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Painting problem solver
Rai tingproblem sover ACCURATELY ESTIMATING THE
Cost ($) = Cost of paint (Sigal) x Number of gallon s used.
AMOUNT OF PAINT
PAINT PEELING ON ROBOTIC MACHINES My client manufactures robotic equipment weighing approximately 5 tons. Initially) the equipment had been pretreated with a zinc phosphate and this was followed by an epoxy system. However, this wassubsequently changed toaliphaticpolyurethane, which ismoreappropriate when an aestheticfinish isdesired. The machines arestored outdoorsfor weeks ormonths before they are ready for shipping; then they are packaged in Vapor Corrosion Inhibiting (VCI) paper. Once shipped) the machines can remain onboard a ship for 3-4 months. The polyurethane tends to be easily damaged) especially at edges) and we have observed that sometimes theentirepaint system peels offonce the machinearrives at its destination. I have advised nry client to upgrade the pretreatmentprocess from 3-in-] phosphatetoa sprayapplication. My client wantsto revert to the oldepoxy system) which appears tohavebeen more reliable. Moreover, his production has increased many [times over] in recent y ears and he wants a faster curing. Can y ou please suggest an approach to his problem?
CALCULATING COST OF PAINT USED ON A PROJECT Can you please help me calculate the material cost to paint any size surface? SometimesI use an HVLPspray gun; other times an electrostatic gun. Transfer efficiency varies according to configuration of thepart) painter's technique, etc.
: Q
A: The following equation is in American un its, but if
necessary you can convert them
to
metric:
Coverage (tt2/gal) =Gals x 1604 (tt2/gal/mil) x % volume solids x % transfer efficiency Dry film thickness (mil) Thickness is expressed in mil = 0.001 inch 1604 is the coverage of on e gallon of "solid" at a film th ickness of 1 mil. If a can of paint contains no solvents, water or other volatiles, it is said to be 100% solid. Although it might be in liqu id form in th e can, it will solidity into a solid after it has cur ed. You will find the volume solids of the paint on th e technical data sheet, or on th e MSDS. If th e paint manufacturer gives you the % volume volatiles: % volume solids = 100 - % volum e volatiles
The average dry film thicknesses rang e for a primer and top coat is typically 1.0-2.0 mil per coat , but the technical datasheer should provide the vendor's recommendations. As you mentioned, rransfer efficiency (TE) varies according to several parameters, and unle ss you actually measur e it, you will need to guess. For spray application, 50 % is a conservative number, but if you are painting small diameter tube s or small parts, TE can be in the 10-25% range. To calculate number of gallons used, we modify the equation as follows:
Gals
=
Coverage (tt2lgal) x Dry film thickness (mil) 1604 (tt2/gal/mil) x % volume solids x % transfer efficiency
If you know the cost of 1 gallon of pain t:
www.metalfinishing.com
Q:
A:
Your comments and observations regarding th e epoxy system are correc t. Epoxy is not the best choice as a finish coat for outdoor equipment, but it is rhe besr choice as a primer. The use of polyurethane as a top coat is a good choice and is probably the most popular system currently being employed for the coating of heavy outdoor machinery. Th e cause of the paint failure is probably not due to th e selection of the two coatin gs, but rather due to application procedures. I'm not sure if a change from th e 3in-I phosphate system to a zinc phosphate spray system of the will im prove th e performance polyurethane. Certainly, zinc ph osphate provides bette r corros ion resistance than 3-in-l iron phosphate, but th at might not influence th e poor performance of th e polyurethane at the edges. Before you make any changes, I suggest thar you first try to understand why rhe polyurerhane is chipping ar rhe edges: Perhaps your client is nor using high-quality aliphatic polyurerhane? Are you gerring sufficient film thickness alon g th e edges? Are you following the corr ect procedures when mixing the two components? Peeling of a coating system is often not due to surface preparation. Instead, it can be cau sed when the painter applies too many coats of paint, one over the other, before the solvents of the previous coats have evaporated. Th is is known as "solvent entrapmen t." September 2010 I metalfinishing 123
paintingproblemsolver Since your client's production rate has increased dramatically in recent years, I assume that his painters are being rushed to get the machines out of the spray booth at a faster rate and this is often accomplished by applying too many coats wet-an-wet. Add to that your statement that the machines are packaged in vcr paper, and I am even more confident that he is not allowing sufficient time for solvents to evaporate from the paint film. There are potentially more questions I could ask, but I'm constrained by not having seen the failure; please send me digital photographs for review. If you do decide to send photos, please send some that show the entire machine and then get closer so that I can see exactly what the failure looks like.
In the meantime, here are some suggestions:
· Each coat of paint mustbe applied within the range of the manufacturer's recommended dry film thickness. When rushed to get the jobdone quickly there is a tendency to apply thicker coats to achieve more coverage in a single coat. · Allow sufficient time forthe coating to tack-dry before applying the nextcoat. · If the machines are dried in an oven, don't apply too much heat too soon afterpainting. Instead, allowsolvents to flash off before increasing the temperature. When you apply heattoo soon,the top coat starts to form a skin that prevents solvents from escaping. • Don't wrap the machines in vel
paper until the coatings have fully cured and the solvents have escaped from the film. . If the coating tendsto damage more easilyat the edges, apply an additional stripe coat of epoxy primer alongalledges prior to application of the topcoat.
810
Ron Joseph, organic coating editor of Metal Finishing, is a full-time paint and coating consultant at Exponent Failure Analysis Associates in Menlo Park, Calif He has worked in thepaints and coatings industry for more than 38 years.
www.metalfinishing.com/advertisers
24 I metalfinishing I September2010
www.metalfinishing.com
Cost ($) = Cost of paint (Sigal) x Number of gallon s used.
AMOUNT OF PAINT
PAINT PEELING ON ROBOTIC MACHINES My client manufactures robotic equipment weighing approximately 5 tons. Initially) the equipment had been pretreated with a zinc phosphate and this was followed by an epoxy system. However, this wassubsequently changed toaliphaticpolyurethane, which ismoreappropriate when an aestheticfinish isdesired. The machines arestored outdoorsfor weeks ormonths before they are ready for shipping; then they are packaged in Vapor Corrosion Inhibiting (VCI) paper. Once shipped) the machines can remain onboard a ship for 3-4 months. The polyurethane tends to be easily damaged) especially at edges) and we have observed that sometimes theentirepaint system peels offonce the machinearrives at its destination. I have advised nry client to upgrade the pretreatmentprocess from 3-in-] phosphatetoa sprayapplication. My client wantsto revert to the oldepoxy system) which appears tohavebeen more reliable. Moreover, his production has increased many [times over] in recent y ears and he wants a faster curing. Can y ou please suggest an approach to his problem?
CALCULATING COST OF PAINT USED ON A PROJECT Can you please help me calculate the material cost to paint any size surface? SometimesI use an HVLPspray gun; other times an electrostatic gun. Transfer efficiency varies according to configuration of thepart) painter's technique, etc.
: Q
A: The following equation is in American un its, but if
necessary you can convert them
to
metric:
Coverage (tt2/gal) =Gals x 1604 (tt2/gal/mil) x % volume solids x % transfer efficiency Dry film thickness (mil) Thickness is expressed in mil = 0.001 inch 1604 is the coverage of on e gallon of "solid" at a film th ickness of 1 mil. If a can of paint contains no solvents, water or other volatiles, it is said to be 100% solid. Although it might be in liqu id form in th e can, it will solidity into a solid after it has cur ed. You will find the volume solids of the paint on th e technical data sheet, or on th e MSDS. If th e paint manufacturer gives you the % volume volatiles: % volume solids = 100 - % volum e volatiles
The average dry film thicknesses rang e for a primer and top coat is typically 1.0-2.0 mil per coat , but the technical datasheer should provide the vendor's recommendations. As you mentioned, rransfer efficiency (TE) varies according to several parameters, and unle ss you actually measur e it, you will need to guess. For spray application, 50 % is a conservative number, but if you are painting small diameter tube s or small parts, TE can be in the 10-25% range. To calculate number of gallons used, we modify the equation as follows:
Gals
=
Coverage (tt2lgal) x Dry film thickness (mil) 1604 (tt2/gal/mil) x % volume solids x % transfer efficiency
If you know the cost of 1 gallon of pain t:
www.metalfinishing.com
Q:
A:
Your comments and observations regarding th e epoxy system are correc t. Epoxy is not the best choice as a finish coat for outdoor equipment, but it is rhe besr choice as a primer. The use of polyurethane as a top coat is a good choice and is probably the most popular system currently being employed for the coating of heavy outdoor machinery. Th e cause of the paint failure is probably not due to th e selection of the two coatin gs, but rather due to application procedures. I'm not sure if a change from th e 3in-I phosphate system to a zinc phosphate spray system of the will im prove th e performance polyurethane. Certainly, zinc ph osphate provides bette r corros ion resistance than 3-in-l iron phosphate, but th at might not influence th e poor performance of th e polyurethane at the edges. Before you make any changes, I suggest thar you first try to understand why rhe polyurerhane is chipping ar rhe edges: Perhaps your client is nor using high-quality aliphatic polyurerhane? Are you gerring sufficient film thickness alon g th e edges? Are you following the corr ect procedures when mixing the two components? Peeling of a coating system is often not due to surface preparation. Instead, it can be cau sed when the painter applies too many coats of paint, one over the other, before the solvents of the previous coats have evaporated. Th is is known as "solvent entrapmen t." September 2010 I metalfinishing 123
paintingproblemsolver Since your client's production rate has increased dramatically in recent years, I assume that his painters are being rushed to get the machines out of the spray booth at a faster rate and this is often accomplished by applying too many coats wet-an-wet. Add to that your statement that the machines are packaged in vcr paper, and I am even more confident that he is not allowing sufficient time for solvents to evaporate from the paint film. There are potentially more questions I could ask, but I'm constrained by not having seen the failure; please send me digital photographs for review. If you do decide to send photos, please send some that show the entire machine and then get closer so that I can see exactly what the failure looks like.
In the meantime, here are some suggestions:
· Each coat of paint mustbe applied within the range of the manufacturer's recommended dry film thickness. When rushed to get the jobdone quickly there is a tendency to apply thicker coats to achieve more coverage in a single coat. · Allow sufficient time forthe coating to tack-dry before applying the nextcoat. · If the machines are dried in an oven, don't apply too much heat too soon afterpainting. Instead, allowsolvents to flash off before increasing the temperature. When you apply heattoo soon,the top coat starts to form a skin that prevents solvents from escaping. • Don't wrap the machines in vel
paper until the coatings have fully cured and the solvents have escaped from the film. . If the coating tendsto damage more easilyat the edges, apply an additional stripe coat of epoxy primer alongalledges prior to application of the topcoat.
810
Ron Joseph, organic coating editor of Metal Finishing, is a full-time paint and coating consultant at Exponent Failure Analysis Associates in Menlo Park, Calif He has worked in thepaints and coatings industry for more than 38 years.
www.metalfinishing.com/advertisers
24 I metalfinishing I September2010
www.metalfinishing.com