Model Engineers’ Workshop – August 2019

52 http://www.model-engineer.co.uk Model Engineers’ Workshop

RAL colour range.

Triboelectric Powder Coating

for the Home Workshop

R

ich vibrant colours more robust

than paint, photo 1. No chipping or

peeling. Perfect fi nishes for many of

our model engineering projects. No solvent

release, thick coatings and fast processing

times of less than 30 minutes are common.

Plastic coating powder colours are linked

to the RAL colour standard and the range

can exceed that of spray cans, photo 2.

Colours come in matt, gloss and satin as

well as transparent. There are fi nishes

which mimic chrome. Bright undercoats

can be covered with translucent overcoats

to produce more funky colours sometimes

associated with motorcycle and car

customising styles. The powder used in the

following work was triboelectric polyester

powder from Electrostatic Magic, photo 3.

Building a Triboelectric

Powder Gun

The challenge was to make my own

powder coating gun and to be able to coat

workshop projects in a manner that is as

convenient as liquid paint, but with the

additional advantages of powder coating.

I hoped to build a powder spray gun that

was safe and easy to use without using

high voltages or expensive kit, photo 4.

There is excellent information on the

web which enabled me to understand the

physics of electrostatic powder coating.

Some of the early patents described how

different forms of powder spray guns

worked. There is material on YouTube

as well describing DIY powder coating

techniques.

Corona Guns and Triboelectric Guns

All powder coating works by statically

charging the coating powder which will

then stick to any grounded object. Two

systems are in use. The corona system uses

high voltages to charge the powder while

Triboelectric guns use a friction eff ect to

induce the charge on the powder. Both

guns are available for DIY use. I chose to

develop my own version of a triboelectric

gun, due to its simplicity and ease of use.

The Corona Gun:

Although corona and triboelectric powder

guns look similar, they diff er in how

they work. The fi rst, and the one usually

found in commercial applications, is the

corona powder coating gun. This works

by exposing the powder stream to a high

voltage, usually 30,000 to 100,000 volts as

it passes through the gun. The high voltage

Chris Gabel looks at a practical alternative to paint.

creates a cloud of excited electrons at the

tip of the gun. This blue glow is a corona

and is an electrical discharge brought on

by the ionization of the air at the tip of

the spray gun electrode. This corona is an

intense cloud of negative ions. The powder

picks up a charge as it passes through that

cloud, fi g. 1.

Eastwood produces a corona gun for

home use. Self-building a corona gun

is probably not too diffi cult, but the

alternative triboelectric powder gun

described next is so much better with no

Health and Safety High Voltage dangers.

How Triboelectric Guns Work:

It is quite easy to grasp that in a corona

gun, powder becomes charged when

zapped with 30,000 volts. The triboelectric

gun however works just by rubbing

together two dissimilar materials. Surface

friction does it all.

The “Triboelectric Series” fi gure shows

the relative charging characteristics of

some common materials, fi g 3.

The red items tend to gather a positive

charge, the blue items gather a negative

charge. Note that polyester is well above

1

Workshop powder coated work.

2

3

Half Kg bags and powder gun jars.