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The tool is as hot as the resin
PEEK processes at a mould temperature of 160–190 °C. PPS runs at 130–150 °C. LCP and PEI sit in that same band. At those cavity temperatures a pre-hardened block at 30 HRC softens plastically — the cavity surface deforms from the combined heat and pressure, and the tool fails by plastic flow before it fails by wear.
The steel has to hold its hardness at the mould temperature the resin demands. That means a through-hardened hot-work grade — H13, H11 or DH31 — hardened to roughly 48–52 HRC and tempered specifically for the application temperature. The temper cycle has to finish below the mould temperature the tool will see in service, or the block continues to temper in service and softens progressively.
The three hot-work grades, and the heat each can take
H13 (AISI H13, DIN 1.2344) is the most widely used hot-work grade. Tempered to 50 HRC at roughly 550 °C, it retains roughly 44–46 HRC at 600 °C service temperature. That makes it the standard for PPS, LCP and PEI. For PEEK the mould temperature pushes 180 °C and the cavity surface temperature can spike higher — H13 is still the answer but the temper cycle and cooling channel design matter more than on a lower-temperature job.
H11 (AISI H11, DIN 1.2343) is slightly lower in alloy content than H13, which buys better toughness at the cost of some hot strength. Tempered to 48–50 HRC, it is the preferred grade for high-temperature tools that also have thin walls or deep ribs — geometries that see impact or thermal cycling stresses that might crack a harder H13 block.
DH31 (JIS SKD61, sometimes referred to as "enhanced H13") is a refined composition with better cleanliness and a more consistent hardness through the block than standard H13. It is the premium answer for PEEK tools where the cavity surface has to hold its finish for a production run without thermal fatigue cracking.
Thermal fatigue, and why it kills high-temperature tools
Thermal fatigue is the network of fine cracks that appears on the cavity surface after repeated heating and cooling cycles. Every shot heats the cavity to the mould temperature, cools it down for packing and hold, then repeats. The result is a craze pattern that starts fine and grows. The crack network does not cause immediate scrap, but it holds resin and burns at the surface, and the polish cannot be restored once it is present.
Preventing thermal fatigue is the reason high-temperature tools run a cavity surface temperature as uniform as possible. Cooling channels that vary the surface temperature across the cavity accelerate the fatigue process. A baffled or bubbler cooling design that keeps surface temperature within ±10 °C halves the fatigue life of the tool.
Cooling circuit design for hot moulds
A hot mould does not mean a hot cooling circuit — it means the mould is designed to hold a set temperature uniformly. Water cooling at roughly 80–100 °C maintains the 160–190 °C cavity target through a balanced circuit, rather than chasing the setpoint with a chiller that cycles on and off.
The common mistake: running a high-temperature tool with room-temperature water. The temperature differential between cavity and cooling circuit creates a thermal shock on every cycle, and the fatigue life of the tool is shortened. A temperature-controlled fluid (water or oil) that runs close to the target mould temperature extends the fatigue life materially.
Welding, repair and surface treatment
Welding a high-temperature tool needs a matching hot-work filler — not a general-purpose weld wire. The weld area has to be preheated to roughly 300–400 °C and tempered after cooling, or it will crack from the combined welding stress and service temperature stress.
Nitriding or PVD coatings on high-temperature tools have to be rated for the service temperature. A standard TiN PVD coating softens above 400 °C. AlTiN coatings hold hardness above 600 °C and are the correct choice for PEEK tools. The coating itself does not solve the steel selection — it is a hard-surface addition to the correct base grade.
Common mistakes
- Running PEEK or PPS in a pre-hardened block. The cavity surface softens plastically and deforms after a few hundred shots.
- Preparing a high-temperature tool with a room-temperature chiller. Thermal shock shortens the fatigue life.
- Choosing a low-temperature temper cycle on a high-temperature tool. The block continues to temper in service and softens progressively.
Frequently asked questions
How do I know this category is the right starting point?
Start with the failure mode. If the resin is corrosive, go to the corrosion category before volume. If the resin is abrasive, go to the wear category before polish. If the cavity has to run above 150 °C, go to high-temperature before anything else. The category you land in decides the steel family, and the grade is then chosen within that family.
Can a tool span multiple categories?
Yes — a PEEK tool with a glass-filled resin runs both the high-temperature and abrasive categories at the same time. Where categories overlap, pick the more restrictive one first. A corrosive resin with abrasive filler starts with corrosion resistance; the wear requirement is then layered on top with inserts or coating.
Why not just pick the hardest grade in this category?
Hardness buys wear resistance and indentation. It does not buy toughness, hot strength or corrosion resistance, and it costs schedule and money. The categories deliberately trade one property for another — that trade is what each is designed around.
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