Single-screw extruder and a method for extrusion

A single-screw extruder and method are disclosed wherein the extruder includes a cylindrical rotor member arranged in a barre, and a drive system for rotation of the rotor member in the barrel. The extruder includes an outlet-provided with a die, having an outer ring arranged to the barrel, and an inner die part, the die establishing a flow channel that is continuously circular in all its cross-sections.

BACKGROUND

The invention relates to a single-screw extruder, comprising a cylindrical rotor member having diameter and length and comprising a feeding zone, the rotor member arranged in a barrel.

The invention further relates to a method for extrusion.

The demand for processing recycled materials is continuously growing. This type of materials are in their density, physical state, particle size and shape etc. extremely heterogeneous materials. A problem arising with these materials is that heterogeneous materials are extremely difficult to be processed in commonly known extruders and compounding devices.

BRIEF DESCRIPTION

Viewed from a first aspect, there can be provided a single-screw extruder, comprisinga cylindrical rotor member having diameter and length and comprising a feeding zone,the rotor member arranged in a barrel,the cylindrical surface of the rotor member carrying cavity/cavities and/or projection(s) arranged in helically extending rows,the helically extending row(s) of the rotor member having a pitch and depth in the feeding zone of the rotor member, and the extruder further comprisinga drive system for the rotation of the rotor member (1) in the barrel, whereinthe relation of the depth to the diameter of the rotor member, i.e. d:D, is not more than 1:20, andthe relation of the pitch of the rotor member to the diameter (of the rotor member, i.e. P:D, is not more than 1:4, wherein an outlet of the extruder is provided with a die, comprisingan outer ring arranged to the barrel,an inner die part arranged to the cylindrical rotor member and rotate therewith,the die establishing a flow channel that is continuously circular in all its cross-sections.

Thereby an extruder for processing heterogeneous materials may be achieved.

Viewed from a second aspect, there can be provided a single-screw extruder, comprisinga cylindrical rotor member having diameter and length and comprising a feeding zone,the rotor member arranged in a barrel,the cylindrical surface of the rotor member carrying cavity/cavities and/or projection(s) arranged in helically extending rows,the helically extending row(s) of the rotor member having a pitch and depth in the feeding zone of the rotor member, and the extruder further comprisinga drive system for the rotation of the rotor member in the barrel, whereinthe relation of the depth to the diameter of the rotor member, i.e. d:D, is not more than 1:20, andthe relation of the pitch of the rotor member to the diameter of the rotor member, i.e. P:D, is not more than 1:4, whereinthe rotor member comprises a cylindrical channel, the diameter of which is at least 75%, preferably 80 to 85%, of the diameter of the rotor member, thatan outlet of the extruder is provided with a die, the die comprising an outer ring and an inner die part creating a flow channel that is continuously circular in all its cross-sections, thatthe outer ring is attached to the barrel, and thatthe inner die part is supported to a support structure that is arranged inside said cylindrical channel,the rotor member being arranged rotatable in respect to the inner die part.

Thereby an extruder for converting heterogeneous material flows into raw material for further processing may be achieved.

Viewed from a third aspect, there can be provided a method for extrusion, the method comprising steps of:

A) feeding the material in a single-screw extruder, the extruder comprisinga cylindrical rotor member having diameter and length and comprising a feeding zone,the rotor member arranged in a barrel,the cylindrical surface of the rotor member carrying cavity/cavities and/or projection(s) arranged in helically extending rows,the helically extending row(s) of the rotor member having a pitch and depth in the feeding zone of the rotor member, and the extruder further comprisinga drive system for the rotation of the rotor member in the barrel, whereinthe relation of the depth to the diameter of the rotor member, i.e. d:D, is not more than 1:20, andthe relation of the pitch of the rotor member to the diameter of the rotor member, i.e. P:D, is not more than 1:4,

B) heating the material in said single-screw extruder to a flowable state, and

C) feeding the material in outlet of the extruder, the outlet being provided with a die, comprisingan outer ring arranged to the barrel,an inner die part arranged to the cylindrical rotor member and rotate therewith,the die establishing a flow channel that is continuously circular in all its cross-sections, and

D) allowing the material to exit between the outer ring and the rotating inner die part.

Thereby a method for converting heterogeneous material flows into raw material for further processing may be achieved.

Viewed from a fourth aspect, there can be provided a method for extrusion, the method comprising steps of:

A) feeding the material in a single-screw extruder, the extruder comprisinga cylindrical rotor member having diameter and length and comprising a feeding zone,the rotor member arranged in a barrel,the cylindrical surface of the rotor member carrying cavity/cavities and/or projection(s) arranged in helically extending rows,the helically extending row(s) of the rotor member having a pitch and depth in the feeding zone of the rotor member, and the extruder further comprisinga drive system for the rotation of the rotor member in the barrel, whereinthe relation of the depth to the diameter of the rotor member, i.e. d:D, is not more than 1:20, andthe relation of the pitch of the rotor member to the diameter of the rotor member, i.e. P:D, is not more than 1:4,

B) heating the material in said single-screw extruder to a flowable state, and

C) feeding the material in outlet of the extruder, the outlet being provided with a die, comprising an outer ring and an inner die part creating a flow channel that is continuously circular in all its cross-sections, whereinthe outer ring is arranged to the barrel, andthe inner die part is supported to a support structure that is arranged inside said cylindrical channel,the rotor member being arranged rotable in respect to the inner die part, and

D) allowing the material to exit between the outer ring and the unrotating inner die part.

Thereby a method for converting heterogeneous material flows into raw material for further processing may be achieved.

The arrangements and the methods are characterised by what is stated in the independent claims. Some other embodiments are characterised by what is stated in the other claims. Inventive embodiments are also disclosed in the specification and drawings of this patent application. The inventive content of the patent application may also be defined in other ways than defined in the following claims. The inventive content may also be formed of several separate inventions, especially if the invention is examined in the light of expressed or implicit sub-tasks or in view of obtained benefits or benefit groups. Some of the definitions contained in the following claims may then be unnecessary in view of the separate inventive ideas. Features of the different embodiments of the invention may, within the scope of the basic inventive idea, be applied to other embodiments.

In the figures, some embodiments are shown simplified for the sake of clarity. Similar parts are marked with the same reference numbers in the figures.

DETAILED DESCRIPTION

FIG.1ais a schematic side view of a screw extruder in partial cross-section,FIG.1bis a schematic view of a detail of the extruder shown inFIG.1ain partial cross-section,FIG.1cis a schematic view of a detail of the extruder shown inFIG.1ain partial cross-section, andFIG.1dis a schematic view of a detail of the extruder shown inFIG.1ain partial cross-section.

The term “screw extruder” or “extruder” means here not only extruders for plastic extrusion but also compounding extruders, and extruders for other materials that may be in flowable form at least partly or that may be rendered, at least partly, in flowable form, such as recycled materials originating e.g. from industrial activities or households.

According to an aspect, the extruder100may be used for converting waste or recycled materials into raw material for use in subsequent processes. One aspect of the invention is that raw material can be used or stored essentially easier and/or in more compact physical state and/or more cleanly, etc. than the material fed in the extruder. The subsequent process may be another extrusion process, molding process etc.

According to another aspect, the extruder100may be used for compounding process, e.g. in manufacturing polymer-fibre compositions.

The extruder100is a single-screw extruder comprising a cylindrical rotor member1, a barrel2, a feed opening3, and a drive system4for the rotation of the rotor member1in the barrel2.

The rotor member1has diameter D and length L. In an embodiment, the relation of the length to the diameter, i.e. L:D, is in range of 2:1 to 4:1.

The cylindrical outer surface of the rotor member1comprises projections5that realizes a screw thread comprising at least one screw flight6and at least one screw channel7between the at least one screw flight6.

The screw thread constitutes a structure that transfers the material to be processed in the barrel from the feed opening3towards the outlet9while the rotor member1is rotating in the barrel2. In an embodiment, the profile of the screw thread is invariable. In another embodiment, the profile of the screw thread is variable such that the volume of the screw channel decreases towards the outlet16of the extruder. In an embodiment, the depth d of the cavity/cavities and/or projection(s)5arranged on the surface of rotor is arranged to decrease after feed zone (10).

The decreasing of the screw channel volume causes a compression ratio which is typically 1:2 to 1:4. The profile may change step by step, or it may comprise at least one section where the profile changes rapidly from one profile to another profile.

The screw flight6has pitch P and the screw channel7has depth d in the feeding zone14of the rotor member1. The “feeding zone” means that part of the rotor member1that is directly under the feed opening3and one to five lap(s) of the flight6following the feed opening.

According to an aspect, the relation of the depth d to the diameter D, i.e. d:D, is not more than 1:20, and the relation of the pitch P to the diameter D, i.e. P:D, is not more than 1:4. An advantage is that the volume of the screw channel7is very low compared to the screw diameter D. Thus it is possible to provide the extruder100with a very large feed opening3compared to the volume of the screw channel7. Following this, light density and/or heterogeneous materials, such as materials comprising fibres, waste/recycled plastic, can be fed in the extruder100in amounts sufficient to fill optimally the screw channel7. Another advantage is that the low volume compared to the screw diameter D decreases power requirement of the extruder.

In an embodiment, d:D is in range of 1:300 to 1:20 (in the feeding zone14of the rotor member1). An advantage is that the temperature of the material to be processed in the screw channel7may be controlled very precisely due to large surface area of the screw channel compared to volume of the material, and thus e.g. materials or processes highly sensitive to temperature may be processed by the extruder.

In an embodiment, P:D is in range of 1:60 to 1:4. An advantage is that the material to be processed may be transferred towards the outlet7by low power usage.

In an embodiment, the rotor member1realizes equation
CL·d≤RD·0.01, whereinCL=CL=channel length measured in direction of length (L) of the rotor member1,d=channel depth measured in radial direction of the rotor member1, andRD=cross-sectional area of the rotor member, including also the cross-sectional area of the channel8.

An advantage is that the volume of the screw channel7is very low compared to the screw diameter D, and thus easily filled with the material to be processed due to a large feed opening3. For instance, the diameter of the rotor member1may be 350 mm whereas the length following the feed opening is 400 mm.

In an embodiment, the feed opening3or the feeding zone comprises shearing or cutting means for reducing the particle size of the material being fed in the extruder100. This means may comprise e.g. cutting teeth arranged on the rotor member1and counterpart(s) arranged in the barrel2.

According to an aspect, the rotor member1is hollow. In an embodiment, the rotor member comprises a cylindrical channel8, such as circular cylinder, the diameter of which is at least 75%, preferably 80 to 85%, of the diameter D of the rotor member. This means that the diameter D of the rotor member may be increased compared to the known rotor members without increasing the weight and expenses thereof.

In an embodiment, the rotor member comprises one screw flight. In another embodiment, the rotor member comprises two, or even more, screw flights.

Thanks to the large diameter of the rotor member1, the drive system4may be construed to transmit high torques in the rotor member1. Also the structure of the rotor member1may stand high torques without risk for damages. The extruder100may be realized many alternative ways. For instance, in an embodiment the rotor member1comprises helically extending rows of plurality of separate cavities, instead of the screw thread. Said cavities constitute a structure that transfers the material to be processed in the barrel from the feed opening3towards the outlet9.

The shape of the cavities may be e.g. dome, hemispheric, a section or calotte of tear-drop, oval or combinations thereof.

According to an aspect, the relation of the depth d to the diameter D in the feeding zone14of the rotor member1, i.e. d:D, is not more than 1:20, and the relation of the pitch P to the diameter D, i.e. P:D, is not more than 1:4. An advantage is that the volume of the cavities is very low compared to the screw diameter D, and thus light density and/or heterogeneous materials, such as materials comprising fibres, waste/recycled plastic, can be fed in the extruder100in amounts sufficient to fill optimally the volume between the rotor member1and the barrel2.

In an embodiment, d:D is in range of 1:300 to 1:20.

In an embodiment, P:D is in range of 1:60 to 1:4.

In another embodiment, the rotor member1comprises projections that realize helically extending rows of plurality of discrete projections, the row comprising pitch P.

According to an aspect, the relation of the depth d, or height of the projections, to the diameter D, i.e. d:D, is not more than 1:20, and the relation of the pitch P to the diameter D, i.e. P:D, is not more than 1:4.

In an embodiment, d:D is in range of 1:300 to 1:20.

In an embodiment, P:D is in range of 1:60 to 1:4.

According to an aspect, the cylindrical inner surface of the barrel2may comprise barrel cavity/cavities and/or projection(s). In an embodiment, said barrel cavity/cavities and/or projection(s) are arranged in helically extending rows. In another embodiment, the barrel cavity/cavities and/or projection(s) are arranged parallel with longitudinal axis of the rotor member1. In still another embodiment, the barrel cavity/cavities and/or projection(s) are arranged perpendicular with the longitudinal axis of the rotor member1.

In an embodiment, the barrel2comprises a barrel screw thread comprising at least one barrel flight10and at least one barrel channel11between the at least one flight, the barrel flight having a barrel pitch BP and the barrel channel having a barrel depth BD.

An advantage is that transfer of the material to be processed towards the outlet9may be enhanced.

In another embodiment, the cavities arranged in the barrel2realize helically extending rows of plurality of separate cavities.

An advantage is that the mixing and blending properties of the extruder may be enhanced.

The cavities arranged in the barrel2may be invariable in their shape and size in all the length of the barrel they exist. In another embodiment, the barrel2may comprise variable sized and/or shaped cavities.

In an embodiment, the barrel cavity/cavities and/or projection(s)5are not continuous, such that there are several cavities or grooves side by side.

The outlet16of the extruder is provided with a die23. The die23comprises an outer ring24that is attached to the barrel2, and an inner die part25attached to the cylindrical rotor member1. The inner die part25thus rotates with the rotor member1.

The die23establishes a flow channel26that is continuously circular in all its cross-sections, as can be seen best inFIG.1c.

In an embodiment, the outer ring24is removably attached to the extruder100by e.g. bolts. In another embodiment, the outer ring24is attached permanently to the barrel e.g. by welding.

In an embodiment, the inner die part25is removably attached to the extruder100, more precisely to the rotor member1. In another embodiment, the inner die part25is attached permanently to the rotor member1.

In an embodiment, both the outer ring24and the inner die part25are fixedly attached to the extruder100. In another embodiment, both the outer ring24and the inner die part25are removably attached to the extruder100.

The outer ring24and the inner die part25has a flow surfaces27that define the flow channel26. In an embodiment, the flow surface27of the outer ring24is essentially smooth. In another embodiment, the flow surface27of the outer ring24comprises corrugations that differ from surface structure the inner surface of the barrel2.

In an embodiment, the flow surface27of the inner die part25is essentially smooth. In another embodiment, the flow surface27of the inner die part25comprises corrugations that differ from surface structure arranged on the rotor member1.

In an embodiment, deepness of said corrugations in the flow surface27mentioned above are in range of 1:300 to 1:20, preferably 1:160 to 1:300, compared to the diameter of the corresponding flow surface27.

In an embodiment, the cross-sectional area of the flow channel26of the die is constant all the length of the die23. An advantage is that the geometry is easy to manufacture. The pressure before the die is easy to control by the die section length. Additionally, when the channel is constant, it is easy to connect one or more die sections sequentially.

In another embodiment, said cross-sectional area is reducing towards the exit of the die23. An advantage is that the die can be used to increase the pressure of outcoming material flow and thus improve the mixing process.

In still another embodiment, said cross-sectional area is expanding towards the exit of the die23. An advantage is that the die is self-cleaning, any material which access the die can come out. In the case of cooled die even poorly flowing material can exit the die.

FIG.2is a schematic view of a piece of an extruder barrel. According to an aspect, the barrel2is construed from two or more barrel modules12that are separately manufactured and then connected consecutively.

An advantage is that the barrel cavity/cavities and/or projection(s), such as barrel screw thread comprising at least one barrel flight10and at least one barrel channel11, may be manufactured in short pieces of the barrel more easily into short and large in diameter barrel module12than they would be manufactured in one monolithic barrel having equal length. It is to be noted, however, that the barrel2may also be manufactured in one piece. In the latter case, the barrel cavity/cavities and/or projection(s) can still be manufactured extremely easily due to high D:L relationship of the extruder.

FIG.3ais side view of another extruder in partial cross-section, andFIG.3bis a schematic view of a detail of the extruder shown inFIG.3ain partial cross-section.

The feed opening3is substantially big comparing to the capacity of the extruder and the depth of the rotor cavities. In an embodiment, the projected area of the feed opening is about 50 square centimeters per kilowatt.

In an embodiment, the feed opening3is cut to the halfway of the diameter of the barrel2, i.e. approximately D/2. Thus the feed opening3potentially weakens the structure of the barrel2such an extent, that the barrel2is not able to carry the load caused during an extrusion process. Especially when the diameter of the barrel is large and the barrel is short, the axial forces tend to bend the barrel at the feeding zone14.

In an embodiment, the barrel2is supported by a support structure15that lies outside of the barrel2. The support structure15comprises a first support part17that is attached to a section of the barrel2between the feed opening3and the discharge end16of the barrel, a second support part18attached to the drive system side19of the extruder, and a load transmit structure20connecting the first support part17to the second support part18. In the embodiment shown inFIG.4, the second support part18has been fixed to a bearing housing22that covers the drive system4. The load transmit structure20may be composed of one or more beam(s), plate structure or trussed construction, for instance.

The support structure15bears a part of the loads and stresses caused in an extrusion process and prevents the structure of the extruder from bending.

In an embodiment, there is an axial slot21arranged between the feeding zone14of the barrel and the bearing housing22of the extruder for receiving material (if any) flowing from the feeding zone backwards. Thus the axial slot21is to prevent the material to enter in the bearing housing22. Instead, the material will drop through the slot in a room where it does not cause any problems to the extruder or the extrusion process.

In an embodiment, the barrel2is totally separated from the bearing housing22, i.e. the axial slot21extends 360° around longitudinal axle of the extruder. In another embodiment, there are plurality of axial slots21that are separated by short sections of material.

In the embodiment shown inFIGS.3aand3b, the outlet16is provided with a die23, the outer ring24of which is attached to the barrel2, and the inner die part25is supported to a support structure28that is arranged inside of the cylindrical channel8. Thus the rotor member1rotates in respect to the inner die part25, or in another words, the inner part does not rotate.

In the embodiment shown inFIGS.3aand3b, the inner surface of a section of the cylindrical channel8lying in proximity of the die23comprises a counter thread29, best shown inFIG.3b. The pitch of the counter thread29is arranged in the same direction as the pitch of the cavity/cavities and/or projection(s)5arranged in the helically extending rows. The counter thread29establishes a counter screw channel30with an outer surface of the support structure28. The counter thread29and the counter screw channel30create a counteracting means that prevents or essentially inhibits the processed material to penetrate in the channel8.

In another embodiment, counter thread29is arranged on outer surface of the support structure28. In this embodiment, the pitch of the counter thread29is arranged in opposite direction as the pitch of the cavity/cavities and/or projection(s)5arranged in the helically extending rows.

In still another embodiment, the extruder comprises both counter threads29mentioned above, i.e. in on outer surface of the support structure28and the inner surface of the cylindrical channel8.

FIG.4is side view of an extruder die in partial cross-section.

In the embodiments discussed above, the exit of the die23is coaxial with the longitudinal axis of the rotor member1. In another embodiments, the outer ring24is provided with at least one radial opening32as a material output port. There is also a blocking means33arranged to block the end of the flow channel26after the radial opening32such that the processed material may exit only through the at least one radial opening32.

In the embodiment shown inFIG.4, the inner die part25rotates with the rotor member1. The blocking means33comprises a reverse thread34arranged in the inner die part, the pitch of which is arranged in opposite direction as the pitch of the cavity/cavities and/or projection(s)5arranged in the helically extending rows. The reverse thread34establishes a reverse screw channel35with an inner surface or flow surface27of the outer ring24.

In another embodiment, the reverse thread34is arranged in the outer ring24. In this embodiment, the pitch is in same direction with the pitch of the cavity/cavities and/or projection(s)5.

In a third embodiment, both the inner die part25and the outer ring24comprises the reverse thread34mentioned above.

The reverse thread(s)34direct(s) material to flow towards the radial opening32.

Additionally, the blocking means33may comprise a cooling arrangement36arranged to cool the outer ring24. Cooling of the die23rises the viscosity of the processed material, and thus reduces its ability to intrude into gaps and joints in the die. The cooling arrangement36may comprise e.g. channels in which cooling liquid may flow.

Embodiments comprising at least one radial opening32may have a scraper arrangement37that is arranged to remove raw material on a rotating inner die part25(as shown) or on a rotating rotor member1. This may help exit of the material out of the extruder.

FIG.5is side view of another extruder die in partial cross-section. In the embodiment shown inFIG.5, the inner die part25does not rotate with the rotor member1. The outer ring24is provided with at least one radial opening32as a material output port, and a blocking means33for blocking the end of the flow channel26after the radial opening32being arranged in the die23.

The blocking means33comprises here a simple plate or similar element that closes the flow channel26. The plate may be fixed removable or permanently to the outer ring24and the inner die part25.

Also this embodiment of the blocking means33may comprise a cooling arrangement36.

The invention is not limited solely to the embodiments described above, but instead many variations are possible within the scope of the inventive concept defined by the claims below. Within the scope of the inventive concept the attributes of different embodiments and applications can be used in conjunction with or replace the attributes of another embodiment or application.

The drawings and the related description are only intended to illustrate the idea of the invention. The invention may vary in detail within the scope of the inventive idea defined in the following claims.

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