H13 Steel for Injection Molds

What H13 is

H13 is a chromium-molybdenum-vanadium hot-work tool steel, corresponding to DIN/EN 1.2344. It is normally supplied annealed, machined in the soft state, then hardened and tempered to 48–52 HRC. That hardness and its hot strength are why it is the standard choice for a long production campaign or an abrasive resin.

The grade was developed for hot work, which means it keeps its strength and temper resistance at temperatures that would soften a plastic mould steel. In injection moulding that matters mainly for engineering resins with high melt temperatures and for glass-filled grades that generate heat and wear at the gate.

It is not a stainless steel. It resists wear, not corrosion, and that distinction decides a large share of the applications it is wrong for.

Hardness, and what it does not buy you

At 48–52 HRC, H13 is hard enough to resist glass-fibre wear at the gate and vent land through a production campaign. That is the function it is bought for. The hardness also resists indentation at the parting line and holds a cavity dimension through a long run.

It does not buy toughness. A hardened H13 block is more sensitive to a sharp internal corner than a pre-hardened one, and an unrepaired EDM recast layer is a crack initiation site. The tool design has to avoid sharp corners, and the EDM surfaces have to be dressed or stress-relieved before the tool goes into service.

It does not buy polishability either. H13 can be polished to a good cosmetic standard, but the hardness makes the work slower and the steel is not as clean as a mirror-grade. For an optical part, a stainless grade with better polish is the correct choice, even if it is softer.

And it does not buy corrosion resistance. A hardened H13 cavity will still rust, and a PVC or aggressive flame-retardant resin will still attack it. Hardness and corrosion resistance are separate properties; neither substitutes for the other.

Corrosion and polishability

Corrosion resistance is low. H13 has enough chromium to resist tempering at high temperature, not enough to resist a wet tool room or a corrosive resin. A cavity left wet between runs will show rust, and a PVC job will corrode the steel regardless of hardness. For those resins, a stainless grade is the correct family and hardness alone does not solve the problem.

Polishability is good but not exceptional. The steel takes a fine cosmetic finish, and a textured cavity holds detail well. The work is slower than on a pre-hardened block because of the hardness, and the polish has to be done after heat treatment. A mirror finish is possible with effort, but a stainless mirror grade will reach it more reliably and hold it better.

The practical combination is a hardened cavity for wear, with a polished surface only where the part needs it. Polishing the entire cavity to a mirror standard when only one face is visible is a cost that buys nothing.

Because the cavity is hardened after machining, any corrosion or damage that appears later is more expensive to repair: welding requires a preheat and a post-weld temper, and re-polishing a hard surface takes longer. That raises the value of storage discipline and of a replaceable insert at the wear locations.

By production volume

Below roughly 100,000 shots in an unfilled resin, H13 is usually more steel than the programme needs, and a pre-hardened grade is the lower total cost. Between 100,000 and 500,000 shots it becomes competitive where the resin is abrasive or the part tolerance is tight.

Above 500,000 shots, H13 is the standard choice for a production tool: the hardness holds the cavity, the wear surfaces survive, and the maintenance interval is longer. The heat-treatment step adds schedule, but it is amortised over a long run.

The resin can move the threshold in either direction. A glass-filled tool may justify H13 at 100,000 shots because the gate would otherwise wear out. An unfilled commodity part may stay in a pre-hardened grade well beyond 500,000 shots if the tool is maintained.

By resin

H13 is the standard answer for glass-filled engineering resins such as PA+GF. The fibre is abrasive, the melt temperature is high, and both point to a hardened grade with a replaceable gate insert.

It is also used for unfilled PC in production, where the mould runs hot and the cavity has to hold its hardness and polish through a long campaign. For a short PC run, a pre-hardened grade is usually enough.

Unfilled commodity resins such as ABS do not need H13. A pre-hardened block at 30–34 HRC will outlast the campaign at a lower cost, and the extra hardness buys nothing against an unfilled melt.

PVC and aggressive flame-retardant grades are a mismatch for a different reason: H13 is not stainless, so corrosion arrives before wear. A stainless grade is the correct answer there, even at a lower hardness.

Heat treatment, welding and repair

H13 is supplied annealed and hardened after machining. The hardening and tempering cycle has to produce 48–52 HRC with the toughness the application needs, and the tool has to allow for the distortion that comes with it. Finish machining and polishing happen after hardening, and the process plan has to sequence the work accordingly.

Welding is possible but stricter than on a pre-hardened steel. Preheat the area, use a matching filler, control the heat input, and temper after welding to restore hardness and toughness. An untempered weld on a hardened cavity can crack in service, and the crack usually starts at the weld toe.

Repair at the gate is common. A replaceable gate insert is the better design: it can be made in the same grade, hardened separately, and replaced without welding the cavity. That keeps the repair small and protects the rest of the tool.

EDM surfaces need attention. The recast layer is hard and brittle, and a sharp corner under it is a crack starter. Dress or stress-relieve the EDM surface before the tool goes into production, especially on a cavity that will see impact or thermal cycling.

Surface treatment and coatings

Nitriding is a common addition to an H13 cavity. It hardens the surface further and improves wear resistance at the gate and the vent land, and it is compatible with the hardened substrate. The case is thin, so it is a wear treatment rather than a structural one.

PVD coatings work well on a hardened H13 substrate because the coating has a hard base to support it. They reduce friction and wear at the gate and can extend the maintenance interval on an abrasive job. The coating adds a process step and a lead time, and it has to be removed before a weld repair.

Chrome plating is used for release and corrosion resistance, but it is a weaker fit for a wear application than a PVD coating. Where corrosion is the real problem, a stainless grade is a better answer than a coating on H13.

Cost per shot vs cost of steel

H13 costs more than a pre-hardened grade, and the heat-treatment step adds both cost and schedule. On a short campaign that premium is hard to recover. On a long campaign it is recovered through a longer maintenance interval and fewer unplanned stops.

The comparison should include the cost of a worn gate. A pre-hardened tool in a glass-filled resin may need a gate repair or insert within the campaign; an H13 tool with a replaceable insert may not. That difference often decides the choice more than the block price.

Where the resin is also corrosive, H13 is not the cheap option at all: it is the wrong one, and a stainless grade has to be priced in from the start. A coating on H13 is not a substitute for the right base steel.

Common selection mistakes

• Using H13 for a short unfilled campaign. The heat-treatment cost and schedule are not recovered, and a pre-hardened grade would do the job.
• Treating H13 as stainless. It rusts, and PVC or aggressive flame retardants will corrode it. Hardness is not corrosion resistance.
• Welding without a post-weld temper. The repair can crack in service, and the crack usually starts at the weld toe.
• Leaving an EDM recast layer at a sharp corner. That is a crack initiation site, and it fails after the tool is in production.

What to send us

Send the 3D part, the resin grade and glass or mineral loading, the annual volume and campaign length, the surface requirement and the cavity count. If the resin is also corrosive, say so, because that moves the recommendation from H13 to a stainless grade before the tool is designed rather than after the first repair.