Note: Descriptions are shown in the official language in which they were submitted.
1~)3766Z
This is a division of copending Canadian patent
application Serial No. 127,238 which was filed on 9 No~ember 1971.
Background of the Invention
1 Field of the Invention
This invention relates to apparatus for advancing
and working thermoplastic materials, and more partic~larly
apparatus for advancing thermoplastic materials successively
through feed, compression, relief, and metering zones with
facilities being provided in the metering zone without
interrupting the helical flight of the extruder screw for
obtaining a high degree of thermal uniformity by an improved
mixing of the thermoplastic materials.
2. Description of the Prior Art
In the extrusion art, and especially in the
extrusion of thermoplastic materials for insulating conduc-tors
~or communications needs, there is an increasing demand for
equipment of higher output rates. The output rates for
extrudates covering conductors, which have somewhat thin
cross section, are governed somewhat by the maximum rate
at which extrusion can be performed without introducing
defects in the products due to a lack of uniform temperature.
For example, an extruder for spinning yarn ends is operated
beyond a practical rate thereof when the filaments passing
therefrom are susceptible to breakage during processing or
exhibit an unacceptable variation in denier. When the
extrudate is a sheet, film variations in the thickness of the
film are indicative of an improper rate of extrusion.
Generally, the lack of temperature uniformity which manifests
itself by defects such as nonuniform dimensions or reduced
strength characteristics evinces a failure to achieve a
thorough mixing of the thermoplastic material or materials
within the extruder.
66~
For experimental purposes, a mixture of pellets
of a clear thermoplastic material of a polyethylene or
polyvinylchloride base together with a very small percent,
e.g., one per cent, of color concentrate pellets may be
fed into a barrel of an extruder. The small amount of
color concentrate mixes with the clear compound material
when melting occurs so that the melt regions become colored
- and are easily distinguishable from the unmelted material.
Once meLting has begun, three distinct regions are noted
in a cross section of a channel formed by a helical flight
on an extruder screw. These are (1) the unmelted plastic
or solid bed, (2) a thin melt film between the solid bed
and the barrel, and (3) a melk pool where melted material
collects. The percentage of unmelted plastic can be
evaluated as a function of position in the extruder.
The thermoplastic material begins to melt along
the interface with the inner surface of the barrel. Then,
as the flight of the extruder screw advances, the flight
wipes off the melt and forms a melt pool on the upstream
side of each section of the channel formed by the turns
of the flight. Some of the solid materials become tacky
but may resist mixing and being transferred into a molten
state thereby detracting from the homogeneity of the mix.
It has been found that better mixing and
temperature distribution are possible through the use of
relatively expensive extruders of increased barrel length
to diameter ratios. A discussion of several available
extruder screw designs is given in a paper, "An Operating
Evaluation of Various 8" Extruder Screws Using an Infra-Red
~hermometer", by R.V. DeBoo and W.B. Beck, prepared for
the Sixteenth Annual Symposium on Wire and Cable, November
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29 and 30 and December 1, 1967.
In one screw design, commonly referred to as an
immediate compression design, the root diameter of the screw
; increases uniformly for approximately fourteen turns along
the axis of the screw followed by a metering section of
approximately six turns. The depth of the compression
section at the small diameter end thereof is approximately
859 mils whereas the depth along the metering secti~n which
has a uniform diameter is approximately I72 mils.
- 10 In another prior art design screw, commonly
referred to as a conventional metering screw, an initial
section thereof has a constant root diameter for approximately
eight turns followed by a uniformly increasing root
diameter section ~or six turns Eollowed by a six-turn
metering section. In this conventional metering screw, the
depth of the feed section is approximately 688 mils and
the depth of the metering section approximately 172 mils.
The conventional metering screw has a sli~ht advantage
at higher revolutions per minute since the presence of the
feed section insures an adequate supply of resin while at
the same time providing additional time for the resin to
pick up barrel heat. This minim~zes subjecting the resin
prematurely to compressive shear forces. Considering the
limiting depth of the metering zones, which amount to
approximately;:thirty per cent of the total screw length,
the output for these screws has been regarded as acceptable.
Heat is usually applied from an external source
to the extruder barrel in the compression section to raise
the temperature of the material. In the metering section,
the temperature of the material is increased over that of
the barrel because of the energy that has been imparted
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to the material and hence, in that section, the barrel acts
as a heat sink. As the successive portions of the material
are advanced into the metering section, the materials have
been generally thoroughly mixed. In the metering section,
heat is distributed throughout the materia] so that the
material is homogenized with respect to temperature, i.e.,
thermal uniformity.
A still further prior art screw design commonly
referred to as a compression relief design is characterized
in having a high output of extrudate at a low temperature
which is variable. The compression relief design has a
feed section of six turns with a depth of 750 mils, a
compression section of six turns with a minimum depth of
150 mils, a compression relie section of one-half -turn
with a maximum depth of 250 mils and a metering section of
seven and one-half turns with a constant depth of 250 mils.
The compression relief design screw with its relatively
deep meteringsection is significantly better in performance
in terms of greater output and has better temperature
control than the two priorly described screws.
The compression relief design screw is the genus
screw design, whereas the compression screw is a species
thereof with a compression relief section of zero length.
The compression screw design is somewhat disadvantageous
since it is difficult to manipulate independently the three
sections or the metering section without affecting:-the
others. However, with the cor.~pression relief screw design,
the metering section can be adjusted and the compression
relief section designed to join the metering section to
the compression section.
A still further prior art screw design is commonly
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referred to as a slotted ring screw design which includes a
feed section of four turns for a constant depth of 675 mils
followed by a compression section of four turns with a
minimum depth of 270 mils and a metering section of twelve
turns with a constant depth of 270 milsu The slotted ring
screw design is characterized by a generally high output
with good mixing and a high temperature, the temperature
being constant. Additionally, the slotted ring screw design
has a broken flight to permit mounting the slotted ring to
the root diameter portion of the screw. However, the
slotted ring screw design has the disadvantage of having
a high shear heating because of the longer metering section,
the bro};en flight and the hydrodynamic action of the
slotted ring.
The presence of the slotted rings in the just
described design ~urtail somewhat the output capability of
the deep metering sections of this screw. Without the
rings, a higher output may be achieved, but the temperature
of the material tends to be less and not as uniform as with
the rings present. Also, with the slotted ring design,
and the broken flight, there occurs so-called "dead spaces"
which tend to cause the thermoplastic material to back-up,
especially when using polyvinylchloride as the extrudate.
The action of the screw, in addition to carrying
the material through the bore, effects a physical blending
of the thermoplastic particles and a shearing type of
mixing between the materials at the cylinder bore walls
and screw flight edges. A thorough mixing and blending
of the material is necessary to provide a homogeneous melt
and to obtain a uniform extrudate. It is desirable to be
able to use the material in a form having substantially the
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same properties of the material which is purchased and
tested. In order to achieve this goal, it is desirable
to avoid any change in melt index.
In at least one prior art patent (see U.S. Patent
3,486,193) a melt dispersing means is positioned in the
root of the screw at least one screw flight upstream of the
discharge end of the metering section and extending outwardly
into the annular space between the screw root and the flight
diameter to form alternating open and closed portions in
the annular space. The dispersing means may be positioned
one-half screw flight upstream of the discharge end. of the
first metering section and may include a plurality of
cylindrical pins ~ositioned a circumferential distance about
the screw root and extending in a plane perpendicul~r to
the screw axis out to the flight diameter of the screw, the
screw flight being interrupted thereàt. Alternately, the
dispersing means may include a plurality of spaced apart
square pegs extending radially from the root of the screw
and positioned a full flight length before the discharge
end of a first metering section and extending to wi~hin
about .015 inch of the flight diameter.
In still another prior art patent (U.S. Patent
3,487,503), an extruder for plastic material has a screw which
is provided with pegs arranged crosswise of a channel
between adjacent turns of one flight of the screw along a
section thereof sufficiently near the discharge end of the
extruder so that the material received thereby will be in
a plastic condition. The row of pegs may be aligned
parallel to the axis of the screw or may follow the shortest
direction between flight portions defining the adjacent
turns of the flight. In this latter arrangement, the rows
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extend perpendlcular to the direction of the channel rather
than parallel with the extruder axis. The pegs of each row
are in staggered relation with those of the adjacent row.
Moreover, the pegs are of approximately the same height
as the flight in all cases. Also, although some of the pegs
in a portion of the screw lie in a plane that may be
perpendicular to the axis of rotation of the screw, there
are other ones of the pegs in that portion which lie
outside the plane and which have the axes thereof parallel
to the plane. The arrangement of the pegs in such a
fashion may restrict somewhat the flow of the material.
Also in the prior art peg arrangements, say
transverse across the channel width, the number of pegs
that may be used is somewhat restricted since the distance
crosswise of the channel may be less than the circumference
of the screw. If it would be possible to use an arrangement
of pins about the circumference of the screw, more pins
could be mounted on the screw so as to achieve a finer
division with less restriction resulting in finer homogenizing
or a better mixing up of the material.
A feature which the prior art appears to lack is
the provision of an extruder screw having facilities for
breaking up the solids in the material so that the solids
are dissipated in with the melt pool formed in front of the
screw flight.
The term "mixing" as commonly used in the extruding
art may be regarded as an action which effects the random
scattering of minute portions of the melt in the condition
as discharged from the extruder. The melt may be in a
thermally uniform state because mixing has been carried out
to a degree that any non-uniformity of heating is readily
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corrected by transfer of heat from small hotter portions of the
melt to the adjacent cooler portions.
The term "dispersing" involves mixing on a more
microscopic level in which particles of various compounds
in the melt are uniformly distributed. Dispersions may be
prepared within the particles approach sizes on the order
of a few molecules of thickness.
As used herein, the term "flight diameter" refers
to a value equal to twice the distance from the center of
the screw to a point in the edge surface of a screw flight
in a plane perpendicular to the axis of the screw. The root
diameter of the screw is the diameter of the shaft or shank
or core about which the helical flight is formed. The flight
diameter is constant so as to maintain a contact clearance
in the cylindrical core with the flight depth or root
character being varied to provide different degrees or blending
in the extruder.
Summary of the Invention
The illustrative embodiments of this invention
provide
(a) apparatus for advancing and working thermoplastic
materials to homogenize the materials;
~b) apparatus for advancing an extrudate and for working
the extrudate optimumly without overly restricting
the flow of the material;
(c) apparatus for improving existing conventional extruder
screws to achieve greater output and extrudates of
improved quality with structural modifications involving
minor costs;
0 (d) improved capacity of conventional extruder of conventional
length-over-diameter (L/D) ratio to effect complete mixing
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of a material after reaching a plasticized state, and
high thermal uniformity within the material just prior
to being discharged or extruded; and
(e) apparatus for achieving greater output rates of
extrudates by using extruder screws having a deeper
~- channel between the walls of the flight of the extruder
screw, especially in the metering section thereof,
without increasing the barrel diameter or length.
In accordance with one aspect of the present invention
there is provided a screw for advancing and working
thermoplastic materials, which comprises: a core having an
axis of rotation for advancing a thermoplastic materlal;
at least one flight connected to and extending outwardly
from the core to a surface of revolution concentric with the
axis of rotation of the core, the flight being gener~ted
helically about the axis o~ rotation; and at least one
plurality of force-producing components connected to the
core for subjecting the material to a plurality of forces,
each of the force-producing components extending beyond the
core along an axis associated with the component which
intersects the core and being spaced from the surface of
revolution, all of the force-producing components along the
core in any one turn of the flight having at least some
portion of their axes in the flight lying in a plane which plane
is perpendicular to the axis of rotation; the flight intersecting
; with and being continuous through the plane.
In accordance with another aspect of the present invention
there is provided a mixing screw for working heat-sensitive
thermoplastic material, such as PVC, through an extrusion
process comprising: a root, a continuous helical flight
extending outwardly from said root with all of the adjacent
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turns of said helical flight defining a channel for the
passage of thermoplastic material, and a plurality o~ rows
of mixing pins, said mixing pins extending radially from said
root through a distance which is slightly less than or equal
to the distance through which said helical flight extends
from said root, each of said rows extending circumferentially
about said root in planes which are perpendlcular to the axis
of said root, more than one of said plurality of rows being
interrupted by said continuous helical flight, and each of
said interrupted rows diagonally bisecting a separate portion
of the channel as defined by the two adjacent turns of said
continuous helical flight.
Brief Description of the Drawings
The present invention taken in conjunction with the
invention described in copending Canadian patent application
Serial No. 127,238 which was filed on 9 November 1971, will be
described in detail hereinbelow with the aid of the
accompanying drawings, in which:
FIG. 1 is an elevation view, partially in section
of an apparatus which embodies certain principles
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of this invention and showing a conventional compression
relief design extruder screw modified with a pin arrange-
ment;
FIG. 2 is an enlarged fragmentary detail view
of a portion of the extruder screw of FIG. 1 and
showing one group of pins connected to a core of the
screw;
FIG. 3 is an enlarged sectional view of the
extruder screw and associated barrel of FIG. 1 taken along
lines 3-3 showing a plurality of pins directed outwardly
radially from a longitudinal axis of the screw and lying
substantially in a plane perpendicular to the axis,
the flight of the screw being uninterrupted in the
section of the screw containing the pins;
FIG. 4 is a detail view of a portion of a prior
art extruder screw of a slotted ring design in which
the screw flights are interrupted to permit mounting
or forming of the slotted rings;
FIG. 5 is an enlarged sectional view of the
slotted ring prior art screw design of FIG. ~ taken
along lines 5-5 thereof;
FIG. 6 is a detail view of a portion of a prior
art extruder screw having a pin arrangement with the
pins arranged generally crosswise of the channel formed
by the flight;. and
FIG. 7 is a graph showiny extruder screw output for
compression relief, slotted ring screw designs and the screw
design which embodies the principles of this invention.
DETAILED DESCRIPTION
Referring now to FIG. 1, there is shown one type
of extruder arrangement which is used commercially in the
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extrusion art. There is shown an extrusion apparatus,
designated generally by the numeral 20, which includes a
hopper 21 into which at least one thermoplastic material
in the form of pellets is fed. The hopper 21 communicates
with an extrusion cylinder, designated generally by the
numeral 22, so that the thermoplastic materials are advanced
from an inlet or receiving end 23 of the cylinder to an
outlet or delivery end 24 thereof where the extrudate is
formed into a covering on a cable core (not shown),
successive sections of which are advanced continuously
through an extruder head (not shown).
As can best be seen in FIG. 1, the extrusion cylinder
22 includes a barrel or casing 26 having an internal surface
of revolution in the form of a cylinder bore 27 of uniform
diameter formed therethrough and connecting the receiving
end 23 to the delivery end 24. The extrusion cylinder 22
also includes a flange 28 at the delivery end 24 thereof
which facilitates the attachement of adapters, dies and other
auxiliary equipment (none of which are shown).
In order to advance the thermoplastic material
from the hopper 21 to the delivery end 24 of the extruder 20,
an extruder screw, designated generally by the numeral 31,
is disposed concentrically within the bore 27. The
extruder screw 31 includes a core 32, has an upstream end 33
thereof adjacent the hopper 21, and a downstream end 34
adjacent the delivery end 24. Moreover, the extruder screw
31 is of a design commonly referred to as a compression
relief design. As such, and beginning at the upstream end
33 thereof, the extruder screw 31 includes, successively,
a first constant root diameter section 36 of the core 32
referred to as a feed section (see FIG. 1), a uniformly
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: 1037662
increasing root diameter section 37, referred to as acompression section, a uniformly decreasiny root diameter
section 38, referred to as a compression relief section,
and a uniform diameter root section 39, commonly referred
to as the metering section.
The extrusion screw 31 is manufactured to have
a thread or flight 41 formed helically about and extending
longitudinally along the core 32. The flight 41 is formed
to provide a groove or channel 42 formed by the root diameter
surface of the core 32 and facing side walls 43-43 of the
flight. The external diameter and pitches of the flight 41
are generally identical and constant along the length of
the extruder screw 31 from a point just beyond the entrance
end 33 of the screw to the delivery end 34 thereof. However,
if desired, the pitch of the flight 41 may be made to
decrease slightly from the portion of the screw adjacent
the receiving end 23 of the bore 27 to the delivery end 24
thereof. The leading face of the flight 41 is substantially
perpendicular to the root diameter surface of the core 32
to provide for an improved delivery action.
The channel 42 formed between the opposing walls of
the 1ight 41 and the surface of the core 32 is generally
rectangular in shape. It should be clear that the area of
the channel 42 is constant from the receiving end 33 to the
beginning of the compression section 37. Then the area of
the channel 42 decreases to the compression relief section 38
whereat the area increases for one-half turn and then remains
constant throughout the metering section 39.
In order to homogenize the thermoplastic material
or materials which are being advanced through the extruder
20 with respect to say temperature or physical mix, the
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extruder screw 31 is provided with ~acilities, designatedgenerally by the numeral 46, for subjecting the materials
to a plurality of forces (see FIG. 2). As can best be seen
in FIG. 2, the homogenizing facilities 46 include a plurality
of force-producing components 47-47 in the form of pegs or
pins which are mounted individually in holes 48-48 formed
in the core 32 of the extruder screw 31. I'he holes 48-48
are formed so that the centers thereof lie substantially in
a plane which is perpendicular to a longitudinal axis of
rotation of the core 32. Additionally, the holes 48-48 are
formed in the core 32 so that when the pins 47-47 are
mounted in the associated ones o~the holes, khe pins are
directed radially outward from the longitudinal axis oE
the core 32.
It should be observed that the arrangement of
pins 47-47 of this invention differs from that of prior
art arrangements such as that shown in FIG. 6. For example,
all of the pins 47-47 in any portion of the extruder screw
31 have at least some portion of the axes thereof or of
the pins themselves lying in the so-called plane of pins
which is perpendicular to the axis of rotation of the screw.
The structural arrangement of the pins 47-47 with
respect to the flight 41 is established so that the co-
operation therebetween minimizes the "dead spaces" and
maximizes the homogenizing actions. In order to accomplish
this, the flight 41 of the extruder screw 31 is uninterrupted
at least in that portion of the screw whereat the pins 47-47
are located. This overcomes some of the disadvantages of
the interrupted pattern of flight typical of slotted ring
design screws (see FIGS. 4 and 5).
The walls 43-43 of the flight 41 of the screw 31
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are formed by surfaces which intersect with the planecontaining the pins 47-47 such that the surfaces are
continuous through the plane. ThiS feature of an uninter~
rupted flight 41 is shown clearly in FIGS. 2 and 3.
It has also been found that the positioning of the
pin planes along the longitudinal axis of the screw 31
is important in order to optimize the homogenizing action
of the pins. Most desirably the pins 47-47 are located
along the metering section 39 of the extruder screw 31.
The pins 47-47 are placed in the metering section 39 since
the pins are more effective there with the least restriction
to the pumping action of the screw. Additionally, our planes
of pins 47-47 spaced evenly along the metering section 39
appear to yield the best results to date.
Of course, the number of pins 47-47, their location,
diameter and spacing may vary according to a particular ap-
plication of the extruder 20, the melt temperature, type of
plastic shape extruded, type of materials fed to the extruder,
diameter of the screw 31, and other pertinent variables.
The passage of the thermoplastic material through a plane
o the pins 47-47 beings about substantial mixing to achieve
thermal uniformity throughout the mix. The number of groups
o~ pins 47-47 may be increased or decreased as necessitated
by the degree of responsiveness of heating and mixing.
The holes 48~48 may be drilled to a diameter
requiring press fitting of the pins 47-47. The pins 47-47
may be positively anchored in the core 32 by, prior to
the insertions thereof, placing solder powder and flux
in the associated hole 48 and thereafter pressing the
pin into the hole and applying heat to the pin and adjacent
core area until bonding has taken place. As the pins 47-47
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are made or~inarily oversized with respect to length, theouter end surfaces are grouna, machined or otherwise trimrned
to a contour conformity with the surface of revolution
swept by the flight 41. Of course,-the pins 47-47 can be
connected to the core 32 in any feasible man~er which
does not otherwise disrupt the cross-sectional area of
the channel 42.
In one typical arrangement, the extrusion ap-
paratus 20 includes a screw 31 having a barrel diameter of
eight inches. The feed section 36 extends for six turns of
the flight 41 and has a depth of 750 mils. The compression
and compression relief sections 37 and 38, respectively,
extend for six and one-half turns respectively and have
minimum depths of 150 mils. Finally, the metering section
extends for seven and one-halE turns and has a uniform
depth of 250 mils. If the compression section 37 is too
short, the materials are compressed in too short a time
which results in excessive heat build-up that could burn and
degrade the therrnoplastic material.
Four planes of pins 47-47 are used with the up-
stream one of the planes being located one-half turn or
one-half pitch downstream of the cornpression relief section
38 of the screw 31. Alternatively, the upstream one of the
planes is three-sixteenths inch downstream of the compres-
sion relief section. The downstream one of the planes
is positioned at the downstream end o the screw 31 with
the other two planes spaced uniformly between the other
two planes.
As for the pins 47-47, the pins may be cylindrical,
three-sixteenths inch diameter with the centers of the holes
48-48 thereof spaced at least one-quar-ter inch a2art on
a circumferential circle about the core 32. The pins 47-47
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extend into the predetermined path of the thermoplastic
materials along the channel 42 with the height of the pins
approximately, but not necessarily, the height of the flight 41.
Of course, all of the pins 47-47 in a portion of the
metering section 39 need only have a portion thereof in the
plane associated with that portion of the metering section.
The pins 47-47, instead of lying substantially in the plane
with the pins directed radially outward, could proiect transversely
out of the plane where it intersects the flight 41. Or the
pins 47-47 could be included in the plane but not necessarily
be directed radially outward from the axis of rotation of the
core 32. And finally, it is within the scope of this invention
that the force-producing components 47-47 need not be in the form
of pins but could be in the form of vanes such as is common in
impeller wheels.
It may also be important to the operatlon of the
extruder apparatus 20 in a particular application to have a
specified ratio between (1) the distance along the root
surface between the intersections of adiacent ones of the
pins 47-47 and the root surface of the core 32 and (2) the
diameter of the pins 47-47. The extruder screw 31 which
embodies the principles of this invention could have the
pins 47-47 arranged so that this ratio lies in the range of
O to 1.
Operation
Thermoplastic material, such as polyethylene,
polymerized vinyl chloride or the like in granular, powder
or pellet form with suitable fillers and/or pigments, is
introduced into the hopper 21 of the extruder 20. Pacilities,
including a motor and gear reduction unit (not shown) are
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1~3761E;Z
provided to turn rotatably the extrusion screw 31 to
advance the thermoplastic material from left to right,
as viewed in FIG..l. The thermoplastic material is
advanced through the channel 42 between the walls of the
flight 41.
As the thermoplastic material is advanced into
the compression section 37, compacting, softening, melting
and mixing takes place therein as the cross section of the
channel 42 decreases. The material in the compression
section 37 tends to be drawn out with a change in velocity.
Then when the material enters the compression relie,f
section 38, the material tends to be retracted somewha-t
with accompanying change in velocity. The metering
section 39 functions to tend to bring about uniformity
throughout the material advanced therethrough with
respect to the tempera-ture, composition and coloring.
The barrel 26 may be heated at selected portions thereof
to increase the rate of plasticization of the material.
Thermoplastic materials generally have maximum
temperatures at which they resist deco~position or other
degradation. This is important so as to avoid over-heating
within the extruder 20. Heat resulting from the work
expended or the material processed by the extruder 20 may
be sufficient to be the exclusive source of heat for
e~ecting plasticization. Where the temperature between
the melting point or melting range of material and the
decomposition temperature is small, facilities (not shown)
for heating or cooling portions of the barrel and screw
core may be required.
The general direction of the melted material
relative to the screw 30 is lengthwise of the helical
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channel 42. For purposes of explanation, the channel 42
` may be regarded as having a helical axis extending
lengthwise of the channel midway between adjacent turns of
the flight 41. In addition to this,movement, the material
~' flows transversely and in a curvilinear fashion about the
axis. Each minute element of material traverses a path
which is a helix having con~olutions centered about the
axis which is also a helix. This movement is generated
by the frictional engagement of the inner barrel surface
27 with the outer surface of the plastic material. Because
of heat transmission at the interface of the screw flight
41 and the surface of revolution resul~ing from frictional
heating, or by heating or cooling equipment, a temperature
~ gradient normally exists which varies outwardly from~the
'~ axis to the interface.
As the material is advanced through each of the
circles or planes of pins 47-47, the pins, depending upon
the height thereoE, penetrate corresponding heights of the
material contained in the channel 42 to disrupt the normal
cross section currents of the material and cause mixing
of the material. By using the pins 47-47 in the manner
described, a high degree of thermal uniformity of the
extrudate is obtained. The pins 47-47 tend to overcome
the tendency of the melt to migrate upstream to the leading
or pushing face of the flight 41. By using the pins 47-47,
the melt is urged toward the trailing faces of the flight
to mix the melt with the solids and achieve a homogeneous
extrudate.
It should be observed that in the past,
achievement of thermal uniformity of an a,cceptable degree
was obtained principally through a reduction of the depth
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of the channel 42 within the metering section 39. This
of course had the unfortunate corollary effect of reducing
the delivery capability of the extruder 20. In FIG. 7
are shown output curves for extruder screws of typical
compression relief and slotted ring designs.
The present invention avoids a reduction in
delivery capability that would otherwise be necessary to
homogenize the material. Rather than thin out the flow
path of the melt stream to a low-capacity output, the
extruder 20 which embodies the principles of the present
invention divides the melt stream into a number of smaller
streams thereùpon exposing the molten material to high
shear rates for a short period of time after which the
small streams o~ material are merged again in a mixed
condition. As shown by the solid curve in FIG. 7, the
screw 31 which embodies the principles of this invention
utilizes a deeper metering section 39 to obtain high output
capability while at the same time having good mixing to
obtain thermal uniformity and overcome the tendency for the
melt to drift upstream.
It should be realized that an additional benefit
of this;invention accrues in that presently used screws
may be easily modified to include the pins 47-47. This
permits the continued use of present investment in plant
and at the same time being able to increase the output
of the present equipment.
In one typical arrangement, in an 8-inch, 20/1
extruder for low-density polyethylene, the barrel
temperature is maintained at 400F, and the melt temperature
at 450F. The speed of the extruder screw 32, which
includes the pins 47-47, is 46 revolutions per minute to
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,
give an output of 1400 pounds per hour.
In another typical arrangement, in a 10-inch,
8 1/2 / 1 extruder, the length of the feed section 36
is 27.9 inches, of the compression sectionl 1905 inches, of
the relief section 5.0 inches, and the metering section
30.0 inches. The depth of the feed channe:L is 0.75 inches,
of the metering section 1.210 inches with a screw lead
of 6.5 inches. This design screw with the pin arrangement
gives an output of 1100 lbs. per hour at 43 revolutions
per minute.
It is to be understood that the above-described
arrangements are simply illustrative of the in~ention.
Other arrangements may be devised by those skilled in the
art which will embody the principles of the invention
and fall within the spirit and scope thereof.
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