1. Raw materials
The wave-absorbing coating is composed of absorbent, adhesive and filler. At present, the commonly used absorbents in China are carbonyl iron powder and superfine alloy powder. Coatings with high density, high absorbent content, poor dispersion of coatings, and large design thickness of coatings can obtain uniform coatings with uniformly mixed coatings. Therefore, the requirements for raw materials are: choose adhesives with good solubility, A diluent with strong dissolving power and an anti-settling agent added to the filler. In order to ensure good dispersion of absorbent and resin system, JSF450 agitating sand-milling and dispersing multi-purpose machine is used to agitate the paint. To judge whether the paint is evenly mixed, use a stirring rod to pick up the paint visually, there are no obvious particles on the stirring rod, and the paint color is uniform as qualified.
Excellent pre-treatment is the guarantee of good adhesion of the absorbing coating, and a clean, rough surface to be sprayed must be provided. Usually choose anodizing, phosphating, coupling agent treatment and other methods for pre-treatment. For workpieces without surface treatment, use emery cloth with d=150gm to grind horizontally and vertically until there are obvious lines on the surface of the substrate. Generally, water-based cleaning agents or organic solvents are used for cleaning operations. The cleaning agent should be neutral or weakly alkaline. It should be cleaned with clean water after staying on the surface of the substrate for 3 to 5 minutes; the organic solvent should be gasoline or acetone for aviation washing, and wipe it with a white cloth without lint when the gasoline or acetone has not evaporated. Dry to avoid the high boiling oily substance in the organic solvent from contaminating the substrate again. Taking the water film continuous 30s as the qualification criterion, the surface after cleaning with the water-based cleaning agent should be tested with pH test paper to test the surface acidity and alkalinity, and the neutrality is qualified. It is forbidden to touch the surface after the pre-treatment and cleaning with bare hands.
3. Spraying process
(1) Spray primer
Primer is the basis of the overall coating. Due to the large thickness of the wave-absorbing coating, strong solvent solubility, and long curing cycle, the primer is required to have strong solvent immersion resistance. Generally, single-component primers are not used. The quality requirements for the primer are: ① Ensure the effectiveness of the primer. Mix in proportion, use after curing for 30 minutes, and use up within the applicable period. ②The primer should be fully cured. Generally, the curing cycle of the primer is 24h at room temperature and no less than 8h at 60℃. If the production cycle permits, prolong the curing time as much as possible. ③The primer viscosity cup test is controlled within 16-20s, and the cross is sprayed again, and the dry film 8 is controlled at about 20gm.
(2) Spraying absorbing paint
The spraying of absorbing materials must ensure the uniformity of the coating on the one hand, and control the thickness of the coating on the other hand. ①The uniformity of the coating. The compressed air pressure used by the air spray method can reach 0.6MPa2. Under the push of high pressure, the coating will rebound when it is attached to the surface of the substrate. Uniformity of coating. In the spraying process of the wave-absorbing paint, the pressure of the spray gun and the amount of paint can be adjusted, and the high flow and low pressure state can be selected to reduce the rebound of the paint. During the spraying process, in order to avoid layering of the paint in the spray gun tank, the gun tank should be shaken frequently. You can also block the nozzle of the spray gun with your fingers to allow compressed air to circulate in the gun tank to stir the paint. When the paint is placed for 20 minutes, the gun tank should be opened and stirred. coating. It is not easy to control the spray volume of the paint when starting and stopping the gun. You can set the position of starting and stopping the gun outside the spraying surface and minimize the frequency of starting the gun. When spraying small parts, rotate the workpiece 90° each time you spray. When spraying large parts, the operator can move the body steadily along with the profile of the product, and try to spray a full stroke in one shot. ②The thickness of the coating. The reflectivity of the stealth coating is closely related to the thickness of the coating. In some frequency ranges, when the coating does not reach the critical thickness, as the thickness of the coating increases, the overall reflectivity of the coating becomes better; after the critical thickness is exceeded, as the thickness of the coating increases, the reflection performance of the low frequency band will be affected. Improved and lower high frequency reflection performance. Therefore, the thickness of the coating should be controlled according to the characteristics of the absorbing coating to meet the reflectance requirements. The reflectance curve of a workpiece obtained by the RCS test method in a microwave darkroom is shown in Figure 3-11. Figure 312 shows the reflectance curve obtained by reducing the thickness of the coating by 0.05mm by grinding. The thickness of the absorbing coating is closely related to the stealth performance, but the accurate measurement of the thickness can only be carried out after the coating is solidified, which is not conducive to the production operation. In the actual spraying process, the quality of the coating can replace the thickness to control the spraying amount. In the process of spraying products, you can spray a follower plate of 180mm×180mm at the same time, and measure the mass increase of the surface coating of the plate to be analogous to the thickness of the surface coating of the product. For example, the surface density of a certain type of wave-absorbing coating is 3.0kg/m2, and the dry film mass increase of the surface coating of a follower plate of 180mm×180mm will reach 97.2g. Considering the solvent volatilization factor during the drying process, when the wet film quality of the surface coating of the follower plate increases to 105g, it can be considered that the thickness of the surface coating of the product has reached the specified value. The same operator uses the same spraying process parameters to prepare at the same time, and the difference between the paint rate of the product and the plate must be considered.
4. Coating detection
After the coating is cured, a special thickness gauge needs to be used to measure the thickness, and the selected measuring points should be representative. For the plane part, the measuring points can be selected uniformly, generally not less than 2cm from the edge of the coating. There will be errors in the measurement data for parts with a small radius of curvature. You can also use the stealth material reflectance on-site measuring instrument to complete the detection of the stealth coating reflectivity. When using this instrument, the calibration is required to be accurate, and the accuracy of the measurement is mainly guaranteed by the calibration. During the measurement process, attention should be paid to the close contact between the test antenna and the test surface of the instrument to reduce the influence of the surrounding environment on the reflectivity. When the local thickness or reflectivity of the coating does not meet the requirements, it can be repaired by grinding or spraying.