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. |
Checklist for a problem-free powder coating process
Problems during application
Fluidisation
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 |
|
|
Optimise plant set-up | ||
|
Make particle size distribution compatible with application plant and item being coated | ||
|
Replace worn friction elements | ||
|
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 |