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Manufacturing Processes - Coatings Surface Finishing

 

Manufacturing: Surface Finishing


Finish Machining

Surface Finishing Coatings

Industrial Spray Painting Application Processes

Air Atomized Systems
Spray Painting & Curing Booth

In general, traditional systems combine compressed air and paint, producing small atomized particles. This approach can be applied at a rapid rate without sacrificing finish quality and it can cover complex shapes with recessed areas. Bounce back has been a problem in the past that has lowered the transfer efficiency. However, newer gun designs allow for greater control of the direction of emerging air jets and heating of the paint allows the viscosity to be reduced and thus the atomization can take place at lower air pressures, thus less bounce back (see spray application processes).

High Volume Low Pressure Guns

HVLP is similar to the traditional method, except that the air and fluid nozzle are designed to reduce the spray velocity. Higher air volumes are used at lower pressures (8-10 psi). The disadvantage of this approach is that it sometimes has difficulty in reaching high quality atomization at higher rates of application (above 200 cc/min.) which effects the quality of the finish. It also can have clogging problems with higher solids paints. Non-turbine HVLP guns do not offer metallic control of conventional style guns, making the spraying of metallic topcoats difficult to master.


Airless Spray Guns

Paint is forced through a very small orifice using high hydraulic pressure. The internal passage of the atomizing nozzle, however, is shaped so that paint passing through it is mechanically forced into a highly unstable form--usually a fan shaped film. As with air guns, heat may be used to lower paint viscosity. It also produces a soft spray. One of the biggest problems with this system is that the nozzles tend to wear out faster, creating tails in the pattern; and the very high pressure (1000 psi) in the hydraulic hoses restricts the ease of movement of the hand gun.

Air-assisted Airless Guns

Compressed air is introduced through orifices in the airless spray tip holder where the hydraulically formed paint film is atomized. This allows hydraulic pressure to be reduced which helps lessen nozzle wear and the formation of tails in the spray pattern.

Electrostatic Spray Guns

Liquid electrostatic spray uses one of two methods to charge the paint: bombardment charging or contact charging.

Bombardment charging happens to atomized paint from a spray gun as it enters an ionizing zone. The particles are charged by air molecules in the intense electrical field created by a thin grid wire network. The charged atomized paint particles are then attracted to the parts which are grounded by a conveyor. The main advantage of this approach is the relatively high transfer efficiency and ease of color change. A comparative disadvantage is that this system requires a much higher voltage source than direct charging. Contamination of the charging system can occur if the parts to be paointed are not properly grounded.

Contact charging uses rotary disks (sometimes called bells) to form a thin film of paint by centrifugal action. The paint film is charged directly by high voltage contact on the rotator assembly.

Both bombardment and contact methods have high transfer efficiencies. One disadvantage of the liquid electrostatic method is the Faraday Cage Effect which does not allow uniform coverage on narrow recesses and sharp interior corners.

 

Spray Application Processes
Process
Description
Advantages
Disadvantages
Air Atomization Most commonly used for industrial finishing.

Complete pattern control.

Finest atomization.

Uses more air

Creates most fog

Lowers transfer efficiency

Siphon Fed (external atomization) Uses vacuum created at nozzle to draw material from cup.
External atomization.
Lowest cost

Least maintenance

Change colors quickly
Operators carry the weight of the material at the gun.

Sprays light materials only

One quart maximum

Spray position limited
Gravity Fed Material is fed to a gun via attached cup
Ideal for most refinishing

Easy to clean

Improved atomization versus siphon feed
Operators carry the weight of the material at the gun

Sprays light materials only

One quart maximum

Spray position limited

Unfamiliar technology to some
Pressure Fed (external atomization) Uses external pressure source, tank or pump to force material from nozzle; material and air mix outside nozzle Delivers more material than siphon fed
Wide viscosity range sprays most materials
No air nozzle wear
Spray in any position
Independent control over air and fluid pressures
Consumes most air

Creates most fog

More controls to learn
Process
Description
Advantages
Disadvantages
Pressure Fed (internal atomization) material and air mix inside nozzle

less fog than external atomization

larger patterns

less air consumption

coarse atomization

fixed patterns

nozzle wear

fast drying coatings not recommended
Pressure Fed (external atomization with heated material) uses external pressure source, tank or pump to force material from nozzle

material and air mix outside nozzle

material is heated to reduce viscosity

better control

reduced air and fluid pressures

limits overspray and rebound

finer atomization for a better finish

better adhesion

more film build per coat

reduces blushing

reduced solvent use

not all materials can be heated

special paint formulations are required

additional equipment to maintain

equipment must be explosion proof (electrical)

high electrical demand

reduces pot life for catalyzed coatings
Low Pressure, Low Volume Atomization material is atomized by soft jets of air optimally positioned to impinge onto the fluid stream of elliptical cross secton fluid stream exiting the spray gun high transfer efficiency (65% to 75%)

sprays well into recesses and cavities
atomization not as fine as that of air spray

not recommended for heavy materials or where light production is required
High Volume, Low Pressure (HVLP) Atomization available for siphon fed, gravity fed, pressure fed, air assisted airless and heated air assisted airless high transfer efficiency (65% to 75%)

sprays well into recesses and cavities

complies with most air quality regulations
atomization not as fine as that of air spray

not recommended for heavy materials or where light production is required

some systems for generating HVLP air may be expensive
Process
Description
Advantages
Disadvantages
Airless (hydraulic atomization) atomization caused by release of high fluid pressure through small orifice

most widely used by painting contractors and maintenance painters
high fluid capaibility

large patterns

fastest spray application process

low air consumption

limited fog and bounce back

permits spraying cavities
strict maintenance

potentially hazardous hydraulic injection

higher rate of overspary

sharp patters difficult to blend

expensive nozzles may flood surface

equipment requires top maintenance
Airless (hydraulic) Atomization-Heated atomization caused by release of high fluid pressure through small orifice

most widely used by painting contractors and maintenance painters

heat is used to reduce viscosity

used by furniture manufacturers and industrial finishers
better flow of material

higher solids per pass of gun

viscosity control

finer atomization than with unheated
strict maintenance

potentially hazardous hydraulic injection

higher rate of overspary

sharp patters difficult to blend

expensive nozzles may flood surface

equipment requires top maintenance
Air Assisted Airless lower fluid pressures than airless (normally below 1,000 psi)

low pressure air is added via the air nozzle to further atomize the already preatomized spray

used by furniture and industrial finishers
material savings 50% better than air spray

lower overspray and fog

less tip and wear

longer pump life than airless

higher film build per pass of the gun
atomized not as fine as air spray

hydraulic injection may occur

tip plugging

strict maintenance required
Air Assisted Airless - Heated lower fluid pressure than airless (normally below 1,000 psi)

low pressure air is added via the air nozzle to further atomize the already preatomized spray

heat is added to improve viscosity and flow
better flow of material

higher solids per pass of the gun

viscosity control

finer atomization than air assisted airless
atomized not as fine as air spray

hydraulic injection may occur

tip plugging

strict maintenance required
Process
Description
Advantages
Disadvantages
Electrostatic Atomization voltage difference between the paint dispenser (low speed rotating disc or bell) and work causes the paint to be attracted to the grounded work

used mostly by appliance manufacturers

high production of uniformly shaped parts
high transfer efficiencies (65% to 95%) depending on the shape of the part

wrap-around effect can be used to coat other surfaces

minimum overspray
parts must be conductive

limit to shapes that may be coated

high production rate required

high voltage and spinning cup or disc may be hazardous
Electrostatic Attraction material is atomized using conventional air, airless or air assisted principles

particles are electrically charged and attracted to the work

electricity may be turned off to permis normal spraying
"wrap around" effect

material savings through minimized overspray

use with or without electrical charge
some conductive materials will require special equipment

parts must be conductive

difficult to penetrate cavities or recesses with power supply on
Electrostatic Attraction -Heated heated material is atomized using conventional air, airless or air assisted airless principles

particples are electrically charged and attracted to the work

electricity may be turned off to permit normal spraying
combines advantages of other heated application processes same as above, plus not all materials can be heated

special paint formulations are required

strict maintenance

potentially hazardous hydraulic injection

higher rate of overspray

sharp patterns difficult to blend

expensive nozzles may flood surface

equipment requires top maintenance
High Speed Rotation Atomizers high speed (10,000 - 70,000 rpm) rotating disc or bell gives exiting paint particles velocity and direction

voltage differences then take over and allow the electrically charged paint particles to attract themselves to the grounded part

"wrap around" effect coating other surfaces

works well with high solids coatings

minimum overspray

parts must be conductive

limit to shapes that may be coated

high production rates required

high voltage and spinning cup or disc may be hazardous




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