The selection of mold steel, particularly for the core and cavity, is largely determined by the desired surface finish of the final injection molded product. The higher the required surface gloss, the greater the demands on the steel’s purity, homogeneity, hardness, and polishability.
I. Classification of Surface Requirements for Injection Molded Products
Typically, the surface requirements for injection molded products can be categorized into the following main types, listed from low to high demand:
Low-Requirement Surfaces (No Cosmetic Demand)
Description: Internal structural components, hidden parts, with no cosmetic requirements.
Surface Treatment: None required, or simple blasting (e.g., coarse grit like VDI 3400) to mask machining marks.
Typical Applications: Gears, bearings, internal brackets.
Medium-Requirement Surfaces (Cosmetic Parts with Texture)
Description: Visible housing parts not requiring a mirror-like gloss, often featuring a fine texture.
Surface Treatment:
EDM Texture: Creates a matte, grainy finish.
Fine Blasting: Produces a uniform matte surface.
Low-Level Chemical Etching: e.g., fine leather grain patterns.
Typical Applications: Power tool housings, non-primary surfaces of home appliances, automotive interior structural components.
High-Requirement Surfaces (High-Gloss / Fine-Textured Cosmetic Parts)
Description: Parts with extremely high cosmetic demands, requiring a mirror finish or very fine, complex textures.
Surface Treatment:
Polishing to Mirror Finish: e.g., mobile phone casings, transparent lenses.
Precision Chemical Etching: e.g., premium leather grains, wood grains, geometric patterns.
Typical Applications: High-end consumer electronics (phones, laptops), automotive high-gloss grilles/trims, cosmetic packaging.
Surfaces with Special Requirements
Description: Parts requiring specific performance characteristics that influence surface needs.
Typical Scenarios:
Transparent Parts: e.g., optical lenses, light guides, requiring absolute freedom from defects and streaks.
Corrosive Plastics: e.g., PVC, flame-retardant materials (containing halogens/phosphorus), which release corrosive gases.
High-Wear Plastics: e.g., engineering plastics filled with glass fibers (GF).
II. Mold Steel Material Selection
Based on the surface requirements above, corresponding mold steel materials can be selected. Key properties for mold steel include:
Polishability: Ability to be polished to a mirror-grade finish.
Corrosion Resistance: Ability to resist attack from plastic melt and corrosive gases.
Wear Resistance: Ability to resist abrasion from plastics and fillers (e.g., glass fibers).
Toughness: Resistance to cracking.
Hardness: Generally, higher hardness improves wear resistance and polishability.
Selection Guide Reference Table
Product Surface Requirement Recommended Steel Type Specific Grade Examples Rationale for Selection
Low-Requirement Surfaces Pre-Hardened Plastic Mold Steels China: P20 (3Cr2Mo)
Sweden: 718 (ASSAB)
Germany: 1.2738 (GS) Supplied pre-hardened to ~HRC 30-40, can be machined directly without heat treatment. Cost-effective, suitable for general service life and surface needs.
Medium-Requirement Surfaces
(EDM Texture, Fine Etching) Advanced Pre-Hardened Steels /
Corrosion-Resistant Pre-Hardened Steels Sweden: 718H (ASSAB)
Japan: NAK80 (Daido)
Germany: 1.2738HH (GS) Higher hardness (HRC 37-42), more uniform structure. NAK80 is renowned for its excellent etchability and mirror machinability, making it very suitable for texturing.
High-Requirement Surfaces
(High-Gloss Mirror, Precision Etching) High-End Mirror Polish Steels
(Often Martensitic Stainless Steels) Sweden: S136H, S136 (*)
Japan: HPM38 (Hitachi)
Germany: M238 (Groditz)
Premium Choice: ELMAX (ASSAB) High purity and excellent corrosion resistance ensure capability for
mirror-grade polishing (#10000+), prevent rusting during prolonged production, and maintain surface finish.
Transparent / Optical Parts Super Mirror, Ultra-Pure Steels Sweden: S136 (High Purity)
Japan: HPM38, PAC5000 Extreme purity, free from sulfur and other impurities, prevents pitting and orange peel effect during polishing, ensuring optical clarity.
Corrosive Plastics
(e.g., PVC, Flame-Retardant) Corrosion-Resistant Mirror Polish Steels Sweden: S136, S336
Germany: M300, M310 (Groditz) High Chromium content (>13%) forms an effective passive layer, resisting attack from corrosive gases, preventing mold rusting and corrosive failure.
High-Wear Plastics
(e.g., with Glass Fibers) High Hardness / High Wear-Resistance Steels Sweden: 8407 (ASSAB)
Germany: 1.2344 (GS)
Superior Choice: Powder Metallurgy Steels like VIKING (ASSAB) Achieve high hardness (HRC 48-52) after heat treatment, providing high wear resistance and extending mold life. Powder steels offer a better combination of wear resistance and toughness.
Note: S136 is available in pre-hardened (S136H, HRC 30-33) and through-hardening (S136, HRC 48-52)