Flexible voltage cable for electrostatic spray gun

An electrostatic spray system including a spray device having an electrode for electrostatically charging coating particles emitted from the spray device toward an article to be coated, a high voltage electrostatic supply, and a high voltage insulated electrical cable interconnecting the electrostatic supply and the spray device electrode. The high voltage cable includes a fibrous resistive core, preferably fabricated from silicon carbide fibers, a fiber-restraining layer of insulative thread tightly wrapped spirally around the entirety of the core to prevent fiber ends from projecting outwardly from the core, an outer dielectric sheath, and an intermediate sheath sandwiched between the spirally wound inner fiber-restraining layer and the outer dielectric sheath and having a resistivity lying between that of the inner resistive fiber core and the outer dielectric sheath. The intermediate sheath provides uniform voltage stress distribution in the region surrounding the resistive core, thereby avoiding internal corona sites which cause degradation of the outer dielectric sheath, which might otherwise occur due to the very small diameter of the resistive core coupled with the very high operating voltages used in electrostatic spray coating operations. The intermediate sheath also avoids high voltage stresses at the ends of stray resistive fibers projecting through the spirally wound layer, which might otherwise occur due to the extremely small diameter of the projecting fibers.

This invention relates to electrostatic spray coating systems, and more 
particularly to a high voltage cable particularly adapted for use in such 
systems. 
This application discloses and claims subject matter related to that of 
pending application Ser. No. 602,974, filed Apr. 23, 1984, in the name of 
Donald R. Hastings and John Sharpless, entitled "Electrostatic Spray 
Coating System", now U.S. Pat. No. 4,576,827, assigned to the assignee of 
the present application. 
The above-referenced Hastings et al application discloses, among other 
things, a high voltage insulated electrical cable for use in 
interconnecting the particle high charging electrode of a spray coating 
device with a high voltage electrostatic supply. In a preferred form, the 
Hastings et al cable includes a resistive core of continuous silicon 
carbide fibers for conducting current longitudinally along the length 
thereof, a carbon-loaded polypropylene sheath extruded around the silicon 
carbide fiber core, and a dielectric sheath of polyethylene extruded 
around the carbon-loaded polypropylene. The resistivity of the 
carbon-loaded polypropylene sheath lies between the resitivities of the 
resistive silicon carbide fiber core and the outer dielectric sheath. To 
facilitate pulling the resistive silicon carbide fiber core through the 
extruder, three strands of 1100 denier Dacron brand polyester is twisted 
with the silicon carbide fibers around the core such that there is a full 
twist every 0.5 inches of core length. 
The function of the carbon-filled polypropylene intermediate sheath in the 
Hastings et al cable is to avoid large voltage gradients at the location 
of a broken silicon carbide filament should a silicon carbide filament 
break somewhere along the length of the cable. At the location of a broken 
silicon carbide filament, the broken ends of the filament may project 
radially outwardly from the resistive core between the twisted Dacron 
strands. In view of the extremely small diameter of the silicon carbide 
filament, which may be on the order of 0.0005 inches, or 15 microns, the 
projecting ends of the broken silicon carbide filament create very high 
voltage gradients. By embedding the outwardly projecting ends of the 
broken silicon carbide filament in the relatively highly resistive 
intermediate sheath, the high voltage gradients that would otherwise tend 
to occur are markedly reduced. This, in turn, reduces the tendency of the 
outer dielectric sheath used to insulate the silicon carbide core for high 
voltage operation to prematurely fail at the site of the broken silicon 
carbide filament ends. The relatively high resistive intermediate sheath 
surrounding the silicon carbide fiber core of the Hastings et al 
application also serves to reduce voltage stresses in the outer dielectric 
sheath occasioned by the very small diameter of the core itself, which in 
a preferred form is 0.035 inches. 
It has been discovered that while the intermediate sheath of carbon-loaded 
polymer disclosed in the Hastings et al application does tend to provide 
uniform voltage stress distribution around the extremely small diameter 
silicon carbide fiber core, thereby reducing internal corona sites which 
can degrade the outer dielectric sheath, there is still some tendency for 
stray silicon carbide fiber ends occasioned by broken filaments to create 
internal corona discharges at localized sites within the cable, eventually 
creating pin holes in the outer dielectric sheath, which ultimately lead 
to cable failures. 
Accordingly, it has been an objective of this invention to provide a high 
voltage insulated electric cable having a fibrous resistive core which is 
substantially free of outwardly projecting fiber ends which create 
internal corona discharge sites and ultimately degrade the outer 
dielectric sheath, resulting in cable failure. This objective has been 
accomplished in accordance with certain principles of the invention by 
encasing the entire inner fibrous silicon carbide core with a tightly 
wound layer of insulative thread, for example, Dacron polyester, in which 
the adjacent convolutions of the spirally wound thread are in close 
physical contact with each other. By making the pitch of the spirally 
wound Dacron thread equal to the thread diameter, the entire silicon 
carbide fiber core is encased in thread. This mats down the stray ends of 
any broken silicon carbide fibers, preventing them from projecting 
outwardly from the resistive core which, as noted, if permitted to occur 
can create local corona sites which lead to degradation of the outer 
dielectric sheath and, in turn, cable failure. 
With the cable of the Hastings et al application, another potential source 
of internal corona, tending to degrade the outer dielectric sheath, and 
eventually producing cable failure, was found to exist. More particularly, 
it has been discovered that in the region of the interface between the 
carbon-loaded polypropylene intermediate layer and the outer dielectric 
polyethylene layer, voids or spaces exist. Notwithstanding that these 
voids or spaces are extremely small, they nevertheless tend to create 
voltage stress concentrations whereat corona can occur, degrading the 
outer dielectric sheath, leading to failure of the cable. 
It has been a further objective of this invention to provide a high voltage 
cable having a carbon-loaded polymer layer between the inner resistive 
silicon carbide fiber core and the outer dielectric sheath, which is free 
of corona-inducing voids or spaces at the interface between the 
carbon-loaded sheath and the outer dielectric layer. This objective has 
been accomplished in accordance with certain further principles of the 
invention by bonding the carbon-loaded intermediate sheath and the outer 
dielectric sheath at their interface such that the outer surface of the 
intermediate sheath and the inner surface of the outer dielectric sheath 
are in intimate physical contact throughout, thereby providing a void-free 
interface between them. In a preferred form of the invention the desired 
void-free interface condition is achieved by using the same polymer for 
both interfacing layers allowing the two layers to blend together at their 
interface. This interface blending eliminates corona-inducing voids.

The electrostatic spray coating system depicted in FIG. 1, which 
incorporates the improved insulated high voltage cable of this invention, 
includes an electrostatic spray gun 10 having a charging electrode 12 
extending forwardly from the gun nozzle whereat the coating material 
particles are emitted in a spray pattern 14 toward an article to be coated 
16. High voltage electrostatic potential is supplied to the electrode 12 
from a remotely located high voltage electrostatic supply 18 via the 
electrically insulated cable 20 of this invention and a gun resistor 22 
located between the forward end of the cable 20a and the electrode. The 
gun resistor 22 typically has a value of approximately 75 megohms, and the 
cable 20 has a resistance of approximately 200 megohms. In addition to the 
gun resistor 22 located in the barrel 24 of the spray gun 10, there may be 
an auxiliary discrete gun resistor of lesser value, for example, 12 
megohms, which is not shown in FIG. 1. The auxiliary resistor, if 
provided, is electrically connected between the gun resistor 22 and the 
electrode 12 immediately rearwardly of the electrode. Plural discrete gun 
resistors in electrostatic spray devices are known in the art. An 
illustration of such is disclosed in Kennon U.S. Pat. No. 4,182,490, 
granted Jan. 8, 1980, entitled "Electrostatic Spray Gun", assigned to the 
assignee of the present application. Also included in the spray coating 
system depicted in FIG. 1 is a source of pressurized coating material 26, 
such as powder or liquid, which is transported to the spray device 10 via 
a hose or conduit 28. To facilitate convenient maneuvering of the spray 
device 10 relative to an article 16 to be coated, the hose 28 is 
fabricated of flexible material. For the same reason, the cable 20 is 
flexible, particularly in the region between the high voltage supply 18 
and the butt 30 of the spray device handle 32 whereat the cable enters the 
spray device 10. 
In operation, when a trigger 34 of the spray device 10 is actuated, high 
voltage is applied to the electrode 12 from the supply 18 via the cable 20 
and gun resistor 22 for electrostatically charging coating particles 
emitted from the nozzle 13 supplied to the gun from the pressurized supply 
26 via hose 28. The electrostatically charged particles in the spray 
pattern 14 are directed toward the article being coated 16, which is 
maintained at a potential, such as ground 36, which is substantially 
different than that of the electrode 12. By reason of the grounded nature 
of the article 16 and the charge on the coating particles in pattern 14, 
the charged coating particles are electrostatically attracted toward the 
article being coated 16 whereat they are deposited with enhanced coating 
transfer efficiency. 
The improved electrically insulated cable 20, which is shown in detail in 
FIGS. 2 and 3, includes a relatively flexible elongated core 40 fabricated 
of a plurality of substantially continuous silicon carbide fibers which 
conduct electrical current substantially longitudinally along the length 
of the core. In a preferred form, the core 40 comprises four 500-filament 
strands, with each filament being a substantially continuous silicon 
carbide fiber having a diameter of approximately 0.0005 inches. The 
resistive silicon carbide fiber core 40 in the preferred form has a 
diameter less than 0.05" preferably between 0.03 and 0.04 inches and most 
preferably 0.035 inches and can be constructed in accordance with the 
teachings of the Hastings et al application referenced earlier, the entire 
disclosure which is specifically incorporated herein by reference. The 
resistivity of this core 40 should be about 10.sup.3 ohm-cm. 
Completely encasing the resistive silicon carbide fiber core 40 is a layer 
of insulative thread 42 which is spirally wound, or served, around the 
outer surface of the core 40. The thread, which is preferably Dacron brand 
polyester, is tightly wound around the exterior surface of the resistive 
fiber core 40, with the adjacent convolutions of the spirally wound thread 
being in physical contact with each other, thereby snugly embracing in 
radially inwardly compressive fashion the entire exterior surface of the 
silicon carbide fiber core 40. In a preferred form the Dacron brand 
polyester thread has a denier of 1100 providing a pitch for the spirally 
wound thread of 3/8 inches. The outside diameter of sheath 42 in the 
preferred embodiment is 0.060 inches. 
The spirally threaded layer 42 snugly encasing the entire resistive fiber 
core 40 serves several important functions. More particularly, the layer 
42 restricts, restrains, inhibits or prevents the confronting end of a 
broken filament of the silicon carbide fiber core 40 from projecting 
radially outwardly from the generally cylindrically shaped core 40, which 
if permitted to occur would create very high voltage gradients or stresses 
due to the extremely small diameter of the broken silicon carbide 
filament. In addition, the threaded layer or sheath 42 protects the 
silicon carbide fiber core 40 from damage when it is pulled through an 
extruding die in the course of extruding the next outermost sheath 44 to 
be described. The threaded layer 42 provides the further function of 
preventing broken silicon carbide filaments of the core 40 from 
accumulating upstream of the extruding die orifice, that is "balling up", 
when the core 40 is pulled through the extruding die in the process of 
extruding the sheath 44. 
While the sheath 42 is preferably fabricated of Dacron brand polyester 
thread, other functionally equivalent layers can be used providing, among 
other things, that they matt down broken filament ends of the silicon 
carbide fiber core 40, preventing the broken ends from projecting radially 
outwardly from the core. It is also desirable that the layer 42 be 
flexible, relatively non-absorbent with respect to moisture, and capable 
of withstanding the extruding temperature of the sheath 44 which surrounds 
it. Illustrative of suitable substitutes for Dacron brand polyester for 
the layer 42 are polyamides such as nylons, polyurethane, Mylar and other 
polyesters such as PET. Instead of thread, the sheath 42 can be fabricated 
of spirally wound flat ribbon. 
The sheath 44 which is extruded directly over the spirally wound threaded 
layer or sheath 42 preferably is fabricated of carbon-loaded polyethylene. 
The carbon-loaded polyethylene sheath 40 tightly embraces the outer 
surface of the threaded layer or sheath 42, and in the preferred 
embodiment has an outside diameter of 0.11 inches. The resistivity of the 
layer or sheath 44 is selected to lie between that of the resistivity of 
the core 40 and the resistivity of the dielectric sheath 46 described 
hereafter. Generally it will have a resistivity of 10.sup.6 -10.sup.8 
ohm-cm and preferably 10.sup.7 ohm-cm. 
With the resistivity and diameter of the sheath 44 selected as described 
above, the relatively high resistive sheath functions to provide uniform 
voltage stress distribution in the region immediately surrounding the 
thread-covered core 40, thereby avoiding internal corona sites which cause 
degradation of the dielectric sheath 46 which might otherwise occur due to 
the very small diameter of the silicon carbide core 40 and the very high 
operating voltages at which the core is energized during operation. The 
sheath 44 also functions to eliminate high voltage stress points produced 
by any stray broken fiber ends which may project from the core 40 through 
the threaded layer 42. As noted earlier, the extremely small diameter of 
the silicon carbide filaments used in the core 40, if permitted to project 
outwardly from the core, should a broken filament occur, can create very 
high voltage gradients. By embedding in the relatively highly resistive 
extruded sheath 44 the stray ends of a broken silicon carbide filament, 
which ends may stray and project through the threaded layer 42 from the 
core 40, the high voltage gradients which would otherwise tend to occur 
are markedly reduced. This in turn reduces the tendency of the dielectric 
sheath 46, used to insulate the high voltage resistive core 40, to 
prematurely fail at the site of the ends of the protruding broken silicon 
carbide filament. 
Although the voltage stress reducing sheath 44 is preferably fabricated of 
carbon-loaded polyethylene, such as high molecular weight, low density 
Alathon brand polyethylene, other functionally equivalent materials may be 
used which are suitably doped or loaded or otherwise formulated or 
fabricated to provide the desired resistivity and which exhibit the 
requisite flexibility, non-absorbency, and thermal stability at the 
extrusion temperature of the dielectric sheath 46. 
The dielectric sheath 46 principally performs the function of electrically 
insulating the resistive silicon carbide fiber core 40 at the high 
voltages encountered during operation, such as 50 k.v. or more. The sheath 
46 is extruded over the sheath 44 to a thickness which, depending upon the 
dielectric properties of the material, is sufficient to insulate the 
resistive core 40 for the high voltage encountered in operation. In the 
preferred embodiment of the invention designed to operate at voltages of 
150 kv, the dielectric sheath 46 has an outer diameter of 0.205 inches and 
a resistivity in the range of 10.sup.14 -10.sup.16 ohm-cm. To minimize 
voids or spaces, at the interface 48 between the outer surface of the 
sheath 44 and thereby the inner surface of the sheath 46, and the 
resulting localized corona sites which degrade the sheath 46, ultimately 
causing cable failure, it is desirable that the mating surfaces of the 
sheaths 44 and 46 be in intimate physical contact throughout the entirety 
of the interface therebetween. In a preferred form of the invention, this 
void-free condition is achieved by bonding the mating surfaces of the 
sheaths 44 and 46 throughout the entirety of their interface 48. This 
bonding is achieved, in the preferred embodiment, by selecting a material 
for the dielectric sheath 46 which will blend or chemically cross-link 
with the material of the sheath 44 at the interface 48. The requisite 
cross-linking or blending occurs, in the preferred embodiment, by 
fabricating the sheaths 44 and 46 of the same polymer, for example, 
polyethylene. Other forms of bonding at the interface 48 between the 
sheaths 44 and 46 may be employed, such as, melting the mating surfaces 
into each other, ultrasonic welding, adhering, or wetting the mating 
surfaces to compatibilize the materials, etc. 
While the sheaths 44 and 46 of the preferred embodiment are both fabricated 
of polyethylene, with only the sheath 44 being carbon-loaded, blending of 
the mating surfaces of the sheaths 44 and 46 at the interface 48 thereof 
can be achieved using other functionally equivalent materials. For 
example, both sheaths 44 and 46 can be fabricated of polypropylene, or of 
different but yet compatible co-polymers such as ethylene propylene 
copolymer and ethylene propylene diene terpolymers. Sheaths 44 and 46 can 
be formed of two different materials which are reactive with each other to 
form a chemical bond or crosslink at this interface. Further, if sheaths 
44 and 46 are formed of incompatible materials, a compatibilizing layer 
can be incorporated between the two layers to avoid any interfacial void. 
Surrounding the dielectric sheath 46 is an electrically grounded conductive 
braid layer or sheath 50 having an outside diameter of approximately 0.233 
inches. Surrounding the conductive braid sheath 50 is a two-mill thick 
layer of Mylar brand polyester ribbon 52 wrapped to provide a 50% overlap, 
producing an outside diameter of 0.241 inches. The Mylar layer 52 is 
provided with a layer of polyurethane 54 having a thickness of 
approximately 0.036 inches, providing an outside diameter of 0.313 inches. 
The electrically grounded conductive braid 50 is provided for safety 
reasons in the event the dielectric sheath 46 should fail to electrically 
insulate the high voltage core 40. The polyurethane outer layer 54 
provides a tough, abrasion-resistant protective cover for the cable.