Powder spraying 1

GENERAL FINISHES

  • Solid colour 1
  • Solid colour 2
  • Solid colour 3

General Considerations

Film Thickness

60-80 um is the recommended optimum thickness range for smooth (C, CS, CM) and sandtex (ST/CT) finishes. For sandtex finishes, the overall appearance and finish obtained is influenced by the applied film thickness; large thickness variations result in inconsistent finish.

80-100 um thickness range is recommended for texture/wrinkle (TX/CW), antique (CA) and hammer (CH) finishes. The results obtained are influenced by the applied film thickness; too low or too high a thickness results in undesirable or poorly formed finishes.

Higher thicknesses may be required for sufficient hiding for some bright, exotic colours eg. certain bright yellow, orange and red shades, and also for coatings exposed to more corrosive environments (see Item 6 below).

(see Item 6 below). 

Deviation in Colour, Gloss, Finish and Mechanical Properties

A certain degree of deviation in colour, gloss, finish and mechanical properties between powder batches may sometimes occur and is not totally avoidable. It is thus important to use powder from a single batch for parts that are subsequently assembled together. However, factors like spraying parameters, curing conditions (temperature and time), substrate temperature pick-up rate, homogeneity within the oven, substrate variation etc. all have an influence on the coating results. The setting up of mutually agreed tolerance samples is recommended to address the issue of acceptable deviations.

Powder Incompatibility

This can cause gloss reduction, blurring, haziness, pinholes, craters or a combination of these.  It is the result of incompatibility between different binder systems of powders of different grades, or between powders from different suppliers.  To avoid cross-contamination, the applicator must ensure thorough cleaning of the system (gun, spray booth, powder hoses, fluidising tank etc.) when changing products.  

Spot Contamination

This may occur especially between colours of very contrasting shades.  Thorough cleaning must be ensured when changing colours. 

Reclaim

It is possible to mix up to 5-10% of reclaim powder into virgin powder for use, depending on the product and end-use requirement. 

Suitability Testing

Where the coated items will be subjected to mechanical stresses (post-bending etc.) or exposed to chemical substances (chlorine, salt, sealants, adhesives, lubricants, cleaning agents etc) which may affect or damage the coating, the applicator should carry out pre-testing to determine the suitability of the coating for use under these situations. 

Requirements for Coastal Projects

For outdoor projects in coastal and off-shore environments with salty atmospheres, it is extremely important to ensure good corrosion protection by ensuring sufficient film thickness, proper pre-treatment and the use of suitable substrates. 

Single-coat systems

Film thickness
This is to be controlled within the range 75-100 um; it is extremely important to achieve sufficient thickness along edges and in corners of the parts. 

Pre-treatment
For aluminium substrates this must be carried out strictly in accordance with the requirement of chromate treatment as per ISO 10546. Any chrome-free treatment used must be Qualicoat-approved.  All chemicals and processes used must be with the approval and recommendations of the chemical supplier and in strict compliance with their instructions.

An enhanced etching of the aluminium substrate is critical; a deeper pre-treatment etch of at least 2.0g/m2 is required. 

Substrate         
For aluminium substrates, high quality alloy 6063 or equivalent alternatives are to be used.

Two-coat systems

Epoxy powders are usually recommended as primer basecoats as they offer excellent corrosion resistance in such environments.   

For optimal intercoat adhesion it is important that the primer coat is only partially cured, by reducing the curing time from what is otherwise required for the primer’s full cure as stated in its Technical Data Sheet.  The topcoat is then applied and fully cured according to its TDS. Intercoat adhesion should be checked during the batch run. 

Touch-Up

Minor defects on powder coated surfaces such as scratches and other surface irregularities can be touched-up with a suitable liquid paint as follows:

    1. Lightly sand or grind with abrasive paper on and around the affected area; this roughens the surface for better inter-coat adhesion.
    2. Wipe with alcohol or dry-wipe to completely remove the sanding dust.
    3. Apply the repair paint and allow to dry as per paint manufacturer’s instructions. 

The paint systems usually recommended are:   

    • Solvent-based 2-component polyurethane paint system, or
    • Solvent-based 1-component acrylic paint system  

In any case, the applicator should consult the touch-up paint supplier and test its suitability for the intended use.

If necessary, mask before painting.
The repair paint may closely match the appearance but not necessarily the performance properties of the powder coating. 

Packaging of Coated Items

Materials used for packaging (plastic wrappings or films, tapes, labels etc) must be free of plasticizing agents or strong adhesives and be easily removed when required, without leaving any residue or affecting the appearance of the coating.  The materials should be tested by the applicator for their suitability for use in this respect. 

Coated items exiting from the curing oven must be allowed to cool down to room temperature before packaging, so as to avoid markings due to the plastic or tapes and to prevent subsequent condensation effects that may occur within the wrapping. 

Storage of Coated Items

Caution must be exercised when storing items (panels, extrusions, profiles etc.) coated with powders. 

    1. Coated items that have been packed, by wrapping with plastic, should always be stored indoors. Avoid any possible contact with chemicals, water, moisture, excessive heat and direct sunlight. This applies to both indoor-grade powders (pure epoxy or epoxy-polyester) and outdoor-durable, pure polyester grades.
    2. When coated items that have been packed are stored outdoors in open air, without any further protection, they are potentially exposed to water or moisture that may form due to atmospheric condensation effect, as well as to direct rain water.  These will get trapped and remain  underneath the plastic used in wrapping the coated items.Under the influence of elevated outdoor temperatures, the trapped water or moisture may be absorbed through the upper portion of the coating, causing a local discolouration of the coating surface.This water absorption through the coating surface occurs in all colours. However, as the discolouration always presents itself as a ‘whitening’ effect, appearing as whitish or light  patches and streaks on the film (commonly known as water marks), it is usually more obvious on dark-coloured coatings than light-coloured ones.
    3. Packed items that are left outdoors for brief periods of time while awaiting despatch must be sheltered from the rain and from dampness or moisture caused by atmospheric condensation.  For added protection, suitable water-proof sheets must be used.
    4. Whenever possible, it is recommended to use perforated-type materials for the packing of coated items, as these facilitate ‘ventilation’ of the items and greatly reduce the possibility of the trapping of water or moisture due to condensation effect.
    5. Water marks can largely be removed by locally heating the affected areas of the coating for a short time to approximately 120°C.  This may be done by means of a powerful hot-air dryer. Alternatively, the whole coated item may be put through the baking oven set at approximately 120°C.  Here, care must be taken so as not to cause permanent colour change or gloss reduction of the coating due to overbaking.This procedure will not have any negative effect on the adhesion, mechanical and protective properties and durability of the coating.
    6. Applicators and users of Oxyplast powder coatings are advised to ensure proper storage of coated items as mentioned.  Lea Hin Co. (Powder Manufacturers) Pte. Ltd. shall not be held responsible for any incidence of water marks arising from careless or improper storage of coated items.   

Maintenance of Coated Surfaces

To maintain the aesthetic properties of powder coated surfaces located outdoors, they should be cleaned on a regular basis ie. every six months.  In harsher environments with high levels of salts or pollutants such as seaside or industrial areas, more frequent cleaning is recommended ie. every three months.  A record of all cleaning schedules shall be documented. 

Powder coated surfaces may be cleaned as follows:

    1. Remove coarse and loose dirt and other loose deposits with a wet, soft cloth, sponge or non-abrasive brush. 
    2. Clean the coating with the soft cloth, sponge or non-abrasive brush using a warm mild detergent solution.  Any scrubbing must be done carefully. 
    3. After cleaning, rinse thoroughly with water to remove any residual detergent.

Caution:
Neither harsh scrubbing nor the use of steel brushes, steel wool or items that will scratch the surfaces are recommended.  In view of its mineral content, hard water is not recommended for cleaning.  Do not clean the surface when it is hot due to being in direct sunlight.

The following products are not to be used for cleaning:
Strongly acidic or alkaline products; Oxidizers; Strong polar solvents and thinners; Abrasive products; Xylene; Ketones; Toluene; Trichloroethylene; Turpentine 

If solvents need to be used, only the following are recommended, under careful use on small areas:
Methylated spirits; White spirits/mineral spirits; Ethylalcohol/ethanol; Isopropyl alcohol/isopropanol

Keep the contact between the coating and solvent brief, no longer than necessary to remove the dirt or contaminant, and rinse off completely. It is strongly recommended to first test out a particular cleaning agent on a small, insignificant area of the coating to ensure it does not damage the coating or affect its appearance before using it on the main surface.  

METALLICS & SPECIAL-FINISHES

  • Vein
  • Gold & Silver
  • Silver
In general, metallic-finish and special-finish (eg. polychromatic and antique-finish) powder coatings are more prone to variation in colour and appearance compared to normal colours. Such powders consist of two or more different components which behave differently during the application process; this makes it relatively more difficult to apply these powders.

The following guidelines serve to highlight the limitations of these powders pertaining to consistency of finish and application problems and how, despite these limitations, reasonably good results can be achieved by proper control of the application process. 

Important Remarks
These guidelines notwithstanding, metallic-finish powders are in general more suitable for the coating of items with smaller surface areas such as furniture, aluminium extrusions, bicycle frames, accessories etc.  

They are not generally recommended for larger surfaces such as panels, where inconsistencies that may occur in the finish eg. unevenness (patchiness) in the metallic appearance, are more obvious than on smaller parts.  For the coating of larger surfaces, a specific range of metallic-finish powders is recommended. 

Powder Application

  1. Pre-batch testing with actual equipment is highly recommended before mass production.
  2. Perfect consistency between batches is very difficult to achieve.  Mutual agreement on acceptable tolerance samples will be necessary in some cases.
  3. Results depend very much on the application equipment and processes used. Thus, it is imperative that a particular coating job be carried out on the same production line, with consistent, optimized settings of the equipment and processes throughout. Any change inequipment requires readjustment of the settings to achieve the same results.
  4. Likewise, colour changes require readjustment and optimization of equipment and process 0settings as different powders behave differently during application.
  5. Corona guns are recommended for applying metallic-finish powders (smooth, textured or metallic-antique types). These powders generally do not work well with tribomatic guns due tothe different charging characteristics of the metallic and powder components.
  6. Film thickness is a key factor influencing the quality of the finish obtained. For smooth and textured-finish metallics, film thickness determines the ‘density’ of the metallic component which appears on the coating. Higher thicknesses generally give a more dense and therefore, a more brilliant metallic look, and vice versa.For antique finishes (both metallic and non-metallic 2-tone types), higher thicknesses give a larger, more open and flatter wrinkle pattern while lower thicknesses, a smaller and tighter wrinkle  pattern.  In general, recommended film thickness:– 60 – 80 mm for smooth and textured metallics
    —     80 – 100 mm for antiques (metallic or non-metallic 2-tone types)
    — 150 – 200 mm for some metallic antiques
  7. In view of the importance of maintaining consistent film thickness, automatic spraying is highly recommended. Manual spraying is suitable for touch-up and coating of difficult corners, which should be done prior to coating of significant surfaces.
  8. Other spraying parameters also influence the results: gun voltage, air pressure, powder delivery, spray pattern etc.
  9. Depending on the gun system, the use of a metallic nozzle or insert designed for spraying metallic powders, or selecting the metallic powder function (for newer versions), will produce better results for metallic finishes. Cloud pattern must be uniform. In general, flat-jet nozzles are recommended for spraying metallic-finish powders as they give a wider spray pattern and  better results.
  10. To avoid striping and patchy appearance on extended surfaces, ensure sufficient overlapping of powder layers. Automatic installations must have sufficient number of guns for each side of the panel.  Conveyor and gun speeds must be optimized to give sufficient and consistent film thickness.
  11. To avoid ‘picture framing’ (film thickness and metallic effect more pronounced at the edges of the panel), optimize the gun settings and gap between workpieces for automatic installations.
  12. Gun-to-workpiece distance: As a general guide, about 15 cm for manual spraying, and 25-50 cm for automatic spraying.
    Generally, bringing the gun closer to the workpiece produces a denser metallic finish, and vice versa. When using manual gun, the operator must keep this distance as constant as possible to minimize variation in metallic effect.
  13. When spraying metallic powders, metallic particles tend to build-up quickly at the gun nozzle and within the gun barrel; this must be removed / cleaned off more frequently to avoid blocking and subsequent ‘sputting’, which results in patchiness or appearance of metallic ‘lumps’ on the coating.
  14. Voltage: 60 – 70 kV is generally recommended. Higher voltages may give a denser metallic output (more brilliant) on the workpiece but also result in orange-peel and back-ionization craters on the coating as well as more metallic accumulation at the nozzle and sputting powder cloud. Some new gun models come with metallic nozzles or insert which allow to operate at higher voltages (to give  better metallic effect) without the increased orange-peel, back-ionization craters or metallic build-up.
  15. Fluidized beds/hoppers are recommended as opposed to direct box feeders as they give better results. The compressed air used for the fluidization must be regulated to impart consistent fluidity to the powder.  Inconsistent powder fluidization can also lead to sputting powder clouds during application.
  16. In general, reclaiming/recycling of metallic-finish and special-finish (polychromatic and antique-finish) powders is not recommended. Due to component separation during application, the reclaim powder usually has different composition, particle size distribution and spraying characteristics from the virgin powder, resulting in different and unpredictable results. If recycling is a must (usually due to cost reasons), apply a constant reclaim to virgin powder ratio; maximum of 5% reclaim is allowed. Do not recycle more than once.Bonded-type metallic-finish powders fare better than normal dry-blended powders with regards to recycling.
  17. To ensure high transfer efficiency during application, all coating equipment and workpieces must be properly grounded.
  18. For certain metallic finishes, horizontal and vertical orientations of the coated panels give different colours or effects;  it is recommended to coat all workpieces for a particular job in a single orientation.The angle of incidence of powder coming from the gun also influences the results; the best practice is to orientate the workpieces perpendicular to the gun.
  19. Pick-up time (heat-up time) for coated workpieces entering the curing oven can influence the results eg. gloss, and wrinkle effect in antique finishes (metallic or non-metallic 2-tone type antiques). Different oven loads and substrate thicknesses must be taken into account when setting the oven parameters and line speed, as these have direct effect on the pick-up time. Avoid coating parts with very different substrate thicknesses and sizes at the same time.
  20. Powder storage: generally metallic-finish and special-finish powders have lower storage stability than regular powders. However, although the aesthetic aspects of the powder (especially the metallic brilliance) may change during storage, its functional aspects remain the same. When in doubt it is advisable to do a pre-batch testing on old powders before using them. 

Coating Durability

  • Depending on the finish, Q-type metallic-finish powder coatings have a tendency for tarnishing during service life, ie. a dulling of metallic effect (reduced brilliance) due to atmospheric oxidation.A clear top-coat is necessary in order to prevent tarnishing and to enhance the coating durability, especially for outdoor applications, and also for some indoor applications where the coated items are subjected to constant abrasion, rubbing, finger-contact or high-humidity environment.
  • It is to be noted that the presence of a clear top-coat will usually add some depth to the metallic effect of the metallic coat and also change its colour to a certain extent.
  • R-type metallic-finishes are non-tarnishing, so require no clear top-coating, but these are usually less brilliant than the tarnish-prone Q-type finishes.
  • Metallic-finish coatings may exhibit a darkening effect if subjected to a continuous service temperature exceeding 120°C. 

Storage & Handling of Coated Items

  • Caution must be exercised when storing and handling all coated items in general, but this is even more critical in the case of metallic-finish coatings.
  • Coated items must be properly wrapped with suitable wrapping material before storage.
  • Coated surfaces must not be allowed to come into contact with moisture or chemicals, or exposed to excessive heat and humidity during storage, be it in wrapped or unwrapped state.
  • Water or moisture trapped within the plastic used in wrapping coated items (due to condensation effect or trapping of rain water etc.) can cause discolouration (darkening or colour fading) and loss of metallic effect. These can occur in a random or regular pattern on a particular item.
  • To minimize the appearance of finger marks coated items must be handled using gloves at all times.
  • Coated surfaces must never be cleaned with any chemicals other than a mild, non-corrosive detergent solution, carefully and briefly using a soft cloth or sponge and quickly dried.

TROUBLESHOOTING

Though tremendous advances and improvements have been made in powder formulations and application equipment technology over recent times, powder coating application, as is the case with coatings application in general, is not free from occasional troubles.  However, with good handling procedures, housekeeping and systematic preventive maintenance of the equipment, these troubles could be avoided, controlled and minimized. 

Listed in the followings are some of the commonly encountered problems in an electrostatic powder spray system and their possible causes and solutions, in terms of Powder Application and Film Defects. 

Powder Application

Problem

Possible Causes

Possible Solutions

Poor Charging –  
inadequate powder

build 

1. High-voltage source not providing enough KV at electrode 
 
2. Poor grounding 

3. Powder delivery too high  
 
4. Powder too fine, possibly too much reclaim added 

1a. Check high-voltage source, electrode, cable 
and resistors. Replace defective or missing 
parts if necessary.


b. Clean electrode insulated by powder build or impact fusion. 
 
2. Check ground from conveyor rail through hanger to substrate. All contact areas must be free of powder build, grease or other insulating materials. 
 
3. Turn down powder feed by reducing air velocity until all material passing through charging field is charged. 
 
4. Reduce the percentage of reclaim added. 

Poor penetration –  
powder not coating on Faraday Cage areas 

1. Powder delivery too low 

2. Poor grounding 
 
3. Powder spray pattern too wide 
 
4. Voltage too high 
 
5. Poor gun placement 

1a. Increase powder delivery air velocity. 
  b. Use gun-barrel extension. 
 
2.  Check grounding. All contact points must be free of insulating materials.  
 
3.  Select smaller deflector. 
 
4.  Reduce voltage, so powder builds on substrate edges and surfaces do not repel powder from reaching recesses. 
 
5.  Adjust gun position so that powder cloud has a direct path to recess areas. 

Back-charging – powder layers repelled from substrate in spots 

1. Voltage too high 
 
2. Gun position too close to substrate 
 
3. Poor grounding 

1.  Reduce voltage setting. 
 
2.  Adjust gun position away from substrate. 
  
3.  Check grounding from conveyor rail through hanger to substrate. 

Powder feed spurting or slugging –  
interrupted  
powder feed 

1. Insufficient air pressure or 
    volume 
 
2. Powder hoses kinked, flattened or too long 
 
3. Hoses, pump venturies or guns are clogged with powder 
      

1.  Check air supply. Ensure enough supply when other equipment are also using the compressed air. 

2.  Check powder feed hoses. 
 
3a. Check hoses, venturies and guns for blockages. Clear if necessary. 
  b. Check free-flow properties of powder. 
  c. Check air supply for moisture that causes powder compaction. Air supply must be dry. 

Poor spray 
pattern –  
 asymmetrical 
 powder cloud 

1. Worn-out gun parts 
 
2. Impact fusion build-up 
 
3. Air delivery too low 

1.  Replace worn-out parts. 
 
2.  Clean gun parts, especially deflector. 
 
3.  Increase air supply. 

Poor adherence 
of powder to 
substrate –  
powder falls 
easily 

1. Poor electrostatic charging of  the powder 
 
2. Powder too old 
 
3. Powder too fine or too coarse 

1.  Adjust the electrostatic voltage supply. 
 
2.  Check with powder supplier. 
 
3a. Reduce the amount of reclaim powder added to virgin powder. 
  b. Check with powder supplier.

Film Defects

Problem

Possible Causes

Possible Solutions

a. Poor impact resistance, poor flexibility or poor adhesion.  

1. Coating undercured 
 
2. Poor cleaning or pre-treatment 
 
3. Coating thickness too high 
 
4. Change in substrate 

1. Increase oven temperature, or increased well time in oven. 
    
2. Check pre treatment chemicals and equipment. 
 
3.  Reduce coating thickness by adjusting application equipment. 
 
4. Check with substrate supplier. 

b. Poor corrosion resistance 

As in a1, a2 or a4 

As in a1, a2 or a4. 

c. Poor chemical resistance 

Coating undercured 

 Increase oven temperature, or increase dwell time in oven. 

d. Poor pencil hardness or poor abrasion resistance 

1. Coating undercured 

2. Powder formulation 
      

1.  Increase oven temperature, or increase dwell time 
in oven. 
 
2. Check with powder supplier. 

e. Poor surface flow-out, too much orange-peel 

1. Coating thickness too thin 

2. Heat-up rate too slow 

 
1.  Increase coating thickness during spraying. 
 
2.  Increase oven temperature. 

f. Gloss appears low or film hazy for a full-gloss powder 

1. Contamination by incompatible 
    powder 
 
2. Micro-pinholing from degassing 

1a. Clean application equipment before changing powder. 
  b. Check powder compatibility between virgin and  reclaim powders. 
   
2a. Check substrate for porosity. 
  b. Check substrate for moisture. 
  c. Check film thickness.  Reduce if too thick. 
  d. Check air supply for moisture.  Air must be dry. 

g. Gloss too low for a semi-gloss 
or matt finish powder  
 
Gloss too high for a semi-gloss 
or matt finish powder 

1. Coating overcured 
 
2. Coating undercured 

1.  Decrease oven temperature. 
 
2.  Increase oven temperature.  

h. Inconsistent film thickness 

1. Wrong gun position 
 
2. Reciprocator not 
    matched to line speed 
 
3. Defective spray 
    equipment 

1.  Check and reposition gun so that spray patterns over ap slightly. 
 
2. Adjust reciprocator or line speed. 
 
3. See Powder Application (e) 

i. Pin-holing, gassing  through coating surface 

See (f) 

See (f) 

j. Craters

1. Contamination by silicon 
    oil or other oils
 
2. Poor pre-treatment –  
    remaining grease and oil 
 
3. Contamination by  
    other materials 

1a. Check air supply for oil contamination. 
b. Check spray booth, oven and surrounding for oil contamination. 
2.  Check pre treatment chemicals and equipment. 
 
3. Check for presence of incompatible materials. Clean up installation if necessary. 

k. Dusts, bits and/or other contaminations (eg. fibre) 

1. Contaminants on substrate surface 
 
2. Contaminants from environment 
 
3. Contaminants in powder 

1.  Check substrate surface and pre-treatment of  substrate. 
 
2.  Check spray booth, oven and surrounding for contamination source. 
 
3a. Check virgin powder for any contaminants. 
b. Check reclaim powder for any contaminants. 

l. Contamination spots 

1. Contamination from spray system 
 
2. Contaminants from environment 
 
3. Contaminants in powder  

1. Check system (spray gun, hoses, tank etc.) 
 
 
2. Check spray booth, oven and surrounding for contamination source. 
  
3a. Check virgin powder for any contaminants. 
b. Check reclaim powder for any contaminants. 

m. Sagging 

1. Coating thickness too high 

2. Flow-out too good 

1. Decrease coating thickness during spraying. 
 
2a. Reduce oven temperature with increased dwell time. 
b. Check with powder supplier. 

n. Poor edge coverage 

1. Coating thickness too thin 
 
2. Change in substrate edge radius 
 
3. Powder formulation 

1.  Increase coating thickness during spraying. 
 
2. Check with substrate supplier. 
 
 
3.  Check with powder supplier. 

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