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ELECTROSTATIC BASICS

Electrostatic Basics

Terms

types of power supplies

electrical output of power supplies

inhibited vs. uninhibited (stiff) systems

ionization

electrostatics and T.E.

resistivitymeasurement

I load I i nes

~

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)-=based on 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 vo I t s) .

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 ca I led 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.

Little or no resistance is placed in the path of e 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

I

POWER SUPPLY

I (14VDC) 1 BOARD (12 VDC)

1 RESISTOR I I (200 MICROAMPS) I

I I RESISTOR I I(200 MICROAMPS)

(65,000 VDC)

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

K w L $5

2

o r L 1

INSULATOR I

- 0 PAINT - a

EARTH 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

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.

I

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

I 1

SAFETY FENCE

DISCHARGE SYSTEM:

ISOLATION STAND

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-=divide by .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 9 Shock

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

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 I

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 = Barehands - Grounding straps (3M) - Proper Shoes (no sneakers) = Clean floor and electrically conductive

Ground path = Gun and operator = Object being sprayed = Pump grounding = 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