What are the Causes of Outgassing in Powder Coating?May 1, 2019
Outgassing incidents make process managers feel as if they're living on their last nerve. The curing oven has perfectly fluidized the powder, and those tiny particles are uniformly covering a complex piece of metal-based geometry. It feels like a celebratory pat on the back is in order, then that ugly pinholing effect is discovered. It's back to square one, then. What in the world is causing the bubbling, anyway?
A Back to Basics Approach
There's the clue, the notion that something is making the fluidized powder coating bubble. And what produces bubbles? Gasses, they're the medium that spawns the tiny coatings blisters. The gasses can't be coming from the dry powder, though. Apart from the material's fluidizing talents and surface sealing capabilities, the tiny resin-loaded grains are practically inert. No, to identify the causative factors here, the sleuthing procedure goes back to a point before the powder coating was applied. It's here, with the metal still naked and unadorned, that the pockets of gas are hiding. Let's identify the main culprits and list them:
• Trace quantities of solvents
• Oil and grease deposits
• Oxidisation issues
• Trapped moisture
• Cast metal gasses
• Coating thickness
A bulleted list lets readers quickly scan through the worst offenders, but it lacks detail. Clearly, we need those details so that corrective actions can be developed.
A Detail-Oriented Look at Outgassing Causes
Contaminants are a real issue, one that can severely hamper a quality-assured powder coating procedure. On the plus side, this type of outgassing can be corrected without too much effort. The contaminants, be they grease or dirt, are probably adhering to a workpiece after it leaves the pre-processing room. No worries, improvements to the workpiece preparation stage will soon clean-up this problem. For trapped moisture, a brief pre-treatment phase in a warm oven will evaporate any signs of trace humidity. Incidentally, there's one other corrective procedure that's become prominent in powder coating activities. Back at the pre-treatment room, with cast iron parts outgassing producing the bubble rupturing energies this time around, special primers are used to create a gas-sealing barrier.
Contaminants trap gasses. Depending on the chemicals contained in those dirty patches, some contaminants produce their own gasses. Then there are oxidized patches and trapped fluids, which evaporate when the curing heat is applied. The pockets of rising gas/vapour pass through the melted powder, then they rupture on the powder coating surface. Ugly little pinholes remain after the fluid cures. Worse than ugly, though, those little patches of popped bubbles physically weaken powder coated finishes.
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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