Metal Painting and Coating Operations
Table of Contents Background
Regulatory Overview Planning P2 Programs
Overview of P2 Surface Preparation
Appendix C Economic Analysis
Selection of a particular paint system (paint and application method) for a specific application depends primarily upon the products to be coated and production requirements. Before selecting a system, a comprehensive economic analysis considering the following items should be performed:
The following section provides one method for conducting a comprehensive economic analysis that considers all four factors. Technical assistance providers can help companies conduct this analysis to compare P2 options for painting operations.
Conventional liquid paints are comprised of both volatile and nonvolatile components. When paint is applied to the part, the volatile components evaporate, leaving the nonvolatile components to form the actual finish. In order to evaluate a coat of an applied finish, one must consider: 1) the nonvolatile fraction of the paint versus the product cost and 2) the efficiency of the paint application method (i.e., transfer efficiency).
Cost per Volume of the Nonvolatile Fraction of the Paint
The cost of a paint based on its nonvolatile (solid) fraction can be calculated from product information (generally the product Material Safety Data Sheets [MSDS]). For example, a paint that costs $15 per gallon and contains 33% solids actually costs $15 divided by 0.33 or $45.45 per gallon of solids.
If a desired film thickness is known, this cost can be further broken down into a cost per applied surface area using the following equation:
Cost of paint solids per gallon x film thickness in mils x 0.0006233 = paint cost per square foot of applied finish (where 0.0006233 is a unit conversion factor)
Using the paint cost of $45.45 per gallon of solids and a 2 mil (1 mil = 0.001 inch) finished film thickness, the paint cost per square foot of applied finish (assuming a 100% transfer efficiency) would be:
$45.54 x 2 x 0.0006233 = $0.057 per square foot (ideal)
Transfer Efficiency Versus Paint Cost
The above calculation gives the minimum or ideal cost of paint per square foot of applied finish because it assumes that 100% of the paint product adheres to the part being painted. In order to get an actual cost, one must also include transfer efficiency. In most spray painting operations only a portion of the product reaches the part to be painted. The remainder (overspray) is collected in the paint booth filters or settles to the floor of the paint area. The amount of paint reaching the product versus the total amount of paint sprayed is referred to as transfer efficiency. A 50% transfer efficiency means half the paint adheres to the product and the other half is wasted. To calculate the actual cost of paint per square foot of applied finish, one must include the estimated transfer efficiency of the paint operation into the above formula as follows:
(Ideal paint cost per square foot x 100)/TE = Actual paint cost per square foot where: TE equals transfer efficiency
Using the previous example and a transfer efficiency of 50%, the actual paint cost would be:
($0.057 x 100)/50 = $0.114 per square foot (actual) (IWRC, p. 10-12)
A small manufacturer of metal cases for consumer electronics currently coats its products with conventional solvent-borne coatings. The firm is considering changing its current coating and application system to one containing lower VOC content and higher transfer efficiency, and they want to know what the coverage, total reduction in emissions and materials cost would be for the new system.
Calculating Material Savings and Emission Reductions
Coverage = (paint volume x % volume solids x % transfer efficiency)/dry film thickness
If the surface to be covered is the same for both production scenarios, then:
(G2 (gallons) x % VS2 x %TE2)/FT2 (mils) = [(G1 (gallons) x % VS1 x % TE1)/FT1 (mils)]/FT1(mils)
G2 (gallons) = (G1 (gallons) x FT2 (mils)% VS1 x % TE1)/(FT2 (mils) x % VS2 x % TE2)
Emissions = paint volume used x VOC content of paint
E (pounds) = G (gallons) x VOC (pounds per gallon)
Total materials cost = paint volume used x cost per gallon of paint
TMC ($) = G (gallons) x C ($ per gallon)
Substituting values, we get: G2 = 4400 x 1 x 35 x 28/.8 x 30 x 65 = 2764 gallons
VOC emissions (E) = G (gallons) x VOC (pounds per gallon)
Current system: 4400 gallons x 3.5 pounds per gallon = 15,400 pounds per year VOC
Proposed system: 2764 gallons x 2.5 pounds per gallon = 6910 pounds per year VOC
Reduction in VOC: 8490 pounds per year VOC (NJTAP, p. 11)
Relative Costs of Various Coating Process Equipment
Because of the various painting requirements present in the broad category of metal manufacturers, providing a realistic cost comparison between one paint application method and another is nearly impossible. In order to provide some degree of comparative information the following table is offered.
Cost/Benefit Summary for Spray Application Methods
NOTE: Capital cost refers to the cost of the system in comparison to conventional air spray. The higher the process complexity, the higher the associated costs (i.e., training for employees and maintenance).
Energy consumption should also be a consideration when selecting a paint and application method. Energy consuming operations include pretreatment (i.e., parts washing), ventilation, and makeup air/heat for curing. All three of these factors are directly related to the type of paint and application method selected. For comparative purposes, powder coating and waterborne paints might have higher energy requirements because of increased curing demands (IWRC, p. 10-12).