Controlling Coating Thickness

Q. I am related mechanical engineer with basket production line in Arcelik Dishwasher Machine Inc. in Turkey. In our line, we use fluidized bed coating process to coat the baskets, made of 1008 cold drawn wires. (The dimensions of  the wires have a range of  2.6–4.5 mm.)

Our coating line consists of five main divisions, i.e. degreasing, rinsing (25°C degrees), pre-heating (370°C degrees), coating and  post-heating (270°C degrees). In addition, an acidic iron phosphate compound is used in the degreasing bath (1% mixed with tap water). pH is generally around 3.0–3.5 and the bath is kept at 60°C degrees. This compound is sprayed on wires through nozzles. After pre-heating at approximately 370°C degrees, the wires are dipped into polyamide powder. Powder is polyamide 12 fluidized bed coating powders.

I have some problems with the coating line and wonder if there is something we can do. In our production, the thickness of the coatings are different from each other. To solve this problem, I want to determine the relationships of coating thickness with temperature (pre-heating and post-heating), time in the fluidized bed,  number of shakes in the fluidized bed, pH, etc. parameters. Another problem is that there are some pin-holes on the baskets. Some points are not covered by powder.

1. Could you explain the relationships of the above parameters with each other?
2. How can they affect the thickness of the coating? 
3. How can we prevent the pinholes on the coatings?

Thank you for your help. S. C.

 

A. There are several factors that affect coating thickness consistency on your parts. First, the wires of the baskets are of different diameters and have different mass. The thinner wires will heat and cool more rapidly then the thicker wires. Therefore, the thinner wires will have less coating on them then the thicker wires as they will have cooled more rapidly after exiting the preheat oven and entering the fluidized bed. There is no way around this phenomenon, as it is related more to physics than to the powder coating process.

Another influencing factor with film thickness control and fluid bed coating is related to the density of the powder within the fluid bed. For instance, the powder coating in a fluid bed is denser towards the bottom then at the top. As the density of the powder increases, so does the thickness of the coating on the part. Fixturing your part to minimize this effect is the best way to reduce this problem. For instance, you can fixture your part to have the shortest dimension in the vertical plane, therefore minimizing the depth that the part is immersed into the fluidized bed.

As for the shaking of the part after coating, this will remove the excess build-up of the powder from the wire intersection points and other areas that hold the powder onto the part by gravitational forces alone. It has little influence on the powder that has already sintered onto the surface of the part. It is still a necessary step, however, to reduce powder build-up on the horizontal surfaces and wire intersections.

Out-gassing is the primary cause of pinholes in powder coating. Out-gassing on wire forms can occur anywhere there are residual contaminants left on the surface by the pretreatment system. When these contaminants volatize (evaporate) during post heating, the resultant gas pierces the coating causing a pinhole. This same phenomenon can occur with entrapped moisture, but this is highly unlikely given your preheat temperature. However, if you are using a liquid primer, common in the application of thermoplastic powder coatings, then the solvent can “pop” in the post-heating oven causing a pinhole in the coating. Increasing the flash-off time will eliminate this problem.

Powder coating thickness control using a fluid bed is directly related to the temperature of the part, the depth of immersion and the time within the bed. Controlling these factors closely with reduce the variance of the coating thickness on your parts. All the other process parameters have little, if any, affect on the coating thickness or coating consistency.