistance, no frequent anti-rust treatment is needed; NAK80 (non-stainless steel) has better rust resistance than P20, suitable for moderately humid environments, with lower cost than stainless steel.
Key Notes for Steel Selection
Match PP material characteristics: PP has good fluidity and a shrinkage rate of 1.0%–2.5%, so mold steels with uniform structure are selected, and pre-hardened steels are preferred to avoid heat treatment deformation.
Balance cost and performance: Do not over-select high-end steels for ordinary products, and select high-performance steels for high-requirement products to control mold costs.
Consider machining and maintenance: Pre-hardened steels are easy to process and maintain, while quenched steels require professional heat treatment equipment.
Pay attention to surface treatment: The cavity of PP mold shall be polished to Ra≤0.8μm (Ra≤0.05μm for high-gloss parts) to improve product quality and extend mold life.
