Selection Reference Standard of Mold Steel for Modified Plastics

Modified plastics with glass fiber reinforcement, flame retardant treatment and mineral filling have far higher wear, corrosion and high-temperature adhesion risks than ordinary plastics in injection molding process. The performance of mold steel directly determines product dimensional accuracy, mold service life and mass production stability. Improper material selection will easily cause cavity wear, surface corrosion, product flash and continuous dimensional deviation in long-term mass production. Therefore, standardized and classified mold steel selection is essential for stable production of modified plastic products.

1. Core Selection Performance Based on Modified Plastic Categories

Glass fiber reinforced modified plastics belong to high-wear molding materials. Hard glass fibers continuously scour the cavity and flow channel during melt flowing, which is the main cause of mold pulling and size out-of-tolerance. The selection must focus on high hardness and high wear resistance, preferring steel with heat treatment hardness above HRC50 to avoid gradual cavity loss in mass production. Flame-retardant modified plastics will precipitate acidic corrosive gas at high temperature, causing mold pitting and oxidation blackening. Corrosion resistance is the primary selection index, and ordinary tempered steel and low-grade pre-hardened steel are strictly prohibited. Mineral-filled modified plastics contain talc powder and calcium carbonate, which will aggravate mechanical wear and form local stress concentration. The steel must balance wear resistance and impact toughness to prevent mold corner chipping and cavity collapse. Toughened modified plastics have mild molding conditions with no obvious wear and corrosion, so conventional high-quality pre-hardened steel is sufficient.

2. Steel Grading Matching Based on Production Batch

For small-batch trial production within 100,000 shots, P20 and 718 pre-hardened steel are the best choices. They require no quenching treatment, feature fast processing cycle and stable polishing performance, and can fully meet the requirements of sample making and trial production with low cost. For medium-batch mass production of 100,000 to 500,000 shots, NAK80 and pre-hardened S136 steel are preferred. With uniform hardness, small deformation and better anti-corrosion performance than ordinary steel, they adapt to continuous production of modified plastics with glass fiber content within 30% and conventional flame retardant formula. For large-batch mass production over 500,000 shots, high-grade heat-treated steel such as H13, quenched S136 and STAVAX is required. After complete quenching and tempering, the steel has compact structure, excellent thermal fatigue resistance, wear resistance and corrosion resistance, which can adapt to long-term production of high-damage modified plastics and greatly extend mold life.

3. Accurate Selection According to Mold Structure and Product Precision

Conventional simple structured molds have flat cavities and uniform stress, so general pre-hardened steel can meet basic molding and wear resistance requirements and control mold cost effectively. Complex precision molds with thin walls, deep cavities, fine ribs and small holes bear uneven injection pressure and large thermal load. High-toughness and thermally stable H13 and FDAC steel are selected to resist deformation, cracking and mold sticking, solving molding defects of complex structural parts. High-gloss transparent appearance molds have extremely strict requirements on surface finish. High-purity S136 and STAVAX mirror stainless steel with excellent polishing and corrosion resistance are adopted to avoid surface pitting and corrosion lines and stabilize high-standard appearance quality.

4. Performance and Cost Balance Selection Principle

Blind high-grade matching should be avoided in material selection. For low-wear and low-corrosion ordinary modified plastics, high-priced stainless steel is unnecessary, and economical pre-hardened steel can effectively reduce mold manufacturing cost. Low-cost low-grade matching is also forbidden. For high-glass-fiber and strong-corrosion modified materials, special wear-resistant and corrosion-resistant steel must be used to avoid frequent mold damage, shutdown and maintenance, which will cause higher comprehensive losses. Meanwhile, steel selection should match subsequent nitriding, polishing and heat treatment processes to maximize material performance.

Summary

The core of modified plastic mold steel selection is classified matching according to material characteristics, production volume and mold precision. Scientific selection can effectively avoid mold wear, corrosion and deformation problems, stabilize product molding accuracy and production stability, and reduce long-term operation and maintenance costs of injection molding production.