Processing methods painting

Electrodeposition This method of paint application is basically a dipping process. The paint is water-based and is either an emulsion or a stabilised dispersion. The solids of the paint are usually very low and the viscosity lower than that used in conventional dipping. The workpiece is made one electrode, usually the cathode, in a d.c. circuit and the anode can be either the tank itself or suitably sized electrodes sited to give optimum coating conditions. The current is applied for a few minutes and after withdrawal and draining the article is rinsed with de-ionised water to remove the thin layer of dipped paint. The deposited film is firmly adherent and contains a minimum of water and can be stoved without any flash-off period. This process is used for metal fabrications, notably car bodies. Complete coverage of inaccessible areas can be achieved and the corrosion resistance of the coating is excellent (Fig. 14.1). 

SABRE resins are easily fabricated on conventional plastic processing equipment using established plastics industry techniques. Parts fabricated from SABRE resins are also readily finished or decorated by a variety of conventional methods (painting, hot stamping, vacuum metallizing, and plating). 

Application Methods and Surface Preparation. Eor good durabiHty and performance, proper surface preparation and correct appHcation of house paints are as important as the formulation of high quaHty paint. Proper surface preparation prior to painting involves several considerations. Eor new constmction, proper installation and protection of the substrate material are necessary. Eor previously painted surfaces, preparation involves mostly cleaning and removing any existing paint that is unstable. Once surface preparation is complete, the appHcation process can begin. 

Additional complications can occur if the mode of deformation of the material in the process differs from that of the measurement method. Most fluid rheology measurements are made under shear. If the material is extended, broken into droplets, or drawn into filaments, the extensional viscosity may be a more appropriate quantity for correlation with performance. This is the case in the parting nip of a roUer in which filamenting paint can cause roUer spatter if the extensional viscosity exceeds certain limits (109). In a number of cases shear stress is the key factor rather than shear rate, and controlled stress measurements are necessary. 

Electrostatic method This is also a w et method like the conventional process, except that the paint is now electrostatically charged, similar to the powder ptiint in a dry method as discussed later. The paint, being highly charged electrostatically, is wrapped around the object automatically. 

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