Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
7~
B~CKG~OUND OF_THE INVENTIUN
I~ is known to form sealed packages from flexible
plastic film ~y sealing the film wit'n heat and pressure.
Some of the more important techniques to seal plastic film
include:
1) "hot bar" or "jaw" sealing,
2) "impulse" sealing,
3) "rotary" sealing or "roller welding".
Hot bar or jaw sealing is a process in which
plastic film is sealed under pressure between sealing bars
(or "jaws"). At least one of the sealing bars is heated,
usually with electric power. Normall~, either the
temperature or the electrical power supplied to the
apparatus is controlled. An apparatus supplied with a
controlled, constant source of power may be described as a
"constant heat" sealing device. Alternativelyg the
temperature of the sealing bar may be the parame~er t'nat
is controlled, and the sealing appara~us is thPn termed a
"constant temperature" sealing apparatus. When the
temperature is the controlled parameter, a sealing device
of the prior art normally incorporates a separate
temperature sensing device, such as a thermocouple, and a
separate temperature controller. The ~h~rmocouple senses
the temperature of the sealing bar and provides a feedback
signal to the temperature controller. There is a time lag
inherent in such a control scheme which tends to limit its
effectiveness. In order to dampen the effect of the time
lag, the sealing bar is often designed to be of
substantial mass ~such as a heavy stèel bar). This causes
the heated bar to function as a radiator, wasting heat
into the working environment of the opera~or of the
sealing device.
Impulse sealing is a film sealing process in
which layers of plastic film are subjected to an impulse
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of heat while being sealed under pressure betwe~n sealing
bars. Heat is provided to the sealing bars only for a
short period during the sealing cycle. One use of an
impulse sealing device is in a packaging apparatus used to
package bales o~ synthetic rubber with flexible plastic
film. Using this apparatus, bales of synthetic rubber are
pac~aged between two layers of plastic Eilm by sealing the
layPrs of plastic film together with sealing bars
constructed ~rom me~al tubing approximately 1/4" - 1/2" in
dia~eter. An impulse of high current electricity is
passed through the tubing ~o provide the heat used for
sealing. A-fter the seal is formed, the sealing bars are
cooled by flushing a cooling fluid, such as water or air,
through tlle tubing. A large amount of energy is wasted
into the cooling fluid and thus this apparatus is not
highly energy efficient. Furthermore, the apparaLUS is
prone to temperature fluctuations due to changes in the
quality of the cooling fluid or changes in the ambient
environment. The apparatus also is known to severely
overheat if problems are encountered with the supply of
cooling fluid.
Roller sealing or roller welding incorporates the
use of pressure rollers to seal plastic film. The plastic
film may be heated prior to being passed through the
rollers or alternatively the rollers may be heated.
Temperature control systems, if incorporated into a rotary
welding apparatus, normally contain an external
thermocouple and feedback system.
The above film sealing techniques all require the
application of heat and pressure to the plas~ic film to
form a seal. The sealing devices are usually constructed
from heavy materials capable of withstanding the rigorous
heat and pressure conditions used in heat sealing
processes.
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The properties o~ the plastic used in the ~ilm
can ~reatly a~fec~ the sealing operation. It is known
that such characteristics as the molecular weight and
crystallinity of the plastic af~ect the temperature at
which the plastic ~ilm softens and the tem-perature at
w~ich the plastic film degrades sufficiently to cause the
film to fail. There is a ran~e of temperatures suitable
for sealing plastic film, above t'ne temperature at which
the plastic softens but below the temperature at which the
plastic degrades. If tle sealing process is undertaken at
too low a temperature, a proper seal will not be Eormed
and the improper seal will break apart. If the sealing
temperature is too high, the plastic fi'lm will s~ick to
the sealing bar and/or be degraded rather than sealed.
The sealing time is also important. It is known
that longer sealing tim~s are generally required at lower
sealing temperatures. Alternatively stated, for a given
sealing temperature and pressure, there is a minimum
sealing time that is required for the film being sealed,
or else the ~ilm will not become properly sealed.
The productivity o~ a film sealing apparatus, as
measured by the number of seals formed, may obviously be
improved by reducing the time required to form a seal.
However, a reduction in the sealing time normally must be
accomp~nied by an increase in the sealing temperature
because it is known that time required to seal the plastic
is affected by the temperature of ~he sealing operation.
The increase in sealing temperature which accompanies the
reduction in sealing time causes the sealing operation to
be conducted at a temperature closer to the degradation
temperature of the plastic. It is therefore apparent that
excellent temperature control is required at minimum
sealing times to ensure that the film is 'heated enough to
form a seal but not subjected to a temperature that will
degrade the plastic. Thus i~ is apparent that a film
sealing apparatus that provides excellent control of the
sealing temperature should be a useful addi~ion to the art.
OBJECT OF THE II~NENTION
It is an object of the invention to provide an
improved apparatus for sealing flexible plastic polymer
wrapping film. The improvement relates to the excellent
temperature control achieved by the present apparatus.
SUMMARY OF THE INVENTION
__
There is provided an apparatus for sealing
flexible plastic polymer wrapping film, which comprises:
(i) means for supplying an upper layer and a lower
layer of said wrapping film to a sealing station;
(ii) means for film sealing comprising separately an
upper component located above the upper layer o-f
film and a lower componen-~ located below the
lower layer of filmj
(iii) wherein said upper component comprises a sealing
bar, said sealing bar comprising a frame support,
a high temperature resistant polymeric layer
detachably attached to tne bottom of said
support, a circular cross-section hea~ing wire
detachably attached to essentially the centre of
the bottom of said polymeric layer, and a high
temperature resistant covering tape detachably
at~ached to the frame support and covering the
polymeric layer and the heating wire;
(iv) wherein said lower component of said film sealing
means comprises a seal forming bar, sald seal
forming bar matching the size and shape of said
sealing bar of said upper component, said seal
forming bar comprising a frame support and
located on or within said frame support a high
temperature resistant low compression set rubber
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bar,
(v) means for supplying electrical power to said
heating wire of said upper sealing bar, ~he
electrical power supplied being controll2d by a
controller to maintain an essentially constant
pre-set tempera~ure of said heating wire by
proportional supply of power in response to the
electrical resistance of said heating wire;
(vi) means for moving said upper component of said
film sealing means downwardly into contact with
said lower component whereby the upper layer of
film is ~rought into contact with the lower layer
of film a~ the area for forming the seal, whereby
the two layers of wrapping film are sealed
together.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE l is a film sealing device according to
the prior art;
FIGURE 2 is a side view of a film sealing0 apparatus according to ~he presen~ invention;
FIGURE 3 is a side view of a preferred embodiment
of the present invention;
FIGURE 4 is a top -view o-f the upper sealing
component of a preferred em`bodiment of the present
invention;
FIGURE 5 is a sectional view of the transve:rse
sealing bar of the upper component of a preferred
embodiment of the present invention;
FIGURE 6 is a top view of the lower sealing
component of a preferred embodiment of the present
invention;
FI&URE 7 is a sectional view of ~he transverse
sealing bar of the lower component of a preferred
embodiment of the present invention.
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DETAILED DESCRIPTION OF THE _NVENTION
The heat for the sealing apparatus of the present
lnvention is provided by a controlled temperature
electrical resistance heating wire. By careful choice of
the material of constructlon, the electrical resistance of
the wire in the heating wire is a known function of the
temperature of the heating wire. rne electrical
resistance of the heating wire is used as a control signal
by a controller whlch controls the electrical resistance.
As the elec~rical resistance is a known func~ion of the
temperature, the temperature oE the wire may be indirectly
controlled by using a suitable controller to directly
control the electrical resistance. Thus the control
signal to the controller comes directly from changes in
the electrical resistance of the heating wire. This
eliminates the use of an e~ternal sensing device, such as
a thermocouple, to provide control signals. By
eliminating the requirement for a separate temperature
sensing device~ excellent feedback response to the
controller is obtained such that the temperature of the
heating wire may be controlled to an essentially constant
temperature.
~ le heating wire may be constructed -from a
relatively light gauge wire. This is an advantage as the
heating wire has a relatively low radiant heat loss in
comparison to the more massive steel sealing bars used in
the sealing devices of ~he aforesaid prior art.
The use of a change in electrical resistance to
provide a control signal for a controlled temperature wire
30 heater is known. For example, ~atlow Winona, Inc.
manufactures a controlled temperature heating wire of ~his
type, which is suitable for use in the appara~us of the
present invention. This controlled temperature heating
wire device has been previously used to trace piping to
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prevent ~e pipes from freezing in cold ~eather. It 'nas
also been used ~o control the ~emperature oE fluid
~ontained in a storage vessel, but :it has not previously
been used in an apparatus to seal plastic film.
One type of con~rolled temperature heating wire
manufactured by Watl~w Winona Inc. is capable of
maintaining temperature control in the heater wire to
within about +1/2% of the temperature scale of the
heater. A heater of this type with a 0 to 600~F
~o temperature scale is suitable for the apparatus of the
presellt invention.
The apparatus of the presen~ invention may be
used to form seals between two layers of flexible plastic
film by bringing the layers o~ film into contact with one
anotl~er between the sealing components of the apparatus
under suitable conditions of pressure and temperature.
Suitable conditions of pressure and temperature are those
conditions which form a satisfactory seal during t'ne time
period of the sealing step.
Suitable conditions for specific film are
normally not known to the operator of the apparatus until
some trial and error sealing step attempts have been made.
The apparatus of the present invention allows the
operator to establish suitable conditions as follows. The
sealing time and pressure are set by the operator and then
are held constant for each seal forming step. (The
pressure and sealing time controls of sealing machines
known in the art are adequate to provide reasonably
reproducible sealing time and pressure between different
seal forming steps). The operator estimates a suitable
sealing temperature for the film being sealed and sets ~he
temperature o the controlled temperature heating wire of
the present apparatus accordingly. ~1 attempted sealing
step is then made. If the seal is too weak or incomplete,
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the opera~or increases the sealing temperature ~or the
next attempt by increasing the temperature of t~e
controlled temperature heating wire. Conversely, if the
plastic film is degraded, the operator decreases the
sealing temperature for the next attempt by decreasing t1ne
temperature o-f the controlled ~emperature sealing wire.
It will be apparent that adjustable tempera~ure
control of the sealing temperature will thus help ~o
establish suitable sealing conditions. It will be further
apparent that reproducible sealing control, in cooperation
with reproducible time and pressure control, will
contribute to the formation of reproducible sealing steps.
An apparatus used to seal plastic film is subject
to strenuous operating conditions due to the heat and
pressure required for forming seals. Thus i~ is necessary
to protect the heating wire from being damaged during the
sealing process. Through experimentation, it has been
determined that the heating wire used in the apparatus of
the present invention may be reasonably well protected
from damage by using a low compression set rubber mat~rial
in the construction of at least one of the sealing bars.
Figure 1 illustrates a schematic drawing showing
a film sealing device of the type known in the art.
~ayers of plastic film (1 and 1') are sealed between
heated sealing bars (2) and (3).
Figure 2 is a schematic drawing showing a side
view of one embodiment of the apparatus of the present
invention. Two layers of flexible plastic polymer
wrapping film (10 and 10') are supplied to the sealing
station9 one supplied below the upper sealing component
and the other supplied above the lower sealing component.
The upper seallng component is a sealing bar which
comprises a frame support (11), a high temperature
resistant polymeric layer (12) detachably attached to the
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bot~om of said support (11), a circular cross-section
heating wire (13) detachably attached to essentially the
centre of -~'ne bottom of said polymeric layer (12) and a
high temperature resistant covering tape (14) detachably
attached to said frame support (11) and covering sald
polymeric layer (12) and said heating wire (13).
Polytetrafluoroethylene (PTFE) and fiberglass are suitable
materials of construction Eor said high temperature
resistant tape (14). Fiberglass bar stock is a suitable
material of construction for said high temperature
resistant polymeric layer (12).
Said heating wire (13) is connected to means t~
supply electrical power such that the electrical power
supplied to said heating wire (13) is controlled by a
controller (17) to maintain an essentially constant
pre-set temperature of said heating wire (13) by
proportional supply of power in response to the electrical
resistance of said heating wire (13). The temperature of
said heating wire (13) may be adjusted by adjusting the
set point on the controller ~17).
The lower sealing component matches the size and
shape of said upper sealing component. Said lower sealing
component is a seal forming bar which comprises a frame
support (16) and located on or wlthin said frame support a
high temperature resistant low compression set rubber bar
~15). Silicone rubber is a preferred material of
construction for said bar (lS). It is preferred that said
lower frame support (16) and said upper frame support (ll)
be constructed from a material of sufficient stiffness to
prevent gross distortion during the sealing process~
Metal channel, such as steel or aluminum channel, is an
excellent material of construction for said frame support
(11 and 16~.
It should be apparent to a person skilled in the
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art that the relatively simple procedures used to
construct the sealing bars of the present apparatus have
several advantages, including ease of construction and
ease of repair.
Suitable means are requi-red to move the sealing
bars together, thus sealing the layers of plastic film
together with heat from the controlled temperature heating
wire ~13) and pressure from the sealing bars. It is
apparent to a person skilled in ~he art that the upper
sealing bar may be moved downwardly or the lower seal
forming bar may be moved upwardly or the two sealing bars
may both be moved towards each other to achieve the
desired result. Known means to move the sealing hars
together are suitable for the present apparatus. For
example, the mechanical means used to move the sealing
components of a known device to package synthetic rubber
are suitable for the present apparatus.
It ~ill be apparent to a person skilled in the
art that ~he sealing device of the present invention may
be readily incorporated into some of ~he sealing devices
of the prior art. This may be done, for example, by
replacing the "jaws" o~ a jaw sealer or the impulse bars
of an impulse sealing device of the prior art with the
aEoresaid upper sealing component, lower sealing component
and temperature controlled heating wire of the present
invention. Thus, a sealing device of the prior art may ~e
retrofitted with the present invention to produce a
sealing device with e~cellent temperature control. The
relatively simple construction procedures used to
fabricate the present sealing device allow such
"retrofits" ~o be easlly completed.
Figures 3, 4, 5~ 6 and 7 illustra~e a preferred
embodiment of the present invention which is suitable ~or
enclosing produc~ into a sealed package o~ flexible
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77
plastic film. Bales oE synthe~ic rubber may suita'bly be
packaged with this apparatus.
Figure 3 illustrates a side view of said
apparatus. Suitable means (20 and 20') supply 1exible
plastic film to the wrapping station. The lower layer of
plastic film (21) is supplied 'below the product to be
wrapped but above the lower sealing component and the
upper layer of plastic film (22) is supplied above the
location of the product to be wrapped but below the lower
sealing component.
Conveyor means (23) convey the product to the
wrapping station such tha~ the product is conveyed to a
position located essentially centrally above the lower
layer of film. (Conveyor means 23' convey the package
forwardly out of the wrapping station.)
~ e upper sealing component comprises,
sequentially in the dlrection of movement of the product
to be packaged, a transverse sealing bar (24) of
suEficient leng~h to provide ~he forward seal of the
sealed package, and spaced forward of said transverse
sealing bar (24) a wrapping film cu~ting means (25) and
spaced forward of said cutting means a second sealing bar
(26) shaped such as ~o provide the remainder of the seal
in con~unction with said forward seal. The sealing bars
are further described below.
The lower sealing component comprises,
sequentially in the direction of movemen~ of the product
to be packaged, a ~ransverse seal -forming bar (27) and
forward of sa~d transverse seal forming bar (27), a second
seal forming bar (28), 'both said seal forming bars
matching the size and shape of said sealing bars of said
upper sealing componen~.
Means are provided (29) fo-r moving said upper
component of said apparatus downwardly into contact with
said lower component whereby the upper layer of film is
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brought about ~he product to be wrapped and into contact
with the lower layer of film at the area for forming a
seal, whereby the two layers of wrapping film are sealed
together about the product to be wrapped. Known means for
moving said upper component are sui~able for the apparatus
of the present invention. For example, the means for
moving the components of t'ne film sealing devic~s of the
prior art are suitable for the present apparatus.
Figure 4 is a top view showing only the upper
sealing component. The upper sealing component comprises
a transverse sealing bar (24), a second sealing bar (26)
and film cutting means (25). ~e transverse sealing bar
(24) is of sufficient length to provide the forward seal
of the sealed package. The plastic film cutting means
~25) are of a type well known in the art. A hot wire is
suitable for said plastic film cutting means.
Figure 5 is a view through a section of the
transverse sealing bar (24) of the upper component at the
plane represented by a-a' in Figure 4. Figure 5 also
represents a view through a section oE the second sealing
bar (26) of the upper component at the plane represented
by b-b' in Figure 4. A high temperature resistant
polymeric layer (34) is detachably attached to a frame
support (35). Fiberglass bar stock is suitable for said
polymeric layer (34). Heating wire ~36) is detachably
attached to essentially the centre of the bottom of said
polymeric layer (34). A high temperature resistant tape
~37) is detachably attached to the frame suppor~ (35) and
covers the polymeric layer (34) and the heating wire (36).
The electrical resistance of the temperature
heating wire (36) is a known function of the temperat-ure
of the heating wire (36). The heating wire is connected
to means to supply electrical power such that the
electrical power supply is controlled by a controller to
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maintain an essen~ially constan~ pre-set temperature in
said heating wire ~35) by proportional supply of
electrical power in response to the electrical resistance
of said heating wire (36). The heating wire temperature
may be adjus~ed by adjusting the se-t point temperature on
tne controller.
Said frame support is constructed from a mat~rial
of suitable s~iffness to prevent gross distortion during
the sealing operation. Metal c'nannel, such as steel or
~luminum channel, is a preferred material for said frame
support (35). PTFE or fiberglass is a suitable material
o construction for said high temperature resistant tape
(37).
Figure 6 is a top view of the lower sealing
component of Figure 3. The lower sealing component
comprises a transverse seal -forming bar (27) and a second
seal forming bar (28). The seal forming bars of the lower
sealing component match the size and shape of the sealing
bars of the upper sealing component. Thus the transverse
seal forming bar (27) of the lower sealing component
cooperates with ~he transverse sealing bar of the upper
sealing component to provide the forward seal of a sealed
package. Similarly, the second seal forming bar (~8) o-f
the lower sealing component cooperates with the second
sealing b~r o the upper sealing component to form a seal
which is the remaining part of a sealed package (in
conjunction with the forward seal).
Figure 7 is a cross-section view through
transverse seal forming bar (27) o the lower component at
the plane represented by y-y' in Figure 6. Flgure 7 also
represents a cross-section view ~hrough the second seal
~orming bar (28) of the lower sealing component at the
plan represented by z-z' in Figure 6. A high temperature
resistant low compression set rubber (43) is detachably
attached to support frame means (44). Low compression set
silicone rubber is a suitable material for said layer of
high temperature resistant material (43). Said support
~rame means ~44) is constructed ~om a material of
sufficient stiffness to prevent gross distortion during
the application of pressure during the sealing process.
~letal channel, especially steel or aluminum channel, is a
suitable material of construction for said support ~ame
means (44).
The film sealing apparatus shown in Figures 3, 4,
5, 6 and 7 may be conveniently described as a film
wrapping station. This film wrapp;ng station may be used
in a process to enclose a produc~ into a sealed package of
flexible plastic polymer wrapping film.
The process of forming an enclosed package with
said film wrapping station starts with t'ne formation of a
straight seal in the film. The product is then introduced
into tl~e wrapplng sta~ion. A second seal, which
cooperates with the initial straight seal to form a
20 package which encloses the product, is then formed. The
film is cut between the enclosed package and the straight
seal and then the package is removed from the wrapping
station.
The process may be e~plained in more de~ail by
referring to Figure 4. The shape of the seals formed
during each sealing step corresponds to the shape of
sealing bars 24 and 26. T~e film is cut be~ween the seals
by film cutting means 25 during each sealing step.
One set of seals is formed before the first
30 package is made. Two layers of plastic film are inserted
into the film wrapping station in the direction moving
from right to left (with reference to Figure ~) and then
the layers of film are sealed. The film to t~e let of
film cut~ing means 25 is removed from the film wrapping
'~
station, but thP straight seal formed in the s'nape of
sealing bar 24 remains. Thus, the first set of seals
formed produces only one useful seal, namely the straight
seal .
Subsequently formed seals are all utllized, as
explained below. Product is then conveyed into the film
wrapping station, again moving in a right -~o left
direction (with reference to Figure 4). The product
pushes against tne straight seal, thus drawing the two
10 lnyers o wrapping film into the wrapping station suC'LI
that the straight seal is -forward of the product. Thus
the straig~t seal formed during the previous sealing step
becomes the Eorward seal of the sealed package. Seals are
then formed wi th the product in the wrapping s tation . The
seal shaped according to sealing bar 26 cooperates wit'n
tl~e straight seal formed during the previous step to form
a package which encloses the product. Simultaneously, a
straight seal is formed, behind the package, in the shape
oE sea`ling bar 24. This straig'nt seal will become the
20 forward seal of the next package. The :Eilm cutting means
~5 cuts the film between the package and the straight
seal. The enclosed package is then forwardly removed from
the film wrapping station, leaving behind the straighc
seal. Conveyor means are suitable for removing -the
enclosed package from the wrapping station.
It is important to note tnat the s traight seal
formed during each sealing cycle becomes the orward seal
for the following enclosed package. Means are t~erefore
required for bringing the two layers~of film having the
30 forward seal formed into the wrapping station. This may
be simply done by conveying the product into t'ne wrapping
s tation such that the product pushes agains t the s trai ght
seal and the movement of the product draws the two layers
of film into the wrapping station such that the straight
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seal ls forward of the product.
Polyethylene is a commonly used material for said
flexible plastic polymer wrapping ~ilm. Other sui~able
materials include poly(ethylene vinyl acetate),
polystyrene and polybutadiene.
Example
The film sealing apparatus illustrated in Figures
2, 3, 4, 5 and 6 is used in a process to package synthetic
rubber with flexible plastic film.
To start the process, two layers of flexible
plastic polyethylene film, approximately 1.7 mil thick,
are provided to the wrapping station. The temperature of
the controlled ~emperature heating wire is set at 175C.
The upper and lower sealing components are brought
together so that a pressure of approximately 40 pounds per
square inch is exerted between the sealing bars, for a
period oE 0.5 seconds, thus sealing the film. The cutting
means cuts the sealed plastic film between the seal formed
by the transverse sealing bars and the seal -Eormed by the
second sealing barsO ~le result is that the film is
sealed together only at the seal formed by the transverse
sealing bar. The plastic film ahead of the seal formed by
the transverse bars is removed by the operator of the
packaging apparatus, thus leaving the two layers of
plastic film ~oined together by the seal formed by the
transverse sealing bars.
A bale of rubber is then conveyed into the
wrapping station. Ihe rubber bale pushes the film into
the wrapping station such that ~he seal from ~he initial
step becomes the forward seal of the package, ahead of the
rubber bale. The sealing and cutting components are then
brought together again in the manner described above. The
second sealing bars form a sealed plastic film package
around the rubber bale, in cooperation with the front seal
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77
formed by the initial sealing step. rne transverse
sealing bars form a seal behind the packaged rubber baleg
which will become the forward seal of the next package.
Simultaneously, t~e cutting means cut ~he plastic film
betw~en the packaged rubber bale and the seal formed by
the transverse sealing bar. The packaged rubber bale is
then conveyed out of the wrapping station9 leaving the
plastic fi.lm sealed together only a~ ~he seal ~ormed by
the transverse sealiLlg bars. The process described in
thLs paragraph may -then be repeated to form further
packages.
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