In Search of a Polymeric Coating

Question:

I have an application that needs a 0.005-0.006-inch thick insulative coating on an aluminum sheet metal part. The coating has to withstand a 1,500 volt surge between the circuit card and the metal guides that the card slides in. I have tried powder coat but am not very happy because of a lack of adequate coverage in certain delicate areas due to the part geometry. Are there any polymeric coatings or other materials that are suited for my application that will guarantee a close tolerance on the coating thickness? R.R.

Answer:

I can’t envision the part you are talking about, but you have discovered one of the few problems with powder coating—lack of penetration into recessed or internal areas. This is due to the Faraday cage effect. Powder particles are attracted to the nearest ground, so even if they are “aimed” into a corner, for instance, most will tend to deposit on the metal grounded surface closest to the gun. This effect is true with any electrostatic paint process, but is more pronounced with powder.
The same effect would be realized with any immersion electrolytic process as well, but likely not to the same extent. Electro-deposition painting (EDP or e-coat) and anodizing would fall into these categories. Some process variables could be changed to better coat a part, such as current density, solution agitation, etc. However, this still may not be adequate to get the part coated in the recesses you need. In that case, a standard spray paint process or dip painting process (non electrolytic) could provide you with an insulating coating. To obtain the 0.005-0.006-inch thickness may require application of more than one coat, though.