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by Rick Anderson, Engineering, Graco Inc.

In recent years, the use of spray foam has increased significantly within the building and construction markets. This increase is due to the escalating cost of energy to heat and cool our homes and businesses, as well as the material's durability, insulating capabilities and ease of application. But even as interest in foam increases and it becomes a bona fide alternative to more common insulating materials, questions still arise about the product, application methods and equipment. One of these questions pertains to selecting the right spray gun, chamber and tip for a contractors' specific type of work.

Gun Selection

There are two categories of spray guns used to apply foam. They are external mix and internal mix spray guns. The external mix gun relies on a disposable static mixer element to mix the two fluid components. The two materials flow separately into the gun when it is triggered but do not mix until they enter the mixer element. The static mixer completes the mixing process before the material is atomized and sprayed through the spray tip. These disposable static mixer elements are available in different lengths and baffle configurations. The operator must replace the element whenever the mixed material inside the static mixer cures. If the gun features a solvent purge the operator can extend the life of the element and get multiple uses from it by purging the static mixer with solvent.

Typically, solvent purge is preferred with materials having set times of 30 seconds or more (vs. typical three to five seconds for most fast-set products). This provides the operator with enough time to purge the mixed material from the element before the material cures in it.

The drawbacks of an external mix/solvent purge system include the use of hazardous solvents to purge the elements, longer cleanup, and the added cost to purchase the disposable static mixer elements.

The internal mix gun is designed with the mix chamber inside the body of the spray gun. When the gun is triggered the two fluids blend inside the gun's mix chamber and the mixed material is propelled through the chamber or spray tip, atomizing the fluid. These guns are suited for fast-set fluids with set times typically under six seconds. Internal mix guns are the most common spray guns used to apply fast-set materials, such as foam.

There are three types of internal mix guns: air purge, fluid purge and mechanical purge guns. Each type of gun purges the mixed material from the gun differently.

Air purge guns use a blast of air to purge the mixed material. Occasionally, the air purge gun requires the mix chamber be cleaned out with a drill bit. Air purge guns are considered easy to use because the operator doesn't need to make any gun adjustments and there are fewer parts, reducing maintenance. This is the most popular spray gun for applying foam.

Fluid purge spray guns were recently introduced and can be used with both foam and polyurea coatings. This new technology allows operators to clear mixed material from the mix chamber using significantly less purge air than an air purge gun. This means less foam blowback in tight spaces. The technology features a non-toxic liquid in a disposable, non-reusable cartridge that fits into the handle of the spray gun. The liquid is automatically dispensed into the mix chamber at each trigger pull; removing any built-up foam and keeping the mix chamber clean.

The mechanical purge gun features a higher flow rate and improved mixing. It also features a wider choice of spray tips. An air piston-driven purge rod within the gun is used to purge mixed material from the spray tip. The mechanical purge gun is the preferred gun for spraying coatings, such as polyureas.

All three of these spray guns feature round and flat spray patterns.

Air Purge & Fluid Purge Spray Chambers & Tips

Most air purge and fluid purge spray guns feature a round pattern mix chamber that does not require a tip. Foam contractors prefer the guns' simplicity because there are no adjustments required to operate it. The round pattern mix chamber also stays cleaner longer because there are no small spray orifices to plug. It also provides the longest life before wear or leakage occurs.

Some air purge guns offer a "wide pattern" mix chamber for a larger round pattern. The operator can spray the foam over a larger area and get a smoother finish. An operator can also get better pattern overlap and finer stipple effects with this system. For contractors wishing to increase production, mix chambers with larger chambers are also available, increasing the size of the spray pattern.

Since air purge guns release a high volume of air through the chamber after the trigger is released to clean the mix chamber, the operator must continue to move the gun away from the work surface so that he doesn't blow back the foam he has just sprayed.

Contractors who like the air purge gun's simplicity but spray non-foam coatings such as polyurea can use flat spray tips designed for this gun. Because of the chamber, the gun doesn't clog as easily as a mechanical purge gun. The flat tips also provide a smoother finish and uniform film build. Different tip sizes allow the contractor to change pattern size.

The drawbacks of an air purge gun include higher air consumption and some tip plugging. The liquid purge gun, on the other hand, reduces air consumption and eliminates tip plugging.

Mechanical Purge Tips

Contractors who want optimum mixing performance and the smoothest surface for foam and coatings prefer a mechanical purge gun. The mix module features multiple impingement fluid ports that mix the two components more thoroughly. Atomizing through a round or flat tip provides a smoother finish than an air purge gun. These guns also feature a wider choice of tips, allowing the contractor to easily change pattern size without changing the mix module. Mechanical purge guns also require less air than an air purge gun. Operators who want the best mixing and smoothest finish for polyureas prefer flat tips. Foam contractors typically use the round tips to minimize plugging.

The drawbacks of a mechanical purge system include the added gun adjustments. In addition, the round tip will generally plug quicker than an air purge round mix chamber. The mechanical purge mix module's life is typically shorter than an air purge guns hardened metal mix chambers.

With any state-of-the-art application method, your best resource for matching the best system, including gun and tip selection, is to discuss your requirements with an authorized foam equipment supplier. They can help you customize a system to meet your specific need. Graco Inc., Minneapolis, MN is the leading manufacturer of spray guns, pump and proportioners for foam and protective coatings. Graco's Fusion spray guns are available in mechanical-, air- and liquid-purge. External mix guns are also available. You can find more information on this technology and these products at www.graco.com.