Apparatus for pressure molding firebrick

A fluid operated press having opposed first and second fluid chambers is employed for compressing a mixture of desired brick ingredients in a mold. Pressurized fluid, preferably hydraulic oil, is first delivered to the first fluid chamber of the press to cause the same to exert a prescribed pressure on the mixture. Then, with this prescribed pressure maintained, the pressurized fluid is directed alternately into the first and second fluid chambers at rapid intervals, with the result that the press imparts vibratory motion to the mixture being held under pressure. In a preferred embodiment, the selective delivery of the pressurized fluid to the fluid chambers of the press is controlled by a servomechanism, constantly comparing a reference pressure signal with an actual pressure signal representative of the actual pressure being exerted by the press and actuating a servovalve so as to make the difference between the reference and actual pressure signals zero.

BACKGROUND OF THE INVENTION 
This invention belongs to the art of brickmaking and pertains more 
specifically to a method of, and apparatus for, pressure molding 
refractory brick commonly termed firebrick. Still more specifically, the 
invention is directed to such a method and apparatus whereby a mixture of 
desired firebrick ingredients in a mold is subjected to both pressure and 
vibrations for more efficient fabrication of high quality firebrick, 
particularly of that containing graphite, than heretofore. 
The pressure molding of firebrick by presses such as a friction press and 
hydraulic press has been known and practiced in the brickmaking industry. 
With the recent advent of graphite-containing firebrick, however, the 
conventional brickmaking equipment has proved to be in need of improvement 
for higher production. Standard methods of molding graphite-containing 
firebrick have been either to increase the molding pressure to 1.0 to 3.0 
tons per square centimeter, as compared with 0.5 to 1.0 ton per square 
centimeter required for the fabrication of common firebrick, or to repeat 
the application of pressure a number of times by a friction press. 
Generally, for the pressure molding of firebrick, there is first prepared a 
mixture of refractory aggregate in the form of both coarse and fine 
particles and a binder with air entrapped in the mixture. Placed in a 
mold, the mixture is pressed to cause the coarse and fine aggregate 
particles to be bound to one another in practically the most closely 
packed state. High bulk density (metric units) or bulk specific gravity 
and low porosity are the requisites of high quality firebrick. 
Conventional measures for the attainment of these properties have been to 
increase the packed density of the mixture of brick ingredients within a 
metal mold at the time of pressing and forming thereby to obtain 
uniformity and to carry out deaeration by degassing. 
However, such conventional measures have proved unsatisfactory in the case 
of graphite-containing firebrick. The mixture to be processed into this 
class of firebrick contains from 15 to 20 percent by weight of graphite as 
particles of uniform and balanced distribution. When the mixture is 
pressed, these graphite particles exhibit a unique behavior not found in 
the fabrication of common firebrick. Not only are the fine graphite 
particles themselves elastic, but they also possess even greater 
elasticity as a mass. This is because of a large volume of air trapped in 
the interstices of the graphite particles and of their great surface 
energies. Having a low coefficient of friction, moreover, the graphite 
particles very easily slip relative to each other. For these reasons the 
graphite-containing mixture subjected to pressing tends to regain its 
initial state as an elastic body. 
Thus, for a higher bulk specific gravity of graphite-containing firebrick 
fresh from the press, it is essential to reduce the elastic properties of 
the firebrick mixture before it is pressed. (It is to be noted that the 
term "bulk specific gravity" as used in this specification means that of 
the green brick that has just been pressed.) The brickmaking industry has 
recently expended much research effort for the solution of this problem. 
We have conducted a series of exhaustive experiments and have amassed data 
concerning how the bulk specific gravity of a graphite-containing 
firebrick is affected by the compositions and particle sizes of the mixed 
raw materials, the molding pressures, and the number of bumping impacts or 
pressures exerted on the mixtures. The data indicate that the repeated 
exertion of impacts or pressures serves to improve the bulk specific 
gravity, but up to a limit of approximately 20 times from the standpoint 
of production engineering. Also, the higher the molding pressure, the 
greater is the bulk specific gravity, and this tendency becomes even more 
pronounced in cases where the mixtures of raw materials contain a large 
proportion of fine particles. At the same molding pressure, however, the 
bulk specific gravity decreases in inverse proportion with the percentage 
of fine particles contained; that is, a higher molding pressure is 
required for the same bulk specific gravity. 
Both friction press and hydraulic press have their own restrictions and 
limitations as heretofore used for pressure molding firebrick. The 
friction press is an inertia operated machine, translating the rotation of 
a flywheel into linear motion of a screw shaft. It has generally been 
employed for applying a series of impact forces or blows on the mixture in 
a mold. Problems arise, however, in increasing the size of the friction 
press to an extent required for the exertion of sufficiently high molding 
pressures for the fabrication of graphite-containing firebrick. The 
operating principle of the friction press unavoidably gives rise to 
considerable energy losses. No negligible proportion of the mechanical 
energy created by the rotation of the flywheel is wasted in the form of 
the heat of friction between the screw shaft and the mating part and of 
vibration and noise upon application of blows. Moreover, as the bulk 
specific gravity of the mixture being pressed rises close to the limit, 
the impact of each blow is transmitted amost directly to the machine 
itself, possibly resulting in its damage. Any attempt to increase the size 
of the friction press to an extent necessary for the production of 
graphite-containing firebrick is, therefore, impractical. 
The hydraulic press, on the other hand, exerts semistatic pressures and 
operates with little noise and little energy loss. The manufacture of 
large size hydraulic presses is also relatively easy. The problem is that 
graphite-containing firebrick of truly satisfactory physical properties is 
not obtainable no matter how high the semistatic pressures of the 
hydraulic press are made. 
Accordingly, a method known as "bumping", which comprises the repeated 
(approximately 20 times) application of impact vibration caused by the 
maximum pressure of the hydraulic press on the mixture in a mold is being 
used. For the application of such repeated blows, a solenoid actuated 
directional control valve has been employed for alternately directing 
hydraulic oil under pressure into the pair of opposed fluid chambers of 
the press and hence for causing the repeated up-and-down motion of the 
ram. 
An objection to the known bumping method is the prolonged length of time 
necessary for pressing each brick. Each blow has ordinarily required a 
period of five to six seconds, so that a total of as much as 100 to 120 
seconds has been necessary for imparting 20 blows to each brick. 
SUMMARY OF THE INVENTION 
The present invention overcomes the problems heretofore encountered in the 
pressure molding of firebrick, particularly of that containing graphite, 
and makes possible the manufacture of firebrick of remarkably high bulk 
specific gravity in a shorter period of time than heretofore. 
Briefly summarized in one aspect thereof, the invention provides an 
improved apparatus for carrying out the method of pressure molding 
firebrick, which method employs a press having opposed first and second 
fluid chambers to be selectively pressurized for respectively exerting 
pressure on, and releasing the pressure from, a mixture of desired brick 
ingredients in the form of fine particles in a mold. A fluid under 
pressure is first supplied into the first fluid chamber until the press 
exerts a prescribed pressure on the mixture. Then, with the mixture held 
substantially under the prescribed pressure, the pressurized fluid is 
alternately directed into the opposite fluid chambers of the press at such 
rapid intervals that the mixture is subjected to vibratory motion from the 
press while being thereby held under pressure, instead of to a series of 
discrete blows by the prior art bumping method, and so is rapidly 
compacted to a high bulk density or specific gravity. 
According to another aspect of the invention, there is provided an 
apparatus for pressure molding firebrick in conformity with the above 
summarized method. The apparatus comprises, in addition to the fluid 
operated press, a directional control valve for selectively placing the 
opposed fluid chambers of the press in and out of communication with a 
source of fluid under pressure and with a fluid drain, and means for 
actuating the directional control valve so as to cause the same first to 
direct the pressurized fluid from the source to the first fluid chamber 
until the press exerts a prescribed pressure on the mixture and then, with 
the mixture held under the prescribed pressure, to direct the pressurized 
fluid from the source alternately to the fluid chambers of the press at 
rapid intervals. 
Preferably, an electromagnetically actuated servovalve is employed for 
directing the pressurized fluid alternately to the opposite fluid chambers 
of the press. For actuating the servovalve, there is provided signal 
generator means which generates a reference pressure signal representative 
of the initial pressure to be exerted by the press on the mixture of 
powdered firebrick ingredients and of the intervals at which the 
pressurized fluid is to be alternately delivered to the opposite fluid 
chambers of the press. A pressure sensor is also provided for generating 
an actual pressure signal representative of the actual pressure being 
exerted by the press on the mixture. The reference and actual pressure 
signals are both directed into servo control means, which actuates the 
servovalve in accordance with the difference between the two input 
signals, so as to make the actual pressure of the press equal to that 
represented by the reference pressure signal. 
Thus, in accordance with the invention, the mixture of powdered firebrick 
ingredients in a mold is subjected to vibrations or repeated pressure by 
the press while being thereby held under pressure. The vibrations of the 
press can be readily generated as the pressurized fluid is alternately 
delivered to the opposite fluid chambers of the press by the servovalve, 
which is faster in operation than its counterpart employed by the 
conventional bumping method. Indeed, in one preferred construction of the 
servovalve, the period of time required for one complete reciprocation of 
the press ram (i.e., for one vibration) is as short as from 1.0 to 1.5 
seconds. If 20 such vibrations are to be imparted to the mixture for the 
production of one firebrick, the total time is only from 20 to 30 seconds, 
which is from 1/2 to 1/3 the time required conventionally by a friction 
press, and from 1/4 to 1/5 the time required by the bumping method 
employing a hydraulic press. 
The machine for use in the practice of this invention can, of course, be a 
hydraulic press. The hydraulic press can be easily increased in size for 
the exertion of pressures sufficient for the purposes of this invention 
without much noise production. 
As an additional feature of the invention, the signal generator means for 
the production of the reference pressure signal can be so constructed, as 
in the preferred embodiment disclosed herein, to predetermine various 
conditions of pressure molding according to the compositions and particle 
sizes of the firebrick materials. Various types of firebrick may therefore 
be automatically produced under the predetermined optimum conditions. 
It will thus be appreciated that the apparatus of the present invention is 
well adapted for the manufacture of firebrick which requires high molding 
pressures and the repeated application of such pressures. In particular, 
the invention will be of immense utility in increasing the production of 
graphite-containing firebrick. 
The above and other features and advantages of this invention and the 
manner of realizing them will become more apparent, and the invention 
itself will best be understood, from a study of the following description 
and appended claims, with reference had to the attached drawings showing 
the preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The general organization of the apparatus in accordance with the invention 
will be understood from a consideration of FIG. 1. Included is in this 
apparatus a fluid-operated, preferably hydraulic, press 10 having an 
upstanding cylinder 12. A piston 14 is reciprocatively fitted in this 
cylinder 12 to define a pair of opposed fluid chambers 16 and 18 therein. 
A ram 20 depends from the piston 14 and projects out of the cylinder 12 
for acting on a mixture of desired firebrick materials in the form of fine 
particles contained in a mold (not shown). 
A servovalve 22 is provided for selectively placing the opposed fluid 
chambers 16 and 18 of the press 10 in and out of communication with a 
hydraulic pump 24 and with a fluid drain or reservoir 26. The servovalve 
22 is a four-way, three-position, closed-center, directional-control valve 
capable of infinite positioning. A preferred construction of the 
servovalve 22 includes a torque motor for electromagnetically translating 
an electric pilot signal into mechanical motion, a hydraulic amplifier for 
amplifying the mechanical motion, and flow and direction control means 
actuated by the fluid output of the hydraulic amplifier. 
Normally held centered as shown, the servovalve 22 holds the opposed fluid 
chambers 16 and 18 of the press 10 out of communication with either of the 
pump 24 and drain 26. When actuated to the right, the servovalve 22 places 
the pump 24 in communication with the upper fluid chamber 16 of the press 
10 by way of a conduit 28, thereby causing the descent of the piston 14 
with the ram 20 for the exertion of pressure on the mixture in the mold. 
When actuated to the left, on the other hand, the servovalve 22 places the 
pump 24 in communication with the lower fluid chamber 18 of the press 10 
by way of a conduit 30, thereby causing the ascent of the piston 14 with 
the ram 20 for the release of the pressure from the mixture in the mold. 
According to the method, the press 10 first exerts a predetermined pressure 
on the mixture in the mold and then, with that predetermined pressure 
substantially maintained, applies vibrations to the mixture by the rapid 
reciprocation of the piston 14 with the ram 20. 
In order to cause such operation of the press 10, the servovalve 22 is 
pilot operated by a servo system comprising a signal generator section 32 
for generating an electric reference pressure signal representative of 
desired pressures to be exerted by the press, a pressure sensor 34 for 
generating an actual pressure signal representative of the actual pressure 
being exerted by the press, and a servo control section 36 for activating 
the servovalve 22 in response to the reference and actual pressure 
signals. 
The signal generator section 32 comprises a variable-frequency oscillator 
38 and its controller 40. The oscillator 38 is capable of generating an 
electric signal with frequencies ranging from 0.1 hertz to 1.0 megahertz. 
Preferably, the oscillator 38 is also capable of generating the 
variable-frequency signal with various waveforms, such as a square or 
rectangular wave, triangular wave, and sine wave, to impart 
correspondingly different modes of vibration to the powdered firebrick 
materials being pressed. The controller 40 has an array of digital 
pushbutton switches 42 to be depressed manually to determine such factors 
to be represented by the oscillator output signal as the initial pressure 
to be exerted on the mixture of firebrick materials in the mold, the 
frequency and period of the vibration to be subsequently applied to the 
mixture, etc. The waveform of the oscillator output signal can also be 
selected by the controller 40. The output from the signal generator 
section 32 is delivered as the reference pressure signal to the servo 
control section 36. 
The pressure sensor 34 is communicatively connected to an intermediate part 
of the conduit 28 extending between the servovalve 22 and the upper fluid 
chamber 16 of the press 10 which is to be pressurized for the power stroke 
of the piston 14. The pressure sensor 34 senses the pressure being exerted 
by the press 10 from the pressure of the fluid flowing through the conduit 
28 and puts out the actual pressure signal indicative of the actual 
pressure of the press at every moment. A pressure monitor including a 
conventional strain gage is a preferred example of the pressure sensor 34. 
The servo control section 36 comprises an amplifier 44 for amplifying the 
actual pressure signal from the pressure sensor 34 and a servo amplifier 
46 responsive to the reference pressure signal from the signal generator 
section 32 and to the amplified actual pressure signal from the amplifier 
44, for actuating the servovalve 22 in accordance with the difference 
between the reference and actual pressure signals. The servo amplifier 46 
functions in the known manner so as to make the actual pressure signal 
equal to the reference pressure signal. 
Operation 
The oscillator controller 40 of the signal generator section 32 may first 
be manipulated to determine the above noted conditions of pressure molding 
in accordance with the invention, as represented by the frequencies, 
waveform, etc., of the output signal of the variable frequency oscillator 
38. Generally, there are two different methods for the determination of 
the moment at which the hydraulic press 10 starts imparting vibrations to 
the mixture of firebrick materials in the mold. One is to cause the 
oscillator 38 to start producing a signal at a desired frequency of the 
vibrations when the actual pressure being exerted by the press 10 on the 
mixture builds up to a prescribed value. The other is to cause the 
oscillator 38 to start producing the desired vibration frequency signal 
upon elapse of a preassigned length of time following the moment the 
piston 14 of the press 10 starts travelling on its power stroke. 
The reference pressure signal from the signal generator section 32 enters 
the servo amplifier 46 of the servo control section 36, and is thereby 
amplified and directed to the servovalve 22 for electromagnetically 
actuating the same. The servovalve 22 first places the pump 24 in 
communication with the upper fluid chamber 16 of the pump 10, thereby 
causing the descent of the piston 14 with the ram 20 for the exertion of 
pressure on the mixture in the mold. 
When the pressure on the mixture in the mold rises to a required value, the 
output signal of the servo control section 36 starts to cause the 
servovalve 22 to shift alternately to its right and left hand offset 
positions at rapid intervals prescribed by the frequency of the reference 
pressure signal from the signal generator section 32. The rapid, repeated 
shifting of the servovalve 22 between its two offset positions results in 
the alternate delivery of the pressurized fluid from the pump 24 into the 
opposite fluid chambers 16 and 18 of the press 10 at rapid intervals and, 
accordingly, in the rapid reciprocation of the piston 14 with the ram 20. 
The rapid reciprocation of the ram 20 can be thought of as a kind of 
vibration, which is imparted to the mixture in the mold. Thus subjected to 
vibration from the press 10 while being held under pressure, the mixture 
is rapidly compacted to a required degree of bulk specific gravity. 
During the above process of compaction, the pressure sensor 34 senses the 
pressure being exerted by the press 10 from the pressure of the fluid 
flowing through the conduit 28 and puts out the actual pressure signal for 
delivery to the servo control section 36. Amplified by the amplifier 44, 
the actual pressure signal is directed to the servo amplifier 46, to which 
there is also supplied the reference pressure signal from the signal 
generator section 32. The servo amplifier 46 controls the servovalve 22 in 
accordance with the difference between the reference and actual pressure 
signals so as to make this difference zero by making the actual pressure 
signal equal to the reference pressure signal. It is thus seen that the 
servomechanism functions to cause the press 10 to operate exactly under 
the conditions dictated by the signal generator section 32. 
EXAMPLE 
Some different particle size groups of magnesium oxide (MgO) were mixed 
with graphite in proportions set forth in Table 1 below, to prepare sample 
mixtures A and B to be processed into firebricks in accordance with the 
teachings of the present invention. In this table, the proportion of 
graphite is given in percent by weight with respect to the total amount of 
MgO of the different particle sizes. 
TABLE 1 
______________________________________ 
Sample Compositions 
Sample 
Particle Size A B 
Ingredient 
(mm) (wt. %) (wt. %) 
______________________________________ 
MgO 1.0-3.0 60 -- 
" 0.3-1.0 20 60 
" Less than 0.3 20 40 
Graphite -- *20 *20 
______________________________________ 
*percent by weight with respect to the total amount of MgO 
The above mixtures A and B were processed into common refractory 
firebricks, sized 230 by 114 by 65 millimeters, by the use of a 1,500-ton 
hydraulic press under various conditions as described below. 
As the first experiment, the initial pressure exerted on the mixtures was 
set at various values from 1.0 to 4.0 tons per square centimeter, with the 
number of vibrations or reciprocations of the ram fixed at 20. FIG. 2 
gives the results, graphically representing the bulk specific gravities of 
the firebricks molded from the mixtures A and B against the molding 
pressures. 
The second experiment was to vary the number of vibrations, from one (no 
vibration) up to 25, with the initial pressure fixed at 1.5 tons per 
square centimeter. The results were as graphically represented in FIG. 3, 
in which the bulk specific gravities of the firebricks molded from the 
mixtures A and B are plotted against the various numbers of vibrations 
that were imparted thereto. 
The third and final experiment ran counter to the teachings of this 
invention: No vibration was imparted, and only the molding pressure was 
set variously from 1.0 to 4.0 tons per square centimeter. FIG. 4 
graphically represents the results as the bulk specific gravities of the 
firebricks molded from the mixtures A and B plotted against the molding 
pressures. 
The results of FIGS. 2 and 3 demonstrate that the application of high 
molding pressures and vibrations in accordance with the invention results 
in the production of firebricks of materially higher bulk specific 
gravities than those of the firebricks of FIG. 4 that have been subjected 
to no vibration. The exertion of 20 vibrations (piston reciprocations) 
normally suffices for practical purposes. Since the apparatus of FIG. 1, 
including the servovalve 22, makes it possible to cause the press 10 to 
make one vibration in 1.0 to 1.5 seconds, one firebrick can be pressed to 
a sufficiently high degree of bulk specific gravity in 20 to 30 seconds. 
Table 2 represents by way of comparison the periods of time required for 
bumping or vibration, handling, evacuation, and the sum of such periods, 
in the production of firebricks of like physical properties by this 
invention and by the prior art friction press and bumping hydraulic press. 
Each press tested was furnished with evacuation facilities, and 20 blows 
or vibrations were applied to the mixture in the mold under the same 
maximum pressure. 
TABLE 2 
______________________________________ 
Comparison of Operating Periods 
Invention versus Prior Art 
(sec.) 
Bumping or 
Handl- Evacua- Total 
Vibration 
ing tion Molding 
Time Time Time Time 
______________________________________ 
Friction Press 
50-70 30-40 10-15 90-125 
(Prior Art) 
Bumping Hydrau- 
100-120 30-40 10-15 140-175 
lic Press 
(Prior Art) 
Invention 20-30 30-40 10-15 60-85 
______________________________________ 
Table 3 compares the characteristics of the present invention with those of 
the prior art friction press and bumping hydraulic press. 
TABLE 3 
______________________________________ 
Comparison of Performance Characteristics 
Invention versus Prior Art 
Friction Bumping Hy- Oil Press 
Press draulic Press 
of 
(Prior Art) 
(Prior Art) Invention 
______________________________________ 
Pressing Mechanical Semistatic Pressure & 
Method Impact Force 
Impact Force Vibration 
Molding Medium Long Short 
Time 
Increase in 
Difficult Easy Easy 
Size of 
Machine 
Energy Much Little Little 
Loss 
Vibration 
Much Little Little 
& Noise 
Brick Excellent Good Excellent 
Quality 
(Same pres- 
sure) 
______________________________________ 
Tables 2 and 3 clearly demonstrate the advantages of this invention over 
the prior art that is believed to be closest to the present invention.