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Dacrylate Paints Ltd.

Coating Application Methods

Application Methods

Conventional spray

Compressed air is mixed in large proportions with the paint, which is then projected through a narrow nozzle.

The resultant pressure drop causes atomisation to take place and the projected air/paint mixture is directed at the desired target. In order to flatten the spray pattern into a fan, rather than a cone, additional air outlets are placed at each side of the nozzle. Because of the entrained air and resultant turbulence, the spray pattern is very soft edged allowing very good operator control and thus a very fine finish can be achieved. The softness and reduced paint content of the edges of the spray pattern enables overlapping passes of the spray gun to blend together without double film thickness being applied in these areas. The entrained air does cause large volumes of paint / air mixture to rebound from the surface being coated which results in an 'over-spray' problem. Solvent is lost in the turbulence which can lead to 'dry' films being applied with poor flow-out. Adequate extraction must be employed to remove these large volumes of paint fog from the workplace. The spray gun nozzle can be progressively opened allowing very low application rates to be applied when required. The paint can be fed to the gun by pressure via a separate paint container (a 'pressure pot' or by gravity or syphon feed where the paint container is attached to the gun. Adjustments can be made to the nozzle size, air pressure, fluid pressure (if pressure feed is utilised), fan width and air / paint mixture ratio.

Airless Spray

Commonly used for high volume work where a fine surface finish is not critical eg bridges, structural steelwork etc.

The paint is pumped at very high pressure (1500 lbs/in2 +) through a very small nozzle (typically 9-27 thou). The massive pressure drop as the paint exits the nozzle causes immediate disruption into very fine droplets (termed atomisation). Airless pumps can be air powered (the air operates the pump only, it does not mix with the paint), electric motor or petrol, gas or diesel engine.

Very high flow rates can be achieved with the most powerful pumps making this the fastest spray application method.  As there is no entrained air within the paint stream, there is very little turbulence and the spray mist loses its forward velocity very rapidly which reduces bounce-back from the surface considerably. The absence of entrained air also leads to less solvent loss within the spray pattern and results in a 'wetter' film being applied. As the paint spray is either 'on or off' there is far less operator control over the applied film. Overlapping passes of the gun will result in a double film thickness in these areas, which rules the method out for most industrial applications requiring a fine finish. Adjustments are only possible to the tip size and fluid pressure.

Air-assisted airless

The system comprises a conventional airless gun has an additional air feed which envelops the atomized paint fan with a low-pressure air shroud. This softens the fan edge, allowing better finishes to be achieved together with the high output of the conventional airless gun. It will not give such a fine finish and is not as controllable as conventional spray but is markedly superior in quality of finish to pure airless application. Fluid pressure, tip size and air shroud are operator controllable.

Various modifications can be made to the above equipment to improve the transfer efficiency or film build, two examples are

Electrostatic Spray

Operates on the principal that if an electrostatic charge is applied to the atomized paint spray, then the individual droplets will become electrically charged and be attracted to the nearest earthed object. If the speed of the particles is kept low then 'wrap-round' will take place i.e. any particles which miss the target will be attracted to the rear. As the paint is deposited on the surface, its electrical charge is lost causing a lessened attraction for further droplets, which will be attracted elsewhere. Because of this self-limiting effect, transfer efficiency of the method is very high, particularly on tubular or open meshwork. All three of the spray methods are available in electrostatic form. The paint usually has to be modified (made more conductive) to allow an electrical charge to be induced, even water-borne coatings can be applied using this method (not all equipment is capable of this), although not all paints are suitable for the method. The system is usually employed for the coating of metals, although other substrates e.g. wood and plastics can be successfully coated providing that the substrate is made electrically conductive first.

Hot Spray

Spray application requires a viscosity low enough for atomisation and subsequent flow-out of the droplets into a smooth homogeneous film; this usually entails the addition of thinner with inevitable reductions in film build and increases in VOC emissions. If coatings (solvent based) are heated, a marked decrease in viscosity takes place thus paint can be sprayed at elevated temperature (approx 70°C) without addition of extra solvent. Once the droplets have landed on the surface this heat is lost causing the viscosity to rise and a reduction in the risk of sags. In this way, coatings with higher solids content can be applied, which achieves a higher film thickness with each coat. This technique can be combined with each of the spray methods and can also be used with electrostatic equipment eg hot airless air-assisted airless units are commonly used for finishing large items of construction equipment.

Electrostatic Bell and Electrostatic Disk Application

A method used for mass automated production of such things as radiators, cars, office furniture etc.

Paint is pressure fed to the edge of a quickly rotating (5-20,000 rpm) and reciprocating metal disk or bell. This disk is charged with a very high electrostatic charge, which will concentrate at the disk or bell edge. Centrifugal force will eject the paint from the disk edge and the electrostatic charge will cause disruption into very small, charged particles. These particles will be attracted to the nearest earthed object, arrange to be the item to be coated.

Dacrylate Electrostatic Bell and Electrostatic Disk Application

The method is extremely controllable, very even films can be applied with far more control and accuracy than a spray operation, variations in film thickness can be made down to ± 1 or 2 microns by varying the paint pump pressure, speed of rotation, speed of reciprocator, conveyor speed of object to be painted, rotational speed of object to be painted.

Dacrylate Electrostatic and Electrostatic Disk ApplicationDisks tend to be used for objects which require painting all round e.g. shock absorbers, gas cylinders etc.

Bells work on a similar principal and are generally used for large flat surfaces which are not rotated, each bell covers a wider area with the spray pattern than the narrow fan of the disk.

Roller Coating

Dacrylate Roller CoatingCan only be used for flat surfaces, but extremely high line speeds can be attained, typically used for painting tin plate prior to can manufacture. The flat continuous sheet is fed between a pair of revolving steel rollers, one of which is in contact with a third rubber roller with a paint feed onto its surface.

The pressure between the rubber roller and steel roller can be varied to deposit more or less paint onto the steel roller which will very accurately control the film thickness of paint applied.

Curtain Coating

A method usually restricted to large flat objects eg doors. Two conveyor belts are arranged end to end with a gap between. Above this gap is a paint trough with a narrow, but variable, slit the full width of the conveyor, below this trough and below the conveyor is arranged a second trough to catch the overflow paint, which is re-circulated. Paint is allowed to fall through the slit as a continuous “curtain, through which the items are fed. Narrowing or widening the slit width or changing the speed of the conveyors can vary the film thickness of paint. Very high line speeds and extremely low paint wastage can be achieved.

Dacrylae Curtain Coating

Flow or Flood Coating

Dacrylate Flow or Flood CoatingAt first site, a fairly crude form of application in which paint is poured or flooded onto the object to be coated, usually by passing the items through a chamber containing a series of nozzles emitting paint (as a hose pipe). Any paint that misses the item or drains off, can be collected and re-cycled. Only objects which can drain freely (ie have no blind cavities or which can be rotated during the application, can be coated in this way to ensure that pools of paint do not accumulate.

Barrel Coating

Used principally for the painting of small items such as screws. The items to be coated are placed inside a barrel, together with a small but precisely calculated and measured quantity of paint. The barrel is then rotated for a set period. During the rotation, the items will tumble and distribute the coating evenly throughout the load. The barrel is rotated until the items are dry, this prevents them sticking together and achieves a surprisingly good finish.

Dip Coating

In its simplest form, lowering the item to be painted into a paint bath, a better finish is obtained if the withdrawal speed is less than the flow speed of the coating, ie. runs flow back into the bath as the item is being withdrawn. If the withdrawal rate is matched to the flow rate of the paint, runs will not form or be kept to a minimum. Dipping paints are usually of low viscosity therefore large dip tanks require some form of mechanical agitation to ensure that the pigments do not settle. This agitation is usually in the form of a re-circulation system whereby paint is continually taken from the bottom of the tank and re-introduced at the opposite at the top of the tank via a weir system (to prevent frothing). Dip application is not suitable for items that will not freely drain.

Dacrylte Dip Coating

 

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