Why is Vacuum Impregnation Performed on Powdered Metal Parts?

October 17, 2017

When a powder coated part ‘gasses out,’ its shell-like membrane is compromised. Just where is this trapped gas coming from? The component has been scrubbed clean, so there’s not a speck of dirt to worry about, nor is there a single rough edge. Maybe it’s the metal, a castings issue that’s causing surface porosity? Enter vacuum impregnation technology, a process that’s designed to seal those micro-pores.

Vacuum Impregnation Plugs Leaks 

Powder coating processing has several procedural advantages on its side. Yes, this is an environmentally-friendly finishing technique, one that produces an impenetrable shell. It’s also a relatively simple coating process, at least when the equipment is installed optimally. So where does vacuum impregnation fit into the production chain? Well, this is a pre-production operation, a stage that’s implemented before the electrostatic adherence phase. Basically, this operation is intended to seal the microscopic pin holes in a metal component. It begins by inserting the workpiece into a pressure vessel. Air is drawn out of that containment unit and an anaerobic sealant is channeled into the resulting vacuum. In effect, that microscopic material ‘plugs’ the leak paths on the surface of the cast metal component so that the porosity issue is solved. Once those near invisible porous metal exterior planes are fully plugged, the finish can harden as a smooth membrane.

Why is Component Porosity an Issue? 

Manufacturing technology is incredibly advanced. Unfortunately, there are still process imperfections to deal with in the metal castings industry. Most of the time, those imperfections go unnoticed. The part is machined and polished after the casting has been ejected. It fulfils its purpose in a mechanically tough application, perhaps as a crankshaft or power transmission linkage. However, surface porosity is not an option if the workpiece in question is entering a powder coating oven, a place that relies on uniform temperature distribution during the powder fluidization phase. All of a sudden, those pin holes make themselves known. The heat, a thermal load that melts the powder and sends it flowing across the metal surface, is being interrupted by the gasses escaping from those tiny pores.

Outgassing surfaces corrupt powder coated finishes. They leave small pinpricks in what should be a perfectly sealed thermoplastic skin. Water can penetrate those openings, as can other corrosive liquids. That’s why a vacuum impregnation pre-processing phase is employed. It seals the part and ‘plugs’ those holes before the powder coating is applied. Found sometimes in aluminium and zinc metal castings, and as gas cavitation defects and oxide film production errors, casting porosity is a curing oven’s enemy, but a solution exists in the form of vacuum impregnation technology.

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