1.1 ELECTROSTATIC ATOMIZATION METHOD

 

• The Blade Method 9Ransburg’s No. 1 process),

- no spraygun used

- a straight sharp edge is used charged to about 90 KV

- the work is grounded

- the paint is slowly pumped onto the blade, it spreads out, and by the time it reaches the tip it comes off as small charged droplets.

- the shape of the workhead encourages a linear dispersion pattern

- advantages,

- high efficiency/low wastage

- simple controls

- suited to flat surfaces

- disadvantages,

- limited dispersion patterns

- least cost effective and flexible

 

 

 

• The Bell Method (Ransburg’s No. 2 process)

- uses a rotating bell or funnel to disperse the paint

- the bell head is about 1 ft. from the workpiece

- a potential of 90-100 KV is applied to the head while the work is grounded

- the head rotates at 900 r.p.m. to cause good paint dispersion

- advantages

- good coverage for rounded surfaces

- better efficiency than blade method

- suited to spherical irregular shapes

- disadvantages,

- extra power/equipment to run motor

 

 

 

• ASIDE: the edge effect in electrostatic phenomenon leads to higher concentration of charges at corners and edges.

 

• The Disk Method (also Ransburg’s No. 2 process)

- similar to Bell method except that a flat disk is used.

- the geometry of the disk allows a much wider spray coverage

- advantages (compared to bell),

- wider coverage area

- well suited to irregular and long parts

 

• Rotating Bell handgun

- can be used for manual/movable spraying

- is well insulated to prevent shocks

- well suited to complex shape because of operator aiming

- most effecient and cost effective method

 

• Consider the comparative tables [source unknown],

 

 

• Air/Hydraulic Electrostatic Spraying

- Type I

- large negatively charged (100KV) electrodes are placed at the side of a spray paint line

- work on the central conveyor is rounded,and uncharged paint is sprayed in.

- the sprayed paint enters the electic field, becomes negatively charged and is drawn to the work.

- can be upgraded to a full electrostatic method easily

- a higher paint wastage results.

- Type II

- a typical spray gun head is modified to deposit a high voltage charge on the paint particles

- the paint is sprayed with a negative charge, and it is attracted to a grounded workpiece

- Type III

- the paint is charged before passing through the spray head

 

• Control systems (power supplies) for spraying a resistive/non-resistive. Resistive systems use resistors to limit the maximum current that can be drawn, thus protecting the users. Non-resistive sysems require added safety precautions and are best suited to automate systems.

 

• The edge phenomenon can result in uneven distribution of paint near edges and corners.

 

• The Faraday cage effect will result in recessed areas that get less paint coverage.

 

• Safety issues include electrical shocks, ventillation (most paints are solvent based), and arcs from poor grounding or unexpected metal contact.

 

• Some automation issues are,

- part recognition on a conveyor

- color changes

- booths are costly, but provide safety and process advantages