Most every commercial Polyurethane Foam Spray Gun uses impingement-mixing technology to mix the chemicals inside the gun. Pouring and injection applications can incorporate a variety of different mixing technologies including static and dynamic mix, 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 the 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. There are three basic purge systems offered by the various manufactures that offer different degrees of advantages and disadvantages, mechanical purge, air purge and solvent purge. Solvent purge is not that common in spraying foams as it is in coatings.
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.
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.