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. 

Q. I am a manufacturing engineer for a company that manufactures various types of hose products. We use textile braid as a reinforcing layer in rubber hose. In our manufacturing process we apply adhesive coatings over certain textile braid for reinforced hose constructions. On other hose items, we apply a lacquer coating over the textile braid. In order to dry out the lacquer coatings, the hose travels vertically through a 50-ft-high tower, passing through a series of electric heating elements. The liquids are applied by spraying. Naturally, this results in a lot of wasted overspray. This is a somewhat antiquated method of coating hose. We would like to investigate the possibility of a more updated arrangement such as electrostatic spraying. We thought you would be aware of some of the newer technologies in other industries which could prove to be of assistance or benefit to us in changing our method of coating hose braid. What do you suggest? L.C.

A. Electrostatic spraying will increase transfer efficiency by reducing overspray. Liquid coatings, which are applied by electrostatic spraying to metallic substrates, can also be applied to non-metallic substrates if they can be made to be electrically conductive. Special surface treatments are applied to wood and plastics to make them conductive. Heating will often produce the same effect also. Since textiles are usually surface treated at the mill during manufacture, a conductive coating can be applied to the filaments before the braid is woven.

Another approach would be using a UV curable, heat-reactive adhesive to coat the braid. UV curable coatings are 100 percent reactive (no solvents) and can be cured instantaneously in a relatively short UV light tunnel, thereby eliminating the need for a long, heated drying tower.