Application

Film Thickness

Dry Film Thickness (DFT) is measured in microns (millionths of a metre, 1 micron = 1/1000 millimetre) or in mils (thousandths of an inch, 1 mil = 1/1000 inch).  

1 mil is approximately 25 microns.  Manufacturers design coatings with a DFT in mind and being below or above the recommended range can have undesirable results.

DFT too low

Substrate shows through and/or the powder coating has a poor grainy flow.  Dry Film Thickness (DFT) measurement shows that the coating is below the recommended film thickness.

  • Potential Cause

    Spray guns settings are not optimised:

    • Powder delivery rate is too high to adequately charge the powder.
    • Gun air pressure is too high
    • Voltage is too low
    • Worn venturi or pump
    • Poor gun positioning – the powder either is being blown beyond the part, or is being extracted away from the part.

  • Solution

    Review settings and discuss with your spray gun supplier.

    • Reduce powder delivery rate until the powder is sufficiently charged.
    • Reduce forward air pressure
    • Increase kV; also check the electrical continuity throughout the electrostatic system and clean or repair voltage-limiting components as necessary
    • Replace worn parts
    • Adjust and optimize spray gun positioning and air flow rates.

  • Potential Cause

    The electrical earth is not sufficient

  • Solution

    Check contact areas between parts and jigging hooks and ensure there is no build-up of cured powder coating, or any other insulating material.  Jigs should not be too thin and the earth connection to ground should be secure and tested.  Ideally resistance should be less than 0.5 megaohm, but should certainly be less than 1.0 megaohm.

  • Potential Cause

    The powder coating may contain excessive concentrations of small size particles (‘fines’); this is usually accompanied with poor fluidisation.

  • Solution

    Sustain a constant ratio of virgin powder and reclaimed powder.  Reduce the reclaim : virgin powder ratio; if the problem persists, the virgin powder may contain excessive fines for the spray system.  Contact us.

  • Potential Cause

    The powder coating or the local air conditions may contain too much moisture (humidity).  Moisture acts as an anti-static agent

  • Solution

    Monitor and control the local humidity in the spray booth and air supply.  Should the powder coating have excess humidity in the bag, this can be conveniently fixed by opening the bag and leaving it open to the atmosphere for a few hours.  Protect the powder from possible contamination during this period.

  • Potential Cause

    The spray equipment may be a Tribostatic system and powder is not suitable.

  • Solution

    Should the equipment be Tribo, check with us that the coating is suitable for Tribostatic application.  Ensure the tribo tube is not coated with powder particles limiting the creation of friction. In certain circumstances is is possible to enhance the tribo-charging of the coating by addition of an additive to the fluidising container

The coating falls off

The powder coating, which should electrostatically adhere to the substrate, falls off.  In more extreme cases, the powder coating does not apply to the edges of a part, or may not adhere at all.

  • Potential Cause

    There is a poor electrical earth causing first-pass charged powder coating particles to limit further deposition onto the part.

  • Solution

    Check contact areas to ensure there is no build-up of cured powder coating, grease or any other insulating material which would impede electrical earthing.  In particular, check that jigs are thick enough for the job and that the contact of the jig with the part and the jig with the hanging rail is free from any coating build-up.

  • Potential Cause

    Excessive moisture:

    • in the air supply, the fluidized bed and/or powder booth
    • in the powder coating

    Moisture, usually from humidity air acts as an antistatic agent

  • Solution

    Control the humidity in the booth and the air supply.  Store powder in a controlled (cool and dry) environment.  Powder may be dried using a fluidised bed in a controlled (warm & dry) environment, although usually just leaving the bag open in a dry environment will acclimatise the powder.  In exceptional circumstances, additives can be used – these are best applied in a fluidised bed.

  • Potential Cause

    The spray gun is tribostatic and the powder may not be suitable for tribostatic application

  • Solution

    Some powders have the ability to charge by friction (tribostatic) others do not. Contact us.  Ensure tribo tube is not coated with powder particles limiting the creation of friction. In certain circumstances is is possible to enhance the tribo-charging of the coating by addition of an additive to the fluidising container

  • Potential Cause

    The powder coating is a metallic effect and the metallic flakes are adversely affecting the charging process.  This is particularly noticeable in blended ‘chrome’ effect coatings.

  • Solution

    Reduce the powder delivery rate to lower the metallic flake content in the spray gun.  This problem is particularly noticeable in blended metallic effects; we can offer a bonding service which solves the issue.

  • Potential Cause

    High voltage creating a strong magnetic field

  • Solution

    Reduce voltage to the electrical field to eliminate electrostatic rejection

  • Potential Cause

    A gun too close to a part will create a strong magnetic field.

  • Solution

    Increase the distance between the gun and part.

DFT too high

Coating surface is orange peeled and/or hazy.  DFT measurement shows that the coating has been applied above the recommended film thickness.

  • Potential Cause

    Spray guns settings are not optimised:

    • Powder delivery rate is too high
    • Gun air pressure is too high
    • Voltage is too high
    • Unfavourable geometry of parts

  • Solution

    Review settings and discuss with your spray gun supplier.

    • Reduce powder feed
    • Reduce air pressure
    • Reduce kV
    • Change hanging or gun configuration
    • Increase distance between gun and component

  • Potential Cause

    Coating time too long

  • Solution

    Lower the coating time, or reduce the amount of passes when spraying.

  • Potential Cause

    Too much pre-heat (if used)

  • Solution

    Reduce temperature of pre-heat cycle

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