Summary of “Painting makes the difference” by Indra
In the paint-bake process, the typical processes include pre-cleaning, multistage phosphate
pretreatment, electro-deposition of primer coating, underbody PVC and sealer application, filler primer
application, top coat application, and wax application (see graph 1.1).
Graph 1.1
1. Pre-cleaning: Dust and dirt on the body-in-white from previous processes are removed.
2. Multistage phosphate pretreatment: The main objective of phosphating (application of zinc
phosphate) is for good adhesion and assisting the protection of the metal surface against
corrosion. Phosphate coating texture, crystal size, morphology, and coating weight are
controlled and manipulated by proper surface conditioning for the desired quality out of the
pretreatment process. Between these stages, the bodies pass through the rinse zones. At the
end of pretreatment line, passivating and neutralizing are carried out, and a final rinsing is done
with fresh demineralized water. Pretreatment is carried out in number of stages as in the table
below:
3. Electro-deposition coating: Electro-deposition (ED) works similarly as eletrowinning processes.
There are two types of ED: Anodic ED (the metallic car body is made positive electrode) and
Cathodic ED (the car body is made negative electrode). Because of the voltage created between
the electrodes, charged particles from the paint emulsion move to Anode (Anodic ED) or
Cathode (Cathodic ED) under electrical forces. This happens during the period when the metallic
parts are completely submerged in the paint bath. The bath is maintained at a temperature of
25-27◦C (pH-value of 5.6-6.6 and solid contents 16-20%) with deposition voltages between 350500V. The direct current established through the bath makes the pigment and resin base of the
paint wander towards the body surfaces and forms film. As the film formed has high resistance,
further deposition ceases, and coating reaches all the recessed area. The deposition process is
followed by a rinse stage to remove any paint particles that are still not electrically deposited.
The deposited film is further sent to the stoving process, and the ED-coated body is cured at
165-180◦C in the oven to set the paint prior to sanding operation. The typical process sequence
of ED system is shown as below:
4.
PVC underbody coating &sealing application: Underbody coating and sealer application is
aimed to weld joints and other areas that are prone to corrosion. The purposes are to increase
impact resistance and to provide a protective function with regard to weather effects. Executed
by programmed robots, sealants are applied to gaps, cracks and seams. The seam seals must be
placed within seconds and with an accuracy of approximately 1mm. At the end, the sealer is
baked at about 160◦C.
5. Filler primer & base and/or top coat applications: Generally, paint baking of aluminum
automobile body involves 3 coat (primer surface, base coat, top coat)-3 bake or 2 coat (primer
surface, top coat)-2 bake process. Some unique differences in the processes are observed from
manufacturer to manufacturer. The thick filler primer application is used to remove any
contaminations from previous processes and smoothen any roughness of the surfaces. After the
“flash-off,” the body is sent to an oven for about 30 minutes at about 170◦C for baking of primer
layer. Before moving to the base and/or top coat, the metallic body is wet-sanded, rinsed, and
dried off. Top coat operation determines the visual appearance of the car. Depending on the
manufacturer’s process plan, the body is either moved through a top coat application booth or a
combination of basecoat and topcoat booths. Between each coat, a flash off is applied. The
body is then baked for about 30 minutes at around 160◦C.
6. Wax application: This application is used to protect the body against corrosion by applying a
protective film of wax either by the manual spray method or by automatic flooding method.
 Manual method: a solvent containing preservative wax is sprayed into the box sections
and cavities in the body with spray nozzles.
 Automatic flooding method: the body is first preheated to 50-80◦C and then hot wax
(solvent free) at about 120◦C from a flooding tank is pumped via automatic nozzles into
specific box section openings of the body.
**Reference
“Painting makes the difference.” Indra’s drishtikona (Viewpoint) 4 Aug. 2007. 11 Sept. 2011
<http://www.drishtikona.com/books/automobile-manufacturing/ch7.pdf>