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BACKGROUND OF THE INVENTION
This invention relates to an apparatus for producing a thermoplastic resin
foam.
It has been known that the thermoplastic resin foam can be effectively
produced, continuously, by means of an extrusion method. (Therefore, it
has been usual to use the extrusion method for this purpose.) In this
case, in order to obtain a highly expanded foam, an extrudate is usually
introduced immediately into a vacuum chamber connected to a die member of
the extruder to enhance the expansion of the foam under reduced pressure.
In such a case, the foam accomplished expansion is produced continuously
from the outlet of the vacuum chamber. Therefore, it is necessary to
prevent air from being introduced into the reduced pressure device through
an annular gap possibly formed between an outer surface of the foam and an
inner wall of an outlet hole of the vacuum chamber. In order to realize
the prevention of air immigration into the vacuum chamber, U.S. Pat. No.
3,822,331 discloses either a flexible flap on an outer end wall of the
chamber or a flexible ring packing provided on an inner wall of the outlet
hole of the chamber which acts to prevent air from passing through an
annular gap between the inner wall of the hole and the outer periphery of
the extruded resin foam.
The flexible packing is of natural or synthetic rubber or soft resin and is
provided so that it covers the outlet hole of the chamber except a center
portion thereof through which the extruded resin passes against a
resilient force acting to minimize the open area of the center portion. It
has been found that such packing provided in the outlet of the chamber is
insufficient to provide a satisfactory air tightness thereof. That is,
during a run of foam expansion, the volume of the foam increases at one
variable rate and thus the cross-sectional configuration and size of the
foam are not always constant. The packing itself cannot follow these
physical variations of the foam. Therefore, it has been desired to improve
the air tightness of the outlet of the vacuum chamber.
SUMMARY OF THE INVENTION
This invention was made in view of the above mentioned state of the art.
In order to obtain a good air tightness of the outlet and/or inlet of the
vacuum chamber, the inventor has developed a cylinder made from a flexible
sheet material which is disposed at the outlet and/or inlet of the vacuum
chamber such that the extruded resin foam passes through the flexible
cylinder.
Further, an air chamber is provided around the flexible cylinder so that,
when pressurized air is introduced into the air chamber, the flexible
cylinder is inflated radially inwardly to eliminate the annular air gap
between the outer surface of the extruded resin foam and the inner wall of
the outlet and/or inlet hole of the vacuum chamber.
It has been confirmed that the above air shielding construction provides a
good result. Therefore, the present invention resides in an apparatus for
producing thermoplastic resin foam, comprising a series connection of an
extruder, a die and a vacuum chamber, the vacuum chamber including a
cylinder body and an air shielding device provided at least one end of the
cylinder body, the air shielding device comprising an annular tube having
an inside wall formed of a flexible sheet, the flexible sheet being
capable of being inflated radially inwardly by a pressurized fluid
introduced into the annular tube.
In the above construction of the apparatus, it is possible to maintain an
interior of the vacuum chamber at reduced pressure when the extruded resin
foam is slidingly moved over the surface of the flexible sheet. However,
the flexible sheet is abraded by the extruded and foamed resin and easily
worn out. Therefore, it is necessary to periodically replace the sheet
with a new one at short intervals of time. Since it is impossible to use
the vacuum chamber during the replacement of the flexible sheet, the
extrusion which operates inherently continuously must be stopped. Thus,
the frequent replacement of the flexible sheet produces new problems. In
order to resolve the problem, an improvement was made on the basis of a
fact that, by providing a protective sheet such as a metal mesh which is
durable against abrasion on an outer surface of the flexible sheet which
is adapted to be in contact with the extruded resin foam so that the
protective sheet directly contacts with the latter, it is possible to
totally maintain the air tightness with an improved abrasion durability of
the sheet.
Therefore, the present invention resides also in the apparatus as mentioned
above, wherein a flexible protective sheet is provided on the flexible
sheet so that the flexible sheet, together with the flexible protective
sheet, is inflated by a pressurized fluid introduced into the air
shielding device, with only the protective sheet being in direct contact
with the foamed resin.
BRIEF DESCRIPTION OF THE DRAWINGS
The above mentioned apparatus of the present invention will be described
with reference to the accompanying drawings.
FIG. 1 is a cross-sectional view showing an embodiment of the present
invention;
FIG. 2 is an enlarged cross-sectional view taken along a line II--II in
FIG. 1;
FIG. 3 is a cross-sectional view of an air shielding member and associated
portion of another embodiment of the present invention;
FIG. 4 is a cross-section taken along a line IV--IV in FIG. 3; and
FIGS. 5a, 5b, 6a, and 6b are cross-sections of flexible sheets to be used
in this invention, respectively.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIGS. 1 and 2, a reference numeral 1 depicts a die, 2 a sizing die, 3 a
connecting plate, 4 a vacuum chamber, 5 a vacuum suction inlet, 6 a
support for a foamed resin, 7 a pressure regulator valve, 8 a connecting
plate, 9 an air-shielding device, and 10 an extruded and foamed resin. The
air-shielding device 9 comprises a frame 91, a flexible sheet 92, and a
pressurized fluid inlet 93.
In the apparatus shown in FIG. 1, the sizing die 2 is provided at an end of
the die 1 which is fixedly secured to any conventional extruder (not
shown), to which an outer end of the vacuum chamber 4 is connected.
In the shown embodiment, there may be no need of an air-shielding at an
inlet side of the vacuum chamber 4 since it is connected air-tightly
through the connecting plate 3 to the sizing die 2. However, there may be
a case where the extruded resin from the die 1 is firstly exposed under
atmosphere and then introduced into the vacuum chamber 4. In such a case
it is necessary to provide the air-shielding device even in the inlet side
of the vacuum chamber 4.
In any case, however, the outlet side of the vacuum chamber must be opened
to derive an extruded and foamed resin. Therefore, it is always necessary
to provide an air-shield at the outlet side thereof. In order to do so,
the air-shielding device 9 is provided in the outlet side of the vacuum
chamber 4.
The frame 91 takes an annular form so that the extruded and foamed resin
can pass through the center portion thereof. An outer configuration of the
frame 91 should be determined such that it can cover the outlet and/or
inlet hole of the vacuum chamber 4. Further, the frame 91 has a U-shaped
cross-section when taken along an axis thereof, with the inside surface
portion thereof being open. The inside annular surface portion means a
surface which faces the surface of the extruded and foamed resin passing
through the annular frame. The fluid pressure inlet 93 is provided at a
suitable portion of an outer surface of the frame 91. The frame 91 may be
of any rigid material and, preferably, of a metal material.
By fixedly securing the frame 91 at one side thereof to the outlet of the
cylinder body constituting the vacuum chamber 4, the outlet of the
cylinder body is opened indirectly through the center hole defined by the
flexible sheet 92 to atmosphere. Further, by introducing a pressurized
fluid such as pressurized air through the fluid inlet 93 into the frame
91, the flexible sheet 92 is inflated inwardly to reduce the
cross-sectional area of the center hole. Therefore, when the extruded and
foamed resin 10 is passing through the hole, the inner surface of the
flexible sheet 92 contacts the surface of the extruded and foamed resin
10.
FIG. 2 shows the hole in cross-section under the latter conditions. That
is, the surface of the flexible sheet 92 is in intimate contact with the
surface of the extruded and foamed resin 10 except portions thereof
corresponding to corners 94 of the flexible sheet 92 where it is folded.
However, the folding thereof is made very tight and, therefore, there is
substantially no leakage of air therethrough.
The flexible sheet 92 may be of natural or synthetic rubber or soft
synthetic resin, etc. The synthetic rubber may include chloroprene rubber,
ethylene-propylene rubber, styrene-butadiene rubber, urethane rubber,
silicon rubber and fluoro rubber, etc. The soft synthetic resin may
include soft vinyl chloride resin.
The cross-sectional shape of the flexible sheet is not limited to those
shown in FIGS. 1 and 2. It may be possible to take a shape shown in either
one of FIGS. 5a and 6a, which can be inflated by introducing pressurized
fluid as shown in FIGS. 5b and 6b, respectively.
The apparatus shown in FIGS. 1 and 2 operates as follows: A melted foamable
thermoplastic resin composition is extruded from the die 1 and is
initially foamed in the sizing die 2. The resin 10 thus foamed is
introduced into the vacuum chamber 4. The vacuum chamber 4 is evacuated
through the suction hole 5 and simultaneously cooled by a suitable means
(not shown). Therefore, if the interior of the vacuum chamber 4 is kept at
a predetermined reduced pressure, the resin 10 is further expanded and
cooled in the vacuum chamber 4 resulting in a highly expanded foam. Since
the chamber 4 is provided at the outlet and/or inlet thereof with the
air-shielding device 9, the maintenance of the pressure is achieved after
an end of the resin 10 reaches the air-shielding device 9.
That is, the air-shielding device 9 is provided such that it closes the
outlet hole of the vacuum chamber 4 except the center portion thereof
defined by the flexible sheet 92. The resin 10 is introduced through the
inlet hole into the vacuum chamber 4 and then leaves it out through the
center hole of the air-shielding device. At this time, since a pressurized
fluid is introduced through the fluid inlet 93 into the air-shielding
device 9, the flexible sheet 92 is inflated thereby to reduce the area of
the center hole. The degree of inflation of the flexible sheet 92 is
regulatable arbitrarily by controlling an amount of the pressurized fluid
to be introduced thereinto. The frame 91 is formed with a fluid discharge
port and/or provided with a relief valve to control internal pressure of
the air-shielding device 9 and to prevent excessive pressure increase
therein. Therefore, the innermost portion of the flexible sheet 92 can be
always in resilient contact with the surface of the extruded resin 10, so
that air cannot be introduced into the interior of the vacuum chamber.
Thus, the inside of the vacuum chamber 4 is kept at the reduced pressure
regardless of continuous passing of the resin 10 having variable
configuration and size therethrough. Thus, the resin 10 is further foamed
within the vacuum chamber 4 and cooled, resulting in a favorably expanded
resin foam.
In the apparatus shown in FIGS. 1 and 2, the flexible sheet 92 is in direct
contact with the foamed resin 10. Therefore, as mentioned previously, the
flexible sheet 92 is abraded thereby and the life thereof is shortened,
causing the necessity of stoppage of the manufacture of the foamed resin
due to the replacement thereof by a new one. Another embodiment of the
present invention improves this point.
The second embodiment has the same construction as the first embodiment
shown in FIG. 1, with the air-shielding device being improved. FIG. 3
mainly shows the improved air-shielding device.
The air-shielding device 9 in FIG. 3 is different from that in FIG. 1 only
in that a protective sheet 11 is provided on and along the flexible sheet
92. In detail, in FIG. 3, at the outlet of the vacuum chamber 4, the
protective sheet 11 is provided, with one end thereof being supported in
between a flange of the cylinder constituting the vacuum chamber 4 and the
connecting plate 8. The other end of the sheet 11 extends overlapping the
surface of the flexible sheet 92.
The protective sheet 11 may be more flexible than the flexible sheet 92.
Therefore, the protective sheet 11 is capable of protecting the flexible
sheet 92 against abrasion with the resin 10 and of easily following a
variation of the varying cross-sectional shape of the resin 10.
A fine metal mesh is one example of a protective sheet 11, which can be
made by weaving fine metal wires. Stainless steel wire is suitable for
this purpose, and a stainless steel mesh of 50 meshes or more is
preferable.
Another example of the protective sheet 11 is a metal foil. Aluminum, lead
or copper foil, etc., may be used for this purpose. Such foil can be thin
enough to provide a desired flexibility.
A further example of the protective sheet 11 is a cloth of metal fibers.
"Nasron" which is a registered trademark of Nippon Seisen Co. for a
commercially available stainless steel cloth has a flexibility similar to
that of the usual cloth and a very high abrasion durability.
A mesh of plastic filaments may be also used as the protective sheet 11.
The plastic material for the mesh should have high abrasion durability and
heat resistivity. For example, a polyester resin mesh or polyamide resin
mesh may be used.
It is further possible to use an abrasion durable plastic film as the
protective sheet 11. Such film may be of ultra-high-molecular-weight
polyethylene, polypropylene, polyamide, polyethylene-terephthalate or
polytetrafluoroethylene. Such resin is relatively stiff. However, since
they may be provided as a very thin film, it can provide a flexibility
enough to follow the physical variation of the flexible sheet 92.
The protective sheet 11 may be also of a natural or synthetic fiber cloth.
The cloth may or may not be a woven one. The natural fiber may include
cellulose such as cotton or jute and the synthetic fiber may include that
of vinylon, polyamide, polyethyleneterephthalate, etc. Particularly, a
cloth of aromatic polyamide is most preferable. The cloth should be as
thin as possible.
The protective sheet 11 is slit to form a plurality of webs connected
together at one end thereof. For example, it may comprise four webs as
shown in FIG. 4. In FIG. 4, the protective sheet 11 includes upper and
lower webs 111 and 113 which are in contact with upper and lower surfaces
of the extruded and foamed resin, respectively, and left and right webs
112 and 114 which are in contact with left and right side surfaces of the
resin, respectively. The upper and lower webs 111 and 113 are
substantially flat in cross-section and webs 112 and 114 are curved in
cross-section so that side edges thereof overlap with side edges of the
webs 111 and 113, respectively.
The overlapping portions are relatively movable and these webs of the
protective sheet always cover the surface of the extruded resin even if
the cross-sectional shape and size of the extruded resin 10 varies.
The latter embodiment employs the protective sheet between the flexible
sheet and the extruded and foamed resin and, therefore, has a merit of
providing an improved durability of the flexible sheet which makes an
effective and continuous manufacture of the foamed resin possible, while
the merits of the preceding embodiment are maintained as they are.
That is, since the protective sheet is highly flexible, it can easily
follow the inflation of the flexible sheet by the pressurized fluid to
keep the periphery of the extruded resin air-tight. Further, since the
resin contacts with not the flexible sheet but the protective sheet, the
abrasion of the flexible sheet due to the rubbing between the latter and
the extruded resin does not occur. Therefore, according to this
embodiment, there is no need of frequent replacement of the flexible sheet
by a new one and thus a continuous manufacture of foamed resin can be
realized effectively.
This invention can be applied effectively to the cases where a foaming
agent is added to a thermoplastic resin and the mixture is extruded to
produce a foamed resin. That is, various thermoplastic resins such as
polystyrene, polyethylene, polyvinylchloride and polypropylene, etc., may
be used for this purpose. Further, any foaming agent can be used. For
example, the aliphatic hydrocarbons such as ethane, propane, butane,
pentane, etc., the halogenous hydrocarbons such as methylchloride,
methylenechloride, ethylchloride, ethylfluoride, chlorodifluoromethane,
dichlorodifluoromethane, etc., or the inert gases, such as carbon dioxide,
nitrogen, etc., may be used as the foaming agent. The melted, foamable
thermoplastic resin composition means any of these resins which contains
any of these foaming agents and is melted by heating.
In this invention, any flat die such as a T die, etc., a circular die or a
rod die, etc., may be used as the die.
Although, in the embodiment shown in FIG. 1, the sizing die 2 is disposed
between the outlet die 1 and the vacuum chamber 4, the sizing die 2 may be
omitted by directly connecting the vacuum chamber 4 to the die 1. In such
a case, the extruded resin 10 may be more expanded.
According to the apparatus of this invention, the extruded and foamed resin
10 is kept in contact with the flexible sheet 92 or the protective sheet
11 thereon of the air-shielding device 9 and thus it is possible to keep
the vacuum chamber air-tight at the inlet or outlet thereof. Therefore, it
is possible to sufficiently foam the extruded resin within the vacuum
chamber 4 and to produce the foamed resin effectively continuously. In the
case of the embodiment in FIG. 3, the flexible sheet 92 does not directly
contact with the extruded resin and thus the abrasion of the flexible
sheet is avoided. Therefore, the life of the flexible sheet is elongated
and thus the extrusion may be performed continuously for a long time
without replacement of the flexible sheet by a new one.
The present invention will be described in more detail with reference to
the following examples in which the term "%" means "% by weight".
EXAMPLE 1
The apparatus shown in FIG. 1 together with a tandem type extruder having a
diameter of 65-90 mm was used. Polystyrene composition containing 1% talc
as a cell size controlling agent was fed into the extruder. From an inlet
port provided in the extruder, 8% butane was added into as a blowing
agent.
The foamable melt uniformly kneaded within the extruder was extruded at
122.degree. C. through a flat die 1 having a 2.0 mm.times.100 mm orifice
and a sizing die 2. The outlet area of the sizing die 2 was 42
mm.times.200 mm. A rectangular cylinder 10 m long was used as the vacuum
chamber 4, whose inner cross-sectional area was 150 mm.times.400 mm. An
air-shielding device 9 such as shown in FIG. 1 was attached to the outlet
of the vacuum chamber. The frame 91 of the air-shielding device 9 was made
of iron and the flexible sheet 92 thereof was made of translucent rubber
whose thickness was 1 mm. The center open area defined by the flexible
sheet 92 through which the extruded and foamed resin 10 passes was 80
mm.times.280 mm with corner curvature being 10R.
Firstly, a foamed resin was obtained without evacuation of the vacuum
chamber and without introducing pressurized air into the air-shielding
device. The take-off speed was 2.1 m/minute. The foamed resin was 50 mm
thick and 233 mm wide with density being about 0.038 g/cm.sup.3.
Then, the pressurized air of 0.65 kg/cm.sup.2 was introduced through the
pressurized fluid inlet 93 to inflate the flexible sheet 92 to thereby
position the latter into intimate contact with the resin 10 while the
resin was continuously taken off at 2.4 m/minute and the vacuum chamber is
evacuated to 460 mmHg (absolute pressure). Then, the air pressure was
reduced to 0.5 kg/cm.sup.2. The foamed resin obtained was 55 mm thick and
260 mm wide with the density being about 0.026 g/cm.sup.3. Such foamed
resin was obtained continuously for 15 minutes.
EXAMPLE 2
Example 1 was repeated except that air-shielding device shown in FIG. 3 was
used.
A stainless steel mesh 11 of 300 meshes and 0.1 mm thick was provided
between the sheet 92 and the resin 10.
Firstly, a foamed resin was obtained without evacuation of the vacuum
chamber and without injecting pressurized air into the air-shielding
device. The take-off speed was 2.5 m/minute and the foamed resin thus
obtained had a thickness of 48 mm and width of 245 mm, with density being
0.034 g/cm.sup.3. Then, pressurized air of 0.65 kg/cm.sup.2 was introduced
through the pressurized fluid inlet 93 to inflate the flexible sheet 92 to
thereby bring the metal mesh 11 into intimate contact with the resin 10
while the resin was continuously taken off at 3.4 m/minute and the vacuum
chamber was evacuated to 260 mmHg (absolute pressure). Then, the air
pressure was reduced to 0.5 kg/cm.sup.2. The foamed resin obtained was 57
mm thick and 275 mm wide, the density being about 0.021 g/cm.sup.3 and was
continuously obtained for 3 hours.
Thereafter, the inner pressure of the vacuum chamber was increased to 360
mmHg (absolute pressure) and the take-off speed was reduced to 3.1
m/minute, resulting in a foamed resin 55 mm thick and 270 mm wide with the
density being about 0.021 g/cm.sup.3.
In any of these examples, the foamed resin obtained was stable and it was
found that there was no need of varying air pressure to be introduced into
the air-shielding device even if the inner pressure of the vacuum chamber
was changed from 260 mmHg (absolute pressure) to 360 mmHg (absolute
pressure). There was no abrasion of the flexible sheet.
It is readily apparent that the above-described apparatus for producing
foamed thermoplastic resin meets all of the objects mentioned above and
also has the advantage of wide commercial utility. It should be understood
that the specific form of the invention hereinabove described is intended
to be representative only,as certain modifications within the scope of
these teachings will be apparent to those skilled in the art.
Accordingly, reference should be made to the following claims in
determining the full scope of the invention.
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