The Coating Process

Parts are generally presented to the coating operation while they are at ambient temperature; the charged particles will be attracted to and accumulate on the article surface. The powder particles approach the surface, being attracted by their electrical charge.

Upon contacting the surface, that portion of the charge in contact with the surface leaks off to ground through the article and its support. Since the powder particles, generally speaking, are plastic and are poor electrical conductors, they will retain a portion of their charge and thus will continue to be attracted to the surface. The particles will be held in place.

Eventually, the accumulated charge on the surface will become sufficiently large and no more particles will be attracted. This represents a self-limiting effect. No amount of continued exposure to the spray will add to the thickness of the accumulated layer once this condition has been reached.

The total consumption of powder materials used will be apparent in the amount of reclaimed materials that are being collected. This will show as an increase in costs. Proper gun adjustments and material selection will help increase efficiencies.

Applying the powder to articles while they are at room temperature seems to be the process most widely adopted by industry at the present time. This is particularly true in those cases involving thin film requirements such as lawn furniture and appliance items.

Heavy film thicknesses for functional requirements such as motor winding insulation, and corrosion protection for pipe coatings may require that the parts be preheated prior to the application of the powder.

The charged powder particles will be attracted to the surface of the part whether the part is cold or heated. If the temperature of the part is above the melting point of the powder, that material attracted to the surface will melt and adhere. The molten material already on the surface will attract additional powder particles equally as well as the bare metal, so material will continue to accumulate on the part surface-so long as there is sufficient heat at the part surface to melt the newly arriving particles.

The material selected is important in this application. The powder selected must allow the high build film thickness to polymerize and cure without sagging or gassing. The powder and process technique must remain constant in order to maintain a given thickness. Proper attention to preheat and post heat dwell times is a must.