Selecting the right steel for your injection mold is one of the most critical decisions affecting tool life, part quality, and production cost. The wrong choice can lead to premature wear, corrosion, heat checking, or even catastrophic mold failure.
This guide covers the most common mold steel grades, their properties, and how to select the right material for your specific application.
P20 is the most widely used mold steel for general-purpose injection molds. It comes pre-hardened to 28-32 HRC, eliminating the need for heat treatment after machining. P20 offers good machinability, reasonable wear resistance, and moderate polishability. Best for: prototype molds, low-to-medium volume production (up to 500,000 cycles), and molds with simple geometries.
H13 is the standard for high-performance injection molds and die casting dies. It offers excellent toughness, high heat resistance, and good thermal fatigue resistance. Through-hardened to 46-52 HRC, H13 maintains its hardness at elevated temperatures. Best for: high-volume production (500,000+ cycles), molds running abrasive materials, and die casting applications.
D2 offers exceptional wear resistance due to its high carbide content. Through-hardened to 58-62 HRC, it is one of the hardest conventional mold steels. However, D2 has lower toughness than H13 and can be challenging to machine and polish. Best for: highly abrasive materials (glass-filled nylon), stamping dies, and applications where wear is the primary concern.
S7 is designed for applications requiring high impact toughness. It offers excellent shock resistance combined with good wear properties. Through-hardened to 54-58 HRC. Best for: molds with thin inserts, sharp corners, high-stress applications, and cold heading tools.
420 stainless steel offers excellent corrosion resistance and the best polishability of any mold steel. It can be polished to a mirror finish (SPI A-1 grade). Through-hardened to 48-54 HRC. Best for: medical molds, optical lenses, clear plastic parts, and corrosive material processing.
| Volume | Cycles | Recommended Steel | Typical Hardness |
|---|---|---|---|
| Low | < 100,000 | P20, 4140 pre-hard | 28-32 HRC |
| Medium | 100,000 - 500,000 | H13, 420SS | 46-52 HRC |
| High | 500,000 - 1,000,000 | H13, D2 | 50-58 HRC |
| Ultra-high | 1,000,000+ | H13 (ESR), D2, powder metal | 56-62 HRC |
Surface treatments can significantly extend mold life beyond the base steel properties: