Hardness Selection of Injection Mold Steels and Practical Application Scenarios
In injection mold design and manufacturing, steel hardness directly determines wear resistance, corrosion resistance, polishability, and service life, affecting product quality, production stability, and cost. Parts with different structures, materials, and volumes have distinct hardness requirements: excessively high hardness causes brittleness and difficult machining; insufficient hardness leads to rapid wear, deformation, and cavity scuffing. Reasonable hardness selection, matched to application scenarios, ensures stable mold performance and cost control—essential knowledge for mold engineers.
Basic Classification and Core Characteristics of Mold Steel Hardness
Mold steel hardness is measured by Rockwell Hardness (HRC), divided into three practical ranges:
Low hardness (HRC 28–35): High toughness, easy machining, and weld repair, suitable for complex, frequently modified molds. Weak wear/compression resistance limits long-term use.
Medium hardness (HRC 35–45): Balances toughness and wear resistance, moderate machining difficulty, and stable heat treatment. The most widely used range for general mass production.
High hardness (HRC 45–55+): Excellent wear resistance, compressive strength, and polishability (mirror finish). Ideal for high-precision, high-volume, corrosion-resistant molds but brittle and costly to machine.
The key principle: adapt to the scenario, not pursue maximum hardness. Transparent/appearance parts prioritize polishability; glass-fiber-reinforced plastics need wear resistance; small-batch/complex molds prioritize machinability. Blindly high hardness increases costs without extending service life.
Hardness Standards and Applications of Common Mold Steels
Pre-hardened Mold Steels (No Further Quenching)
Pre-hardened steels are preferred for small-to-medium batch molds, delivered with heat treatment. Common grades:
P20 (HRC 28–32): Excellent machinability and cost-effectiveness, suitable for simple-structure, medium-volume general parts (home appliance brackets, ordinary housings), service life 300,000–500,000 cycles.
718H (HRC 33–38): Higher purity and polishability than P20 (matte to fine texture), used for home appliance appearance parts and non-high-gloss automotive interiors, stable production up to 800,000 cycles.
NAK80 (HRC 38–42): Good polishability without quenching, suitable for small-to-medium precision molds, non-mirror transparent parts, and high-volume appearance components, shortening manufacturing cycles.
Quenched and Tempered Mold Steels (Heat Treatment Required)
These reach HRC 45–52 after heat treatment, for high-volume, high-requirement molds:
SKD61/DAC (HRC 48–52): SKD61 balances heat resistance, wear resistance, and toughness; DAC has better toughness to avoid chipping. Used for glass-fiber-reinforced plastics (PA, PBT+GF), automotive connectors, and precision structural parts, service life over 1,000,000 cycles.
S136/STAVAX (HRC 50–55, corrosion-resistant mirror steel): Excellent corrosion resistance and polishability (mirror finish), suitable for corrosive plastics (PVC, POM), transparent PC/PMMA parts, medical supplies, and cosmetics, ensuring high cleanliness and appearance consistency.
Hardness Selection by Injection Product Type
1. General Plastic Parts (PP, PE, ABS)
Features: Simple structure, non-corrosive, medium volume (300,000–500,000 cycles), low appearance requirements.
Hardness: HRC 28–38, preferred P20/718H.
Applications: Home appliance internal parts, daily necessities, packaging, balancing cost and machinability.
2. Appearance/Semi-High-Gloss Parts
Features: No scratches/marks, moderate gloss, high volume (500,000–1,000,000 cycles) (air conditioner panels, automotive interiors).
Hardness: HRC 35–42, preferred 718H/NAK80.
Key: Balance wear resistance and polishability to ensure stable mass production.
3. Glass-Fiber/Mineral-Filled Engineering Plastics (PA66+GF, PBT+GF, LCP)
Features: Poor flowability, high wear, high precision, long service life (over 1,000,000 cycles) (automotive connectors, electronic components).
Hardness: HRC 45–52, preferred SKD61/DAC.
Key: High hardness resists fiber erosion, ensuring dimensional stability.
4. Transparent/High-Gloss/Medical/Food-Grade Parts
Features: Mirror finish, no defects, corrosion resistance (transparent masks, medical supplies).
Hardness: HRC 50–55, preferred S136.
Key: High hardness + purity, meeting hygiene and appearance standards.
5. Small-Batch/Prototype Molds
Features: Complex structure, frequent modifications, low volume (<100,000 cycles), rapid prototyping.
Hardness: HRC 25–30, soft machinable steel.
Key: Reduce processing/modification costs, switch to high-hardness steel after product finalization.
Common Misconceptions and On-Site Notes
Misconceptions
Higher hardness = better performance: Increases machining difficulty/cost and risks chipping.
Uniform hardness: Cavity/core use high-hardness steel; mold base/ ejector plates use low-hardness steel to balance performance and cost.
Ignoring heat treatment: Quenched steels require precise heat treatment to avoid uneven hardness/deformation.
Ignoring material compatibility: Corrosive plastics (PVC) need corrosion-resistant steel, not ordinary high-hardness steel.
On-Site Notes
Adjust hardness based on trial runs; use nitriding to enhance wear resistance if cavity wear occurs.
Use wear-resistant tools for high-hardness steel, reduce machining speed to avoid tool damage.
Maintain molds: Avoid violent opening/closing; lubricate/clean cavities regularly.
Control cost: Use pre-hardened steel for small batches; quenched steel for high-volume/high-requirement products.
Conclusion
Hardness selection depends on product material, volume, appearance, and cost. Low-medium hardness pre-hardened steel for general plastics/small batches; medium-high hardness quenched steel for glass-fiber-reinforced/high-volume products; high-hardness mirror steel for transparent/corrosion-resistant scenarios.
Reasonable selection reduces mold failures, extends service life, and ensures stable product quality. Avoid misconceptions, adjust based on on-site trials, and maximize steel performance for reliable injection production.
