Heat treating Mould Steel is a very complex work. Selecting the correct type of heat treatment in plastic injection mold making can also be very difficult if you don’t have the practical experience.
The bottom line is that the heat treatment chosen for an injection mold must keep the mold in a good working situation for the life of the mould.
How to choose the correct treatment for your mould? I am joyful you asked.
The type of heat treatment depends upon the following factors:
Selection of Mould Steel. The choice of steel mainly determines the type of heat treatment. Mould price. This should consider the type of heat treatment. If the quote is too low then there is probably not will be any heat treatment at all. Part design. If the 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 need special attention so that the surfaces don’t wear out rapidly or get broken. Mold design. This is mainly influenced by the part design. Annual production quantities. Small quantities can use softer, more economical materials but high volume moulds require long-term materials and heat treatments. Environmental elements. Does the molding take place in a corrosive environment? Mould repair issues such as corrosion.
Types Of Heat Treatments For Mould Steels
Surface treatment– the outside 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. Through hardening treatment– presents uniform hardness throughout the entire work piece.
Below are 4 of the most popular steels used to build injection molds and their recommended treatments:
P20 Heat Treating Mould Steel
P20– (DIN No. 1.2312) – supplied in the thru hardened and tempered condition at a hardness of 310 HB (34HRC). It has really 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 annually 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 erosion resistance. Because electroless nickel plating takes place in a bath, the plating will also cover internal water cooling channels, that makes mould maintenance easier. The plating will add in 0.005 mm per side or 0.01 mm to plate thickness.
Sliding inserts made from P20 should be nitrided for wear resistance and guard against possible ruin when using with a P20 bolster.
It could be welded which benefits repairs. It can be flame hardened or nitrided for extra resistance to wear and corrosion. A nitride surface also increases the erosion resistance.
H13 Heat Treating Mould Steel
H13– (DIN No. 1.2344)– is a through hardening Mould Steel which has excellent hot tensile properties, high hot wear resistance, enough toughness and resists tempering at high operating temperatures.
These characteristics make this steel an excellent choice for cores, cavities, stripper rings, sliding parts or rotating cores in moulds designed to produce millions of parts each year at fast cycle times. Thin wall molding is an example of this kind of application (containers and cutlery).
H13 can be nitrided. Nitriding is required if an H13 sliding component moves within another H13 component to avoid damage. 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, essentially, weld itself to the H13 core and all movement will stop.
With great difficulty, the centre core will then should be divided 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 could be hard chromium plated which does for mould reconditioning purposes. It can be used to rebuild worn interlocking surfaces between a core and cavity. Be careful about using it around shut off edges because it is prone to chipping at corners during machining.
H13 is more highly-priced than P20 but the additional cost is more than offset by the outstanding performance of this steel.
Advise 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 available but suitable precautions must be taken (heightened working temperature, joint preparation, choice of consumables and welding procedure).
Ramax Heat Treating Mould Steel
Ramax – (DIN No. 1.2085) – is a through hardened stainless-steel that offers good erosion resistance which avoids clogging of water cooling channels that could otherwise affect cycle time consistency and mould maintenance.
It is a holding steel and is provided with a consistent 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 (raised working temperature, joint preparation, selection of consumables and welding procedure).
Polishable, but only advised for parts requiring low to medium polishing demands.
Stavax Heat Treating Mould Steel
Stavax– (DIN No. 1.2083) – is a through hardened superior stainless steel with good erosion resistance, good polishability and good wear resistance.
The combination of these properties gives a steel with excellent production performance. The practical benefits of good corrosion resistance in a plastics mould can be summed up as follows:
Lower mould maintenance costs. The surface of the cavities maintain their original finish over prolonged running periods. Molds stored or operated in damp circumstances require no special protection. Lower production costs. Since water cooling channels are unaffected by corrosion (different from P20 steel) heat transfer features and therefore cooling efficiency are constant throughout the mould life, making sure consistent cycle times. These properties make this steel an outstanding choice for cores, cavities and stripper rings in moulds designed to produce millions of parts per year at fast cycle times (containers and cutlery).
It’s suitable for molding corrosive materials such as PVC and abrasive filled materials.
It’s good polishability makes it suitable for optic parts such as camera and sunglass lenses.