Pretreatment For Powder

The first step in the powder coating process is to prepare or pretreat the parts. The product to be coated is exposed to cleaning and pretreatment operations to ensure that surfaces to be coated are clean and free of grease, dust, oils, rust and other contaminants. Chemical pretreatment normally takes place in a series of spray chambers. Parts are first cleaned using an alkaline, acidic or neutral cleaner. In many cases the part is surface treated with a conversion coating of iron or zinc phosphate or a transitional metal conversion coating such as a zirconium oxide product. Each state is typically separated by a rinse stage to remove residual chemistry. Spray systems enable pretreatment of a wide variety of part sizes and configurations; dip tanks may be used instead of spray for some applications.

The specific pretreatment process selected depends on the characteristics of the coating and substrate materials, and on the end use of the product being coated. Pretreatments most often used in powder coating are iron phosphate for steel, zinc phosphate for galvanized or steel substrates and chromium phosphates for aluminum substrates. In addition to traditional phosphate processes a new group of technologies has emerged that use transition metals and organo-metallic materials or other alternatives. These alternative conversion coatings can be applied with little or no heat, and they are less prone to sludge buildup in the pretreatment bath than conventional iron or zinc phosphate formulations. The result is greater operating efficiencies in terms of lower energy costs, reduced floorspace requirements and decreased waste disposal requirements. Other advances include non-chrome seal systems, which can yield improved corrosion protection on steel, galvanized steel and aluminum alloys.

Dry-in-place pretreatment products, such as a seal rinse over an alkali metal phosphate, can reduce the number of stages required before powder coating application. Chrome dried-in-place treatments are effective on multi-metal substrates, and may be the sole pretreatment required for some applications.

After the chemical pretreatment process is complete, the parts are normally dried in a low-temperature dry-off oven. They are then ready to be powder coated.

For many functional applications, a mechanical pretreatment such as sand or shot blasting can be used. With this method, high-velocity air is used to drive sand, grit or steel shot toward the substrate, developing an anchor pattern on the part that improves the adhesion of the powder coating to the substrate. Mechanical cleaning is particularly useful for removal of inorganic contaminants such as rust, mill scale and laser oxide.