Metal Painting and Coating Operations
Table of Contents Background
Regulatory Overview Planning P2 Programs
Overview of P2 Surface Preparation
Appendix G Measuring Transfer Efficiency
Before conducting any transfer efficiency test, several parameters need to be established:
After identifying the basic parameters, the paint operator must establish a fluid flow rate that is representative of day-to-day production. The operator needs to set the optimum air pressure for coating atomization and to adjust the coating viscosity and temperature to be representative of typical operating conditions. If the operator is using electrostatic equipment, they must confirm that the parts are properly grounded, the coating is adjusted so that resistivity meets manufacturers recommendations, and the air velocity through the spray booth is neither too high nor too low. A decision also needs to be made in selecting the proper transfer efficiency test.
Guidelines for Choosing Transfer Efficiency Methods
The Weight (Mass) Method
Determining transfer efficiency on a weight or mass basis, as is usually the case, requires purchasing or renting an electronic balance capable of measuring within 0.5 grams. There are balances available that can weigh parts up to 70 pounds (154 kilograms) with this accuracy. The balance must sit on a hard surface, such as a metal table, concrete floor, or cement slab. Operators should never place a piece of cardboard under the balance, as this will lead to inaccurate results.
In addition, the operator must shield the balance from all drafts that occur on a factory floor (this can be accomplished by surrounding the balance with cardboard walls). The operator must also ensure that the pressure pot or coating reservoir is not too heavy for the balance and that the part to be coated falls within the acceptable limits of the balance.
The balance should be set so that the air bubble in the bubble glass falls within the center of the glass. In addition, all four feet of the balance must be in firm contact with the ground or surface. Finally, the operator must calibrate the balance using standard weights, which are often supplied by the balance manufacturer.
The paint operator should follow the steps below to determine the weight of coating used during the operation. This process begins by measuring the liquid coating, then calculating the weight of the solid coating.
1. Before beginning the test, appropriately label each part and then accurately weigh the parts on the balance. Record the weights.
2. Place the pressure pot or coating reservoir on the balance and slowly fill it with coating, ensuring not to exceed the limit of the balance even after tightening the pressure pot cover.
3. Before commencing the actual test, apply the coating to several dummy parts to ensure that the coating process is representative of actual operating conditions.
4. To begin the test, disconnect the fluid and air hoses from the pressure pot. Do not allow any paint to drip to the floor, as it is imperative that the coating fills the line all the way up to the spray gun. Record the coating weight and then replace the air and fluid hoses and begin the spraying operation.
5. For accurate results continue spraying until at least one quart of paint has been used (approximately 2.2 pounds or 1 kg). After applying the coating to the selected parts, immediately disconnect the fluid and air hoses from the pressure pot, weigh the pot and record the second reading. Repeating this entire procedure three times will help determine an average transfer efficiency.
At any time during the test, the operator should take a small grab sample (approximately one pint of the coating) from the pressure pot. The operator should be sure to close the container to prevent solvent evaporation. The facility should send the sample to an analytical laboratory that will conduct a percent weight solids test in accordance with ASTM D2369 (this is the standard test method for volatile coatings).
The company should not bypass the sampling procedure by simply calling the coating manufacturer to request information on the percent weight solids or referring to the MSDS. Even a small discrepancy between the manufacturer's value and the actual value obtained from the pressure pot sample will make a large difference in the transfer efficiency calculations.
The weight of solids used is calculated by following this equation:
As noted earlier, before starting the transfer efficiency test, each part must be labeled and weighed. After applying the coating, the operator should allow thorough curing before weighing the part again. If the coating is normally air or force-dried, allow extra time for all of the solvent to evaporate. Curing the parts in an oven set at 230°F will result in a more accurate transfer efficiency reading, even if this is not the normal method for curing. This oven curing schedule is identical to what a laboratory will use to determine the percent weight solids of the one pint sample taken earlier.
After the coating has thoroughly cured, the operator should weigh the parts. The difference between the weights of coated and uncoated parts represents the weight of solid coating deposited. Knowing the weight of solid coating used, and the weight of solid coating deposited, the operator can calculate the transfer efficiency as follows:
The credibility of the results depends entirely on the accuracy of the weighing. If the factory has drafts or vibrations that could affect the balance, the operator may wish to take two or three readings before recording any one weight. In addition, the laboratory determination of percent weight solids must be accurate. Finally, the accuracy of the results will increase if a number of parts are coated during any one test.
When using this method for a large part with a relatively simple geometry the operator can still use the weight method by "wallpapering" the surface with pre-weighed aluminum foil. At the conclusion of the test, the operator should weigh the dried coating on the foil to complete the calculations.
The cost to conduct a transfer efficiency test can be minimal. Companies can usually rent an electronic balance for less than $300/week. A laboratory charge might be $150/ sample. The other "in-house" expense is labor. If a consultant is retained costs might range from $3,000 to $5,000, depending on the complexity of the operation.
The Volume Method
The volume method is not as accurate as the weight method. To measure transfer efficiency using the volume method, a laboratory must determine the percent solids of the coating as applied, as described in the weight method. To determine the volume of solid coating deposited, a lab measures the average film thickness of the deposited coating, as well as the total surface area of the coated parts.