CA1037662A - Helical extrusion screw having force producing components - Google Patents
Helical extrusion screw having force producing componentsInfo
- Publication number
- CA1037662A CA1037662A CA297,765A CA297765A CA1037662A CA 1037662 A CA1037662 A CA 1037662A CA 297765 A CA297765 A CA 297765A CA 1037662 A CA1037662 A CA 1037662A
- Authority
- CA
- Canada
- Prior art keywords
- flight
- screw
- core
- pins
- axis
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
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Landscapes
- Extrusion Moulding Of Plastics Or The Like (AREA)
Abstract
HELICAL EXTRUSION SCREW HAVING FORCE PRODUCING COMPONENTS Abstract of the Disclosure The present invention relates to a screw for advancing and working thermoplastic materials. The screw is comprised of a core which has an axis of rotation for advancing the thermoplastic material and at least one flight connected to and extending outwardly from the core to a surface of revolution concentric with the axis of rotation, the flight being generated helically about the axis. At least one plurality of force-producing components is connected to the core for subjecting the material to a plurality of forces. Each of the force producing components extend beyond the core along an axis associated with the component which intersects the core and which is spaced from the surface of revolution.
Description
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.
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 a376~z 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 1C)3766Z
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 103766;~
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 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
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 a376~z 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 1C)3766Z
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 103766;~
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 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 -~037662 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 ~03766Z
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 a3~66z 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 11)3766Z
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 -~ ~3766Z
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 `` 103766Z
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 c : 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 ~03766;~
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 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 1~3761~Z
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 - ~037662 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 ~ 17 ~
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 1~3766Z
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 ~37~i~iZ
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 lV3766~ ~
, 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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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 -~037662 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 ~03766Z
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 a3~66z 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 11)3766Z
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 -~ ~3766Z
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 `` 103766Z
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 c : 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 ~03766;~
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 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 1~3761~Z
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 - ~037662 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 ~ 17 ~
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 1~3766Z
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 ~37~i~iZ
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 lV3766~ ~
, 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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Claims (15)
1. A screw for advancing and working thermoplastic materials, which comprises:
a core having an axis of rotation for advancing a thermoplastic material;
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 generated helically about the axis of 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.
a core having an axis of rotation for advancing a thermoplastic material;
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 generated helically about the axis of 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.
2. A screw for advancing and working thermoplastic materials, which comprises:
a core having an axis of rotation for advancing a thermoplastic material;
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 generated helically about the axis of rotation; and at least one plurality of force-producing components connected to the core for subjecting the material to a plurality of forces, the force-producing components extending beyond 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 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.
a core having an axis of rotation for advancing a thermoplastic material;
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 generated helically about the axis of rotation; and at least one plurality of force-producing components connected to the core for subjecting the material to a plurality of forces, the force-producing components extending beyond 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 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.
3. A screw for advancing and working thermoplastic materials, which comprises:
a core having an axis of rotation;
at least one helical flight extending outwardly from the core to a surface of revolution concentric with the axis of rotation of the core, the at least one flight defining a channel measured in a transverse direction between adjacent turns of the flight, and extending in a helical path lengthwise of the core; and a plurality of means for subjecting a thermoplastic material to forces to homogenize the material; all the means in any one turn of the flight being disposed substantially in one plane which is perpendicular to the axis of rotation of the core;
the helical flight being continuous at least in that portion of the screw in which the subjecting means is positioned.
a core having an axis of rotation;
at least one helical flight extending outwardly from the core to a surface of revolution concentric with the axis of rotation of the core, the at least one flight defining a channel measured in a transverse direction between adjacent turns of the flight, and extending in a helical path lengthwise of the core; and a plurality of means for subjecting a thermoplastic material to forces to homogenize the material; all the means in any one turn of the flight being disposed substantially in one plane which is perpendicular to the axis of rotation of the core;
the helical flight being continuous at least in that portion of the screw in which the subjecting means is positioned.
4. The screw for advancing and working thermoplastic materials as set forth in claim 3 wherein:
the core is formed with at least a metering section, and the subjecting means is disposed in at least one plane positioned along the metering section.
the core is formed with at least a metering section, and the subjecting means is disposed in at least one plane positioned along the metering section.
5. An extruder screw for advancing and working thermoplastic materials, which comprises:
a core having an axis of rotation;
a helical flight extending outwardly from the core to a surface of revolution concentric with the axis of rotation of the core, the flight defining a channel measured in a transverse direction between adjacent turns of the flight and extending in a helical path lengthwise of the screw from an upstream end to a downstream end thereof;
the screw being formed with at least a feed section at the upstream end thereof, a compression section, a compression relief section and a metering section at the downstream end of the screw, and at least one plurality of pins extending radially outwardly from the core towards the surface of revolution, all of the pins in any one turn of the flight being arranged substantially in a plane, the plane being perpendicular to the axis of rotation of the core, the helical flight being continuous in at least that section of the screw containing the pins.
a core having an axis of rotation;
a helical flight extending outwardly from the core to a surface of revolution concentric with the axis of rotation of the core, the flight defining a channel measured in a transverse direction between adjacent turns of the flight and extending in a helical path lengthwise of the screw from an upstream end to a downstream end thereof;
the screw being formed with at least a feed section at the upstream end thereof, a compression section, a compression relief section and a metering section at the downstream end of the screw, and at least one plurality of pins extending radially outwardly from the core towards the surface of revolution, all of the pins in any one turn of the flight being arranged substantially in a plane, the plane being perpendicular to the axis of rotation of the core, the helical flight being continuous in at least that section of the screw containing the pins.
6. The extruder screw of claim 5, wherein at least one plurality of pins are located in the metering section of the screw.
7. The extruder screw of claim 6, wherein the pins disposed in the metering section all lie substantially in a plane located one-half turn downstream of the compression relief section.
8. The extruder screw of claim 6, wherein the pins are arranged in four planes spaced evenly along the metering section of the screw, the first one of the planes being one-half turn downstream of the compression relief section and the downstream one of the planes being at the downstream end of the screw.
9. The extruder screw of claim 6, wherein the height of the pins is less than that of the flight.
10. The extruder screw of claim 6, wherein the height of the pins is the same as that of the flight.
11. The screw of claim 6, wherein the upstream one of the planes is positioned approximately three-sixteenths inch downstream of the compression relief section.
12. The screw of claim 6, wherein the pins are three-sixteenths inch in diameter and the axes of the pins are spaced at least one-quarter inch apart along an axis circumferentially of the core.
13. The screw of claim 6, wherein the ratio between the distance along the root surface between the intersections of adjacent ones of the pins and the root surface of the core and the diameter of the pins is between 0 and 1.
14. 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 turns of said helical flight defining a channel for the passage of thermoplastic material, and a plurality of 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 perpendicular 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.
a root, a continuous helical flight extending outwardly from said root with all of the adjacent turns of said helical flight defining a channel for the passage of thermoplastic material, and a plurality of 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 perpendicular 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.
15. A mixing screw according to claim 14, wherein said plurality of interrupted rows of mixing pins are spaced axially along said root one from the next by a distance greater than the distance covered by one flight turn taken axially along said root but less than the distance covered by two flight turns taken axially along said root.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US13357571A | 1971-04-13 | 1971-04-13 | |
| CA127,238A CA1031915A (en) | 1971-04-13 | 1971-11-09 | Extrusion screw with continuous flights intersected by rows of radial pins |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1037662A true CA1037662A (en) | 1978-09-05 |
Family
ID=25666944
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA297,765A Expired CA1037662A (en) | 1971-04-13 | 1978-02-27 | Helical extrusion screw having force producing components |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1037662A (en) |
-
1978
- 1978-02-27 CA CA297,765A patent/CA1037662A/en not_active Expired
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |
Effective date: 19950905 |