Perfect booth

In the April 2010 issue of Overspray, we looked at the basics of designing an efficient powder spray booth. Overspray decided to revisit spray booth design to discover how designs are changing to meet the finishing market’s needs.

What would be the main features of the perfect booth?

Some obvious answers come to mind:

• The booth would maximize the transfer efficiency of the powder guns.

• The booth would require no cleaning between colours with 100 per cent powder recovery.

• A contamination-free booth.

Achieving all of these features though is not possible and the best we can hope for is improving the technology to come close to these objectives.

To meet objectives that are sometimes in opposition with each other, we sill study the evolution of materials used in the construction of modern powder booths, specifically the products offered by three of the industry’s major players—ITW Gema, Nordson and Wagner.

Powder booths are available in various shapes including square, rectangular and round. Two different concepts are used for operator touch-ups in an automated booth:

• “vestibule coating

• the classical approach of having the openings on the sides of the wall.

The ¨vestibule coating¨ is usually in the square and the round type of booth. The mode of painting in what is normally the vestibule requires linear velocities higher than those we are familiar with i.e. between 135 and 160 linear fpm, to ensure containment of the powder. Another reason for these higher speeds is in the “less powder in¨ concept. With less paint in suspension in the cabin there is less paint on the walls, ceiling and floor, therefore less cleaning and possible contamination during colour changes. It is also the main reason why the cabin with side openings also works with greater air velocity.

Making a booth

If you look at the different building materials used from the beginning of electrostatic powder to the present day, you will find steel painted panels, galvanized steel followed by stainless steel, PVC, Polypropelene, the polycarbonate family Lexan and now the sandwich-type construction of new generation plastic. In the past there were glass paint spray booths and the electric curtain booths, but because of  price, weight, safety issues and installation elements, these have virtually disapeared.

The three manufacturers Overspray contacted currently advocate the concept of self-supporting panels of plastic sandwich-type construction. While Wagner and Gema promote a panel made of two layers of plastic with a central air core, Nordson opts for a panel with a foam core. The Nordson panels are made and shipped in one piece, (Nordson Apogee). Gema’s Luminaire concept and Wagner’s CPVC 2 provide panels that are welded and polished in place during booth erection.

Why this walls-type concept? The electrostatically charged powder is attracted to anything that is grounded, so gounded conductive surfaces are prime targets. By contract, plastics are dielectric in nature. Some cabins are made of single thickness material plastic with reduced attraction of powder. This feature is sometimes used to improve paint application, for example in a brake pad painting system. Brake pads are placed on a conveyor that has a plastic surface with a conducting bar allowing contact of the target with ground. Powder is sprayed vertically onto the surface. The charged particles are repelled by the plastic surface, which is also charged.

In the case of powder booths with single thickness plastic walls, when the resistivity of the dielectric material is not sufficient to stop the search for ground completely, there is attraction and accumulation of paint on the surfaces where the booth’s grounded steel or stainless steel reinforcements and structure are in direct contact or very close to the wall.

This characteristic is used in car industry for liquid electrostatic painting of plastic bumpers by inserting a grounded metal copy or mask of the part under the bumper. The mask attracts the paint particles through the plastic enough to justify the added cost of expensive electrostatic bells. This is known as using the ¨shadow effect.¨

The use of plastic sandwich panels reduces the paint’s attraction to the walls dramatically. The accumulation of free ions from the electrostatic applicators on the surface of the plastic makes the highly charged particles repel almost  all of  the paint since they have the same charge. Powder paints have a Volume Resistivity in the range of  10 ¹ ³ Ω while CPVC ¹ ³  is .034 x 10 ¹ ³ Ω (ohm-cm).

Painting this sandwich is like trying to recoat a 3 5/8 in. coat of paint over a 3/8 in. piece of plastic with no ground.

These walls do not attract the charged powder and isolate it from external grounds. They all increase the transfer efficiency of applicators by not being targets.

Quick cleaning

Cleaning these cabins is done using compressed air at high air volume. Because of restrictions on the pressure blow guns issued by OSHA, efficiency and power increases via the air volume.

Another area to consider when it comes to spray booth design and cleaning is to look at the materials used and air flow once the booth has been thoroughly blown off; whatever paint particles remain on the walls, floor and ceiling will stay in place. There is not enough energy to remove the paint particles during the paint process.

At one customer’s facility during a colour change from red to light grey, I noticed some red on the booth walls. The paint could not be removed unless I touched it. After the cleaning cycle and the application of the grey paint, we checked but couldn’t find any red powder on the parts.

Why not use de-ionizing guns to improve the cleaning? Many believe this would be an improvement on the process and could reduce cleaning time and quality. Consider though that clean dry air usually does the job efficiently. There is the added cost of the de-ionizing guns and a limitation with higher air volumes.

When it comes to maintenance of the panels in a booth, suppliers offer different solutions. Some say that clean dry air is sufficient for cleaning between colour changes. One manufacturer recommends that the walls be cleaned once per week with deionised water and another suggests the booth be cleaned thoroughly once a year with an application of a recommended wax product for buffing.

A few weeks ago I visited a client that purchase a used booth and noticed from the terrible condition of the walls that the personnel used ¨squeegees¨ in the clean up process. That’s like using a Ferrari on Quebec’s roads: it’s a waste!

These newer generation of booths come with a higher price tag, but they offer better performance and more efficiencies. CM

Pierre Bachand is president of  Quebec-based Chromatech Inc., and a regular contributor to Overspray.

www.itwgema.us

www.nordson.com

www.wagnersystemsinc.com