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Description  |
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FIELD OF THE INVENTION
The present invention relates to the manufacture of extruded articles (e.g.
sheets or tubes) of thermoplastic synthetic foam, for example of
polystyrene, polyethylene or polypropylene.
BACKGROUND OF THE INVENTION
According to conventional technique, a thermoplastic resin is melted
continuously under pressure in the barrel of an extruder having one or
more screws, terminating with an extrusion head having a narrow extrusion
orifice which may be flat (for sheets) or circular (for tubes). In an
intermediate section of the barrel there is continuously injected into the
molten resin a volatile expansion agent, usually in liquid state, for
example a "Freon" (R.T.M.) or pentane and the extruder is designed in such
a way as to produce a possibly uniform dissolution of the agent in the
molten resin. The resin may also contain advantageously suitable
nucleating agents, such as, for example, talcum, citric acid and sodium
bicarbonate in the form of very fine particles uniformly dispersed in the
molten resin. On the way to the extrusion orifice the molten material is
subjected to a high pressure, which is necessary to prevent volatilization
of the expansion agent. On leaving the extrusion orifice, the material
undergoes decompression to atmospheric pressure, as a result of which the
expansion agent separates within the material in the form of bubbles,
giving rise to the desired foam.
It is known that the quality of the foam thus obtained is heavily dependent
on the extrusion temperature. If the extrusion temperature is too high,
the foam collapses or, at least, its specific gravity (density) is
undesirably high in relation to the value theoretically obtainable, and
its mechanical strength is poor. In principle, the higher the percentage
of the expansion agent in the molten material, the lower the extrusion
temperature should be, since otherwise the viscosity of the resin just
extruded is insufficient to resist the disruptive pressure of the gas
which is liberated in the resin. Since, in order to obtain foams of low
density (less than 0.1 g/cc), substantial percentages of expansion agent
are necessary, the problem of lowering of the extrusion temperature
assumes great importance.
PRIOR ART
Cooling of the extrusion head is until now resulted insufficient to achive
the desired result, mainly because the cellular structure of the foam thus
obtaines is coarse and not at all uniform. A method commonly used until
now to obtain a uniform cellular structure relies upon cooling of the last
section of the barrel of the extruder. For example, the Italian Pat. No.
831,699 (and the corresponding English Pat. No. 1,231,535 and French Pat.
No. 1,600,010) describes an extruder for thermoplastic foams comprising at
least one injector of the expansion agent into an intermediate zone of the
barrel containing the molten material, this zone being followed by a first
cooling zone, by means of a water jacket, and by a second (final) zone for
cooling by means of a coil fed by a refrigerating system. However, even
this intense cooling is insufficient to lower the temperature of the
molten material to the level which would be necessary to obtain a low
density foam. In fact, as the material travels through the cooled zones
its viscosity increases and therefore the frictional heat due to the
action of the screw or screws also increases and a steady state is
therefore reached in which the temperature of the material no longer
decreases, whilst it is still far from the low level desired for the
extrusion. This difficulty may be overcome in part by reducing
appropriately the speed of rotation of the screws; however, this also
reduces the hourly productivity of the extrusion press. Another remedy is
to effect the cooling in another extruder, which is fed with the molten
mass from the first extruder and in which the screw or screws rotate at
low speed. The quality of the foam obtained in this way is acceptable.
However, the costs of running the second extruder are only rarely less
than that of the first, apart of its high investment cost.
According to U.S. Pat. No. 2,669,751 the molten material supplied by an
extruder is conveyed under pressure through a cooled cylinder enclosing an
axially extending, internally cooled tubular shaft equipped with a
multitude of mixing blades, the discharde end of the cylinder being
connected to the extrusion head. However, in practice, it is impossible
with this system to have incorporated in the molten flow more than about
7-8% only of the liquid expansion agent and, at the same time, the
operational costs are very high due to high power necessary for rotating
the bladed shaft under high viscosity conditions of the material.
According to U.S. Pat. No. 3,751,377 synthetic thermoplastic foams of low
density (such as 0.026-0.029 g/cc in case of polystyrene) would be
obtainable by interposing between the extruder barrel and the extrusion
head a "static mixer" or "interface surface generator", previously known
as such by being disclosed by a number of prior patents, e.g. U.S. Pat.
No. 3,286,992, owing to which it would be possible to successfully process
melts containing high proportions expanding agent (10 wt.% and even more).
In a "static mixer", which belongs to the general class of mixers having
no moving parts, the flow of the melt containing the expanding agent is
subdivided into a plurality of partial flows by means of a stationary flow
divider and the partial flows are subsequently recombined together under
modified contact conditions ("modified" in respect to a contact surface
and/or mutual positions of the partial flows); the design and operational
conditions of the mixer are such as to possibly accurately avoid turbulent
mixing (which latter is characteristic of the non-static mixer belonging
to the abovementioned class). In practice, a substantial number (even 20
or more) of dividing/recombining stages is necessary, operating in series
in a common tubular casing. The resulting structure is complicate in
manufacture, presents a relevant length and a purposely designed
supporting system must be provided (in addition to the extruder) to firmly
support both the mixer and the extrusion head. It is also a matter of fact
that, since laminar-flow conditions must be respected both in the design
and operation of a "static mixer", the flow velocity must be kept low by
adopting a sufficiently large cross-sectional area of the tubular casing.
The consequence is that a great number of stages is necessary to throughly
subdivide a flow of material of large cross-sectional area. The prior
patents mentioned in the specification of the aforesaid U.S. Pat. No.
3,751,377 show that, in order to increase efficiency of a "static mixer",
several forms of flow-dividers were excogitated, with the result that the
structure and manufacture became still more complicated than before. Still
moreover, presently known "static mixers" do not comprise a cooling jacket
or other cooling means but, rather, imply natural dispersion of heat from
the inside to the outer atmosphere through the wal of the tubular casing.
Experimental tests conducted in connection with the present invention have
shown that at least with the static mixer disclosed by U.S. Pat. No.
3,286,992 the temperature of the material flowing through the mixer cannot
be controlled at will and, even, the material often tends to heat up by a
few .degree.C. instead of cooling down. Further experimental tests,
wherein a cooling jacket was added, have shown that, at least with high
production rates desirable commercially, the homogeneousness of the foam
was unacceptably worsened, most probably because the mixer was unable to
adequately intermix the external cool layer of the flow with the hot
internal layers.
Summarizing, as far as obtention of low-density extruded thermoplastic
foams is concerned, the presently most reliable technique resides in the
use of a primary and a secondary extruder in tandem arrangement, in
accordance with the aforementioned U.S. Pat. No. 3,151,192 notwithstanding
the high investment costs and operating costs.
THE INVENTION
It is an object of the invention to provide a device for cooling the molten
material containing the expansion agent, in which the flow of material is
efficiently cooled and homogenized with a very low power consumption. An
additional object is to provide the said device in a form which is
compact, robust and obtainable by simple machining steps rendering the
device particularly inexpensive. Yet another object of the invention is to
make the said device in a form which may be readily applied to a
conventional pre-existing extruder for thermoplastic synthetic foam. Other
objects and advantages will emerge from the description which follows.
In accordance with the above, the invention provides an extruder with one
or more screws, for thermoplastic synthetic foam, comprising means for
injecting into the barrel of the extruder a liquid volatile expansion
agent and an extrusion head fed by the barrel with a flow of molten
thermoplastic synthetic resin in which is thoroughly dispersed under
pressure the said expansion agent, the said extruder being characterised
by a device for cooling the said flow of resin, connecting the said head
with the said barrel, this cooling devive comprising:
(a)--a cooled metal block, having two opposed faces and at least one row of
passages passing through the said block from one to the other of the said
faces;
(b)--an inlet manifold channel for the input of the said flow to a first of
the said faces and an outlet manifold channel extending from the second of
the said faces;
(c)--the said outlet manifold channel being directed at least substantially
in the same direction as the row of passages and having one of its lateral
walls formed by the said second face of the said block whereby the
individual passages in the said row open into the outlet manifold channel
transversely to the direction of the latter and in sequence with respect
to the said direction; and
(d)--at least one cooling channel adjacent the block for cooling the latter
by a liquid refrigerant.
Advantageously the inlet manifold channel is also directed at least
substantially in the same direction as the said row of passages and has
one of its lateral walls formed by the first of the said faces of the
block, whereby the individual passages branch off from the said inlet
manifold channel transversely to the direction of the latter and in
sequence in relation to the said direction. The passages are
advantageously of constant circular cross-sectional shape.
In a particularly advantageous embodiment, the length/diameter ratio of the
passages does not substantially exceed 10:1, with the result that the
pressure drop through the passages is maintained within particularly low
values. Furthermore, instead of a single block, the device may comprise a
further cooled metal block, in accordance with what has been indicated
above under (a), with the respective input and output manifolds according
to (b) and (c) and cooling channel according to (d), the outlet manifold
channel of the first block being connected to the inlet manifold channel
of the second block, whereby a plurality of cooling stages in series with
each other is obtained. In each case, according to a particular
characteristic of the invention, the overall cross-sectional area of the
passages interconnecting an inlet manifold channel with the respective
outlet manifold channel is preferably greater than the effective
cross-sectional area of the inlet manifold channel, and preferably also
than that of the outlet manifold channel.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 in the appended drawings illustrates an extruder for thermoplastic
foam equipped with one embodiment of the device accordin to the invention;
FIG. 2 illustrates in longitudinal cross-sectional view an elementary
embodiment of the device;
FIG. 3 is a plan view of one of the cooled blocks used in the device of
FIG. 2;
FIG. 4 is an axial cross-sectional view of a device according to the
invention in a preferred embodiment, and
FIG. 5 is a frontal view, partially in cross-section, of one of the cooled
blocks used in the device of FIG. 4.
DETAILED DESCRIPTION
The extrude 10 illustrated in FIG. 1 comprises a hopper 12 for a granulate
of thermoplastic synthetic resin, which latter is brought to molten state
under pressure in the extruder barrel 14 containing the screw or screws
not illustrated. The present invention is particularly advantageous in
application to extruders with two intermeshing screws (in particular with
co-rotating screws, according to the so-called "Colombo system"), which
have until now been more suitable for obtaining foam of medium density
(0.15-0.4 g/cc) and not foams with low density (0.03-0.15 g/cc). An
injector 16 leads into an intermediate section of the barrel 14 for the
injection of the liquid volatile expansion agent. In this zone the
temperature of the molten resin in the barrel considerably exceeds (by as
much as 90.degree.-100.degree. C.) the melting point, such that the
viscosity of the molten resin is sufficiently low for the purposes of a
quick, homogeneous dissolution of the expansion agent. The degree of
compression of the resin by the screws in this zone is high and depends
mainly on the nature of the agent and on the percentage injected; in
general, the pressures involved conveniently are between 200 and 300
kg/sq.cm. A terminal section 14' of the barrel is preferably cooled; to
this end it is sufficient to have a simple external oil circulation
jackets, if necessary in combination with internal cooling of the screws
in this section. In this way the temperature of the molten resin is
prereduced to a convenient level, which usually exceeds by
20.degree.-50.degree. C. the desired extrusion temperature, depending on
the resin treated and on the expension agent used. By way of an example,
in the case of polystyrene with high molecular weight (for example DOW
686) loaded with 7-8% of a 50/50 mixture of "Freon 11" and "Freon 12", the
recommendable extrusion temperature seems to be approximately 125.degree.
C. With the same polymer, loaded with 7-8% of pentane, the recommendable
extrusion temperature appears to be 110.degree. C., whereas in the case of
low density polyethylene (e.g. QG1 of Montedison) with 12-14% of "Freon
114" the extrusion temperature should be approximately 100.degree. C. The
viscosity values corresponding to these temperatures are practically
impossible to obtain in the section 14' of the barrel for the reasons
already explained above. With limited pre-cooling, as suggested above, the
viscosity of the material in the section 14' reaches only those levels
which are still compatible with the mechanical strength of the barrel and
of the screws and which in any case correspond to a limited frictional
heat, dissipatable by cooling means mentioned. Many commercial extrudes
comprise a terminal section of the barrel equipped with cooling means. The
extruder RC 41/E of Applicant's Company is an example.
The continuous flow of molten resin thus obtained, with the expansion agent
uniformly dissolved in the resin, is fed from the barrel 14 to an
extrusion head 18, which presents in a way known per se a narrow extrusion
orifice the shape of which is adapted to the foam profile it is desired to
obtain. In the particular case which will be looked at in grater detail
below, reference will be made to a circular orifice, adapted for the
extrusion of a foam tube.
According to the invention, between the extrusion head 18 and the barrel 14
there is interposed a cooling device 20, which is fixed to the free
extremity of the barrel and which supports, in its turn, the head 18. The
device 20 is a heat exchanger without moving components, to the "cold"
side of which there is continuously fed cooling liquid (e.g. oil) at the
necessary temperature, and one of the objects of the present invention is
to controllably cool with this device the said flow of resin in a
thoroughly homogeneous way to a temperature which is possibly close to
that of the cooling liquid, that is with high efficiency.
In FIGS. 2 and 3, numeral 22 indicates a flat, rectangular block of a metal
which is a good heat conductor, preferably aluminium, having two opposed
large faces 22A, 22B parallel with each other. In the block 22 there are
drilled parallel rows (A, B, C, D, E) of through holes 24 of circular
section, the said holes being at least substantially perpendicular to the
said faces and being preferably identical to each other. The block 22 is
mounted, in peripherally sealed condition, in a housing 26 in which there
is formed all round the perimeter of the block a channel 28 for the
circulation of cooling oil. The face 22A of the block 22 constitutes a
longitudinal lateral wall of a manifold channel 30, formed in the housing
26, for the inlet flow of molten resin to the block 22. The longitudinal
direction of the inlet manifold channel 30 is indicated by the arrow 32 in
FIG. 3 and corresponds to the direction of the rows A . . . E of the holes
24. The holes in each of the rows branch off therefore in sequence from
the manifold channel 30, transversely to the direction of the latter, that
is, in practice, transversely to the flow of resin in the manifold
channel. In other words, the flow of resin in the manifold creeps on the
face 22A of the block 22 to reach in sequence the individual holes of each
of the rows A . . . E. It is preferable if, as illustrated, the width of
the manifold channel 30 measured in axial direction of the holes 24
decreases progressively down to zero in the direction of the flow, that is
in the direction of the rows A . . . E.
In substantially similar conditions, there extends from the opposite face
22B of the block 22 an outlet manifold channel 34, the longitudinal
direction of which corresponds to that of the rows A . . . E and is also
indicated by the arrow 32. Thus, the face 22B constitutes a longitudinal
lateral wall of the manifold channel 34 and the holes 24 of each of the
rows A . . . E open in sequence into this manifold channel transversely to
its longitudinal direction. Consequently, in this case too the flow of
molten resin in the manifold channel 34 creeps on the face 22B of the
block 22, in the direction 32; in these conditions, as is also apparent
from the arrows given, the cooled partial flows of resin discharged by the
successive holes 24 of each row interfere substantially perpendicularly
with the general flow in the manifold channel, giving rise to an effective
mixing of the material and therefore to the homogenization of the
temperature. The width of the outlet manifold channel 34 measured in
direction of the holes 24 increases progressively from zero along the face
22B of the block 22. The "progression" of this increase of the section of
the manifold channel 34 is in proportion (more or less) to the flows
received from the successive holes in each row, substantially in such a
way that the specific flow rate (in g/sec/sq.cm) in any point of the
manifold is practically the same. A similar consideration of the whole is
valid for the progressive decrease of the section of the inlet manifold
channel 30. The "effective" cross-sectional areas of the manifold channel
are those which receive the total flow of material and are indicated in
FIG. 2 by S1 and S2 respectively. According to the invention it is
preferable that the total cross-sectional area of the holes 24 is greater
than the cross-sectional area S1, and is preferably greater also than the
cross-sectional area S2, such that the total of holes does not constitute
a constriction (in terms of area) in the passage of the material from one
manifold channel to the other. The diameter of the holes 24 may be
selected within a relatively broad range, usually of approximately 3 mm up
to about 10 mm, more or less in proportion to the production capacity of
the extruder; obviously for particularly high capacities, greater than
about 250 kg/hr, diameters greater than 10 mm may be adopted. Since the
passage through each hole involves an increase in the viscosity of the
resin, which is all the greater the longer is the hole, it is evident that
the pressure drop (in kg/sq.cm) produced by the passage through a hole
depends to a large extent on the length/diameter (l/d) ratio of the hole,
and it is also evident that the greater the pressure drop, the greater
would be the power absorbed by the device according to the invention. On
the other hand, however, given the presence of the volatile expansion
agent in the flow of resin, it is necessary that the device in question
should introduce a certain counter-pressure, that is, give rise to a
certain presence drop. Practical tests have shown that there exists a
range of optimum values for the said counter-pressure, namely between
about 15 kg/sq.cm and about 35 kg/sq.cm. This unexpected circumstance is
extremely favourable insofar as these values constitute only a small
fraction of the counter-pressures (200-300 kg/sq.cm) already necessary in
the extruder to produce and maintain the dissolution of the expansion
agent in the molten resin, and, therefore, the device according to the
invention involves only a correspondingly small increase in absorbed
power, so that it may be applied even to a pre-existing extruder without
prejudice to the latter. Further experimental tests have demonstrated
that, under the circumstances discussed above, the values indicated in the
following table are advisable:
______________________________________
Hole diameter
(mm) 1/d 1/d preferred
______________________________________
3 8-14 9-13
5 11-19 13-17
6 13-22 15-20
8 13-23 15-21
10 14-24 16-22
______________________________________
With these values, the flow of resin may be cooled to a temperature very
close to the optimum extrusion temperature. It may also be noted from the
above table that for hole diameters exceeding 6 mm the l/d values vary
only very slightly.
In practice, however, it is not advisable to use high l/d values with a
single cooled block, for example with a block having holes of 6 mm
diameter and 120 mm length. It is preferable, instead, to effect the
cooling in two stages, that is with two consecutive cooled blocks,
realizing jointly the desired 1/d ratio. Such a second block is indicated
by 22' in FIG. 2 and is identical to the first block 22, the two blocks
being coplanar and orientated in the same direction already indicated
previously by 32 (FIG. 3). The block 22' sealingly is mounted in the
housing 26 previously mentioned and is surrounded by a channel 28' for the
circulation of cooling oil. Numerals 22'A and 22'B indicate the two
opposed flat faces between which extend the holes 24', analogous to the
faces 22A and 22B of the block 22 with the holes 24. The outlet manifold
channel 34 connects directly, in the direction 32, with the inlet manifold
channel 30' relating to the second block 22', the outlet manifold channel
34' leads off from the face 22'B. For the manifold channels 30', 34' and
for their relation to the block 22', the same description applies as has
been made with reference to the manifold channels 30, 34 and the block 22.
The manifold channel 34' may lead into an extrusion head or connect with
the inlet manifold channel of a further cooled block (if necessary).
Assuming that there are only two blocks, as illustrated in FIG. 2, the l/d
ratio realized by each block is preferably half the selected overall
ratio; thus, to realize the ratio 20:1 with holes of 6 mm, each of the
blocks 22, 22' will have a thickness of 60 mm.
The device according to FIGS. 2 and 3 is particularly useful for small flow
rates of resin. For relatively large rates, in particular of 100 kg/hr and
more, the embodiment illustrated in FIG. 4 and 5 is preferable.
In this embodiment, a first cooled block is formed by a circular
cylindrical sleeve of aluminium, advantageously formed by two rings 40,
40' arranged end to end. Similarly, a second cooled block is formed by a
circular cylindrical sleeve of aluminium advantageously formed by two
rings 42, 42' arranged face to face. The external cylindrical surfaces of
all the rings 40, 40', 42, 42' have the same diameter, for example, 260
mm, and the internal cylindrical surfaces of all the rings have the same
diameter, for example 140 mm. The radial thickness of the rings,
therefore, amounts to 60 mm, and this is the length of each of the radial
holes 44 bored in the rings. Each of the rings has a number of
circumferential series of these holes 44; in the case illustrated, each
ring has six series of holes, and each series comprises 40 holes
equidistant from each other. The diameter of the holes is for example 7
mm, so that the overall area of all the 480 holes in the block 40-40' (and
in the block 42-42') amounts to 184.8 sq.cm. The l/d ratio realized by the
two blocks is 60/7.times.2=17.14 and corresponds therefore to the table
given hereinbefore. Between the blocks 40-40' and 42-42' there is
interposed a circular disc 46, the external diameter of which is equal to
that of the rings, and the pile thus formed is centred on the axis X of a
tubular casing 48 of circular cross-section, to the ends of which are
sealingly bolted two circular head plates 50, 52 which firmly clamp
between them the said pile. The head plate 50 has a circular central
aperture 50' which connects with the internal cavity of the ring 40, and a
tubular hub 50" by means of which the device of FIG. 4 is screwed axially
into the section 14' of the extruder barrel of FIG. 1 to constitute the
device 20 indicated in the latter Figure. Inside the hub 50" there
penetrates radially a thermometric probe 51. Similarly, the head plate 52
has a circular central aperture 52' which connects with the internal
cavity of the ring 42', and a tubular hub 52" for screwing into the
extrusion head 18 of FIG. 1. A thermometric probe 53 radially penetrates
into the hub 52".
From the disc 46 there extend axially towards the apertures 50', 52' two
generally conical torpedos 54, 56 respectively. The torpedo 54 defines
with the internal cylindrical surface of the block 40-40' a tubular inlet
manifold channel 58 of circular cross section, the radial width of which
decreases progressively down to zero from the axially external extremity
towards the axially internal extremity of the block. Similarly, the
torpedo 56 defines with the internal surface of the block 42-42' a tubular
outlet manifold channel 60 of circular cross section, tha radial width of
which decreases progressively from the axially external extremity towards
the axially internal extremity of the block 42-42'. The maximum
cross-sectional area of the manifold channels 58, 60 amounts (in the
embodiment illustrated) to 98,6 sq.cm, and is therefore less than the
overall area (184.8 sq.cm) of the holes in the respective blocks. The
internal surface of the casing 48 defines with the radially external
surfaces of the rings 40, 40', 42, 42' and of the disc 46 a tubular outlet
manifold channel 62 of circular cross section, a tubular inlet manifold
channel 64 of circular cross section, and a tubular direct connection 66
between the two manifold channels. The radial width of each of the
manifold channels 62, 64 decreases down to zero from the connection 66
towards the axially external extremity of the respective block 40-40',
42-42'. The cross-sectional shape of the connection 66 is constant and has
an area of 167 sq.cm, this area too being less than the overall area
(184.8 sq.cm) of the holes in the respective blocks.
As may be seen in FIG. 4, analogously to the rows A . . . E of the block 22
of FIG. 3, the holes 44 in the four rings 40, 40', 42, 42' of FIG. 4 also
form rows (40 rows per block) extending at least substantially in a common
direction, parallel to the axis X and comparable with the direction 32 in
FIG. 3. In particular, the said holes 44 form fourty rows situated at
least substantially in angularly equidistant planes containing the axis of
the relevant block. As a result, the material which flows into the device
of FIG. 4 undergoes a treatment (cooling and mixing) substantially similar
to that already described with reference to FIG. 3.
Within the area of support of the ring 42' against the internal face of the
head plate 52 there is formed in this face a circular flat cavity 70,
accessible from the exterior through a threaded connection 72. The cavity
70 communicates, through at lest one longitudinal passage 74 in the ring
42', with a similar cavity 76 formed in the ring 42. In its turn, the
cavity 76 communicates, through at least one longitudinal passage 78 in
the ring 42, with a similar cavity 90 formed in the adjacent face of the
disc 46, from which there branches off at least one longitudinal passage
96 communicating with an identical cavity 94 formed in the other face of
the disc. The cavity 94 communicates, through at least one longitudinal
passage 96 in the ring 40', with an identical cavity 98 formed in the ring
40'. Finally, this latter cavity communicates, through at least one
longitudinal passage 100 in the ring 40, with an identical cavity 102
formed in the internal face of the head plate 50 and accessible through a
threaded connection 104. Thus, a controllable flow of cooling oil may be
fed continuously to the connection 72 to cool the four aluminium rings and
then discharged by the connection 104; in this way, the flow of resin is
cooled in counter-current, in accordance with the preferred usage of the
present invention. The control of the flow of cooling oil is effected
under the supervision of the two thermometric probes 51, 53. The probe 51
indicates the temperature reached with the cooling in the final section
14' of the barrel of the extruder (FIG. 1), whereas the probe 53 indicates
the final temperature (extru | | |
