Mold Steel Surface Coating — How Do TiN, CrN and DLC Coatings Extend Mold Life, Reduce Friction and Improve Part Release?

Last updated: 2026-09-03 · Reading time: about 6 min

A surface coating is a thin (2–5 μm) hard layer — TiN, CrN or DLC — deposited on mold steel by PVD or CVD. Match the coating to the resin: it runs 1,500–4,000 HV against the steel’s 700–900 HV, cuts friction 30–70% and extends mold life 2–10×.
See also related article.

Key facts

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  • A coating is a hard, low-friction barrier that keeps glass, mineral filler and flame-retardant additives from wearing or attacking the steel — hardness 1,500–4,000 HV against the substrate’s 700–900 HV.
  • Pick by resin: TiN for general and abrasive, TiAlN for high-temperature, CrN for corrosive, DLC for stick-prone and optical, electroless nickel for corrosion plus a mirror finish.
  • PVD runs at 300–500 °C and adds only 2–5 μm, so dimensions hold — but the steel must be at least 48 HRC and polished first, or the layer cracks and the scratch shows through.

    • Parting line — The line where the two mold halves (A-side and B-side) meet. Parting line location affects: gate placement, ejector pin marks, witness lines, and cosmetic appearance. A well-designed parting line minimizes visible witness marks. See also related guide.

    What are the coating types — and how do you choose the right one?

    Five coatings cover most mold work. Selection follows the resin, the mold temperature and the defect you are solving.

    Coating Hardness (HV) Friction Max temp Best for
    TiN (titanium nitride) 2,000–2,500 0.4–0.6 600 °C General purpose; abrasive GF-filled resins
    TiAlN (titanium aluminum nitride) 3,000–3,500 0.4–0.6 800 °C High-temperature resins (PPS, PEEK, PEI)
    CrN (chromium nitride) 1,800–2,200 0.3–0.5 700 °C Corrosive resins (PVC, flame-retardant)
    DLC (diamond-like carbon) 3,000–4,000 0.05–0.15 200 °C Stick-prone resins (TPE, TPU); optical parts
    Ni (electroless nickel) 500–700 0.3–0.5 400 °C Corrosion protection; mirror finish

    TiN is the workhorse — the gold-colored layer on molds running glass-filled PP or PA. TiAlN takes the high-temperature resins that would oxidize TiN above 600 °C. CrN is chemically inert, so it resists the acid attack from PVC and brominated flame-retardant decomposition. DLC has the lowest friction coefficient of the group, so a TPE or TPU part releases cleanly and an optical face comes off without flow marks.

    What is the coating process — and does it change the mold dimensions?

    Four steps, and one number that decides whether the coating holds: the substrate hardness.

    1. Polish and clean first. The coating replicates the surface underneath, so the components are polished to their final finish, then ultrasonically cleaned and solvent-degreased. Any oil, grease or polishing compound left behind is trapped under the layer and turns into a pinhole, a blister or a delamination.
    2. Deposit by PVD, 3–6 hours at 300–500 °C. Titanium, chromium or carbon is vaporized by an arc or sputtering source in a vacuum and builds up as a uniform 2–5 μm layer, even in deep holes and complex geometry. The temperature stays below the tempering point of most mold steels, so hardness is unaffected.
    3. Inspect and test. Coated parts are checked for pinholes and delamination, measured for thickness by eddy current or X-ray fluorescence, tested for adhesion by a Rockwell C indentation at 150 kgf (cracking or peeling around the indent means poor adhesion), and measured for hardness by Vickers microhardness.
    4. Re-coat when it wears. The layer eventually wears off, and the components can be stripped and re-coated. Re-coating costs 50–70% of the original because of the stripping, and a component tolerates 3–5 cycles before the steel dimensions shift — each strip removes 1–2 μm of steel.

    A coating preserves a surface; it does not create one. PVD adds only 2–5 μm that copies whatever finish is underneath, so a scratch still shows through and a soft substrate still fails. Harden the steel to at least 48 HRC and polish it to its final state before coating — below that hardness the steel deforms under injection pressure and the hard, brittle layer cracks instead of flexing with it.

    “We treat the coating as a wear budget, not a cosmetic add-on. In the DFM we match the layer to the resin — TiN for glass-filled, CrN for corrosive, DLC for TPE release — and we specify hardened, polished inserts so the coating has something to hold onto. The payoff shows up as documented shot count and fewer re-polish stops, not a shinier tool.”

    — Project Engineer, aCatMould

    Frequently asked questions

  • Key parameter identified — critical variable confirmed against spec.
  • Tolerance within achievable range — commercial ±0.15mm or tighter as needed.
  • Material matched to application — resin grade selected for end-use environment.
  • Process window validated at T1 — parameters locked after first trial.
  • Inspection method defined — CMM, visual, or functional test specified.

    • What does a PVD coating cost — and when does it pay back?

    Coating runs $5–15 per square inch, and DLC $15–30 because the process is slower.
    A typical cavity of 100–300 square inches works out to $500–4,500 per cavity.

    It pays back for high-volume molds above 500,000 shots a year (life up 2–10×), abrasive resins (wear rate down 5–10×) and stick-prone resins (ejection force down 30–70%, no release spray at $0.01–0.03 a part). Payback lands in 50,000–200,000 shots.

    Do you coat the whole mold — or only specific components?

    Only the components that touch the melt and wear: cavity inserts, core pins, slides and ejector pins. Base plates are left uncoated because they never contact the melt.

    Parts are removed, coated in a vacuum chamber — most chambers take pieces up to 400 × 400 × 400 mm — and reassembled. The 2–5 μm layer is negligible against a 0.01–0.05 mm fit tolerance, so the fit does not change.

    What are the coating limits — and when should you skip it?

    A coating does not raise steel hardness (it needs a substrate at 48 HRC or above), does not fill scratches (polish first), and has a temperature ceiling — TiN oxidizes above 600 °C, DLC above 200 °C.

    It also adds $500–4,500 per cavity and 1–2 weeks of turnaround. Below 100,000 shots the mold will not wear out before production ends, so the coating is not worth it.

    Key takeaways

    1. Match the coating to the resin — TiN general and abrasive, TiAlN high-temperature, CrN corrosive, DLC stick-prone and optical, electroless nickel for corrosion plus a mirror finish.
    2. PVD runs at 300–500 °C and adds 2–5 μm, so dimensions hold; hardness reaches 1,500–4,000 HV and mold life goes up 2–10×.
    3. Harden the steel to 48 HRC and polish before coating; the $500–4,500 per cavity is justified above ~500,000 shots or for abrasive, stick-prone and corrosive resins.

    A coating is a targeted fix, not a substitute for the right steel or a sound design. Match it to the resin, harden and polish the substrate first, and it buys 2–10× the shot life with lower ejection force and no release spray. If a mold is wearing, sticking or corroding, a DFM review picks the coating before the tool is cut — or before it goes back for re-coating.

    Sources
    Plastics Technology — mold surface coatings, wear and maintenance practice
    MoldMaking Technology — PVD/CVD coatings and surface engineering for tooling
    Plastics Industry Association — plastics processing and tooling resources
    ASTM International — hardness and coating adhesion test methods (Vickers, Rockwell)
    Society of Plastics Engineers — molding materials and process engineering

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet