Multilayer modular extrusion die

An extrusion die is disclosed including a first member having a surface which includes a plurality of spiral channel segments formed therein and a second member having a surface which includes a plurality of spiral channel segments formed therein. The surfaces of the first and second members are positioned with respect to each other to form a leakage gap therebetween, wherein the spiral channel segments of the first and second members are mated to form a plurality of discrete spiral channels having spiral centerlines which serpentine back and forth across the leakage gap. A spiral channel is formed by a spiral channel segment in the first member aligned with a spiral channel segment in the second member, where the depths of the respective spiral channel segments are substantially 90.degree. out of phase. The respective spiral channel segments have a plurality of interconnected spiral channel segment portions which each gradually increase in depth from their surface to a maximum depth point and thereafter decrease in depth back to such surface.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to extrusion dies utilized in blown plastic 
film applications and, more particularly, to extrusion dies including a 
unique spiral channel configuration which improves mixing and residence 
time distribution of material flowing therethrough. 
2. Description of Related Art 
Conventional extrusion dies for blown film and other annular extruded 
applications have generally been of the spiral mandrel type, as disclosed 
in U.S. Pat. No. 4,201,532 to Cole. As seen therein, a plurality of 
plastic feed spiral channels are cut into the inner flow surface thereof 
and the outer flow surface remains flat, with an overflow gap between the 
inner and outer flow surfaces acting as an annular passage to the annular 
extrusion orifice. More recently, extrusion dies having a disk or 
frusto-conical configuration have been utilized, particularly in stacking 
arrangements which permit several types of material to be joined together 
as a multi-layer film. Examples of this type of extrusion die are 
disclosed in U.S. Pat. No. 5,076,776 to Yamada et al. and U.S. Pat. No. 
4,798,526 to Briggs et al. In all of these die designs, it has been well 
known to utilize spiral channels to promote uniformity of the various 
material layers, as well as better mixing of materials within each layer. 
Spiral channels utilized with the various kinds of extrusion dies in the 
prior art generally have been formed in only one of the two flow surfaces 
adjacent an overflow gap therebetween (see FIG. 7). Further, such spiral 
channels generally have a depth which uniformly decreases from beginning 
to end (see FIG. 8). This spiral channel configuration promotes flow 
kinematics in which material at the top of such spiral channel (adjacent 
the die surface) flows into the overflow gap much more quickly than 
material flowing at the bottom of the channel. Consequently, the residence 
time for material flowing at the bottom of the spiral channel is much 
greater and leads to material degradation. Moreover, the leakage flow into 
the overflow gap is such that it does not involve mixing between adjacent 
spiral channels to a very great degree. This lack of mixing can cause one 
or more diagonal weld lines to be formed across the final film product, 
which affects its uniformity, structural integrity, and appearance. 
As seen in U.S. Pat. No. 3,809,515, a stacked-type extrusion die is 
disclosed having spiral channels formed by mating grooves provided within 
both flow surfaces. In this design, the top and bottom grooves have a 
different radial pitch, causing the two halves of the spiral channel to 
move out of phase from one another. Therefore, the half-channel of one 
flow surface mates with part of one half-channel downstream and part of 
another half-channel upstream formed in the corresponding flow surface. 
Although the arrangement claims to promote inter-spiral mixing, the 
leakage through the overflow gap always comes from the middle of the 
spiral channel. Accordingly, material moving at the very top of the top 
groove and at the very bottom of the bottom groove continues moving 
through the spiral channel and does not participate in leakage flow until 
the end of the spiral channel. This leads to the same negative 
consequences relating to mixing and residence time uniformity as the 
spiral channel design formed on only one flow surface discussed above. 
In light of the foregoing, it would be desirable for extrusion dies, 
particularly those utilized in blown plastic film applications, to include 
spiral channels which promote improved mixing of material between adjacent 
spiral channels and improved residence time uniformity of material flowing 
through the spiral channels. Further, it would be desirable for such 
extrusion dies to include spiral channels having flow kinematics where 
material at the top and bottom of such spiral channels is forced to 
participate in leakage flows into the overflow gap due to channel 
geometry. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of the present invention, an extrusion die is 
disclosed including a first member having a surface which includes a 
plurality of spiral channel segments formed therein and a second member 
having a surface which includes a plurality of spiral channel segments 
formed therein. The surfaces of the first and second members are 
positioned with respect to each other to form a leakage gap therebetween 
from an entry end to an exit end, wherein the spiral channel segments of 
the first and second members are mated to form a plurality of spiral 
channels having centerlines which serpentine back and forth across the 
common leakage gap. The extrusion die may be of the disk-type design, 
wherein the first member is an upper die plate with a lower surface having 
the plurality of spiral channel segments formed therein and the second 
member may he a lower die plate with an upper surface having the 
corresponding plurality of spiral channel segments formed therein. 
Alternatively, the first member may he a central mandrel with a 
cylindrical outer surface having the plurality of spiral channel segments 
formed therein and the second member would then he a cylindrical die body 
surrounding the central mandrel with an inner surface having the 
corresponding plurality of spiral channel segments formed therein. 
In accordance with a second aspect of the present invention, each spiral 
channel is composed of a spiral channel segment formed in a surface of the 
first member aligned with a spiral channel segment formed in the abutting 
surface of the second member. Each of the spiral channel segments of the 
first and second members have a plurality of interconnected spiral channel 
segment portions, including at least an inlet spiral channel segment 
portion and an end spiral channel segment portion. The interconnected 
spiral channel segment portions of the spiral channel segments gradually 
increase in depth from the respective surface to a maximum depth point and 
thereafter gradually decrease in depth back to the respective surface. The 
interconnected spiral channel segment portions also have a substantially 
arcuate cross-section in a plane transverse thereto, with the spiral 
channels having a substantially cylindrical cross-section in a plane 
transverse to the centerline thereof. The maximum depth point of each 
succeeding interconnected spiral channel segment portion diminishes from 
the inlet spiral channel segment portion to the end spiral channel segment 
portion, thereby causing the cross-sectional area of each spiral channel 
to diminish from an inlet end to a terminating end. 
In accordance with a third aspect of the present invention, a flow channel 
is formed between the first and second members by their abutting surfaces. 
The flow channel is defined by the leakage gap between the first and 
second members, as well as a cross-section across the spiral channels 
formed thereby. Although the width of the leakage gap between the abutting 
surfaces remains substantially constant, the overall depth of the flow 
channel decreases from the entry end of the extrusion die to the exit end. 
A primary object of the present invention is to provide an extrusion die 
having a spiral channel configuration which improves residence time 
uniformity of material flowing therethrough to prevent material 
degradation. 
It is another object of the present invention to provide an extrusion die 
having a spiral channel configuration which improves mixing between 
adjacent spiral channels to homogenize temperature and composition. 
Yet another object of the present invention is to provide an extrusion die 
having a spiral channel configuration which has a flow pattern that 
promotes leakage flow across the entire channel. 
Another object of the present invention is to provide a spiral channel 
configuration which is compatible with several extrusion die designs. 
Yet another object of the present invention is to provide an extrusion die 
having a spiral channel configuration where material at the top and bottom 
of such spiral channels is forced to participate in leakage flows by 
virtue of the channel geometry. 
Still another object of the present invention is to provide an extrusion 
die having a spiral channel configuration which diffuses weld lines formed 
by adjacent layers of material. 
Another object of the present invention is to provide an extrusion die 
having a spiral channel configuration which promotes improved film quality 
for an end product having multiple layers. 
These objects and other features of the present invention will become more 
readily apparent upon reference to the following description when taken in 
conjunction with the following drawing.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings in detail, wherein identical numerals 
indicate the same elements throughout the figures, FIG. 1 depicts a radial 
feed extrusion die 10, otherwise known as an extrusion head, for use in 
film blowing and other annular extrudate applications. Extrusion die 10 is 
shown as preferably having a plurality of disk-shaped die modules 12 in a 
stacked arrangement and designated as die modules 12a-g. In order to 
provide thermal isolation between each adjacent die module 12, to allow 
materials of varying temperature characteristics to be utilized, an air 
gap 13 is provided therebetween. More specifically, it will be seen in 
FIGS. 1-3 that each die module 12 includes an upper die plate 14 and a 
lower die plate 16, where a lower surface 18 of upper die plate 14 is 
positioned adjacent to (or in abutting relation to) an upper surface 20 of 
lower die plate 16. The relationship between upper and lower die plates 14 
and 16 is preferably such that a leakage gap 24 is formed therebetween 
(see FIGS. 4 and 5). 
It will be noted that extrusion die 10 includes a hollow central mandrel 25 
having a lower portion 26 below a lowermost die module 12g and an upper 
lip portion 28 positioned above an uppermost die module 12a. Mandrel 25 
forms a common annular extrusion passage 29 with the inside surfaces of 
modules 12 through which the combined polymer layers move upwardly to an 
annular extrusion orifice 30. A lower plate member 27 is preferably 
located below mandrel base portion 26 to facilitate connection with an air 
supply (not shown) and an upper plate member 31 is located between lip 
portion 28 and die module 12a. It will be understood that extrusion die 10 
may be configured to include any number of die modules 12 to be stacked 
therein, whereby a corresponding number of layers of thermoplastic 
material may be utilized to form the finished film product. 
A corresponding number of outer die body feed modules 33 (designated as 
33a-g) surround die modules 12 to form an overall cylindrical die body 34. 
Annular feed modules 33 include passageways by which the melt or polymer 
is delivered from an extruder to die modules 12, as described hereinafter. 
The stacking arrangement of modules 12 and 33 in extrusion die 10 is then 
accomplished by fastening each outer die body feed module 33 to a die 
module 12 adjacent thereto by means of a fastener 32 (see outer die body 
feed module 33g and die module 12g in FIG. 1). Each die module 12 
preferably includes an annular flange 39 extending therefrom (from lower 
die plate 16 in FIG. 1) to facilitate this attachment. Additionally, a 
plurality of bores 35 are provided (see left portion of FIG. 1 and FIG. 
10) within each die module 12, lower portion 26 of central mandrel 25, and 
upper plate member 31 so that they may be aligned and a tie bolt or other 
fastening arrangement (not shown) can interconnect them. A plurality of 
bores 37 are also provided in die modules 12 so that a fastener (not 
shown) can interconnect each pair of upper and lower die plates 14 and 16 
(see right portion of FIG. 1 and FIG. 10). 
With respect to each die module 12, it is seen in FIG. 2 that lower surface 
18 of upper die plate 14 includes a plurality of spiral channel segments 
36 formed therein. Likewise, FIG. 3 depicts a plurality of spiral channel 
segments 38 formed in upper surface 20 of lower die plate 16. As seen 
therein, spiral channel segments 36 and 38 preferably have substantially 
the same arcuate length and are arranged with other spiral channel 
segments formed in their respective die plate to construct a swirling 
pattern. Eight separate spiral channel segments 36 and 38 are shown in 
FIGS. 2 and 3 with respect to upper and lower die plates 14 and 16, the 
inlets thereof being circumferentially spaced along an outer periphery and 
leading inwardly to their radially inner terminating ends. Depending upon 
the needs and requirements of extrusion die 10 (e.g., a desired flow 
rate), however, any number of spiral channel segments 36 and 38, 
respectively, may be provided. 
It will be noted that each spiral channel segment 36 has a plurality of 
interconnected spiral channel segment portions, including at least an 
inlet spiral channel segment portion 44 and an end spiral channel segment 
portion 46. Preferably, one or more intermediate spiral channel segment 
portions 48 are provided therebetween. Likewise, each spiral channel 
segment 38 has a plurality of interconnected spiral channel segment 
portions including at least an inlet spiral channel segment portion 45 and 
an end spiral channel segment portion 47, as well as one or more 
intermediate spiral channel segment portions 49 therebetween. Although 
spiral channel segments 36 and 38 are each shown as including three spiral 
channel segment portions in FIGS. 2 and 3, any number of interconnected 
spiral channel segment portions may be utilized for desired residence 
times of material therein and the frequency of periodic leakage by 
material flowing along the channel surface into a flow channel described 
in detail herein. 
Most importantly, when upper die plate 14 and lower die plate 16 are 
positioned so that lower surface 18 and upper surface 20 are adjacent each 
other, a plurality of spiral channels 50 are produced in which centerlines 
52 thereof serpentine above and below leakage gap 24 between upper and 
lower die plates 14 and 16 (as best seen in FIG. 6), giving spiral 
channels 50 a wavy configuration. It will be understood that each spiral 
channel 50 is defined by a matched pair of spiral channel segments 36 and 
38 in upper and lower die plates 14 and 16, respectively. 
The wavy design of spiral channels 50 is accomplished by both the design of 
spiral channel segments 36 and 38, as well as their relationship to each 
other. As depicted in FIGS. 5 and 6, each interconnected spiral channel 
segment portion of spiral channel segments 36 and 38 has a substantially 
arcuate cross-section of varying size in a plane transverse thereto. 
Accordingly, each interconnected spiral channel segment portion of spiral 
channel segments 36 and 38 gradually increases in depth along a first 
arcuate surface 54 and 56, respectively, to a maximum depth point 58 and 
60. Thereafter, a second arcuate surface 62 and 64 for each spiral channel 
segment portion of gradually decreasing depth extends from maximum depth 
points 58 and 60 to lower surface 18 and upper surface 20, respectively. 
It will be seen in FIG. 6 that an included angle .alpha. exists between 
first arcuate surface 54 and second arcuate surface 62 and an included 
angle .beta. exists between first arcuate surface 56 and second arcuate 
surface 64. Preferably, angles .alpha. and .beta. increase between 
succeeding interconnected spiral channel segment portions of spiral 
channel segments 36 and 38 from inlet spiral channel segment portions 44 
and 45 to end spiral channel segment portions 46 and 47. Maximum depth 
points 58 and 60 for succeeding interconnected spiral channel segment 
portions preferably decrease from inlet spiral channel segment portions 44 
and 45 to end spiral channel segment portions 46 and 47. Therefore, it 
will be understood that each spiral channel 50 decreases in 
cross-sectional area from inlet spiral channel segment portions 44 and 45 
to end spiral channel segment portions 46 and 48. 
In order for spiral channels 50 to have the desired wavy design, each 
matched pair of spiral channel segments 36 and 38 is preferably 
substantially 90.degree. out of phase (depth-wise). It is seen in FIG. 6 
that maximum depth points 58 and 60 of spiral channel segments 36 and 38 
are not in alignment, but rather are positioned across from a high point 
of the opposite die plate, known as leakage points 70 and 71, located 
between adjacent spiral channel segment portions. This is accomplished by 
varying the arcuate lengths of interconnected spiral channel segment 
portions for spiral channel segments 36 and 38. 
In particular, the arcuate lengths of the interconnected spiral channel 
segment portions for spiral channel segments 36 preferably decrease in 
arcuate length from inlet spiral channel segment portion 44 to 
intermediate spiral channel segment portion 48 and then from intermediate 
spiral channel segment portion 48 to end spiral channel segment portion 
46. With respect to the corresponding interconnected spiral channel 
segment portions for spiral channel segments 38, the arcuate length 
preferably increases from inlet spiral channel segment portion 45 to 
intermediate spiral channel segment portion 49 and then from intermediate 
spiral channel segment portion 49 to end spiral channel segment portion 
47. It will be understood that this is only one design for accomplishing 
the wavy design of spiral channels 50 and is not meant to limit the scope 
of the invention. 
As seen in FIG. 4, a flow channel 72 is formed between the opposed surfaces 
of upper and lower die plates 14 and 16 from an outer perimeter 74 to an 
inner perimeter 76 of die module 12. It will be seen that flow channel 72 
is definedby leakage gap 24 between upper and lower die plates 14 and 16, 
which opens radially inwardly into annular extrusion passage 29, as well 
as a cross-section across the plurality of spiral channels 50. It is 
preferred that the width of leakage gap 24 between upper and lower die 
plates 14 and 16 remain substantially constant, although the overall depth 
of flow channel 72 preferably decreases from outer perimeter 74 to inner 
perimeter 76. Although not shown, it will be understood that leakage gap 
24 may increase in size between inner and outer perimeters 76 and 74, with 
the direction of increase being the same as the flow direction. 
The cross-section across spiral channels 50 inherently includes the 
cross-section of several spiral channel segments 36 and 38 in upper and 
lower die plates 14 and 16, which occurs at varying positions in the 
arcuate lengths thereof (and therefore has varying depths). As depicted in 
FIG. 4, this includes cross-sections across both spiral channel segments 
at outer and inner perimeters 74 and 76 (in order to promote symmetrical 
flow of material into and out of flow channel 72) and alternating 
cross-sections of spiral channel segments 36 and 38 therebetween. 
As stated previously, leakage points 70 and 71 are high points of spiral 
channel segments 36 on lower surface 18 of upper die plate 14 and spiral 
channel segments 38 on upper surface 20 of lower die plate 16, 
respectively, which are located between each adjacent spiral channel 
segment portion of spiral channel segments 36 and 38. For example, as seen 
in FIG. 6, leakage points 70a and 71a are located between inlet spiral 
channel segment portions 44 and 45 and intermediate spiral channel segment 
portions 48 and 49, and leakage points 70b and 71b are located between end 
spiral channel segment portions 46 and 47 and intermediate spiral channel 
segment portions 48 and 49. It is at leakage point 70 that a portion of 
the material flowing along arcuate surfaces 54 and 62 and at leakage point 
71 that a portion of the material flowing along arcuate surfaces 56 and 64 
is forced to flow into flow channel 72, which is substantially transverse 
to the direction of flow through spiral channel 50. The flow kinematics of 
material through spiral channels 50 is produced by the V-shaped 
configuration of each spiral channel segment portion, where the material 
flows from the surface of the die plate to the maximum depth point and 
then back to the die plate surface. Therefore, the maximum residence times 
for material within each interconnected spiral channel segment portion is 
substantially uniform and prevents the degradation of material occurring 
in prior spiral channel designs. 
More specifically, it will be understood from the diagrammatic depiction in 
FIG. 6 that since the material flowing through spiral channel 50 is a 
viscous material (e.g., a polymer melt), it will have laminar flow 
characteristics. As seen therein, a portion 73 of material 75 flowing 
through inlet spiral channel segment portions 44 and 45 exits into flow 
channel 72 at leakage point 71a. Material 75 continues along first arcuate 
surface 54 of inlet spiral channel segment portion 44 to maximum depth 
point 58 and then second arcuate surface 62 of inlet spiral channel 
segment portion 44. Thereafter, a portion 79 of material 75 flows into 
intermediate spiral channel segment portion 49 and a portion 81 flows into 
flow channel 72 at leakage point 70a. Material 79 is directed toward 
second arcuate surface 64 of intermediate spiral channel segment portion 
49 of spiral channel segment 38, which causes material 79 to flow to upper 
surface 20 of lower die plate 16. A portion of material 83 flows into flow 
channel 72 at leakage point 71b and a portion 85 continues toward second 
arcuate surface 62 of intermediate spiral channel segment portion 48 of 
spiral channel segment 36. This process continues throughout each spiral 
channel segment portion until any remaining material at terminating end 68 
of spiral channel 50 is then finally forced into flow channel 72. 
Accordingly, material entering flow channel 72 occurs across the entire 
length of spiral channels 50, with leakage from material flowing adjacent 
arcuate surfaces 54, 56, 62 and 64 occurring periodically at leakage 
points 70 and 71. It should also be noted that material flowing into flow 
channel 72 mixes with other material flowing through adjacent spiral 
channels located radially inward of spiral channel 50. This mixing between 
adjacent spiral channels promotes diffusion of weld lines, which may be 
prevented completely if sufficient mixing takes place. 
Returning now to FIG. 1, it will be recalled that extrusion die 10 includes 
center mandrel 25 extending through die modules 12a-g and die body 34 so 
that an annular extrusion passage 29 is produced between center mandrel 
and inner perimeter 76 of die modules 12a-g to annular extrusion orifice 
30. A plurality of openings 82 are provided at an outer circumferential 
portion of upper and lower die plates 14 and 16 for each die module 12 
(see FIG. 10), wherein material is encouraged to flow through upper and 
lower die plates 14 and 16 to annular extrusion passage 29. Feed openings 
82 of die modules 12 are in flow communication with a feed nozzle 84 
within die body 34 as described below. 
In order to promote equal path length distribution of material through die 
modules 12, and thereby promote a uniform thickness of material in the 
layers of the finished film product, a network 86 of passages for feeding 
material to die module openings 82 is provided. As best seen in FIGS. 9 
and 10, network 86 includes a single inlet 88 connected to feed nozzle 84 
and a plurality of radial exit passages in which each one is in flow 
communication with one of die module openings 82. The configuration of 
network 86 is such that the distance between network inlet 88 and each die 
module opening 82 is substantially equal. 
More specifically, it will be understood that network 86 of passages 
includes a first arcuate passage 92 in flow communication with network 
inlet 88 where first arcuate passage 92 extends approximately one-quarter 
the circumference of die block module 12 from inlet 88 in each direction. 
A first connector passage 94 and a second connector passage 96 are in flow 
communication with the ends of first arcuate passage 92, with first and 
second connector passages 94 and 96 being oriented substantially 
perpendicular to first arcuate passage 92 and positioned in directly 
opposite relation to each other across die module 12 (see FIG. 10). It 
will be noted that the transition between first arcuate passage 92 and 
first and second connector passages 94 and 96, as with the transition 
between all passages in network 86, is radiused to promote proper flow 
therebetween. 
A second arcuate passage 98 is provided in flow communication with first 
connector passage 94, wherein second arcuate passage 98 extends 
approximately one-eighth the circumference of die module 12 from first 
connector passage 94 in each direction. Likewise, a third arcuate passage 
100 is in flow communication with second connector passage 96, with third 
arcuate passage 100 extending approximately one-eighth the circumference 
of die module 12 from second connector passage 96 in each direction. 
A third connector passage 102 and a fourth connector passage 104 is in flow 
communication with the ends of second arcuate passage 98, with third and 
fourth connector passages 102 and 104 being oriented substantially 
perpendicular to second arcuate passage 98. A fifth connector passage 106 
and a sixth connector passage 108 is in flow communication with the ends 
of third arcuate passage 100, with fifth and sixth connector passages 106 
and 108 being oriented substantially perpendicular to third arcuate 
passage 100. It will also be noted in FIG. 10 that fourth and fifth 
connector passages 104 and 106 and third and sixth connector passages 102 
and 108, respectively, are positioned in directly opposite relation to 
each other across die module 12. 
A fourth arcuate passage 110 is in flow communication with third connector 
passage 102, wherein fourth arcuate passage 110 extends approximately 
one-sixteenth the circumference of die module 12 from third connector 
passage 102 in each direction. A pair of exit passages 112 and 114 extend 
radially inward from fourth arcuate passage 110 to be in flow 
communication with two feed openings 82 in die module 12. Likewise, a 
fifth arcuate passage 116, a sixth arcuate passage 118, and a seventh 
arcuate passage 120 are provided which are in flow communication with 
fourth connector passage 104, fifth connector passage 106, and sixth 
connector passage 108, respectively, wherein each such arcuate passage 
extends approximately one-sixteenth the circumference of die module 12 
from its respective connector passage in each direction. A pair of exit 
passages (not identified) extend radially inward from the ends of fifth 
arcuate passage 116, sixth arcuate passage 118, and seventh arcuate 
passage 120 and are in flow communication with other feed openings 82 in 
die module 12 as with those exit passages 112 and 114 for fourth arcuate 
passage 110. 
It will be understood that greater or fewer arcuate and connector passages 
may be required depending upon the number of feed openings 82 in die 
module 12. Accordingly, such number will also affect the circumferential 
length for each arcuate passage as should be understood by those skilled 
in the art. 
Having shown and described the preferred embodiment of the present 
invention, further adaptations of the spiral channel design for the die 
modules can be accomplished by appropriate modifications by one of 
ordinary skill in the art without departing from the scope of the 
invention. In particular, while the extrusion die described herein 
involves a plurality of matching disk-shaped die plates, a central mandrel 
and surrounding die body with a leakage gap therebetween may employ the 
unique spiral channel design described herein. In such case, both the 
outer surface of the central mandrel and the inner surface of the die body 
will have the plurality of spiral channel segments formed therein which 
together comprise the spiral channels having centerlines that serpentine 
back and forth across the leakage gap. Further, material flowing through 
the extrusion die (as shown and described herein) may be either side fed 
or center fed into the spiral channels depending upon die design. 
While the form of apparatus herein described constitutes a preferred 
embodiment of this invention, it is to be understood that the invention is 
not limited to this precise form of apparatus, and that changes may be 
made therein without departing from the scope of the invention which is 
defined in the appended claims.