Powder coating defects and solutions - problems related to the application of powder coating

 07 June 2020   13:04
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Powder coating defects and solutions - problems related to the application of powder coating

Some problems occur during the application of powder coating which depend on many factors. In this excerpt, we have tried to briefly state these problems and the reason they occur and the solutions for them.

Problems with powder coating application

  • Poor fluidization in the storage container or feed hopper

When applying powder coating, in order to facilitate the spraying process, the powder is poured into a receptacle and becomes suspended in air, causing a fluid effect. This fluidization and transfer of powder and air from the tank to the hoses and feed tubes and then to the spray gun can be weak because of the following.

  1. Air pressure adjustment
  2. Defects in the porous membrane of the feed hopper
  3. Blockage in the porous membrane
  4. Presence of oil in the air pressure chamber
  5. Powder coating particle size being too small
  6. High humidity in powder coating
  7. High temperature in the coating system
  8. Clogged vent hose

  • Improper powder flow because of clogging in the feeding tubes

Deposits and clogging of powder in the feeding tubes cause spitting, these spits on the surface can cause spotting on the coating finish.

  1. Improper hose installation
  2. Improper mixing of virgin powder with reclaim powder
  3. Eroded venture nozzle
  4. Inappropriate feeding tube thickness
  5. The length of the feed hose
  6. Low-quality tubes
  7. Poor fluidization of powder

  • When powder doesn’t adhere to the substrate

Electrostatic powder coating that is sprayed onto the substrate flows down and does not adhere to the substrate.

  1. Incomplete charging of the substrate because of disconnection
  2. Low voltage
  3. Powder does not receive enough electrostatic charge
  4. Large powder particles
  5. The conveyor belt does not work smoothly and vibrates
  6. High velocity of air current
  7. High powder outflow
  8. Close proximity of gun to part
  9. Improper orientation of coating substrate
  10. Powder builds up on racks, electrodes and hooks and must be cleaned away regularly
  11. Humidity in the coating cabin

  • Poor coating

In case powder cannot cover the surface of the parts properly, poor coating occurs.

  1. Outflow of powder from gun being too low or too high
  2. Insufficient grounding
  3. Substrate being too hot after preheating
  4. Air pressure being too low or too high while spraying
  5. Distribution of improper powder particle size
  6. High voltage in spray gun
  7. Inadequate charging of powder
  • Powder escape from container

  1. High air pressure
  2. Powder particle size being too small
  • Sprayed powder being too thick

In this case, the powder coat before curing is uneven. This causes problems such as orange peel, waves and cratering.

  • Film thickness too low

You will be able to see the substrate and powder appears in spots.

  1. Insufficient powder charge
  2. Improper distribution of powder particle size
  3. Low outflow of powder
  4. Powder supply hose being too long
  5. Poor fluidization of powder
  6. Short stay of parts in front of spray gun
  7. High velocity of powder suction in cabin
  8. Reduction in powder outflow resulting from powder settling on transfer system such as hose and nozzle.
  9. Transfer system blocked (presence of fibers, remnants of detergents)
  10. Improper installation of parts on the rack
  11. Poor grounding
  12. Using non-Tribo powder for Tribo system
  13. Not enough powder in the receptacle (feed hopper)
  14. Powder particles being too small
  15. First layer of coating acting as insulator, later layers will not adhere
  16. Long gun to part distance
  17. Friction in the hose causing powder to get a charge opposite to the charge in the spray gun
  18. High humidity level in the surrounding environment

  • Poor penetration into recesses and gaps (Faraday cages)

Powder cannot penetrate into recesses and gaps with the effect of Faraday cage.

  1. High air pressure in feeding tubes or in Tribo
  2. High air velocity
  3. High powder outflow
  4. Inadequate powder outflow
  5. Improper application
  6. High voltage
  7. Faraday effect resulting from electrical field
  8. Inadequate grounding
  9. Large angle of spray gun with part (nozzle not adjusted properly or gun too far from part)
  10.  Improper distribution of powder particle size
  11. Gun to part distance
  12. Improper gun placement  
  • Powder spitting and wavy outflow

              Powder spurts out in spit form.

  1. Inefficient powder feed rate
  2. Inappropriate hose thickness
  3. Poor powder fluidization in the receptacle (feed hopper)
  4. Improper air pressure or volume
  5. Twists and turns in the hoses and feeding tubes
  6. Improper powder particle size
  7. Presence of humidity
  • Inconsistent powder feed

             Powder is not sprayed onto part with a consistent rate.

  1. Insufficient powder in the receptacle (feed hopper)
  2. High humidity in powder
  3. Poor fluidization of powder
  • Caking of powder coating

Caking is a defect that is caused when powder coating is stored for a long time in unsuitable warehousing conditions.

  1. Unsuitable warehousing condition
  2. Improper powder coating transport
  • Layering of powder in the storage container

  1. Too much powder in the container
  2. Powder particles being too small
  3. Low air pressure in the container
  • Back ionization

  1. High voltage in spray gun
  2. Close gun to part distance
  3. Part poorly connected to electricity
  4. Too much powder applied
  5. Improper part installation

Prepared by: Mojtaba Zeinal