Deposition of Charged Powder Particles

Abstract

The problems brought to the Study Group by Courtaulds Coatings concern the electrostatic deposition of powder paints onto an earthed metal workpiece. The process involves charging the paint particles as they pass through a 'gun'; they then travel towards the workpiece under the influence of aerodynamic and electrostatic forces. At present two types of gun are used, these are:
- the Corona gun in which particles are forced past a high voltage electrode which ionises the surrounding air and hence particles are charged by collecting ions (Paulthier Charging).
- the Tribo gun in which particles are charged in an earthed gun directly by friction.

The main differences between these two types of gun are:
- when using a corona gun the electric field is set up not only by the space-charge but also by the applied potential difference between the gun and workpiece.
- free ions constitute 99% of the charge in a corona system, whilst no free ions are present when using a tribo gun.

As the corona gun involves many more complicated physical processes, it is the tribo gun, with a single species of particle, which is considered in this report.

The main questions asked by Courtaulds were
- What factors affect deposition efficiency?
- How is deposition efficiency maximised?

This report is organised into three main sections.
Firstly, a tribo gun model is presented and non-dimensionalised, revealing that electrostatic and aerodynamic forces are in balance, whilst particle inertia and gravity may be neglected for the system in question.
Then, a one-dimensional model is considered which gives vital information about the orders of magnitude of relevant quantities at the workpiece and the effect of charge saturation.
Finally, a model is proposed for a narrow 'jet' of particles impinging on an earthed workpiece and it it shown that geometry is by far the most important factor affecting deposition efficiency.