Shrinkage, usually centered around a droplet or small piece of impurity, forms a circular edge around it. The primer can be seen as the substrate, and the topcoat can be seen as the primer. The edge paint film shrinks towards the center, and there is no paint film on the edge or the paint film is very thin. This phenomenon is mainly related to surface tension. When the surface tension of the coating is high and the surface tension of the substrate is too low, it can cause shrinkage.

The generation of shrinkage not only affects the appearance quality of the coating film, but also may expose the substrate and damage the integrity of the coating film. Moreover, the phenomenon of shrinkage can seriously affect production efficiency, as it cannot be eliminated by ordinary polishing and polishing methods after shrinkage occurs. If necessary, rework is also required. Severe exposed shrinkage usually requires polishing to the bottom layer before painting. Therefore, preventing shrinkage is crucial for improving production efficiency and ensuring film quality.

The main reasons for shrinkage are:

1. The reason for the coating itself;

① In the process of making coatings, the amount of leveling agent and defoamer used is insufficient or unevenly dispersed.

② The underlying coating contains organosilicon additives, and the surface tension of organosilicon compounds differs greatly from the coating used, which can also cause shrinkage.

2. Improper handling of coated workpieces

① The surface treatment of the coating is not clean and there is oil stains. When the oil removal process before coating is not clean, resulting in low local surface tension, the resulting coating is prone to shrinkage.

② The surface of the coating is uneven, too rough, or has pinholes. During the coating process, when the coating flows smoothly, the air in the pores will expand and bulge out the curing paint film, forming shrinkage pores.

③ Residual moisture on the surface of the coating after surface treatment. The evaporation of residual moisture during baking after coating can cause shrinkage of the coating film.

3. Poor construction environment

① When adding curing agent or diluent, the container is not clean, resulting in uneven preparation or impurity pollution during coating preparation.

② When changing the coating variety, the equipment was not thoroughly cleaned. New and old coatings are mixed, interfering with each other, and prone to shrinkage.

③ The compressed air used in the coating assembly line during painting contains oil and moisture, which seriously damps the coating.

④ When painting, it should be done in a harsh environment with dust.

⑤ Employees' clothing and low surface tension substances on their hands may cause shrinkage when brought onto the vehicle body during operation.

4. The process requirements for coating construction are unreasonable,

① The spraying film thickness is required to be too thin. The low surface tension part will migrate towards the high surface tension area and drive some paint materials to migrate together. If the liquid film is thin and there is no liquid to supplement, permanent shrinkage holes will be formed.

② Low viscosity. The components with low surface tension inside the coating will adsorb to the surface layer and drive some materials to migrate towards the surrounding area. If the viscosity of the system is small and the surface adsorption and material flow process are fast, the material flow driven by this flow will form shrinkage pores.

③ The air pressure of the spraying equipment is too low, which can easily cause shrinkage.

Solution:

1. Add surfactants with low surface tension and surface control agents with good compatibility to reduce the surface tension of the liquid film. Defoamers can be added to reduce the surface tension of the coating during the leveling process, allowing air to escape from the pores of the coating in a timely manner.

2. Pay attention to the amount of additives added, as excessive amounts may cause shrinkage due to incompatibility.

3. Add an appropriate amount of diluent (water) for dilution to avoid low viscosity.

4. Strictly control the pre-treatment of the substrate, polish it, and fully treat oil stains, moisture, etc. before construction

5. Therefore, when changing the coating variety, it is necessary to clean the spray gun, spray chamber, and other equipment thoroughly.

6. Preheat the coated material before painting to avoid air expansion and film breakage.

During the process of coating production, strict operation must be carried out according to the process requirements to avoid pollution problems.

8. The air compressor often releases water to remove oil stains and installs an oil-water separator.

9. Adjust the construction process to ensure appropriate film thickness for spraying

10. Construction in a clean construction environment to avoid dust and other impurities pollution