Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 322 of 331

PAINT USAGE PER SQUARE FOOT Paint usage per square foot can be calculated by totaling the amount of paint used then dividing it by the square footage of parts produced. Calculating paint per square foot using this method is accurate if the filmbuild is the same through- out the part and from part to part. If the filmbuild varies, the above method calculates the average paint used per square foot for all the pieces produced. To accurately calculate the paint per square foot, for the different pieces pro- duced, the filmbuild needs to be considered. Calculating the paint per square foot using the filmbuild is more complicated then the above method, but the results are more useful. The method using filmbuild is listed below. Calculating Paint Usage per Square Foot The following formula uses the filmbuild to calculate paint usage per square foot: One square foot is equal to 144 square inches. The factor 231 is used to con- vert cubic inches to gallons. Percent volume solids are available from the paint supplier. The above formula provides the theoretical usage for a perfect paint system; the amount of paint used is equal to the amount applied. Real-world system losses are not considered. When calculating the actual usage, which takes into ac- count the system losses, use the following formula: Transfer efficiency is the percentage of paint that actually makes it to the part. How to determine transfer efficiency will depend on the type of paint and equipment used to apply the paint. Contact the equipment supplier and paint supplier for assistance on determining transfer efficiency. Knowing paint usage per square foot can be very beneficial. Usage for a new part can be calculated. The cost impact of a process modification could be calculated. The actual cost of coating a part could be determined. An example of how this formula follows. Example A plant produces 1,000,000 ft2 one and a small one. The large part accounts for 750,000 ft2 part 250,000ft2 of painted parts a month. It coats two parts, a large a month, the small . The large part has a paint thickness of 1.0 mil. The small part spec- ification is a thickness of 1.0 mil, but because of the way the parts are mixed the small part has a paint thickness of 1.2 mils. If the parts where processed dif- ferently and the paint thickness of the small part was reduced to 1.0 mil, what reduction in paint would occur? Assume the percent volume solids are 35, the transfer efficiency is 90%. Large part usage per ft2 = [(144in2 /ft2 = 0.001979 gal/ft2 x 0.001 in)/(35/100 x 231 in3 @ 1.0 mils /gal)]/90/100 x 1.0 mil 321

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