9/13, Gandhi Nagar, Irugur, Coimbatore - 641 103
ERROR CONTROL
ERROR CONTROL IN POWDER COATING PROCESSES
Powder coating are gaining in popularity because of their high quality, the apparent ease of application and the technology. Whilst powder coatings as materials are now developed, their practical application is not always ideal Powder coating plants like other finishing plants are based on a combination of techniques and each plant is designed individually according to the demands of the buyer. However there is a need to identify the best Possible equipment & identify causes of recurring faults.
Faults & defects are mostly associated with:
- Poor design and operation or pretreatment process and equipment
- Inaccurate powder application resulting in product defects, uneven and excessive recycling of powder.
Simple maintenance of powder coating plant is not sufficient to remain competitive while day find tuning is valuable, the user must keep abreast of new development s in the technology and must be prepared to rebuild or replace facilities whenever practical as demands and product design changes. Powder coating offer very limited possibilities in altering the powder that in supplied. However, good routines and alternate will largely prevent problems from occurring.
In most cases, the powder will be custom - made for a specific use or a specific process. Even though powder and equipment can vary from one application to another, there are a number of common problems areas. The following are systematically presented with references to possible causes and suggestions for their correction. For the best possible result and a problem - free powder coating process it is vital that all instructions from both the suppliers of materials and the equipment be followed exactly.
POWDER DELIVERY
| PHENOMENON | POSSIBLE CAUSE | SUGGESTED SOLUTION |
|---|---|---|
| Powder spills out of the powder hopper | Fluidization air pressure is too high | Reduce fluidization air pressure | Proportion of the fine-grained particles is Excessive |
Reduce the proportion of recycled relation to virgin powder in the powder Hopper Discuss with the powder supplier the Possibility of supplying powder with a Lower proportion of fine-grained particles |
| The powder is not fluidized | Insufficient air pressure | Check the compressed air supply. Examine and if necessary increase the Pressure in the compressed air network. |
| Clogged membrane in the bottom of The powder hopper | Examine the condition of the supply lines empty the hopper and check if the membrances is clogged | |
| Powder packed down in the bottom of the hopper. | Manually stir the powder in the hopper and increase the air pressure in order to start fluidization. | |
| Poor fluidization | Too little powder in the hopper | Add virgin powder so the hopper is Approximately 2/3 full under fluidization |
| After a while, only "eruptions" that Blow air in certain places | Compacted or damp powder |
Increase the air pressure and manually stir the powder in the hopper Check the moisture in the Com pressed Air (dew point < 3 deg C) |
| Clogged or damaged membrane | Empty the hopper and examine the membrane | |
| Lumps of powder in the hopper | The powder has been stored too long or under unfavourable conditions (Temperature / humidity) | Sift the powder through a 200-300 μm powerful fluidization with clean, dry compressed air |
| Accumulation of powder in the Hoses - powder delivery in bursts | Air velocity is too low in relation to the amount of powder |
If possible to increase the supplemental air flow pressure on the ejector Change to hoses with a smaller diameter |
| Too much wear on the ejector plugs |
Install new ejector plugs consider using ejector plugs made of another material |
|
| Dust from the spray booth insufficient ventilation | Unnecessarily large opening in the booth | Close unused openings |
| Adjust the entrance and exist to the needed, replace filters of size of the objects | ||
| Clogging of filters in the booth or of the final filter |
Clean and, if needed, replace filters or cartridges If possible, increase the ventilation in the spray booth |
|
| Amount of powder sprayed is too large | Reduce the number of spray guns or reduce the amount of powder per gun | |
| Powder in not sufficiently charged | The high - voltage generator is not providing enough voltage to the spray gun's electrodes |
Ensure that cables and fuses are intact Ensure that the electrodes are in good condition Increase the generator voltage Ensure that the actual spray gun voltage is the same as the shown on the control panel |
| Poor earthing connection |
Check the connection between the object and the conveyer and the earth All contact points must be free of powder coating and other insulating materials |
|
| Too much powder per spray gun | Reduce the amount so that as much of the powder as possible that passes the spray gun is charged | |
| Proportion of fine - grained particles is too high |
Reduce the proportion of recycled powder inrelation to virgin powder Check with the powder supplier on the possibility of supplying powder with a lower proportion of fine - grained particles |
|
| Inappropriate type of powder | Check with the powder supplier if the powder is appropriate for the specific use / powder application equipment | |
| Insufficient penetration in corners and recesses | Air velocity in the spray gun is too high-the powder is blown off |
Reduce the supplemental air flow |
| Incorrect distance between the spary gun and the object | Increase or reduce the distance between spray gun and the object | |
| Amount of powder the air pre spray Gun is too small | Increase the air pressure on the ejectors | |
| Poor earthing connection | Check the connection between the object and the conveyer and between the conveyer and the earth. | |
| Spray cloud too wide | Change smaller deflector or to a flat jet beam nozzle | |
| Voltage is too high on the voltage generator | Reduce the voltage in order to even out the differences in coating thick ness on interior and exterior corners | |
| Air velocity is too high | Reduce the supplemental air flow pressure and if necessary increasethe distance between the spray gun and the object to prevent the powder from being blown off | |
| Incorrect spray gun adjustment | Adjust the spray guns so that the cloud of powder is directed as much as possible towards the most difficult areas of the object | |
| Proportion of fine-grained Particles is too high |
Reduce the amount of recycled powder optimising the spraying process Check with the powder supplier on the possibility of supplying powder with a lower proportion of fine-grained particles |
|
| Difficult to build up sufficient film thickness | Not enough charging / charging effectiveness |
Ensure that the cables, fuses and electrodes are working Increase the voltage from the generator Check the spray gun voltage |
| The powder slides off | Poor earthing connection |
Check the connection between the object and conveyer and between the conveyer and the earth All contact points must be free of powder coating and other insulation materials |
| Air velocity is too high | Reduce the air pressure, and if necessary increase the distance between the spray gun and the object. | |
| Adverse moisture in the work Area | Avoid relative humidity below 70% the ideal range is 45 - 55% | |
| Back - ionisation/uneven powder layer | Unnecessarily high voltage | Reduce the voltage |
| Insufficient distance between the spray gun and the object | Increase the distance | |
| Poor earthing connection | Check the connection between the object and the earth | |
| Large variation in film thickness | Incorrect spray gun adjustment | Check the adjustment and ensure proper over - lapping |
| Incorrect distance between spray gun and object | Increase or reduce the distance | |
| Improperly adjusted conveyer and speed of the reciprocator | Adjust the conveyer speed and / or the stroke and speed of reciprocator | |
| Improper hanging technique | Ensure that objects are as uniform as possible, correctly hung, and at the proper distance | |
| Uneven poder delivery |
Ensure proper fluidization and that the powder hopper is more than half full Examine the diameter and length of the hoses |
|
| Spray problems after mixing recycled powder | Change the particle size distribution | Reduce the amount of recycled powder as much as possible through the the hanging technique, spray gun adjustemin, and charging effective ness |
| Spitting | Accumulation of fine-grained powder in spray guns/delivery system |
Reduce the amount of recycled powder as much as possible Reduce the time between each cleaning / blow through Check with the powder supplier on the possibility of supplying powder with a lower proportion of fine - grained particles |
SURFACE/APPEARANCE
| PHENOMENON | POSSIBLE CAUSE | SUGGESTED SOLUTION |
|---|---|---|
| Poor flow | Film thickness is too low | Measure the thickness with appropri ate and calibrated equipment adjust the thickness by changing the air pressure on the ejectors |
| Heating up metal too slowly | Increase the oven temperature, or adjust the temperature profile if possible, using IR heating in the entrance zone. | |
| Spray gun voltage too high | Reduce the voltage or increase the Distance between the spray gun and the object. | |
| Inappropriate type of powder (Reactivity is too high or powder particles are too coarse) | Check with the powder supplier if the powder is appropriate for the specific use / powder application equipment. | |
| Low gloss or a high gloss finish | Contamination with another product that is not compatible | Through cleaning of the spray booth, delivery system and recycling system before adding virgin powder |
| Extreme temperature stress during heating | Reduce the effect in th IR - zone | |
| Pinholes in the finish | Film thickness is too high | Reduce the film thickness |
| Moisture | Check if there is moisture in the compressed air or if drying after the pre - treatment is insufficient | |
| Check the degree of porosity in the substrate (cast iron) | ||
| High gloss or a low gloss finish | Insufficient curing | Increase the temperature and/or time in the oven |
| Discoloration in the finish | Contamination virgin powder | Check with the powder supplier |
| Specs in coating finish | Sintered powder in the spray guns loosens and adheres to the object | Stop and be thorough cleaning avoid too much air pressure |
| The sieve in the recycling system is not functioning | Inspect and repair / replace the sieve if necessary | |
| Impurities from the conveyer or Hanging system fall down in the spray booth | Routine cleaning of the conveyer and stripping the hooks | |
| Craters in the finish / surface | Contamination with another product that is not compatible | Stop and do a thorough cleaning before adding virgin powder. |
| Grease on the metal surface after Insufficient pre-treatment | Check and replenish the pretreatment baths | |
| Moisture and / or oil in the Compressed air | Examine the oil and water separator | |
| Porosity in the coating | Moist substrate | Check the material after pre-treatment / drying |
| The powder quality is unsuitable for the specific substrate | Contact the powder supplier | |
| Incorrect or uneven colour | Curing conditions exceed the specified range | Refer to the product data sheet and make necessary adjustment |
| Variation in film thickness | Measure the thickness and make nessary adjustment | |
| Varying substrate thickness | Switch to polyester-based powder coatings | |
| Unsuitable type of powder | Check with powder supplier if the powder is appropriate for the specific use and type of oven | |
| Drop formation / sagging | Film thickness is too high | Check the thickness and make necessary adjustment |
| Type of coating is unsuitable for the specific use (flow too good) | Contact the powder supplier | |
| Poor coverage of edges | Sharp cutting edges on the metal | Contact the metal supplier for Changes alternative hanging technique (sharp edges down) |
| Type of coating unsuitable for the specific use (flow too good) | Contact the powder supplier |
PHYSICAL CHARACTERISTICS
| PHENOMENON | POSSIBLE CAUSE | SUGGESTED SOLUTION |
|---|---|---|
| Poor mechanical properties Impact strength / flexibility | Insufficient curing |
Take measurements of the metal's temperature profile Increase the temperature and / or time in the oven |
| Film thickness is too high | Check the thickness and reduce it by reducing the amount of powder or possibly increasing the conveyor speed | |
| Poor adhesion due to insufficient pre-treatment | Check the thickness and replenish the pre-treatment baths | |
| The nature of the powder coating | Check with the powder supplier | |
| Poor adhesion to the substrate | Inadequate pre-treatment | Check and replenish the pre-treatment baths |
| Insufficient curing | Increase the temperature and / or time in the oven | |
| Poor corrosion resistance | Inadequate pretreatments | Check and replenish the pre-treatment baths |
METALLIC POWDER COATINGS
| PHENOMENON | POSSIBLE CAUSE | SUGGESTED SOLUTION |
|---|---|---|
| PHENOMENON | POSSIBLE CAUSE | SUGGESTED SOLUTION |
| No charging | Short circuit in the spray gun | Change to a special nozzle for metallic powder coatings |
| Change in the metallic effect over time | Improper spraying equipment uneven consumption of base and metal pigment (dry mixed powder) |
Contact the powder supplier Reduce the amount of recycled powder as much as possible by optimising the spraying process Frequent addition of virgin powder |
TEXTURE POWDER COATINGS
| PHENOMENON | POSSIBLE CAUSE | SUGGESTED SOLUTION |
|---|---|---|
| The substrate is visible under the texture | Film thickness is too low | Increasing the thickness |
| Uneven texture | Uneven film thickness | Optimise the spraying process for the most even thickness possible |
| Variation in texture over time | Variation in particle size distribution | Reduce the amount of recycled powder as much as possible by optimising the spraying effectiveness |
RECOATINGS
| PHENOMENON | POSSIBLE CAUSE | SUGGESTED SOLUTION |
|---|---|---|
| Poor flow | The first layer insulates thus limiting the earthing connection | Reduce the spray gun voltage to 30 - 40kv |
| Poor adhesion between layersing | Impurities on the surface of the first layer | Ensure thorough process removal and rinsing |
| The powder coating is unsuitable for re spraying | Contact the powder supplier. |
OVEN PROBLEMS
| PHENOMENON | POSSIBLE CAUSE | SUGGESTED SOLUTION |
|---|---|---|
| Smoke / order from the curing process | Dry powder is blown off and comes into contact with the heating elements | Reduce air speed in the oven's entrance zone check damper and fans to ensure that the oven is properly aired out |
| Overheating of the powder coat |
Take measurements of the metal's temperature profile and reduce the temperature if necessary Check the effect and distance between lamps and materials in the IR - zone if one exists. |
