Paint Spray Guns

Transfer Efficiency

GRAVITY-FED HVLP VS. NON-HVLP SPRAY GUNS

Q:  How do gravity feed HVLP guns compare (with respect to transfer efficiency) to non-gravity feed HVLP guns?

A:  As the name implies, gravity feed guns (not necessarily only HVLP guns) supply paint to the orifice solely be means of gravity.  This means that the flow rate of fluid (paint) to the orifice is limited.

On the other hand, when a pressure pot is used, the flow rate of paint is limited only by the pressure applied to the pressure pot.  For instance, if the painter applies 5, 10 or 20 psig compressed air to the paint inside the pressure pot the flow rate will increase accordingly.

How does this affect transfer efficiency, (TE)?  Atomization of the paint takes place when compressed air leaving the cap of the gun breaks up the fluid stream emerging from the orifice.  As the fluid flow rate increases, more atomizing air is required to achieve an acceptable spray pattern and TE decreases.  Because of the limitations of HVLP guns, the painter is limited (by air pollution regulations) to a maximum atomizing pressure of 10 psig.  Hence, if he continues to increase the fluid flow rate, there will come a point at which his limited atomizing air pressure will preclude his ability to get an acceptable paint finish. 

In the real world many if not most painters do not observe the 10 psig regulatory limit.  Hence, as they increase the fluid flow rate, they correspondingly increase the atomizing air pressure to achieve a nice looking finish.  Transfer efficiency suffers.

The same arguments apply to non-HVLP spray guns. 

The efficient application of paint to a surface is dependent on the painter more so than on the gun itself. It is possible for one painter to use a non-HVLP gravity-fed gun and get a higher TE than his colleague who uses an HVLP gravity-fed or pressure-fed gun. On the other hand, painters who use HVLP guns are more likely to get a higher TEs than those who use non-HVLP guns.

Q: If the gravity fed HVLP and pressure pot HVLP are sprayed in accordance with air pollution regulations, and are tested side-by-side using the same coating, robot, target, etc., which type of HVLP (gravity or pressure pot) will perform the best in regards to transfer efficiency?

A: If the fluid and atomizing air pressures are the same, then both guns should give you the same TE.  Unfortunately, I don't know how you can easily measure the fluid pressure for the gravity-fed gun.  Since the gravity-fed gun usually uses less fluid pressure than the pressure pot, I will GUESS that it will also give you a higher TE.  If I sound liike a lawyer covering my rear, it is because of the paramerters that affect how the gun itself is set up.  (I'm ignoring the external parameters, such as gun-target distance, robot, etc.)

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