Transfer Efficiency of Coatings during spray application is becoming an important issue for the railcar industry, Although the reduction of solvent emission has been in fist instance the driving force, economics and quality are playing a role as well. Type and use of equipment as well as the skill of the applicator can greatly influence the percentage of the coating actually applied to the substrate.
This paper will address the different types of application equipment, skills and techniques of the applicator.
Up to recently railcar coating manager’s main concerns were keeping the equipment operational, minimizing production downtime, and ensuring a quality finish Today other factors such as solvent emissions, waste reduction, selection of equipment and materials, are equally important.
Over the years applied coating cost have increased considerably There are several ways in which these cost can be reduced. One way is to minimize coating consumption. This can be achieved while maintaining required film thickness, by increasing transfer efficiency Higher transfer efficiency requires less coating to paint a particular object. When the transfer efficiency is improved it will decrease solvent emission, bating sludge, exposure to personnel, coating cost, and coating handling in general.
To accomplish this the railcar coatings manager must study what effect transfer efficiency has on the overall coating operation A better understanding will improve his judgments regarding current finishing equipment and materials, and assist him in making the coating operation more productive, profitable, and efficient.
WHAT IS MEANT BY TRANSFER EFFICIENCY
Transfer Efficiency is defined as the amount of coating that actually lands on the coated object, compared with the total amount of coating sprayed from the gun.
HOW TO CALCULATE TRANSFER EFFICIENCY
There are basically two methods for the determination of the transfer efficiency. Although they are simple in concept, they are not easy to execute under practical conditions. In controlled laboratory situations transfer efficiency can be measured quite accurately and reproduced within 1.5% accuracy, However on production lines dealing with wide variances in physical conditions (pump pressure, viscosity of coating material, fluid settings, tip sizes and f-m widths etc.) besides the variety of cars in size and design) make it difficult to compare so that results may differ as much as ± 10%. While those wide variations tend to reduce the value of such a transfer efficiency test, by trying to keep the conditions as uniform as possible, it is still worthwhile to study the influencing factors and maximize the transfer efficiency.
The first method to determine the transfer efficiency uses a procedure in which the part being coated is carefully weighed before application of the coating and again after sufficient curing of the material. At the same time the weight of the can holding the coating before use as well as the can holding the coating left after the spraying are weighed. Assuming that the weight solids of the coating at spray consistency is known as well, the transfer efficiency can be calculated.