Calculation Method to Transfer Efficiency for Spray Paint Operations
RAPID RESPONSE QUESTION:
Do you have an easy yet specific method to calculate transfer efficiency for spray paint operations?
Requested by: Washington Department of Ecology (on behalf of cabinet maker)
Very simply, transfer efficiency (TE) of a spray operation or finishing process is the amount of material that adheres to the substrate compared to the amount of material that was sprayed through the applicator toward the substrate. Transfer efficiency is expressed as a percentage.
Ken Grimm, PPRC, who trains facilities and operators in spray efficiency, and is certified in Spray Painting Efficiency Training (STAR®), says that initial reaction from operators is that 37% is a low rate of transfer efficiency. In fact, he rarely sees rates above 50% in automotive painters and almost never sees rates above 37% in industrial facilities.
Transfer efficiency is important with respect to productivity, and cost and environmental savings. A facility had a vendor consultant come in and measure their TE to a specific standard, and the result came in at about 17% TE. This translates to the following:
- For every $100 the company spends on paint as a raw material, they are, in a sense, throwing $83 of that paint away as overspray.
- They are paying to dispose of the overspray (as solid waste or hazardous waste), to take that $83 away from them.
- The lower the TE, the more airborne paint particles are captured in the air filters, requiring more frequent change of air filters (than a higher TE operation).
- The lower the TE, the more frequently new paint will be needed, increasing labor.
- Lower TE can help achieve compliance with air permits.
There are many variables to transfer efficiency, listed below. Painter technique is believed by many experts in spray efficiency to have more of an affect than any of the other variables.
- Painter technique
- Part size
- Part geometry
- Gun-target distance
- Coating viscosity
- Ease with which coating can be atomized
- Spray gun design and method of atomization
- Fluid pressure
- Atomizing air pressure
- Fan size
- Overlapping of successive spray gun strokes
- Orifice diameter of spray gun cap
- Air velocity in the spray booth
- Air balance in the spray booth
- Lead and lag triggering times
- Conveyor line speed
- Speed of spray gun travel
- Painter fatigue
- Lighting in the spray booth
- Space constraints on the spray booth
- Attitude of management
- Attitude of the painter
For electrostatic guns additional parameters include:
- Coating conductivity (or resistivity)
- Grounding of the parts
- Voltage potential between electrode and ground
When training painters, STAR® also addresses build efficiency (BE), related to finished build thickness of the paint on the substrate. This is important because, like TE, it is a measure of wasted paint (or efficient application). For example, if a customer specification calls for a finish thickness of 3 dry mils, and a part ends up with 6 dry mils of finish thickness, an operator who (theoretically) achieves 100 percent TE, would still have wasted 50% of the paint due to the extra 3 mils of paint coated on the part. All painters will likely exceed the 3 dry mils to be sure the coating thickness is not under spec. However, keeping the coating under, say, 4 dry mils (for a 3 mil spec) can be almost as important as ensuring a good TE rate.
Calculating Transfer Efficiency (TE)
In spray painting operations, a clear and specific description of how to calculate TE depends on desired accuracy of the result. Accuracy may range from a “back of the napkin” calculation to expensive laboratory tests when required to qualify an operator to a specific TE performance, and/or ratings for particular spray apparatus to ensure they meet NESHAP requirements (including methods by the South Coast Air Quality Management District (SCAQMD).
The method below is a simple way to determine an adequately accurate TE for “in-house” use.
 The South Coast Air Quality Management District (SCAQMD) has a certifiable procedure to verify that new guns meet NESHAP requirements before being authorized for use in the market, The US EPA specifically refers to use of this method to determine TE ratings for new guns.
A simple formula, adequate for “in-house” accuracy, is as follows:
TE = Weight gain of the sprayed part (WS)/Weight of material sprayed (WM) * Percent solids in coating (S)
In more detail, as items correspond to lettered steps below:
TE = Weight gain of the sprayed/cured part (f) – Weight of the unpainted part (a)/Weight of gun and paint before spray (b) – weight of paint and gun after spray (d) * % of solids in coating (S)
The procedure itself involves weighing a part or test panel before painting, then again after the paint has cured, and determining the total weight of the amount of paint sprayed for the operation. The percent solids (S) of the paint is also required for the calculation.
The following steps are required:
a) Weigh a part or test panel before it is painted, record this number.
b) Weigh the initial paint apparatus (with paint): If an HVLP or other type of cup gun is used, simply weigh the gun before spraying. If a pressure pot is used, weigh the pot before spraying. If a line system is used, weigh the 5 gallon bucket (or other container) before.
c) Spray the part and allow to fully cure.
d) Weigh the paint apparatus after painting. If an HVLP or other type of cup gun was used, weigh the gun after spraying. If a pressure pot was used, weigh the pot after spraying. If a line system is used, weigh the 5 gallon bucket (or other container) after spraying. Note that getting a weight from a line system will be a less accurate value unless there is a way to account for the paint remaining in the pump and the fluid line.
e) Subtract the result of (d), the weight of the paint apparatus and remaining paint, from (b), the initial weight of the paint and paint apparatus to determine WM.
f) Weigh the part after the coating has cured, record this number.
g) Subtract the weight of the original, unpainted part (Result from # 1) from cured part (Result from #4) to find WS.
h) Find the percent solids in the coating (S). The solids content weight can be obtained from either the MSDS or the data sheet supplied by most manufacturers, or by calling the manufacturer directly. This number will also have to be adjusted if the user adds solvent to reduce the coating viscosity.
i) Use the first formula above to calculate TE.
As an example, say that the test panel weighed 56 grams before painting and 68 grams after the coating was sprayed and then cured, so WS = 12 grams.
Let’s also say the weight of the coating used (WM) was measured at 54 grams and the percent of solids (S) is 60.
TE = 12 grams/54 grams x 0.60 = 37%
- Spray Painting Efficiency Training (STAR®), Pacific Northwest Pollution Prevention Resource Center
- Transfer Efficiency, Concept and Theory Training, by Graco (a spray equipment manufacturer)
- Transfer Efficiency, by Ron Joseph
 Manufacturer-supplied data for paint solids can be inaccurate because manufacturers tend to operate in ranges versus absolutes. For instance, if a data sheet says “above 50% solids”, it could be quite a bit higher than 50%, or a stated content of 60 percent solids could mean it is somewhere above 58% and below 62%. Hence, some may require laboratory testing for TE determinations.