Film Build Control

What are some ways to control a manual electrostatic powder application in order to meet strict min and max film thickness orders?

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Q. We have an application that specifies both a minimum and maximum film thickness and we are having difficulty keeping within the limits. This is a smooth epoxy powder being applied to electroless nickel plated copper.  We are applying the powder with a manual electrostatic spray system.  The limits are 3.5 to 7 mils thickness.  We try to remain as consistent as possible with the number, speed and distance of passes, but still get a tremendous amount of variation in final cured thickness. Any suggestions on how to better control this process so we can meet the customer requirements? We have asked some of our powder suppliers about controlling to a min/max thickness specification and were told they had never heard of such a request. – B.D.

A. Many products are judged by full coverage and do not have a film build tolerance. However, there are specifications from some manufacturers for minimum and maximum film thickness. The minimum thickness is related to appearance (complete coverage) and performance. The maximum thickness could be related to fit or in some cases appearance. Heavy films build up more ions in the film and are more likely to show excessive orange peel.

Control of film-build with a manual gun is somewhat challenging. Your approach to consistent application is the right direction and should be reasonably effective. If your pattern is smooth and consistent and your technique is smooth and consistent you should be able to meet the specification of 3.5 to 7.0 mils. Gun-to-target distance is particularly important. Consistent hanging patterns are also important. There are a couple of other factors that could be affecting your film build control.

First of all, you must have good earth ground. Poor ground will have a big impact on film build control. You need clean hooks and clean path to earth ground. You need to measure earth ground with a good ohm meter and show resistance below 1 mega-ohm. 

The geometry of the part is also critical. If the surface is irregular you will have more difficulty getting consistent thickness. One thing that will help with control over irregular shapes is limiting the current draw. A current draw above 20 micro-amps may have more influence on the film thickness due to areas of resistance on the part surface (Faraday Cage effect). You can experiment to find the optimum level of current and you should be able to improve the film build consistency. A typical effective current level for a manual gun over an irregular surface is between 10 and 40 micro amps. A level of current that is below 20 micro-amps is usually best.

Consistent gun settings, consistent hanging, consistent technique, good ground and current limiting should help you get better film build control. Be sure the powder pattern is uniform and the velocity of the powder is low. Make sure the parts are spaced correctly. If the parts are too close you may have trouble building up powder along the edges. If the parts are too far apart the edge of the part may have strong voltage lines and create some picture framing.

One thing you can be certain of is that anyone who says they have never heard of a min/max film build requirement is not very well informed.


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