Electrostatic wood finishing

Electrostatic field finishing was originally used for metal: housings of household appliances (washing machines, refrigerators), car bodies, metal doors. Gradually, because of its advantages, the method was adapted to wood finishing. Electrostatic field finishing is now frequently used to coat chairs or other objects where ordinary spraying generates very high losses.

Principle of the method

Electrostatic field spraying is based on the movement of atomised particles along the lines of an electromagnetic field formed between a negatively charged gun and a positively charged object. Electrically charged lacquer or paint particles are attracted to the object to be lacquered along the lines of force of this field formed between the gun and the object to be finished, which is grounded. The particles give up their electrical charge and are retained on the surface of the object to be finished.

The strength of the electromagnetic field depends on the distance between the gun and the object and the potential difference (voltage). Normally the voltage between the charged object and the gun is 80 kV. The distance depends on the equipment and varies between 0.3 and 0.5 m.

Wood is a poor conductor of electricity. Therefore, to have an optimal electrostatic effect, the wood humidity should be 10-12% and the air humidity no lower than 60-70%. To improve its conductivity, wood can be treated with saline solutions or kept for a short time in special steam rooms.

Electrostatic field application guns

It goes without saying that not just any gun can be used for such applications and not just any varnish. There is special equipment for electrostatic field application, which transmits the electric charge to film-forming materials specifically formulated to conduct electric current.

The equipment can be similar to regular spray guns or can be application discs. The latter rotate and spread the negatively charged paint, which is attracted to the positively charged coating objects. The disc system is older and less used recently.

Electrostatic field finishing

Electrostatic finishing materials are different from the usual ones. The solvents used to solvate the resin, surface tension, viscosity and electrostatic charge capacity differ. Products that can be used are those that form a film (primer, varnish, enamel). In the composition of the materials there are substances that increase the conductivity and by forming the film, the conductivity of the object increases. The higher the number of layers, the higher the conductivity. Glazes, glazes or similar products cannot be applied in this way.

Both solvent-based and waterborne products can be applied electrostatically, provided they are formulated for this type of application. The application of solvent-based products is not dangerous because high flash point solvents are used.

Viscosity is also very important in this type of application. If the recommended viscosity is not met, the application will not perform as expected.

The person doing the electrostatic field application must be trained for such work. They must also know how to maintain the gun. Both the lifetime of the equipment and the avoidance of accidents depend on this.

Advantages of electrostatic field application

The main advantage of the process is the considerable reduction in material consumption by reducing losses.

In conventional spraying the lowest losses are when finishing straight panels without mechanical machining. Losses increase as machining occurs and the lacquering surface narrows. This is the case for frames, carved elements or chairs. The theoretical losses for panels are 20% in this case, rising to 60% for chairs. In reality, however, the losses of finishing material are much higher, reaching and even exceeding 100%.

In the case of electrostatic finishing, the reduction in losses is spectacular. Material consumption is reduced by 50%, even 80%. Because of the electric charges, even if the spraying is done from the front of the object to be finished (seat, frame), the particles also settle on the back, being attracted by the electromagnetic field formed. This reduces the amount of material that ends up in the atmosphere (over spray) or on the floor and increases the amount deposited on the wood.

Another advantage derived from the above is improved working conditions. By reducing losses, the atmosphere in the hall will be much cleaner, and environmental protection and work quality problems will be significantly reduced.

Other advantages are the uniform application of the finishing material and the possibility of automating the entire system, thus increasing productivity.

The best performing equipment is that which combines electrostatic spraying with anthropomorphic robots. These robots apply the material by following the contours of the part, an operation carried out using special software. Combined with the electrostatic effect, material losses are further reduced.

For advice or specialised assistance in industrial finishing projects you can contact the specialists at Falk Consulting.