December 2005

Ready, Take Aim, Spray

Industry experts discuss ways to increase transfer efficiency and improve atomization of spray guns.

By J.D. Piland

The finish on a cabinet or a piece of furniture is one of, if not the, first things a consumer notices when looking to buy.

Of course, the design and construction have to work in harmony, but a beautiful finish can sell the piece - sometimes in spite of the price. So why would a company use equipment that would give anything less than a perfect finish?

Wood & Wood Products spoke with industry experts about the last thing finishing materials encounter before hitting the substrate: the spray gun.

Whether it is handheld or part of some larger finishing equipment, the spray gun helps determine just how fine a finish can be achieved. Several things factor into that, but without a properly configured spray gun, things can get messy real quick.

Spray Quality = Finish Quality

When asked what they consider to be the most important spray gun feature, the experts agree it is spray quality. Two aspects greatly determine a quality spray-on finish: atomization and transfer efficiency.

Atomization is the process by which air pressure breaks up the finishing material into "very small particles, basically mistifying it," says James Berry, technical manager at AccuSpray Application Technologies.

Spray Gun Cleaning Tips

Because transfer efficiency is greatly determined by the atomization setup, among other things, here are some tips to keeping it clean and functional.

  1. All nozzles and needles are precision-made. Handle them carefully.
  2. Do not make any alterations within the gun. Doing so may cause difficulties during finishing operations.
  3. Nozzles should be cleaned by submersing them in solvent to dissolve any dried material. Blow them clean with air afterward.
  4. Do not use metal instruments to probe any orifice of a nozzle. Use only a tool made of a material softer than brass, if necessary.

    Also, always remember to check and record air and fluid pressures and fluid viscosities on a regular basis, says Mitch Drozd, atomization product manager with ITW Industrial Finishing - Binks and DeVilbiss products. "This especially helps when various gun operators will be using the same gun to spray the same products," he adds.

Information courtesy of ITW/Binks-DeVilbiss

This ties into transfer efficiency, which is the measurement of how much finishing material adheres to the substrate; a quality transfer is 65 percent, though that is a relative number and is affected by nearly all mechanical and environmental factors. Atomization affects the transfer efficiency because as the material becomes more atomized, the greater the overspray cloud becomes. Therefore, there needs to be a balance between the atomization setup - the tip, nozzle and aircap - and the air and fluid pressure to ensure proper atomization and minimized overspray.

The appropriate nozzle size that is used depends on the viscosity of the finishing

material, plus air and fluid pressure.

While all parts play a role in the amount of atomization and transfer efficiency, the nozzle size is important because "it determines the flow rate, hence, the speed of application when spraying," says Mitch Drozd, atomization products manager with ITW Industrial Finishing - Binks and DeVilbiss products.

Dale Stitt, national sales manager with Anest Iwata, agrees, adding, "The larger the orifice, the more material output will increase with a resultant decrease in fluid speed at a given pressure," he says. "This can also be seen as an increase in productivity as long as the user can keep up with the increased speed necessary to assure a good finish."

Most spray patterns are in a "V" shape, but a "U," or "tulip," shape now is on the market, Stitt says. "Our GÇÿU' shape allows our spray patterns to be more uniform while increasing transfer efficiency," he adds. "Transfer efficiency is greatly affected by external factors and painter skill level."

Tips, nozzles and aircaps are so vital that Wendy Hartley, product manager at Graco, says, "[The company's] main focus is on design of the tips and aircaps.

"The aircap and tip can be designed to improve transfer efficiency by allowing for lower fluid and air pressure usage," she adds.

Choosing an atomization setup is not so simple, though. All spray system factors must be considered when choosing a setup. This includes the type of gun, air pressure, fluid pressure, viscosity, desired finish, environmental factors and more. Users must find "a happy medium" of all these elements to achieve the desired look, Berry says.

The same goes for transfer efficiency.

Transfer efficiency has become more of a cost-savings result, however. While it is important for the finishing materials to adhere correctly on the substrate, they have to make it there first.

The gun most recommended for this, and the one that is gaining in popularity for a myriad of reasons, is the high-volume, low-pressure (HVLP) gun, Hartley says.

The high volume of air passing through the finishing material atomizes it at low pressures. This, in turn, decreases the overspray cloud, which also helps a company meet its regional volatile organic compounds (VOCs) emission standards, says Mark Hebbeler, marketing manager at Anest Iwata. Some shops, mainly those in Southern California, are required to use HVLP guns, he adds.

Manufacturers continually focus on improved ergonomics of their spray guns. Because of the nature of the spray motion, operators can get fatigued quickly if the gun has inferior ergonomics.

Photo courtesy of Graco Inc.

According to an article in the October issue of Custom Woodworking Business, a sister publication of W&WP, there are many other factors that go into improving transfer efficiency.

These factors, which include things like maintenance (see sidebar on p. 89) and the distance from which the substrate is being sprayed, are dependent upon the manufacturer of the gun, so it is a good idea to follow the material safety data sheet provided with the gun, Hebbeler says.

Drozd thinks these factors, especially the operator's spray techniques and training, are the most important factors in transfer efficiency, after choosing the appropriate atomization setup.

Changeover

While changeover from one finishing material to another using the same gun is something more of an issue in custom shops, it still affects the downtime during any shop's finishing process. The flushing and cleaning process has to be dealt with, however, but there are some steps to reduce the time spent doing so.

Because small shops may not offer as many finishes, a couple of spray guns may only be warranted, or afforded, unlike their larger counterparts that have multiple guns for each step of the finishing process. This forces small shops to seek ways to decrease downtime, and eventually increase their output.

One way to do so is to utilize a disposable cup system. If all the material is not used, the cup can either be thrown away or cleaned for further use later. After that, only a small amount of solvent would be needed to clean the gun. Gravity guns are ideal for this application, Hebbeler says.

"If you have a diaphragm pump system or pot system, you may have 20 feet of fluid line that is going to a pressure gun," Stitt says. "Switching from material to material, you are going to have to clean that whole line out and use GÇÿX' amount of solvent. To clean that line out, whereas, if you are using a gravity or siphon gun, you use a smaller amount of solvent to clean it."

"There are some smaller shops I have heard of that will use a gravity gun because they can keep a clearcoat in a pitcher, and as they run low, they can just add material as needed," Hebbeler adds.

If a pump system is already in use, one way Stitt recommends reducing clean time is to use smaller-diameter hoses. By doing so, less material flows through the hose, meaning less solvent is needed to flush it out.

"Air-assisted airless systems and piston-type systems typically have less volume in the pump at any one time than a diaphragm system, so if you are trying to minimize the amount of fluid, use smaller diameter hoses on an air-assisted airless versus a conventional system with a diaphragm pump," Stitt says.

Getting a Handle on Things

Ergonomics, as the experts again agree, has become a more important feature of spray guns.

"When designing a new gun, ergonomics is one of the top priorities," Hartley says.

The gun has to feel good in the user's hand, for which many guns now are being designed for small-handed users, and it has to be lightweight. This is in hopes of preventing any repetitive-motion injuries, such as carpal tunnel syndrome. While the expert panel was not aware of many instances of such ailments, the lingering threat is there.

"The last thing you want to do is hold a gun in your hand that doesn't feel good for eight hours a day," Berry says.

One feature manufacturers have focused on as of late is the trigger pull.

"The trigger pull needs to be not only comfortable for the user, but also less resistant, so there is a reduced amount of muscle strain when triggering the gun," Hartley says.

Adds Hebbeler: "Operator comfort is paramount to their success. If a gun is too heavy and cumbersome, their performance drops and quality is greatly affected."

Aimed in the Right Direction

The automotive finishing market is making in-roads in the wood finishing markets, with regard to spray guns.

The gravity gun once was considered exclusive to the automotive market. The reason for this, Hebbeler says, is that the industry is regulated by the VOC emissions standards. With the quality finish and little overspray produced, gravity guns were ideal for automotive finishing purposes.

Now, they have made their way into the wood shop. The potential for speedy changeover - think of the pitcher example - and the finish and overspray aspects have made these guns handy for small shops.

Also making strides is the air-assisted airless gun. This, as Berry explains, provides the best of both worlds: the transfer efficiency of airless guns with the finish quality of conventional air guns. It does this by allowing a lower fluid pressure, and using air to shape the spray fan, "instead of using the nozzle to try and ram it through."

"Air-assisted airless can get you very close to a very smooth finish, which is why it's so attractive to a lot of these wood shops," Berry adds.

Hartley adds that she has heard of users demanding that air-assisted airless guns be more HVLP compliant with the emissions standards.

                                                                                                                                                                                           

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