Heat Treating Tool Steel - How To Choose The Right Treatment For Injection Molds

Heat treating tool steel is a very complicated task. Choosing the right type of heat treatment in plastic injection mold making can also be very difficult if you don’t have the knowledge.

The bottom line is that the heat treatment selected for an injection mold must keep the mold in good working condition for the life of the mould.

So how to choose the right treatment for your mould? I am glad you asked.

The type of heat treatment depends upon the following factors:

• Choice of tool steel. The choice of steel largely dictates the type of heat treatment.
• Mould price. This should take into account the type of heat treatment. If the price is too low then there is probably not going to be any heat treatment at all.
• Part design. If part has deep undercuts or threads then mould design will need some moving components in order to release the part from the mould. Moving components require special attention so that the surfaces don’t wear out quickly or get damaged.
• Mold design. This is heavily influenced by the part design.
• Annual production quantities. Small quantities can use softer, less expensive materials but high volume moulds require long lasting materials and heat treatments.
• Environmental factors. Does the molding take place in a corrosive environment?
• Mould maintenance issues such as corrosion.

Types Of Heat Treatments For Tool Steels

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1. Surface treatment – the surface of the work piece becomes harder than the inside. Treatments include case hardening, nitriding, flame hardening, hard chromium plating, nickel plating, titanium nitride and titanium carbide.
2. Through hardening treatment – gives uniform hardness throughout the entire work piece.

Below are 4 of the most common steels used to build injection molds and their recommended treatments:

P20 Heat Treating Tool Steel

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P20 – (DIN No. 1.2312) - supplied in the thru hardened and tempered condition at a hardness of 310 HB (34HRC). It has good polishability, photo-etching properties (for surface texturing).

This steel is mainly used for the mold bolster (it is a holding steel) but can be used for core, cavity, gate inserts, sprues and sliding inserts in moulds with shot production quantities less than 500,000 per year such as in automotive and home ware products.

P20 can be hard chromium plated which is useful for mould reconditioning purposes.

P20 can also be electroless nickel plated for extra corrosion resistance. Because electroless nickel plating takes place in a bath, the plating will also cover internal water cooling channels, which makes mould maintenance easier. The plating will add 0.005mm per side or 0.01mm to plate thickness.

Sliding inserts made from P20 should be nitrided for wear resistance and guard against possible damage when using with a P20 bolster.

It can be welded which is good for repairs. It can be flame hardened or nitrided for extra resistance to wear and erosion. A nitride surface also increases the corrosion resistance.

H13 Heat Treating Tool Steel

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H13 – (DIN No. 1.2344) – is a through hardening tool steel which has excellent hot tensile properties, high hot wear resistance , adequate toughness and resists tempering at high operating temperatures.

These properties makes this steel an excellent choice for cores, cavities, stripper rings, sliding parts or rotating cores in moulds designed to produce millions of parts per year at fast cycle times. Thin wall molding is an example of this type of application (containers and cutlery).

H13 can be nitrided. Nitriding is required if an H13 sliding component moves within another H13 component to prevent damage. For example, if an H13 moving centre core is fitted to an H13 core block then the normal procedure is to nitride the centre core. Without nitriding, the centre core will, in effect, weld itself to the H13 core and all movement will stop.

With great difficulty, the centre core will then have to be separated from the core and all damage (also called “pick up”) will have to be machined out. There is a good chance that the centre core will not be recoverable and a new one will have to be made.

H13 can be hard chromium plated which is useful for mould reconditioning purposes. It can be used to rebuild worn interlocking surfaces between a core and cavity. But be careful about using it around shut off edges because it is prone to chipping at corners during machining.

H13 is more expensive than P20 but the extra cost is more than offset by the outstanding performance of this steel.

Recommend through hardness 48-52 HRC. A hardness of 52 HRC will give longer mould life but it is harder to perform finish machining operations at this hardness compared with 48 HRC. 

Welding is possible but proper precautions must be taken (elevated working temperature, joint preparation, choice of consumables and welding procedure).

Ramax Heat Treating Tool Steel

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Ramax - (DIN No. 1.2085) - is a through hardened stainless steel that offers good corrosion resistance which prevents clogging of water cooling channels that could otherwise affect cycle time consistency and mould maintenance.

It is a holding steel and is supplied with a uniform hardness of 340HB(38HRC) (which is more than P20 ) so it is a more durable steel for mould bolsters and gives a longer life time.

Welding is possible but proper precautions must be taken (elevated working temperature, joint preparation, choice of consumables and welding procedure).

Polishable, but only recommended for parts requiring low to medium polishing demands.

Stavax Heat Treating Tool Steel

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Stavax – (DIN No. 1.2083) - is a through hardened premium stainless steel with good corrosion resistance, good polishability and good wear resistance.

The combination of these properties gives a steel with outstanding production performance. The practical benefits of good corrosion resistance in a plastics mould can be summarized as follows:

• 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.
  
• Lower production costs. Since water cooling channels are unaffected by corrosion (unlike P20 steel)heat transfer characteristics and therefore cooling efficiency are constant throughout the mould life, ensuring consistent cycle times.

These properties makes this steel an excellent choice for cores, cavities and stripper rings in moulds designed to produce millions of parts per year at fast cycle times (containers and cutlery).

Its suitable for molding corrosive materials such as PVC and abrasive filled materials.

Its good polishability makes it suitable for optical parts such as camera and sunglass lenses.

It can be photo-etched and welded.

It is more expensive than H13 steel.

Recommended through hardnes is 45 – 54 HRC

Click here to learn about another type of tool steel called A-11

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Reference

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Bohler Uddeholm, Australia, Steel reference chart 2008.

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