Underbody Coating

SIGNIFICANCE OF UNDERBODY COATING IN CARS

AIM:

• To know about the coating of the cars and their importance.

NEED FOR COATING:

1. Protects against rust and corrosion:

Underbody of the car will be highly exposed to debris and water on the roads and most unnoticed part of the car as we cannot be able to inspect the underbody frequently. So, when it is continuously exposed to the water leads to the rust and corrosion of the underbody. This corrosion will make the floor of the car weaker and affects the resale value.

1. Protects vital components:

Some of the vital components of the car like exhaust, Transmission shafts will be under the car. If the were affected by the environmental and the road condition they become weaker may also fail and the need to change of those parts will arise. Considering Indian roads which have uneven roads, potholes with water stagnating need for replacing these parts will arise frequently which is not good for car and not economical.

STEPS IN COATING:

1. Pre-treatment
2. Electro Deposition

• Sealer Application

1. Primer Application

Pre-treatment:

Pre-treatment is preparing the surface for the application of primer. Component will have dirt and oil residue from Stamping and welding processes. This process consists of degreasing followed by Surface conditioning. Degreasing is cleaning of the surface done by Spray stage and a dip sequence where high pressure degreasing solution composed of alkaline salts and surfactants. Surfactants were the detergents to disperse the oil and grease in the surface. After the spray stage a dip is made into a same solution. Last stage of pre-treatment is phosphate coating where thin and dense layer of phosphate is applied over cleaned surface.

Electro Deposition:

Electro deposition is the coating process where the part is dipped into a solution containing coating material. A closed circuit is made between the part and the solution by dipping the part inside the solution where the solution consists of positive terminal and the part connected to negative terminal as we know opposite charges attract the anode which is the paint solution here will get deposited on the part.

Generally, in this process a rust prevention layer or anti corrosion layer is applied to the surface with the minimum thickness of 20 microns.

Sealer Application:

Sealant is applied using the machines or by manually. Sealants like PVC or acryl/urethane were used to as a sealant in underbody. Sealant will have the maximum thickness than any other layer in coating process to improve the anti-chipping property. Primary function of the sealant is to prevent chipping and to prevent the water ingestion to the metals which will result in rusting if exposed to longer period to water. In addition to above said process the sealants acts as a damper which will reduce road noise and the engine and transmission noise coming into the passenger cabin. Sealant will be applied in the lower areas of the body that are prone to chipping due to stones and debris on the road. The figure below shows thickness of the various layers of painting process.

Primer application:

Primary application of Primer is to leveller that is to fill and smoothen the minor imperfections such as scratches that are created during the previous processes of the coating. They improve the chipping resistance and binding between the electro deposition coating and the topcoat there by increasing the durability of the painting. Different types of primers such as powder based, Solvent based and water based were used in this method as the need to satisfy wide range of requirements such as the environmental conditions, Manufacturing processes and application process and associated coats.

Topcoat:

Topcoat consists of a base coat and a clear coat. The basecoat contains the primary colouring pigment, and the clearcoat provides a protective coating against environmental effects, corrosion, and UV light degradation, promotes unmatched colour retention, and provides a smooth, unblemished, and even finish. The body is first coated with either a water-borne or solvent-borne basecoat. Then if the manufacturer is using wet on wet processes where a second coat is applied over a first coat without curing the first coat if not then the part is cured in a over before applying the topcoat. As a final step a clear coat is applied over the topcoat which improves the durability, scratch resistance of the coating. It mainly protects the paint from the UV light which will fade the colours if clear coat is not applied. It protects the base coat from etching by the water marks, Bird droppings etc and makes maintenance easier.

Image below shows thickness of various layers of the coating processes.

CONCLUSION:

The processes of coating is well sophisticated now a days by automation where the difference in the thickness will almost be negligible. Manufacturers still try to maximize efficiencies and meeting environmental regulations with new processes and optimising the existing processes which improves the quality of the coating at low cost.