Plastic Mold Steel Nitriding Specification

Plastic molds are core equipment in injection molding. Key components such as cavities, cores, ejector pins and slides operate under repeated heating, high pressure, friction and corrosive environments, leading to wear, scratches, fatigue cracking and dimensional deviation. Nitriding is a widely used surface strengthening process that forms a high-hardness, wear-resistant and corrosion-resistant layer on mold surfaces with minimal deformation, significantly improving service life and product quality. This specification standardizes nitriding processes for stable and reliable results.

Scope and Material Requirements

This specification applies to gas nitriding and ion nitriding for critical mold components including cavities, cores, slides, ejector pins, guide pillars and bushes. Applicable materials include H13, SKD61, DAC, FDAC, 718H, P20 and NAK80. It is not recommended for ultra-precision mirror surfaces or components requiring later welding. S136 and other stainless steels are not recommended for conventional nitriding due to potential brittle phases.

Molds must complete final quenching and tempering before nitriding, with hardness stabilized at 46–52 HRC. Parts with high internal stress or insufficient tempering are prohibited from nitriding to prevent cracking and deformation.

Pre-Treatment and Machining Requirements

Machining allowances must be reserved before nitriding: 0.03–0.06 mm for general surfaces and 0.015–0.03 mm for precision fitting surfaces. Sharp edges and corners must be rounded to R0.3 or above to avoid excessive nitrogen penetration and chipping.

Surfaces must be free of oil, scale, welding marks, paint and ink. Non-nitriding areas such as mounting surfaces and threaded holes must be protected with high-temperature resistant masking agents or copper foil to ensure uniform penetration.

Nitriding Process Parameters

Gas nitriding is the most common method, using ammonia as nitrogen source. Process temperature ranges from 480–530°C. H13 and SKD61 are recommended at 510–525°C, while pre-hardened steels are controlled at 480–500°C to reduce brittleness. Holding time is 8–20 hours, with effective layer depth 0.10–0.30 mm and compound layer 10–25 μm.

Ion nitriding features uniform heating and minimal deformation, suitable for precision molds. Process temperature is 490–520°C, pressure 200–800 Pa, time 6–15 hours. Controlled cooling must be applied to avoid cracking.

Quality Inspection Standards

Nitrided surfaces should be uniform gray or dark gray without oxidation, rust, spots or peeling. Surface hardness shall be ≥850 HV for H13/SKD61 and ≥700 HV for 718H/P20, with hardness variation ≤100 HV. Layer depth is measured by micro-hardness method. Dimensional deformation of precision molds shall not exceed 0.02 mm.

Post-Treatment and Application Notes

High-temperature tempering is generally not recommended after nitriding. Low-temperature tempering at 300–350°C for 1–2 hours can improve toughness. Grinding and polishing must retain sufficient nitrided layer. Welding, re-heat treatment or severe impact are prohibited. Proper handling, lubrication and regular maintenance ensure long-term performance.

Nitriding is essential for enhancing mold performance. Strict implementation of this specification ensures stable quality, reduces failures and lowers production costs.