The heart of any successful plastic injection molding project is the mold itself. While the plastic resin gets most of the attention, the type of steel used to create the mold is a critical factor that affects everything from the part's quality and cost to the mold's lifespan. Selecting the right steel is crucial for an efficient and long-lasting tool.
Here is a quick guide to some of the most common steels used in mold making.
P20 Steel: The General-Purpose Workhorse
P20 is by far the most widely used steel for plastic molds. It's a pre-hardened alloy, which means it comes ready to be machined, saving significant time and cost by eliminating the need for further heat treatment. P20 is an excellent choice for low to medium-volume production runs (typically up to 500,000 parts) and is easy to machine and polish. It offers a good balance of properties for most general-purpose applications. A variant, P20 HH (High Hardness), is available for projects that require better wear resistance and a higher polish.
- Best for: General-purpose molds, low to medium production volumes, and prototypes.
- Key advantages: Cost-effective, good machinability, no heat treatment required.
- Considerations: Lower wear resistance compared to other tool steels, not ideal for highly abrasive or filled plastics.
H13 Steel: The High-Volume & High-Performance Champion
H13 is a hot-work tool steel renowned for its exceptional toughness and resistance to heat and wear. It's the go-to choice for high-volume production and for molding materials that are abrasive (like glass-filled nylon) or require high processing temperatures (e.g., PEEK). H13 can be heat-treated to achieve very high hardness, which gives it excellent durability and a long lifespan, often lasting for millions of cycles.
- Best for: High-volume production, molds for abrasive or high-temperature plastics.
- Key advantages: Superior toughness, excellent wear resistance, thermal fatigue resistance.
- Considerations: More expensive and more difficult to machine than P20.
420 Stainless Steel: For Corrosion and Cleanliness 🧼
420 stainless steel is a great option when corrosion is a concern. Its high chromium content makes it highly resistant to rust and other forms of corrosion, which is essential when molding corrosive resins like PVC or when the mold will be operating in a humid environment. Its resistance to corrosion also makes it a top choice for medical, food-grade, and cleanroom applications. A key benefit of 420 stainless steel is its ability to achieve a beautiful, mirror-like polish, making it perfect for optical components and other parts requiring a pristine surface finish.
- Best for: Medical and food-grade molds, corrosive resins, molds exposed to moisture, and parts requiring a high-gloss finish.
- Key advantages: Excellent corrosion resistance, high polishability.
- Considerations: Can be more challenging to machine due to its hardness.
S7 Steel: For Impact and Shock Resistance
S7 is a shock-resistant tool steel that offers an impressive combination of toughness and strength. It's particularly useful for molds that have complex mechanical actions such as slides, lifters, or ejectors that are subject to heavy impact and stress during the molding cycle. Its air-hardening properties also make it dimensionally stable during heat treatment, reducing the risk of warpage.
- Best for: Molds with intricate moving parts, tools requiring high impact and shock resistance.
- Key advantages: Outstanding toughness, high impact resistance, good machinability.
- Considerations: Less wear resistance than H13 and less corrosion resistance than stainless steels.