Screw and twin screw assemblies for use in elastomer mixture extruders and related methods for extruding elastomer mixtures

Abstract

This application discloses a screw (21; 22) for a twin-screw assembly in an elastomer mixture extruder, comprising a threaded portion having a single-start thread that defines at least three distinct sections (20, 30, 40) of the screw from upstream to downstream along an axially extending longitudinal direction (X-X), wherein the at least three sections include: an inlet section (30) for capturing and pushing a mixture supplied from the outside along the longitudinal direction (X-X), the inlet section having a cross-section (S) including a flow channel between adjacent sides of the thread, the cross-section being at least... The two pitches are constant or rotate at 720 degrees; a transition section (40) downstream of the inlet section has a flow channel cross section (S) that is variable and smaller than the flow channel cross section of the inlet section, and is designed to increase the propulsion pressure acting on the mixture being conveyed in the longitudinal direction (X-X); a high-pressure section (50) downstream of the transition section has a flow channel cross section that is minimal, constant at at least one pitch, and is designed to compress the mixture in order to obtain the maximum pressure of the mixture.

Description

[0001] This invention relates to a threaded screw for use in a twin-screw extruder having intermeshing screws for extruding and / or filtering elastomer-based mixtures, and also to a twin-screw extruder with intermeshing screws for elastomer mixtures and a method for extruding elastomer-based mixtures.

[0002] Elastomer materials (e.g., elastomer-based mixtures or compounds) are known to be amorphous materials with a glass transition temperature below ambient temperature. In other words, elastomeric materials are highly viscous (viscoelastic) fluids at temperatures above or equal to ambient temperature; they are already "rubber-like" and therefore do not require melting for further processing.

[0003] These materials, commonly referred to as rubber, include, for example, natural rubber, polybutadiene, polyisoprene, EPDM, NBR, and SBR.

[0004] It is known in the technical field of producing elastomer-based compounds that it is necessary to “filter” the compound; this operation involves using suitable machinery to allow the material to be processed to flow through a “filter”, which typically consists of one or more metal mesh screens with appropriately sized mesh orifices.

[0005] Typically, the value of this aperture ranges between 0.1 mm and 1 mm.

[0006] The purpose of this operation is to retain any possible "objects" (impurities, particles of unmixed material, etc.) in the filter and thus eliminate them from the compound, provided that the size of these "objects" is larger than the flow aperture of the filter screen.

[0007] A typical example is a compound used to produce some visible contours of a car (such as window seals), for which a "perfect" surface appearance is an important characteristic, requiring the elimination of any possible sources of surface inhomogeneity from the compound used for this purpose.

[0008] Another important area of ​​filtration is the use of compounds in power supply cables, which must be completely free of impurities, especially metallic impurities.

[0009] In order to filter the compound, it must be “pushed” or “forced” to flow through the filter. This operation is only possible when the compound is in a fluid state, i.e., with a “viscous” component that predominates compared to the “elastic” component. This state occurs when the compound is not cross-linked (unvulcanized) so that the polymer chains are not chemically bonded together.

[0010] In its uncrosslinked (so-called "naïve") state, an elastomer compound can be considered a "fluid" capable of "flowing" or "moving freely".

[0011] Even in this state, the fluid has a relatively high viscosity, which sometimes causes an undesirable temperature rise during the flow of the compound due to the high friction within the material.

[0012] Therefore, filtration must be carried out under conditions where the compound is not cross-linked and the compound absolutely cannot cross-link during the filtration process; thus, filtration is greatly constrained by two main factors: temperature and pressure.

[0013] Therefore, temperature rise (e.g., due to friction) must be avoided throughout the process. Thus, temperature must be controlled by reducing friction and / or by efficiently dissipating the generated heat.

[0014] The pressure required to supply the compound during processing also depends on the velocity of the "fluid" passing through the filter. All other things being equal, to achieve an increase in productivity and therefore an increase in flow rate, this velocity needs to be increased precisely, which in turn increases the pressure and therefore the temperature.

[0015] It is also known that in various treatments involving elastomers subjected to velocity gradients, it is precisely because of these “gradients” that mechanical stresses are generated within the treated material, which can often cause undesirable changes in properties (e.g., a decrease in viscosity due to the mechanical breakage of macromolecules).

[0016] Therefore, from a rubber technology perspective, there is a problem with filtering elastomer-based compounds:

[0017] Under high flow / productivity conditions;

[0018] Limit the temperature rise of the compound to a relatively low value and avoid any initial cross-linking in any case;

[0019] To avoid any degradation of the mechanical properties of the material, such as undesirable breakage of polymer chains.

[0020] From an industrial perspective, filtration processes typically must ensure that:

[0021] The processing operation is economically sustainable, therefore its cost is low (i.e., machinery, labor and energy costs).

[0022] Easy and safe to operate;

[0023] The equipment layout is simple and highly automated.

[0024] "Environmental" sustainability refers to emissions and waste treatment with limited low values.

[0025] Different types of single-screw or multi-screw extruders are known in various technical fields involving the processing of plastic compounds such as PVC and PP.

[0026] Plastic compounds have glass transition temperatures above ambient temperature, therefore they must be introduced into the extruder in a solid state and heated within the extruder to "melt" and thus be processable. Consequently, the methods and extruders designed for plastic materials are incompatible with the proper handling of elastomeric compounds, in which the heating must be avoided as much as possible. Therefore, these two technical fields are considered quite different from each other.

[0027] For example, known extruders for processing plastic materials in the presence of heated compounds are described in US20150184655A1 and US2508495. US20150184655A1 proposes the use of a molding constant pitch screw, while US2508495 proposes the use of a screw whose pitch and width change continuously and gradually between the compound inlet and outlet.

[0028] GB1359672 describes a single-screw or twin-screw extruder having non-meshing screws for processing solid plastic materials (e.g., PET) and a heating device for melting the plastic material, wherein the volume included between adjacent tooth crests of the screw thread varies by a "platform" protruding from the bottom of the thread. The protruding platform prevents the use of screws in twin-screw extruders with meshing screws for processing elastomeric compounds.

[0029] Such screws and extruders are unsuitable for the extrusion and filtration of elastomer compounds because they have: a heating device (heat-conducting oil or resistor) designed to heat the processed plastic material to the high temperatures required to melt (which would damage the elastomer material, which, conversely, must not melt); and non-intermeshing screws (resulting in clearance between the screws, thus causing excessive flow loss and preventing the achievement of sufficient pressure for extruding / filtering elastomers). Furthermore, non-intermeshing screws operate along essentially separate and independent compound flow channels.

[0030] Devices for filtering elastomer compounds include known intermeshing twin-screw / multi-screw extruders, which typically rotate in opposite directions and include screws configured to generate high-pressure compounds at the filter.

[0031] However, these known extruders have handling issues because:

[0032] When a compound is loaded into an extruder under ambient pressure, a large amount of air may remain trapped in the compound and may remain "trapped" in the compound until the filtration stage, resulting in the final product having air bubbles inside it and making it difficult / unusable for downstream filtration.

[0033] They have relatively high screw lengths to reduce backflow opposite to the main motion and achieve the desired pumping effect. However, this length is detrimental to controlling the temperature rise of the compound, as the heat generated by friction between the compound and the screw surface and cylinder will increase with the increase of the surface area.

[0034] Moreover, the larger the contact area with the compound, the greater the likelihood that the properties of the treated compound will change significantly, precisely because of the friction between the compound and the surface.

[0035] Therefore, the proposed technical problem is to overcome or at least reduce the disadvantages of the prior art by specifically providing a screw with an improved design suitable for use in a twin-screw extruder for elastomer mixtures, having axial discharge, wherein the screws are arranged intermeshingly with parallel axes of rotation.

[0036] The specific problem raised is the geometry of the screw to allow air present in the elastomer mixture to escape before it is filtered, and / or to reduce temperature rise and changes in the physical properties of the mixture.

[0037] The proposed technical problem also lies in providing a twin-screw extruder for elastomer mixtures, wherein the twin-screw extruder:

[0038] It has a loading system that operates under environmental pressure; and / or

[0039] Materials that do not require lubrication (i.e., no mixture loss); and / or

[0040] High productivity; and / or

[0041] Ensure improved heat exchange, i.e., generate less heat; and / or

[0042] It features a screw design to ensure high voltage only in the region of interest and along a relatively short section; and / or

[0043] Able to expel air still trapped inside the mixture before it reaches the filtration zone; and / or

[0044] It does not cause any change in the physical properties of the mixture.

[0045] Regarding this issue, it is also required that twin-screw assemblies and / or extruders be easy and inexpensive to manufacture and assemble, have small dimensions, and be easily installed at any user location.

[0046] According to the present invention, these results are obtained by means of a screw according to the present application and a twin-screw assembly according to the present application.

[0047] The present invention also relates to a method for extruding elastomer mixtures according to this application.

[0048] Further details can be obtained from the following description of non-limiting examples of embodiments of the subject matter of the invention, with reference to the accompanying drawings, in which:

[0049] Figure 1 This shows a side view of a screw according to the invention, wherein three different longitudinal sections of the screw are highlighted.

[0050] Figure 2 : indicates according to Figure 1 The side view of the screw shows various characteristic parameters;

[0051] Figure 3 : This shows an exploded view of the twin-screw assembly according to the present invention;

[0052] Figure 4 : indicates the basis for assembly Figure 3 A perspective view of a twin-screw assembly with a cylinder opening for receiving the cylinder;

[0053] Figure 5 : indicates that it is based on Figure 3 A perspective view of the extruder of the present invention, in which the twin-screw assembly is assembled.

[0054] Figure 6 : This shows a schematic cross-sectional view of an extruder according to the invention, wherein the clearance between the screws and between the screw and the cylinder is shown;

[0055] Figure 7a 7b: A schematic diagram showing the clearance between the screws and the corresponding view of the pressure obtained in each section;

[0056] Figures 8a-8c : This shows a perspective view and a partial side view of a pair of screws in a twin-screw assembly, with the C-shaped chamber highlighted and the different flow rates involved shown.

[0057] Figure 9 : A view showing an example of a percentage change in the volume of a C-shaped chamber, which depends on the axial position of the chamber in the passage from the mixture inlet area of ​​the extruder to the high-pressure / output area;

[0058] Figure 10 : This shows an example of the progression of actual flow along the axis of rotation of the screw and the volume change of the C-shaped chamber in a preferred embodiment of the twin-screw assembly of the present invention, wherein the local width (W) of the thread crest is indicated along the X-axis;

[0059] Figure 11: This shows a schematic cross-sectional view of the flow channel of the screw according to the present invention as the rotation angle of the thread changes;

[0060] Figure 12 a) to e): illustrate examples of the geometry of the screw according to the invention;

[0061] Figure 13 a) to e): These illustrate examples of intermeshing and counter-rotating screw pairs in a mirror arrangement, each screw pair being determined according to... Figure 12 The corresponding screw is used to form it;

[0062] Figures 14A-14E : indicates according to Figure 13 An example of the characteristic progression of the flow passage section of a twin-screw assembly, depending on the thread winding angle.

[0063] As shown in the figure, a set of three reference axes is assumed for ease of explanation and not as a limitation: the longitudinal direction XX corresponds to the axial length dimension of the screw and the direction of mixture supply; the transverse direction YY corresponds to the radial width dimension of the screw and is parallel to the interaxial plane between the rotation axes of the two screws when used in a twin-screw assembly; the vertical direction ZZ is perpendicular to the other two directions. The screw according to the invention has a threaded portion, the thread of which is of the single-start type, the thread being raised relative to the core, and defining three different longitudinal sections, namely 30, 40 and 50 respectively.

[0064] refer to Figure 2 and 6 The following describes some characteristic parameters of the screw according to the present invention, and these characteristic parameters will be discussed in the continuation of the specification.

[0065] P = The screw pitch, measured as the axial distance between the center lines of the two tooth crests of the thread, which are arranged 360° apart (the thread completes one full revolution around the screw axis). In the application of this invention, the pitch is generally constant along the entire screw, and preferably coincides with the outer diameter D;

[0066] D = outer diameter of the screw. It is usually constant along the entire screw.

[0067] d = inner diameter of the screw, corresponding to the diameter of the core; d can vary along the length of the screw, but it is preferred to keep it constant in the inlet and high-pressure sections, and even more preferably in the transition section;

[0068] Flow channel: includes the free volume between adjacent sides of the thread (corresponding to the groove of the thread).

[0069] Tooth crest: The top surface that connects two consecutive side surfaces

[0070] (Channel) Flow cross section: The cross section of the flow channel (or channel) along the axial plane that passes through the axis of rotation of the screw;

[0071] W = Width measured along the axial direction of the thread crest;

[0072] H = Height of the mixture flow channel;

[0073] L = Length of the threaded portion of the screw

[0074] The profile of the thread can preferably be trapezoidal or flattened triangular.

[0075] To illustrate the twin-screw assembly according to the present invention, the following further definitions are provided:

[0076] l = the interaxial distance between the screws in the screw assembly ( Figure 6 );

[0077] σ = the distance between the crest of one screw and the core of another screw ( Figure 6 );

[0078] δ = the distance between the crest of a screw thread and the inner surface of the housing cavity containing the cylinder. Figure 6 ).

[0079] C-shaped chamber: A C-shaped chamber defined by the free volume between the screws inside the housing cylinder and included in a single rotation (in other words, "pitch") of the thread of a single screw.

[0080] (Flow) Channels: The free volume between the screws inside the receiving cylinder defines the flow channels for the mixture. The flow channels for flow are formed by the combination of all the C-shaped chambers of the two screws.

[0081] refer to Figure 3 and Figure 4 Furthermore, an upstream portion M corresponding to the area into which the mixture to be filtered enters and a downstream portion V corresponding to the output area for filtering the mixture are defined. The twin-screw assembly according to the invention essentially comprises:

[0082] The cylinder 10 has a body 11 with a top upstream opening 13 for the entry of the mixture and a downstream axial outlet opening 12; advantageously, the cylinder can be divided into two half-cylinders 11a and 11b for easy assembly.

[0083] The cylinder has a suitable internal cavity that is shaped to accommodate two screws 21 and 22, which are arranged such that their axes of rotation are parallel, meshing with each other and rotating in opposite directions during use.

[0084] The output region 60 is located at the downstream end of the twin-screw assembly, and the region includes a filtration region 70. Figure 5 The mixture passes through the filter (not shown) in the supply direction toward the discharge outlet and enters the filtration zone 70.

[0085] For ease of explanation, this specification will always refer to a twin-screw assembly in which the screws are arranged in a mirror image of each other, but different configurations of the two meshing and counter-rotating screws of the twin-screw assembly according to the invention are also conceivable.

[0086] The cylinder and screw assembly are along the longitudinal direction of the twin-screw assembly. Figure 3 Three distinct sections are defined, corresponding to the three longitudinal sections of the threaded portion of the screw, namely:

[0087] In the upstream section 30, the mixture is introduced into the cylinder 10 under ambient pressure and is “captured” by the rotation of screws 21 and 22;

[0088] - Intermediate or transition section 40, which is located downstream of inlet section 30, wherein the two screws 21, 22 act to gradually increase the propulsion pressure of the mixture pushing downstream;

[0089] - High-voltage section 50, which is located between the transition section and the subsequent output region 60.

[0090] As shown in Figure 7, and as will become clearer below, along the mixture inlet section 30, there is a large clearance between the screws to create a large "free" volume that allows for easy entry of a large amount of mixture. Along the transition section 40, the clearance between the screws gradually decreases, and along the high-pressure section 50, there is a very small clearance between the screws, smaller than that of the inlet section 30 and the transition section 40. This creates smaller flow channels or free volumes, which minimizes backflow and achieves the high pressure required for filtration.

[0091] The three longitudinal sections of the screw can be determined based on the change of the flow channels formed in the free volume inside the cylinder relative to the rotation angle.

[0092] In this context, reference will also be made to "C-shaped chamber" ( Figures 8a-8c The known concept of ) is used to identify channel Cx ( Figure 8c The channel Cx has a free volume in the form of C, which is defined between a pair of meshing screws and is included in a single turn of the thread of the individual screw (in other words, the "pitch").

[0093] In detail, according to the present invention, the twin-screw assembly of the present invention is characterized by a single flow channel comprising at least three distinct sections (20, 30, 40) of the screw along the longitudinal direction (XX) from upstream to downstream, the section comprising:

[0094] The inlet section (30) has sufficient volume of C-shaped chambers C1-C3 forming flow channels, which is optimized to capture the mixture supplied from the outside and push the mixture downstream in the longitudinal direction (XX), and it remains constant at at least two pitches of the threads of each screw.

[0095] In the transition section (40) downstream of the inlet section, the transition section (40) has C-shaped chambers C1-C3 with variable volume, thereby reducing (in the direction of forward movement XX) and smaller than the C-shaped chamber volume of the inlet section 30, so as to specifically generate an increase in the propulsion pressure acting on the conveyed mixture in the longitudinal direction XX, and to discharge the air trapped during loading of the mixture;

[0096] High-pressure section 50, located downstream of transition section 40, has the volume of C-shaped chambers C6 and C7 forming flow channels. These flow channels have a constant pitch in at least one section and are smaller than the volumes of the inlet section and transition section. Therefore, this section 50 with its minimum volume of C-shaped chambers is optimized to compress the mixture and achieve maximum pressure of the mixture in the output region 60.

[0097] According to a preferred embodiment of the invention, the geometry of the three longitudinal sections of each screw is developed to generate a variation pattern in the flow channel and thus a variation pattern in the C-shaped chamber, which ensures an optimized configuration for the specific function of each section. In particular, the geometry maximizes inlet performance at atmospheric pressure, corresponding to the inlet section required for the large volume (and therefore low tooth crest width W) of the C-shaped chamber, thereby maximizing flow rate and maximizing the compression of the mixture in the high-pressure section 50 arranged immediately upstream of the filtration area.

[0098] In particular, in a preferred embodiment of the screw used in the twin-screw assembly according to the invention, the inlet section has the following parameters:

[0099] W = (0.025-0.20)D, preferably (0.05-0.10)D for at least two, three, or four constant pitches;

[0100] With P constant, the preferred value is =D;

[0101] σ = (0.0025-0.030)D, preferably (0.005-0.015)D

[0102] δ = (0.0025-0.030)D, preferably (0.005-0.015)D

[0103] H = (0.3-0.8)D, preferably (0.54-0.6)D

[0104] (Axial length of Li inlet region) = (3-4)D

[0105] The preferred geometric variables of the screw in the inlet section result in the following in a single-thread screw:

[0106] The relatively low W value and high H value enable the achievement of the maximum volume of the C-shaped chamber;

[0107] Lower values ​​for σ and δ, as well as the axial transmission value (P=D), maximize the mixture “capture” performance at the extruder input and mixture supply.

[0108] In order to generate the required pressure only in the high-pressure section immediately adjacent to the filter, in order to reduce heat generation and backflow losses, the C-shaped chamber in this region has a smaller constant volume than in other regions of the screw, so that the clearance is as small as possible mechanically.

[0109] Specifically, the high-voltage section preferably has the following parameters:

[0110] W = (0.3-0.4)D, preferably (0.33-0.37)D, for one or two constant pitches;

[0111] With P constant, the preferred value is =D;

[0112] σ = (0.0025-0.020)D, preferably (0.005-0.015)D

[0113] δ = (0.0025-0.030)D, preferably (0.005-0.015)D;

[0114] H = (0.3-0.8)D, preferably (0.54-0.6)D;

[0115] Lp (axial length of the high-voltage section) = 1 - 2D;

[0116] Typically a constant flow cross section

[0117] The geometric variables in the high-pressure region cause:

[0118] Lower values ​​of δ and σ, and higher values ​​of W, maximize pumping performance, that is, maximize the ratio between the main flow (in the direction of supply) and the counterflow opposite to the main flow.

[0119] Under high H conditions, the value of P=D simultaneously maximizes the flow rate.

[0120] Therefore, the screws of the twin-screw assembly according to the invention can preferably be configured to simultaneously achieve: high pressure in the section 50 near the filter, wherein the clearance between the screws and between the screw and the cylinder is relatively very small (in order to reduce backflow opposite to the main motion); and high capture and flow rate of the mixture in the inlet region, due to the higher free volume (i.e., the space that can potentially be filled by the mixture).

[0121] Considering the different performance characteristics required for the two sections (i.e., the upstream / ambient pressure inlet section and the downstream / high-pressure section), the transition section between the inlet section (with a low-pressure and high-volume C-shaped chamber) and the high-pressure section (with a low-volume C-shaped chamber upstream of the filtration area) is preferably configured to have a variable C-shaped chamber volume, specifically so that:

[0122] When the mixture enters the extruder, it eliminates the air trapped in the mixture.

[0123] Avoid sudden and unexpected changes in geometry, which can lead to potential inhomogeneities and localized pressure spikes in the material being processed.

[0124] Therefore, preferably, the transition section 40 has a reduced C-shaped chamber volume. In particular, the cross-section of the mixture flow channel preferably decreases along the direction of the mixture's forward movement according to a variation law that is substantially continuous, and particularly at least locally approximately linear and / or quadratic and / or of order greater than 2.

[0125] Preferably, the variation of the mixture flow channel in the transition section is achieved by changing the geometry of the screw thread crest width W, while other parameters of the screw can remain constant in the transition section.

[0126] Based on a particularly preferred geometry of the screw used in the twin-screw assembly of the present invention, the transition section has the following parameters:

[0127] W can vary continuously between the W value in the inlet section and the W value in the high-pressure section.

[0128] With P constant, the preferred value is =D;

[0129] σ = (0.0025-0.030)D, preferably (0.005-0.015)D

[0130] δ = (0.025-0.030)D, preferably (0.005-0.015)D;

[0131] H = (0.3-0.12)D, preferably (0.54-0.6)D;

[0132] Lt (axial length of the transition section) = 1 - 3D

[0133] This allows for a gradual transition between high-pressure and low-pressure performance. Variations in the flow channels can follow appropriate patterns to optimize the desired performance.

[0134] It is particularly preferred to limit the axial length of the threaded portion of the screw in order to obtain a ratio L / D ≤ ​​8, i.e. a shorter length, in order to limit the undesirable temperature rise typically seen in long extruders (L / D > 10 is known in the art).

[0135] Based on the above rules, a screw is provided, which has a geometry obtained based on the variation of the internal channels, and is designed to define the three different processing regions.

[0136] Referring again to Figure 8C and the mixture flow channel defined by the C-shaped chambers of the twin-screw assembly, it can be seen that, without flow loss, the theoretical maximum flow rate Qth of a twin-screw extruder with mirror-arranged, meshing, and counter-rotating screws will be equal to the volume Vc of the two C-shaped chambers multiplied by the rotational speed N of the screws:

[0137] Maximum theoretical flow rate: Qth=2 Vc N

[0138] Vc = C-shaped cavity volume

[0139] N=rpm

[0140] Moreover, it is known that the theoretical maximum flow rate can never be reached due to the flow loss caused by the clearance between the screws and between the screw and the cylinder, as well as the counterflow opposite to the main motion. The larger the clearance, the higher the flow loss, which also depends on the pressure development between the loading area and the filtration area.

[0141] Referring again to Figure 8C, the main traffic loss is as follows:

[0142] Qc = Calendar leak

[0143] Qt = Tetrahedral Leakage

[0144] Qf = Flight gap leakage

[0145] Qs = Side gap leakage

[0146] Therefore, the effective flow rate Q is obtained from the algebraic sum of the theoretical maximum flow rate (Qth) and the total flow loss (Ql), which is derived directly from the volume of the C-shaped chamber, and the total flow loss (Ql) is due to the clearance between the screws and between the screw and the cylinder, as well as the backflow caused by pressure.

[0147] Total loss of flow / backflow:

[0148] Ql = Qf + Qs + Qt + Qc

[0149] Effective flow: Q = Qth - Ql

[0150] According to the preferred configuration of the twin-screw assembly, the assembly is configured to obtain a substantially constant effective flow rate Q along the longitudinal direction of the forward movement of the mixture from the input section to the end of the high-pressure section. Therefore, the efficiency is calculated as the ratio Q / Qth, which gradually increases toward the output region.

[0151] Figure 9 The diagram illustrates a particularly preferred example of this configuration, showing the geometry of the counter-rotating screw and the variation of the C-shaped chambers forming the flow channels for the mixture; the corresponding effective flow rate Q = Qth - Ql through the flow channels is constant along the extrusion direction. Figure 10 In the view, it is represented by a black dashed line.

[0152] Figure 11 This diagram shows the cross-section S of the flow channel as the rotation angle of the screw thread changes according to the present invention.

[0153] In another preferred embodiment of the screw and corresponding twin-screw assembly according to the present invention, respectively in... Figure 12 , 13 The Chinese side indicated that...

[0154] Figure 14 shows the cross-sectional variation of the mixture passage for the corresponding twin-screw assembly.

[0155] In more detail, Figure 12 The screw of a has a flow channel cross section which is constant (and maximum) over three 360° rotations (3l) (three pitches) of the thread in the inlet section 30, decreases over three rotations of the thread in the transition region, and is constant (minimum) over two pitches (2P) in the high-pressure region.

[0156] Used according to Figure 13 The C-shaped chamber and cross-section of the mixing passage of the twin-screw assembly a follow a similar variation pattern, as shown in Figure 14a.

[0157] Figure 12The screw of type b has a flow channel cross-section that is constant (and maximum) over four 360° rotations of the thread in the inlet section 30, decreases over one 360° rotation of the thread in the transition region, and is constant (minimum) over two pitches in the high-pressure region. According to... Figure 13 The C-shaped chamber and cross-section of the mixing passage of the twin-screw assembly b follow a similar variation pattern, as shown in Figure 14b.

[0158] Figure 12 The screw of type c has a flow channel cross-section that is constant (and maximum) over four 360° rotations of the thread in the inlet section 30, decreases over the two pitches (2L) of the thread in the transition region, and is constant (minimum) over the two pitches in the high-pressure region. According to... Figure 13 The C-shaped chamber and cross-section of the mixing passage of the twin-screw assembly in Figure 14c follow a similar variation pattern.

[0159] Figure 12 The screw and twin-screw assemblies in d, 13d, and 14d are similar to Figure 12 The screw and twin-screw assembly in c, 13c, and 13d, but in this case, the change in cross-section in the transition section 40 has a reduced second-order number (2L).

[0160] Figure 12 The screw of type e has a flow channel cross-section that remains constant (and is at its maximum) over four 360° rotations (three pitches) of the thread in the inlet section 30. This cross-section decreases over the three rotations of the thread in the transition region and remains constant (at its minimum) over the 360° rotations of the thread in the high-pressure region 50.

[0161] Used according to Figure 13 The C-shaped chamber and cross-section of the mixing channel of the twin-screw assembly in Figure 14e follow a similar variation pattern.

[0162] Advantageously, all the preferred examples shown (these preferred examples are just some possible geometries) are able to keep the length of the threaded portion of the screw within ten pitches, preferably within eight pitches.

[0163] In addition, it can also:

[0164] High pressure (even above 300 bar) can be achieved with screw length and a finite ratio L / D = length / diameter of transition zone + pressure zone equal to 5 and high flow rate, so it is also possible to use filters with very fine sieve holes (<0.1 mm) for filtration;

[0165] Controlling and limiting the temperature rise of the mixture at high RPM and flow rates;

[0166] Use relatively low drive torque because the screw is short;

[0167] To prevent significant bending of the screws and to prevent them from contacting the cylinder;

[0168] Air is discharged through the inlet opening, which can be retained and captured as the mixture enters.

[0169] In a preferred embodiment of the extruder according to the invention, the extruded mixture filtration / output section 70 is envisioned to include a filter retainer plate 71 connected to a connecting flange 60 and closed by a forming head 73.

[0170] A “filter” (not shown here) typically comprising one or more metal meshes is arranged between the connecting flange 61 and the filter retainer plate 71; the mixture is propelled by the thrust generated by the rotation of the screw to flow through the filter, which retains any impurities larger than the mesh openings.

[0171] Preferably, one or more pressure and temperature sensors 61 are arranged in the flange 60 and are capable of continuously monitoring the pressure and temperature of the mixture being processed in order to obtain complete control over the filtration step.

[0172] Figure 6 7 represents the cross-section of the housing passing through the screws 21 and 22 inside the cylinder 10.

[0173] It can be seen how the clearance δ between the tooth crest of each screw and the cylinder 10, and the clearance σ between the tooth crest of one screw and the core of another screw, are made very small, while ensuring pumping action and no contact between the screw and the cylinder under any circumstances.

[0174] Therefore, it is clear how the screw, twin-screw assembly, and extruder equipped with the twin-screw assembly according to the present invention provide a solution to the problems of the prior art, resulting in:

[0175] There are no other auxiliary devices for loading the elastomer mixture to be filtered under environmental pressure;

[0176] High pressure is generated in a small, defined area, thereby limiting the temperature rise, which can advantageously keep it below the vulcanization temperature of the elastomer mixture (typically less than 100-120°C).

[0177] Overall performance is optimized through three zones, each dedicated to precise tasks and configured accordingly;

[0178] Due to the special geometry of the transition region, there are no local temperature peaks;

[0179] When the mixture is introduced into the inlet area, it effectively removes the air trapped in the mixture.

[0180] Although various embodiments and preferred examples of the present invention have been described in conjunction with implementations of the invention, it should be understood that the scope of protection of this patent is determined only by the following claims.

Claims

1. Screw (21, 22) suitable for use in a twin-screw assembly with intermeshing screws of an elastomer mixture extruder, the screw comprising a threaded portion with a thread, the thread determining different at least three sections (30, 40, 50) of the screw from upstream to downstream in a longitudinal direction (X-X) extending in axial direction, wherein, The at least three segments include: An inlet section (30) is used to capture the mixture and push it downstream in the longitudinal direction (XX), the cross section (S) of the flow channel of the inlet section being included between adjacent sides of the thread, the cross section being constant over at least two pitches of the thread or over a 720-degree rotation; The transition section (40) is downstream of the inlet section. The cross-section (S) of the flow channel of the transition section is variable and smaller than the cross-section of the flow channel of the inlet section, and is designed to increase the propulsion pressure acting on the mixture conveyed in the longitudinal direction (XX). The high-pressure section (50), located downstream of the transition section, has a flow channel with a minimum cross-section, constant at least one pitch, and designed to compress the mixture in order to achieve maximum pressure. Wherein, the threaded portion along each of the at least three sections is a single-start thread, and the geometry of the screw in the high-pressure section (50) has a thread crest width (W), wherein the thread crest width W = (0.3-0.4)D and the thread crest width W is constant over one or two pitches, where D is the outer diameter of the screw.

2. The screw of claim 1, wherein: The cross-section of the flow channel in the inlet section and / or the crest width (W) of the thread tooth are constant over at least three pitches.

3. The screw according to claim 1, characterized in that: In the inlet section, the screw geometry has the following characteristics: The thread pitch P is constant; and / or The height H of the flow channel, H = (0.3-0.8)D; and / or The axial length of the inlet region is Li = (3-4)P and / or (3-4)D.

4. The screw according to claim 1, characterized in that: In the high-pressure section (50), the screw geometry has the following characteristics: The tooth crest width W = (0.33-0.37)D, is constant over one or two pitches; and / or The pitch P is a constant; Where D is the outer diameter of the screw.

5. The screw according to claim 1, characterized in that: The screw has a constant pitch along the entire length of the threaded portion.

6. The screw according to claim 1, characterized in that: The length of the threaded portion of the screw is less than or equal to 10D, where D is the outer diameter of the screw, and / or less than or equal to 10P, where P is the thread pitch.

7. The screw according to claim 1, characterized in that: In the transition zone, the variation of the flow channel for the mixture is achieved by changing the geometry of the screw thread crest width (W).

8. The screw according to claim 7, wherein: In the transition section, the tooth crest width W can continuously vary between a minimum value and a maximum value, where the minimum value of W corresponds to the tooth crest width in the inlet section, and the maximum value of W corresponds to the W value in the high-pressure section; wherein, the geometry of the screw in the transition section has: Constant pitch (P); and / or The height H of the flow channel is constant, and / or included between (0.3-0.12)D; and / or The axial length of the transition section is Lt = (1-3)D; Where D is the outer diameter of the screw.

9. A twin-screw assembly for an elastomer mixture extruder, the twin-screw assembly comprising: Two screws (21, 22) having threaded portions having single-start threads are arranged inside a cylinder (10) to mesh with each other and rotate in opposite directions with parallel longitudinal axes of rotation (XX) to form a flow channel for the passage of a mixture, the flow channel being composed of a plurality of C-shaped chambers (Cx) joined together, each C-shaped chamber being determined by the free volume inside the cylinder (10) and included in a single rotation of the thread for a single screw. The twin-screw assembly is provided with an upstream opening (13) for allowing the mixture to enter the flow channel. Its characteristic is that: the flow channel for the flow to pass through extends axially along the screw and the longitudinal direction (XX) of the forward movement of the mixture from upstream to downstream includes at least three different sections (30, 40, 50), said sections comprising: An inlet section (30) is used to capture a mixture supplied from the outside and push the mixture downstream in the longitudinal direction (XX), the inlet section having a C-shaped chamber volume forming a flow channel for flow passage, the C-shaped chamber volume of the inlet section being constant over at least two pitches of the threads of each screw. Transition section (40), which is downstream of the inlet section, has a C-shaped chamber volume forming a flow channel, the C-shaped chamber volume of the transition section being variable, decreasing and being smaller than the C-shaped chamber volume of the inlet section; High-pressure section (50), the high-pressure section being located downstream of the transition section, the high-pressure section having a C-shaped chamber volume forming a flow channel, the C-shaped chamber volume of the high-pressure section being constant over at least one pitch and smaller than the C-shaped chamber volume of the inlet section and the C-shaped chamber volume of the transition section, the high-pressure section being used to induce compression of the mixture in order to obtain the maximum pressure of the mixture; Wherein, at least one of the two screws is a screw according to any one of claims 1-8.

10. The twin-screw assembly according to claim 9, characterized in that: The two screws are arranged in a mirror image.

11. The twin-screw assembly according to claim 9, wherein: The high-voltage section has a constant C-shaped chamber volume at at least two pitches of each screw.

12. The twin-screw assembly according to claim 9, wherein: The inlet section has a constant C-shaped chamber volume at at least four pitches of each screw.

13. The twin-screw assembly according to claim 9, wherein: According to the law of change, the transition section has a C-shaped chamber volume that decreases along the direction of the mixture's forward movement, and / or the cross-section of the flow channel through which the mixture flows decreases along the direction of the mixture's forward movement, wherein the law of change is at least locally approximately linear and / or quadratic and / or has an order greater than 2.

14. The twin-screw assembly according to claim 9, wherein: Two screws and cylinders are arranged and configured to obtain an effective flow rate (Q) through the flow channel, which is constant from the inlet section to the end of the high-pressure section along the longitudinal direction of the forward movement of the mixture.

15. The twin-screw assembly according to claim 9, wherein: In the import segment: σ = (0.0025 - 0.030)D; δ = (0.0025 - 0.030)D; and / or In high-voltage sections: σ = (0.0025 - 0.020)D; δ = (0.0025 - 0.030)D; and / or In the transition section: σ = (0.0025 - 0.030)D; δ = (0.025 - 0.030)D; Where σ is the distance between the crest of one screw and the core of another screw, and δ is the distance between the crest of one screw and the inner surface of the housing cavity of the cylinder.

16. The twin-screw assembly according to claim 9, wherein: The upstream opening is configured to supply the elastomer mixture to the flow channel in a direction (ZZ) substantially orthogonal to the longitudinal direction (XX).

17. An extruder for elastomer mixtures, characterized in that: The extruder for the elastomer mixture includes a twin-screw assembly extending in the longitudinal direction (XX) as described in any one of claims 9-16, and an extrusion head including a downstream end.

18. The extruder according to claim 17, characterized in that: The extruder of the elastomer mixture includes a filtration zone, which is provided with a screen filter downstream of the high-pressure section (50), through which the elastomer mixture passes.

19. A method for extruding an elastomer mixture, the method comprising the steps of: The twin-screw assembly according to any one of claims 9-16 supplies the elastomer mixture to the mixture extruder to form a flow channel for the elastomer mixture flow from upstream to downstream, the flow channel being composed of a plurality of C-shaped chambers combined, each C-shaped chamber being determined by the free volume between corresponding screws inside the cylinder and included in a single rotation of the thread for a single screw; The elastomer mixture is supplied through an upstream opening (13) into a flow channel for the elastomer mixture to flow through a twin-screw assembly; An elastomer mixture is captured in the inlet section (30) of the twin-screw assembly and pushed downstream through a flow channel for the flow of the elastomer mixture, the inlet section (30) having a C-shaped chamber volume forming the flow channel, the C-shaped chamber volume being constant at at least two pitches of the threads of each screw; The elastomer mixture is advanced through a transition section (40) of a flow channel for the mixture to flow through, the transition section (40) extending axially along the screw and arranged downstream of the inlet section in the longitudinal direction (XX) of the advance of the elastomer mixture, and having a C-shaped chamber volume forming the flow channel, the C-shaped chamber volume of the transition section being variable, decreasing and being smaller than the C-shaped chamber volume of the inlet section; The elastomer mixture is propelled and compressed in a high-pressure section (50), which is located downstream of the transition section and has a C-shaped chamber volume forming a flow channel. The C-shaped chamber volume of the high-pressure section is constant at at least one pitch and is smaller than the C-shaped chamber volume of the inlet section and the C-shaped chamber volume of the transition section, so as to cause compression of the elastomer mixture and obtain the maximum pressure of the mixture. The elastomer mixture is passed through an extruder located downstream of the high-pressure section (50) under maximum pressure.

20. The method of claim 19, further comprising: The elastomer mixture is filtered by passing it through a screen filter downstream of the high-pressure zone (50).

Images