Q&A: Spray Gun Flow Rate Problem

Q. We have problems achieving consistent paint flow through our spray guns (typical rates range from 10 to 50 g/min). We spray small round parts up to 50 mm in diameter by mounting the parts on spindles and moving them past the spray guns using a chain-on-edge conveyor system, producing about 3-5k parts per hour. The paint is supplied pre-mixed in 2-L tins, and we pump this using a regulated diaphragm pump set at 60 psi. The paint flow is controlled by a flow regulator set at 20 psi and back pressure regulator set at about 18 psi so that the diaphragm pump runs at 3-4 sec per cycle. The fluid flows through air-operated fluid regulators to a maximum of three spray guns.

We find that the fluid flow will alter during spraying at the low flow rates. We are debating changing the fluid hose from the regulator to the gun from a 4-mm bore to 2-mm bore, the theory being that we will require more pressure for the same flow rate. The paint in question is medium solids content, approximately 40 percent, and viscosity of 78 sec using a DIN cup. K.F.

A. Following your theory of stabilizing flow rate by decreasing the hose size and increasing pressure may work. However, there may be other problems such as a faulty fluid pump, faulty regulators and out-of-adjustment gages. Differences in viscosity could also account for the flow rates. You should also consider checking the viscosity of each can of paint before adding it to your system

Rather than trying to solve this problem yourselves, however, your equipment salesman or service representative should be in your plant trying to solve it for you. When you bought the equipment you also paid for technical service, so I recommend calling the equipment supplier for help.