Everything You Need to Know About Mold Steel for Plastic Injection Molding
Plastic injection molding is the backbone of modern manufacturing, enabling the mass production of plastic components used in automotive, electronics, medical devices, consumer goods, and more. Central to the success of this process is the mold steel—the material used to create the molds that shape molten plastic into precise, complex parts.
Choosing the right mold steel is crucial. It affects not only the lifespan of the mold but also the dimensional accuracy, surface finish, and overall quality of the final product. In this blog, we’ll walk you through the types of mold steel, their applications, key selection criteria, and best practices for using them in injection molding.
What is Mold Steel?
Mold steel refers to specially formulated grades of steel used in manufacturing molds for injection molding processes. These steels are engineered to withstand:
- High pressure from injection cycles
- Constant thermal cycling (heating and cooling)
- Wear and abrasion from plastic resins, some of which are highly corrosive or abrasive
Mold steels are generally classified by their hardness, machinability, corrosion resistance, thermal conductivity, and ability to hold a polished or textured surface.
Why Mold Steel Selection Matters
Using the wrong mold steel can lead to:
- Premature mold failure
- Surface defects on the plastic part
- Inaccurate tolerances
- Longer lead times due to poor machinability
- Increased production costs
Correct selection ensures:
High-dimensional stability
Long mold life (often exceeding a million cycles)
Better cooling and cycle time
Reduced maintenance
Consistent quality of the molded parts
Types of Mold Steel Used in Plastic Injection Molding
1. P20 Steel (Pre-Hardened Tool Steel)
- Hardness: ~28–32 HRC
- Applications: Ideal for general-purpose plastic injection molds
- Advantages:
- Excellent machinability
- Good surface polish
- Pre-hardened—no need for post-machining heat treatment
Common Uses: Consumer goods, low- to medium-volume production molds.
2. H13 Steel (Hot Work Tool Steel)
- Hardness: Can be heat-treated up to ~50–54 HRC
- Properties:
- High-temperature resistance
- Good wear and impact resistance
- Great for high-volume production
Common Uses: Molds for engineering plastics, automotive parts, and components with thin walls.
3. 420 Stainless Steel
- Hardness: Up to 50 HRC
- Properties:
- Excellent corrosion resistance
- Highly polishable
- Ideal for medical and optical applications
Common Uses: High-humidity environments, cleanroom production, and food-grade components.
4. S136 (Stainless Tool Steel)
- Origin: Developed by Uddeholm
- Properties:
- Superior corrosion and wear resistance
- Excellent mirror polish capability
- Long mold life
Common Uses: Medical devices, cosmetic packaging, high-gloss components.
5. NAK80 (Pre-Hardened Mirror Polish Mold Steel)
- Properties:
- Pre-hardened
- High polishability
- Good machinability and dimensional stability
Common Uses: Transparent plastic products, such as optical lenses and light covers.
How to Choose the Right Mold Steel
When selecting mold steel for your plastic injection molding project, consider these factors:
1. Type of Plastic Resin Used
Some plastics, especially those with glass fibers or flame retardants, are abrasive or corrosive. Choose steel accordingly:
- Use corrosion-resistant steel (like 420 or S136) for PVC or flame-retardant materials.
- Use wear-resistant steel (like H13) for glass-filled resins.
2. Production Volume
- Low-volume (<100,000 cycles): P20 or aluminum molds may suffice.
- Medium-volume (100,000–500,000 cycles): P20, NAK80.
- High-volume (>500,000 cycles): H13, 420SS, or S136.
3. Cooling Efficiency
Steels with better thermal conductivity reduce cooling time, which speeds up cycle time and increases productivity.
4. Surface Finish Requirements
Transparent or high-gloss parts (e.g., lenses, cosmetic components) require steels that can achieve a mirror polish, such as NAK80, 420SS, or S136.
5. Budget Constraints
- P20 is cost-effective for general use.
- High-alloy stainless steels like S136 are more expensive but offer extended mold life and better performance.
Heat Treatment and Surface Treatment
Post-machining treatments play a crucial role in mold steel performance.
Heat Treatment
- Hardens the mold surface
- Improves wear resistance
- Must be done properly to avoid warping or cracking
Surface Coatings
- Nitriding: Enhances surface hardness and reduces wear
- Chrome plating: Improves corrosion resistance
- PVD coatings: Add anti-stick properties and further hardness
Common Defects Related to Poor Mold Steel Selection
- Flash or Burrs – Due to poor dimensional stability
- Warping or Cracks – Result of uneven heat treatment
- Short Mold Life – Due to using non-hardened or low-grade steel
- Rust or Corrosion – If low corrosion resistance steel is used with corrosive plastics
- Poor Surface Finish – Caused by incompatible or low-polish steel
Best Practices for Working with Mold Steel
- Always use certified and traceable steel from reliable suppliers
- Use Computer-Aided Engineering (CAE) simulations to predict wear and thermal cycles
- Maintain molds regularly to prevent buildup and surface degradation
- Store molds in controlled environments to prevent rust and oxidation
Mold Steel Suppliers in India
Steel Mart is one of India’s trusted suppliers for premium mold steels such as P20, H13, NAK80, and more. With over 50 years of experience in specialty steel, we serve tool rooms, automotive part makers, medical equipment manufacturers, and consumer goods industries across the country.
We stock a wide range of mold steels in various sizes and grades and offer cutting, machining, and testing services.
Frequently Asked Questions (FAQs)
1. What is the best steel for plastic injection molds?
It depends on your application. P20 is good for general use, while H13 is ideal for high-volume or high-temperature resins. For corrosion resistance, choose 420SS or S136.
2. How long does a mold last?
Mold life varies based on steel, resin type, and maintenance. With proper material and care, molds can last from 100,000 to over 1 million cycles.
3. Can I use stainless steel for molds?
Yes, stainless steels like 420 and S136 are often used when corrosion resistance and high polishability are required.
4. Is P20 steel pre-hardened?
Yes, P20 steel comes pre-hardened and does not require additional heat treatment after machining.
5. Can mold steels be recycled?
Yes, most mold steels are recyclable and can be repurposed or remelted for further processing.
Conclusion
Mold steel is the foundation of reliable, efficient, and high-quality plastic injection molding. Choosing the right steel grade ensures precision, productivity, and long-term value in your manufacturing operations.
Whether you’re producing 10,000 parts or 10 million, investing in the correct mold steel—combined with proper maintenance and supplier support—will give your operation a competitive edge.
If you're looking for expert guidance or reliable supply of mold steel in India, Steel Mart is ready to assist you. Get in touch with us today to find the perfect mold steel solution for your project.
