Electrostatic application of insulative refractory dust or powder to casting belts of continuous casting machines--methods and apparatus

Electrostatic application of a dusting of dry, self-adhering, thermally and electrically insulative powder particles over a work face of an endless, thin, flexible, water-cooled, metallic casting belt advantageous for use in a continuous metal-casting machine. A dry dusting of protective powdery refractory substance is applied to the belt after being rendered airborne and electrostatically charged by various embodiments of suitable electrostatic apparatus. The casting belt to be dusted is electrically grounded for attracting the charged powder particles for adhering them to the casting belt. The dusting so deposited is remarkably uniform over a substantial area, a phenomenon explainable by mutual electrostatic repulsion of the dry powder particles being deposited. Continuously re-applied dusting over the work face of an endless casting belt during a cast provides an immediately useful repair of lost dusting powder. The dusting may be removed at will by means of an air knife. Certain powders that are effectively soft afford high thermal insulativity for the metallic casting belt, and desirably they cause only minimal interference with further mechanical processing of a cast metallic product into which they might inadvertently become entrained.

FIELD OF THE INVENTION 
The present invention is in the field of continuous casting of molten metal 
in belt-type machines using one or more relatively wide and thin-gauge, 
flexible, metallic casting belts for defining a moving mold cavity. More 
particularly, this invention relates to method and apparatus for 
electrostatic application of insulative refractory dust or powder, notably 
soft powders, to such relatively wide, thin-gauge, flexible metallic 
casting belts and to the resulting dusted casting belt itself. 
BACKGROUND OF THE INVENTION 
This invention is for the improvement of the operation of belt-type 
continuous metal-casting machines. The specification will proceed in terms 
of describing a twin-belt casting machine as disclosed in U.S. Pat. Nos. 
4,588,021 and 3,937,270. 
In a thin-gauge-belt-type casting machine, one or more moving, endless, 
thin, flexible, metallic, water-cooled casting belts successively enter 
and leave a moving mold cavity. During the casting of molten metal, a flat 
casting belt is very important. The problem of belt flatness vs. 
distortion or warping arises because of thermal heat expansion of the 
belts when the belts enter into contact with molten metal. The casting 
belt or belts must remain flat and free from distortion or warping, lest 
the freezing metal lose contact here and there and thereby interrupt the 
heat transfer locally, causing metallurgical problems. The warping problem 
is discussed in U.S. Pat. Nos. 3,937,270, 4,537,243 and 4,749,027, all 
assigned to the same assignee as the present invention. 
Insulative, non-wetting belt coverings have been, and continue to be, part 
of the strategy to eliminate this problem of belt warping or distortion. 
These include permanent precoverings or base coverings (hereinafter called 
"basings"). These are described in U.S. Pat. No. 4,588,021 of Bergeron et 
al. and the more or less temporary top deposits or top dressings or 
temporary insulative deposits or toppings or mold-release agents, which 
are applied on top of a basing. All prior-art top or temporary insulative 
deposits known to us wear and compact and flatten unevenly and thus soon 
require replenishment or replacement. Manual replenishment of the unevenly 
worn or flattened spots does not in practice result in re-establishing a 
top deposit that affords uniform heat transfer. Nor has it been feasible 
to strip and reapply the prior-art insulative toppings, which usually 
comprise a binder. 
Most of the prior-art top deposits were applied wet. Thus, residues of 
liquid resulting from such wet applications would sometimes flash into gas 
and cause porosity or other problems in the cast product. In the casting 
of copper bar or copper anodes, synthetic oils upon otherwise bare 
metallic casting belts have been customary, sometimes resulting in similar 
porosity problems. None of the prior art known to us can achieve the 
unique results disclosed herein. 
SUMMARY OF THE DISCLOSURE 
The problems of an easily applied and maintained top insulative deposits 
for water-cooled, thin-gauge flexible casting belts and for edge dams are 
solved or substantially overcome by the present invention, in which 
suitable, finely-powdered refractory substance is applied and re-applied 
by means of high-voltage electrical apparatus which imparts charge to the 
dry powder or dust particles in flight, such that they disperse from each 
other in a generally uniform distribution before being attracted to the 
casting belt and landing upon it. The dry particles adhere evenly to the 
casting belt in a self-leveling fashion over a wide area. Electrostatic 
re-application of more powder particles results in the beneficial, uniform 
self-healing of wear spots. Yet all the powder particles can be removed 
and replaced continually according to need.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1 is shown a belt-type of continuous casting machine, 
illustratively shown as a twin-belt caster 10. Molten metal is fed from a 
tundish 11 into a moving mold cavity M at the entry end (upstream) E. Cast 
metal product P issues from the downstream or discharge end D. (The plane 
of product P is also denominated spatially as the pass line.) Upper and 
lower casting belts 12 and 14 define between them a moving casting mold 
cavity M and are supported and driven by means of upper and lower carriage 
assemblies U and L respectively. Multiple, freely-rotatable back-up 
rollers 15 in both carriages U and L guide and support the casting belts 
12 and 14 as they move (arrows 17 and 19) along the moving mold cavity M. 
For clarity of Illustration, only a few of these back-up rollers are 
shown. 
The upper carriage U, as shown in this embodiment of the present invention, 
includes two main roll-shaped pulley drums 16 (nip pulley drum) and 18 
(tension pulley drum) around which the upper casting belt 12 is revolved 
as indicated by the single-line arrow 17. Similarly, the lower casting 
belt revolves as shown by arrow 19 around a lower nip pulley drum 20 and a 
tension pulley drum 22. Two laterally spaced multiple-block, revolving 
edge dams 28 (only one is seen) travel typically around rollers 30 to 
enter the moving casting mold cavity M. Coolant water is applied to the 
inside surfaces of the casting belts 12 and 14, and this coolant travels 
longitudinally along the inside surfaces of the casting belts 12 and 14, 
as is known in the art. 
The reference numbers henceforth usually apply identically to the 
components of both upper and lower carriages U and L. The description will 
generally be in terms of the equipment on the upper carriage U. with the 
understanding that similar equipment will normally be at an equivalent 
place in the lower carriage L. As to the apparatus that is attached to the 
lower carriage, supporting structures will differ from those shown for the 
upper carriage, partly because the lower belt 14 sags when slack, and it 
is necessary to keep a slack belt clear of the lower dusting equipment 32 
when withdrawing the slackened lower belt to replace it periodically. 
Electrostatic corona-discharge-type paint-powder spray guns 50 (FIG. 2) and 
150 (FIGS. 5, 7 and 8) serve well in the first two embodiments of the 
present invention. Six embodiments of the invention will be distinguished 
later herein. Voltage that is direct current, or at least unidirectional 
in polarity, is applied by the spray gun 50 or 150. The gun we use is 
adjustable to about 100 kilovolts, charging the air and the entrained 
powder or dust on its way out of the respective gun 50 or 150 by means of 
a charged electrode, namely, a small-diameter central "corona-discharge" 
wire 51 (FIGS. 2, 7 and 8), typically 0.55 mm (0.021 of an inch) in 
diameter, protruding forward at an exit tip of the respective gun. The 
negative high voltage at the corona-discharge wire 51, which voltage we 
have used successfully, is produced electrically in a multiplying circuit 
located right in the gun from a low-voltage, 17 kilohertz 
alternating-current electric supply line 42 which comes out of the rear of 
a remotely-located operating console 300 (FIG. 9). A corona discharge is 
thereby produced at the exit of the gun where the airborne powder 
particles emerge around the corona-discharge wire 51 which is centrally 
placed in the exit of the gun 50 or 150. 
This corona discharge is a key to the charging of the powder particles (see 
article by Miller). The voltage required to produce corona discharge is 
related to the sharpness or rather the radius of the corona-emitting round 
wire or of the edges of an electrode of other configuration than a round 
wire. That is, the greater the sharpness of the sharpest parts, or the 
less the radius on the small-radius parts of the electrode, the less the 
voltage that is required to produce corona discharge. The work (that is, 
casting belt 12 or 14) to be dusted is grounded to Earth as indicated at 
48 (FIG. 2). else a powder-repelling charge accumulates on the work, and 
an operator may get a shock. 
A single fluidizing hopper 302 (FIG. 9) near a console 300 supplies powder 
or dust to all electrostatic equipment in an installation. A rigid 
horizontal suction tube 304 (FIG. 9) draws (following arrows 305) 
fluidized powder 323 from near the bottom of the hopper 302 and feeds It 
to an aspirator pump 310, which is here located on the side of the hopper 
302. The conveying air for the aspirator pump 310 comes from the console 
300 to the aspirator pump through an air-supply hose 312. Emerging from 
the aspirator pump 310 and carrying a feed of powder is a powder-feed hose 
314, which conveys powder-laden air 318 through a hose 44 (FIG. 2) to the 
respective gun 50 (FIG. 2) or to the single-tube gun 150 (FIGS. 5, 7 and 
8) or to tubular dispenser 172 (FIGS. 5A, 6, 6A, 6B, 6C, 6D, 7A and 8A). A 
suitable rate of flow of air through each gun was measured to be roughly 
70 cubic inches per second (70 liters per minute). 
In the multiple-gun direct-applying "high-rise" station 52 (FIG. 2) there 
are shown five electrostatic spray guns 50. For each gun there is a 
remotely-located operating console 300 and an aspirator pump 310. In other 
words, for supplying these five guns 50, there is one hopper 302, and 
associated with this one hopper there are five consoles 300, five 
aspirator pumps 310, five powder-feed hoses 314, five suction-reducing 
hoses 316 and a fluidizing hose 320 for keeping the mass of powder 323 
suitably fluidized. It is noted that the five respective powder-feed hoses 
314 to the guns are indicated in FIG. 2 by the reference numbers 44. 
For providing fine control, a supplementary hose 316 from a console 300 
adjustably reduces suction of the aspirator pump 310 by adjustably 
"breaking the vacuum" therein. A fluidizing line 320 from a console 300 
leads into the bottom of the hopper 302 for passing air or other suitable 
gas into plenum 301 and upwardly through a fluidizing porous sheet 322 
extending across the whole area of the bottom of the hopper 302. The 
thickness of porous sheet 322 is for example 6 millimeters (0.25 inch). A 
reference number 323 indicates the fluidized mass of powder shown dotted 
within the hopper 302. The pressure of the fluidizing air or gas (arrow 
321) flowing through the hose 320 and then passing up through the porous 
sheet 322 is a fraction of atmospheric pressure, for example, being in a 
range from about 4 to 5 pounds per square inch (0.27 to 0.34 bars). The 
air or gas 321 that fluidizes and the air or gas that conveys the powder 
or dust 318 must be quite dry and quite free from oil. 
The electric aspect of the guns is easily managed and easily rendered 
uniform among more than one gun in use at a time. The rate of powder flow 
(arrow 318, FIG. 9) from the fluidized hopper 302 has not been so easily 
managed. At the extremely low rates of powder flow required for 
electrostatic application to even a relatively wide, thin-gauge flexible 
metallic casting belt 12, control of the rate has been difficult and 
hardly repeatable, when employing fluidizing hoppers 302 as shown in FIG. 
9. 
Our further improvements to the above-described apparatus of FIG. 9 are 
shown in FIG. 10, where a fluidizing hopper is indicated by 402. A novel 
concept is to draw powder or dust from a "dust cloud" (instead of directly 
from a fluidized mass of powder 323, FIG. 9). This dust-cloud provides 
powder-flow dynamics which Improve control at our extremely low 
powder-flow rates. Fluidizing air or gas 421 from a fluidizing line 420 
passes through plenum 401 and then upwardly through a porous sheet 422 
extending across the whole bottom of the hopper 402 for producing a mass 
of fluidized powder 423. Some of the fluidizing air or gas 421 from the 
line 420, though it goes through plenum 401, does not pass through the 
porous sheet 422 but passes instead through a "fountain" consisting of a 
small-diameter nozzle exit 430 that is here shown as extending upward from 
the center of the porous sheet 422 so as to be buried in the mass of 
fluidized powder 423. The exit orifice 430 is here shown aimed vertically 
upward for creating a kind of fountain powder-cloud 432. An emerging jet 
of air or gas 437 from nozzle orifice 430 renders some of the mass of 
fluidized powder 423 in the hopper airborne in an elevated fountain-like 
cloud of dust 432 which is suspended turbulently above the fluidized mass 
423. The exit of the nozzle 430 is conveniently about 0.8 mm (0.030 of an 
inch) in internal diameter. The pressure of the air flow for creating the 
fountain jet 437 is the same as that for fluidizing the powder 423 through 
air passing up through porous sheet 422, that is, in the range from about 
4 to 5 pounds per square inch (0.27 to 0.34 bars). Hence, no separate air 
supply need be made for the nozzle 430. However, the fluidizing air or gas 
through sheet 422 and jet 327 ultimately tends to come out of the top of 
the hopper 402. Under some conditions, this air or gas entrains some 
powder and so must be exhausted and filtered. The connections and 
equipment to accomplish this are not shown. 
A relatively short, rigid suction tube 405 projecting down from the top of 
the hopper draws some powder-containing air (arrow 433) from this 
dust-cloud 432. This powder-containing air 433 goes to an aspirator pump 
410. The conveying air for the aspirator pump 410 comes from a console 400 
to the aspirator pump through an air-supply hose 412. Emerging from the 
aspirator pump 410 carrying an extremely low feed-rate (arrow 419) of 
powder is a powder-feed hose 414. This powder-feed hose is conducting air 
or gas, carrying an extremely diluted flow of powder 419 to the respective 
gun 50, as shown by the connections 44 in FIG. 2 or to the single-tube gun 
150 or to the tubular dispenser 172. For providing fine control, a 
suction-reducing hose line 416 from the console 400 can reduce suction of 
the aspirator pump 410 by adjustably breaking the vacuum therein. A single 
hopper 402 and a single dust-cloud 432 and a single orifice 430 and a 
single jet 437 serve to supply all of the guns in an installation. 
However, there are as many consoles 400, as many aspirator pumps 410 as 
there are guns 50 or 150 or tubular dispensers 172 in an installation. 
Each aspirator pump 410 has its accompanying hose lines 412, 414 and 416 
and its accompanying suction-intake short-tube 405. 
Casting belts that are ready for applying dustings according to the present 
invention may be either bare or else precoated notably with thermally 
sprayed refractory substances which we call "basings," according to U.S. 
Pat. Nos. 4,537,243, 4,487,790 or 4,487,157. These patents are assigned to 
the same assignee as the present invention. Such thermally applied basings 
underly the presently disclosed temporary insulative deposit of a dust 
cushion of dry insulative particles. However, limited success has been 
attained by using a top deposit according to the present invention and 
without any underlying basing, i.e. on a bare metallic casting belt. 
We have been successful in using six embodiments of the invention for 
electrostatic application of thermally-insulative refractory powder or 
dust onto relatively wide casting belts 12. We shall describe all six 
embodiments. 
The first embodiment of the invention involves a multiple-gun, 
direct-applying, "high rise" station 52 (FIG. 2). To cover a "casting 
width" of 52 inches (1320 mm), five electrostatic powder spray guns 50 
emit charged powder or dust within the confines of solid interior walls of 
a bottomless spray box 54 (FIG. 2). This box 54 is about 10 inches (250 
mm) wide and is as long as the "casting width" on a casting belt 12 to be 
dusted. This box 54 is mounted so that its length extends across the 
moving casting belt 12 to be dusted. Each gun 50 applies dust or powder to 
about 10 inches of casting-belt width. Thus, there are five such guns to 
cover a "casting width" of 50 inches. The total width of the casting belt 
12 is at least about eight inches wider than the "casting width." 
Plastic non-conducting mounting nipples 56 hold the guns onto the box 54. 
The box 54 is made of nonconductive material such as a suitable plastic, 
or at least the box 54 is internally lined with a suitable non-conductive 
material. We have successfully used relatively rigid sheets of commercial 
polyvinyl chloride plastic material for constructing the box 54. We have 
found that a box 54 made from such PVC plastic material does not "compete 
with" the casting belt 12 for attracting the charged powder or dust. The 
guns are mounted by the nipples 56 in a roof 58 of the box 54 and are 
pointed toward the moving casting belt 12 to be dusted. The discharge 
snouts 53 of the guns 50 are positioned about 10 inches (about 250 
millimeters) above the belt 12. 
A dispensing gun 50 or 150 which can be employed to advantage in the 
multi-gun, direct-applying station 52 or in a single-tube powdering or 
dusting station 152A or 152B (described later) is an electrostatic powder 
spray gun as described earlier herein, which has a sharp-ended 
corona-discharging electrode in the form of wire 51. A hose line 46 (FIGS. 
2, 9 and 10) extends to each gun 50 in the station 52 for supplying 
"rinsing air" to each gun to keep the electrode unclogged, as is known in 
the art. 
A bottomless but otherwise enclosed arcade or buffer region 60 extends 
along or around the perimeter of the bottomless spray box 54. The enclosed 
arcade 60 is subject to continuous suction through a large-diameter 
exhaust hose 80 which is connected into remote filtering and 
dust-collecting equipment (not shown). In such remote filtering equipment, 
we use dry, surface-treated filters that are self-cleaning by discharge 
into a hopper located below the filters. Frequent, programmed puffs of 
back air pressure dislodge the dust or powder so accumulated. 
The purpose of the enclosed corridor or arcade 60 coupled to the exhaust 
hose 80 is for preventing airborne particles from escaping into the 
atmosphere from clearance gaps 62 preferably about 0.12 of an inch (3 
millimeters) located between the moving (arrow 17 or 19) casting belt 12 
or 14 being dusted and the bottom edges of the walls of the bottomless 
spray box 54. An approximately 0.08 of an inch (2 millimeters) clearance 
gap 62 is about the minimum gap practicable as a predetermined clearance 
for protecting a dust-cushion distribution 70 from being undesirably 
scraped. Over about 0.32 of an inch (8 millimeters), the effectiveness of 
the gap is compromised. 
In FIG. 2 the casting belt 12 or 14 is illustratively shown moving in a 
right-to-left direction as indicated by the arrow 17 or 19. The casting 
belt emerges from beneath the multiple-gun direct-applying station 52 with 
an applied uniform distribution 70 of thermally-insulative refractory dust 
or powder electrostatically adhered thereto. The suction (arrows 61) of 
air drawn by the large-diameter hose 80 from the external corridor or 
arcade 60 is enough to draw air (arrows 64) from the environment reliably 
into the corridor or arcade, both at gaps 63 located below the bottom 
edges of the periphery of the corridor and also to draw air (arrows 65) 
through similar gaps 62 located below the perimeter of the inside 
bottomless spray box 54. A suction of no more than about three inches 
(about 75 mm) of water-column is sufficient. Too much suction undesirably 
disturbs the uniformly distributed powder 70 on the moving casting belt 12 
or 14. 
For narrower casting belts, the multiple-gun, direct-applying station 52 is 
arranged with less than five guns, such that there is one gun for each 
nine to ten inches of casting width. Powder is normally applied also to 
the working surfaces of the edge dams 28 (FIG. 1), for example for the 
continuous casting of copper bar. 
The second embodiment of the invention involves a single-tube, 
gun-injection powdering or dusting station, indicated generally at 152A 
and 152B (FIGS. 5. 7 and 8). Only a single electrostatic spray gun 150 per 
casting belt is normally used. Its snout 153 (FIGS. 7 and 8) is inserted 
into a nipple 156 of plastic non-conductive material, whence the airborne 
stream of charged powder or dust passes through nipple 156 which is 
plastic acting as an insulator. The powder arrives into a tubular 
dispenser 172 which is preferably ungrounded and electrically conductive, 
most conveniently metal. This tubular dispenser 172 has a row or series of 
suitably sized and spaced powder-dispensing holes, slots or other 
apertures, for example a row of 0.062-inch (1.58 mm) diameter holes 174 
(FIG. 6) spaced along the length of the tube at a pitch of about 0.5 inch 
(about 13 mm). This row of holes 174 extends along a length of the tube 
172 corresponding to the casting width on the belt 12 or 14. 
A curved deflector 176 (FIG. 6) made from a quadrant of a pipe is attached 
to the tubular dispenser 172, going the working length of the tube. The 
curved deflector 176 directs an air or gas flow of powder or dust 167 
toward the belt 12 or 14 to be dusted. We have found that direct 
impingement of the Jets 175 without the deflector 176 usually results in 
undesirable streaks on the belt, a streak for each hole 174. The holes 174 
are not aimed to shoot powder directly straight toward the casting belt 
12. Instead, the holes 174 are now directed generally parallel with a 
nearby portion of the inside surface of the curved deflector 176. As shown 
by an arrow 175 (FIG. 6), the holes 174 are directed to shoot the powder 
or dust generally tangentially along the inside surface 177 of one of the 
curved walls constituting curved deflector 176 wherein each stream 175 
spreads out laterally in a direction across the belt width. The powder so 
emitted leaves the curved deflector inside surface 177 tangentially. The 
curved deflector 176 stands off from the belt 12 or 14 by the gap 162 
which is about 1.4 inch (about 35 mm) in size. 
The third embodiment of the invention is the single tube, 
long-separate-electrode mode. No gun 50 or 150 is used; this is a 
significant variation from FIGS. 5, 7 and 8. A transversely oriented 
corona-discharge-producing electrode--for instance, one or more 
corona-discharge wires 179 (FIGS. 5A, 6A and 8A)--is placed near to curved 
deflector 176 and is spaced from the work face of the casting belt in the 
path of the powder particles (arrow 175) that come airborne out of tubular 
dispenser 172. The wire 179 may conveniently be made of 0.012-inch (0.3 
millimeter) diameter wire of austenitic stainless steel. The 
corona-discharge wire 179 is stretched the length of the curved deflector 
176 (FIGS. 5A, 6A and 8A) in such a way that the oncoming powder (arrow 
175) to be adhered to the casting belt passes close by it. The wire 179 
lies conveniently near the concavity 177 near its powder-guiding exit edge 
191, as shown in FIG. 6A and is spaced about 0.4 of an inch (10 
millimeters) away from edge 191. This long corona-discharge wire 179 is 
charged by a high-voltage power supply 450. Either polarity has been 
satisfactory. When the corona-discharge wire 179 is used without any gun, 
the hose line 314 or 414 goes directly to the tubular dispenser 172 which 
may in this embodiment be made of either conductive or nonconductive 
material, though it should not be grounded lest extra corona-discharge 
current unduly load the power supply 450. 
The air or gas pressure within distributing tube 172 should not be greater 
than about one inch (about 25 millimeters) of water column. Rinsing air 
hose 46 is not used in this single-tube, long-separate-electrode 
embodiment shown in FIG. 6A. The corona-discharge wire (or wires) 179 may 
be removed and one (or more) conductive grids or plates placed in its 
stead as another kind of electrode, but the wire 179 is our most preferred 
mode. According to electrostatic theory, a smaller-diameter wire electrode 
179 would enable lower voltages to be used. In any case, the electrode 
voltages used for electrostatic application of insulative refractory dust 
or powder to a casting belt are corona-discharge-producing voltages. A 
corona-discharge-producing power supply 450 has its high voltage terminal 
connected to corona wire 179, as indicated via a conductor 451, having a 
suitable insulation jacket 452. 
In FIG. 6B is shown the fourth embodiment of the invention, which is the 
single-tube-as-electrode mode. It is like the single-tube, 
long-separate-electrode embodiment described above and illustrated in 
FIGS. 6A and 8A, except that the corona-discharge wire 179 is omitted. 
Instead, an electrically conductive, ungrounded, insulated tubular 
dispenser 172 itself functions as the electrode. It is electrified as was 
the corona-discharge wire 179 in FIG. 6A. It is our theory that the burrs 
or sharp edges at the entrances and exits of holes 174 may act as 
desirable emitters of corona discharge and so should not be quite blunted. 
In all of the single-tube modes described herein, an external buffer region 
or arcade 60 or 160 is internally kept at a below-atmospheric pressure of 
about 3 inches (about 75 millimeters) of water column through a flow of 
air 165 by means of exhaust tube connections 178 (FIGS. 5, 6 and 7), 
regulating valves 182 and exhaust ducts 180 which lead into a large 
diameter exhaust hose 280 going ultimately to a dust collector (not 
shown). The dust collector intermittently cleans itself by puffs of 
reverse air pressure as described above in connection with the multi-gun, 
direct-applying station 52 (FIG. 2). As occurs with bottomless spray box 
54 described above, this buffer region (or arcade) 160 prevents airborne 
particles from escaping into the atmosphere through clearance gaps 163 of 
about 0.08 to 0.32 inch (about 2 to about 8 millimeters) between the 
bottom edges of the walls 154 of the buffer region 160 and the casting 
belt 12 or 14. Air flow 164 through gaps 163 prevents the escape of powder 
particles into the atmosphere. 
Powder will settle out and pile up in the lower portion of tubular 
dispenser 172 under the influence of gravity if not prevented. It is 
desirable to limit accumulations of powder, since accumulations may emerge 
untimely, resulting in uneven deposition. Moreover, accumulated stagnant 
powder sometimes has an undesirable electrical influence on other powder 
particles. 
There are two general ways to meet the dust-settlement problem in 
dispensing tube 172. The first way is the split-tube mode, which is to 
manufacture the tube 172 in two pieces: an antechamber tube part 172A and 
an exit chamber tube part 172B, as shown in FIG. 6C. Hence, dispensing 
tube 172 becomes split along its length in a direction that conveniently 
may be perpendicular to the pass line P. Powder feed hose 314 or 414 goes 
into antechamber tube part 172A as indicated by a legend and bears a 
powder-charged airstream 318 or 419. An intermediate baffle plate 181 
separates the two dispensing-tube parts 172A and 172B and is conveniently 
sandwiched between these two tube parts fastened by machine screw 187. A 
row or series of holes or other apertures 192 in baffle 181 is positioned 
low, i.e., near the bottom of the baffle 181. The low position of these 
intermediate holes 192 causes the powder-charged airstream 194 to entrain 
settled powder particles 196 and 198 and hence desirably to limit their 
accumulation. The total area of the holes or apertures 192 in baffle 181 
is comparable to the total area of the exit holes 174 discussed below; 
this evenness brings about even distribution of powder regardless of the 
location of the inlet from lines 314 or 414. 
The exit holes 174 are placed low in the exit tube part 172B. Their low 
placement similarly enables airstream 175 to entrain settled powder 
particles 198 in the bottom of exit chamber 172B and so to limit their 
accumulation. However, it must be noted that the above description has 
been in terms of the apparatus for depositing powder onto the upper belt 
12 by means of the assembly of FIG. 6C, also shown in FIG. 1 as 152A. 
Gravity enters into the operation of the apparatus. The adaptation of FIG. 
6D is required. If the equipment of FIG. 6C were simply inverted for use 
under lower belt 14 as in assembly 152B and FIG. 6D, the holes or 
apertures 174 and 192 would not entrain settled particles 196 and 198, 
since the particles would settle downward, away from the holes. 
Accordingly, in apparatus for the lower belt 14, the holes 192 and 174 
must be low as in FIG. 6D. It will be seen that this move entails a 
repositioning of tubular dispenser 172 as in FIG. 6D, in order that the 
exit holes 174 will aim the airborne powder along the deflector surface 
177. The corona-discharge wire 179 of either FIG. 6C or 6D is electrified 
as was the corona-discharge wire 179 of FIG. 6A. 
The split construction of tube 172 into two parts 172A and 172B (FIGS. 6C 
and 6D) also makes possible the removal of internal burrs at the holes 174 
during manufacture and, further, to regularize the sharpness of the 
entrance edges of these holes in order to regularize the corona-discharge 
effect when the tubular powder dispensing tube 172 is also the charging 
electrode as employed in the fourth embodiment (powered as in FIG. 6B). 
Split tube 172 comprising 172A and 172B with baffle 181 as just described 
is our most preferred embodiment to prevent powder accumulation in a 
tubular powder dispenser 172. 
The problem of powder accumulation in dispensing tube 172 is also met in a 
second way, that is, by the internal-circulation mode which is that of 
increasing the speed of the powder-bearing air stream where it goes 
through a one-chamber tubular dispenser 172 (FIGS. 6 or 6A). However, this 
speeded air flow must be accomplished without incurring the undesirable 
local jet effects on powder deposition that would be caused by a high 
speed of the powder-charged air stream 175 going straight out of the holes 
174. Moreover, the effect of this high speed air proceeding directly in 
reducing the settling of powder would be uneven, being nil at the closed 
far end 184 (FIG. 7) of tubular dispenser 172. Hence, to simply increase 
the amount of air in the airstream 318 or 419 is not a satisfactory way of 
preventing accumulation of powder in dispensing tube 172. 
A satisfactory way to increase the air speed in one-chamber tubular 
dispenser 172 and so to keep powder airborne involves the providing of 
internal circulation through making the tube 172 part of a local loop 190, 
shown as tubing (FIGS. 5A, 7A and 8A). The return reach 186 is plumbed at 
183 and 185 into opposite ends of the tube 172 for forming an inclusive 
flow circuit that includes the whole length of tube 172 in the loop 190. 
At one corner of the new plumbing 186 near the end connection 185, a 
small, electrically-driven squirrel-cage centrifugal blower 188 placed 
in-line in the loop 190 speedily moves the powder-bearing air flow 318 or 
419 all the way through the tube 172 and through return reach 186 thereby 
preventing settling of the powder. The principle is to supply about the 
same circulating flow speed 183 to all regions along the length of the 
dispensing tube 172 without ever allowing the remote regions of the flow 
183 to stagnate and in this way to avoid powder settlement within the 
dispensing tube 172. A fluidizing plenum similar in principle to that 
described above in connection with the hopper 302 (FIG. 9) is a sometimes 
useful option (not shown) to prevent certain powders from stagnating in 
the dispensing tube 172. 
All of the above-described embodiments for adhering powder to casting belts 
incorporate the four following elements: (1) a conductive corona-discharge 
electrode, (2) powder dispenser, (3) bottomless spray box, and (4) arcade 
or buffer region along the perimeter of the bottomless spray box. 
There is air-knife equipment for removing the powder or dust from a belt, 
as is generally indicated at 252 (FIGS. 1, 3, 4, 5 and 8). Air 249 (FIGS. 
4, 5 and 8) from a single-stage centrifugal blower (not shown) at a 
pressure, for example, in the range of about 18 to about 26 inches of 
water column, enters a pair of air knife chambers 248, as shown in FIG. 4. 
This 249 air from the blower is fed into these air knife chambers through 
hoses 244 and creates air-knife jets 256 (FIG. 4A), thereby loosening the 
powder or dust which has previously been applied to the casting belt 12 or 
14 and which already has been cast upon. A series of inclined jet slots 
240 (see also FIG. 3) is cut in the wall 250 of each chamber 248 near a 
belt, alternating in two staggered rows (FIGS. 3 and 4). These slots as 
shown are about 0.025 of an inch (0.6 mm) wide. They are typically 3 to 4 
inches (75 or 100 mm) long, with the effective part of the slots 
overlapping each other about 0.08 of an inch (2 millimeters) to ensure 
that no streaks of undislodged powder are left on the casting belt. The 
air knife chambers 248 are set at a gap of about 0.25 of an inch (6 
millimeters) from the work face of the casting belt per gap 264. Removable 
end caps 260 on the chambers 248 enable cleaning the interior surfaces and 
also make possible the leveling of interior burrs during manufacture. 
The air knife chambers 248 are enclosed in a non-conductive open-bottom 
plastic suction box 254 (FIGS. 5 and 8). Metal framing 268 with associated 
machine screws and brackets supports the apparatus of this invention on 
the casting machine 10 near the upper and lower casting belts 12 and 14. 
Between a casting belt and this open-bottom suction box 254 is a gap 262 
(FIG. 5) of about 0.08 to 0.32 of an inch (about 2 to about 8 millimeters) 
through which air enters this suction box under an exit vacuum of about 12 
inches (about 305 mm) of water column below atmospheric pressure inside 
the box 254, in order to keep dust from entering the atmosphere. As shown 
in FIG. 4A, there is about a 60-degree inclination of the slots 240 
relative to the belt, and their relative converging inclinations direct 
most of the air jets 256 toward a plenum region 242 located within the 
suction box 254 between the two air knife chambers 248, from whence the 
dust-laden air is readily extracted through hose 280 to the aforesaid air 
filtering and collecting equipment (not shown). 
FIGS. 1, 5, 7 and 8 show, as an upper-carriage assemblage 270 and a lower 
carriage assemblage 271, both the powder removal apparatus 252 and the 
powder distribution apparatus 152A and 152B in the single-tube method. The 
upper-carriage assemblage 270 is secured to carriage structure 272 of the 
machine 10 by means of cable assemblies 282 turnbuckles 284, brackets 288 
and a pair of rollers 286 (FIG. 8). The relative height of the 
powder-distribution station 152A or 152B and the air-knife apparatus 252 
is adjustable by means of screw slots 274 (FIG. 5) in the metal framing 
2-6.8 while the whole assemblage 270 is adjusted down or up, toward or 
away from a casting belt by means of the turnbuckles 284. The pair of 
rollers 286 (FIG. 8) accommodate such up or down adjustment. 
The corresponding lower assemblage 271 is supported by a cylinder 36 and a 
lever 34 with a rocker 38 interposed, turning on pivot pin 40. 
An initial powder or dust distribution 70 (FIG. 2) or 171 (FIG. 6) is 
itself strikingly uniform, a fact that is visually observable when the 
film thickness of the distributed dust is adjusted to be semi-transparent. 
Unless continually replenished, the dust deposit or cushion becomes 
thinner and nonuniform as the casting belts turn and are cast upon 
repetitively. The normal mode of maintenance of the dust deposit 70 or 171 
is by the electrostatic application of minute additional dustings. Such 
electrostatic re-depositings of dust particles afford the surprising and 
very advantageous quality of re-establishing a uniform, immediately useful 
self-healing of wear spots and scuffs without any interrupting of an 
ongoing casting operation. 
If the resulting dust-cushion deposit 70 or 171 becomes contaminated or 
becomes too thick, it may be removed without difficulty, most conveniently 
with air jets 256 provided by the air-knife apparatus 252 described above. 
The dust deposit is then immediately renewed as for instance by the 
single-tube distributing station 152A or 152B, and the casting of 
desirable product is continued. With some powders, the air-knife removal 
is done routinely and is immediately followed by re-application. 
However, we have observed that a continuous, very light reapplication of 
dust (without intentional removal) will automatically-self-adjustably 
patch over, and effectively repair, even a gross bare spot and will do so 
within a few revolutions of the casting belt. The patched area may not at 
once appear uniform, but the effect on the cast product is about as though 
it were uniform. Advantageously, the all-important requirement of an 
approximately uniform rate of heat transfer, in or out of the re-dusted 
previously bare spot, is evidently met by this overall touching-up 
procedure. This desirable uniformity is in marked contrast to prior-art 
top deposits or top dressings, where uniformity of heat transfer could not 
well be regained after a treated area of a casting belt had become worn. 
Many finely divided refractory ceramic powders or dusts perform acceptably 
in the present method and apparatus. Powders or dusts should be refractory 
to the temperatures involved and non-wetting to the molten metal 
concerned. They should be thermally insulative and electrically 
insulative, at least at room temperature. Among the substances meeting 
these requirements on occasion are aluminum oxide, titanium dioxide, 
zirconium oxide, zircon, boron nitride, magnesium zirconate and aluminum 
silicate. 
The above substances are relatively hard. Hard powders can be used but 
should preferably be of minute particle size. Some refractories are soft 
enough to ensure that subsequent rolling or drawing will crush them and 
break them into lesser, harmless minute pieces. Talc, mainly a magnesium 
silicate, is not hard and it is serviceable. Talc as sold for personal use 
has a laminated structure. Under our microscopic examination, the larger 
talc particles were seen microscopically as having a thin delicate 
three-dimensional structure of warped sheet material, rather like some 
dried leaves. Another soft substance is pyrogenic amorphous silicon 
dioxide (CAS Registry no. 112945-52-5 or no. 7631-86-9, where CAS stands 
for Chemical Abstracts Service). Although silicon dioxide is an inherently 
hard substance, it is rendered effectively soft in this form. Generally, 
the particles of these two soft substances are translucent or 
semi-transparent. Identifiable particles of these substances at 90 X 
magnification were seen to be within a size range of about 3 to about 300 
micro-meters in their major dimension, with the vast majority of particles 
by count being below 50 micro-meters in their major dimension. When either 
of the above soft, large-particle substances are electrostatically 
applied, the collective tops of the particles present to the molten metal 
a roughness that we believe helps to account for their insulativity. 
Another suitable, effectively soft substance is zinc oxide. 
Users of any of the above powdered substances should ask the manufacturers 
for health data. Electrostatic application of the above dry substances as 
dusts is not only convenient; it leads to results more uniform and 
serviceable in casting on flexible belts than through other methods of 
application. 
OUR THEORY OF ELECTROSTATIC ADHERENCE OF REFRACTORY CERAMIC POWDERS OR 
DUSTS TO METALLIC CASTING BELTS 
Corona discharge from a high-voltage thin-wire electrode is a means whereby 
refractory ceramic powder particles become charged as they go through and 
issue from an electrostatic spray gun or otherwise pass close to a charged 
electrode capable of creating corona discharge. In the several successful 
embodiments of the invention which have been described, and in the 
experiments performed to validate them, electrons are emitted from a 
negatively polarized electrode. 
The electrons may charge the particles directly; however, they may rapidly 
become attached to air molecules to make negative Ions of molecules of 
nitrogen and perhaps especially of oxygen. These negative ions then 
bombard the refractory ceramic powder particles, which become negatively 
charged thereby. 
According to our most recent experiments, a positively charged electrode 
will often serve as well as a negative one. This result corroborates the 
theory that only one stray electron will start an avalanche involving it 
(see reference by Miller). Under these conditions, the electrons flow to 
the electrode and the positive ions bombard the particles. We usually 
prefer the negative voltage; the behavior of the two kinds of charge is 
not quite identical. 
Air flow through the electrostatic charging apparatus carries the charged 
powder particles and ions toward the casting belt. Electrostatic forces 
become important in the deposition of such charged particles only when 
they have reached a point about 0.8 of an inch (20 millimeters) from a 
ceramic-covered casting belt surface to be dusted, the metallic substrate 
of which is grounded. 
In our attempts to design powder distribution apparatus, electrostatically 
charged powder particles in free flight away from the electrostatic 
charging apparatus lose their charge in two seconds or less under any 
condition known to us. This loss of charge occurs also when nitrogen or 
argon or carbon dioxide is used as the carrier gas in place of air. High 
humidity is thought to accelerate the loss but, in our observation, it 
happens even when the humidity is reduced to one part per million of water 
vapor. 
When the electrostatically charged particles strike the belt being coated 
within less than about a second of free flight, many of the particles 
stick, being presumably still charged when they land. Once stuck, they 
remain stuck, resistant to moderate mouth-blowing apparently forever or 
until they are mechanically detached. This clinging persists on the work 
faces of either bare belts or thermally sprayed ceramic-coated belts. 
Once the particle contacts the surface, one would expect the electrostatic 
charge to be dissipated either through contact with a grounded substrate 
or by continued bombardment by uncharged air molecules. The loss of the 
electrostatic charge is supported by the observation that, if the 
particles are detached from the substrate, by scraping for example, they 
have lost the ability to reattach themselves to the substrate. To account 
for the ability of the particle to remain attached to the belt after the 
electrostatic charge has been dissipated, the van der Waals force affords 
a plausible explanation. The van der Waals force is effective only at a 
distance on the order of atomic dimensions: the force is said to decrease 
inversely at something like the sixth power of the distance. Van der Waals 
force may be regarded as a special kind of electrostatic force, a kind 
that results from charges that stay dynamically captured within individual 
molecules and atoms of the refractory ceramic powder particles. 
It follows from electrostatic theory that the inverse-square-law 
electrostatic force becomes strong as the refractory ceramic powder 
particles come in for a landing on the casting belt. During such a landing 
approach, the inverse-square force becomes large enough to cause a 
significantly high-speed impact, such that the van der Waals forces can 
then become effective in retention of the adhered charged particles on the 
casting belt. The high-speed-impacting particle thus would penetrate 
adsorbed air films and other obstructive films and would thereby come into 
intimate contact with the casting belt such that the van der Waals 
attractive force would become an effective adherent force. Perhaps optimum 
final alignments of each impact are piloted by local van der Waals forces. 
In tending to accept this theoretical explanation, it is economical to 
suppose that the usual electrostatic forces do not persist anywhere but 
that they quickly dissipate themselves even from particles attached to the 
casting belt surface as the charges do when in free flight in mid-air, 
while the van der Waals forces remain effective. This theory based upon 
the effect of van der Waals forces avoids a paradox between sticking and 
nonsticking. 
Alternatively, the molecules that originally held the electrostatic charge 
as Ions may continue to aid the attachment of the particle, even after the 
electrostatic charge is lost, by becoming part of the film of adsorbed 
gases attached to both the particle and the substrate. 
Nevertheless, we cannot entirely rule out the existence of continuing 
electrostatic charges acting from the adhering particles and acting from 
protected positions thereon. A highly insulative particle is not charged 
uniformly. Rather, the charges remain at the points of random ionic or 
electronic impact and do not spread themselves around the particle (see 
reference by Hughes). This situation conceivably could allow for protected 
positions of electric charges. 
Regardless of whether our explanatory theory is correct or not, the 
described advantageous successful results are obtained by employing the 
methods and apparatus of the present invention. We believe that the 
above-described advantageous results are not limited to the casting of any 
particular metal product. Our experiments show that these advantageous 
results of application of insulative refractory dust or powder are 
achieved in casting aluminum alloys and in casting copper in a twin-belt 
casting machine 10. 
Although specific presently preferred embodiments of the invention have 
been disclosed herein in detail. It is to be understood that these 
examples of the invention have been described for purposes of 
illustration. This disclosure is not to be construed as limiting the scope 
of the invention, since the described methods and apparatus may be changed 
in details by those skilled in the art of continuous casting of metals, in 
order to adapt these methods and apparatus to be useful in particular 
casting machines or situations, without departing from the scope of the 
following claims.