Plastic Mold Steel Selection in China: Common Grades and Application Scenarios
Mold steel selection is the core link of plastic injection mold design and manufacturing in China’s mold industry. Reasonable steel grade matching directly determines mold processing performance, surface finish quality, wear resistance, service life and mass production stability. Different plastic materials, product appearance requirements, dimensional tolerance standards and production batch scales correspond to different mold steel grades. Improper selection will easily cause mold wear, rust, poor polishing effect, product surface defects and frequent mold maintenance, which seriously affects production efficiency and product yield. Combined with the mainstream application habits and industry standards of China’s plastic mold industry, this article systematically sorts out common mold steel grades, core characteristics and applicable scenarios to provide standardized selection reference for mold design and mass production.
1. Pre‑hardened Mold Steel for Medium and Small Batch Production
Pre‑hardened steel is the most widely used mold material in China’s civil plastic mold industry. It is delivered with finished hardness treatment and can be directly processed by CNC, drilling and polishing without secondary quenching and tempering. It features short processing cycle, stable performance and moderate cost, fully meeting the production needs of most medium and small batch plastic products.
P20 steel is a basic general‑purpose pre‑hardened plastic mold steel with a hardness of 28 to 32 HRC. It has excellent cutting performance and stable processing performance, suitable for ordinary ABS, PP, PS and other non‑reinforced general plastics. It is mostly used in daily necessities, common shell parts and low‑requirement structural plastic molds, with a production life of about 300,000 shots. It is not suitable for glass fiber reinforced materials, corrosive plastics and high‑gloss mirror products.
718H is an upgraded version of P20 steel with higher material purity and uniform internal structure, with a hardness range of 33 to 38 HRC. It has better polishing performance and texture etching stability, which can meet the appearance requirements of medium and high‑grade plastic parts. It is widely used in home appliance shells, consumer electronic accessories and ordinary transparent plastic parts, and is the mainstream steel grade for medium batch injection molds in China’s mold factories.
NAK80 is a high‑grade pre‑hardened mirror mold steel with a stable hardness of about 40 HRC. It has ultra‑high polishing performance and fine texture restoration effect, and can achieve high mirror finish without heat treatment. It is mainly used for high‑appearance products such as cosmetic packaging, optical accessories and high‑gloss electronic shells. It has stable dimensional accuracy and low deformation rate, and is the preferred material for high‑precision appearance molds with tight delivery cycles.
2. Quenched and Tempered Hardened Steel for High‑wear Mass Production
For large‑batch and long‑life injection molds, especially glass fiber reinforced and mineral filled engineering plastics, pre‑hardened steel is difficult to resist long‑term material erosion and wear. High‑hardness quenched mold steel must be adopted to improve mold wear resistance and thermal fatigue resistance.
H13 and SKD61 are mainstream hot‑work mold steels in China’s plastic industry. After quenching and tempering, the hardness reaches 48 to 52 HRC, with excellent thermal stability, thermal fatigue resistance and mechanical impact resistance. They are widely used in PA66+GF, PBT+GF and other high‑wear engineering plastic products, suitable for new energy parts, automotive structural parts and other million‑shot mass production molds. They can withstand long‑term high‑temperature injection and repeated cold and hot alternation, effectively avoiding mold cavity cracking and surface wear.
3. Corrosion‑resistant Stainless Mold Steel for Special Plastics
When molding corrosive plastics such as PVC, POM and flame‑retardant modified materials, acidic corrosive gas will be decomposed at high temperature, causing ordinary mold steel to rust and produce surface pits. Corrosion‑resistant stainless steel represented by S136 is required for production.
S136 stainless steel has high chromium content, with excellent anti‑corrosion and anti‑rust performance. After heat treatment, it can achieve ultra‑high mirror polishing effect, which is suitable for optical transparent parts, medical plastic parts and food‑contact plastic products. It can keep the mold cavity smooth and rust‑free during intermittent production and long‑term storage, and greatly extend mold service life. The pre‑hardened version S136H cancels quenching treatment, which is suitable for high‑appearance and anti‑corrosion molds with tight construction periods.
4. Scientific Selection Principles for Domestic Plastic Molds
Combined with the actual production experience of China’s mold industry, steel selection should follow the principle of matching demand rather than blindly pursuing high‑grade materials. For ordinary non‑glass fiber and non‑corrosive medium and small batch products, P20 and 718H are cost‑effective choices. For high‑gloss and fine texture appearance parts, NAK80 is preferred. For high‑wear glass fiber modified materials and million‑shot mass production, H13 and SKD61 are matched. For corrosive and high‑cleanliness products, S136 series stainless steel must be selected.
Conclusion
The domestic plastic mold steel system has formed a complete graded matching system covering general, appearance, wear‑resistant and anti‑corrosion scenarios. Standardized steel selection can effectively balance mold manufacturing cost, processing cycle, product appearance quality and mass production stability. Reasonable matching of steel grades according to plastic material characteristics, surface requirements and production batch is the key to improve mold yield and reduce later maintenance costs in long‑term injection production.