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Claims  |
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Having described the invention, it is claimed:
1. A system for electrostatically spraying highly electrically conductive
water-based coating material with improved safety, comprising:
a spray device for directing electrostatically charged water-based coating
material toward an object to be coated which is maintained at a potential
different from that of the charged coating material,
a container for highly electrically conductive water-based coating
material,
a fluid conduit interconnecting said container and said spray device for
transporting water-based coating material from said container to said
spray device, said fluid conduit having a bore through which coating
material is transported from said container to said spray device, said
conduit including:
a. an inner zone surrounding said bore, said inner zone including material
which is substantially chemically inert and impermeable to said
water-based coating material,
b. an outer zone of dielectric material surrounding said inner zone, said
outer zone having an exterior surface which is substantially free of
electrically conductive material to minimize the capacitance of the
conduit, and hence the electrical energy stored in the conduit in
capacitive form, when the conduit is physically spaced from a grounded
object,
said inner and outer zones having dielectric constants and radial
thicknesses to avoid dielectric breakdown therein when subjected to a
potential difference equal to the difference in potential between ground
potential and the potential existing at the interface of the transported
coating and the inner zone, and
a source of electrostatic charging potential for charging water-based
coating material in said container, conduit and spray device,
said fluid conduit having a length and cross-section dimensioned to provide
a very low electrical resistance between said spray device and container
whereby said water-based coating material in said container, conduit and
spray device is charged to an electrostatic potential substantially the
same as output from said electrostatic charging source.
2. A system for electrostatically spraying highly electrically conductive
water-based coating material with improved safety, comprising:
a spray device for directing electrostatically charged water-based coating
material toward an object to be coated which is maintained at a potential
different from that of the charged coating material,
a container for highly electrically conductive water-based coating
material,
a fluid conduit interconnecting said container and said spray device for
transporting water-based coating material from said container to said
spray device, said fluid conduit being hollow and having a dielectric wall
with a zone in contact with the coating which is substantially chemically
inert and impermeable to the water-based coating, said wall constructed to
withstand dielectric breakdown when subjected to an electrical potential
by electrostatically charged coating material in said conduit, said
conduit having an exterior surface which is substantially free of
electrically conductive material to minimize the capacitance of the
conduit, and hence the electrical energy stored in the conduit in
capacitive form, when the conduit is physically spaced from a grounded
object, and
a source of electrostatic charging potential for charging water-based
coating material in said container, conduit and spray device,
said fluid conduit having a length and cross-section dimensioned to provide
a very low electrical resistance between said spray device and container
whereby said water-based coating material in said container, conduit and
spray device is charged to an electrostatic potential substantially the
same as output from said electrostatic charging source.
3. A fluid conduit for minimizing safety hazards when transporting
electrostatically-charged water-based coatings between a coating container
and a spray device, said coating being charged from an electrostatic
source, said fluid conduit comprising:
a hollow hose having a dielectric wall with a zone in contact with the
coating which is substantially chemically inert and impermeably to the
water-based coating, said wall constructed to withstand dielectric
breakdown when subjected to an electrical potential by electrostatically
charged coating material in said hose, said hose having an exterior
surface which is substantially free of electrically conductive material to
minimize the capacitance of the hose, and hence the electrical energy
stored in the hose in capacitive form, when the hose is physically spaced
from a grounded object,
said fluid hose having a length and cross-section dimensioned to provide a
very low electrical resistance between said spray device and container
whereby said water-based coating material in said container, hose and
spray device is charged to an electrostatic potential substantially the
same as output from said electrostatic charging source.
4. A fluid conduit for minimizing safety hazards when transporting
electrostatically-charged water-based coatings between a coating container
and a spray device, said coating being charged from an electrostatic
source, said fluid conduit comprising:
a hose having a bore through which coating material is transported from
said container to said spray device, said hose including:
a. an inner zone surrounding said bore, said inner zone including material
which is substantially chemically inert and impermeable to said
water-based coating material,
b. an outer zone of dielectric material surrounding said inner zone, said
outer zone having an exterior surface which is substantially free of
electrically conductive material to minimize the capacitance of the hose,
and hence the electrical energy stored in the hose in capacitive form,
when the hose is physically spaced from a grounded object,
said inner and outer zones having dielectric constants and radial
thicknesses to avoid dielectric breakdown therein when subjected to a
potential difference equal to the difference in potential between ground
potential and the potential and the potential existing at the interface of
the transported coating and the inner zone,
said fluid hose having a length and cross-section dimensioned to provide a
very low electrical resistance between said spray device and container
whereby said water-based coating material in said container, hose and
spray device is charged to an electrostatic potential substantially the
same as output from said electrostatic charging source.
5. The system of claim 2 wherein said zone is tetrafluroethylene.
6. A system for electrostatically spraying highly electrically conductive
water-based coating material with improved safety, comprising:
a spray device for directing electrostatically charged water-based coating
material toward an object to be coated which is maintained at a potential
different from that of the charged coating material,
a container for highly electrically conductive water-based coating
material,
a fluid conduit interconnecting said container and said spray device for
transporting water-based coating material from said container to said
spray device, said fluid conduit having a bore through which coating
material is transported from said container to said spray device, said
conduit including:
a. an inner zone surrounding said bore, said inner zone including material
which is substantially chemically inert and impermeable to said
water-based coating material,
b. an outer zone of dielectric material surrounding said inner zone, said
outer zone having an exterior surface which is substantially free of
electrically conductive material to minimize the capacitance of the
conduit, and hence the electrical energy stored in the conduit in
capacitive form, when the conduit is physically spaced from a grounded
object,
said inner and outer zones having dielectric constants and radial
thicknesses to avoid dielectric breakdown therein when subjected to a
potential difference equal to the difference in potential between ground
potential and the potential existing at the interface of the tranported
coating and the inner zone, and
a source of electrostatic charging potential for charging water-based
coating material in said container, conduit and spray device,
said inner zone being tetrafluroethylene and said outer zone being
fabricated of material which exhibits substantially a greater flexibility
than said inner zone.
7. The system of claim 6 further including a skin of material surrounding
said outer zone which exhibits substantially greater abrasion resistance
than said outer zone, said skin having an exterior surface substantially
free of electrically conductive material to minimize the capacitance of
the hose, and hence the electrical energy stored in the hose in capacitive
form, when the hose is physically spaced from a grounded object.
8. The hose of claim 3 wherein said zone is tetrafluroethylene.
9. The hose of claim 4 further including a skin of material surrounding
said outer zone which exhibits substantially greater abrasion resistance
than said outer zone, said skin having an exterior surface substantially
free of electrically conductive material to minimize the capacitance of
said hose, and hence the electrical energy stored in said hose in
capacitive form, when the hose is physically spaced from a grounded
object. |
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Claims |
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Description |
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This invention relates to electrostatic spray coating and more particularly
to an apparatus and method for minimizing safety hazards when
electrostatically spraying highly electrically conductive water-based
coating material.
In the recent past there has been a considerable increase in the emphasis
placed on providing employees, particularly in the industrial field, with
working conditions which are free from recognized safety and health
hazards. This trend has, in part, been the result of the Occupational
Safety and Health Act which requires employers to provide their employees
with safe places of employment. In the field of electrostatic paint
spraying, in an effort to comply with various safety and health
regulations issued pursuant to the Occupational Safety and Health Act,
there has been a marked trend among employers to switch from solvent-based
coatings to water-based coatings. Coating materials of the solvent-based
variety, such as varnishes, lacquers and the like create an atmosphere
which is both explosive and toxic. The explosive nature of the environment
presents a safety hazard should a spark inadvertently be generated, such
as by accidentally electrically grounding the nozzle of the spray gun,
which can then ignite the solvent in the atmosphere, causing an explosion.
The toxic nature of the workplace atmosphere creates a health hazard
should the employee inhale the solvent vapors which are present.
Because both of the foregoing hazards are eliminated by switching to
water-based coatings, the increased emphasis on providing employees with
safe and healthful workplaces has resulted in a shift by industry to
electrostatic spray coating products of the water-based type.
Unfortunately, the switch from electrostatically spraying solvent-based
coatings to those of the water-based type has sharply increased the risk
of electrical shock, which risk was relatively minor with solvent-based
coatings. Specifically, the increased risk of electrical shock
accompanying electrostatic spraying of water-based paints is occasioned by
the fact that water-based paints are extremely electrically conductive,
with resistivities often falling in the range of 100-10,000
ohm-centimeters. This is in contrast to resistivities of 200,000-1,000,000
ohm-centimeters for moderately electrically conductive coatings such as
metallic paint, and resistivities exceeding 1,000,000 ohm-centimeters for
solvent-based lacquers, varnishes, enamels, and the like.
By way of background, with coating materials which are either not
electrically conductive or only moderately electrically conductive, as is
the case with metallic and solvent-based paints, the charging electrode,
which is maintained at a high electrostatic potential, is usually placed
near the nozzle of the spray device thereby electrostatically charging
coating material as it is emitted. Due to the relatively high resistivity
of the coating material, when the electrode is so placed, the column of
coating material in the hose, which connects the spray device to the
coating supply tank, has sufficient electrical resistance to prevent any
significant electrostatic charging of the material in the tank or the tank
itself. However, when the coating material is highly electrically
conductive, as is the case with water-based paints, the resistance of the
coating column in the hose is very low, with the result that a high
voltage charging electrode located in the vicinity of the spray device
nozzle, electrostatically charges not only the emitted particles, but the
coating material in the hose and supply tank, as well as the supply tank
itself. Under such circumstances, operating personnel inadvertently coming
into contact with an exposed supply tank risk serious electrical shock.
To avoid the aforementioned electrical shock problem associated with a
system in which highly conductive water-based coating material in the hose
and tank becomes charged as an incident to spraying, it has been proposed
in the past to enclose the supply tank in an electrically insulative
housing. However, even when this is done electrical shock hazards still
exist, particularly when the insulative enclosure is opened to expose the
tank and gain access thereto for replenishing the coating supply. Such
hazards exist, notwithstanding that the electrostatic charging source may
have been de-energized prior to accessing the coating tank, due to the
fact that the electrostatic charge on the coating material in the tank and
hose does not immediately dissipate, or discharge, following
de-energization of the electrostatic charging source, but rather requires
a finite time, dependent on existing current leakage paths, to discharge
to a nonhazardous level. Thus, a residual charge of a hazardous level,
albeit gradually decreasing, remains on the tank for a finite time, often
as long as 40 seconds, after the electrostatic charging source is
de-energized. Hence, operating personnel contacting the coating container
after de-energization of the charging source, but prior to reduction of
the residual electrostatic charge to a safe level, are exposed to shock
hazards.
Accordingly, it has been an objective of this invention to provide an
apparatus and method for electrostatically spraying highly conductive
water-based coating material which minimizes shock hazards heretofore
existing when a normally enclosed tank of conductive coating material is
accessed following de-energization of the charging source, but prior to
discharge of the residual electrostatic charge on the coating material
through the normal current leakage process. This objective has been
accomplished in accordance with certain principles of this invention by
providing, in combination with an insulative enclosure surrounding a tank
containing electrostatically charged water-based coating material, means
for automatically electrically grounding the system as a concommitant to
removal of the enclosure cover to access the tank. As a consequence,
residual charge in the system is automatically and immediately dissipated
to ground as an incident to accessing the paint tank, eliminating shock
hazards which heretofore existed when a tank was accessed shortly after
de-energizing the charging source, but prior to dissipation of the stored
residual charge.
In a preferred form of the invention the automatic electrostatic grounding
arrangement includes a high voltage grounding switch located in a
compartment of the insulative enclosure separate and apart from the
compartment in which the paint tank is housed. Upon removal of the cover
of the tank compartment, the high voltage switch, which is connected
between the tank and ground, automatically discharges the system to ground
potential. Placement of the high voltage switch in a separate compartment,
vis-a-vis in the same compartment as the tank, provides a number of
advantages. For example, by reason of the high voltage switch being absent
from the tank compartment, the switch is protected against possible damage
due to paint spillage, jarring or the like, as an incident to tank
replenishment and/or removal. In addition, the high voltage switch
compartment, by reason of being separate from the tank compartment, can be
sealed against unauthorized tampering or the like without inconveniencing
personnel needing access to the tank compartment for replenishment
purposes, etc. Finally, the size of the tank compartment can be made
smaller since the requisite electrical standoff between the tank and
compartment walls can be provided by measuring directly from the tank to
the compartment walls, rather than from the compartment walls to the high
voltage electrical switch which, if located in the tank compartment, would
be mounted on the tank, thereby increasing the actual distance required
between tank and compartment walls to provide a predetermined electrical
standoff.
In accordance with another aspect of the invention, the hose which supplies
paint from the tank to the spray device passes through the high voltage
switch compartment whereat an electrode communicating with the bore of the
hose carrying the conductive water-based paint is alternatively
selectively connected to either (a) the high voltage source for charging
the paint as it flows through the hose or (b) ground potential via the
high voltage switch for discharging the system. By virtue of this
arrangement, the paint hose serves multiple purposes. Specifically, it
transports paint from the tank to the gun, as well as establishes a
conductive path for both charging the paint and grounding the system.
Thus, separate electrical conductors interconnecting the tank to the
charging source and high voltage grounding switch are unnecessary. Not
only does this reduce cost, but it makes tank removal for maintenance or
like purposes easier since once the tank compartment cover is removed and
the tank accessible, the only fitting or connecting to the tank requiring
disconnection is the fitting interconnecting the tank and paint hose. A
second hose, which supplies air to the tank for pressurizing the paint, is
typically connected to the tank cover. However, the tank can be removed
without disconnecting this hose by merely removing the cover from the tank
before removing the tank itself.
A further advantage of this particular aspect of the invention is that
separate openings between the tank and high voltage switch compartments
need not be provided for electrical conductors which might otherwise be
needed were separate conductors designed to charge the paint and discharge
the system provided. The only openings in the wall separating the
compartments which are needed are those necessary to accommodate the paint
and air hoses. Accordingly, only two seals need be provided in the wall
between the compartments, reducing initial equipment manufacturing cost as
well as operating problems occasioned by paint leakage from the tank
compartment into the high voltage switching compartment.
In the preferred form of the invention, the high voltage switch includes an
electrically grounded rod pivotally mounted within the switch compartment.
The rod, upon removal of the tank compartment cover, is moved into
electrically conductive contact with the paint hose electrode by a linkage
which interconnects the cover and rod, thereby grounding the system
through the paint column in the hose and dissipating the residual charge
on the coating material in the tank hose and gun.
Another aspect of this invention contemplates electrically grounding the
system means of the high voltage grounding switch under remote control
from the gun trigger, whenever the user of the gun releases the trigger
for an abnormal period of time exceeding the normal, momentary-type
release typically encountered in the course of spraying an article. A
typical example of such an abnormal, long-duration trigger release is when
the user must interrupt spraying for a protracted period to replenish the
tank. Since a user is more likely to inadvertently touch the gun nozzle
and receive a shock when he has finished spraying and, for example,
contemplates replenishing the tank, than he is when engaged in spraying an
article, by grounding the system automatically each time spraying is
concluded, shock hazards are minimized. Of course, momentary trigger
releases, since they do not result in discharging the system, do not
interfere with normal spraying, as might otherwise occur were the system
discharged every trigger release, due to the lag time necessary for the
charge to build up to the desired operating level following turn-off.
In accordance with a further aspect of the invention, also designed to
minimize shock hazards, an electrical switch in the energization circuit
of the high voltage power supply is provided which, upon removal of the
insulative cover from the enclosure surrounding the tank, disables the
high voltage charging supply. Thus, when the cover of the insulative
enclosure is removed, thereby exposing the tank, the supply of
electrostatic charging potential to the coating material is automatically
terminated.
In accordance with a still further aspect of the invention, also designed
to minimize safety hazards, a second electrical switch is connected in the
energization circuit of the high voltage power supply. The second switch
is designed to be placed in an open circuit condition, thereby
automatically de-energizing the high voltage supply, when the spray gun
trigger is released to stop the flow of coating material to the gun.
Should the operator, after concluding spraying, accidentally cause the
nozzle of the gun, which is typically charged due to the conductive nature
of the coating material, to come into contact with a grounded object, such
as in the course of placing the gun on a support, the likelihood of
electrical shock due to inadvertent grounding is minimized.
In accordance with another aspect of the invention, also designed to
minimize electrical shock hazards and extend the life of the hose used to
interconnect the gun and tank, the hose is constructed in a very unique
manner never before used in water-based coating spray systems.
Specifically, the hose is provided with a dielectric wall which has a zone
in contact with the water-based coating which is substantially chemically
inert and impermeable to the coating, as well as constructed to withstand
dielectric breakdown when subjected to the electrostatic charging
potential by the charged paint flowing through the hose. In addition, the
hose has an exterior surface free of electrically conductive material. The
impermeability and inertness of the dielectric wall to the water-based
paint prevents it from physically deteriorating and becoming water-logged
which, if permitted to occur, would eventually lead to arcing through the
wall and ultimately failure of the hose. The absence of an outer cladding
of conductive material minimizes the capacitance of the hose, and hence
the electrical energy stored therein in capacitive form, particularly when
the hose is physically spaced from a grounded object, such as a factory
floor. Reduction in stored energy reduces shock hazards should an operator
inadvertently contact the gun nozzle before the system has been
discharged, such as while spraying. Even when the conductively unclad hose
of this invention is in contact with a grounded surface, such as a factory
floor, its capacitance, and hence energy storing capability, is still
significantly less, e.g., 50%, than conductively clad hoses. Thus, shock
hazards are significantly reduced with the conductively unclad hose of
this invention even when the hose rests on a grounded factory floor or
like grounded surface.
These and other advantages and objectives of the invention will become more
readily apparent from a detailed description of the preferred embodiment
taken in conjunction with the drawings in which:
FIG. 1 is an elevational view, partly in cross-section, of the spray
device, coating container and interconnecting hose 1 showing the
electrical interlock and automatic system grounding features of this
invention;
FIG. 2 is a bottom view taken along line 2--2 of FIG. 1;
FIG. 3 is an elevational view of a modified form of grounding mechanism;
and
FIG. 4 is a schematic view of a portion of a second embodiment of the
invention which is operative to ground the system when either the cover of
the insulative tank enclosure is removed or the flow of coating material
to the spray device terminates for longer than a predetermined interval.
Electrostatic spray coating systems of the general type to which this
invention relates are designed to spray highly electrically conductive
water-based coating materials, i.e., having resistivities in the range of
100-1,000,000 ohm-centimeters. By way of illustration only, one such
water-based coating material frequently used is water-soluble bake enamel,
manufactured by Muller Industries, having a resistivity of 318
ohm-centimeters. Typically these systems include, as a principle component
thereof, an electrostatic spray device such as an electrostatic spray gun
10. The gun 10 has an electrically grounded handle 11 of electrically
conductive material designed to be manually grasped in use by the operator
and an electrically insulative barrel 12 which at its forward end
terminates in a nozzle 13. A spray 14 of finely divided, or atomized,
particles of highly conductive coating material flows from the gun nozzle
toward an object 15 being coated when a manually-operated actuator on the
handle, such as a trigger 18, is actuated by the operator. A source of
coating material 21 is connected to a coating inlet 16 of the gun 10 via a
flexible conduit, hose or supply line 24. The coating inlet 16
communicates with the nozzle 13 via fluid passages 19 and 20 in the barrel
13 and handle 11, respectively. Actuation of the gun trigger 18 opens a
normally closed flow valve 17 in the gun 10 via an interconnecting plunger
18a to permit the flow of conductive coating material to the nozzle 13
whereat it is atomized and emitted as the spray 14.
Electrostatic spray systems also include an electrical power pack, or
booster supply, for transforming commercially available low voltage AC
power, e.g., 60 Hertz - 115 volts, to high DC voltage, e.g., 50KV - 100KV,
and an electrode connected to the high voltage power supply and in contact
with the conductive coating material for electrostatically charging the
coating material such that as coating particles are emitted from the gun
nozzle their attraction to the article 15 being coated, which is typically
maintained at a potential different than that of the charging potential,
for example, at zero or ground potential, will be enhanced.
Depending upon whether or not the gun is of the "air" type, wherein
atomization of the coating material is effected by impact of an air stream
with the liquid coating material, a source of air may or may not be
connected to the gun via an air line for impinging air on the coating
stream in the region of the nozzle. If the spray gun is of the "airless"
type, wherein atomization of the coating particles in the region of the
nozzle is effected hydraulically, the air line may be omitted. The
electrostatic spray gun 10 which is illustrated is of the airless type.
However, it should be understood that the invention is equally applicable
to other types of electrostatic, manual and automatic, spray guns and
systems.
The fluid conduit or hose 24 interconnecting the coating inlet 16 of the
gun 10 and the source of coating material 21 preferably includes an inner
dielectric layer 24a and an outer dielectric layer 24b.
The inner layer or zone 24a is preferably chemically inert with respect to
the water-based coating being transported within the central bore 24c
defined thereby such that the surface of layer 24a will not be
significantly corroded, dissolved, eroded, or otherwise physically or
chemically deteriorated by chemical interaction with the coating being
conveyed through the bore. The inner layer 24a is also preferably
essentially impermeable with respect to the coating conveyed through bore
24c, effectively establishing a fluid-tight barrier between the
coating-transporting bore 24c and the remaining layer or zone 24b of the
conduit 24. Tetrafluroethylene has been found to be both chemically inert
and impermeable to water-based paints and therefore a good material from
which to construct layer 24a. When tetrafluroethylene is used, a wall
thickness of 40 mils is preferable.
The establishment of a barrier between the bore 24c and zone 24b which,
with respect to the coating being transported, is substantially
fluid-tight limits possible permeation of the dielectric zone 24b by the
coating should the latter zone be permeable, which is often the case where
the zone 24b is fabricated of flexible dielectric material since many
flexible dielectric materials are permeable to water-based coatings. Were
significant permeation of zone 24a permitted to occur, an electrically
conductive path through the zone 24b could be established, assuming the
latter is permeable, leading to undesirably high electrical current
leakage in a radial direction through the wall of conduit 24.
The layer or zone 24b functions, in combination with the inner zone 24a, to
establish a dielectric breakdown resistant barrier in the radial direction
which withstands dielectric breakdown when the interface between the inner
zone 24a and the coating in bore 24c is subjected to a high voltage as
necessarily occurs when the electrostatic charging voltage is applied to
the interface via the column of highly conductive water-based coating in
the bore 24c which is in electrical contact with the electrostatic
charging source. In a preferred form of the invention, the dielectric
breakdown resistant zone 24b is fabricated of extruded hollow low density
polyethylene tubing having a thickness of approximately 60 mils. Such a
construction is relatively flexible, in addition to having the desired
electrical properties of low radial current leakage and high dielectric
breakdown resistance. Specifically, polyethylene has a resistivity of
10.sup.15 - 10.sup.16 ohm-centimeters and a dielectric breakdown
resistance of approximately 700 volts per mil and, like zone 24a, resists
substantial radial electrical current leakage flow and dielectric
breakdown when subjected to voltages thereacross on the order of the
charging potential. Other dielectric materials could be utilized for layer
24b depending upon the degree to which it is desired that the zone
material be chemically inert and impermeable to the water-based coating.
For example, polypropylene and vinyl plastics may be used.
It has been found desirable to fabricate the zones 24a and 24b of material
which provides a combined, or average, dielectric strength of
approximately 800 volts per mil, although average, or combined, dielectric
strengths ranging between 250 volts per mil and 1,000 volts per mil are
satisfactory for specific applications. If composite dielectric strengths
of lesser values are used, the thickness of the conduit wall measured in
the radial direction may become undesirably large, increasing the bulk and
stiffness of the coating conduit.
If desired, the hose layer 24b may be provided with a tough outer layer or
skin of polyurethane (not shown) for abrasion-resistance purposes. A
polyurethane skin thickness of 25 mils has been found to afford
satisfactory resistance to abrasion.
The outer surface of the hose 24, whether it be layer 24b or an
abrasion-resistant polyurethane skin (not shown), is free of electrically
conductive material, i.e., does not contain a layer or cladding of
conductive material. It has been discovered, when conductive cladding is
omitted in construction of a hose used to spray highly conductive
water-based coatings, that the capacitance of the hose is significantly
reduced, particularly when the hose is spaced from a ground surface such
as a factory floor, in comparison to the capacitance of a conductively
clad hose. By reducing the capacitance of the hose, its ability to store
electrical energy in capacitive form is reduced, in turn reducing shock
hazards should an operator inadvertently contact the nozzle of the spray
device when the system is in charged state, such as when the operator is
spraying with the power supply energized. Even when the conductively
unclad hose of this invention is in contact with a grounded surface, such
as a factory floor, its capacitance, and hence energy-storing capability,
is still significantly less, e.g., 50%, than a conductively clad hose.
Thus, shock hazards are reduced with the conductively unclad hose of this
invention even when the hose rests on a grounded factory floor or like
grounded surface.
The coating material source 21 includes an inner metallic container,
preferably aluminum, of conventional design capable of holding anywhere
from one gallon to 50 gallons of highly electrically conductive coating
material 29. The coating material container or tank 28 is provided with a
removable aluminum cover 30 which in use is held in sealed engagement with
the coating-containing tank via circumferentially spaced clamps 31. An air
conduit 32, preferably of electrically insulative material, is connected
between the tank cover 30 and a pressurized air supply 34 to subject the
interior 35 of the tank 28 above the level of the coating 29 to
pressurized air for maintaining the coating material under pressure. With
the coating material 29 in the tank 28 pressurized, the coating is
pressure fed to the gun 10 via the hose 24 and a suitable fitting 28a in
the container wall. If desired, an air-operated agitating mechanism (not
shown) having an impeller immersed in the coating material 29 can be
provided for the purpose of insuring that the coating material in
container 28 is maintained in a homogenous state.
As noted, the coating material 29, with respect to which this invention
possesses a particularly high degree of utility, typically has a
resistivity in the range of 100-1,000,000 ohm-centimeters. While a
specific resistivity range has been used to define high conductivity
water-based coating, it is understood that such a resistivity value is
arbitrary and relative, and employed only for the purpose of illustration.
Accordingly, a coating material having a resistivity above 1,000,000
ohm-centimeters could conceivably be considered as highly conductive
notwithstanding that it falls near, although without, the specific
numerical value given.
Connected in the hose 24 is an electrically conductive fitting 36 which
communicates with the hose bore 24c and hence with the coating material
pressure fed from the container 28 to the spray gun 10. Electrically
connected to the fitting 36 is an electrically conductive plate 38. The
plate 38 is connected via a current limiting resistor 40 to an
electrically insulated high voltage line 42 output from a source of high
voltage DC electrostatic charging potential 44. Conductive coating
material pressure fed from the container 38 to the gun 10 via the hose 24
is electrostatically charged to the desired high voltage potential, e.g.,
50KV - 100KV, as it passes through the electrically conductive fitting 36.
Since the coating material is highly electrically conductive, the column
of coating material within the hose bore 24c and gun passages 19 and 20 is
also at the electrostatic charging potential output from the voltage
supply 44 on line 42. Thus, the atomized coating material 14 emitted from
the gun nozzle 13 is electrostatically charged at the potential of line
42, and the charged coating particles will be attracted to the grounded
article 15 being coated. In addition, the coating material 29 within the
metallic container 28, as well as the metallic container itself, are at
the potential of high voltage line 42 due to the conductive nature of the
coating material in the container and in the section of hose between the
electrostatic charging fitting 36 and the tank outlet fitting 28a.
An electrically insulative enclosure 45, including a removable electrically
insulative cover 46 and an electrically insulative base 48, is provided to
completely enclose, and hence electrically insulate from the surrounding
environment, the container 28. Specifically, when the cover 46 is located
with its lower rim 46a seated on a cooperating peripheral ledge or lip 48a
of the base 48, a chamber 50 is provided within which the metallic
container 28 is located in electrical isolation from the environment.
Preferably, the insulative base 48 is configured to provide a well 48b
within which the lower portion of the container 28 nests. Obviously, when
the enclosure cover 46 is removed from seating engagement with the base 48
to expose the container 28, access to the container is provided such that
the cover 30 can be removed and the container replenished with coating
material. The enclosure 45, and particularly cover 46, are sized such that
the point whereat the tank 28 comes closest to the enclosure is spaced
from the enclosure by a distance sufficient to provide an "electrical
standoff" between the tank and the enclosure.
The enclosure 45, particularly the region enclosed by the base 48 located
below the well 48b, defines a second chamber 54. Located within the
chamber 54 is the electrically conductive plate 38 and fitting 36.
Preferably fitting 36 and plate 38 are spaced from the floor or the like
support 56 on which the base 48 sits by a distance X sufficient to provide
an "electrical standoff" between the floor 56 which is typically at ground
potential and the conductive elements 36 and 38. This prevents arcing
between ground and the conductive elements 36 and 38 when the latter are
electrostatically charged from the high voltage power supply 44 via
electrically insulative cable 42 which passes through a suitably located
aperture 49 in the base.
Also located within the chamber 54 is a system grounding assembly 58 which,
in a manner to be described, functions as a high voltage switch to
electrically ground the system when the enclosure cover 46 is removed from
the base 48, i.e., as a concomitant to cover removal. The system ground
assembly or high voltage switch 58, in a preferred form, includes an
electrical conductor 60, preferably in the form of an elongated conductive
rod, which is connected to ground potential via an electrical line 61. The
electrically conductive rod 60 is mounted for pivotal movement by a
bracket 62 secured to the inner wall 63 of the base 48. One end 60a of the
pivotally mounted conductive rod 60 extends vertically upwardly through an
aperture or hole 65 in the lip 48a of the base 48 at a point which
underlies the cover rim 46a, while the other end 60b of the conductive rod
60 extends vertically downwardly. The rod 60 is biased by suitable spring
means (not shown) such that the rod tends to pivot in a clockwise
direction about bracket 62 as viewed in FIG. 1.
When cover 46 is in its closed position with respect to the base 48, such
that cover rim 46a seats on base lip 48a, the rod end 60a is urged
downwardly by the cover rim to the solid line position shown in FIG. 1.
With rod end 60a in the solid line position due to depression thereof via
the cover rim 46a seating on base lip 48a, the rod 60 is located at its
counterclockwise limit of travel (shown in solid lines) in which the rod
end 60b is spaced from the conductive plate 38. Thus, with the cover 46 in
place on the base 48 and the conductive rod 60 in its solid line position
shown in FIG. 1, the electrically grounded conductive rod end 60b is
displaced from the electrically conductive plate 38, with the result that
the system is not grounded.
When the cover 46 is removed and its lower rim 46a no longer seats on base
lip 48a, the rod end 60a is free to move upwardly through aperture 65 to
the dotted line position shown in FIG. 1 due to the spring bias action,
with the result that grounded rod end 60b makes electrical contact with
the electrically conductive plate 38. Since the electrically conductive
plate 38 also makes electrical contact with the conductive coating
material in the tube bore 24c via the conductive fitting 36, and in turn
with the coating material 29 in the container 28, the system is
electrically grounded when cover 46 is removed.
While in the preferred embodiment the system is grounded when the cover 46
is removed by electrically grounding the conductive plate 38, it will be
apparent to those skilled in the art that electrical grounding of the
system could be accomplished in other suitable manners. For example, the
grounded conductive rod 60 could make contact directly with the metallic
container 28 by relocating the pivot 62 and rod element 60b as shown in
FIG. 3, such that element 60b moves through an aperture 66' in the well
48b underlying container 28' in response to removal of the cover 46' which
permits rod end 60a' to rise through aperture 65' in lip 48a' underlying
cover rim 46a'. A still further variant comprehends providing a
microswitch in the base 48 which, upon removal of the cover 46, is
actuated to energize a relay, solid state switch or the like. The relay,
in turn, when energized, would complete an electrical circuit between the
metallic container 28 or conductive plate 38 to a source of electrical
ground potential.
By virtue of providing automatic system grounding, safety hazards, such as
electrical shock, are minimized when the cover 46 is removed by operating
personnel to gain access to the container 28 for refilling it with coating
material or the like. As previously noted, since the coating material is
highly conductive, the contents of the metallic container, as well as the
metallic container itself, are electrostatically charged as an incident to
electrostatically charging the coating material during spraying, whether
the electrostatic charging electrode is in | | |
