Powder coating is a different method of painting. It starts out as a liquid but is formulated and processed differently. It is mixed, extruded and baked, then ground into a powder with the consistency of talcum powder. Paint is comprised of pigments or dyes, binder and fillers suspended in solvent. As most people have spray bombed

or brushed a part or two, you understand that you have to wait for the solvent to evaporate for the paint to "dry", taking anywhere from minutes to days. With powder coating, the powder uses heat as a catalyst to cure the powder, usually

200 degrees C (390 degrees F).

With most powder finishes, once the part cools, you can assemble or install your parts immediately without fear of damaging the finish. How does the powder stick to the part? Powder is applied by an electrostatic charge. Simply put, as the powder passes out of the nozzle of the gun it picks up a charge and is attracted to the grounded part. The thing with powder is if something goes wrong while coating a part, you can blow it off and start again. From there the part is put into the oven to cure.

The items or parts to be powder coated can be dipped in powder or the powder can be sprayed onto the part manually with a powder coating spray gun.

Preparation

It is important to assess your needs at this point. Rust protection, decorative or a combination of both.

There are a few different ways to prepare the substrate to accept the coating.

Hot rolled steel has a mill scale which should be removed before coating. With older rusty material we sandblast to a clean grey finish. New parts are either degreased in a degreasing fluid or treated with phosphate to promote adhesion. Sometimes a combination of the two.

We have experienced problems with aluminum parts that are chromed. It is advisable to have the chrome removed at a plating shop. Steel chrome parts usually coat just fine. Although we have done many chromed parts, sometimes it comes

off when we media blast it and sometimes it does not. Problems can occur when only half of it comes off. Because it has a thickness, it can be seen through the powder coating resulting in extra time spent on sanding and blending. It is recommended that you have the chrome removed before taking it in to a powder coating company to be painted. Use high temperature tape and plugs to protect threads and bearing surfaces. Some surfaces are obvious like bearing and machined areas but if you need something protected you can mark it with a felt pen

or tape.

There are some advantages of powder coating over conventional liquid coatings:

1. Powder coatings emit zero or near zero volatile organic compounds (VOC).

2. Powder coatings can produce much thicker coatings than conventional liquid coatings without running or sagging.

3. Powder coating overspray can be recycled and thus it is possible to achieve nearly 100% use of the coating.

4. Powder coating production lines produce less hazardous waste than conventional liquid coatings.

5. Capital equipment and operating costs for a powder line are generally less than for conventional liquid lines.

6. Powder coated items generally have fewer appearance differences between horizontally coated surfaces and vertically coated surfaces than liquid coated items.

7. A wide range of specialty effects is easily accomplished which would be impossible to achieve with other coating processes.

Thermoset or Thermoplastic?

There are two main categories of powder coatings: thermosets and thermoplastics. The thermosetting variety incorporates a cross-linker into the formulation.

When the powder is baked, it reacts with other chemical groups in the powder polymer and increases the molecular weight and improves the performance properties. The thermoplastic variety does not undergo any additional reactions during the baking process, but rather only flows out into the final coating.

The most common polymers used are polyester, polyurethane, polyester-epoxy (known as hybrid), straight epoxy (fusion bonded epoxy) and acrylics.

Production:

1. The polymer granules are mixed with hardener, pigments and other powder ingredients in a mixer

2. The mixture is heated in an extruder

3. The extruded mixture is rolled flat, cooled and broken into small chips

4. The chips are milled to make a fine powder

Electrostatic fluidized bed coating:

Electrostatic fluidized bed application uses the same fluidizing technique and the conventional fluidized bed dip process but with much less powder depth in the bed. An electrostatic charging media is placed inside the bed so that the powder material becomes charged as the fluidizing air lifts it up. Charged particles of powder move upward and form a cloud of charged powder above the fluid bed. When a grounded part is passed through the charged cloud the particles will be attracted to its surface. The parts are not preheated as they are for the conventional fluidized bed dip process.

Electrostatic magnetic brush (EMB) coating:

A coating method for flat materials that applies powder coating with roller technique, enabling relative high speeds and a very accurate layer thickness between 5 and 100 micrometers. The base for this process is conventional copier technology . Currently in use in some high- tech coating applications and very promising for commercial powder coating on flat substrates (steel, aluminum, MDF, paper, board) as well in sheet to sheet and/or roll to roll processes.

This process can potentially be integrated in any existing coating line.

With most powder finishes, once the part cools, you can assemble or install your parts immediately without fear of

damaging the finish.

Authored by:

Rob Chalmers, 2010

http://vicpowder.com