Types of Guns

Tribostatic guns

If a powder is brought into contact with a surface and is caused to move along that surface, the powder particles acquire an electrical charge.  This frictional charging is a function of, among many parameters, the chemical and physical structure of both powder and surface of contact, relative humidity and particle velocity.

Powder is fed to the gun from a fluidised bed powder hopper, from which it is drawn by vacuum created by a variable choke air injector.

The vari-choke controls the volume of powder delivered to the gun and the air injector pressure governs powder velocity, which is approximately proportional to the resultant charge.  No electrical controls are required, so that the operator has only three variables:

  • fluid bed pressure
  • injector air pressure
  • powder injector choke control to adjust

Advantages

1)    Low cost – no high voltage generator required.
2)    Good penetration characteristics into recessed areas.
3)    Spark free.
4)    Self-limiting coating thickness tends to be greater that that obtained from conventional electrostatic spray guns.

Disadvantages

1)    Because powders are of different composition in relation to polymer, pigment type and concentration, the charge potential varies greatly and in some cases so small is the charge pick-up that little or no powder deposition takes place.
2)    Charging efficiency falls off with increasing relative humidity.
3)    Charging efficiency decreases with increasing time of use.
4)    Powder deposition rate is slower than that of a conventional electrostatic spray gun, therefore production line throughput rates could be slower.

Electrostatic powder spray efficiency

For powder spray application, charging of particles is usually accomplished by corona discharge or ion bombardment.  This takes place at or near the point of exit of the powder, where the discharge electrode is situated at the gun head.  A high voltage is applied to the electrode so that a high electrical gradient is produced.  This creates an electrical corona, or air breakdown, in the vicinity of the electrode.  The gas molecules in the air become conductive when they are subjected to bombardment by the electrons which move freely at the discharge head.  Those particles which are charged to the sign opposite to that of the electrode are immediately drawn to the electrode.  Those of the same sign are repelled into the space about the electrode.  The powder particles are propelled through this space and, by collision or ion bombardment, the charge on the air particles is transferred to the powder particles and so they in turn become charged.  Once charged they move to deposit on the earthed workpiece.

Any spray gun will exhibit variations in performance as:

  • Its voltage is adjusted upwards and downwards.
  • Its powder flow rate and exit velocity are adjusted.
  • The distance from the gun exit to workpiece varies.
  • The powder particle size varies.

In addition guns of differing structure, such as guns from two different manufacturers, will behave differently when adjusted to identical delivery rate conditions.

The specific gravity of the powder, the volume, resistivity, the shape of the particles and particle size distribution are all recognized as being significant features affecting powder deposition efficiency.

The powder particle must be able to accept the maximum charge as it passes through the ion cloud and to achieve this as long a dwell time as possible in this area is recommended.  A powder particle of high resistivity is preferred as powders with a low resistivity will not deposit on the workpiece in a ready manner and dissipation or leaking away of charge once deposited on the article could lead to powder particles falling off and therefore not producing a uniform coating.

The ‘Transfer Efficiency’ of a powder can be expressed as a ratio of weight of powder transferred to the workpiece in a given operation, divided by the total weight of powder that is passed through the application spray gun in the same operation.

The efficiency of any given operation is affected, not only by the powder properties and spray equipment, but also by the size, configuration and dwell time of the workpiece in the electrostatic powder spray.  For flat panel components Transfer Efficiency is higher than, for example, parts fabricated from bent and welded rod or tubing.

In electrostatic spray application, the charged particles move towards and deposit on the earthed workpiece.  A charged layer of powder therefore builds up on the surface of the workpiece.  Since the charged powder which has already deposited on the surface will tend to repel those particles which arrive later during the building up of a heavy film, the transfer efficiency will fall off as the coating becomes heavier and heavier until a point is reached where no further powder can be deposited.  Although this self-limiting factor was considered as being the fundamental factor in causing progressive repulsion of the charged powder particles, it has been suggested that a reverse discharge occurs in the powder layer as deposited, when ions stream out of the deposited layer towards the gun.

It has been shown that this discharge is accompanied by constant ejection and eruption of the powder particles.  If the discharge is localised, ‘pin-holing’ or moon-cratering’ can result.