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Checklist for a problem-free powder coating process

Problems on the coated object

Insufficient mechanical properties (adhesion, elongation, surface hardness); insufficient corrosion resistance and resistance to chemicals

Identification

Possible cause

Remedial action
Performance expectation of the coating is not achieved Film thickness beyond nominal values Comply with nominal values of film thickness
  Insufficiently cross-linked powder Check crosslinking/curing, comply with curing conditions
  Heating rate too low Increase heating rate

Insufficient intercoat adhesion in two-coat procedures (peeling)

Identification

Possible cause

Remedial action
Insufficient mechanical properties Insufficient pre-treatment of objects (phosphating, chromating) Optimise pre-treatment
  First coating overcured Set up temperature of object and holding time optimally (temperature-time-diagram)
  Inappropriate powder type, film thicknesses extremely high Coordinate assembly, use correct powder type
  Use of a directly heated gas oven Use different heating method

Levelling (orange peel) all areas or parts of the object

Identification

Possible cause

Remedial action

  
Performance expectation of the coating is not achieved (nominal value of levelling) Film thickness of powder coating too high or low Optimise powder output  
  Temperature profile on the object not optimal Optimise temperature on object  
  Usage of inappropriate powder (too reactive, too high-viscous) Use appropriate powder  
  Particle size distribution of powder inappropriate Use powder with changed particle size distribution  
  Incompatibility with other powders Clean plant thoroughly, do not mix different powders  
  Back ionization effect (voltage too high, temperature and relative humidity at the plant location changed) Reduce film thickness, reduce voltage; check air conditioning; divert “superfluous” charge by means of special spraying elements or additional equipment such as rings for diverting ions (e.g. Super Corona. Corona Star)  
  Powder stored too Iong/too warm Check storage conditions  
  Relative humidity too low Increase relative humidity  

Gloss variation

Identification

Possible cause

Remedial action

  
Performance expectation of the coating is not achieved (nominal value of gloss) Curing conditions are not being adhered to Set up temperature of object and holding time optimally (temperature-time-diagram)  
  Special curing technology (heating with gas direct, infrared, near-infrared, inductive heating) Use appropriate powders  
  Bad levelling (powder stored too long/too warm) Optimise curing conditions, use new powder  
  Incompatibility with other powders Clean plant thoroughly, do not mix different powders  
  Powder thermalIy unstable Use thermalIy-stable powder  
  Film thickness too high/too low Optimise film thicknesses  
  Emissions from the powder or the substrate in the course of the curing process Use appropriate powders, check substrate/pre-treatment  
  Deposit or condensate (from the air inside the oven) Clean oven, increase oven airflow if necessary  

Contaminated surface

Identification

Possible cause

Remedial action
Appearance of coated surface is not acceptable Missing or too large mesh in reclaim sieve Use sieve with correct mesh size in machine
  Insufficient sieving Use sieves with small mesh size
  Dirt from the air inside the application plant is deposited on the coated object due to air flow Use screening machine with various sieves; use screening machine with automatic dirt and deposit discharge
  Dirt from the conveyor Clean conveyor
  Dirt from the jig Use conveyor with dirt trap
  Emissions from the powder inside the oven Clean oven regularly; increase oven air flow if necessary
  Polluted air is being sucked into the booth or the oven Provide for separation of coating plant from other factory floors; isolate coating plant with overpressure filtered air
  When operating two or more booths mutual suction of the powder-air mixture (especially during cleaning processes) Provide for appropriate physical separation of the booths
    Check after filter regularly
  Powder hose insuffiently cleaned after change of colour Clean powder hose with compressed air and stopper
  Clothing fibres are being sucked in by the recovery unit Use lint free clothing
  Fibres from the filter of the recovery unit Use filters that will not leave fibres
  Adhering dirt particles on the object prior to the coating process Provide for clean objects before the coating process
  Objects "cleaned" with fluffy cloth Do not use fluffy cloths in application plant
  Powder contains contamination when delivered Contact manufacturer of the powder

Contamination

Colour variation

Identification

Possible cause

Remedial action
Objects do not match in terms of colour Curing conditions changed (oven temperature/time conditions, overcuring during breaks or downtimes, IR contribution) Set up curing conditions correctly
  Combustion products in directly gas-heated ovens Use stabilised powders
  Film thickness beyond nominal values Comply with nominal values of film thickness
  Object or operation mode changed (dimensions, mass) Sort objects
  Insufficiently degreased or pre-treated objects Optimise degreasing, pre-treatment
  Irregular film thickness of a priming coat Apply primer with uniform film thickness in order to achieve even substrate in terms of colour
  Mixture of two powders Separate powders carefully; clean plant thoroughly when changing powders
  Specification tolerances of the powder insufficiently defined Check or specify tolerances together with the powder manufacturer

Colour variation

Colour variation

Colour variation

Blisters (closed or burst), "pinholes"

Identification

Possible cause

Remedial action
Performance expectation of the coating is not achieved Curing conditions not optimal (heating rate) Optimise curing conditions
Weak spots when mechanically stressed Surface of objects not cleaned sufficiently (grease residues, rust, water, emissions from the substrate) Clean surface of objects perfectly; contact manufacturer of pre-treatment
  High film thickness (air pockets, emissions from the powder, e.g. water) Reduce film thickness
  Surface of the object has cavities (cracks, indentations, overlaps) Improve surface of objects, change construction
  Blow holes on surfaces with hot-dip galvanized coat; porous surface, e.g. die casting Degas objects appropriately, use appropriate primer
  Grade of the powder Use powders for outgassing substrates
  Incompatible powders mixed together (contamination) Separate powders carefully; clean plant thoroughly when changing powders

Craters

Identification

Possible cause

Remedial action
Performance expectation of the coating is not achieved Objects not sufficiently degreased, pre-treated Optimise degreasing, pre-treatment
Weak spots when mechanically stressed Incompatible powders mixed together (contamination) Separate powders carefully; clean plant thoroughly when changing powders
  Influences of products containing silicones Avoid products containing silicones within the coating area (check air ducts of connected production units)
  Blow holes on surfaces of cast parts, hot-dip galvanized or zinc-sprayed coatings; porous substrates as e.g. die casting Degas objects appropriately, use powders suitable for outgassing substrates, use appropriate primer
  Oily dirt from the conveyor chain Clean conveyor chain
  Oil from the compressed air Use and check oil separator, refrigerant type drier
  Condensates from the oven Clean oven, increase oven airflow if necessary

Cratering

Cratering

Cratering under microscope

Beading, sagging and drop formation

Identification

Possible cause

Remedial action
Performance expectation of the coating is not achieved Film thickness too high or variable Optimise film thickness distribution
Fitting of components is compromised Curing conditions not optimal (heating rate, object and oven temperature) Optimise curing conditions
  Geometry and surface profile of the objects (powder-drawing areas) Introduce individual coating processes of objects
  Specification of the powder incorrect Define specification profile of powder
  Wetting disturbance: "slipping" of the powder coating Clean substrate residue-free (salts, oils, greases, residues of rust and separating agents)

Wetting disturbance

Wetting disturbance