Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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MELT DISTRIBUTION BLOCK AND EXTRUSION DIE
Field of the Invention
This invention relates to co-extrusion dies for extruding multilayer polymer
materials into a single tubular form.
Background of the Invention
Many areas of polymer processing require multiple layers of different
polymers to be co-extruded into a single tubular form. One example is the
blown
film process which is used to make most of today's commodity bags and also
high barrier food packaging. Although multi-layer packaging can be made from
co-extruded flat film, using a tubular form presents fewer sealing operations,
results in less trim scrap and is more conducive to certain product shapes.
Tubular forms are used in many applications including the production of
multilayer pipe or tubing, pipe coating, wire coating, and the production of
multi-
layer parisons for blow molding. Tubular parisons are used in making
containers
of various shapes as annular dies are typically easier to manufacture than
dies of
other shapes, such as oval or square. Annular co-extrusion dies are commonly
used to process high volume commodity resins as well as relatively low volumes
of barrier type resins.
Annular co-extrusion dies are generally of one of two arrangements;
namely axially fed and radially fed. In either type of arrangement, melt is
introduced into an inlet port from where it has to be evenly distributed about
the
circumference of an annular outlet. Good flow distribution is essential to
forming
film having layers which are uniform in thickness, appearance and structural
integrity. In axially fed co-extrusion dies, melt is fed in a direction
parallel to the
axis of the tubular form to be extruded. Each layer is formed between
respective
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die elements which are generally concentrically disposed in a manner analogous
to cups of different diameter stacked one within an other. The individual
layers
are merged upstream in an extrusion passage through which the co-extruded film
is discharged.
In radially fed co-extrusion dies, melt distribution blocks are stacked one
behind another along a die axis and melt is fed radially relative to the die
axis into
a respective inlet port in each melt distribution block. The melt distribution
blocks
distribute the melt about a central mandrel and discharge the melt in an axial
direction into an extrusion passage between the melt distribution blocks and
the
mandrel. Each consecutive melt distribution block applies an overlying melt
layer
to the melt moving along the extrusion passage.
Axially stacked radially fed co-extrusion dies are advantageous in that it is
relatively simple to vary the number of layers by varying the number of
"modules"
stacked along the die. Furthermore, each level presents a similar area and the
levels are more easily thermally isolated than possible with axially fed co-
extrusion dies in which heat from one die element is difficult to isolate from
adjacent die elements. Even melt distribution is however a much more
challenging problem with radially fed co-extrusion dies because of a much
shorter axial distance being available for melt equalization and the
requirement to
redirect melt flow from a radial to an axial direction after the melt has been
distributed into a thin film.
It is an object of the present invention to provide a radially fed multilayer
extrusion die which is effective in providing a uniformly thick film of melt
to an
extrusion passage.
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It is a further object of the present invention to provide a melt distribution
block for a radially fed multilayer extrusion die which can accept and combine
two different types of melt.
It is yet a further object of the present invention to provide a melt
distribution block for an extrusion die having a matched pair of distribution
passages so configured and oriented as to cause an averaging of extruded film
thickness by matching high flow areas of one of said pair of passages with
lower
flow areas of the other of said pair of passages.
Summar>r of the Invention
A melt distribution block for feeding melt through an extrusion die to an
extrusion passage. The melt distribution block has a generally annular body
with
an inner face extending about the extrusion passage, an outer face radially
outward of the inner face and opposite front and rear faces. The front and
rear
faces each have a series of flow divider channels thereon which extend in a
generally radially inward direction from an inlet through a series of flow
diverting
bifurcations which terminate in a plurality of feed spirals. Each of the feed
spirals
substantially encircles the inner face and narrows toward a radially inwardly
disposed end. An inlet port extends into the outer face to fluidly communicate
with the inlet of the flow divider channels.
A melt distribution die has an axially stacked array of melt distribution
blocks of the type described above interspersed with separator blocks
extending
radially about a centrally disposed mandrel. An extrusion passage is defined
between the mandrel and the stacked array of melt distribution and separator
blocks. The separator blocks cover the flow divider channels to maintain melt
flow within the flow divider channels. The separator blocks are spaced apart
from
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the feed spirals to define a generally continuous melt outlet passage
extending
into the extrusion passage.
In order to thermally isolate adjacent feed spirals" the separator blocks
may be provided with a radially extending insulating zone generally
corresponding in location to the feed spirals.
Descr~tion of Drawings
Preferred embodiments of the invention are described below with
reference to the accompanying drawings in which:
FIG. 1 is a perspective view of a melt distribution block according to the
present invention;
FIG. 2 is a perspective view corresponding to FIG. 1 but showing an
opposite face of a melt distribution block according to the present invention;
FIG. 3 is a top plan view of a melt distribution block according to the
present invention;
FIG. 4 is a bottom plan view of a melt distribution block according to the
present invention;
FIG. 5 is a section on line 5-5 of FIG. 4;
FIG. 6 is an axial section through a melt distribution die according to the
present invention;
FIG. 7 is an axial sectional view illustrating one half of a melt distribution
block according to the present invention mounted between two separator blocks;
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FIG. 8 is an axial section through a melt distribution die according to an
alternate embodiment of the present invention;
FIG. 9 is a view of a groove-face similar to that shown in FIG. 1;
FIG. 10 is a diagrammatic view showing another grooves layout;
FIG. 11 is a diagrammatic view showing a further grooves layout;
FIG. 12 is a diagrammatic view showing yet another grooves layout; and,
FIG. 13 is a close-up of a portion of FIG. 9.
Description of Preferred Embodiments
A melt distribution block according to the present invention is generally
indicated by reference 10 in the accompanying illustrations. The melt
distribution
block has a generally annular body 12 with an inner face 14 which, in use,
extends about and defines part of an outer surface an extrusion passage 16 in
FIGS. 6 and 7. The melt distribution block 10 has an outer face 18 radially
outward of the inner face 14, a front face 20 in FIG. 1 and a rear face 22 in
FIG.
2, opposite the front face 20.
The front and rear faces, 20 and 22 respectively have a series of flow
divider channels 24 extending into their surfaces. The flow divider channels
24
are generally concentrically disposed and define a flow path which extends in
a
generally radially inward direction from an inlet 26 through a series of flow
dividing bifurcations 28.
Each of the flow dividing bifurcations 28 is located at a juncture of the end
of a flow dividing channel 24 and the midpoint of an adjacent, radially
inwardly
disposed flow dividing channel 24. The direction of melt flow is initially
from an
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inlet 26 into the outermost flow divider channel 24 and then through the flow
dividing bifurcations 28 into adjacent radially inwardly disposed flow
dividing
channels 28. Upon passing through each flow dividing bifurcation 28, which are
in effect "T" junctions, melt flow is divided into two generally oppositely
directed
melt flow paths of similar configuration and therefore similar flow rate.
The flow dividing channels 24 distribute melt from the inlet 26 evenly
about the mold block 10 which is important as a first step in ensuring
uniformity
of flow from the mold block 10 into the extrusion passage 16 about its
circumference.
The flow dividing channels 28 terminate in four feed spirals 30 which are
radially inward of the flow dividing channels 28. Although four feed spirals
30 are
illustrated, other numbers may be selected as being more desirable in some
applications, keeping in mind however that the number of flow dividing
channels
28 would have to be selected accordingly.
Each feed spiral 30 is "fed" by (i.e., fluidly communicates with) two of the
flow divider channels 24 at references 31 to obtain as consistent as possible
a
flow of melt into each of the feed spirals 30 by "averaging" the melt flow
between
adjacent flow divider channels 28.
Each feed spiral 30 substantially encircles the inner face 14 in a loop of
diminishing radius and breadth. The feed spirals 30 are basically channels of
diminishing width and breadth, the purpose of which is to evenly spread melt
over a thin, even layer before it enters the extrusion passage 16. Using a
plurality
of circumferentially spaced apart feed spirals 30 evens out the flow from each
feed spiral. The front and rear faces 20 and 22 have respective feed spirals
30
which curve in the same direction relative to each other as viewed through the
melt distribution block 10.
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As can be seen in FIGS. 3, 4 and 5, an inlet port 32 extends into the outer
face 18 of the melt distribution block 10 and fluidly communicates within the
inlets
26 of the flow divider channels 24. As illustrated, the inlet port 32 feeds
the inlets
26 on the front face 20 and rear face 22 of the melt distribution block 10.
Alternatively, as suggested by dashed line 34 in FIG. 5, the inlet port 32 may
be
divided into separate ports, 32a and 32b respectively, to feed different types
of
melt to the front and rear faces 20 and 22 respectively.
A melt distribution die according to another aspect of the present invention
is generally indicated by reference 40 in FIG. 6. A segment of a melt
distribution
die 40 is shown in larger scale in FIG. 7. The melt distribution die 40
includes an
axially stacked array of melt distribution blocks 10 as described above
interspersed with separator blocks 42, extending about a central mandrel 44.
The
extrusion passage 16 is defined between the separator blocks 42, melt
distribution blocks 10 and the central mandrel 44.
The flow divider channels 24 are completely covered by the separator
blocks 42 in FIGS. 6 and 7. The feed spirals 30 are not completely covered by
virtue of a space between the separator blocks 42 and the front and rear faces
20 and 22 respectively adjacent the feed spirals 30 to define a melt outlet
passage 46 extending into the extrusion passage 16. Melt will therefore
overflow
the edges of the feed spirals 30 to form a film of melt which is continuous
about
the circumference of the melt distribution block 10 at least leading into the
extrusion passage 16.
In the arrangement illustrated, the front face 20 tapers toward the rear
face 22 adjacent the feed spirals 30 to provide the space for the melt outlet
passage 46. Alternatively, the front face 20 and rear face 22 may be
substantially
parallel across the melt block 10 and the space for the melt outlet passage 46
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may be accommodated by relieving the corresponding faces of the separator
blocks 42.
Although two melt distribution blocks 10 are illustrated in the stacked array
of the melt distribution die 40, it will be appreciated by those skilled in
such
structures that more or less melt distribution blocks may be accommodated
depending on the number of layers required to be co-extruded.
Creating multi-layer films requires processing different materials and
different layers. This means that dissimilar materials are processed at
different
temperatures. While not always the case, some materials can be damaged or
degraded if they are exposed to temperatures above their respective optimal
processing temperatures. When a temperature degradeable material in a co-
extrusion die is placed adjacent a material that is processed at a higher
temperature, as is often the case, the hotter processing material will
transfer
thermal energy to the heat sensitive material. This will tend to raise the
temperature of the heat sensitive material and potentially cause its
degradation.
According to a preferred embodiment of the present invention, the
temperature transfer problem may be addressed by adding an insulation layer
between adjacent melt distribution blocks 10 to inhibit thermal energy
transferred
between layers. FIG. 8 illustrates one manner in which adjacent melt
distribution
blocks 10 may be thermally isolated. In the FIG. 8 embodiment, one of the
separator blocks, indicated by reference 60, includes an insulating zone 62.
According to this embodiment, the insulating zone 62 may be a gap between
adjacent faces 66 of adjacent parts 64 of the separator block 60. The gap 62
may
be formed by machining at least one of the adjacent faces 66 of the parts 64
of
the separator so as to form a recess extending into the face(s). In FIG. 8,
corresponding recesses extend into each adjacent face. It will be appreciated
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however that only one of the faces 66 need be provided with a recess to form
the
gap comprising the insulating zone 62.
In the FIG. 8 embodiment, a spacer 68 is provided between the parts 64 of
the spacer block 60 adjacent the mandril 44 to provide support and maintain
the
parts 64 in a spaced apart relationship. The gap which comprises the
insulating
zone 62 may extend between the parts 64 and a radially outward face 70 of the
spacer 68. The spacer 68 may be of a metal, but preferably one with a lower
thermal conductivity than that of the separator block 60.
Other approaches to thermal isolation may be used as alternative or in
addition to the use of a gap for the insulation zone 62. For example, a
ceramic
insulator may be used between the parts 64 either within or in lieu of the
gap.
FIG. 9 is a plan view of a die-member 80. Groove-face-A is shown, having
melt-conveying-channels-A. Groove-face-B lies on the opposite side of the die-
member, i.e. on the underside of the die-member in the view of FIG. 9. The
groove-face-B has melt-conveying-channels-B, which lie exactly underneath the
melt-conveying-channels-A. That is to say, even if the die-member 80 were made
of glass, the melt-conveying-channels-B would not be visible in the FIG. 9
view,
because they lie underneath the melt-conveying-channels-A.
The melt-conveying-channels-A in FIG. 9 include: one entry-channel; two
divider-grooves; four supply-grooves; eight feed-grooves; four start-grooves;
and
four spiral-grooves.
FiG. 10 shows a similar plan view of another die-member 82. Here, the
whole set of melt-conveying-grooves-A as in FIG. 9 is shown diagrammatically,
in
that the set shown in FIG. 9 is represented by the line-A. In FIG. 10,
underneath
the die-member 82, melt-conveying-grooves-- B are represented by the line-B.
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In FIG. 10, the melt-conveying-grooves-A are staggered, i.e
circumferentially or orientationally offset, with respect to the melt-
conveying-
grooves-B. The line-A is spaced, in the circumferential or orientational
sense, an
arc-A relative to a datum-point 83 on the outer-face 84 of the die-member 82.
The line-B lies spaced at an arc-B relative to the datum-point 83, which is
different from arc-A.
When arc-A and arc-B are almost the same (FIG. 10), the designer may
find it convenient to arrange for both sets of melt-conveying-grooves-A and -B
to
be fed from a single point. So, in this case, melt-entry-channel-A 86 and melt-
entry-channel-B 87 both open into a unitary melt-entry-port 89. A pipe-
connector
90 enables the port 89 to be connected to a source of hot pressurized melt.
When arc-A and arc-B differ greatly (FIG. 11), preferably the melt-entry-
port is in two sections, having separate pipe-connectors 92,93. In this case,
the
melt-entry-channel-A 94 is not in communication with the melt-entry-channel-B
95. The two separate pipe-connectors 92,93 may be connected, either both to
the same source, or to two separate sources--which is appropriate, for
example,
if different melts are being fed respectively to the two sets of grooves.
In FIG. 9, the spiral-grooves-A have an anti-clockwise spiral sense, when
the die-member is viewed face-on to the groove-face-A. In FIG. 9, spiral-
grooves-
B lie exactly underneath, and are therefore hidden by, the spiral-grooves-A,
whereby the spiral-grooves-B also have an anti-clockwise spiral sense, when
viewed face-on to groove-face-A. (Of course, spiral-grooves-B, in that case,
have
a clockwise spiral-sense when viewed face-on to groove-face-B.)
In FIGS. 10,11 the grooves-A and the grooves-B both go clockwise, but
they do not overlie. In FIG. 12, the grooves-A lie in the opposite spiral-
sense from
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the grooves-B, and the grooves-A also are circumferentially offset from the
g rooves-B.
The configuration in which grooves-A lie exactly over grooves-B, as in
FIG. 9, is less preferred by the designer, because of the possibility that
some
slight unevenness on one face might be repeated in the other face, and thus
perhaps cause banding to appear in the resulting plastic film. Some manner of
staggering, or de-equalizing, the upper and lower grooves is preferred.
It is not essential that the melt-conveying-grooves-B underneath the die-
member be identical to the melt-conveying-grooves-A on top. For example, the
number M of spiral-grooves-B below need not be the same as the number N of
spiral-grooves-A above.
It is easier to ensure that the melt is divided evenly between the spiral-
grooves if the number of spiral-grooves is a multiple of two, since then each
junction is a simple T- or Y-junction. But, if the designer is bound to avoid
the
restriction to a multiple of two, an incoming-melt channel can be spilt into,
say,
three streams, rather than two, with careful engineering.
FIG. 13 illustrates a measure that is aimed at ensuring evenness in the
flow of melt to all the spiral-grooves. FIG. 13 concerns the lands between the
melt-conveying-grooves, and especially between the spiral-grooves. Two
adjacent spiral-grooves, F and G, are shown. Spiral-land-FG is the land lying
between spiral-groove-F and spiral-groove-G. The spiral-grooves-F and -G
receive melt from start-groove-F and start-groove-G respectively. Base-land-FG
is the land lying between start-groove-F on the left, start-groove-G on the
right,
spiral-groove-F to the inside, and feed-groove-FG-F and feed-groove-FG-G to
the outside. (Feed-groove-FG-F and feed-groove-FG-G both receive melt from
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supply-groove-FG, as shown.) More precisely, the base-land-FG is bounded by
the edges of the said grooves.
The groove-face of the illustrated die-member in FIG. 13 is overlain by a
smooth flat surface, for example by the smooth flat undersurface of another
die-
member stacked above the illustrated die-member. The spiral-land-FG is
machined to lie well clear of this overlying surface; for example, the spiral-
land-
FG typically is a millimeter, or more, clear of the overlying surface. Thus,
melt in
the spiral-grooves spills over the edges of the spiral-grooves, as the melt
passes
inwards, towards the inner-edge of the illustrated groove-face.
It is recognized that, preferably, the melt should not be allowed to spill
over in the region of the base-land-FG. The base-land-FG should be clamped
tightly against the said overlying surface: that is to say, the base-land-FG
should
not be clear of the overlying surface, such that melt from the feed-grooves-FG-
F
and -FG-G could spill over the base-land-FG, into the spiral-groove-F.
The function of the spiral-grooves is to ensure that the melt flows as
evenly as possible over the inner-edge of the annular groove-face, i.e. that
the
flow over the inner-edge is the same at every point on the circumference of
the
inner-edge. This evenness-function might be compromised if the melt were
allowed to flow over the base-land-FG (and, where there are four spiral-
grooves,
the corresponding three other base-lands), since the comparatively large area
of
the base-land would make it difficult to ensure that all the base-lands
between
the spiral-grooves allowed exactly equal amounts of melt to pass. By keeping
the
base-lands tight against the overlying surface, no melt can pass over any of
the
base-lands, whereby that source of possible unevenness is eliminated.
Evenness of melt flow over the inner-edge is very important to the quality
of the finished plastic film, and designers of blown-film extrusion dies
strive to
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ensure that the melt passing into and through the spiral grooves is the same
for
all the spiral-grooves.
Thus, the spiral-land-FG is machined away, and lies well below the level
of the base-land-FG. This difference preferably is accommodated by providing a
step-FG. The step-FG preferably is at least approximately contiguous with, and
in
line with, the left edge 98 of the start-groove-G. The step-FG, like the rest
of the
melt-conveying-channels of the die, should be profiled to avoid impeding the
flow
of melt, and especially to avoid even tiny pockets that might cause melt to
slow
down and hang up.
There are several kinds of channels or grooves included in the melt-
conveying-channels-A as shown in FIG. 9. On entering the die-member, the melt
passes first through the flow-divider-grooves, where the incoming flow from
the
melt-entry-port is divided among four supply-grooves, into four separate
streams.
It might be considered that these four streams may be fed straight into the
four
spiral-grooves. However, it is recognized that this direct connection should
be
avoided. Liquid polythene, and many other plastics from which blown-film is
made, have the unfortunate property that the viscosity of the liquid is very
temperature-dependent. Even a slight cooling of the liquid causes a large
increase in viscosity. In a melt-conveying-channel, the faster-moving liquid
in the
middle of the channel tends to be the less viscous liquid, i.e. tends to be
the
hotter liquid, than the slow liquid adjacent to the walls of the channel.
Thus, the
cooler portions of the liquid flowing in the channels tend to migrate to the
walls of
the channels, the hotter portions to the centers. As a result, the channel can
serve, indeed, almost as a temperature separator, separating the hot stream in
the center of the channel from the surrounding cooler stream on the walls.
Attention is directed to patent publication U.S. Pat. No. 5,261,805 (Gates,
Nov 1993). This patent shows the use of flow-mixing-channels to a center-fed
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cylindrical die. Now, it is recognized that flow-mixing-grooves can be
provided in
an annular flat die, of the kind as shown herein, which is fed from a single
melt-
entry-port on the outer-face thereof. In the flow-mixing-grooves, the melt is
subdivided and recombined, as it flows inwards towards the inner-edge of the
die, and this can serve to even out some temperature differences.
The flow-mixing-grooves are interposed between the four supply-grooves
and the four spiral-grooves, whereby the grooves convey melt from the melt-
entry-port in the outer-face inwards first through the flow-divider-channels
to the
supply-grooves, then inwards through the flow-mixing-channels-A, then inwards
through the spiral-grooves-A, and finally inwards towards the inner-edge-A of
the
g roove-face-A.
As shown in FIG. 9, the flow-mixing-channels include subdivides-junctions
and recombines-junctions. At the subdivides-junctions, the four incoming-
streams
from the flow-divider-channels are sub-divided into respective left and right
subdivided-streams. The recombines-junctions tie inwards of the subdivider-
junctions, and between adjacent subdivides-junctions, in the sense of being
positioned to receive the subdivided-streams moving inwards from the adjacent
subdivides-junctions.
Each recombines-junction receives the left subdivided-stream from the
adjacent one of the subdivides-junctions to the right of that recombines-
junction,
and receives the right subdivided-stream from the adjacent one of the
subdivider-
junctions to the left of that recombines-junction. The recombines-junction
combines the said left and right subdivided-streams into one recombined-stream
respective to that recombines-junction.
By this arrangement, the streams from the supply-grooves are split, the
hot center portions thereof going to one wall of the feed-grooves, and the
cold
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outer portions going to the other wall of the feed-groove. When the portions
recombine, in the start-grooves, the cold portions now lie to the inside, and
the
hot portions to the outside, of the start-grooves. As a result, the melt in
the start-
grooves, and hence in the spiral-grooves, tends to have a more even
temperature profile than the melt in the supply-grooves.
The die-members shown in the previous drawings are flat. Alternatively,
the melt-conveying-grooves may be provided on a convex conical surface, in the
manner as illustrated, for example, in patent publication U.S. Pat. No.
5,779,959
(Tuetsch, July 1998). Conical die-members may be provided in a stack, and the
concave conical surface of one die-member serves as the overlying surface for
the melt-conveying-grooves provided in the next die-member below, in the
stack.
Many of the aspects of groove layout as described herein can be applied
to conical dies. However, it can be difficult to machine grooves in a concave
(i.e.
inwards-facing) surface, and therefore providing grooves only on the convex
side
of the die-member is preferred, when the die member is conical.
