Automobile exterior bumper coating process
The painting process is the same as the car body. The car bumper painting process also has 3C2B and 3C1B. The factory uses the 3C1B process. The process flow is as follows: Pretreatment → moisture drying → flame treatment → primer spraying → leveling → color paint spraying → leveling → varnish spraying → leveling → drying → lower part polishing
The painting process is the same as the car body. The car bumper painting process also has 3C2B and 3C1B. The factory uses the 3C1B process. The process flow is as follows: Pretreatment → moisture drying → flame treatment → primer spraying → leveling → color paint spraying → leveling → varnish spraying → leveling → drying → lower part polishing
01 Paint
Like the types of paint used for the car body, the paint used for the bumper is also divided into water-based paint and solvent-based paint. Water-based paint is not widely used at present due to factors such as high cost, difficult construction process and poor weather resistance. Solvent-based paint (3C2B or 3C1B) is mainly used. Since plastic parts are not resistant to high temperatures, the paint used must be low-temperature drying type, and the drying temperature must be lower than 80℃. Solvent-based 1K primer and 1K color paint, 2K solvent-based low-temperature varnish are usually used. Since plastic parts are insulators, conductive primers are required to improve the utilization rate of paint. Air spraying is used when spraying conductive primer. The primer connects the bumper and the bracket to form a conductor. Electrostatic spraying can be used when spraying the subsequent color paint and varnish to improve the paint utilization and improve the spraying effect of the metallic paint. For the conductivity of the primer, the primer resistance is generally required to be <0.3MΩ. The higher the viscosity of the paint, the worse the atomization effect, which is not conducive to improving the spraying efficiency. Usually 13~20s (DIN4#/23℃) can be taken.
02 Flame treatment
The surface tension of plastic is lower than that of metal, and it belongs to low surface energy materials, so it is not conducive to the adhesion of the coating. In order to enhance the adhesion of plastic parts or release the internal stress of plastic parts, flame treatment, pre-baking, and wiping the surface of plastic parts with solvents to remove internal stress are required. If these surface treatment methods are selected appropriately, the construction performance of plastic parts will be greatly improved. The poor adhesion of plastic to paint is because the plastic molecules have small polarity, large crystallinity, low surface tension, poor wettability, and smooth surface. Direct spraying of paint can easily lead to quality defects such as paint film shedding. Using a robot plasma flame treatment process, a high-temperature propane or natural gas flame of 1000~2800℃ quickly contacts the workpiece surface, which can locally oxidize the surface molecules of the plastic parts to improve the wettability and adhesion of the surface. Another function of flame treatment is to eliminate burrs on the plastic surface and improve the spray pass rate. The characteristics of plastics and metals are fully utilized in one, and plastic electroplating products are used in electronics, automobiles, household goods and other industries. The effect of flame treatment needs to be tested with dyne solution. Apply dyne solutions of different concentrations to the surface of the bumper after flame treatment. If the solution remains uniform within 3s, it means that the surface tension after treatment is better than the surface tension of the corresponding concentration solution. If the solution shrinks into droplets, the surface tension is lower than the surface tension of the solution of this concentration. Generally speaking, the surface tension of the bumper is between the surface tension of the dyne solution with a concentration of 38%~44%. If the flame treatment is excessive, the surface tension of the bumper is too large, which is easy to cause the paint film to sag. It should be noted that after 20 minutes of flame treatment, the surface tension of the bumper will gradually recover, so it is necessary to spray within 20 minutes.
03 Process conditions
Painting environment: temperature requirement (23±2)℃, relative humidity requirement (65±5)%. According to experience, the temperature is low at night in autumn and winter in the north, and the temperature of the paint room is not easy to control. During nighttime production, the varnish flow rate can be reduced by about 3%, which can effectively avoid defects such as sagging and ensure continuous production. Leveling time: After spraying primer and color paint, a leveling time of 7~10 minutes is required to ensure that the paint is fully leveled and flash-dried. After varnishing, a leveling time of 10~15 minutes is required. Adequate leveling time can not only ensure a good appearance effect, but also avoid defects such as prickly heat. If the leveling time is insufficient, the amount of quick-drying solvent added to the paint can be adjusted to adjust the flash-drying effect of the paint film. Drying temperature: Plastic parts are not resistant to high temperatures, and the drying temperature generally needs to be <80℃. The drying conditions are 80℃/30min or 70℃/45min, and the heating rate is preferably less than 7℃/min. Wind speed: The wind speed must be greater than 0.3m/s, otherwise the overspray mist is difficult to control and easily contaminates the sprayed parts.
【Bumper painting process popular science】
Currently, typical bumper painting can be divided into the following processes: 1. Loading ⇒ 2. Water washing ⇒ 3. Blowing ⇒ 4. Drying ⇒ 5. Cooling ⇒ 6. Electrostatic dust removal ⇒ 7. Flame treatment ⇒ 8. Primer spraying ⇒ 9. Primer flashing ⇒ 10. Color paint spraying ⇒ 11. Color paint spraying ⇒ 12. Color paint flashing ⇒ 13. Varnish spraying ⇒ 14. Varnish spraying ⇒ 15. Varnish flashing ⇒ 16. Varnish drying ⇒ 17. Cooling ⇒ 18. Polishing ⇒ 19. Inspection ⇒ 20. Unloading
Loading
The main task of loading is to place the workpiece to be sprayed on the painting hanger, so that the workpiece to be sprayed passes through the various process sections of the entire painting line and completes the spraying process.
Water washing
The main function of the water washing process is to clean the surface of the product coating. Water washing can remove dirt, dust, grease, etc. on the surface of the parts. The standard water washing process is divided into five levels of water washing, namely, the first level of heated degreasing water washing, the second level of water washing, the third level of water washing, the fourth level of circulating pure water washing, and the fifth level of fresh pure water washing. The above liquids are used to wash the surface of the product at a certain pressure and temperature, thereby playing a cleaning role.
Water blowing
After the product is washed, the water droplets on the surface of the product need to be removed. The water blowing process is located after the end of the water washing channel. The fan blows clean high-speed air on the surface of the product through the air duct to blow away the residual water droplets on the surface of the parts. To ensure the water removal effect, the wind speed is usually as high as 30m/s or more.
Drying
Although there are no dense water droplets on the surface of the product after the water blowing process, there is still moisture on the surface that affects the spraying. The drying process is to use hot air to completely remove the moisture on the surface of the product.
Cooling
The surface temperature of the product coming out of the drying room is high and cannot be sprayed directly, so it must be cooled through the cooling channel.
Dust removal
Electrostatic dust removal The high-speed deionized air blown by the deionized air knife removes the static electricity on the surface of the product. Since the surface of plastic parts is a poor conductor of electricity, the static electricity accumulated on the surface of the bumper will absorb dust and affect the spraying, and static electricity will bring great safety hazards during spraying, so it is necessary to neutralize the static electricity on the surface of the product before spraying.
Flame
Flame treatment: The polarity of plastic parts is much worse than that of metals. The main purpose of flame treatment is to increase the surface tension of the plastic substrate surface so that the paint can better combine with the bumper substrate. The increase in the surface tension of the substrate improves the wetting performance of the paint during spraying.
Primer
Primer spraying: In this process, the primer is mainly sprayed on the product. The main functions of the primer are two points. The first is to provide adhesion between the entire paint layer and the plastic layer. The second is that for electrostatic spraying, the conductive primer layer provides a conductive base. Flashing Before spraying the next coat of paint, the product that has just been primed needs to have a flashing process. Flashing is actually to volatilize the solvent in the paint. Before spraying the next coat of paint, the surface of the previous coat of paint is basically dry, avoiding the influence of the solvent in the newly sprayed paint on the previous coating.
Color paint
One coat of color paint: The main function of color paint is to provide different colors for the product. The principle of spraying and the equipment used are similar to those of primer. In order to achieve the required paint thickness, color paint needs two coats of spraying.
Color paint
Two coats of color paint spraying: In addition to making the color paint film thickness reach the required film thickness, the second coat of color paint spraying has a great impact on the final product color. At present, the color of passenger cars contains a high proportion of colors with metallic effects, and the electrostatic effect of electrostatic spraying will make the metal powder in the color paint stand up, affecting the color effect, so the second coat of color paint spraying mainly uses an air spray gun as an atomizer. Flash dry Before the product sprayed with color paint enters the varnish spray room, most of the solvent in the paint on the surface of the parts needs to be volatilized in this area, and the color paint is basically dry before spraying the varnish. Varnish One coat of varnish spraying: Varnish is usually also called topcoat. Its main function is to make the product surface have a high gloss and provide the product with basic performance requirements such as weather resistance. The equipment for spraying varnish is basically the same as that for color paint.
Varnish
Two coats of varnish spraying: To meet the overall film thickness requirement of varnish, if two coats of varnish spraying are electrostatically sprayed, the general construction film thickness is about 70% of the total required film thickness, which can effectively improve the overall paint coating rate.
Flash drying
Flash drying of varnish: Before the product sprayed with varnish enters the topcoat spray room, it needs to pass through the flash drying area to make the varnish on the surface of the parts level and some of the solvent evaporate. The flash drying channel structure of varnish is the same as that of primer and color paint.
Topcoat
Topcoat drying: In this process, the product enters the drying channel, and the solvent of the sprayed paint layer gradually evaporates. Not only the varnish, but also the residual solvent in the primer and color paint on the surface are completely evaporated in this process. At the same time, the chemical functional groups in the paint react chemically to finally form a complete paint surface.
Cooling
After the product is dried, it passes through the cooling channel. The cooling of the product is mainly for the subsequent polishing process.
Polishing
Generally, impurities larger than 0.2 mm can be observed on the high-gloss paint layer. Therefore, polishing is required for defective paint surfaces. The polishing process can eliminate and reduce defects through grinding and polishing paste. Therefore, the coating pass rate directly affects the coating cost. Inspection Carry out appearance inspection and use different marks to distinguish qualified products, return spray parts, and scrapped parts for workers at the unloading station to treat them differently.
Unloading
The worker removes the qualified products from the hanger and leaves an empty hanger for the next round of loading.
