Selecting best tool steel for injection molding tool cavity is a critical and very important task. It makes a huge difference on tool cost, tool life and quality of part in long run.
Following points need to be considered for tool steel selection.
- Injection molded part Surface finish.
- Tool Life
- Raw Material properties.
- Expected cycle time.
- Thin steel areas venerable to cracking.
Tools steels can be broadly classified in two categories :
- Heat Treating Tool Steel
- Pre-Hardened tool steel
1. Heat Treating or Hardening Tool Steels
H-13 Tool Steel
H13 is a through hardening tool steel. It has excellent hot tensile properties, wear resistance, adequate toughness and resists tempering at high operating temperatures. After hardening, H-13 tool steel typical hardness range is approx. 44 to 48 RC. Welding is also with possible on H-13 tool steel.
H-13 tool steel is a good choice for high volume molds. For example, H-13 insert can give 1 million cycles with a 33% glass filled nylon without any wear issue.
It becomes very good choice where tool steel need to address wear and while working with abrasive materials.
- H-13 tool steel can be nitrided. It is required to avoid welding of H-13 tool steel components during relative motion between them.
- Hard lating is possible in H-13 tool steel.
H13 tool steeel is more expensive than P20.
Stavax Tool Steel
Stavax is a through hardened stainless steel with good corrosion resistance, good polishability and good wear resistance. Photoetching and welding is possible on stavax tool steel.
Recommended stavax through hardness range is 45 – 54 HRC
Following are the advantages of good corrosion resistance in Stavax Tool Steel for plastics
- Lower mould maintenance costs.
- The surface of the cavities maintain their original finish over extended running periods.
- Molds stored or operated in humid conditions require no special protection.
- Water cooling channels are unaffected by corrosion. As a result, heat transfer characteristics and cooling efficiency are constant throughout the mould life.
Stavax Tool steel is an excellent choice for cores, cavities and stripper rings for moulds with high production rate. It is also suitable for molding corrosive materials such as PVC and abrasive filled materials.
Because of good polishability, it is suitable to mould optical parts such as clear glasses.
Stavax tool steel is more expensive than H13 steel.
Ramax Heat Treating Tool Steel
Ramax heat treating tool steel is a through hardened stainless steel with good corrosion resistance. Ramax is a holding steel and is supplied with a uniform hardness of 38HRC that is more than P20.
Welding is also possible on ramax tool steel with required expertise.
This tool steel has application only for parts requiring low to medium polished parts.
S7 Tool Steel
S7 is very durable, impact-resistant tool steel that can be hardened up to 56 RC. It is an air or oil hardening shock resisting (cold work) tool steel offering a unique combination of toughness and wear resistance.
S7 tool steel is primarily used in applications that require resistance to shock and impact such as slides and lifters. This material is also used for
very tight tolerance and high-volume part requirements.
Stainless steel is a Post hardened tool steel with hardness range of 50-52 Rc
Stainless steel is used for low production volume products.
2. Pre-hardened Steel For Injection Molding
P 20 Tool Steel
P-20 is a pre-hardened mold steel with good polishability and photo-etching properties. It is available in hardness range of 28-32 Rc.
P20 mould steel is hard chromium plated for mould reconditioning purposes. It can also be electroless nickel plated for extra corrosion resistance.
It can be welded which is helpful for repairs. It can be flame hardened or nitrided for extra resistance to wear and erosion. A nitride surface also increases the corrosion resistance.
P-20 pre-hardened tool steel is used for middle volume molds with materials without abrasive additives and part finish up to SPI B-2 .
This material is mainly used for the mold base but can also be used for core, cavity, gate inserts, sprues and sliding inserts in moulds with low and medium production.
NAK 80 Steel
NAK-80 is a Pre-hardened tool steel, available in uniform through hardness of approximately 40 HRc.
It is super clean, Vacuum-Arc Remelted steel with uniform grain structure, uniform hardness, and excellent dimensional stability. When welded, NAK 80 leaves no witness lines after re-aging.
NAK 80 machines up to 20% faster than 30 HRC P20 mold steels and never needs stress relieving, even after heavy machining.
NAK-80 tool steel has application in middle volume molds. This can be even polished over welded areas. NAK 80 steel can be used for critical diamond finish applications. For example, Clear lens molds.
NAK 55 is a pre-hardened mold steel with uniform microstructure. It is available in hardness range of 38-42 HRC.
NAK 55 Machines upto 30 to 40 % faster than P20 mold steels.
It had good weldability and EDM properties.