Powder Coating Shrinkage on Edges

A. Hi Cal!

I am not sure why your coating supplier is telling you this information. During the powder coating application, the electrostatics usually tend to apply more powder to the edges than the flats. You can usually see this on most powder type properties. Especially when using a TGIC as I assume your using for outdoor use. The more likely candidate for suspect in your case would be that the 1/4" stiffener/rib was laser cut.

The plasma machine was adequate until customers started asking for closer tolerances and straighter edges. The running speeds are faster too. Plasma usually leaves burrs that needs grinding before topcoats. The two most common methods for removing laser smut/carbon are mechanically and chemically. The residual material that is left by the laser must be removed before topcoating or you will have premature topcoat failure. I think the chemical (acid) removal method is probably the easiest to implement. My guess is that once you remove this material, ! you will notice the powder attracting to entire areas previously suspect and you will eliminate the premature paint failure.Many companies use hydrochloric acid to eliminate the laser smut. Lastly, remember that if you use high acid concentrations to clean with, you need to watch out for premature flash rusting.

Bob Utech Benson, Minnesota

Ed. note: Bob is the author of: "High Performance powder Coating" on eBay or AbeBooks or Amazon [affil links]

"You don't need a weatherman to know which way the wind blows" -- Bob Dylan

A. This by way of saying that although Bob Utech is far more qualified to answer your question than me, I have seen this thin-at-sharp-outside-edges problem. When parts are powder coated then plated, forexample, you can count on seeing plating "trees" building up along the edges/corners. I understand that outside edges get the most plating current, but still, the trees can't be there unless the coating is perforated, and that always seems to happen along sharp outside edges as the powder melts and fuses. Surely something about the cohesive contraction causing a tension and thinning at corners.

A. Cal,

This problem is not real unusual in the brighter colors: Reds, oranges and yellows. Because of the cost of pigments especially in heavy metal free powder formulas, some manufacturers try to keep powder cost down by reducing the amount of pigment and using less expensive fillers.

I have seen the edges show through on Safety Yellow that was applied at 4 mils. Other powders will have full edge coverage in Safety Yellow at 1.5 to 2 mils, BUT the price of the powder is significantly higher. Because you are having the work done through a job shop who most likely uses the same color for other customers, they are probably reluctant to change to a higher cost powder because they would not be able to increase the price to their other customers.

If you have a large enough volume of work, they should be able to work something out. If not, then you either need to find someone receptive to doing the work and finding a powder to solve the problem or bring the work in house, which would require purchase of a powder system, but you would have total control.

This problem occurs on sheared edges as well as laser cut. We have been seeing a lot of laser cutting shops adding an additional two stages to their washers to chemically remove the burr/slag left by the laser cutting. This is generally on only one side. This will not stop the edge show through except if it is due to the slag. It is the nature of all powders to pull away from the edge somewhat during cure due to surface tension issues.

A. Although I am not a plater, I am convinced that Ted "hits the nail on its head"(old dutch proverb). It is a well known fact that the thickness (DFT) of a coating - at a sharp edge- is less than the thickness at the flat. This effect is stronger with liquid paint than with powder. During the drying/curing the coating "shrinks" (solvent evaporates, due to melting and crosslinking the spec. density of the layer increases and therefore less volume is left).

This means that a) the dft at the sharp edge is less and b) there exists an internal tension in the coating-layer possibly causing micro-cracks. Probably the cause of the trees Ted described. Yes, one can notice a layer building up at the edges of thin sheets, but also there the dft AT THE EDGE is less than the one at the rest. To see this yourself, simply make a crosscut and look at it through a magnifier (20x will do). But..... we are not talking about thin sheet but material of about 6 mm thick, an enormous distance compared to the dft of the powder (60 -100 microns).

Remedy: ROUND THE SHARP EDGE before powder coating! How? The powder coater has to remove the laser scale/oxides anyhow (provided lasercutting was done under "normal" gas)before applying the coating. I presume they did, otherwise Cal would have more corrosion problems than just at the sharp edge. If the coater uses any mechanical system (grinding, blasting) normally the edge is rounded sufficiently. If he uses a chemical system like pickling the rounding effect will NOT be enough. In such a case the job-shop and the client must come to an agreement, one of the two has to do it. Normally it ends up that the producer of the part performs this because he is better equipped (a simple hand-grinder is enough) and he knows what is allowable for his product.

A. I agree that some non-electrostatic and many liquid paint systems have less edge coverage. You are right that it can be caused by solvent evaporation and paint shrinkage that pulls away from the edge, however this is not true of powder systems. In general, powder will "usually" have more edge coverage than not. This is especially true if the part has a recess or a channel on its surface, the electric field will follow the path of the lowest resistivity to ground (i.e. the edges). Therefor, with most of the electric field concentrating on the edges, powder deposition will be greatly enhanced in these areas and the edge coating layer will build up very rapidly. This is one reason that you find back -ionization occurring at the edges first. There are instances of Faraday Cage areas that may not accept powder no matter how much powder is applied. Take some 3x5 test panels and spray up 2 or 3 at different film thicknesses and you will visually see the increased edge thickness.

Bob Utech Benson, Minnesota

Ed. note: Bob is the author of: "High Performance powder Coating" on eBay or AbeBooks or Amazon [affil links]

A. Dear Faribia

if you want make a good control of the powder thickness specially at the sharp edge you have to make a control in KV of the guns , distance of guns from the surface of parts , your earth of the conveyer , the quality of the powder , the speed of the gun reciprocator , the speed of conveyer chain , the quality of the air line
if all these point are ok you will have a good film thickness in all points at the part surface
best regards

Black Wrinkle Paint

on eBay or

Amazon

(affil links)

A. According to my experience, deburring is not solution but you have to do it. The easy way is after curing the paint, paint the edges again and cure it. The second solution is, rounding edges with oscillation sanding machine. And after applying the paint. The sand will be 50 or 80 (aggressive sand). Another way is wrinkle paint ⇦ this on eBay or Amazon [affil links] . Wrinkle or textured paint can be used after deburr operation.

Okay, the solution is very interesting but I will share with you. First you have to take the burrs with oscillating sand machine, standard sanding machine is inefficient. Second you will speak with good static paint manufacturer they will increase the viscosity of paint with extra materials. In my country 3 different paint manufacturers did it. After if you are using primer type static powder you will check the paint thickness, if thickness is so much, you will clean with pressurized air, after you will make painting again. After conclusion is excellent. Durability of paint under salt test 1000 hr without scratch. Regards,

A. The layers of powder will flow together until a full cure schedule is done and crosslinking occurs. I hardly ever do a full cure on basic polyesters or TGIC powder until my final coat including clear. This basically makes all the powder bond together. There are some exceptions to this, Chrome being the biggest. I found that a full cure on a chrome base before the clear increases the brightness of the chrome tremendously. Metallics that are dry mixed with a lot of flakes need a full cure. And dormants, Neons and fluorescents seem a bit brighter with a full cure before clear but not much. And super durable polyesters.

I can tell a big difference when I'm stripping parts I've done a partial cure until the final coat. It takes forever in the best strippers where with a full cure on multiple coats it still takes a while but you can see the different layers stripping off. That's just the way I do it and it seems to work very well for me. I've had people tell me to scuff the powder before recoating but I couldn't tell any difference when I did. I keep a small hand held MAP gas torch by my booth and it's been a life safer on corners and welds. If it gives me problems I'll hit the corner for a second with the gas and remove my deflector tip and powder will lay right in there. Again this is just what I've found works for me .