Underbody Coating
UNDERBODY COATING OF BIW:
i) CHEMICAL CLEANING :-
The bodyshell (BIW) comes from the BIW construction facility and are chemically pre-treated in order to ensure optimal cleaning of the BIW from grease, Dirt, Aluminium residues and prepared for the next step. key parameters from the chemical manufactures are given which have to be met by adjusting the dosage of the chemicals. if the dosage isnt met according to the manufacturers specifications, it may lead to ugly flaws in the paint work.
ii) IMMERSION BATH:
Following the chemical cleaning, is a very special "Immersion bath". in this the BIW is immersed in a tank, the vehicle receives a special corrosion protection.
In the Immersion bath, there is a special "Electro-diposition" coating in a completely "De-salinated" water at a temperature around 33 degrees. Between the tank walls and the BIW, an electric current of 330V is applied, intiating and electrochemical process. as a result the "Positively" charged paint particles are automatically attracted to the "Negatively" charged car body. The result of the "Cathodic Electro-deposition", an extrememly uniform coating of cavities and metal surfaces is acheived. after the Electro-deposition coating, the BIW dries at 175 degrees for 50 mins. The car body is immersed in an angled manner and rotated 360 degrees to ensure that every hole and cavity of the body get even coating of the ED coated.
iii) PRIMING:
Absolute cleaniness is the highest priority. Not a speck of dust is allowed to come between the car body and the coat of paint. Orelse paint flaws may occur. The robots apply the so called "filler Paint" or "Primer coat". The color of the primer is based on the final paint color of the color. "White Filler" for light exterior colors, "Anthracite" for "Dark" Exterior colors and "Medium Gray" for the Hues in between are used. The Filler balances out any remaining uneven surfaces in the body and has UV protective charecteristics. It also enhances the brilliance of the paint finish. The filler needs to dry for about 40 mins and may vary with different manufacturers before applying the final coat of paint.
iii) SEALING WELDS:
Robots in "splash guard covers" apply sealant to the welds using sperately calibrated "Nozzle's". In addition, the seal the underbody in order to protect it from stones and other road conditions. for sports cars, the sealant used is a low density PVC material. It has the advantage that it can be applied at the same volumebut lower weight for the the carbody, which relavant specifically to the sports car segment. Between 6-8 kilograms of PVC sealant applied per car body.
In this process, the sealin gjoints need to have perfect coverage in order to prevent moisture from later penitrating into the BIW. In order to check how the sealant welds look on the BIW. "Splash Guards" are produced. In this process the robot applies weld to a sheet metal and the welds are compared 1 to 1. the "look of the weld", "join of the weld to the Base material:, and the "Weld Width" is inspected. Once Underbody protection is done, the body advances to the next stage which is painting.
