CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from Italian Patent

Application No. 102017000100955 filed on September 8, 2017, the disclosure of which is incorporated by reference.

TECHNICAL FIELD

The present invention relates to a material extruder assembly.

BACKGROUND ART

One of the most important issues limiting the screw extruders functionality sources from the poor interfacial surface occurring between the extrusion flow channel walls and the material to be conveyed. Such an issue becomes even more severe when the material to be conveyed is solid and has low bulk density.

For the above reasons is popular to equip the screw extruder with a pumping device fixed upstream the extruder and arranged to pump the material to the screw as to increase substantially the bulk density.

There are known pumping devices comprising moving walls capable to drag the material by a tangential friction contact. For example documents US 52231909 and US 5356289 describe pumping devices housed in a cylindric body. In such solutions the rotational axis of the rotor rotates normal to the plane supporting the screw extruder. Said in different words, the axis of the rotor rotates substantially normal to rotational axis of the extruder's screw.

In the referred solutions the pumping device is necessarily connected to the extruder through a static conduit .

The walls of the static conduit generate a friction resi stance, i.e. generate a negative friction. As a consequence the most mechanical energy developed by the drag action of the pump to pressurize the material gets dissipated over the static conduit, with the obvious drawback that the net force acting on the material at the discharge location of the static conduit becomes much smaller than the pushing force acting at the feeding material inlet location of the same static conduit .

To avoid such drawback, the pumping device needs be oversized in such a way to make the net pushing force, active at the discharge location, sufficient.

DISCLOSURE OF INVENTION

Therefore a purpose of the present invention is to provide an extruder assembly free from the inconvenients of the known art .

In particular it is a purpose of the present invention to provide an extruder assembly which is efficient and easy and has a low construction cost. In accordance with the above purposes the present invention relates to an extruder assembly to pump material according to the Claim 1.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the present invention will become evident from the following description of a not limiting embodiment example thereof, with reference to the figures of the accompanying drawings, in which:

figure 1 is a lateral schematic view, with parts in section and parts removed for clarity, of the extruder assembly according to the present invention;

figure 2 is a front schematic view, with parts in section along the plane II-II of figure 1 and parts removed for clarity, of the extruder assembly of figure 1;

- figure 3 is a sectioned front view along the plane III-

III of a detail of the extruder assembly of figure 1; figure 4 is a lateral schematic view, with parts in section and parts, removed for clarity, of the extruder assembly according to the present invention in accordance to a second embodiment of the present invention referred to a first operative position

figure 5 is a lateral schematic view, with parts in section and parts, removed for clarity, of the extruder assembly of figure 4 referred to a second operative position figure 6 is a front schematic view, with parts in section along the plane VI-VI of a detail of the extruder assembly of figure 4;

BEST MODE FOR CARRYING OUT THE INVENTION

In figure 1 the reference numeral 1 indicates an extruder assembly comprising an extruder 2 and a pumping device 3 designed to pump material to the extruder 2.

The material can be solid, whether thermoplastic or non thermoplastic, in the particulate form of film or pellets or fibers or in the form of semiliquid or liquid paste.

Material which can be usefully processed by the inventive device comprises for example plastic pellets, reground plastic film, e.g. poyolefine, regrind plastic fibers, e.g. polyamide, plastic powder e.g. Polyvinyl Cloride in powder form, fluffy natural fibers, and any other particulate solids characterized by any physical form, provided showing a sufficient coefficient of friction. For sake of clarity a coefficient of friction of the material relative to metal is defined sufficient, in view of the use of the inventive device, when it exceeds 0,01 in the physical form occurring during conveying.

Moreover also semisolid pastes can be advantageously processed with the inventive device and for example irregular, not yet fully formed, extrudates, to be conveyed to more downstream extrusion steps of the process. An example is provided by pasty material, often discharged by continuous mixers (e.g. Buss Co-kneader or Farrel Continuous Mixer) to be further guided from the discharge port of the continuous mixer to an end-process extruder.

The extruder 2 is a screw extruder and is provided with a hollow body 5 extending over a longitudinal axis A (figure 2), usually identified by skilled persons as extrusion barrel, containing at least one extrusion screw 6, which is housed within the hollow body 5 and is rotatable about a first rotation axis Bl along a first direction VI and a processing channel 7, which is defined by the space included between the extrusion screw 6 and the cylindrical inner surface 8 of the hollow body 5.

In the non limiting example described and illustrated here, the rotation axis Bl equals to the longitudinal axis A.

A variation not shown provides the screw extruder with two or more extrusion screws housed within the hollow body. Such screws may be either intermeshing or non intermeshing, as well as, either co-rotating or counter-rotating.

The pumping device 3 is provided with a cylindrical rotor, which is rotatable about a second rotational axis B2 along a second direction V2 and has an outer face 12 provided with at least one groove 13 (better visible in figures 2 and 3) , with a stator 15, provided with a cylindrical seat 16 wherin the rotor 11 is housed and coupled in sliding manner to the rotor 11, and with a pumping channel 17, which is substantially defined by the space comprised between the rotor 11 and the cylindrical inner surface 19 of the stator 15.

The external face 12 of the rotor 11 and the cylindrical inner surface 19 of the stator 15 are concentric and face each other and show respective radiusses of curvature as to reduce to a minimum the clearance between the rotor 11 and the stator 15 in the range suitable to allow an easy rotation for the rotor 11 relative to the stator 15.

The processing channel 7 and the pumping channel 17 communicate directly through a passage 20 which is defined by the intersection of the extension of the cylindrical inner surface 19 of the stator 15 and the extension of the cylindrical inner surface 8 of the hollow body 15.

The first rotation direction VI of the screw 6 may be indifferently either co-rotating or counter-rotating relative to the second rotation direction V2 of the rotor 11.

In the non-limiting example described and illustrated here the first rotation direction VI of the screw 6 is counter-rotating relative to the second rotation direction V2 of the rotor 11.

Preferably the hollow body 5 and the stator 15 are defined by more elements coupled together. The pumping channel 17 extends circumferentially by an angle less than 360°, between a feeding material inlet 23 and the passage 20.

Particularly the pumping channel 17 extends in a

circumferential direction orthogonal to the second rotation axis B2.

Preferably, the feeding material inlet 23 is connected to a feed hopper 24 which supplies material to the pumping device 3 in a continuous fashion.

Preferably the feed hopper 24 is designed to supply the material by gravity fall from the top.

The feed hopper 24 has depth s, while the feeding

material inlet 23 has a flow section F.

For a feed hopper designed so as to eliminate in full or almost in full the friction acting over the lateral walls (e.g. when designed as truncated-cone with increasing lower basement area or manufactured with materials having very low coefficient of friction) the material flow rate QI through the feed port 23 gets:

QI = V _g Fp

where : is the falling speed of the material defined as: F is the flow section of the feeding material inlet 23

p is the density of the falling material discharged from the feed hopper 24.

With reference to figure 2, the processing channel 7 extends longitudinally between the passage 20 and an extrusion discharge port 27.

In the non-limiting example described and sketched here, the first rotation axis Bl is parallel to the second rotation axis B2.

A variation, not shown, provides the feeding material inlet 23 to include at least a twin roll unit designed to press and drag the material, when supplied to the pumping channel 17.

With reference to figure 3, the external face 12

preferably is provided with one or more grooves 13, each designed so as to define a respective indentation 26.

Preferably the groove 26 has a section which is rectangular or triangular or diverging toward the outer rotor circumference (comprising diverging lateral faces toward the outer rotor circumference) so as to make an easy material release from the groves 13, at the channel discharge.

Preferably the grooves 13 extend over a circumferential direction so as to define the grooves 13 as annular.

Preferably the external face 12 of the rotor 11 is provided with at least one central groove 13a and at least one side groove 13b, arranged side the groove 13a. In the non-limiting example described and sketched here, the external face 12 of the rotor 11 is provided with a central groove 13a and two side grooves 13b arranged aside the central groove 13a.

The central groove 13a defines a respective indentation

26a characterized by a flow section greater than the flow section of the indentation 26b defined by each of the side grooves 13b.

The central groove 13a is designed so as to define a central indentation 26a providing with a bottom face 28 and two lateral faces 29.

Preferably the flow section of the central indentation 26a is constant.

The radial distance between the cylindrical internal surface 19 of the stator 15 and the bottom face 28 of the central indentation 26a defines substantially the depth of the pumping channel 17 or the depth of the circumferential flow section. The axial distance between the two lateral faces 29 defines the width W of the pumping channel 17, orthogonal to the faces 29.

By use, the material supplied through the feeding

material inlet 23 engages the central indentation 26a, while the lateral indentations 26b allow to escape the air or gas entrapped in the supplied material or exhaling from the material when heated or melted downstream. In other words the lateral indentations 26b define venting channels having radial depth h (defined as the radial distance between the cylindrical inner surface 19 of the stator 15 and the bottom face 28 of the central indentation 26b), which is preferably shallower of the depth H which defines substantially the depth of the pumping channel 17.

Preferably the lateral channels 26b are connected to ventilation ducts 30 machined in the stator 15 and shown in figure 2.

With reference to figure 1, the pumping device 3 is provided with a scraper 32 arranged in a sliding manner with the external face 12 of the rotor 11.

The scraper 32 shows a profile suitable to match in the at least one groove 13 of the rotor 11 with the function to scrape away the feestock from the external face 12 of the rotor 11.

Preferably, the scraper 32 is provided with a

complementary profile relative to the profile of the external surface 12 of the rotor 11, thus can effectively scrape away all material from all portions of the grooves 13.

The scraper 32 is fixed substantially close to the passage 20 so as to scrape out the feedstok from the external surface of the rotor 11 and simultaneously to make the feedstok to easy flow through the passage 20.

Preferably the volumetric flow rate [m ³ /s] of the pumping device 3, defined as the product of the rotational speed of the rotor 11 [m/s] with the flow section of the pumping channel 17 [m ² ], is higher than the volumetric flow rate of the extruder 2 [m ³ /s], defined as the product of rotational speed of the screw 6 [m/s] and the flow section of processing channel 7 [m ² ] . More preferably the ratio of the volumetric flow rate of the pumping device 3 to the processing channel 17 is at least 1,5, preferably at least 3 and more preferably at least 10.

In figures 4-6 a second type of embodiment of the present extruder assembly is shown with the reference numeral 50.

In the following and in figures 4-6 the same reference numerals will be used as used in figures 1-3 to indicate either identical or similar parts.

Essentially the extruder assembly 50 differs from

extruder assembly 1, for the piston 52 housed in the pumping channel 17 and reciprocating inside the pumping channel 17. Particularly the piston 52 moves forth and back along a portion of the pumping channel 17.

The piston 52 shows a pushing end face 53 (well visible in figure 6), designed to extend circumferentially and has a cross section, preferably orthogonal to the circumferential direction of the pumping channel 17, designed so as to engage at least one groove 13 of the external face 12 of the rotor 11 to push the material toward the passage 20.

In details, the piston 52 has a cross section orthogonal to circumferential direction of the pumping channel 17 and engages the central groove 13a of the external face 12 of the rotor 11, as better shown in the figure 6.

With reference to figures 4 and 5, the piston 52 is reciprocating inside the pumping channel 17, between a start position (shown in figure 4) and an end position (shown in figure 5) .

In the start position (shown in figure 4) the pushing end face 53 of the piston 52 is positioned upward (as seen along the rotation direction of the rotor 11) relative to the feeding material inlet 23 of the pumping channel 17, to allow the material to enter into the pumping channel 17.

In the end position (shown in figure 5) the pushing end face 53 is positioned between the feeding material inlet 23 and the passage 20 (as seen along the rotation direction of the rotor 11) .

The forth movement of the ram 52, between the start position and the end position, is effected along the opposite rotational direction of the second rotational direction V2.

Preferably, the piston 52 shows at least one sharp edge over the pushing end face 53.

The piston 52, the lateral faces 29 and the bottom face 28 cooperate mutually to ensure a high conveying efficiency inside the pumping channel 17. If the lateral faces 29 and the bottom face 28 were stationary, the piston 52 would have very poor efficiency due to the exponential frictional resistance developed by the lateral faces 29 and the bottom face 28. Such frictional resistance can be reduced, or completely eliminated or even converted into a positive frictional drag aid, by making the lateral faces 29 and the bottom face 28 to rotate.

The approximate result can be appreciated from the equations 3 and 4 reported below and available in the book "Principles of Polymer Processing" authored by Zehev Tadmor, Costas Cogos, E. John Wiley, 2006.

where :

Fi=force discharged on the material at the discharge of the stationary pumping channel

Fo=force discharged on the material at the entrance of the stationary pumping channel

C="wetted" perimeter of the stationary walls of the pumping channel

f _w =coefficient of friction solids-stationary walls of pumping channel

K= cross-to-axial distribution coefficient for pressure inside the stationary pumping channel

W= width of the flow section orthogonal to the side walls of the stationary pumping channel

H=depth of flow section of the stationary pumping channel L=length of the stationary pumping channel and

(2) where :

Fi= force exerted on the material at the discharge of the rotary pumping channel 17

Fo= force exerted on the material at the entrance of the rotary pumping channel 17

C2="wetted" perimeter portion of the moving walls of the rotary pumping channel (lateral faces 29 + bottom face 28) Ci="wetted" perimeter portion of the static walls of the rotary pumping channel

fw2= coefficient of friction solids-moving walls of the pumping channel 17

fwi= coefficient of friction solids-stationary walls of the pumping channel 17

K= cross-to-axial distribution coefficient for pressure inside the rotary pumping channel 17

W=width of the flow section normal to the side walls of the rotary pumping channel 17

H=depth of flow section of the rotary pumping channel 17

L=length of the rotary pumping channel 17

The equation 1 describes the force Fi acting on the material at the discharge when the rotor 11 gets stopped.

By the other hand the equation 2 describes the force Fi acting on the material at the entrance, when the rotor 11, which is provided with the pumping channel 17 (as defined by the lateral faces 29 and by the bottom face 29) is set in motion . By the following example, we report a comparison between exit forces of respectively two pumping devices, both equipped with piston 52, but one with stopped rotor and the other with moving rotor.

Assuming, for example, that the rotary channel has rectangular flow section with the following geometrical data: W=0,02 m, H=0,08 m, L=0,3 m, K=0,4, fw ₂ =0,2, fwi=0,2 and the applied force Fo= IN, then with stopped rotor we would obtain a force Fl at the discharge, reduced down to 0,165 N, while with moving rotor we would obtain a force Fl at the discharge, almost doubled to 1,822 N.

In other words with stationary pumping channel (stopped rotor) the initially applied force Fo loses its magnitude down to almost zero (Fi=0,165 N) , while with rotating channel

(rotor 11 in rotation) the force Fi, in the example,

increases, by almost doubling, the initially applied force Fo.

The above physical-mathematical comparison shed light about the efficiency of the two pumping devices, namely of the rotary pumping channel 17 operating in cooperation with the moving piston 52. Said in further different words the arrangement of the rotary pumping channel 17 aided by the moving piston 52, makes the force exerted on the material by the ram 52 at the entrance, to never be dissipated by the negative frictional forces over the walls 29 and 28 but to rise up effectively in a location close to the passage 20, where the pushing force is effectively requested. In the non-limiting example described and sketched here, the piston 52 is moved by an actuator 55 connected with the piston 52 through a type of transmission which can be of any known type and for example electric-mechanical (piston rod) or hybrid pneumatic-mechanical (pneumatic piston), etc. as illustrated in figures 4 and 5.

The reciprocating movement frequency of the piston 52 can be freely adjusted.

For example the piston 52 frequency can be suitably adjusted in a way to make the actual volumetric delivery rate of the rotary pumping channel 17 to match the designed mass delivery rate of the extruder 2. Said in different words if the entrance material density gets lower than expected, then the piston frequency can be made to increase in order to match the designed extruder mass delivery rate and viceversa.

Such an automatic control can be easily arranged by controlling the extruder torque through a master-slave control torque- (piston) frequency and a transmission cam driven by an electric actuator.

It is obvious that the electric actuator is more expensive than the pneumatic actuator but has a higher control efficiency .

The extruder assembly 50 provided with the piston 52 is particularly useful in all those applications where the bulk density is low as well as when the low bulk density is subject to continuous and/or sudden variations. It appears also evident that many modifications or variations can be introduced in the extruder assembly described and sketched here, without coming out of the scope of the alleged claims.