Nitriding Layer Peeling Protection, Surface Treatment Scheme for Plastic Molds
Nitriding treatment can raise surface hardness, wear resistance and anti-sticking performance of plastic mold cavities. However, peeling of nitriding layer is one of the most common failure modes for surface strengthening. Such peeling defects are mainly caused by insufficient substrate pre-treatment, excessively thick white compound layer, concentrated residual stress and cyclic thermal shock during production. Once local peeling occurs, the defect will spread rapidly, resulting in product surface scratching and corrosion, and even premature mold retirement. A complete protection system should cover substrate pre-treatment, nitriding parameter control, composite coating selection, machining assembly specification and production maintenance, to reduce the risk of nitriding layer peeling.
1. Mold Substrate Pre-treatment
The substrate state directly determines the bonding force of nitriding layer, which is the basic link to prevent peeling. Select pre-hardened steel meeting nitriding requirements for mold steel, and the substrate hardness should reach design index. Insufficient substrate hardness leads to poor supporting capacity, and the surface hardening layer is easy to peel off. After finishing all cavities and inserts, remove the remelted layer generated by EDM discharge machining. Microcracks exist inside the discharge white layer, and cracks will expand after nitriding and induce surface peeling. Carry out stress relief tempering to eliminate mechanical residual stress brought by CNC milling and grinding. Insufficient stress release will produce stress difference during nitriding heating and cooling, causing cracking and peeling of nitriding layer. Clean surface oil, polishing paste and rust impurities, and make small round corners at sharp corners. Sharp edges are easy to accumulate and embrittle nitriding layer during nitriding, and round transition above R0.05 is preferred.
2. Optimization Control of Nitriding Process Parameters
The nitriding process focuses on controlling the thickness of white bright compound layer. Excessively thick compound layer has high brittleness, which is the core inducement of peeling. Two-stage ion nitriding process is preferred, and the nitriding temperature is stably controlled at 520~560℃ to avoid over-temperature. The thickness of white bright layer is controlled in the range of 3μm to 8μm, and do not increase the thickness blindly. Thick layer has high brittleness and is easy to peel under alternating cold and heat. Set holding time reasonably according to cavity size, extend the diffusion stage time, improve the diffusion depth of nitrogen in steel, build gentle hardness gradient, and reduce interface stress caused by sudden hardness change between surface layer and substrate. Slow cooling is adopted after nitriding, and rapid air cooling is forbidden to reduce thermal stress. Sample inspection is carried out after nitriding, and metallographic inspection and Vickers hardness test are used to confirm that there are no brittle structures such as network nitride and vein nitride in the penetrated layer tissue.
3. Matching Scheme of Composite Surface Coating
For molds forming glass fiber reinforced plastics and corrosive raw materials, single nitriding protection is not enough to resist long-term scouring and chemical erosion. Nitriding plus PVD composite treatment scheme is recommended. Ion nitriding is carried out first to form diffusion support layer, and then CrN, AlCrN hard coating is deposited. The composite structure has stronger bonding force, improved wear resistance and corrosion resistance, which can greatly reduce the probability of surface peeling. Molds for ordinary non-glass fiber and non-corrosive raw materials can retain single nitriding treatment to reduce processing cost. For raw materials releasing corrosive gas such as PVC and POM, single nitriding has limited corrosion resistance. Passivation treatment can be matched on the basis of nitriding to inhibit medium erosion interface and prevent corrosion from prying up nitriding layer gradually from the edge.
4. Mold Assembly and Polishing Control
After nitriding, the polishing operation of mold needs strict control. The nitriding layer has high hardness and brittleness. It is forbidden to polish the surface of cavity vigorously by hand or grind strongly. The grinding operation will locally cut and damage the nitriding layer, produce microcracks, and gradually expand into peeling in subsequent production. Fine oil stone light polishing is preferred for parting surface and sealing position polishing, control grinding allowance and cannot remove nitriding surface in large area. Protect the cavity surface during assembly to avoid hard object collision and impact. Any impact will cause cracking of nitriding layer. Control the interference magnitude when assembling inserts and cores. Excessive interference assembly will produce tensile stress on the surface layer and induce peeling of nitriding layer.
5. Protection and Inspection During Injection Molding Mass Production
Alternating cold and heat impact during production will accelerate fatigue failure of nitriding layer. Heat up and cool down the mold step by step, and forbid rapid heating and quenching to reduce thermal impact. When forming corrosive raw materials, do a good job in mold exhaust. Decomposed acidic gas will corrode the interface of nitriding layer and weaken bonding force. Carry out regular mold inspection, focusing on gate impact area and cavity position corresponding to weld line. These areas have strong melt scouring and are high-incidence areas of peeling. Soft polishing cloth and mild cleaning agent are used for mold cleaning. Hard scraper is forbidden to directly scrape the cavity surface. Once slight peeling and cracks are found, stop the machine immediately for disassembly and evaluation, and cannot continue mass production to prevent continuous expansion of defects.
6. Failure Repair and Remedial Scheme
Molds with local nitriding layer peeling cannot be directly re-nitrided at damaged positions. It is necessary to polish and remove all peeling and microcrack areas first, carry out stress relief treatment again, check whether there are crack defects in substrate, and re-carry out nitriding or composite coating treatment after confirming that the substrate is intact. For cavities with large-area peeling, evaluate the damage degree of substrate. Welding repair can be adopted if necessary, and re-surface strengthening after polishing to remove defects of welding layer. After repair, carry out small batch verification first to confirm no surface peeling and qualified product appearance, then transfer to mass production. Nitriding layer peeling protection is a full-process management and control system, which does not only rely on surface heat treatment process. Details control in every link from mold steel selection, processing stress relief, nitriding process, assembly and production maintenance can reduce interface stress, improve bonding strength of nitriding layer, extend the service cycle of plastic molds, and reduce shutdown maintenance cost caused by surface failure.
