PAINT APPLICATION TECHNIQUES:
There are number of different methods which can be used for the application of paint coating. The choice of the application technique depends on the number of parts to be painted and the type of parts. The following are the different paint application techniques:
1. Conventional spray painting
2. Electrostatic painting
4. Roller coating
5. Flow coating
6. Dip coating
8. Silk screen coating
CONVENTIONAL SPRAY PAINTING:
It is either done manually using the simple hand gun or automatically using highly automated system with automatic screen washer and elaborate masks for multiple colour decoration. Large complex parts or low cost parts are not suitable for the automatic operation either due to their complexity or economic reasons. Basic components of spray painting are: 1. Gun 2. Mask 3. Spray booth.
Types of Gun:
1. Rotary Guns: Usually two guns are mounted so that they rotate around a part to get at hard to reach spots.
2. Reciprocating Guns: They have a to and fro action and travel on a curved traverse to paint the sides of complex parts.
3. Spindle machines: Here the parts rotate on a spindle while the guns are stationary. It can paint small parts from top and bottom at the same time.
4. Combined motion: There are two types: (i) Rotating guns and reciprocating parts. Or (ii) Rotating parts and reciprocating guns.
Masks: Sections of a part which are not to be painted should be shielded off using masks which are usually made by electroforming. They vary widely in complexity. However they can be classified in following four categories:
1. Lip mask: It is used for painting a depressed name or design. Here a lip of metal extends down the vertical side wall of the depressed design entirely or partially depending on the result required. The lip is thin and strong. The centres of circular letters or designs (A, 0, o, 6, 8 etc.) are securely held in the place by bridges. The fit and the lip of the mask ensure a clean sharp paint line. The draft angle of the depressed design is at least 5o.
2. Cap mask: It is used when the embossed name or design is to be shielded. The lip of the metal covers the vertical side walls entirely till the bottom and thus protecting the embossing.
3. Plug mask: It is used for protecting the depressed design. It is usually used for protecting the depressed design. It is usually used with transparent articles and articles where vacuum plating is required. Positive fitting is accomplished by directly electroforming in to the design. The plugs are cut out and finished with the proper radii and draft angles to facilitate painting. They are then suspended by fine wires, usually attached to a frame so as to provide a unit which can be handled in production.
4. Block cut out mask: It is used in filling depressed letters and calibration marks. For masking small letters, the openings in the masks will be so small that they will be immediately filled with paint, hence a cut out is made to enclose each character/s, (distinctive mark, sign or letter) to confine the paint to the intermediate area. The excess on the surface is removed by wiping or buffing, so that the depressed characters are left filled with paint. The block cut out masks can be designed to accommodate the articles made in multiple cavity mould which do not have very uniform dimensions.
While designing a mould, the probability of masking should be considered. The masks should be regularly washed during its operation or an automatic washing system should be incorporated. Three to four masks should be supplied for each part to facilitate the best production efficiency.
Washing of masks:
Most masks were washed with solvents but the restrictions on the discharge of solvents in to atmosphere require the cleaning of the paint masks with water based material. These are non-toxic, non-flammable and cheaper. Here a water soluble film is applied to the mask surface and when the mask is to be cleaned, it is immersed in hot water or a heated solution in which the protective film dissolves, along with the paint. However the paint does not dissolve and can be taken off mechanically.
In some automated and semi automated operations, a closed chamber is used. Choice of the booth depends on the plastic to be sprayed, the method of application and the rate of production desired. The basic types are:
1. Water wash booth: It provides a continuous water fall in the back of the booth to wash out the paint. It is useful in continuous spray production lines.
2. Filter booth: Here a filtering device to trap over spray particles is provided. It is suitable for long runs where slow drying or light viscosity materials are used. OR For intermittent or short run application. Roll type dispensing units are developed to replace the filters regularly.
3. Baffle booths: These are used where exhaust air do not have to be free of paint particles. It is suitable for intermittent production with quick drying materials. Baffles assure an even air flow distribution through the work area of the spray booth.