yvonne@nydcasting.com 
During the flow coating process of casting coating, some problems often occur. This article will analyze the causes of their formation and provide solutions.
1. Coating accumulation
Reasons:
- The coating flows slowly. Since the coating is a thixotropic fluid, there are mesh structures and yield values in it. Yield value and viscosity are the two main factors causing coating accumulation. The yield value of the coating is too high and the viscosity is too large, resulting in poor fluidity of the coating.
- The flow marks produced by the coating during the flow process flow along the sand mold, and accumulation occurs when encountering the groove, resulting in unclear edges and corners of the sand mold.
- The inclination angle of the sand mold is not appropriate.
- The flow rate is small, and the coating cannot flow down, causing accumulation.
- The flow rate is slow due to insufficient pressure, causing accumulation.
Countermeasures:
- Considering the on-site operation, the coating Baume degree should be reduced. Practice has proved that the fluidity of the coating is best when the Baume degree of the flow coating is between 22-26. Considering the coating’s own factors, the coating yield value and viscosity should be reduced.
- Remove the flow marks by blowing with a duct or using a brush dipped in diluent.
- Sand mold placement angle requirements: Use a crane to lift the sand mold above the paint tank at an angle of 75-90 degrees to the horizontal for flow coating.
- Increase the cross-sectional area of the flow coating rod head and hose to increase the flow rate.
- Increasing the wind pressure can increase the flow rate. In order to obtain a suitable coating thickness, the speed of the paint flowing out of the flow coating machine should be 100-200mm/s. The wind pressure is generally between 0.4-0.6MPa. If it is too large, it is easy to splash.
2. Insufficient coating thickness
Reasons:
- The paint flows directly without forming enough coating thickness.
- The paint penetrates into the molding sand completely, resulting in insufficient coating thickness.
- The surface of the sand mold is adhered to the release agent, which reduces the permeability of the paint and directly affects the thickness of the coating.
Countermeasures:
- 1. Increase the viscosity of the coating (maximum value does not exceed 7 seconds, thereby improving the coating and avoiding excessive flow of the coating.
- 2. Improving the compactness of the sand mold can effectively avoid excessive penetration of the coating. The compactness of the sand mold is more suitable between 45% and 55%.
- 3. Production can be carried out after the mold release agent on the surface of the mold is fully dried. The parts of the sand mold that are stained with the mold release agent should be ground off with fine sandpaper before flow coating.
- 4. Requirements for the thickness of the wet coating of the sand mold for cast iron parts:
Thin-walled castings 0.15mm-0.30mm
Medium-sized castings 0.30mm-0.75mm
Thick-walled castings 0.75mm-1.00mm
Extra thick castings 1.00mm-2.00mm
3. Coating surface shedding
During the assembly process, when the operator blows the floating sand in the mold cavity with an air duct, the surface of the coating layer occasionally falls off.
Reasons:
1. Low coating strength.
2. The coating layers are not fully combined into a whole.
Countermeasures:
1. The small amount of binder in the coating can easily cause insufficient coating strength, so the proportion of binder in the coating should be increased.
2. The coating is not fully burned, which can easily affect the bonding between layers. For castings above 3 tons, the coating surface is prone to fall off. As long as the ignition time is reasonably controlled, this problem can be solved. Generally, it is appropriate to ignite 3-5 seconds after the upper box flow coating, and 5-7 seconds after the lower box flow coating. It is also best to ignite it. The gas torch baking method can also be used, but the time should not be too long, otherwise the coating will crack.
4. Sand sticking to castings
The refractoriness of the coating is not enough, and the coating or sand mold reacts chemically with the high-temperature molten metal, so that a substance that is extremely difficult to clean is formed on the surface of the casting, which is usually called sand sticking. The flow coating method will also cause sand sticking.
Countermeasures:
1. Change the aggregate composition of the coating to improve the refractoriness of the coating. Choose refractory fillers such as high-alumina bauxite powder and zircon powder.
2. Increase the coating thickness, but the thickness cannot exceed the maximum required coating thickness. Too thick coating will cause casting defects such as coating peeling.
3. Increase the Baume degree of flow coating, the maximum should not exceed 28, otherwise the fluidity will be reduced.
4. Some castings are overheated locally, and flow coating is very easy to stick to sand. Applying high-refractory coating on the hot joint before flow coating can effectively prevent sand sticking.
5. Severe flow marks
Reason:
The coating has poor fluidity and high viscosity. The coating cannot drip when it flows downward, resulting in serious flow marks; the pressure is too high when the coating flows out, the distance between the flow coating rod head and the cavity surface is too close, and the coating liquid impacts the coating surface, resulting in uneven marks; the coating flow is small, the flow is unstable, and flow marks will be formed on the surface of the cavity.
Countermeasures:
1. Use a large flow rate to flow quickly from top to bottom during flow coating, and do not stay on the sand mold surface for a long time.
2. Improve the fluidity and leveling of the coating and reduce viscosity.
3. Increase the distance between the flow coating rod head and the cavity surface, generally 18-25mm is appropriate.
4. Use a fan-shaped flow coating rod head.
6. Lamination
Lamination is the superimposed texture produced when the cavity surface is flow coated from top to bottom or from left to right twice or more times.
Reason:
It is mainly caused by high sand mold temperature, high coating viscosity, small flow coating flow rate, and multiple flow coating.
Countermeasures:
1. The sand mold temperature just finished by the sand mixer is high, so do not flow coat immediately. Air cool according to the specific situation.
2. Reduce the Baume degree of the coating and improve fluidity.
3. Increase the flow rate to avoid multiple flow coating. By making flow coating machines of different specifications, the flow rate can be reasonably controlled. When selecting a pump, the head and flow rate should be slightly higher, and the liquid flow pressure is large. The liquid flow can be controlled by controlling the switch and other places to achieve the required application pressure and flow rate.
7. Paint splashing
Paint splashing causes paint drops to splash on the smooth coating surface.
Reasons:
This defect is mainly caused by excessive pressure at the flow coating outlet.
Countermeasures:
1. Reduce the pressure at the flow coating outlet. The thickness, length, surface roughness, and outflow position of the paint flow pipe will have a great impact on the pressure of the flow coating.
2. Do not flow coating the flow coating rod head vertically to the cavity surface to avoid paint splashing.
8. Sanding on the sand mold surface
Commonly known as “pulling”, sanding on the sand mold surface often occurs when the mold is used for a long time or the mold is unstable. After flow coating, the surface flatness of the sand mold is not enough and it is concave, which has a great impact on the appearance quality.
Countermeasures:
1. Repair the sanding part of the sand mold with paint paste before flow coating. The disadvantage of this method is that it needs to be left for a longer time after flow coating, otherwise the part repaired with paint paste will bubble.
2. After flow coating, use paint paste to repair the sanded parts, then use thinner to brush the paint paste flat, and finally ignite. This method is currently widely used, saving manpower and making up for the shortcomings of tooling and previous operations.
9. Uneven coating
During flow coating, the sand mold is often prone to thin coating on the top and thick coating on the bottom. When the viscometer rotor speed is fixed, the apparent viscosity of the coating decreases with the increase of shear time, and it can reach a constant value for a long time. If it is turned to a static state, its apparent viscosity gradually recovers with the increase of static time. This is the thixotropy of the coating. The thixotropy of the coating is too strong, which is conducive to leveling, but it is easy to cause excessive flow, making the coating thin on the top and thick on the bottom. Poor fluidity and small inclination angle will also cause uneven coating thickness. For water-based zircon powder coatings, a thixotropy rate of 9-12% is better.
10. Poor adhesion to the substrate
Countermeasures:
The general practice now is to use an adhesion promoter, which is a special treatment agent that can improve the adhesion between the coating and the substrate. It can effectively combine with the polar groups on the surface of the material to produce high adhesion between the layers and play a good primer role.
Summary:
1. The flow coating process is nearly ten times more efficient than the original brush coating process, which is very suitable for the flow operation of the casting production line.
2. The casting mold after the flow coating operation has a smooth surface, uniform coating thickness, dense, and clear contours. After casting, the surface of the casting is smooth, the surface roughness can reach more than Ra25um, and the casting size accuracy is high, reaching CT9 level or above in GB 6414-1999 “Casting Dimension Tolerance and Machining Allowance”.
3. Because the flow coating is uniform, the coating that flows down can be recycled. According to on-site measurements, the use of flow coating can save about 25% of the coating compared to the original.
4. After many experiments, it was found that when the Baume degree of flow coating is between 22-26, the fluidity of the coating is the best, the coating thickness is appropriate, and the casting defects are the least.
5. Reduce the pollution of the working environment, and the use of flow coating completely solves the pollution of paint dust to the air.
6. The adhesion problem between the coating and the substrate can be quickly solved by using adhesion promoters.