Metal finishing Q&As since 1989

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Low Film Thickness

February 3, 2008

We experience low film thickness complaint now a days.
The humidity of spray room is in the range of 22%. can this be one of the major cause?

1. Particle size within the range
2. Air Flow is fine
3. Improved Spray Techniques
4. Grounding is perfect.
5. Lighting in booth is adequate.
6. Line Speed is comfortable to Painters.

Also what maximum transfer efficiency can be achieved in manual powder coating application

Kaleem Muhammad
Manufacturing Engineer - Holland Landing, ON, Canada

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First of three simultaneous responses -- February 7, 2008

Completely disassemble and clean the guns.
Replace all of the wear parts.
Test them for kv output.
You can achieve >8 mils easily.
Transfer effeciency should be 80%.

Sheldon Taylor
supply chain electronics
Wake Forest, North Carolina

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Second of three simultaneous responses -- February 7, 2008

Dear Kaleem,
In your efforts to inform us of what you think is satisfactory within the confines of your powder coating set up, you have missed the most important information we require to enable a possible solution.
What film weights are you obtaining and what do you require?
What is the substrate? However, this is not the cause of the problem.
What type of powder equipment do you use ø is it Corona or Tribo application?
What Kv are you using when spraying?
Is the flow rate of powder too high? High will lower the powders ability to pick-up the electrical charge it requires to deposit the powder efficiently on the items to be coated. The same goes if the air pressure is too high.
Are you certain that the line speed is comfortable for your coaters, or are they just too scared to admit the lines are too fast.

Terry Hickling
Birmingham, United Kingdom

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Third of three simultaneous responses --

Hi Kaleem,

If everything you say is true, you may well have hit the nail on the head with the question about transfer efficiency. This is more likely to be an effect of the powder rather than the type of gun (unless your are using non tribo powder on a tribo system). Every powder has a different capability of holding a charge. This can depend on additives but is largely the effect of the amount of organic material that a powder contains (polymer), the more the better. Manufactures of powders can help improve their profitability by adding more inorganic material to the powder such as Barium sulphate or calcium carbonate, which is much cheaper than the resin. Ask your powder supplier if they have changed the formulation of their powder. Have you changed to a cheaper powder?

Check your powder is fluidising OK, check your pipes are not clogged and your ventures are not overly worn. The humidity is not bad and should not cause a problem. One other possible cause is you powder storage. If the powder is getting damp in any way before spraying this will reduce the film thickness.

Hope this helps,

Drew Devlin
- North Lincolnshire
February 8, 2008