Most every commercial Polyurethane Foam Spray Gun uses impingement-mixing technology (shown at left) to mix the chemicals inside the gun. Impingement-mixing means the two separate A-side and B-side materials are mixed together using high pressure forces inside a very small chamber inside the spray gun.
Pouring and injection applications can incorporate a variety of different mixing technologies including static and dynamic mix. Many DIY and Professional Foam Kits utilize a low-pressure static mixing technology, however, in this section we will focus on spraying.
It is important to understand that once the two materials come together inside the gun to mix, they begin to react immediately as they mix and exit the spray gun*. If all of the reacted material is not expelled from the gun once the trigger is released, the material will set-up and harden inside the gun, rendering it non-usable. The design function within the spray gun that assures this does not happen is called purging.
* High Pressure Impingement Mixing Most lay people think that the machine or metering pumps create the pressure at the gun. The pumps create material flow. Pressure is created by causing a resistance to flow. Much like using a garden hose water your plants, you put your thumb over the front of the hose to increase the pressure to make the water spray further. Your thumb simply restricts, or resists the water flow, hence creating more pressure. This is the same role the spray gun plays in the spray foam scenario. The gun is essentially your thumb creating a resistance to flow from the machine, in order to create impingement pressure for the A-side and B-side materials to mix together. The gun also uses additional components that make the spray pattern a little better and more consistent than your thumb does.
Mechanical purge serves two functions in a spray gun. A valving rod is set inside the mixing chamber of the gun. When the gun is not triggered, the valving rod remains static and closes of the two chemical ports. When the gun is triggered, the valving rod withdraws, opens up the two chemical ports to let the polyol resin and isocyanate materials into the gun to mix and spray. When the trigger is released, the rod comes forward closing off the two chemical ports and mechanically purges the remaining mixed material out of the gun.
Air purge guns work similar to mechanical purge guns, except they purge the material from mixing chamber and gun block using a quick blast of air. Instead of fixed mixing and a moving purge rod, like a mechanical purge gun uses, the air purge gun uses a moving mixing chamber to valve the material flow on and off.
Solvent purge is not that common and requires flushing of a cleaning solvent through the gun to neutralize the chemical reaction and the wash out the inside of the gun. This not favorable due to the fact that solvents cost additional money, they need to be properly disposed of (costing more money), and they can be harmful to the operators and the environment, if not handled properly.