Sealing materials (glue) can be divided into the following two types in order to meet the needs of general coating operations and improve appearance.
① High-viscosity sealant: the viscosity is 70000~120000mPa`s, and it is coated with a high-pressure gun.
②Low viscosity sealant: the viscosity is 40000~70000mPa`s. Apply with low pressure gun and hand applicator.
The sealant coating process is still mainly manual coating in CHINA, and the rod-shaped nozzle is used to coat the guide device along the side of the plate lap. The automation rate of sealant coating in the highly modernized car body PVC coating line is 50%~80%, and some of them use manipulators for coating parts. In order to confirm the reliable sealing of the board lap, it is necessary to manually use brushes to even out the paint. Recently, Graco Company of the United States has launched a new type of precision swirl welding seam coating gun (graco precision swirl TM), which can be coated more accurately, improve quality of the sealing coating and reduce the cost.
In addition, it is difficult for the manipulator to coat on a continuous production line, which is generally intermittent (that is, coating when the body is stopped). It takes about 10 seconds to transfer the car body to the next process. This transfer time is in vain, and it is also a problem with automation. The operating environment for coating sealant on the underside of the car Some car body painting lines transfer the body on a skid with a swivel frame to pass through the PVC painting line, and the body can be rotated 90° during operation, or the desired angle.
The failures or coating defects that often occur in the sealant coating process are introduced as follows.
① Coating disadvantages caused by moisture absorption of sealing materials
The filler of the sealing material absorbs moisture, and foaming occurs after pre-drying, mid-coating and topcoat drying , not only on the surface but also inside the material. If it is a material with insufficient anti-moisture absorption measures, long-term moisture absorption after the topcoat is completed will cause disadvantages such as cracking of the coating film. The measure is to use calcium carbonate treated with anti-hygroscopic film. European cars use barium sulfate (BaSO 4 ) with low hygroscopicity as filler. In the painting production line, attention should be paid to the environment of the place where the car body is placed after the sealant is applied. During the long-term shutdown, the possibility of moisture absorption accidents caused by placing such a place is very high, and it needs to be placed after the topcoat is completed.
②Disadvantages of coating film cracking
After repeated drying, the coating film on the sealing film surface cracks. Repeated drying, the coating film is further cured. There is a difference in the expansion force between the sealing material of the lower layer and the coating film (the height of the sealant layer), and the coating film cannot follow the expansion of the sealant , and the coating film cracks and cracks due to the resonance with the sealant. In the two-color and three-color painting of automobiles, only the topcoat is baked more than twice, so it is necessary to improve the resistance to multiple baking.
③ Malfunction of the heating part of the sealant conveying device
PVC sealing materials have a high temperature dependence, which causes various failures. The material is high-viscosity, and the viscosity also changes with the temperature change, and the viscosity change causes the change of the discharge amount of the gun. There is a limit to the control of the discharge volume of the gear pump for the manipulator, and it is difficult to control the discharge volume during manual work. Therefore, the temperature of the material must be kept constant, and a temperature adjustment function must be provided on the material supply pipeline. There is also a sealant supply tank that also needs to be .
The supply piping is a circulation method, and the material temperature is set at 25~30°C. The circulation volume drops and the circulation stops. The material may stay in the same place for a long time, and the heating of the temperature adjustment heater will cause local heating, and the material on the side wall of the pipe will undergo a curing reaction (the temperature of the heater is generally around 100°C according to the set circulation amount).
The sealing material starts curing reaction at about 40°C, and particles are produced due to gelation in the piping. The particles accumulate in the small hole of the pressure reducing valve at the outlet of the material, and heat up under pressure, which accelerates the solidification of the particles and soon blocks the pressure reducing valve.
④Troubleshooting example of pressurizing pump
In order to easily obtain high pressure with a large discharge capacity, a plunger pump (drive method is electric or pneumatic) is used. Where material is sucked up slowly, air is sucked into the pump and accumulated. When the pump is pressurized, the air becomes a cushioning air cushion. When the supply of material stops, the air accumulated part will cause the sealing material to solidify in the pump due to the heat of compression.
⑤The filter is partially clogged
In order to prevent foreign matter from entering the pump, a filter is installed at the outlet of the pump and at the inlet of the material . The filter is a structure made of metal mesh that is easy to pass through. The precision is high, and it is difficult for the material to penetrate through the thin one. Due to the high resistance , it will generate heat and the material will start to solidify. In addition, after being left as it is, clogging occurs in a short period of time, and the output of materials stops.
Depending on the material, generally use a 40~60μm filter.
