Method and apparatus for transporting green work pieces through a microwave sintering system

This disclosure sets forth a method and apparatus for microwave processing of green work pieces. Typically, individual green work pieces are formed in a small mold cavity crucible, and individual crucibles are then indexed into and out of a tube for a controlled transit time along the tube. The tube extends in one embodiment through a preheater and then into the microwave cavity, the preheater providing an initial heating step to change the rate of absorption of microwave energy so that microwave sintering is accomplished in the cavity.

BACKGROUND OF THE DISCLOSURE 
As set out in the parent application, microwave sintering is believed to be 
effective for conversion of loose particulate material which is packed 
into a small mold. It is especially useful in the manufacture of cast 
devices which are sufficiently small to fit in the microwave sintering 
process. Examples of devices which benefit from microwave sintering and 
which are especially enhanced by such sintering techniques include drill 
bit inserts. While many examples could be noted, this is an especially 
important device for microwave sintering. A typical drill bit insert 
measures about 1/2 inch in diameter and has a length of about 1 inch. At 
one end, it is normally formed of tungsten carbide particles supported in 
a softer metal alloy which normally is formed of a number of metals but 
especially featuring cobalt. At the exposed or cutting end of the insert, 
the tungsten carbide sintered body is then capped with a diamond particle 
layer. It is secured in place by a cobalt alloy matrix. Quick heating and 
cooling is important to the fabrication of this composite structure. 
Different quantities of cobalt are used to form the tungsten carbide (WC 
hereinafter) body of the drill bit insert while the diamond crown has a 
different level of cobalt in it. The crown is normally called a 
polycrystalline diamond compact or PDC. The PDC capped WC body is later 
inserted into an opening formed in the body of the drill bit. This is 
fastened in place in an interference fit, i.e., the hole is smaller than 
the outside diameter of the cylindrical insert, or brazed in place. 
If heated with conventional sintering techniques, the heat is maintained 
for sufficiently long intervals that grain growth occurs. This damages the 
structural integrity of the completed product. Worse than that, it 
provides a less than acceptable cobalt alloy dispersion in the different 
regions. This is undesirable in all aspects. 
Sintering by microwave achieves modification of the grain boundaries and 
also accomplishes the sintering in such a short time interval that the 
alloy integrity is unchanged. In fact, the finished product exhibits more 
desirable characteristics. Sintering, in this particular instance, is 
directed to the fabrication of loose particulate materials into a solid 
member having structurally sintered yet different regions. This sintering 
process reduces or avoids multiple intermediate sintering steps otherwise 
involved in separate WC and PDC components. It also reduces or eliminates 
the stress that is involved in attaching the PDC layer to the WC 
component. 
Prior to manufacture, the constituent parts of the drill bit insert are 
powders. They are loosely packed in a mold at a nominal pressure. They are 
joined together in the mold either by a slight amount of moisture but 
preferably with a sacrificial wax. This provides just enough adhesive 
benefit to hold the particles together. During sintering, the wax is 
driven off in the form of a combustible gas. If the volume is sufficient, 
the gas can be combusted for easy disposal after it has been vaporized. 
However, it is not involved in the heating process itself; rather, it is 
involved in initially adhering the particles together so that they 
maintain structural integrity at the time of molding and from molding 
through sintering. The finished products hold together as a result of the 
sintering process; the sintered drill bit insert has structural integrity 
as a result of the hard particles and the metal alloy binder which holds 
them together. 
The amount of sintering can be controlled in making a sintered product by 
simply turning the microwave source off, first placing the unsintered 
green molded product in the microwave cavity and then turning it on. The 
present disclosure sets forth a process which is advanced over that. The 
microwave generator is turned on and left on. An elongate tube, hollow and 
having a circular cross-section in the preferred form, extends through the 
microwave cavity and is able to process a series of individual molded 
green inserts. They are assembled in individual molds. The molds provide 
structural definition to the profile and hold the particulate ingredients 
together in the desired profile and shape. That shape is held during the 
sintering process. Each mold is preferably identical in size and shape to 
the others so that they can be serially pushed or dropped by gravity 
through the tube in the microwave cavity. By controlling the velocity, the 
dwell time of each mold, and, hence, each insert in the cavity can be 
controlled. By controlling the velocity and the dwell time, the microwave 
sintering equipment is then simply switched on and left on so long as 
individual molds are sequentially put into and taken out of the sintering 
furnace. 
The pathway for an individual mold is thus along a conveyor tube. They are 
introduced at the same rate and they are removed at the same rate. This 
enables a consistent dwell time to be obtained for every sintered insert. 
In some instances, it will be desirable that the individual molded pieces 
progress through the conveyor tube by gravity and in other instances, they 
can be forced through the conveyor tube with a positive feed and indexing 
mechanism. In some occasions, it is more desirable that the conveyor tube 
be vertical but it will also operate at an inclined angle or horizontally. 
Vertical and horizontal embodiments are both illustrated and described 
below. 
The present apparatus is therefore summarized as a microwave sintering oven 
for multiple small pieces. An example is the molding of a drill bit insert 
which is made of WC and/or PDC in separate layers and which are sintered 
together to form a unitary device. They are held together by an alloy 
(primarily formed of cobalt and other high temperature alloys) and are 
compacted in a small mold. The individual molds are sequentially placed in 
a conveyor tube and conveyed through a microwave cavity. Heat is created 
in them. In one embodiment, a preheater is added to raise the temperature 
of the green molded piece prior to microwave exposure. This helps change 
the reflectivity, therefore increasing microwave absorption as will be 
noted. The conveyor tube is provided with a series of individual molds 
which hold individual work pieces; they progress through it in sequence 
and are treated thereby having a fixed dwell time sufficient to obtain 
full sintering. Different techniques are set forth for feeding including 
gravity feed, operation of an indexing input device, and pushing the mold 
pieces with a rod inserted into the conveyor tube.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Attention is now directed to FIG. 1 of the drawings where the numeral 10 
identifies the microwave sintering apparatus of the present disclosure. It 
incorporates a wave guide 12 which connects to a microwave energy source 
14. The source 14 provides a continuous wave (CW) signal which is 
delivered through the microwave guide into the microwave cavity 16. The 
cavity 16 has a wall which reflects the microwave energy and keeps it 
within the cavity. There is a microwave stirring device 18 driven by a 
motor and which rotates to scatter radiation, thereby creating a 
continuous change in the microwave pattern. This helps provide more 
uniform exposure within the microwave cavity. A tube 20 is extended 
through the cavity 16. The tube 20 is made of ceramic at least partially 
or fully transparent to the microwave energy. It is typically an elongate 
hollow round tube. It is preferably round for the products to be described 
but it can be made rectangular or to any other cross-sectional shape 
depending on the shape of the product and the molds that are placed in it. 
As shown in FIG. 1, the hollow ceramic tube inserts through an insulator 
sleeve 22 which is serially connected to a larger insulator sleeve 24. The 
sleeves define a central region in the conveyor or transport tube 20, 
between the two ends, where the temperature is increased by near proximity 
to the microwave cavity. The microwave cavity is heated substantially by 
the radiation interacting with the sinter material so that cooling is 
needed at various locations around the microwave cavity. An optional 
electric resistance heater wire 25 is located adjacent the tube 20 to 
preheat or supplement the microwave heating resulting from irradiation of 
the unsintered particles. A water jacket 26 fits around the cavity in one 
dimension and another portion thereof is illustrated at 28. Water is 
introduced at the bottom and flows from an outlet 30 at the top. The 
conveyor tube 20 extends through a lower insulative sleeve 32. The several 
insulation sleeves assure that heat is confined within the conveyor tube. 
This helps provide the proper microwave initiated temperature increase to 
the particles for sintering. Also, it may be necessary to include an 
additional cooling jacket 34 on the conveyor tube for subsequent post 
sintering cooling. 
The tube 20 has a specified diameter. It is oversized with respect to the 
individual molds introduced into it. A gas inlet 36 is shown at the bottom 
so that gas can be introduced and flows up through the tube. Gas flow 
upwardly can easily carry a reducing component such as hydrogen along the 
tube. The gas (having a selected make-up) can optionally react in the 
sintering process. Gas flows next to the side wall and moves in an annular 
flow path to exit the elevated end of the tube 20. At the top, the green 
work pieces 40 are inserted in individual crucibles 42. The crucible or 
mold shapes the particles to the desired shape. The preferred or target 
product is an elongate cylindrical body. That is easily formed in the 
crucible 42 which is defined by a simple hollow cylindrical cavity in a 
cylindrical body having a removable lid or cover. The green work piece 40 
initially has the form of compacted powder. Typically, the compacted 
particulate material is placed in it first. The particles are put into the 
mold and include the WC particles and the particles forming the PDC layer 
of the finished product. Binder particles making up the bonding alloy, 
primarily cobalt and lesser quantities of other metals, are added. All of 
these are placed in the crucible 42 in the form of particles. A binder 
element is often added and usually is a sacrificial wax or other petroleum 
product. It is a wax which is tacky and solid at room temperature. It 
preferably has an adequate measure of tackiness so that it holds the 
particles. When heated, it becomes soft and when heated further, it 
preferably vaporizes so that the temperature increase while microwave 
sintering completely expels the sacrificial wax component. The wax is 
optional in the sense that it is not required in the finished product. It 
is helpful to hold the loose particles compacted together. 
The wax put into the mold 42 holds the components together. They are also 
tamped to a sufficient packing density that the particles are in intimate 
contact one with another. They are tamped and slight pressure is applied. 
So to speak, finger pressure will suffice. Hence, the work piece particles 
defining the work piece are place in the crucible 42 and this is done so 
the individual crucible can then be inserted into the conveyor tube 20. 
FIG. 1 further includes an elongate push road 44 extending upwardly into 
the tube 20. A seal 46 is shown at the lower end and permits the rod 44 to 
be retracted by a suitable power source such as a hydraulic retractor 48. 
It is desirable that the rod 44 control crucible velocity. It holds up the 
first crucible inserted into the system. Indeed, FIG. 1 is shown with any 
number of individual crucibles standing on the rod 44. The rod 44 is a 
speed control device. It is retracted at a constant rate such as 1 inch 
per minute. The velocity of the rod retraction can be adjusted. 
Preferably, the rod has a length which is approximately equal to the 
transparent conveyor tube 20. It is extended fully into the conveyor tube 
20 so that it extends well beyond the microwave cavity 16. A first 
crucible is then placed on the top end of the rod in the conveyor tube 20. 
Second and third crucibles are then stacked on it. The conveyor tube is 
commonly filled from the top. The rod 44 is controllably pulled 
downwardly. Gravity movement then gradually carries each of the green work 
pieces 40 into the microwave sintering process, and they move steadily 
through the microwave cavity 16. At individual work pieces, each is 
exposed to a build-up in microwave radiation which achieves a maximum in 
the cavity 16. Then, the microwave energy is decreased for the individual 
work piece as it leaves the cavity 16 progressing from the top to the 
bottom of FIG. 1. Each work piece is microwave treated to thereby sinter 
the particles making up the work piece. Any wax adhesive mixed with the 
particles is driven off in the form of vapors. Any moisture is also given 
off as steam. To this end, the crucible 42 is preferably a loosely sealed 
hollow cylindrical chamber formed of a ceramic material which is partially 
or fully transparent to the microwave radiation. As the rod is pulled 
downwardly, the individual work pieces progress steadily downwardly and 
are removed. The several pass fully through, thereby accomplishing the 
necessary treatment. This is accomplished while simultaneously controlling 
the temperature of the microwave cavity 16 by providing a fixed flow of 
coolant through the jacket around it, and a flow of ventilation gas is 
introduced through the port 36 and flow out of the top end of the conveyor 
tube 20. The process begins by insertion of the metal rod 44 fully into 
the tube. It continues by removing it steadily. The tube 20 is preferably 
extended in length so that the rod guides all of the individual crucibles 
until the last has moved down and out of the microwave sintering region in 
the middle of the cavity. The equipment is then reset by returning the rod 
44 to the raised position. Another batch can then be sintered thereafter. 
Conveniently, the individual crucibles can be recycled and used again. 
Going now to FIG. 2 of the drawings, an alternate embodiment 50 is 
illustrated. This embodiment incorporates the same microwave cavity as 
before. It is shown with a larger insulator 52 in the cavity, and also 
includes a temperature probe 54 which extends to the conveyor tube. In 
this particular instance, the tube 60 is shorter, and has a bend or elbow 
58 at the top end along with a similar elbow 56 at the bottom end. The 
elbows 56 and 58 enable the individual crucibles to be placed in the tube 
and includes an indexing device which pushes them in or out as the case 
may be. From the top, a port 62 enables one individual crucible 42 to be 
dropped into the elbow. An indexing device pushes to the left, and 
incorporates a push rod 64 driven by a rotating cam lobe 66. That controls 
the stroke for pushing one crucible to the left. The length of stroke is 
sufficient to move the crucible 42 from alignment with the port 62 into a 
centerline position above the tube 20. As shown in the drawing, this 
equipment is intended for continuous operation, typically around the 
clock. Individual crucibles are input in the manner just described. When 
they arrive at the aligned position above the conveyor tube 20, they fall 
downwardly in the concentric tube. The tube 60 is filled so that it is 
stacked from the bottom to the top. At the bottom, the bottom most 
individual crucible is delivered out of the tube 20 adjacent to an 
indexing mechanism 68. Again, it functions in the manner of a push rod and 
is operated by a controlled cam lobe 70 which periodically pushes the 
individual crucible to the left. The indexing rod 68 has a stroke which is 
only as long as needed to force the crucible one position to the left. The 
elbow 56 incorporates an outlet port 72 so that the individual crucibles 
are forced ultimately to the far left and drop downwardly through the port 
72. First one and then the next one falls through that port, and each is 
pushed to that position by operation of the push rod 68. 
Consider an example of the sintering time accomplished by the embodiment 
50. Assume, for purposes of discussion, that sintering occurs primarily 
and substantially while the individual work piece 40 is in the cavity 16 
shown in FIG. 2. If the cavity, as illustrated in these drawings, has a 
height spanning approximately five individual crucibles, and it is 
required that each one spend ten minutes in the microwave cavity, then the 
system must operate to remove one crucible every two minutes and add a new 
one at the top end. Once the conveyor tube 20 is filled, each one will 
dwell in the microwave cavity for the requisite ten minute interval. To 
illustrate this further, assume that the conveyor tube is filled and that 
the elbows 56 and 58 are also filled. The push rod 68 is moved to the 
left, thereby forcing the individual crucible at the far left to fall 
through the port 72 so that it can be removed because microwave sintering 
has been completed. The rod 68 is extended and then retracted. After it is 
retracted, the vertical stack of individual crucibles in the conveyor tube 
falls downwardly so that one is returned to the position abutting the end 
of the push rod 68. At this point, the push rod 64 is also operated to 
push to the left. When it pushes to the left, it moves an individual 
crucible and the encompassed work piece into the conveyor tube so that it 
is then standing and supported on the standing column of individual 
crucibles. After the rod 64 has been retracted, another individual 
crucible 42 is then dropped through the port 62. This cycle is then 
repeated to index the next crucible into the system while removing a 
completed work piece. 
FIGS. 1 and 2, taken together, show gravity feed working to advantage in 
the two different embodiments. FIG. 3 shows another embodiment which does 
not use a vertical feed. Going to FIG. 3 of the drawings, the numeral 75 
illustrates another version which has notable added features. In FIG. 3, 
the tube 80 passes through the microwave cavity 82. It also passes through 
a preheater chamber 84. The preheater chamber 84 is provided with B.sup.+ 
voltage for a resistance strip heater 86. A suitable power supply is 
connected for heating. The preheater cavity optionally also includes a 
spark gap 88. Periodically, a spark is provided from the spark power 
supply. The spark jumps through the gap 88 to assure combustion of 
combustible fumes driven from the wax in the individual crucibles. A 
blower 90 introduces a flow of air including oxygen from left to right. 
The spark gap 88 can be omitted and the blower 90 can be provided with 
nitrogen to avoid combustion and also reduce oxygen near the sintered 
materials. In many cases, the sintering occurs in an inert atmosphere. The 
blower 90 forces any of the combustible gas discharged from each crucible 
to flow to the right. Preferably, they flow into the region of the spark 
88 and the spark ignites, thereby combusting any discharge gases. If the 
discharge rate is somewhat erratic, the spark is applied from the spark 
power supply repetitively to keep the spark alive so that the combustion 
continues. As a generalization, the combustion adds some measure of heat 
which has a value as will be set forth. Again, if inert gases flow along 
the tube, the spark is omitted and the inert gas flow suppresses any 
combustion. A push rod 92 at the right hand end controllably forces the 
individual crucibles with the work pieces in them through the system. 
Going momentarily to the individual crucible 94, it will be shown to have 
a removable lid 96. Again, the system is discussed and illustrated in the 
context of making cylindrical drill bit inserts. The crucible 94 is 
therefore a cylindrical upstanding hollow chamber with a circular lid 
having sufficient lip to close. The lip closes at the top, thereby 
defining a chamber for receiving the particulate materials making up the 
green work piece. The rod 92 indexes the individual crucibles as they are 
introduced into the tube 80 and forces them to the left at a controlled 
rate. They move through the preheater region at 84. Then they move into 
the microwave cavity 82 and are exposed to microwave energy for sintering. 
Consider the impact, however, of the preheater stage upstream of the 
microwave cavity. As a generalization, if the loose particles requiring 
sintering are primarily metallic in nature, and that is especially the 
case in the manufacture of drill bit inserts, they reflect microwave 
energy. That is especially more severe at ambient temperatures. As the 
temperature is raised, that characteristic changes with temperature, 
thereby enabling more energy to be absorbed into the particles. As the 
temperature goes up, the reflectivity changes sufficiently that sintering 
can then be accomplished. Thus, a strip heater 86, shown in this 
embodiment, is incorporated to raise the temperature somewhat but not to 
the sintering temperature. Sintering typically is accomplished in the 
range of 1,000.degree. C., and it is not uncommon to operate the microwave 
sintering device as high as about 1450.degree. C. Focusing, however, on 
the initial preheating step, the strip heater 86 raises the temperature of 
the green work pieces from about 20.degree. C. to about 300.degree. C. to 
500.degree. C. Then, on entering the microwave cavity 82 at room 
temperature, the reflectivity is great so that more microwave energy is 
required. At higher temperature, energy is absorbed into the particles, 
and a more rapid sintering process is thus accomplished because of 
improved initial energy absorption. Considering the energy input to the 
strip heater 86 from the power supply and the energy input from the 
microwave generator into the cavity 82, this approach is much more rapid 
and efficient in the use of energy. With external heating in a furnace or 
the like, greater energy expenditures are incurred and the dwell time is 
much longer. In this particular instance, there is a net energy savings 
and the green work pieces are exposed to the energy for a shorter 
interval. This significantly changes for the better the energy 
requirements, shortening the actual high temperature interval, and yet 
providing a better sintered product. Moreover, and especially in the 
instance of forming the insert with cobalt alloy, the cobalt alloy grains 
are relatively small. The enhanced operation with the preheater just 
mentioned enables a more rapid transit time through the microwave cavity. 
Preheating to 200.degree. C. or more de waxes the green part and relocates 
the reflectivity/absorption characteristics so much so that dwell time is 
seriously and significantly improved. 
While the foregoing is directed to the preferred embodiment, the structure 
is determined by the claims which follow.