Techniques to Extend Mold Service Life via Nitriding Treatment
Nitriding treatment forms high-hardness nitride layers on mold surfaces to improve wear resistance, anti-seize property and corrosion resistance, becoming a mainstream process to prolong service life of injection and die-casting molds. Improper nitriding control leads to loose nitride layers, excessive brittleness, uneven hardness and surface cracking, which shorten mold lifespan conversely. Combined with full-cycle management including pre-nitriding pretreatment, nitriding type matching, parameter control and post-treatment maintenance, nitriding advantages can be maximized to extend mold service cycle effectively.
1. Pre-Nitriding Pretreatment Control
Post-processing status before nitriding determines final quality of nitride layers. Finishing allowance of 0.03 mm to 0.06 mm is reserved on cavities, parting surfaces and insert fitting faces, so only light polishing is required after nitriding instead of heavy grinding which would remove hardened layers completely. Surface roughness of all nitriding areas is controlled within Ra 0.8, and oil stains, polishing wax, rust and release agent residues are removed thoroughly via ultrasonic degreasing plus alcohol wiping, since residual contaminants will form nitriding spots and discontinuous hardened layers. Deep holes, slender water channels and narrow insert gaps of molds need dredging for smooth air circulation inside nitriding furnaces to avoid soft spots caused by trapped gas. Quenched and tempered molds must release internal stress sufficiently with base hardness maintained between HRC 28 and HRC 35; nitriding before stress relief will induce mold deformation under heat and damage precision of parting surfaces. Copper plating protective paste is applied to block threaded holes and assembly positions that need reserved fitting size, preventing hardening caused by nitriding and subsequent assembly seizing.
2. Match Nitriding Types According to Mold Working Conditions
Different production environments correspond to different nitriding methods, and wrong process selection results in insufficient wear resistance or spalling of brittle surface layers. Gas nitriding applies to common plastic molds processing non-corrosive materials, forming nitride layers of 0.12 mm to 0.18 mm thickness with hardness ranging HV850 to HV950, balancing toughness and wear resistance for repeated mold opening and closing cycles. Plasma nitriding is adopted for molds producing PVC, glass-filled PA and flame-retardant plastics susceptible to corrosion, featuring dense pore-free nitride layers with superior anti-corrosion performance, and layer thickness is controlled at 0.10 mm to 0.15 mm to prevent acidic gas erosion on cavity surfaces. Soft nitriding is suitable for die-casting molds and frequently impacted inserts, creating surface layers with both hardness and toughness to avoid spalling of overly brittle nitride layers. Low-temperature nitriding (480℃–500℃) is used for precision small inserts to prevent thermal deformation, while nitriding temperature for large templates cannot exceed 520℃ with temperature fluctuation strictly limited within ±5℃.
3. Precise Control of Core Nitriding Parameters
Step heating mode is adopted during temperature rise: hold temperature at 250℃ for 60 minutes first to remove water vapor inside molds, then heat up to nitriding temperature in segments. Sharp temperature rise creates temperature difference between mold interior and exterior, inducing deformation and microcracks. Ammonia decomposition rate serves as the key parameter: 25% to 35% for plastic molds and 35% to 45% for die-casting molds. Excessively high decomposition rate generates loose porous nitride layers with poor wear resistance, while low decomposition rate slows nitriding speed and produces thin hardened layers. Holding duration is set based on requirements: ordinary molds keep warm for 4 to 6 hours, and wear-resistant sliding blocks and sprue bushes requiring thicker nitride layers hold for 7 to 9 hours. Long-time heat preservation causes excessive brittleness of nitride layers. After nitriding, molds cool slowly inside furnaces below 200℃ before discharging instead of rapid air cooling, reducing internal stress of nitride layers and avoiding tiny surface cracks. The furnace chamber is kept clean throughout nitriding to prevent dust adhesion and formation of nitriding defects on mold surfaces.
4. Post-Nitriding Finishing and Stress Relief Techniques
Molds cannot be directly put into production after nitriding, and natural placement for 24 hours at room temperature is required to release nitriding stress. Only abrasive stones above 1200 grit and fine sandpaper are used for light polishing to remove oxide films on cavity surfaces, with grinding thickness controlled below 0.01 mm to retain intact hardened nitride layers. Polished mating positions including mold closing friction surfaces, ejector pin holes and guide sleeve inner walls undergo trial fitting and running-in to prevent galling between high-hardness nitrided surfaces. Low-temperature tempering at 180℃ for two hours is added for molds with minor residual stress after nitriding to further eliminate internal stress, improve toughness of nitride layers and reduce spalling risks during mass production. Copper plating protection layers are cleared completely, and assembly dimensions of threads and mounting holes are inspected before assembly.
5. In-Production Protection to Extend Service Life of Nitride Layers
Nitrided molds are forbidden to open and close without lubrication. High-temperature grease is coated on guide pins, parting surfaces and ejector positions before initial trial runs, and 50 low-speed mold cycles are performed for running-in before formal mass production. When molding glass-fiber reinforced plastics, gate structure is optimized to lower melt flow velocity and slow down abrasive erosion of glass fiber on nitrided cavities. Neutral release agent is sprayed evenly at fixed intervals to avoid acidic release agents corroding nitride surfaces. Copper or plastic scrapers are used to clear residual plastic inside cavities during shutdown instead of hard steel shovels which scratch hardened nitride layers. Even water flow inside cooling channels prevents premature aging of nitride layers caused by local overheating, and anti-rust oil is coated on cavities for long-term shutdown to isolate moisture in air.
6. Specification for Re-Nitriding During Mold Repair to Avoid Scrap
Worn nitride layers cannot receive secondary nitriding directly. Original nitride layers must be fully ground off first, followed by stress relief tempering to eliminate surface fatigue layers. Holding time for secondary nitriding is shortened by 30% compared with initial treatment to prevent over-thick brittle nitride layers. Any single position can be nitrided twice at most, and repeated nitriding reduces substrate toughness and causes mold cracking eventually. Worn local inserts are disassembled for separate nitriding instead of putting the whole mold into furnaces to avoid repeated thermal deformation of templates.
Conclusion
Maximizing mold service life via nitriding relies not merely on nitriding processes, but covers pre-finishing and stress relief, matching nitriding types with production environments, controlling temperature, ammonia decomposition rate and holding time to prevent loose and brittle nitride layers, optimizing surface toughness through light polishing and low-temperature tempering after nitriding, protecting nitride layers from scratching and corrosion during production, and removing old nitride layers properly before re-nitriding during maintenance. Standardized implementation of this system enables nitride layers to exert wear-resistant and anti-corrosion functions without cracking or peeling off. Service life of plastic molds can be doubled generally, and wear cycles of die-casting and high-load injection molds are extended obviously as well.
