Why Mould Surfaces Need Hardening
Mould steel wears where plastic flows and parts eject. Glass-filled resins act like sandpaper on soft steel. Hardened surfaces last far longer.
Surface treatments harden only the outer layer. The core stays tough and machinable. This beats through-hardening for large mould plates.
Nitriding and Tufftride are two proven options. Both work at low temperatures. Both cause little distortion.
What Nitriding Does
Nitriding diffuses nitrogen into the steel surface. Iron nitrides form in the outer layer. Surface hardness rises sharply.
Alloy steels with aluminium, chromium, or molybdenum respond best. They form hard, stable nitrides. Plain carbon steels gain less hardness.
Typical case depths run 0.1 to 0.6 mm. Hardness can exceed 1000 HV on the right grade. Use a hardness conversion chart to compare HV with HRC.
Parts are heated to 500–550 °C in a nitrogen-rich atmosphere. No quench follows the cycle. Dimensional change stays minimal.
Three Nitriding Methods Compared
Gas nitriding uses cracked ammonia. Cycle times run 10 to 100 hours. It suits large batches of simple parts.
Plasma nitriding uses ionised nitrogen in a vacuum chamber. It runs faster and controls case depth well. Masking keeps selected areas soft.
Salt bath nitrocarburizing adds carbon as well as nitrogen. The Tufftride process is the best-known version. Cycles take one to three hours.
| Method | Medium | Typical time | Notes |
|---|---|---|---|
| Gas nitriding | Ammonia | 10–100 h | Deep cases, slow |
| Plasma nitriding | Ionised N2 | 4–40 h | Good control, maskable |
| Salt bath (Tufftride) | Molten salt | 1–3 h | Fast, adds carbon too |
Tufftride in Detail
Tufftride is salt bath nitrocarburizing. Parts soak in molten salt at about 570–580 °C. Nitrogen and carbon diffuse into the surface together.
The result is a thin compound layer over a diffusion zone. The compound layer resists wear and scuffing. The diffusion zone adds fatigue strength below it.
Treated parts often get a black oxide finish. The dark layer improves corrosion resistance. The full sequence is sometimes called QPQ.
Tufftride suits H13, D2, and similar tool steels. It also works on plain carbon steels. Stainless grades need special activation first.
Choosing the Right Steel First
Base steel sets the ceiling for hardness. Nitralloy grades were made for nitriding. H13 and D2 also respond well. See our guide to choosing the right steel for details.
Pre-hardened mould steels need no heat treatment after machining. Nitriding then adds surface hardness directly. This saves a full hardening cycle.
Where Mould Makers Use These Treatments
Ejector pins are classic candidates. They slide thousands of times per day. A hard surface cuts wear and galling.
Hot runner nozzles and manifolds benefit too. They run hot for long campaigns. Nitriding keeps their surfaces hard at temperature.
Slides, lifters, and leader pins see sliding contact. Nitrocarburizing lowers their friction. It also resists corrosion from PVC outgassing.
Cores with fine texture hold polish longer after treatment. Release improves on high-gloss surfaces. Maintenance intervals stretch out.
Limits and Precautions
The compound layer is thin and brittle. Sharp impacts can crack it. Avoid treating parts that take hammer blows.
White layer may need removal on precision fits. Lapping takes off a few microns. This restores exact dimensions.
Growth is small but real. Allow 5–10 microns per surface in tolerancing. Mask threads and press fits before treatment.
Not every steel is a good candidate. High-nickel grades nitride poorly. Check with the treater before committing production parts.
Nitriding vs Carburizing
Carburizing adds carbon at 850–950 °C. Parts then quench and temper. Distortion risk is much higher.
Nitriding works below the tempering temperature. Core properties stay unchanged. Finished dimensions shift far less.
Carburizing gives deeper cases for heavy loads. Nitriding wins where precision matters. Mould work usually favours nitriding.
Mistakes to Avoid
- Treating before final machining. Grinding after treatment removes the hard case.
- Skipping the white-layer check on sealing faces. A brittle layer can flake into the parting line.
- Nitriding stainless without activation. The passive film blocks nitrogen uptake.
- Expecting corrosion proofing from nitriding alone. Add the oxidising step for real rust resistance.
Working With Topworks
Topworks Plastic Mold was founded in 2006. Its headquarters is in Huangyan, Taizhou, Zhejiang. The company also operates a manufacturing plant in Guangdong.
The team advises on steel choice and surface treatment. Send your 3D files and resin details. You will get DFM feedback covering hardening options.
FAQ
What is the difference between nitriding and Tufftride?
Nitriding adds nitrogen only. Tufftride adds nitrogen plus carbon in a salt bath. Tufftride runs much faster.
Will nitriding distort my mould plates?
Distortion is minimal at 500–550 °C. No quench is involved. Large plates usually stay within a few microns.
Can nitrided surfaces be welded for repair?
Welding destroys the case locally. Machine off the layer first, weld, then re-treat. Plan repairs before treatment when possible.
How thick is the Tufftride layer?
The compound layer is about 10–20 microns. The diffusion zone below runs deeper. Total effect reaches a few tenths of a millimetre.
Does nitriding help with plastic corrosion?
It helps, but the oxidised Tufftride finish resists more. PVC and flame-retardant resins attack steel. The QPQ sequence suits these best.
Need hardening advice for your tool? Send the steel grade, part resin, and expected shots. You will get a treatment recommendation with the mould quote.
