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Solutions to ten common problems in foundry coatings (Part 2)

Time：

2022-09-21

source：

Weiye New Materials

The lamination is the superimposed texture produced when the cavity surface is flow coated twice or more times from top to bottom or from left to right.

6、 Lamination
The lamination is the superimposed texture produced when the cavity surface is flow coated twice or more times from top to bottom or from left to right.
Cause:
It is mainly caused by high sand mold temperature, high paint viscosity, low flow rate and repeated flow coating.
Solution:
1. If the temperature of the sand mold that has just run out of the sand mixer is too high, do not apply it immediately, and cool it with air according to the specific situation.
2. Reduce the Baume degree of paint and improve the fluidity.
3. Increase the flow and avoid multiple flow coating. The flow shall be reasonably controlled by making flow coating machines of different specifications. When selecting the pump, the head and flow shall be slightly higher, and the liquid flow pressure shall be large. The liquid flow can be controlled by controlling the discharge at the switch, etc., to achieve the required coating pressure and flow rate.
7、 Paint spatter
Paint splashing makes bright and clean coating surface splash with paint drops.
Cause:
This defect is mainly caused by the excessive pressure at the flow coating outlet.
Solution:
1. Reduce the flow coating outlet pressure. The thickness, length, surface roughness and outflow position of the pipeline where the coating flows will have a great impact on the pressure of the coating, and the outflow pressure P of the coating must not be less than 0.4 × 105Pa。
2. The flow coating rod head and the cavity surface shall not be applied vertically to avoid splashing paint.
8、 Gripping of sand mold surface
It is commonly known as "galling". Sand scraping on the sand mold surface often occurs when the mold has been used for a long time or the mold is unstable. The flatness of the sand mold surface after flow coating is not enough, which has a great impact on the appearance quality.
Solution:
Method 1. The sand scraping part of the sand mold shall be repaired with paint paste before flow coating. The disadvantage of this method is that it needs to be placed for a longer time after flow coating, otherwise the part repaired with paint paste will blister.
Method 2. After flow coating, use paint paste to repair the sanding position, then use thinner to brush the paint paste flat, and finally ignite. At present, this method is widely used, saving manpower, and making up for the shortcomings of tooling and pre sequence operation.
9、 Uneven coating
During flow coating, the sand mold is often prone to thin coating and thick coating. When the rotational speed of the viscometer rotor is fixed, the apparent viscosity of the coating decreases with the increase of the shear time, and can reach a constant value for a long time. If it is turned to the standing state, its apparent viscosity gradually rises with the increase of the standing time, which is the thixotropy of the coating [2]. The thixotropy of the coating is too strong, which is conducive to leveling, but it is easy to cause excessive flowing, making the coating thin and thick. Poor fluidity and small inclination angle will also cause uneven coating thickness. For water-based zircon powder coating, the thixotropy M=9% - 12% is good.
10、 Poor adhesion to substrate, paint peeling
In the process of coating and spraying, due to the poor adhesion between the substrate and the paint, the paint often falls off, resulting in a high rate of defective products and serious damage to the quality and production cycle.
resolvent:
Now the general practice is to use adhesion promoter, which is a special treatment agent that can improve the adhesion between the coating and the substrate. It has special functional groups and can effectively combine with the polar groups on the surface of the material to produce high adhesion between layers and play a good role in primer.
Summary:
1. The efficiency of the flow coating process is nearly ten times higher than that of the original brush coating process, which is very suitable for the flow process of the casting production line.
2. After flow coating, the surface of the mold is smooth, the coating thickness is uniform, dense, and the outline is clear. After casting, the surface of the casting is smooth and clean, the surface roughness can reach above Ra25um, and the dimensional accuracy of the casting is high, reaching above grade CT9 in GB 6414-1999 Dimensional Tolerances and Machining Allowances of Castings.
3. As the flow coating is uniform, the flowing coating can be recycled. According to the field measurement, the flow coating process can save about 25% of the original paint.
4. Through many experiments, it is found that when the Baume degree of flow coating is between 22-26, the fluidity of the coating is good, the coating thickness is appropriate, and the casting defects are less.
5. The pollution of working environment is reduced, and the air pollution caused by paint dust is solved by flow coating.
6. The adhesion between coating and substrate can be solved quickly by using adhesion promoter.