Page 436 - Industrial Power Engineering and Applications Handbook

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uniform electrostatic field. Since the field is the same at between the gun and the object and also maintain the
all points of the metal surface, it attracts the charged paint thickness. Grounding may be performed by the
powder paint particles with equal force and allows only hooks holding the workpiece. The highly charged paint
as much paint at the surface as is actually required and is released through the gun at high pressure and is attracted
for which the spray gun is set (Le. the electrostatic field). by the oppositely charged object, to which it uniformly
adheres. Both sides of the object may be painted at the
Principle of electrostatic technique same time.
Due to the diminishing strength of the electrostatic
Similar electric charges repel and opposite electric charges field, which will depend upon the thickness of the paint
attract each other. This is the principle on which the already coated, the spraying process provides a uniform
process of electrostatic spray painting is based. The paint coating on the entire surface. The process may even be
particles are charged with a strong electrostatic field, made automatic through tracer guns, which may be set
which is created at the tip of the electrostatic spray gun, to move on a set pattern, like robots. For more details
as a result of high potential difference between the tip of contact the manufacturers. This system may be more
the gun and the object. The gun is maintained at a high advantageous for industries that have jobs of a repetitive
negative potential with the help of a power pack unit, nature and in large quantities such as cars and home
generating adjustable d.c. output, in the range of 0-100 appliances.
kV, negative (Figures A13.7(a) and (b)). The variable
potential difference is necessary to adjust the distance Powder paints
Powder paints are dry coating materials and are in the
form of dry polymer powders. They are electrostatically
charged and applied with force to the surface to be painted.
Electrostatic gun On heating, they polymerize and form a tough film. A
surface so produced will generally be better than the
conventional wet lacquer paints, for obvious reasons.
There are no solvents or thinners involved. The powder
is made of resin hardeners, crosslinking components,
?
pigments, extenders and additives etc. The blend is mixed
in an extruder at a temperature of lOO"C, cooled and
granulated and sieved to separate out grits. This process
causes no blisters on the surface. It is a single-coat process.
In one pass a thickness of up to 60-70 microns can be
obtained. The oversprayed powder can be recovered and
re-used.
Curing of paint
The paint is then cured in an oven. The curing time will
depend upon the thickness of coat, shape of the workpiece
and type of oven and its effectiveness. Generally, a coat
of up to 60-70 microns at a stoving temperature of 180-
200°C (depending upon the type of powder) should take
around 10-12 minutes to cure. Contact the manufacturers
for exact details.
Figure A13.7(a) Electrostatic spray gun with the power pack
unit (Courtesy: Statfied) Precautions to be observed
The storage and processing of powder paints must be
carried out under well-controlled conditions, and prefe-
rably away from fire hazardous areas. The powder
on mixing with air becomes inflammable and can cause
an explosion. Powder paints should be stored at about
25°C.

Equipment required
Electrostatic gun, with built-in protection to cut off
power when the gun is too close to the grounded
workpiece.
HT variable d.c. generator (0-100 kV negative) from
a single-phase LT power source. The power required
Figure A1 3.7(b) Electrostatic gun is around 60 W only.