Causes of Peeling and Pinholes in Powder CoatingMarch 29, 2017
The powder coating project is nearing completion. Yet another fine result is making its way to a satisfied customer. But what if the finish is exhibiting strange pinholes? The normally skintight coating is either peeling or there're more pinholes spreading across the surface. What's causing these unsightly, potentially seal corrupting defects? Well, there's a phenomenon called outgassing. Could this be our pinhole precipitating culprit?
What is Outgassing?
Gaseous anomalies occur in powder coating operations when the workpiece metal is slightly porous. This occurs back at the casting stage, the pouring of the liquid metal. Essentially, the quality of the metal casting process is in question. The curing oven releases these trapped gases, at which point the finish is rendered unworkable. A preheating stage that raises the metal's temperature above the curing temperature eliminates outgassing.
Other Pinhole Miscreants
A low-quality workpiece could introduce this gas venting effect, but quality issues are only the beginning. Certain zinc galvanised alloys are subjected to this bleeding-edge finishing method. Oils and greases, moulds and grime, also produce gaseous fall-out when they're exposed to high temperatures. Again, preheat the part. Ensure the workpiece is completely free of such contaminants. As for zinc galvanised surfaces, consider a product sealant that coats the part before the powder is applied.
Solving the Peeling Question
The cured powder has flowed properly. It's adhering to the workpiece surface, but its texture is strangely pebbled. This is the orange peel effect. The smooth finish has cured to form ridges and depressions, a coarse surface that most definitely does not fit the customer's design proviso. A high gun kV setting is a known causative factor here, with the high charge corrupting the smooth surface veneer on the part. A poorly cleaned or prepped initial metal part also spoils any efforts to create a satin finish. Finally, the powder medium needs to be assessed for quality issues. It's just possible that either the brand is wrong for the job, or that the powder reclaim mechanism is mixing improperly with the virgin powder to create a substandard discharge.
No single system asset earns a pointing finger. Granted, operator error is possible, so adjust gun voltage before calling time on the work. The preprocessing stage also needs to be assessed, as does the quality of the powder medium. Then, when all other causes have been eliminated, this is the point that the base metal requires examination. If the metal is found guilty, then apply a preheating stage or a barrier coating, the twin solutions that stop outgassing problems.
GP Industries Pty Ltd
1 Regal Court,
Vermont South VIC 3133
Phone: (03) 9802 1355
Fax: (03) 9802 6027
20 Burgess Road,
Bayswater North VIC 3153
Phone: (03) 9761 7676
Fax: (03) 9761 7671
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