EQUIPMENT PROBLEMS
HOW TO ENSURE SUCCESSFUL POWDER COATING?
1/ Earthing of the object & equipment
The process of Powder coating application might generate a large quantity of negative charges being transfered to the object. Supposed these electrons could not be earthed efficiently, the coated subject surface rapidly develops a powerful negative charge which thus repel the negatively charged powder from the spraying gun. Thus, as a result of unsufficient earthing it becomes impossible to build up rapidly and efficiently a thick layer of powder with good flow and finish.
Inadequate earthing can easily be recognized by little or no powder accumulation around the hanging point of the object being coated.
2/ Air & Compress air quality
Relative humidity within the working environment is of great significant for spraying efficiency. Ideal relative humidity being 45~55%. A quicker build-up of the powder layer and more even coating thickness are two of the advantages achieved by controlled humidity. Relative humidity can be measured with simple and inexpensive equipment.
Only clean, dry compressed air should be supplied to powder coating equipment. The quality of the compressed air can change the electrostatic charging and transporting properties of the powder. Impure compressed air my also cause visul defects in the coating. Pressurized air must be free from oil, water and as dry as possible.
3/ Most frequent encountered problems shown in the table below
| PROBLEMS | CAUSES | ADDITIONAL TEST | SOLUTIONS |
Poor charging Inadequate powder build | Voltage too low
Poor earthing
Powder output is too high Powder too fine | Check electrical continuity from voltage source to electrode, including cable, resistors and fuses. Check earthing of the conveyer, hanger and substrate (<5 Ohm).
| Replace broken or missing components.Clean electrode insulated by powder build. All contact area must be free of powder build, heavy grease and other insulating material. Turn down the powder feed until all material passing through corona field is adequately charged. Reduce ratio of reclaim to virgin powder |
Poor penetration, powder will not coat Faraday cage areas | Output of powder too low. Poor ground. Powder cloud too wide. Poor gun placement. Voltage too high | Check earthing. Use a smaller deflector. Adjust gun position so powder cloud has a direct path to recess area
| Turn up powder delivery air setting. Use gun-barrel extension. All contact area must be free of insulating material,
Turn voltage setting down so powder on part edges and leading surfaces do not repel powder from corner |
Rotating valve of cyclone blocked | Temperature too high Powder too fine Sieve is collated | Vibration of valve Content of hopper after sieve is too high Optimize flow of powder in hopper | Cool valve. |
Poor built-up (crater formation) | Voltage too high Gun positioned too close to piece. Poor earthing | Turn down voltage setting. Change gun placement away from part Check earthing of conveyer, hanging and substrate | |
Powder feet spurting or slugging, interrupted powder feed | Insufficient air pressure or volume Hoses kinked, flattened or too long Poor fluidization Hoses, pump venturies or guns clogged with powder | Check air supply, enough air volume when other equipment make use of compressed air Check supply-tubes See "poor fluidization in fluidized bed" Check filled tubes, venturies and guns on |
Renew tubes if necessary
Clean or renew the tubes, venturies or guns |
Poor spray patterm, not a symmetrical powder cloud | Worn gun parts Build up of agglomeration though friction Delivery air too low | Replace worn parts Clean gun parts, particularly the deflector Increase delivery air | |
Poor fluidization in fluidized bed | Fluidization pressure is too low Porous bottom clogged Powder too fine Moisture content of compressed air too high |
Check particle size (Dew point of atmospheric pressure -40°C) | Increase air pressure Clean or replace porous bottom Reduce the ratio of reclaimed powder Install an air dyer |
Powder falls down from the substrate | Moisture content of powder too high Bad chargeability of the powder Powder output too high or the pressure for the transport air too high, which blows the powder from the object Poor earthing Powder is too old Unsuitable particle size distribution | Check storage parameters (maximum humidity 75 %, max. temperature 25°C) Adjust level of high tension unit Reduce powder output and/or reduce pressure of transport air Check earhing Contact powder supplier Reduce ratio of reclaim to virgin powder. |
