Steel choice decides how long a mould lasts, how well it holds a finish and how much it costs to maintain. A practical comparison of P20, H13, stainless and other grades, matched to volume, material and finish.
Two tools for nearly identical parts. One, in pre-hardened P20, cost noticeably less and ran well for the first year. By month eighteen, the glass-filled nylon had worn the gates, the parting line had begun to flash and the polish on the cosmetic face had dulled. The other tool, in hardened H13 with replaceable gate inserts, cost more up front and was still running to the original approval dimensions three years and several million shots later.
Mould steel selection is one of the decisions that quietly determines the total cost of a tool for its whole life.
What the steel has to do
A mould steel must balance several properties:
- Wear resistance against abrasive fillers and high-volume production.
- Toughness to resist cracking under clamping, injection pressure and thermal cycling.
- Corrosion resistance against corrosive materials such as PVC and some flame retardants, and against condensation and cooling water.
- Polishability for cosmetic and optical surfaces.
- Machinability and heat treatment behaviour, which affect build cost and lead time.
- Thermal conductivity, which affects cooling and cycle time.
The common grades
- P20 and similar pre-hardened steels (around 28–34 HRC): supplied hardened, easy to machine and modify, lower build cost. Good for prototypes, low to medium volumes, large tools and unfilled, non-abrasive materials. Polishes reasonably well, but wears faster with filled materials.
- H13 and similar hot-work steels (typically 48–52 HRC after hardening): tough and wear resistant, good for high-volume production, filled materials and long tool lives. Requires heat treatment after rough machining, adding time and some risk of distortion.
- Stainless mould steels such as 420 grades (around 48–52 HRC hardened, or pre-hardened versions): corrosion resistance and excellent polishability. Common for medical, optical, food-contact and PVC parts, and in humid environments.
- Higher-alloy and powder-metallurgy steels: exceptional wear resistance for heavily filled materials and very high volumes, at a higher price.
- Copper alloys such as beryllium copper, used as inserts: excellent thermal conductivity to cool hot spots and shorten cycles, but softer and more expensive.
Matching steel to the job
Ask four questions:
- How many shots will the tool make over its life? Prototype and short runs suit P20; millions of shots usually justify hardened steel.
- How abrasive or corrosive is the material? Glass and mineral fillers demand hardness; PVC and some additives demand stainless.
- What surface finish is required? High-gloss and optical finishes need steels that polish well and resist corrosion pitting.
- Where are the hot spots? Inserts in high-conductivity alloys can transform cycle time on deep cores.
Mixing steels in one tool
Most good tools are not one steel. The mould base may be a standard holder steel, the cavities and cores hardened tool steel, gate areas replaceable hardened inserts, and deep cores beryllium copper for cooling. This puts the money where it does the most good.
Questions to ask your toolmaker
- What grade and hardness for each cavity, core and insert, and why?
- What guaranteed tool life in shots, for this material?
- Which areas are designed as replaceable inserts?
- What heat treatment and surface treatments, such as nitriding or coatings, are planned?
- How will the steel choice affect lead time and cost?
The steel specification should be explicit in every quote, as we recommended in comparing toolmakers fairly. "Standard steel" is not a specification.
