Understanding the Basics of CED Paint

CED Paint is the most expensive coating process in automotive industry and rightfully so since it is the basis of car’s durability. The life of car depends on it. Also in Pakistan in order to apply for automotive Greenfield status, the company must have ED process.

Like any paint CED paint is a mixture of resin and pigment dissolved in a medium. Generally resin is epoxy, pigment being Titanium oxide and medium is DI water with some traces of thinner also called solvent in this context.

Resin and Pigment constitute the NV part of paint short for Non Volatile aka solid content aka heating residue. Resin forms the paint coating upon crosslinking in oven whereas pigment aka ash gives color to the coating. In a typical ED bath NV constitutes 20% of total composition.

In order to keep the solid content of paint suspended, neutralizing acid is used. The amount of neutralizing agent is monitored as MEQ.

The bath functions on the principle of electroplating where body acts as cathode in other words the body is grounded and voltage is supplied by an array of anodes.

At the application of VDC, the paint migrates towards cathode where paint is discharged and deposits on the surface of body. This is also accompanied by origination of OH- ions at cathode which increases PH of bath. On the other hand acid in bath migrates towards the anode through anode membrane. 

The above chemical phenomenon increases the PH of bath beyond its limit therefore neutralizing agent must be constantly supplied to the bath. MEQ values must also be kept in check as higher MEQs will cause inhibition in the deposition on paint film as columbic efficiency reduces.

Side electrodes are tubular and comes with anolyte circulation system. However bottom electrodes are bare and do not control acid levels in bath therefore bare electrode to tubular electrode ratio must be considered while designing ED system.

The ramp up voltage of rectifier must be set with consideration as voltage without soft start would cause gas pinholes. Gas pinholes may also arise if solvent levels are down which in turn makes the wet film hard.

The above article summarizes the basics of CED painting. For an efficient CED coating an effective UF system is of paramount importance. Also phosphating rinse carryover must be closely monitored.

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