Matching Tool Steel Selection to Initial Production Commitments
Match tool steel hardness and chemistry directly to committed shot counts and resin abrasiveness to prevent premature cavity failure and tooling cost overruns.
Gas Nitriding
Meaning
Thermochemical surface hardening process introduces nitrogen into the surface of steel components at elevated temperatures to increase wear resistance and hardness. Applying gas nitriding to injection moulding components like screws, barrels, and ejector pins extends their service life by creating a hard wear-resistant casing. This process occurs in a controlled ammonia atmosphere at temperatures below the tempering temperature of the substrate steel, which prevents dimensional distortion.
It is widely used on martensitic stainless steels and hot-work tool steels.
Chemical Process
Dissociation of ammonia gas at the steel surface releases active nitrogen atoms that diffuse into the metal lattice. These atoms react with alloying elements such as chromium, aluminium, and molybdenum to form hard nitrides. The resulting surface layer consists of an outer compound layer and an underlying diffusion zone.
This diffusion zone gradually transitions into the soft ductile core, providing excellent support for the hard surface.
Wear Improvement
Raised surface hardness prevents abrasive wear from highly filled plastics containing glass fibers or mineral additives. The treated components can withstand sliding contact without experiencing adhesive wear or scuffing during long moulding runs. This surface modification also introduces compressive residual stresses, which improve the fatigue strength of ejector pins subjected to cyclic loading.
Moulding Application
Treated barrels and screws maintain their dimensional tolerances over millions of cycles when processing aggressive engineering thermoplastics. Maintaining the tight clearance between the screw flights and the barrel wall is essential for consistent injection pressure and melt homogeneity. Without this treatment, the wear of these components leads to erratic shot sizes and increased polymer degradation from excessive shear.