Model Engineers’ Workshop – August 2019

August 2019 53

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Powder Coating

Tefl on (PTFE) meaning that the polyester
powder will become positively charged
when forced into frictional contact with
PTFE. There are no high voltages in a
triboelectric gun. All of the electrostatic
charge is created by friction of diff erent
molecules rubbing against each other.
Using the example of rubbing a toy balloon
on your hair, you can create a triboelectric
charge on your hair and on the balloon. A
charge is imparted in the balloon by the
frictional contact with your hair. As the
balloon rubs your hair the balloon grabs
electrons from the hair leaving the tips of
the hair with a positive charge. The hair
is attracted to the balloon, as dissimilar
charges attract. At the same time strands
of your hair fan and spread out, as each
fi bre is positively charged and like charges
repel each other. For the construction of

this triboelectric gun, we simply have to
force the polyester powder material into
frictional contact with Tefl on (PTFE). The
polyester particles will be stripped of their
electrons, therefore gaining a positive
charge, the moving powder cloud continues
on. It is attracted to and sticks to the
grounded workpiece, fi g. 2.

Reduction of the Farady Eff ect:
There are other advantages gained with the
triboelectric powder gun. The corona gun
can impart unwanted charges in cracks,
pockets and crevices and inside corners of
the target piece. These charges repel the
powder and cause thin or bare areas in the
fi nal coating. In addition surface charging
can cause an unwelcome orange peel
surface. The triboelectric gun does neither
of these. Corners coat well and there is a
reduction of orange peel eff ect.

The Powder Coating Process:
The process itself is not complex, but
the quality of the resulting finish is
dependent upon good craftsmanship at
each step. The first step is to prepare the
object to be coated. This is probably the
most important step. Just as in soldering,
using adhesive, or painting, the object
should be surgically clean and oil free.
Sandblasting is the preferred method,
followed by a wiping down with acetone.
Mechanical cleaning with files and
abrasives works too but consistency is

Fig.1

more difficult to attain.

Sandblasting also leaves a light “tooth”,

which aids in surface adhesion. Nitrile

gloves are suggested in order to prevent

fingerprints, photos 5 & 6.

Items such as cast iron are naturally

porous and therefore oily and should be

de-gassed. This is done by baking the

object for 20 minutes at 220 degrees C.

If this step is missed, the oily inclusions

can release vapor during curing which will

degrade the coating.

The Second step is to hang or securely

support the object on the coating rack

and to attach the ground cable, photos

7 & 8.

All about Grounding:

You can see from the fi gures that being

able to ground both the gun and the target

object is of prime importance and is what

makes powder coating work. There are two

ways to create a good ground. The method

used by almost all of the DIY coating

4

5

A siphon feed powder gun.

Schematic of a Corona powder gun. Schematic of a Triboelectric powder gun

Sand-blasting is the ideal preparation.

Fig.2