page 398
62. ELECTROSTATIC SPRAYING
•Used to coat an object to,
-reduce corrosion
-protect against scratching
-stop contact with air/water/chemicals/etc
-alter appearance
•Electrostatic paintig methods
-increases the production rate
-reduces paint wastage
-are well suited to automation
-give good finishes
•The basic process is,
-before/as the paint is sprayed, it is given a negative charge (extra electrons).
-The workpiece is positively charged, thus attracting the paint particles
-The paint is generally sprayed at the workpiece, the electrostatics even out the dispersion, and reduce wastage.
62.1 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
page 399
pump and paint supply
|
|
sharp edge |
|
||
|
|
|
|
|
|
|
|
|
|
|
high |
work |
|
|
|
voltage |
|
|
|
|
|||
|
|
|
|
||
|
|
|
|
+ |
|
|
|
|
|
- |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
•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
page 400
•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],
PARAMETERS |
TYPICAL VALUES |
Voltage |
90-100 KV |
Distance between gun and work |
8 to 15” |
Paints |
Solvent Based |
•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-resis- tive 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