PRE TREATMENT  SYSTEMS

PRE TREATMENT  is a process in which the surface of the component to be painted is chemically cleaned and prepared for the painting operation.Metallic components can be freed from impurities with proper PRE TREATMENT, using degreasing and derusting solutions, coated with a protective layer of phosphates and passivated for lasting effects.
The process of pre treatment can be broken up into the following stages in Powder Coating System  :-
1. DEGREASING IN PRE TREATMENT  : In this stage, the component is freed of grease, oil and other unwanted contaminants like dirt
and swart adhering to its surface. Degreasing can be achieved by-
a) Spraying hot water or steam on the component. This is generally used where the component is very large and heavy, so that the operator can clean it by walking around it. For example, heavy castings and engines. As an alternative, kerosene or
petrol can also be sprayed.

b) Tri-choloroethylene degreasing. In this the component is suspended in a specially designed tank containing tri-choloethylene liquid or vapour. The tri-choloethylene condenses on the surface of the component, dissolves oil and grease and carries them away with it. The grease and dirt collect at the bottom of the tank in the form of sludge, which can be removed from time to time. This system has excellent oil and grease dissolving capabilities, but since there is very little mechanical rubbing action between tri-chloroethylene and the component surface one often finds a thin layer of dirt remaining adhered to the surface of the component just degreased. Also, tri- chloroethylene cannot dissolve and, therefore, remove any soap which is generally present in the coolants used during machining processes. It should also be noted that tri- choloroethlene, though quite harmless as such, decomposes when heated beyond is boiling point, forming very toxic products of decomposition. These are harmful and care should be taken to avoid this decomposition. With the tri-chloroethylene degreasing system there is a very high percentage of solvent recovery.
C ) Alkali degreasing. In this method the component is cleaned by a solution of alkali which generally contains sodium hydroxide (caustic soda), sodium carbonate(soda ash) and some sodium ortho or metasilicate. The solution often has an additive of detergent or a wetting agent to help the degreasing action. The degreasing is achieved by saponification of oil and grease and is assisted by mechanical agitation which gives a scrubbing action to dislodge the dirt from the component surface. As the degreasing action is a saponification process, the alkali solution is consumed as the process continues.
2. WATER RINSE : Whenever components are degreased by using alkali, they should be rinsed, preferably twice, to remove all traces of alkali which may have remained on the components.
3. PRE TREATMENT FOR  PICKLING OR DERUSTING : In this process, rust and scales are removed from the component by means of acid solution. Acids used are generally dilute hydrochloride or sulpuric along with an inhibitor to minimise their attack on the bare metal.
4. WATER RINSE : The above operation is followed by one or two water rinses to remove all traces of acid
5. PRE TREATMENT IN PHOSPHATING / CHROMATISING In this operation, the clean surface of the component is coated with a thin layer of phosphate or chromate salt by using a phosphating / chromatising solution.
6. PRE TREATMENTS IN WATER RINSE :The above operation is followed by one or two water rinses.
7. PRE TREATMENTS FOR PASSIVATING  :The surface of the phosphated component is chemically very clean and hence very easily susceptible to rusting immediately, unless passivated. The components are, therefore, dipped into a solution of hot chromic acid which tends to passivate the phosphated surface and protect it from rusting for a period of 12 to 24 hours until the components are ready for painting.

This briefly indicates the sequence of operations involved in PRE TREATMENT. The phosphated components can be, if absolutely necessary, stored overnight, but it is recommended not to store, them for too long a period, or else they will start rusting. It will also prevent any rusting underneath the paint film at a later stage after painting.

In most  PRE TREATMENT applications, the series of tanks are lined up in a row with an overhead monorail and hoist.

For very large production for PRE TREATMENT volume, an automatic dipping system with an overhead conveyor can be provided, so that there is no intermediate handling from tank to tank. For large volume PRE-TREATMENT of small items, the components in each basket can be transferred from tank to tank automatically. These are naturally expensive system to install.

One of the modern powder coating system PRE TREATMENT SYSTEM is called also  spray phosphate. The amount of solution required is also less, as it is being re-used by continuous recirculation, but he capital cost involved in setting up this system is very high. As a result, the use of Spray type of PRE TREATMENT can be justified only when the production volume is extremely high.

Enquiries are invited for complete systems consisting of PRE TREATMENT equipment, Powder coating system ,spray booths, flash-off chambers, stoving equipment and conveyors.