How to make the coating process for metal gravity casting

How to make the coating process for metal gravity casting
The coating for gravity casting metal molds has three functions: insulation, demolding, and adjusting solidification speed. The coating thickness is usually controlled at 0.2-0.3mm.
The thicker the coating, the better
Thick coating, strong insulation, slow surface cooling of castings, coarse grains, and decreased mechanical properties.
The coating is too thin, making demolding difficult and increasing the probability of mold sticking and pulling. 0.2-0.3mm is a common equilibrium range.
The thickness should be divided into different parts. It can be thinner near the gate and thicker at the far end, and the temperature gradient can be adjusted with a coating.
Coating ratio and mixing
Coatings are mostly water-based or alcohol based. Low cost and slow drying of water-based materials; Alcohol based drying is fast and suitable for production lines with tight rhythms.
The ratio should be executed according to the supplier's instructions. If the dilution ratio deviates by 10%, the coating performance will change.
Stir evenly. Directly use after coating precipitation, with high concentration in the lower layer and uneven coating thickness.
Selection of Brushing and Spraying
Small batches often use brush coating, resulting in thick but poor uniformity of the coating. Mass spraying with high efficiency but requires masking.
The spraying distance is usually set at 200-300mm, and the gun speed is uniform. Direct spraying at a fixed point can cause local thickening.
When delivering the gravity mold, Xinfeng Mold will attach a schematic diagram of the coating area, indicating the thickness zones.
Drying and preheating
Water based coatings need to be thoroughly dried after application, as a moisture content higher than 0.5% will increase the risk of porosity.
The drying temperature is usually set at 150-200 ℃ for 30-60 minutes. Alcohol based coatings can self ignite and dry, but attention should be paid to ventilation and fire prevention.
Preheat the mold to above 200 ℃ before pouring. The coating will sinter at high temperatures, forming a stable insulation layer.
Maintenance of coatings
Check the coating status after each mold cycle. Local peeling should be promptly repaired, otherwise the mold will stick to the area.
The coating life is usually between 20-50 cycles, depending on the alloy and temperature. After exceeding the lifespan, reapply as a whole.
Remove the old coating before recoating. The residual old layer can lead to insufficient adhesion of the new layer, making it easy to peel off in pieces.
The relationship between common defects and coatings
Adhesive molds are often caused by thin coatings or local detachment. After patching, it can usually be resolved.
Cold insulation is often caused by excessively thick coatings leading to low local temperatures. Thinning the coating or increasing the mold temperature can improve it.
Pores are often caused by high moisture content in the coating. Insufficient dryness is a common cause, especially during humid seasons.
Safety and Environmental Protection
Alcohol based coatings are flammable, and should be stored and used away from sources of fire. The workshop needs to be equipped with fire extinguishers and ventilation facilities.
Wear a mask and protective goggles during spraying operations. Paint droplets can irritate the respiratory tract.
Waste paint and cleaning solution belong to hazardous waste and need to be collected and treated according to regulations. They cannot be directly discharged.
Practical tips
After mixing the paint, let it stand for 5-10 minutes before use. If the bubbles do not disappear before brushing, pinholes may appear in the coating.
When changing paint batches, first make a small sample for trial coating to confirm that there are no abnormalities in adhesion and drying time before using in bulk.
Frequently Asked Questions
Q: What is the appropriate thickness for the coating?
Answer: It is usually 0.2-0.3mm, adjusted according to the location.
Q: How to choose between water-based and alcohol based?
Answer: Water based materials have low cost and alcohol based materials dry quickly.
Q: How long does the paint need to dry?
Answer: 30-60 minutes at 150-200 ℃.
Q: How long does it take to reapply the coating?
Answer: Usually 20-50 cycles, depending on the working conditions.
Q: Is mold sticking a coating issue?
Answer: It is often related to the coating being too thin or peeling off.
Q: How to deal with waste paint?
Answer: It is classified as hazardous waste and requires compliant collection and disposal.
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