As part of my degree I am to do a work-based project whereby I must come up with a scientific solution to a problem. My chosen project is the variation in dry paint film build (often 20-90 microns) currently being experienced on my employer's new automated electrostatic paint system. The system is supposed to paint aircraft stringers. The variation is not consistent, all lines and components are regularly stripped and maintained. How do you calculate what thickness of paint will be put down over a certain area on any system? (i.e., what factors do I need to take into consideration to see if the correct pressures, feed rates, nozzle sizes, etc. are being used to give the correct thickness over a given area?)J. D.
The calculation of paint coverage is simple. One gallon of liquid will cover 1,604 sq ft at a thickness of one mil. To get theoretical coverage of a paint, multiply the volume applied by % solids. Then multiply the product by the % transfer efficiency of the painting process. You will seldom get the theoretical coverage in industrial practice.
In addition to the aforementioned factors, there are others that will be important in your study. There can be a number of factors causing these variations both from part to part and on a single part. Some of these factors are: differences in the Faraday Cage effect at different points on a part, type of part hangers used, gun to part distance variations and differences in grounding from part to part. Since the answer to your question really sets up a research project and that is beyond the scope of the Painting Clinic, I can only give you a list of factors that are important in controlling film thickness variations.
Some other important factors to be measured are: plant relative humidity, spray booth relative humidity, plant air temperature, spray booth air temperature, part temperature, consistency of conveyor line speed, paint temperature, paint density, paint viscosity, paint % solids by weight, paint % solids by volume, spray booth air flow at several locations, use of different solvent blends and transfer efficiency.
I am not surprised that there are film thickness variations. As you know, it will be important to map out locations for film thickness measurements before your study begins. If I tell you any more, I would have to ask that my name be added, as co-author, to the paper you are going to publish.