Core Selection Criteria for Large Injection Mold Steel Strength

Selecting the appropriate steel strength for large injection molds (typically defined as having a mold insert size exceeding 1000mm or a product weight over 5kg) is critical for ensuring dimensional stability, longevity, and production reliability. Unlike small molds, large molds demand a balanced approach to material selection that goes beyond sheer hardness, focusing instead on mechanical properties, resin characteristics, and operational conditions.

1. Mold Stress Analysis: The Primary Benchmark

The forces exerted during injection—clamping force, cavity pressure, and ejection impact—are the fundamental drivers of steel strength requirements. Industry standards indicate that cavity pressures typically range from 80 to 150 MPa.

Standard Large Parts (e.g., appliance housings): Operating at 80-100 MPa, these require steel with a tensile strength of 750-900 MPa and a yield strength of 600-750 MPa to resist plastic deformation.

High-Fill/Thick-Walled Parts (e.g., automotive bumpers, battery cases): With pressures reaching 120-150 MPa and significant abrasive wear from glass fibers, steel must possess a tensile strength ≥ 1000 MPa, yield strength ≥ 900 MPa, and shear strength ≥ 650 MPa to prevent edge failure.

Extra-Large Molds (>1500mm): Bending strength is paramount to control deflection (industry standard ≤ 0.05mm/m). A yield strength ≥ 850 MPa is recommended to maintain flatness under load.

Ejection Zones: Localized impact pressures of 50-80 MPa during ejection necessitate a compressive strength ≥ 800 MPa to prevent pin-hole deformation or crushing.

2. Molding Material Characteristics

The chemical and physical properties of the plastic resin directly dictate the required steel grade:

Unfilled Commodity Plastics (PE, PP, ABS): Low abrasion and pressure (80-100 MPa) allow for cost-effective pre-hardened steels with 750-850 MPa tensile strength.

Filled Engineering Plastics (PA66, PC/ABS with ≥20% GF): High viscosity and abrasive glass fibers demand quenched and tempered steels (1100-1300 MPa tensile strength) with a hardness of HRC 48-52 to withstand wear and high pressure.

Corrosive Plastics (PVC, PMMA): These resins release acidic byproducts. While requiring moderate strength (850-950 MPa tensile), the steel must prioritize corrosion resistance to prevent premature strength degradation.

3. Production Volume & Lifespan

The target number of cycles is a quantitative metric for strength selection:

Low Volume (<100,000 cycles): Pre-hardened steels like P20 (750-850 MPa) are sufficient, offering fast machining and low cost.

Medium Volume (100,000-500,000 cycles): Steels like 718H (850-950 MPa tensile) are preferred for their excellent strength uniformity (deviation ≤ 5%), critical for large, assembled mold inserts.

High Volume (>500,000 cycles): High fatigue strength is essential. H13/SKD61 (1100-1300 MPa tensile, fatigue strength ≥ 400 MPa) is the industry standard for withstanding cyclic loading without cracking.

4. Strength-Toughness Balance

Complex large molds (deep cavities, multiple slides) risk cracking if toughness is sacrificed for high strength.

Shear Resistance: Integral mold inserts require a shear strength ≥ 600 MPa.

Impact Toughness: For intricate geometries, a toughness value ≥ 20 J/cm² (e.g., H13) is mandatory. Avoiding ultra-high strength but brittle steels (toughness < 10 J/cm²) prevents chipping and catastrophic failure during machining or operation.

Summary

Match Requirements: Align steel strength with cavity pressure, resin abrasiveness, and production volume.

Grade Selection: Use P20/718H (750-950 MPa) for standard applications and H13/SKD61 (1100-1300 MPa) for high-pressure or high-volume scenarios.

Prioritize Toughness: Always balance high strength with sufficient toughness to ensure the reliability of large, complex mold structures.