Proper mold steel selection directly decides production stability, product surface quality and service life of plastic molds for automotive, medical and daily plastic parts. Different molding materials and production batches impose diverse requirements on steel hardness, corrosion resistance, polishing performance and wear resistance. Supported by abundant design and processing experience from China plastic mold industry, this document sorts mainstream mold steel grades, compares core performance indicators and summarizes targeted selection rules for different production scenarios to effectively avoid premature mold abrasion, rust and poor surface finish caused by improper steel selection.
1. Core Performance Indicators for Mold Steel SelectionFive key parameters dominate steel selection criteria: hardness, corrosion resistance, polishing property, machinability and wear resistance. Hardness determines anti-deformation capability under long-term high lock-mold pressure; corrosion resistance matters for molding PVC, PA+GF and medical PC which release corrosive small molecules during melting; polishing performance directly influences A-surface finish of transparent and appearance plastic parts; good machinability shortens mold processing cycle and reduces manufacturing cost; wear resistance is essential for high-volume production of glass-fiber reinforced plastic products with strong abrasive effect. Different application priorities adjust indicator weighting to match actual production demand.
2. Detailed Parameter Comparison of Common Pre-hardened Mold Steel
718HH, P20HH and S136H are the most widely used pre-hardened steel in domestic plastic mold manufacturing. 718HH reaches hardness HB280~320 with outstanding polishing performance and moderate cost, suited for medium-batch household appliance and automotive interior non-transparent parts; P20HH owns HB260~300 hardness, superior cutting performance and low material cost, mainly applied for low-volume trial-production molds and internal non-appearance structural molds; S136H is stainless pre-hardened steel with HB300~330, excellent acid-proof and rust-resistant property, ideal for medical-grade plastic, transparent PC and corrosive material molding despite higher procurement price. These three types of steel differ obviously in anti-corrosion and wear indexes, so appearance requirement and molding raw material become core reference for grade picking.
3. Quenched and Tempered High-hardness Mold Steel Application RangeSKD61, STAVAX and NAK80 belong to post-quenching high-performance mold steel. After heat treatment, SKD61 reaches HRC50~54 with strong wear resistance and high-temperature stability, largely used for large-size plastic mold and high-output fiber-filled plastic molds; STAVAX is premium stainless steel achieving HRC52~56 after quenching, top-level mirror polishing and full acid resistance, exclusive for high-end optical transparent parts and medical precision component molds; NAK80 features pre-hardened HRC38~42 without follow-up heat treatment, ultra-fine grain and high gloss surface, popular for high-standard cosmetic plastic shell molds. Higher steel grade brings higher material and processing expense, balanced by prolonged mold service cycle and reduced later maintenance cost for mass production.
4. Selection Rules Based on Product Batch and Material Attribute
Small-batch trial production below 10,000 shots prefer economical P20HH to control mold development cost; medium batch ranging from 10,000 to 300,000 shots selects 718HH for ordinary appearance parts and S136H for transparent or corrosive plastic molding; mass production over 300,000 shots with glass-fiber addition adopts quenched SKD61 for strong anti-abrasion. Medical and food-contact plastic products prioritize stainless S136 and STAVAX to prevent rust-derived product contamination. Large-size integrated mold chooses steel with stable internal organization to lower heat-treatment deformation risk.
5. Later Processing Matching after Steel ConfirmationAfter steel grade confirmation, surface treatments including nitriding, plating and polishing are arranged according to working condition. Nitride treatment improves surface hardness and wear performance of common pre-hardened steel for positions prone to abrasion such as insert and rib cavity; stainless steel usually skips nitriding but demands high-standard mirror polishing for appearance cavity. Reasonable post-processing maximizes inherent material advantages of selected mold steel and further extends whole mold service lifespan under continuous industrial production.
