Powder coating Training Notes
WHY POWDER COAT ?
•Produces a hard, abrasion resistance & tough coating
•Can be applied over a wide range of thicknesses (20-25 mils/500-600 microns)
WHAT IS POWDER COATING
“A mechanical mixture of resin, pigments & hardening agents in powdered form, when applied on to a surface as a thin/thick layer, forms an adherent film after curing”.
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CONSTITUENTS OF POWDER COATING
•Powder coating is composed of finely ground particles of:
•Resins: key component
Thermosetting, Most suitable for thin coating. (Epoxy, Polyester, urethane, Acrylics, etc) & Thermoplastic, Most suitable for thick coating. (PVC, Polyehylene, Polypropylene, nylon, etc)
–Good resistance to chemicals
–Low shrinkage during curing
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CONSTITUENTS OF POWDER COATING
•Pigment: Titanium dioxide, Carbon black, Zinc oxide, etc.
–Provides desired colour & opacity
–Protects from UV-light
–Improves impermeability to moisture
•Fillers: Barytes, French chalk, Talc, Whiting, etc
–Fills voids
–Reduces the cost
•Degassing agent:
–Eliminates gas bubbles
•Accelerators: Copper salts, etc
–Enhances the rate of slow reaction
•Dry flowing agents: Poly acrylates, silicone, fludised alkyl esters, etc
–Improves free flow of powders
•Waxes: –Provides slip
MANUFACTURING POWDER PAINT
•The dry ingredients in exact amounts are blended at high speed to create a homogeneous mixture & heated to the resin melting temp in an extruder.
•The extruded mixture is then cooled & rolled before being broken into small chips.
•The chips are milled to a very fine size ( Average diameter of powder particle varies in between 30-50 micron) & packed.
•Each individual powder particle contains the entire coating formulation.
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COMPRESSED AIR DATA
•Requires good quality of compressed air, free of oil & moisture.
•Pressure ——————- –4 to 5 kg/cm2
•Max. moisture content—-1.4 gm/m3
•Max. oil content———–0.1 ppm
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POWDER COATING PROCESS
•Basic steps:-
–Surface preparation
–Application of powder
–Curing
–Recovery of powder
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SELECTION OF GUN ATTACHMENT & RIGHT POWDER
•No perfect tip for all applications.
•Extension lances help to get into recessed areas.
•Unwanted areas to be covered with high temp tape specially designed for powder coating.
•Right powder to be chosen as per the end use of the component.
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BASIC POWDER COATING PROCESS – Four basic processes:-
•Electrostatic spraying
•Fluidised bed
•Electrostatic fluidised bed
•Flame spray
*Electrostatic spraying is the most commonly used
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OPERATIONAL PROCEDURE OF POWDER COATING
•Ensure proper earthing to the equipment.
•Put powder into the hopper.
•Connect the equipment to the main air supply.
•Switch on the power supply.
•Direct the gun into the spray booth & press the trigger.
•Adjust the fluid bed air pressure to 4-5 kg/cm² till the powder in the hopper appears to ‘boil’.
•Don’t keep higher pressure than necessary otherwise powder may ‘puff’ out the gun.
*By increasing dosing air, powder speed will increase, cloud density is reduced. By reducing dosing air, powder speed will reduce, cloud density will increase.
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ELECTROSTATIC SPRAYING
•An electrical charge is applied to the dry powder particles while the component to be painted is electrically grounded.
•The charged powder & grounded work piece create an electrostatic field that pulls the paint particles to the work piece.
•The coating deposited on the work piece retains its charge, which holds the powder to the work piece.
•The powder coated work piece is placed in a curing oven, where the paint particles are melted on to the surface & the charge is dissipated.
* The gun gives +ve electrical charges on the powder particles by frictioning between them & non- conductive material (air).
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ADDITION OF VIRGIN POWDER
•Virgin powder should be added frequently to the powder hopper because, recycled powder will always have properties that differ from virgin powder.
•In order to maintain a constant composition, always keep the volume of re-used powder to minimum.
•To reduce variations in particle size distribution due to recycling.
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CURING
•The curing process requires a certain degree of temp for a certain length of time in order to reach a full cure.
–Temp…………….180-2000C
–Curing time……45 minutes.
•The application of energy to the product to be cured can be accomplished by:-
–infrared cure ovens (direct exposure to light in the range of IR-region)
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RECOVERY OF POWDER
•Powder coating booth is equipped with powder recovery cyclone and filter.
•It recovers as much as 98% of powder exhausted from the booth.
•Recovered powder is progressively re-used.
•After filter, filters fine powder of powder recovery cyclone and virtually no powder goes to atmosphere.
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POWDER COATING STORAGE CONDITION
•Powder coating must be:-
–flowing easily
–capable of maintaining a good electrostatic charge.
•Proper storage is essential as it:-
–prevents particles agglomeration
–ensures satisfactory application.
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FACTORS AFFECTING POWDER STORAGE CONDITIONS
•Factors:-
–Temperature
–Humidity
–Contamination
–Direct sunlight
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Recommended optimum conditions:-
•Temperature ………………………….250C (Approx).
•Relative humidity…………………….50 to 65 %.
•Away from direct sunlight.
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SAFETY & HEALTH
•Powder & air mixtures can be a fire hazard when an ignition source is introduced.
•The concentration of powder in air must be controlled to maintain a safe working.
•Any finely divided organic material, such as dust, can form an explosive mixture in air.
•Personal cleanliness should be maintained at all times (soap & water clean up to remove powder from the skin)
•Don’t use air blast nozzles to blow off powder from the skin.
•Disconnect the main power supply before cleaning or maintaining the gun
•Operator should hold the gun in his bare hands.
•Avoid inhalation of the powders.
•Use proper PPE
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ADVANTAGES OF POWDER COATING
•Releases zero or negligible amounts of VOC into the atmosphere, resulting in minimal air pollution.
•Easy recycling of over sprayed powder for reuse
•Provides a superior finish, greater durability, improved corrosion resistance.
•Lower raw material (powder) costs.
•Unlimited colour selection with high & low gloss.
•Easy to apply thick coatings
•Requires minimum operator training & supervision
•Parts can be racked closer together.
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DISADVANTAGES OF POWDER COATING
•Application of large areas not feasible.
•Coating touch up not possible.
•Difficult to apply intricate shapes & assembled components
•Can be more prone to orange peel.
•Difficult to apply thin coats (< 1.0 mil) .
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REMOVING POWDER COATING
•Methylene Chloride is most effective.
NB: Most recently the suspected human carcinogen Methylene Chloride is being replaced by benzyl alcohol with great success.
*(Organic solvents such as Acetone, Thinners, etc, are completely ineffective).
•Can be removed by abrasive blasting.
•Can be removed by using 98% commercial grade Sulphuric acid.
