Checklist for a problem-free powder coating process

Problems during application


Fluidisation

Identification

Possible cause

Remedial action

 
Intermittent or no delivery from the application equipment Fluidisation properties of the powder poor, low fluidising airflow, fluidised bed porous plate blocked Increase pressure of fluidisation air, check fluid bed tile. Contact powder manufacturer  
Visually detectable local ratholing and cratering in the fluid bed Compressed air too moist, contains oil; relative humidity is too high inside the room in which the powder is processed. Powder is moist. Check drier is switched on. Check drain valve on drier is open: check oil and water separator  
Poor fluidisation of the powder Fluid bed base tile clogged or damaged Clean or replace fluid bed tile  
  Air temperature too high in the coating plant Comply with storage conditions and climatic processing conditions recommended by the powder manufacturer  
  “Lumps“ in fluidised powder Increase vibration of powder container  
  Reclaim powder too fine; Reclaim system filters blocked. Powder circulation not working effectively Increase ratio of virgin powder in fluid bed. Check powder recirculation equipment working effectively.  

Build-up of sintered deposits in hoses and injectors (impact fusion)

Identification

Possible cause

Remedial action

 
Powder cloud from spray unit looks very uneven Air speed too high; powder flow too high Reduce air speed; Balance powder feed across the spray unit using forward and atomising air controls.  
  Wrong spraying nozzle used Use appropriate spraying nozzles or spray nozzle attachments  
  Inappropriate hoses (hose material, dimension, installation, kinks, narrow radius) Use appropriate hoses, note installation and length of hoses  

Build-up of sintered deposits inside and on spray nozzle gun tips

Identification

Possible cause

Remedial action

 
Powder cloud output reduced from nozzles
Backwashing air too low, mechanically damaged or worn parts
Increase backwashing air, clean injectors, baffles, and nozzles so that the surfaces are not roughened or damaged  
  Circulation powder too fine; insufficient feeding of virgin powder into the circulation system
Continually automatically feed/add virgin powder to the system
 
  Powder too moist
Check operation of refrigerant type drier, in regard to compressed air check oil and water separator
 
  Pressure of conveying air too high Reduce pressure of conveying air  
  Pressure fluctuation inside the compressed-air system Check compressed-air system  
  Room temperature too high, relative humidity too high Respect climatic processing conditions  

Variation of film thickness on the object

Identification

Possible cause

Remedial action

 
Colour difference due to different substrate coating or composition Irregular conveying of powder, space between spray gun and object not balanced Optimise plant parameters  
Surface finish appears irregular Relation between line speed and reciprocator speed of the spray guns gun not optimised (sine curves); high voltage fluctuations
Optimise “sine curves”
 
  Particle size distribution not optimal
Optimise particle size distribution
 
  Powder column on the venturi tube fluctuates heavily
Provide for consistent feeding from the fluid bed container
 
  Construction of object (Faraday cage) and/or jigging inappropriate
Change construction of object and/or jigging
 
  Pressure of conveying air too high; powder coat is blown off due to excessive air speed
Reduce conveying pressure
 
  Earthing insufficient Check earthing  

Powder coating material adheres poorly on flat object (powder coating falls of)

Identification

Possible cause

Remedial action

 
Colour differences due to different substrate coating Objects insufficiently earthed; electrostatic field strength too intense Optimise plant parameters, clean hangers and jigs  
Surface appears irregular Forward and atomising air not balanced correctly Balance set-up between forward and atomising air  
  Voltage of spray guns and their distance to object not optimal Change voltage and space between spray guns and object  
  Voltage too low Increase voltage  
  Output of powder too high Reduce powder output  
  Particle size distribution not optimal Optimise particle size distribution  

Powder coating material adheres poorly on objects with corners and recesses (powder coating falls of)

Identification

Possible cause

Remedial action

 
Parts of the object remain uncoated Inappropriate hose material Use appropriate hose material  
  Wrong spraying nozzle Use the correct spraying nozzle  
  Speed of conveying air too high inside the spray gun Reduce speed of conveying air  
  Geometry of object and object jigging inappropriate for coating process Change construction of object and/or jigging  
Special notes concerning tribo application  
See above Insufficient charging because of:
Constantly monitor specific charge or current flow of the tribo equipment
 
 
  • powder throughput too high
Optimise plant set-up  
 
  • wrong particle size distribution
Make particle size distribution compatible with application plant and item being coated  
 
  • worn friction element inside the spray gun
Replace worn friction elements  
 
  • powder insufficiently compatible with tribo
Use tribo quality powder  

Edge beading

Identification

Possible cause

Remedial action

 
Noticeable build up on corners and edges of objects Powder cloud too large Change powder cloud appropriately  
  Distance between objects too great
Reduce spraying of powder into gaps on conveyor (use gap control); minimise space between objects; mechanically shield hanging equipment
 
  Coarse particle content of powder too high
Optimise particle size distribution
 
  Total film thickness too high
Reduce total film thickness
 

Picture frame effect

Identification

Possible cause

Remedial action

 
Surface appears irregular, border area differs Different deposition behaviour on the surface of the objects Use appropriate spray nozzles or spray nozzle attachments  
  Output of powder too high, voltage too high
Reduce conveying pressure and high voltage  
  Inappropriate distance between spraying elements and object Contact plant supplier, adjust distance between spraygun and object  

Sudden escape of powder from the coating booth

Identification

Possible cause

Remedial action

 
Powder dusts from the booth and contaminates surroundings Extraction capability too low Make powder flow consistent with extraction capability  
  Most common cause is draught inside the plant due to open windows and doors Close windows and doors  
  Fitters inside the recovery unit are clogged Check pressure gauge differential on filters  

Electrostatic Discharge

Identification

Possible cause

Remedial action

Sparking Insufficiently conductive hose material Use hoses recommended by the plant contractor
  Injector insufficiently earthed Pay attention to earthing of injectors
  Objects insufficiently earthed Clean hangers/jigs; remove coating from jigs regularly
  Usage of metallic powders Use appropriate spray nozzle attachments
Discharge (electric shock) on operators Spraying enclosure insufficiently earthed Wear conductive protective clothing
    Check positive earthing grips on the hand-spray gun