Screw and twin-screw assembly for use in elastomer mixture extruders and associated method for extrusion of elastomer mixtures
A Screw for use in a twin-screw assembly of an elastomer mixture extruder with a threaded part with a single-start thread which defines at least three different sectors of the screw. The three sectors have an intake segment to capture the mixture fed from outside to push it downstream along the longitudinal direction, with a cross-section of the through-flow channel between adjacent flanks of the thread which is constant over at least two pitches or a rotation through 720 degrees of the thread. A transition sector which has a cross-section of the through-flow channel variable and smaller than the cross-section of the through-flow channel and is designed to cause an increase of the thrusting pressure acting on the mixture in transit in the longitudinal direction; and a high-pressure sector designed to cause the compression of the mixture to obtain a maximum pressure of the mixture.
1 . A screw ( 21 ; 22 ) adapted for use in a twin-screw assembly with intermeshing screws of an elastomer mixture extruder, comprising a threaded part with a thread and which defines at least three different sectors ( 30 , 40 , 50 ) of the screw along a longitudinal direction (X-X) of axial extension from upstream to downstream, wherein the at least three sectors comprise:
an intake sector ( 30 ), which is adapted to capture and push a mixture downstream along the longitudinal direction (X-X) and which has a cross-section(S) of a throughflow channel comprised between adjacent flanks of the thread which is constant over at least two pitches or a rotation through 720 degrees of the thread;
a transition sector ( 40 ), downstream of the intake sector, which has a cross-section(S) of the throughflow channel which is variable and smaller than the cross-section of the throughflow channel of the intake sector and is designed to cause an increase in thrusting pressure acting on the mixture in transit in the longitudinal direction (X-X);
a high-pressure sector ( 50 ), downstream of the transition sector, which has a cross-section of the throughflow channel which is minimal and constant over at least one pitch and designed to cause compression of the mixture to obtain a maximum pressure of the mixture,
wherein the thread of said threaded part along each sector of the at least three sectors is a single-start thread and a geometry of the screw in the high-pressure sector ( 50 ) has a crest width (W) of a thread crest where W=(0.3-0.4)D, and is constant over one or two pitches, where D is an external diameter of the screw.
2 . The screw according to claim 1 , wherein the intake sector has the cross-section of the throughflow channel and/or the crest width (W) of the thread crest which is constant over at least three pitches.
3 . The screw according to claim 1 , wherein, in the intake sector, the geometry of the screw has:
a pitch (P) of the thread which is constant; and/or
a height (H) of the throughflow channel is H=(0.3-0.8)D; and/or
an axial length (Li) of the intake zone is Li=(3-4)P and/or (3-4)D.
4 . The screw according to claim 1 , wherein, in the high-pressure sector ( 50 ), the geometry of the screw has:
the crest width (W) is W=(0.33-0.37)D, constant over one or two pitches; and/or
a pitch (P) is constant;
where D is the external diameter of the screw.
5 . The screw according to claim 1 , wherein the screw has a constant pitch over an entire length of the threaded part.
6 . The screw according to claim 1 , wherein the screw has a length of the threaded part less than or equal to 10 D, where D is the external diameter of the screw, and/or less than or equal to 10 P, where P is a pitch of the thread.
7 . The screw according to claim 1 , wherein, in the transition sector, variation of the throughflow channel for passage of the mixture is obtained with a geometry which varies the crest width (W) of the screw thread.
8 . The screw according to claim 7 , wherein, in the transition sector, the crest width W is variable continuously between a minimum value of W, corresponding to a crest width of the intake sector, and a maximum value corresponding to a value of W of the high-pressure sector; wherein the geometry of the screw in the transition sector has:
a constant pitch (P); and/or
a height (H) of the throughflow channel is constant and/or comprised of H=(0.3-0.12)D; and/or
an axial length of transition sector is Lt=(1-3)D;
where D is the external diameter of the screw.
9 . A twin-screw assembly ( 10 , 20 ) for elastomer mixture extruders, comprising:
two screws ( 21 , 22 ) with a threaded part having a single-start thread, which are arranged intermeshing and counter-rotating with parallel longitudinal axes of rotation (X-X) inside a cylinder ( 10 ), so as to form a throughflow channel for passage of the mixture flow consisting of a union of a plurality of C-chambers (Cx), each defined by a free volume inside the cylinder ( 10 ) and comprised in a single rotation of the thread for a single screw,
the twin-screw assembly being provided with an upstream opening ( 13 ) for the intake of mixture into the throughflow channel,
wherein the throughflow channel for passage of the flow comprises, from upstream to downstream in the longitudinal direction (X-X) of axial extension of the screws and advancing movement of the mixture, at least three different sectors ( 30 , 40 , 50 ), which include:
an intake sector ( 30 ), which is adapted to capture the mixture fed from the outside and to push it downstream along the longitudinal direction (X-X) and which has a volume of the C-chambers forming the throughflow channel for passage of the flow which is constant over at least two pitches of the thread of each screw;
a transition sector ( 40 ), downstream of the intake sector, which has a volume of the C-chambers forming the flow channel which is variable, decreases and is smaller than the volume of the C-chambers of the intake sector;
a high-pressure sector ( 50 ), downstream of the transition sector, which has a volume of the C-chambers forming the flow channel which is constant over at least one pitch and smaller than the volume of the C-chambers of the intake and transition sectors, and is adapted to cause a compression of the mixture so as to obtain a maximum pressure of the mixture, and
wherein at least one of the two screws is the screw according to claim 1 .
10 . The twin-screw assembly according to claim 9 , wherein the two screws are in a mirror-image arrangement.
11 . The twin-screw assembly according to claim 9 , wherein the high-pressure sector has the volume of the C-chambers which is constant over at least two pitches of each screw.
12 . The twin-screw assembly according to claim 9 , wherein the intake sector has the volume of the C-chambers which is constant over at least four pitches of each screw.
13 . The twin-screw assembly according to claim 9 , wherein the transition sector has the volume of the C-chambers decreasing in a sense of advancing movement of the mixture, and/or cross-section of the throughflow channel for passage of the mixture flow decreasing in a sense of advancing movement of the mixture, according to a variation law which is at least partly approximately linear, quadratic, and/or of an order greater than 2.
14 . The twin-screw assembly according to claim 9 , wherein the two screws and the cylinder are arranged and configured to obtain an effective flowrate (Q) through the throughflow channel which is constant along the longitudinal direction of advancing movement of the mixture from the intake sector to an end of the high-pressure sector.
15 . The twin-screw assembly according to claim 9 , wherein:
in the intake sector:
O′=(0.0025-0.030)D;
δ=(0.0025-0.030)D;
and/or
in the high-pressure sector:
O′=(0.0025-0.020)D;
δ=(0.0025-0.030)D;
and/or
in the transition sector;
O′=(0.0025-0.030)D;
δ=(0.025-0.030)D; and
where O′ is a distance between the crest of one screw and the core of the other screw and δ is a distance between the crest of one screw and the internal surface of the housing cavity of the containing cylinder.
16 . The twin-screw assembly according to claim 9 , wherein the upstream opening is configured to feed the elastomer mixture to the throughflow channel in a direction (Z-Z) substantially orthogonal to the longitudinal direction (X-X).
17 . An elastomer mixture extruder, wherein the elastomer mixture extruder comprises a twin-screw assembly extending in the longitudinal direction (X-X) according to claim 9 and a downstream extrusion head ( 70 ).
18 . The elastomer mixture extruder according to the claim 17 , wherein the elastomer mixture extruder comprises a filtration zone provided with a mesh filter, downstream of the high-pressure zone ( 50 ), through which the elastomer mixture is made to pass.
19 . A method for extruding an elastomer mixture, comprising the steps:
feeding the elastomer mixture to the twin-screw assembly according to claim 9 of a mixture extruder to form a throughflow channel for passage of elastomer mixture flow from upstream to downstream, consisting of the union of a plurality of C-chambers, each defined by the free volume between respective screws inside the cylinder and comprised in a single rotation of the thread for a single screw;
the elastomer mixture being fed inside the throughflow channel for passage of the elastomer mixture flow of the twin-screw assembly through the upstream opening ( 13 );
capturing the elastomer mixture and pushing the elastomer mixture downstream through the throughflow channel for the elastomer mixture flow, in the intake sector ( 30 ) of the twin-screw assembly, which has the volume of the C-chambers forming the flow channel which is constant over at least two pitches of the thread of each screw;
advancing of the elastomer mixture through the transition sector ( 40 ) of the throughflow channel for passage of the mixture flow, the transition sector ( 40 ) being arranged downstream of the intake sector in the longitudinal direction (X-X) of axial extension of the screws and advancing movement of the elastomer mixture and having the volume of the C-chambers forming the flow channel which is variable, decreases and is smaller than the volume of the C-chambers of the intake sector;
advancing and the elastomer mixture in the high-pressure sector ( 50 ) which is arranged downstream of the transition sector and which has the volume of the C-chambers forming the flow channel which is constant over at least one pitch and smaller than the volume of the C-chambers of the intake and transition sectors to cause compression of the elastomer mixture and obtain a maximum pressure of the mixture;
passing the elastomer mixture at a maximum pressure through an extrusion head ( 60 ) arranged downstream of the high-pressure sector ( 50 ).
20 . The method according to claim 19 , further comprising filtering the elastomer mixture by passing the elastomer mixture through a mesh filter downstream of the high-pressure zone ( 50 ).