It is the most exciting finishing method to be introduced in the past 100 years. Rarely does a new technique offer significant performance advantages and at the same time offer reduced costs. It also meets environment regulations. (no VOC emission, no paint sludge disposal).
Quality advantages – These coatings are supplied ready for immediate use without additional pre-mixing, stirring, solvent additions, or viscosity adjustments. Hence you get consistent results. The optimum film thickness is achieved in a single pass without problems of drips, sags or runs. Powders can be formulated to provide a wide range of melt viscosities. Hence powder can provide better edge coverage protection than can be achieved in single-coat liquid coating applications. With only few exceptions almost all liquid paint films, mechanical and corrosion resistant properties can be achieved with powder. However few properties that are readily achievable with powder are difficult or impossible to achieve with conventional liquid paints.
Powder coating booth
Powder coating
Powder coating system with steel hopper
Electro-static Powder Coating There are several powder coating processes meant for different substrates, shapes of components and end use. The following can be the general classifications:-
(i) Most of the powders are applied on the product using electro-static spray guns capable of depositing powder on the object by charging the powder negatively by using an electrostatic gun. The object is grounded with a positive charge, which means the powder is attracted towards the object. This powder coating when melted in the oven forms a cure edge of the film. Although there are developments over the years in this technology, still powder spray reaching recessed areas is a problem due to faraday cage effect. (Powder tends to go to the objects and not into the recessed areas). Of course balancing the high voltage and the air pressure, one can achieve increased penetration, but this technology still suffers problems associated with powder reaching recessed areas.
(ii) Tribo static powder coating – instead of charging the powder electrically using high voltage, the charge is developed in this technology using friction method, thereby eliminating the problems associated with faraday cage effect, and electro static process. However, this technology has its own limitations, since it can be used for only selected powder formulations specially created for Tribo charging. Typical components using tribo charging are radiators used for heating the building, super market shelving, etc.
(iii) Dip powder coating – There are 2 types of dip powder coating. One uses fluidized coating with electro-static charge to deposit thermosetting powders. In the other type, objects are pre-heated to a specified temperature, quickly dipped into a fluidized powder coating tank and subsequently post cured if the product does not have enough mass to retain the heat. The example of products coated in this method is valves, refrigerator freezer shelves, etc.
PVC Powder coating - Is done using similar technique, but instead of dipping, the powders can also sprayed on pipes – used for fencing or wire goods etc.
(iv) Powder enameling – Components like bathtubs, cooking ranges, drums of washing machines, electric geysers, highway sign boards etc need tough coating with heat resistance. Enameling is a process used for many years, using a slurry made of quartz, feldspar etc. This liquid coating process is replaced with powder enameling. The choice of the process depends on the quality of steel used, one coat-one fire, two coats-two fire process needs to be chosen. Normally the temperature for curing exceeds 350 C and this abrasive powder should be sprayed with special abrasive resistant guns and with special powder recovery system.
(v) E-Coat/water based paint/powder – In the section " liquid paint technology " we have explained the reason for using electro coat painting under the heading “b” and need to use water based dip paint under heading “e”. In other words this primer coating method is used to paint recessed areas. On top of this, primer coating, powder is applied for several components requiring powder as a top coat depending on the end use.
(vi) Paint over Powder – Parts like Aluminium die casting wheels are passivated and subsequently powder is applied as primer. Top coat is done with silver metallic wet liquid paint followed by liquid lacquer coat, and subsequent baking. The liquid lacquer reduces the orange peel compared to powder lacquer and also helps in protection.
(vii) Fusion Bonded Epoxy – Pipes for oil and gas industry re-bars for construction industry are coated with fusion bonded epoxy powders with a very high thickness upto 300 microns and above depending on end use. The products are shot blasted, powder sprayed. Polyetheline wrapping is done on this powder coated film for pipes followed by water quenching.
Typical sources of variation of a powder coating system
MATERIAL
Powder (before application)
Composition, particle size, density, moisture
content, flow rate, hiding ability
Powder (Cured Properties)
Film thickness, color, gloss, hardness,
orange peel, uniformity, corrosion resistance
Water (Washing/Pretreatment)
Hardness, purity
Chemicals (Pretreatment)
Composition, Concentration
Chemicals (Applied)
Phosphate coating weights
Part Surface
Metal type, oil type
EQUIPMENT
Pretreatment Systems
Spray pressures, dwell times, bath
temperatures
Ovens
Air temperature, part temperature, dwell
times
Powder Application
KV, flow rates, atomizing air, ground
Tooling
Shape, design
Air Supply
Volume, air quality
Conveyor
Travel rate
ENVIRONMENT
Plant
Air quality, temperature, humidity,
lighting, housekeeping
Inspection Area
lighting
Powder Application
Temperature, humidity, air flow (velocity)
in booths
Ovens
Air quality, cleanliness
Line Set-up
Organization, ergonomics
PEOPLE
Ability
Skill level, fatigue, eyesight,
attention span
Technique
Positioning of product on racks, manual
Film Testing - Surface Characteristics
Films (Powder Coating)
Test Method
Procedure (s)
Primary Test Equipment
Surface Characteristics
Smoothness
PCI # 20
Smoothness Standards
Gloss
ASTM D523
Glossmeter
Color
ASTM D2244
Colorimeter
Distinctness of Image
Visual Observations
Contrast Ratio
PCI #3
Special Substrates,
ASTM D2805
Reflectance Instruments
Film Testing - Physical Characteristics
Films (Powder Coating)
Physical
Test
Primary Test Equipment
Characteristics
Procedure (s)
Film Thickness
ASTM D 1186
Magnetic Film Thick Gauge,
ASTM D1400
Eddy Current Induce Gauge
Impact
ASTM D2794
Impact Tester
Flexibility
ASTM D522
Conical or Cylindrical Mandrel
Adhesion
ASTM 2197
Balanced-Beam Scrape-Adhesion,
ASTM D3359
Cross Hatch Cutting Device and Tape
Hardness
ASTM D3363
Calibrated Drawing Leads or Pencils
Abrasion Resistance
ASTM D4060
Taber Abrader and Abrasive Wheels
ASTM D968
Edge Coverage
ASTM 296
Standard Substrate and Micrometer
Chip Resistance
ASTM D3170
Gravelometer
Films (Powder Coatings)
Test Method
Procedure(s)
Primary Test Equipment
Environmental Characteristics
Solvent Resistance
MEK or other
Stain Resistance
ASTM D1308
Standard Reagents
Chemical Resistance
Immersion or Spot Tests
Humidity
ASTM D 1735
Standard Water Fog
ASTM D2447
Chamber, Test Chamber 100% RH
at 100F
Salt Spray
ASTM B117
Salt Fog Chamber
Weathering
Exterior
ASTM D1014
Panel Racks
Accelerated
ASTM D4141
Special Racks and Reflector
Weatherometer, QUV and
Artificial
ASTM D5031
Equipment ASTM G23,
ASTM D 822
G26, G53
ASTM D4587
ASTM D3361
Film Defects
Investigation and identification of film defects is an important part of quality control. Often identifying a film defect can give insight into the cause and thereby the solution to that defect. A 10X glass, or better, a variable stereo microscope (10X to 50X) can be extremely useful in defect identification.
Blooming
Blooming appears as a whitish haze on a cured surface. It may be possible to remove blooming by wiping the surface with a solvent containing cloth.
Color Specks in Film
Caused by foreign material or other powder coatings in a powder. Color specks may promote cratering.
Craters
Craters are visible depressions in a cured film which may vary in size from less than 1 millimeter to greater than 1 centimeter. Crater size and shape is dependent upon the actual cause of the crater. Examination of craters using some type of magnification is essential to identifying a cause. Craters may or may not contain a small grain or “lump” at the center and may vary in frequency from very low density (1 or 2 defects per sq.cm or less) to very high density which gives a textured appearance to the film. Three broad categories of cratering are listed below:
Fisheyes – Generally it is considered to be the largest of craters possibly being so deep as to expose the bare metal substrate.
Craters – Generally it refers to medium size defects. Most craters will have a grain or “lump” at the center.
Pin Point Craters – Very small, finely distributed defects sometimes referred to as micro pin-holing. Approximately the size of the point of a pin.
Faraday Cage Effects
The Faraday Cage effect is an artifact of corona electrostatic spraying of powder coatings. Cage effects occur in areas where several surfaces are in close proximity to one another such as corners, inside small parts, or at sharp angles. When cage effects occur, the amount of powder deposited is reduced causing thin or incomplete film formation. The mechanism of this phenomenon is relatively simple to understand. As power is deposited onto adjacent surfaces, the charges associated with the individual particles of powder accumulate. Eventually the charge buildup is sufficient to repel particles as they approach the affected area causing reduced deposition and thus thinner films. Using lower charge density application equipment such as tribo charging guns helps to reduce or prevent Faraday Cage effects.
Haze
Haze lowers the distinctness of image and may have many causes. One type of hazing has already been described as blooming. A second type of hazing can be caused by poor pigment dispersion and, unlike blooming, is not a surface effect and cannot be removed by wiping with solvent.
Marring
Damage to a film surface due to rubbing contact is termed marring. Because marring actually affects the continuity of the film surface, manifestations of marring may be areas of changed gloss or slight shade differences (due to different light reflection).
Picture Framing/Fatty Edges
The excessive film build around the edges of a part is called picture framing or fatty edges.
Pull Away
Generally observed when Faraday Cage effects are present. Pull away is characterized by shrinking of the film which exposes the substrate.
Seeds/Bumps
Seeds (also referred to as bumps) are protrusions of the film. Seeds can be considered the opposite of craters which are indentations in a film. As with cratering, determining the cause of the bumps is performed by a magnified examination of the defect’s cross-section. Powder bumps are caused by aggregated powder deposited on the part. The aggregated powder will melt and flow out but at a slower rate than individual powder particles. Powder bumps appear as identically colored, uniformly rounded areas in a film. Gel Bits are characterized by sharp protrusions from a film. These may or may not be the same color as the film. Lint or Fibers are foreign matter contaminants in the powder spray. These defects may appear linear or balled in the film. This type of contaminant may be identifiable prior to baking. Identifying this type of defect prior to curing allows easy repair. Bubbles have an appearance similar to powder bumps but are hollow. Volcanoes, so called because they mimic the shape of a volcano when viewed under magnification, are formed by gas evolution during cure. Commonly found on coated cast metal parts.
Sags
Sags are less commonly seen in powder than in liquid. These defects appear as a drip or ridge in the cured film. It is usually associated with vertically oriented surfaces.
Telegraphing
Telegraphing is the mirroring of a substrate imperfection through the film. Examples of imperfections that can cause telegraphing are forming and sanding marks.
Orange Peel-Excessive
Lack of smoothness of a coating tends to resemble the texture found on oranges and thus the name. Orange peel coatings may be desirable when the hiding of metal defects is necessary.
Water Spotting
This defect appears as a disruption in the smoothness of the film surface as though a spot of water had dried on the film.
