Electrostatic spray technique

Electrostatic spray

The powder is usually contained in a hopper adjacent to the application booth.  It then passes through the electrostatic spray gun, which charges the particles on emission, causing them to deposit on the earthed workpiece.  Subsequent fusion and curing takes place in an oven.  Powder not deposited on the workpiece is collected and re-used.

Powder feed

There are three techniques for delivery of powder from the powder storage hopper to the spray gun, all using air as the carrier medium.

The basic principle is that the powder supply should be controlled so it flows to the gun at a steady rate, free from surges or pulsing.  Controlling the powder and air volume independently usually ensures the proper ratio can be delivered.  The powder feed should be so designed that the discharge pressure and velocity feed to the electrostatic gun, even at some distance from the gun, are maintained.

Most hoppers are similar in design  – they are cone-shaped at the base.  The powder exit zone varies in diameter according to the method used for powder transit.

In the worm-screw feed system, the powder is screw-fed from the hopper into an air stream, where it is picked up and delivered to the gun.  This method of delivery can be finely adjusted to give a constant and reproducible flow of powder.  This technique overcomes the initial puffing which can be experienced with other forms of powder feed from the gun when spraying recommences after a shut-down period.

The venturi is only activated, as in the other methods, when the gun trigger is operated.  Also, an additional air control valve serves to vibrate the powder hopper to eliminate any tendency of the powder to bridge in the centre, so causing insufficient, or preventing altogether, powder from flowing into the venturi.  An alternative method is the use of a rotating bridge-breaker.

In the third technique, the powder contained in a flat-bottomed hopper can be fluidised and delivered to the gun by utilising a vacuum created by a variable choke injector.

It is advantageous that the powder feed line connections, ie. at hopper and at gun, are of the snap-on variety as this facilitates speed of line cleaning and changing from one colour to another.

High voltage supply

The high voltage generator, usually specified to deliver variable voltages between 30 & 100kV, is generally situated in close proximity to the powder feed hopper and spray equipment.  The gun handle should have a total surface area of at least 20cm2 of either metal or a low resistivity material which is connected to the earth terminal.  The high voltage cable should include an earthed metallic screen protected by an insulating sheath.  In most cases the power, powder feed and air lines are all contained within the same outer protective cover.

Some generators are designed to provide negative and/or positive outputs.  All generators should incorporate an overload adjustment for any high voltage malfunction.  Should a short circuit occur in the system the high voltage generator should immediately shut down and in addition some visual or audible alarm should indicate trouble.

Spray gun

A range of spray guns, both manual and automatic, is usually offered by the gun manufacturer.  The internal powder feed passage in the gun should be as straight and as smooth as possible. Any irregularities or bends within the passage could cause powder build-up and blocking which could cause ‘coughing’ or ‘spluttering’ of the powder on emission.

Usually powder spray patterns can be adjusted by diffusers to give a wide fan, cone or fine rod shape.  The width and diameter of the powder fan can be adjusted to suit the workpiece.

Advantages

  1. Intricately shaped articles can be coated.
  2. Film thickness of 35 to 125µ can be achieved in one application.
  3. Automatic operation can be installed for high production rates.
  4. Color changes can be introduced.
  5. Preheating of the workpieces is not required.
  6. Thin substrates (eg. metal foils) can be coated.

Disadvantages

  1. Cost of application equipment is higher than with fluidised bed.
  2. Coating rate is slower than with wet paint.

A typical electrostatic coating line would comprise:

  • A powder hopper.
  • A source of compressed air at controlled humidity to transport the powder from hopper to gun.
  • A high voltage generator (typically 30-100kV).
  • Powder application guns which may be:
      • hand operated
      • automatic, either static, reciprocating or wagging.
  • A specially designed booth, to allow all excess powder to be continuously removed by an air stream to the recovery unit.
  • A recovery unit may consist of:
      • a cyclone unit
      • bag or frame filters
      • or    - a combination of both.
  • A rotary valve and sieve unit to remove contaminants from the recovered powder before returning it to the hopper.
  • An earthed conveyor.
  • A curing oven, normally operating at 160-200oC.

There are a number of diffuser designs available which may be cone-shaped, either concave or convex, and with single or multiple SLOTS.  Diffusers may also be equipped with an air shroud to control and direct the charged powder to the workpieces.

Most guns are designed to permit a high rate of powder delivery.  However, the higher the powder throughput rate the lower the deposition efficiency and the greater the quantity of overspray powder to be collected.

Powder spray guns can usually be adjusted to provide a powder discharge to suit particular circumstances.  At the lower end of the discharge rate, flow of powder can be from 100-200g/min (approximately 6-12kg/hr), and at the higher end , at a rate of 500-600g/min (approximately 30-36kg/hr).  At 14kg/hr, assuming 60% deposition efficiency of the powder ejected from the gun, at a film build-up of 50µ, 1.74m2 per minute of surface can be coated.

In an automatic plant the guns may be:

  • Held in fixed positions but individually adjustable to direct powder to the workpiece.
  • Mounted on mechanical reciprocators which may be on one or both sides of the spray booth and may contain one or more guns on each reciprocator.
  • Pivoted to continuously traverse over a present arc (ie. ‘waggler guns’).

Manual coating is recommended when:

  • The workpieces are of complex shape.
  • The workpieces have semi-enclosed areas.
  • The workpieces have unfavourable width/ depth ratio.
  • The total surface area to be coated is so low that one spray operator can cope adequately.

Automatic coating is recommended when:

  • The workpieces are of relatively simple and flat surfaces.
  • The total surface area to be coated in a given time is large.

In some automatic electrostatic powder spray plants the same basic principles as outlined previously apply.  However, the gun(s) are positioned to move about the workpiece in a predetermined path and are automatically stopped and started as required.  This movement or reciprocator speed should be as slow as possible, probably between 20 & 35m/minute and consistent with the other conditions applying.  It is usually advisable that on an automatic line, a hand touch-up gun should be available.  This can be used to coat areas difficult to spray, eg. recessed areas prior to automatic coating.  It is advisable to incorporate more guns to be able to run at a reduced throughput which improves transfer efficiency and gives greater flexibility in gun adjustment and in the variety of workpieces which can be coated.