Download Electrostatic Applications

87
ELECTROSTATIC
BASICS
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Electrostatic Basics
Terms
types of power supplies
electrical output of power supplies
inhibited vs. uninhibited (stiff) systems
ionization
electrostatics and T.E.
resistivitymeasurement
loadIines
safety
Charged Particles
Like charges will repel each other and
unlike charges will attract each other.
Like Charges m
-,
Unlike Charges +
-
Charged Particles
All matter contains charged particles.
Matter can be positively charged, negatively
charged or have no charge, called neutral.
Neutral Charge
Positive Charge
Negative Charge
(+) positive charge-protons
(-) negative charge--electrons
Not shown is the neutral particles called
neutrons.
Charged Particles
Electrons (negatively charged particles)
can move easier than protons (positively
charged particles)-=basedon their position
within an atom.
Because of this ease of movement,
electrons or negatively charged particles
are generated by the power supply in most
electrostatic systems.
Electrical Flow
Electrical flow in a circuit is like the flow
in a pumping system.
A pump is like a power supply, the hose
is an electrical cable or wire and the valve
is like a switch.
Electrons flow through the wire or cable
to the switch-when the switch is on,
electrons flow, and when the switch is off
the electrons stop.
Voltage
A pump is like a transformer or power supply
in an electrostatic gun.
The power supply:
1. changes the electrical energy to form
negatively charged electrons.
2. increases the electrical force pushing
the electrons.
The force or pressure pushing the electrons is
called volfageand is measured in volts
(typically kilo volts which is a unit in 1000
voIts)
.
Current
The flow of electrons is called currentand is
measured in amps.
The electrons flow through a cable like a
fluid through a hose at a rate based on the
amount of voltage that is applied.
A material that allows easy flow of electrons
is called a conducfor4ike copper.
A material which would prevent the flow of
electrons is called in insulafor.
Resistors
A material where electrons cannot easily
move through is called resisfurs.
Resistors in electrical circuits control the
flow or rate of electrons.
More resistance-less electron flow, less
resistance-more electron flow.
Resistance is measured in ohms (typically
measured in megohms or millions of ohms).
Capacitance
The storage or accumulation of electrons is
caIled capacitance.
Any object with a surface area and does not
provide a path for electrons to leave that
surface will accumulate electrons.
More electrons can be stored with a greater
surface area of an object than a smaller area.
If the electrons accumulate to a point where
the object cannot hold anymore (saturation)
the electrons will discharge or leave the
object usually forming an arc or felt as a
shock.
The uncontrolled release of electrons is the
largest concern in the electrostatic systems.
Uninhibited (Stiff) Systems
Voltage and current is directly applied to the
atomization device.
e
Little or no resistance is placed in the path of
the electrical power charging the entire
electrostatic tool-so an operator cannot touch
the tool while the power is on.
Uninhibited systems are interlocked so
personnel cannot touch the atomization tool.
The atomizer will store electrons (like a
capacitor)-these electrons must be grounded
or bled off before you can touch the tool.
Typical systems will have varying voltage
ranges up to 100 K.V.
Electrical Output of
Internal Power Supplies
POWER SUPPLY
I
(14VDC)
BOARD
(12 VDC)
1
(65,000 VDC)
I
1
RESISTOR
III
I (200 MICROAMPS) I
I
RESISTOR
I(200 MICROAMPS)
The electrode transfers the K.V. output and
current output from the stud resistor to the
electrode needle.
The electrons coming from the electrode
combines with the paint particles making
them negatively charged-this is called
ionization.
Ionization
INSULATOR
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K
w
L
-EARTH
$5 o r
1
L
2
-0
PAINT
a
EARTH
EARTH
As the paint particles become negatively charged
from the electrode they are directed toward the
part.
The negatively charged paint particles will seek
the nearest unlike charge.
The part on the conveyor system should be
grounded-earth ground is neutral and unlike in
charge to the negatively charged paint particles,
so the paint will be drawn to the part.
Some paint will pass the part from the velocity of
the pattern and be pulled back and paint the back
side-this is called "wrap."
Electrostatic
Transfer Efficiency
A number of factors can effect electrostatic
T.E.--so obtaining electrostatic "wrap" does
not necessarily mean you are spraying with
best possible T.E.
Some T.E. factors unique to electrostatics
include:
distance of the gun to the part
humidity
=
fluid conductivity
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Distance of Gun to Part
Air presents a resistance to current flow
between the gun and part.
The shorter the distance between the gun
and part the less resistance to electron
flow causing current flow to increase and
voltage to decrease reducing T.E.
Electrostatic gun distance should be 10 to
12 inches from the part.
Humidity
Moisture in the air supports the flow of
electrons-water is a good conductor.
If the humidity is high in the spray
environment, the electron or current flow will
increase causing voltage to drop which
decreases T.E.
You may see more efficiency on dry days
versus wet days.
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Conductive
(Low Resistivity)
Fluids
Conductive Solvent Based
Metallics
Water Borne
Conductive Solvent
Based Fluids
Some material suppliers may use more
conductive solvents than others when I ley
formulate causing electron flow to be high
and lowering tip voltage for poor T.E.
A common group of conductive solvents are
the polar based solvents-=the keytone family.
Sometimes the material supplier can adjust
the solvent balance using more resistive
solvents in the formulation--this should
always be discussed with the material
supplier and customer.
Highly Conductive
Solvent Based Fluids
Fluids which support the flow of electrons
will cause tip voltage to drop, decreasing
T.E.=-this is where you may have heard the
phrase "the deader the better."
Fluids must have some conductivity for
electrostatics to work-it's a matter of how
much conductivity.
Paint conductivity is often referenced by it's
resistivity.
=
Highly resistive fluids do not support the
flow of electrons causing tip voltage to be
high-high T.E.
=
Low resistive fluids do support the flow of
electrons causing tip voltage to be low
decreasing T.E.
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Metallics
Some of the metallic materials contain
flake which is conductive--like aluminum.
Conductive flake supports the flow of
electrons decreasing tip voltage and T.E.
Metallics are more conductive at higher
voltage than lower-measurement of
conductivity with the meters will not work
due to the low voltage used.
Volts
Non-conductive fluid
between metallic
volts
molecules
High Conductivity
Electrostatic Gun
To spray high conductivity fluids with good
T.E., Graco uses a high conductivity gun
kit--fluids like urethanes or metallics.
e
Electrons can be directed to either the
electrode or supply ground via the fluid
filtering on the gun.
Fluids allow electron flow through the fluid
depending on the basic conductivity of the
fluid and the length/diameter of the fluid
column. Since we cannot change the basic
conductivity we can alter the size/length of the
fluid column.
When the fluid is conductive and the material
supplier cannot use less conductive solvents
in the formulation for electrostatic spray, the
fluid column from the fluid fitting to the front
of the gun can be lengthened.
The extended fluid column acts as a
resistance to current flow allowing more
electron flow to the electrode for electrostatic
spray.
Water Borne Fluids
Materials containing water in their
formulation easily support the flow of
electrons.
As electron flow increases, tip voltage will
decrease lowering T.E.
To spray water bornes electrostatically, a
special system which isolates and controls
the voltage is needed.
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Waterbase Isolation System
ISOLATION STAND
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1
SAFETY FENCE
DISCHARGE SYSTEM:
1 . PNEUMATIC DUMP SWITCH
2 . BLEED RESISTOR
Resistivity Measurement
of Fluids
A number of meters are available from
electrostatic equipment suppliers to
measure suppliers to measure the
resistivity (conductivity) of a fluid.
Graco measures resistivity in units called
Meg Ohm Centimeters.
Generally, a resistivity reading of 20 or 25
Meg Ohm Centimeters or above sprays best
electrostatically.
There are no hard fast rules-some materials
as low as 7 Meg Ohm CM and above have
been successfully sprayed electrostatically.
Resistivity Conversion
Factors
Graco old style meter-multiply by 37 to get
Meg Ohm CM.
Graco old style meter--divide by 3.94 to get
the Ransburg reading
New Graco meter--take the Meg Ohm CM
reading and divide by 146 to get the Ransburg
reading.
Ransburg reading-=divideby .0068 to get the
new Graco meter scale in Meg Ohm CM
Note: These are approximate figures
based on inhouse testing, but
should be very close.
Electrostatic Safety
Personal Safety
Shock
9
Fire or Ignition
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Personal Safety
Voltage vs. Current
Voltage without current cannot hurt
you-like a spark plug wire on a car or
lawn mower.
The combination of voltage and current
may hurt you based on how much current
and how much voltage.
Grounding
Provide a continuous path for
electrons to complete an electrical
path.
b
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What Needs To Be
Grounded
Operator
GunHandle
Object Being Sprayed
Fluid Handling Equipment
(No Plastic Pail Liners)
Any Objects Within the Spray Zone
that are Conductive
Conductive Floors-Properly Clean
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Flammable Liquids
Air Hose
Safe Operation of
E/S Equipment
Handle must be grounded
Use Graco grounded air hose (black with
green ground wire or new flexible gray
hose with green ground wire.
Operator must be grounded
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Barehands
Grounding straps (3M)
Proper Shoes (no sneakers)
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Clean floor and electrically conductive
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Ground path
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Gun and operator
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Object being sprayed
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Pump grounding
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All flammable liquids in FM approved
grounded containers.
Housekeeping
Maintenance
Proper functioning equipment
Remove paint build up which prevents
grounding and reduces T.E.--like
conveyor hooks.
Clean guns properly-consider gun
washers