The influence of mold coating and surface treatment on demolding and service life

The influence of mold coating and surface treatment on demolding and service life
Nitriding increases the surface hardness to above 900HV, and PVD coating reduces friction. The combination of the two can extend the lifespan by 20% -50%.
Nitriding is the most fundamental treatment
The depth of the gas nitriding layer is often taken as 0.05-0.15mm, and the surface hardness can reach 900-1100HV, significantly higher than the substrate's 45-52HRC.
The infiltration layer is too shallow, and wear penetrates quickly; If it is too deep, the brittleness increases and it is easy to peel off. About 0.1mm is a common equilibrium point.
After nitriding, the cavity size will slightly increase, usually 0.005-0.02mm. Precision molds need to reserve this amount of allowance during processing.
PVD coating solves friction
The thickness of AlCrN coating is 2-5 μ m, the friction coefficient can be reduced to 0.3-0.4, and the demolding force can be reduced by 10% -20%.
The coating has limited temperature resistance. Under long-term working conditions above 600 ℃, the coating will gradually fail. The mold temperature for aluminum alloy casting is within the safe range of 200-350 ℃.
Mirror polishing is required before coating. The substrate roughness is higher than Ra0.4 μ m, and the coating cannot form a continuous film, resulting in early peeling in some areas.
The influence of coating on demolding slope
With a low friction coating, the demolding slope can be appropriately reduced. Conventional 1 ° can be reduced to 0.5 ° -0.7 °, leaving more space for styling.
But reducing the slope will amplify the impact of any surface defects. The risk of strain has not disappeared, only the threshold has shifted backwards.
Xinfeng Mold will choose the coating scheme for wheel hub molds based on the difficulty of demolding the wheel shape, and prioritize the combination of coating and nitriding for deep cavity wheel shapes.
Other options for surface treatment
Boronization can increase the surface hardness to above 1400HV, with outstanding wear resistance, but it is brittle and not suitable for areas with high impact loads.
Laser cladding can repair local wear, and the cladding layer is metallurgically bonded to the substrate, with a lifespan close to that of a new product after repair.
Chromium plating is less commonly used in aluminum alloy casting, and the chromium layer is prone to cracking at high temperatures, resulting in less effective results compared to nitriding and coating.
The processing order cannot be wrong
The correct sequence is: rough machining, heat treatment, precision machining, polishing, nitriding, and coating. Nitriding can only ensure size after precision machining.
Nitriding first and then precision processing will cut off the infiltrated layer, which is equivalent to white processing. This is a common process sequence error.
Cleaning and preheating should be done before coating. Residual oil stains on the surface, insufficient adhesion of the coating, and detachment in patches during use.
Maintenance and touch up painting
After local wear of the coating, it can be repaired, but the old coating needs to be removed first. The adhesion of local patching is usually lower than that of the original coating.
After the wear of the nitride layer, it cannot be locally repaired and can only be re nitrided. Before re nitriding, the original infiltration layer needs to be removed, which is a complex process.
It is more economical to conduct preventive examinations according to the pattern rather than waiting for the strain to be treated. It is recommended to check the surface condition of the mold cavity every 5000 molds.
Frequently Asked Questions
Q: How much hardness can nitriding increase?
Answer: The surface can reach 900-1100HV, and the substrate is 45-52HRC.
Question: How thick is the coating?
Answer: PVD coatings are commonly 2-5 μ m thick, which affects replication accuracy.
Q: What is the temperature resistance of the coating?
Answer: Long term temperatures above 600 ℃ will result in failure, and the casting process is safe.
Q: Can the processing order be reversed?
Answer: No, nitriding must be done after precision machining.
Q: Can the coating be partially repaired?
Answer: Yes, but the old coating needs to be removed first as it has lower adhesion.
Q: How often is the surface inspected?
Answer: It is recommended to check the cavity status every 5000 molds.
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