Note: Descriptions are shown in the official language in which they were submitted.
W~ ~23970PCT/US92/03797
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INDUCTION DRYER AND MAGNETIC SEPARATOR
BACRGROUND OF THE INVENTION
1. Field of the Invention
10The present invention relates to a method for
heating or otherwise treating metal objects, and more
particularly, to a method and apparatus for inductively
heating or otherwise treating metal can lids or closures
for drying, curing or other purposes, for maintaining a
spacing between them, and for motivating them along a
path.
2. Description of Related Art
Closures for metal beverage containers are generally
of a circular shape with a flanged perimeter called a
curl. The closures are usually made of aluminum or
steel, and the curl is used in attaching the closure to
a can body through a seaming operation. To aid the
integrity of the seal thus formed between the can body
and the closure, it is a common practice to apply a bead
of sealant within the curl during manufacture of the
closure. Different types of coatings are also
selectively or generally applied to can closures for
various other purpo6es as well, for example, to repair
damaged coatings.
One problem which arises in this manufacturing
operation is the curing or drying of such coatings.
Recently there has been increased interest in the use
of water-based sealants in the container industry, which
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may take up to ten days to dry to an acceptable state
for application of the closure to a can body. This was
not a severe problem for solvent-based coatings, because
the volatile solvent quickly evaporates and is
acceptably dry for application of the closure to a can
body typically within 24 to 48 hours.
In the past, can closures were heated to aid the
drying or curing process typically either by infrared
radiation or convection heating. These systems,
especially the convection heating systems, tended to be
large, bulky and expensive to operate due to inefficient
energy usage.
Metal can closures are typically conveyed into the
heat-treating apparatus in either of two ways. They can
be conveyed by a conveyor belt, in which case the
closures lie flat on the belt with coating side up, or
they can be stacked within a track or cage, in abutting
face-to-face contact with each other. In the latter
case the closures are pushed through the apparatus in a
direction transverse to their faces. The latter
arrangement is shown in U.S. Patent No. 4,3~33,246 to
Sullivan.
In both orientations, the conveyance velocity and
the length of the drying apparatus are chosen to ensure
that a sufficient amount of the water in the coating has
been driven out by the time each can closure emerges
from the apparatus. A problem arises, however, if the
production line should stop for some reason or somehow
become blocked. In this case, the can closures in the
heating apparatus would remain there longer than
originally intended, thereby overheating them and
potentially destroying them. No closed-loop mechanism
has been provided for handling this situation.
Furthermore, for IR systems and high-temperature
93~23970 213 ~ 4 PCT/US92/03797
convection systems, even if such a mechanism were
provided it would be difficult to stop the heating
process quickly enough to avoid damage. Lower
temperature convection heating systems do exist which
avoid the risk of overheating can lids simply because
they never get hot enough to cause damage, but the lower
temperatures undesirably also necessitate longer drying
times and longer conveyance paths.
Another problem with some prior-art heaters for can
closures occurs because of the speed with which the can
closures are conveyed through the heating apparatus.
Can closures are increasingly being produced at rates as
high as approximately 1,600 per minute, requiring
movement at a high rate of speed through the heater.
Especially for conveyor belt conveyance systems, it is
very easy for the can lids to fly off the belt when
moving at that speed. To avoid this, prior-art heating
apparatus typically included vacuum equipment or
permanent magnets for adhering the closures tightly onto
the belt. Such vacuum equipment can be expensive and
bulky.
The heating of certain types of metal objects by
high-frequency induction is known, but has heretofore
not been applied to the manufacture of metal can
closures. See, for example, U.S. Patent No. 4,339,645
to Miller; U.S. Patent No. 4,481,397 to Maurice; U.S.
Patent No. 4,296,294 to Beckert; and U.S. Patent No.
4,849,598 to Nozaki. While some of the systems
disclosed in these references may be usable for heating
can closures, they are not optimal. In particular, for
example, they may be very large and bulky, may require
water cooling, and may be inefficier.t due to unnecessary
wasting of flux energy. The coils in prior-art
induction heating apparatus also typically must be
- 2 1 35294
shaped very carefully in order to ensure adequate energy transfer.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide can closure
heating apparatus which overcomes some or all of the above disadvantages.
According to an aspect of the invention, there is provided a method of
drying water-based sealant compound applied to electrically conductive can ends.Can ends are provided to an inlet of a dryer, the can ends already having the
water-based sealant compound applied thereto. The can ends are transported
through the dryer in face-to-face relationship along an electrically nonconductive
0 support structure. During the transportation of the can ends, the can ends are
passed through an all~ll~Ling m~gn~tic field to induce current flow in the can ends
to induction heat the can ends and thereby heat the compound applied to the can
ends to remove water from the compound.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described with respect to particular embodiments
thereof, and reference will be made to the drawings, in which
Fig. 1 is a side view of apparatus according to the invention.
Fig. 2 is a drawing illustrating certain magnetic flux lines generated in the
apparatus of Fig. l.
2 o Fig. 3 is a top view of one of the cores shown in Fig. 1, together with one
of the can closures of Fig. 1.
Fig. 4 is a front view of another embodiment of the invention.
Fig. 5 is a top view, taken along lines 5-5, of the apparatus of Fig. 4.
Figs. 6 and 7 are a side view and a cross-section, respectively, of another
2 5 embodiment of the invention.
Figs. 8 and 9 illustrate motivational techniques according to the invention.
Figs. 10 and 11 are side views of appalalus according to the invention,
illustrating respective aspects thereof.
Fig. 12 is a top view of a portion of appala~us according to the invention,
3 o for use with a conveyor belt such as that shown in Fig. 1.
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21 35294
DETAILED DESCRIPTION
In Fig. 1 there is shown inductive drying apparatus according to the
invention. It comprises a series of
W O 93/23970 2 1 3 ~ 2 ~ ~ PC~r/US92/03797
E-shaped cores 10, 12 and 14 placed at different
longitudinal positions along the length of a conveyance
path 16. Each of the cores has a center parallel prong
20 and two outer parallel prongs 22 and 24. The cores
are spaced from each other in the longitudinal direction
by respective gaps 26 and 28, for reasons described
below. Each of the parallel prongs 20, 22 and 24 of
each of the E-shaped cores is directed toward the
conveyance path 16.
A conveyor belt 40 is positioned above the cores 10,
12 and 14, and is moved continuously forward in the
longitudinal direction of the conveyance path 16 due to
the rotation of a motor and roller shown symbolically as
42. A series of metal can closures 50 lie flat on the
conveyor belt 40 and are motivated forward along the
conveyance path 16 by the movement of the conveyor belt
40. As can be seen in the drawing, the can closures 50
are resting on their curls 52 which extend downwards
from the major surface of the closures. A bead of
coating (not shown) to be dried may be placed in these
curls prior to being placed on the conveyor belt 40.
The center prong of each of the E-shaped cores 10,
12 and 14 is wrapped with a respective coil 60, 62 and
64 of a wire 66. The opposite ends of wire 66 are
connected to an AC current source 68. The coil 62 is
wrapped in the opposite direction from coil 60, and the
coil 64 is wrapped in the opposite direction from coil
62. The reasons for the different coil winding
directions will become apparent.
Fig. 2 shows the magnetic flux paths which are
generated by the coils 60, 62 and 64 in conjunction with
the cores 10, 12 and 14, for one phase of the AC current
source 68. When the AC current source 68 is in its
positive half cycle, a magnetic field is induced in the
V~93/23970 2 1 3 ~ 2 9. ~
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coil 60 having a north pole at the free (top) end of the
center prong of the core 10 and a south pole at the
opposite end (bottom) of the coil. The cores 10, 12 and
14 are each of high-permeability, however, and therefore
have the effect of containing the magnetic flux lines
emanating from the bottom end of the coil 60 and
carrying them around to the two outer parallel prongs 22
and 24 of the core 10.
Two flux circuits are thereby created. One extends
from the north pole 20 across an air gap to the south
pole 22, around the base of the core 10 and back to the
north pole 20. The other extends from the north pole 20
across the other air gap of the E-shaped core to the
south pole 24, around the base of the core 10 in the
opposite direction, and back to the north pole 20.
Because of the shape of the core 10, the magnetic flux
lines which cross the gaps from the north pole 20 to the
south pole 24 are both generally arcuate paths. And
because of the orientation and position of the core 10,
these arcuate paths pass through the conveyance path 16
and ultimately through each of the can closures 50 as
they pass by. Accordingly, it can be said that the
shape, orientation, and position of the core 10 is such
as to, in essence, concentrate magnetic flux lines from
the magnetic field generated by the coil 60, through the
conveyance path 16. This feature of the invention
greatly improves the coupling efficiency of energy from
the AC current source 68 into the can closures 50, and
does not require any particular accuracy in the winding
or shaping of coil 60.
Additional efficiencies are obtained due to the
aforementioned longitudinal gaps 26 and 28 between the
cores 10, 12 and 12, 14. As previously mentioned, the
coil 62 is wrapped around the center prong 20 of the E-
W093/23970 ~ 1 3 ~ 2 9 ~' PCT/US92/03797
shaped core 12 in a direction opposite to the winding ofcores 10 and 14. Accordingly, whenever the outer prongs
22 and 24 of the E-shaped core 10 are magnetically
south, the outer prongs 22 and 24 of the E-shaped core
12 will be north. The same is true with respect to the
relationship between cores 12 and 14. Accordingly, in
addition to the flux paths generated across the prongs
of each individual core, an additional arcuate flux path
is generated across the nearest adjacent outer prongs of
each pair of adjacent cores.
The AC current source 68 oscillates on the order of
20 KHz, thereby causing the magnetic fields generated by
the coils and cores to oscillate at the same frequency.
This generates AC electrical currents also oscillating
at the same frequency, in the metal can closures 50 as
they move along the conveyance path 16. The figure of
20KHz is chosen as an optimum base frequency for an
optimum depth of heating of the can closures. Because
of skin effects, as is well known, lower frequencies
will induce currents deeper into the can closure,
whereas the currents induced at higher frequencies are
more shallow. Optimally, the AC current source 68 is
intelligent enough to vary the frequency by several
kilohertz in each direction in order to optimize the
energy transfer efficiency for can closures 50 of
different available sizes, shapes, material content, and
position relative to the cores 10, 12 and 14.
Fig. 3 shows a top view of one of the E-shaped cores
10, and one of the can closures 50 with which it is
intended to operate. It can be seen that the width of
the core 10, transverse to the conveyance path 16, is
wider than the diameter of the can closure 50. In
general, in order to ensure that all parts of the metal
V~93/23970 2 1 3 5 2 9 ~ PCT/US92/03797
can closure 50 are heated, the core 10 should be at
least as wide as the largest expected workpiece width.
The cores 10, 12 and 14, as previously stated,
should be made of a material of high permeability in
order to best contain the magnetic flux generated by the
coils 60, 62 and 64. The cores should also have low
electrical conductivity, in order to prevent loss of
energy through the induction of currents within the
core. Ferrite is a suitable material for these
purposes. Similarly, the conveyor belt 40 should be
made of a non-conductive material.
It can be seen that numerous variations on the
embodiment shown in Figs. 1, 2 and 3 are possible. For
example, though all three coils 60, 62 and 64 are shown
as being series connected to the same AC current source
68, some or all of them could be powered by separate
current sources instead. As another example, the cores
could be oriented differently, though still
concentrating magnetic flux through the conveyance path
16. As yet another example, though each of the cores
shown in Fig. 1 have windings wrapped only around their
center parallel prongs 20, it will be apparent that
additional windings in the opposite direction may be
placed around the outer parallel prongs 22 and 24.
Windings may also be placed around the base portions of
the cores. Other shapes or cores are also feasible.
For example, a U-shaped core would also work, as long as
it is positioned and oriented to concentrate magnetic
flux through the conveyance path 16.
The modularity of the construction of the inductive
heating apparatus shown in Figs. 1-3 offers extensive
flexibility with regard to the placement of cores. For
example, it is easy to increase or decrease the length
of the conveyance path along which the can and closures
W093/23970 PCT/US92/03797
21352~4
--10--
are heated simply by adding or removing cores. Such a
need may arise due to, for example, changes in the speed
of the production line, or changes in the water content
of the coatings to be dried. As another example, in
some situations it may be desirable to increase the
temperature of the workpieces more slowly as they enter
the heater apparatus and more quickly as they progress
downstream. This may be accomplished simply by placing
the upstream core or cores at a greater distance from
the conveyance path 16 and the more downstream cores at
a smaller distance from the conveyance path.
It will also be apparent that the invention is not
limited to metal can closures, but can also be used with
other, preferably but not necessarily flat, electrically
conductive workpieces. Many other variations will be
apparent.
Figure 12 is a top view of a different embodiment of
apparatus according to the invention, in which the cores
10, 12 and 14 are omitted. The apparatus comprises a
support 350 on which is mounted a plurality of spiral
windings facing, and arranged sequentially along, the
conveyance path 16. The conveyor belt 40 (Figure 1), on
which the can lids 50 are carried, is not shown in
Figure 12. The spirals 352 are interconnected in a
polyphase manner, in particular, every third spiral
being connected together. Three phases of the AC
current source 68 (not shown in Figure 12) are connected
respectively to the three phases A, B and C of the
spirals. As with the embodiment of Figure 1, the
embodiment of Figure 12 will generate high frequency
oscillating eddy currents in the metal can closures 50
as they move along the conveyance path 16. The use of
polyphase spirals as shown in Figure 12 is appropriate
to provide motivational forces as explained in more
V~93/23970 ~1 3 ~S 2 g I PCT/US92/03797
detail below; a single phase arrangement is all that is
necessary if motivation is provided by some other means,
such as a moving conveyor belt.
Temperature Sensing
As mentioned previously, a problem with prior-art
can-closure drying apparatus has been their tendency to
overheat and damage or destroy can closures which are
inside the heater when the production line becomes
blocked or stops for some reason. Even if a means were
to be provided to turn off the heater when the line
stops, heating can nevertheless continue for an
undesirably long period of time.
In accordance with an aspect of the invention,
closed-loop temperature control is provided for the can
lids 50. In particular, as shown in Fig. 1, a
temperature sensor 80, which may be a conventional IR
sensor, is provided adjacent the conveyance path 16 to
sense the temperature of the closures 50. Should the
temperature be higher than a predetermined temperature,
the AC current source 68 is automatically turned off.
This stops all current flow through the can closures,
thereby almost immediately preventing the closures from
becoming any hotter.
Temperature sensing can also be used as part of
closed-loop temperature control for the ordinary
operation of the inductive heater, even absent failures
such as line stoppage. For example, it is known that a
particular water-based sealant placed in the curl 52 has
been sufficiently heated to reach 98% solids within 10
minutes when the closure 50 has reached a temperature
of 150-220-F. A closed-loop temperature-sensing system
can therefore be incorporated in an induction dryer
which senses the temperature of the can closures
W093/23970 2 1 3 ~ 2 9 l~ PCT/US92/03797
individually and turns off the AC current source 68 when
each closure reaches that threshold temperature. In
this way closures of different size, thickness, position
or orientation can be accommodated, even within a
continuous stream of closures, without changing the
construction of the induction drying portion of the
production line.
Holding Means
The conveyor belt 40 of Fig. 1 typically moves very
quickly, so as to dry on the order of 1,600 can closures
per minute. At this velocity it is common for the
closures to slide off the conveyor belt 40 unless they
are held in place by some holding means. A holding
means should be included also to counteract the magnetic
repulsive forces created between the current in the
windings and the induced current in the can closures.
As previously mentioned, can closures are usually
made either of aluminum or steel. For aluminum can
closures, a holding means may be constructed which draws
air downward through the conveyor belt 40 through holes
punched therein. Such a vacuum apparatus can be
expensive and bulky, however, and it is desirable to
avoid it if possible. Accordingly, for steel (or other
ferromagnetic) can closures, the cores 10, 12 and 14 and
coils 60, 62 and 64 themselves provide the holding
means. That is, the cores are positioned and oriented
such that, in addition to inducing appropriate currents
in the closures 50, they also magnetically attract the
closures toward the conveyor belt 40. The positioning
and orientation of the cores 10, 12 and 14 should be
such that this magnetic attraction more than counteracts
the repulsive forces generated by the induced currents.
~93/23970 2 1 3 S 2 9 4 PCT/US92/03797
Alternative Embodiment
In Figs. 4 and 5 there is shown an alternative
embodiment for the inductive drying apparatus according
to the invention, which the can closures are stacked
face-to-face and pushed through the heating apparatus in
a direction transverse to the major surfaces of the
closures.
Fig. 4 shows a front view of the apparatus, and Fig.
5 shows a projection taken along lines 5-5 shown in Fig.
4. One set of the cores, namely core 120 and the cores
directly behind it, are omitted from Fig. 5 for purposes
of clarity of illustration. In the apparatus, each of
the can closures 100 in a stack rests end-wise on a pair
of guide rods 102 and 104. Two more guide rods 106 and
108 are provided to help hold the closures in place.
The four guide rods 102, 104, 106 and 108 together
define a conveyance path 110 for the stack of closures
100. Though the closures 100 are shown spaced from each
other in Fig. 5, this is only for the illustrative
purpose of showing portions of the apparatus that would
otherwise be blocked from view. In actuality, the can
lids abut each other and, if their shape permits it, are
nested with each other. In this way, the entire stack
of lids may be pushed along the conveyance path 110 by
force from only the rear end of the stack.
Located at three different radial positions around
the conveyance path 110 are a plurality of E-shaped
cores 120, 122 and 124. Each of the cores 120, 122 and
124 has a respective coil 126, 128 and 130 wrapped
around its center prong in the manner described with
respect to the apparatus of Fig. 1. The three cores
120, 122 and 124 are attached to a frame, not shown,
which also rides on the guide rods 102, 104, 106 and
108. In this manner, the three cores 120, 122 and 124
W093/23970 - PCT/US92/03797
213~294
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form a relatively self-contained module (except for the
AC current source) which can be disposed at any
longitudinal position along the length of the conveyance
path 110. These modules can also be added or removed
from an induction heater as desired according to the
changing needs of any particular production line.
Additional modularity can be obtained by including a
separate AC current source in each module.
As shown in Fig. 5, this particular induction drying
apparatus includes three modules located at three
successive longitudinal positions along the length of
the conveyance path 110. In particular, the module
immediately behind the module visible in Fig. 4 includes
cores 132 and 134 wrapped with respective windings 136
and 138. A third core positioned at the same radial
position as core 120 (Fig. 4) is omitted from Fig. 5 for
the purposes of clarity of illustration. Similarly, a
third module including cores 142 and 144, wrapped with
respective coils 146 and 148, is located longitudinally
20 behind cores 132 and 134 along the conveyance path 110.
Again, a third core located at the same radial position
as core 120 (Fig. 4) has been omitted from the drawing
of Fig. 5.
The coils wrapping the center prongs of successive
ones of the E-shaped cores along the longitudinal axis
of the conveyance path 110 are wrapped in opposite
directions, and the cores are spaced from each other for
the same reasons as described above with respect to Fig.
2. Additionally, the guide rods 102, 104, 106 and 108
are made of a non-conductive material such as plastic or
ceramic.
In operation, can closures are treated with
selective coatings and typically pushed onto the rear
end of the stack by a magnetic wheel or other means (not
1~ 93~23970 2 13 5 ?J g ~ PC~r/US92/03797
shown). The act of pushing each new can closure onto
the rear of the stack effectively pushed the entire
stack forward by the width of one can closure. Dried
closures are removed from the front of the stack at the
same rate.
As the closures pass through the AC magnetic fields
generated by the various cores and coils shown in Figs.
4 and 5, high-frequency AC currents are generated in the
closures, thereby heating them in much the same way as
described above with respect to the apparatus of Fig. 1.
Figs. 4 and 5 also illustrate an additional feature,
namely that the cores can be shaped at the ends of the
prongs similarly to the shape of the workpiece, in order
to maximize the amount of flux which passes through the
can lids. This feature is illustrated by the arcuate
shape of the ends 150, 152 and 154 of the prongs of the
E-shaped cores 120, 122 and 124, respectively.
As with the apparatus of Fig. 1, an AC current
source 168 is included with the apparatus of Figs. 4 and
to cause the currents through the windings to
oscillate at approximately 6-20KHz. Additionally, an IR
temperature sensor 180 may be included for closed-loop
temperature control to turn off the AC current source
168 if and when the temperature of the lids increases
beyond a predetermined threshold. The remainder of the
considerations and variations described above with
respect to the apparatus of Fig. 1 also apply to the
apparatus of Figs. 4 and 5.
Additional Embodiment
As can lids or other substantially plate-like
objects are moved through a drying Gr curing apparatus,
it is desireable to keep them separated from each other
to permit air to access all parts of the workpiece.
W093/23970 2 1 3 5 2 ~ ~ PCT/US92/03797
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Sullivan U.S. Patent No. 4,333,246, mentioned above,
describes one technique for separating a series of
workpieces being pushed along a track in face-to-face
relationship in a direction transverse to the major
surfaces of the workpieces. In Sullivan, the workpieces
are pushed through a curvilinear path defined by a
constant width trackwork, allowed to pivot on the
portions of the workpieces in proximity to the shorter
radiuses whereby fan-like separation of the portions in
proximity to the longer radius occurs. Sullivan uses
this trackwork to partially separate can lids as heated
air is directed toward the separated portions.
The Sullivan technique has a number of major
disadvantages. First, though one portion of each of the
workpieces is separated from the other workpieces, there
is always another portion of the workpieces (the
portions in proximity to the shorter radiuses) which are
touching other workpieces. The pieces are only fanned,
not truly separated. Thus, if the apparatus is being
used to cure selectively applied coatings on can lids,
for example, it can be used only where the selectively
applied coating has been applied somewhere other than
around the circumference where the lids are likely to
touch each other. Additionally, the pressure on the
portions of the lids which do touch each other, caused
by the forces pushing the lids along the track, can
soften and/or damage the metal of the lids or their
coating. Moreover, the Sullivan apparatus can generate
only limited separation between the fanned portions of
the can lids, since greater separation requires tighter
curves in the trackwork, which in turn requires greater
force and stronger materials in the equipment which
pushes the lids along the track. Nor can the technique
be used for long conveyance paths, for the same reason,
~93/23970 2 1 3 5 2 ~ ~ PCT/US92/03797
-17-
even if the curves are kept shallow. Still further,
Sullivan's technique will not work well with can lids
which have pull rings, since these can lids do not nest
well and are likely to scratch each other if they touch.
It is well known that a plurality of magnetic
objects free to move within a magnetic field, will
spread out to share the entire available magnetic field
equally. However, this technique has not heretofore
been used in apparatus that heats metal beverage can
lids, since in the past, expensive magnetic materials
with very high curie temperatures would have been
required.
Figure 6 shows a side view, partially cut away, of
inductive heating apparatus which uses permanent magnets
for maintaining a separation between steel (or other
ferromagnetic) beverage container lids 100. Figure 7
shows a cross-section of the same apparatus. The can
closures 100 rest end-wise on a pair of guide rods 202
and 204, and two more guide rods 206 and 208 are
provided to help hold the closures in place. The four
guide rods 202, 204, 206 and 208 together define a
conveyance path 210 for the stack of closures 100. The
guide rods 202, 204, 206 and 208 are oriented axially at
different circumferencial positions along the inside
surface of a guide tube 220. Both the guide rods and
the guide tube are made of non-electrically-conductive
material such as ceramic or teflon. The tube 220
preferably should also be thermally insulating, for
reasons which will become apparent below.Guide rods 202,
30 204, 206 and 208 can be omitted in some embodiments,
their function being replaced by the tube 220 itself.
Mounted on the outside surface of the guide tube 220
is inductive wiring 222 which is connected to an AC
current source 68, such as that shown in Figure 1. The
W O 93/23970 21 3 a 2 9 q P~r/US92/03797
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wiring 222 comprises four parallel regions of spirals
223, each region subtending a little less than one
quarter arc on the circumference of the tube 220 and
extending along substantially the entire length of the
tube 220 within which heating is desired. Various well
known techniques can be used to satisfy electronic
switching requirements in the power supply and permit
higher current carrying capacity in the wiring. The
wiring 222 can also be provided as a series of axially
adjacent wiring sections if desired for modularity or
other purpo B es.
It should be noted that instead of spirals 223, the
wiring 222 can be provided as a single, many-turn coil
(not shown) wrapping the tube 220. The magnetic forces
induced by this arrangement, however, tend to rotate the
can lids about a diameter, making it difficult to keep
their faces oriented transversely to the direction of
the conveyance path. Also, such an arrangement tends to
heat the permanent separator magnets, discussed below,
undesirably.
The tube 220 has holes such as 224 at various
positions along its length for ventilation of the can
lids inside. Air can be circulated through these holes
to provide for moisture scrubbing, cooling or otherwise
treating. The spirals 223 are wound to avoid these
holes 224. This affects the AC magnetic induction field
inside the tube at that point, but the overall heating
process is not significantly affected since the wiring
still extends substantially the entire length of the
tube 220 which is being used for inductive heating.
Located within the gaps between the four regions of
spirals 223, and oriented longitudinally along the
length of the tube 220, are a plurality of rail magnets
230. Only one of the rail magnets 230 is shown in
~93/23970 2 1 3 ~ 2 9 ~ PCT/US92/03797
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Figure 6, for illustrative simplicity. The permanent
magnets 230 are oriented to provide alternating magnetic
north and south poles around the circumference of the
tube 220. Four permanent magnets 230 are shown in
Figure 7, but any number greater than 1 may be used.
Also, the permanent magnets 230 may each run the length
of the tube, or they may be provided in axially adjacent
segments for modularity or other purposes.
The apparatus of Figures 6 and 7 further includes a
vibrator 240 (shown only in Figure 6), which
mechanically vibrates the permanent magnets 230 axially.
In operation, when a particular number of can lids
100 are inside the tube, they will try to equally share
the magnetic fields generated by the permanent magnets
230 along the length of the tube. Friction is overcome
by the mechanical vibrator 240, which vibrates the
magnets 230, and therefore the magnetic fields generated
by them, axially. The vibration frequency may be on the
order of 60Hz, and the wavelength should be shorter than
the spacing between the lids. Vibration can be achieved
instead by other methods, such as by mounting the guide
rods 202, 204, 206 and 208 on flexures and vibrating
them axially, or by using the force oscillations
inherent in the reversing field of the coil 222.
Another alternative would be to wrap a coil (not shown)
around the tube 220 to provide a more slowly oscillating
magnetic field specifically for vibrating the can lids
100. Vibrations would also be effective if transverse
to the direction of travel.
With the can lids inside the tube 220, and spaced
apart by the magnetic fields generated by the permanent
magnets 230, a high frequency AC current is provided to
the wiring 222. A high frequency AC magnetic field is
thereby generated in each of the can lids inside the
W 093/23970 2 1 3 5 2 ~ '1 PC~r/US92/03797
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tube 220, which generates eddy currents to heat and dry
them.
It can be seen that though high temperatures are
induced in the can lids 100 themselves, the wiring 222
remains cool. Water cooling of a few-turn induction
coil is not necessary. Also, since high temperatures
are generally restricted to the lids 100 themselves, and
since the permanent magnets 230 are substantially
outside the fields generated by the spirals 223, the
permanent magnets 230 may be inexpensive ceramic magnets
instead of expensive magnets made of a high-curie-
temperature material. It should also be noted that
though permanent magnets 230 are shown in Figures 6 and
7, AC or DC electromagnets may be used instead to
accomplish spacing.
As long as no other forces are applied, the can lids
100 in the tube 220 will simply space out to share the
field generated by the permanent magnets 230. A
motivating force or motivating means further may be
provided to move the lids longitudinally along the path
of travel 210. One way to apply such a force would be
to tilt the tube such that the entrance end is higher
than the exit end. This method uses gravity to skew the
distribution of can lids along the length of the tube,
so that they are spaced more closely together as they
move toward the exit. When the lids reach some maximum
packing density at the exit, the magnetic fields
generated by the permanent magnets 230 will no longer be
strong enough to overcome the gravitational tendency of
the lid which is closest to the exit to fall out of the
tube. Accordingly, for a given number of can lids
desired in the tube at once, and for given magnetic
field strengths generated by the spacer magnets, a tilt
angle can be determined at which whenever one lid is
~ n 93/23970 21 3 ~~ 2 9 4 Pc~r/US92/03797
added at the entrance of the tube, another lid falls out
the exit. Thus a continuous flow of lids through the
induction dryer can be maintained.
The lids 100 can be motivated through the tube 220
also by other means, such as by mechanically removing a
lid from the exit of the tube each time a new lid is
added to the entrance. For example, Figure 8 shows an
upstream conveyor belt 2S0 transporting can lids 100 to
a magnetic upstacker 252, which periodically adds a new
can lid 100 to the entrance of the tube 220. Each time
such a new can lid is added, a magnetic downstacker 254
removes the can lid then at the exit of the tube 220,
and places it on a downstream conveyor belt 256 for
further processing. Each time one lid is added to the
entrance and another lid is removed from the exit, the
remainder of the lids inside the tube automatically
readjust their longitudinal positions to equally share
the magnetic field generated by the permanent magnets
230 (not shown in Figure 8). A rotating knife (not
shown) may also be used instead of the downstacker 254
to remove individual can lids from the exit end of the
tube 220.
Another method for motivating the can lids along the
conveyance path 210 in the tube 220 is to cause them to
move as if part of a linear induction motor. If the
spirals 223 are connected in, for example, three phases,
and three phases of the AC current source 68 are
provided, then assuming the spirals are properly spaced,
a given one of the can lids 100 will be repeatedly
attracted to the next downstream spiral and repelled
from the previous spiral as the phases of the current
source 68 rotate. The spirals 223 can be connected with
a displacement of any desired number of turns.
W093/23970 PCT/US92/03797
2135294
-22-
Alternatively, a motivating means can be provided by
adding a separate polyphase motivating coil, wrapped
around the tube 220, for motivating the lids 100 along
the conveyance path inside the tube 220. A three-phase
(A,B,C) motivating coil 260 is shown in Figure 9. The
motivating coil 260 can operate at a lower frequency,
for example 60Hz. A separate motivating coil is
disadvantageous in that it requires additional wiring,
but it is advantageous in that the functions of heating
and motivating are kept inductively independent. Thus
the can lids may be kept moving by a separate motivating
coil such as 260 even through a portion of the tube 220
within which inductive heating is not desired. Such a
feature is useful in repair coat dryers, for example, in
which can lids may be moved through an inductive heating
portion of the tube, followed by a hot air soak portion
of the tube, followed by a cool down portion of the same
tube. In such a system, one portion of the tube might
be wound with motivating coil 260, and only the
inductive heating portion of the tube provided with the
induction wiring 222.
A motivating coil should not be used in the same
portion of the tube 220 in which inductive heating will
take place, since the magnetic fields generated by the
inductive wiring may induce undesired currents in the
motivating coil and vice versa.
Any of the above described motivation techniques can
be aided, if desired, by strategic placement of the
separator magnets 230. For example, in Figure 10, two
of the permanent magnets 230 are shown slanting away
from the tube 220 toward the exit end thereof. This
reduces the separating magnetic field within the tube
at the exit end, and thereby permits the lids to space
themselves more densely toward the exit end of the tube.
213;~29~
93/23970 PC~r/US92/03797
-23-
This technique for controlling the density of the lids
100 at various points along the length of the tube 220
may be used as desired for any purpose. For example,
the technique might be useful if it in any way
S simplifies the process of removing can lids from the
exit end of the tube.
The invention permits significant flexibility in the
design of can lid processing equipment. For example,
since the permanent magnets 230 (Figures 6 and 7) do not
need to have a high curie temperature, they can be made
of a flexible material. This permits the use of a
curved tube 220, such as that shown in Figure 11. The
tube 300 in Figure 11, though mainly horizontal, curves
90 at the entrance to form a vertical uptake. The
entrance of the tube 300 is disposed directly above a
conveyor belt 302, which carries the can lids 100 into
position. The can lids are individually attracted into
the tube 300 by permanent magnets 304 (only two of which
are shown in Figure 11), which follow the curve of the
tube 300. An inductive wiring such as 222 (Figures 6
and 7) may be provided on the tube 300, or on only a
portion thereof as shown in Figure 11. This technique
effectively obviates any necessity for an upstacker. A
similar curve at the exit of the tube 300 can obviate
any need for a downstacker.
Aluminum can lids and bodies, since they are not
ferromagnetic, probably cannot be magnetically spaced by
spacer magnets such as 230 (Figures 6 and 7). However,
since they do conduct eddy currents induced in them by
wiring on the outside of the tube 220, aluminum can lids
nevertheless are subject to induction heating by the
wiring 222. The motivational features of the invention
also apply to aluminum workpieces, since the eddy
currents induced in the workpieces generate a magnetic
W093/23970 2 1 3 5 2 9 4 PCT/US92/03797
-24-
field oriented repulsively to the magnetic field
generated by the wiring 222. Thus the workpiece and the
wiring 222 form a repulsive linear motor, propelling the
workpiece longitudinally along the inside of the tube
220. Moreover, whereas for ferromagnetic workpieces,
the magnetic attraction of the workpieces to the spirals
223 may be so strong as to counteract the magnetic
repulsive forces generated, this is not true with
aluminum can lids. Thus, aluminum workpieces will be
repelled inwardly from all sides of the tube with
substantial uniformity, forcing it into the middle of
the tube and thereby minimizing friction as the
workpiece is propelled longitudinally. This minimizes
the need for a vibrator such as 240. Aluminum
workpieces can also be propelled by a polyphase linear
propulsion motor formed with a polyphase winding such as
that shown as 260 (Figure 9).
The invention has been described with respect to
particular embodiments thereof, and numerous variations
are possible within its scope.
