Patent Description:
It is known that elastomer materials, such as elastomer-based mixtures or compounds, are amorphous materials which have a glass transition temperature which is lower than the ambient temperature. In other words, elastomer materials at temperatures higher than or the same as the ambient temperature are highly viscous (viscoelastic) fluids which are already "rubbery" and therefore do not need to be melted for further processing. These materials, which are generally known as rubbers, include for example natural rubber, polybutadiene, polyisoprene, EPDM, NBR and SBR.

It is known that, in the technical sector relating to the production of elastomer-based compounds, there exists the need for "filtration" of the latter; with this operation the material being processed is made to flow, by means of suitable machinery, across a "filter" which is generally composed of one or more metal meshwork screens having a through-flow aperture of the meshes with suitable dimensions.

Typically this aperture has values ranging between <NUM> and <NUM>.

The purpose of this operation is to retain in the filter, and therefore eliminate from the compound, any possible "bodies" (impurities, particles of unmixed materials, etc.). which have dimensions greater than the through-flow aperture of the filter meshes.

A typical example is the compounds used to produce some visible profiles of cars such as, for example, window seals, for which the "perfect" surface appearance is an important feature that requires the elimination of any possible source of surface unevenness from the compounds used for this purpose.

Another sector in which filtration is important is that of the compounds used for the electric power supply cables which must be totally free from any impurities, in particular of a metallic nature.

In order for the compound to be filtered, it must be "pushed" or "forced" to flow through the filter, said operation being possible only if the compound is in a fluid state, namely has a "viscous" component which is predominant compared to the "elastic" component, this condition occurring when the compound is not crosslinked (not vulcanized) such that the polymer chains are not chemically bonded together.

In the non-crosslinked (so-called "raw") state, the elastomer compound may be regarded as being a "fluid" which is able to "flow" or "freely move".

Even in this state the fluids in question have a viscosity which is relatively very high so that, during the flowing movement of the compound, sometimes undesirable temperature increases arise owing to the high friction inside the material.

Filtration must therefore be performed in conditions where the compounds are not crosslinked and where the compounds are absolutely unable to crosslink during filtration; consequently, filtration is greatly conditioned by two main factors: temperature and pressure.

An increase in the temperature, for example due to friction, must therefore be avoided during the whole process. The temperature must therefore be controlled by reducing the friction and/or by means of efficient dissipation of the heat generated.

The value of the pressure, which is required in order to feed the compound during processing, is also dependent on the speed of the "fluid" which crosses the filter. All other conditions being equal, in order to obtain an increase in the productivity and therefore flowrate, it is required to increase precisely this speed, consequently increasing the pressure and therefore the temperature.

It is also known that, during every process which involves elastomers which are subject to speed gradients, mechanical stresses arise inside the material being processed precisely because of these "gradients", said stresses being able to cause in general an undesirable modification of the characteristics (for example, a reduction in the viscosity due to the mechanical breakage of the macromolecules).

From the point of view of rubber technology there therefore exists the problem of filtering elastomer-based compounds:.

From an industrial point of view, the filtration process must be in general such as to ensure:.

<CIT> provides a dual-rotor structure for continuous mastication of block rubber in order to output a continuous and high-quality rubber sheet, on which the preamble of Claim <NUM> is based.

Known extruders for the processing of plastic materials with heating of the compound are for example described in <CIT> which proposes the use of profiled constant-pitch screws and <CIT> which proposes using screws with thread pitch and width which vary continuously and progressively between a compound inlet and outlet.

<CIT> describes a single-screw or twin-screw extruder with non-intermeshing screws for the processing of plastic materials in the solid state such as PET, with heating means for melting the plastic material, wherein the volume comprised between adjacent crests of the thread of a screw is varied by means of "lands" projecting from the bottom of the threading.

Such screws and extruder are not suitable for the extrusion and filtration of elastomer compounds since they have heating means (diathermic oil or electrical resistances) designed to heat the plastic material being processed to high temperatures necessary for melting (which would damage the elastomer material which on the contrary must not be melted) and non-intermeshing screws (resulting in play between the screws and therefore an excessive loss of flowrate such that sufficient pressure for extrusion/filtration of the elastomers cannot be reached). Moreover, the non-intermeshing screws operate along substantially separate and independent compound flow channels.

The apparatus for filtering elastomer compounds include known intermeshing twin-screw/multiple-screw extruders which are generally counter-rotating and comprise screws configured to generate a high pressure of the compound at the filter.

These known extruders, however, are subject to processing problems, since:.

Moreover, the greater the contact area with the compound, the greater the probability that the properties of the compound being processed will change significantly, precisely because of the friction between compound and surface.

The technical problem which is posed therefore is that of overcoming or at least reducing the drawbacks of the prior art, by providing in particular screws with an improved design suitable for application to a twin-screw extruder for elastomer mixtures, with axial discharging, in which they are arranged intermeshing, with parallel axis of rotation.

A particular problem which is posed is that of creating a geometry of the screws such that air present in the elastomer mixture can be evacuated before filtration thereof and/or the increase in temperature and the variations in physical properties of the mixture being processed can be reduced.

The technical problem which is posed is also that of providing a twin-screw extruder for elastomer mixtures which:.

In connection with this problem it is also required that the twin-screw assembly and/or extruder should be easy and inexpensive to produce and assemble, have small dimensions and be able to be easily installed also at any user location.

These results are obtained according to the present invention by a screw according to the characteristic features of Claim <NUM> and by a twin-screw assembly according to Claim <NUM>.

The present invention relates furthermore to a method for extruding elastomer mixtures according to Claim <NUM>.

Further details may be obtained from the following description of non-limiting examples of embodiment of the subject of the present invention, provided with reference to the accompanying drawings, in which:.

As shown and assuming solely for the sake of easier description and without a limiting meaning a set of three reference axes in a longitudinal direction X-X corresponding to the axial lengthwise dimension of the screws and direction of feeding of the mixture, transverse direction Y-Y corresponding to the radial widthwise dimension of a screw and, during use in the twin-screw assembly, parallel to the interaxial plane between the axes of rotation of the two screws; and vertical direction Z-Z orthogonal to the other two directions, a screw according to the present invention has a threaded part, the threading of which is of the single-start type, with the thread raised with respect to a core, and defines three different longitudinal sectors, i.e. <NUM>, <NUM> and <NUM> respectively.

With reference to <FIG> and <FIG>, below some characteristic parameters of a screw according to the present invention are described and will be referred to in the continuation of the description.

The profile of the thread may have preferably a trapezoidal or flattened triangular form.

For the description of a twin-screw assembly according to the present invention the following further definitions are also provided:.

With reference to <FIG> and <FIG> and having also defined an upstream part M corresponding to the zone for intake of the mixture to be filtered and a downstream part V corresponding to the output zone for the filtered mixture, a twin-screw assembly according to the invention comprises substantially:.

The cylinder has a suitable internal cavity with a form suitable for housing two screws, <NUM> and <NUM> respectively, arranged with their axes of rotation parallel and intermeshing and counter-rotating during use.

The output zone <NUM> is arranged at the downstream end of the twin-screw assembly, said zone comprising a filtration zone <NUM> (<FIG>) into which the mixture is made to pass through the filter (not shown) in the feeding direction towards the discharge outlet.

For the sake of easier description, the present description will refer always to twin-screw assemblies in which the screws are in a mirror arrangement with respect to each other, although it is also possible to envisage different configurations of the two intermeshing and counter-rotating screws of the twin-screw assembly according to the invention.

The cylinder and screw assembly defines three different sectors in the longitudinal direction (<FIG>) of the twin-screw assembly corresponding to the three longitudinal sectors of the threaded parts of the screws, i.e.:.

As shown in <FIG> and as will become clearer below, along the mixture intake sector <NUM> there is a large amount of play between the screws so as to create a large "free" volume able to facilitate the entry of large volumes of mixture, along the transition sector <NUM> there is play between the screws which gradually becomes smaller, and along the high-pressure sector <NUM> there is play between the screws which is very small, i.e. less than that of the intake zone <NUM> and transition zone <NUM>, so as to create a small throughflow channel or free volume which is able to minimize the counter-flow and achieve the high pressure necessary for filtration.

The three longitudinal sectors of the screws may be defined depending on the variation of the throughflow channel which is formed in the free volume inside the cylinder, with respect to the angle of rotation.

In this context reference will be made also to the known concept of "C-chambers" (<FIG>) used to identify the channel Cx (<FIG>) with a free volume in the form of a C defined between the screws of a pair of intermeshing screws and comprised in a single rotation (in other words in a "pitch") of the thread of a single screw.

In detail and according to the present invention, the twin-screw assembly according to the present invention is characterized by a single throughflow channel which comprises from upstream to downstream at least three different sectors (<NUM>,<NUM>,<NUM>) of the screw along the longitudinal direction (X-X), which comprise:.

According to preferred embodiments of the present invention, configurations of the geometry of the three longitudinal sectors of each screw have been developed so as to create laws of variation of the throughflow channel, and therefore of the C-chambers, which are such as to ensure an optimized configuration for the specific function for which each sector is intended. In particular, the geometries are such as to obtain at the same time a maximization of the intake performance at atmospheric pressure, corresponding to the intake sector for which C-chambers with a high volume, hence low crest width W, are required, with consequent maximization of the flowrate, and a maximization of the compression of the mixture in the high pressure sector <NUM> arranged immediately upstream of the filtration zone.

In particular, in preferred embodiments of a screw for use in a twin-screw assembly according to the invention the intake sector has the following parameters:.

The preferred geometric variables of the screws in the intake sector are such that, in a single-thread screw:.

In order to generate the required pressures only in the high-pressure sector immediately close to the filter, so as to reduce the generation of heat and counter-flow losses, the C-chambers in this zone have a constant volume less than in the other zones of the screw, so that the play is mechanically as small as possible.

In detail, the high-pressure sector preferably has the following parameters:.

The geometric variables of the high-pressure zone are such that:.

Therefore, the screws of the twin-screw assembly according to the invention may advantageously be configured so as to obtain simultaneously: high pressures in the sector <NUM> close to the filter, where the play between screws and between screw and cylinder are relatively very small (in order to reduce the counter-flow opposing the main movement; and high capture and flowrate of the mixture in the intake zone, owing to the high free volumes, i.e. the space which can be potentially filled by the mixture.

In view of the different performance features required by said two sectors, i.e. the upstream/ambient-pressure intake sector and downstream/high-pressure sector, the transition sector between the intake sector, with low pressure and high volume of the C-chambers, and the high-pressure sector with low volume of the C-chambers upstream of the filtration zone, is advantageously configured with a variable C-chamber volume, in particular so as to:.

Therefore, preferably, the transition sector <NUM> has a decreasing volume of the C-chambers. In particular, the cross-section of the mixture throughflow channel preferably decreases in the direction of advancing movement of the mixture according to a law of variation which is substantially continuous, in particular at least partly approximately linear and/or quadratic and/or of an order greater than <NUM>.

Preferably, the variation of the mixture throughflow channel in the transition sector is obtained with a geometry which varies the crest width W of the screw thread, while other parameters of the screw may reman constant in the transition sector.

According to particularly preferred geometries of the screws for use in the twin-screw assembly according to the present invention, the transition sector has the following parameters:.

A gradual transition between the intake performance and the high-pressure performance is thus obtained. The variation of the throughflow channel may follow suitable laws, such as to optimize the required performance.

It is particularly preferable to limit the axial length of the threaded part of the screw so as to obtain a ratio L/D≤<NUM>, namely short length, in order to limit the undesirable temperature increases typical of long extruders, such that L/D><NUM> known in the art.

In accordance with the aforementioned laws, screws with a geometry obtained on the basis of the law of variation of the internal channel and designed to define the three said different processing zones are provided. With reference again to <FIG> and the mixture throughflow channel defined by the C-chambers of the twin-screw assembly, it emerges that, if there were no flowrate losses, the theoretical maximum flowrate Qth of a twin-screw extruder with single start thread, intermeshing and counter-rotating screws in a mirror arrangement would be equal to the volume Vc of two C-chambers multiplied by the speed of rotation N of the said screws: <MAT>.

In reality it is known, moreover, that owing to the loss of flowrate due to the play between the screws and between the screw and cylinder, and the counter-flow opposing the main movement, the theoretical maximum flowrate is never reached and that, the higher is the play, the higher are the flowrate losses which also depend on the progression of the pressure between the loading zone and the filtration zone.

With reference still to <FIG> the main flowrate losses are as follows:.

The resultant effective flowrate Q is therefore obtained from the algebraic sum of the theoretical maximum flowrate (Qth) which is directly derived from the volume of the C-chambers) and the total flowrate loss (QI) which is due to the play between the screws and between screw and cylinder and the counter-flow due to the pressure:.

According to preferred configurations of the twin-screw assembly, said assembly is configured so as to obtain a substantially constant effective flowrate Q along the longitudinal direction of advancing movement of the mixture from the input sector to the end of the high-pressure sector and, consequently, an efficiency calculated as the ratio Q/Qth which gradually increases towards the output zone.

A particular preferred example of this configuration is illustrated in <FIG> which shows the geometry of the counter-rotating screws and the law of variation of the associated C-chambers which form the mixture flow channel; the corresponding effective flowrate Q=Qth-Ql through the flow channel, which is constant in the extrusion direction, is indicated by means of a black broken line in the diagram of <FIG>.

<FIG> shows a schematic view of the cross-section S of the through-flow channel upon variation of the angle of rotation of the thread of a screw according to the present invention.

Further preferred embodiments of screws according to the present invention and corresponding twin-screw assemblies are respectively shown in <FIG>, <FIG>.

The variation in the cross-section of the mixture throughflow channel for the corresponding twin-screw assemblies is shown in <FIG>.

In greater detail, the screw of <FIG> has a cross-section of the throughflow channel which is constant (and maximum) over three <NUM>° rotations (3I) of the thread (three pitches) in the intake sector <NUM>, decreases over three rotations of the thread in the transition zone and is constant (minimal) over two pitches (2P) in the high-pressure zone.

The C-chambers and the cross-section of the mixture throughflow channel for the twin-screw assembly according to <FIG> follow a similar law of variation, as illustrated in <FIG>.

The screw of <FIG> has a cross-section of the throughflow channel which is constant (and maximum) over four <NUM>° rotations of the thread in the intake sector <NUM>, decreases over one <NUM>° rotation of the thread in the transition zone and is constant (minimal) over two pitches in the high-pressure zone. The C-chambers and the cross-section of the mixture throughflow channel for the twin-screw assembly according to <FIG> follow a similar law of variation, as illustrated in <FIG>.

The screw of <FIG> has a cross-section of the throughflow channel which is constant (and maximum) over four <NUM>° rotations of the thread in the intake sector <NUM>, decreases over two pitches (<NUM>) of the thread in the transition zone and is constant (minimal) over two pitches in the high-pressure zone. The C-chambers and the cross-section of the mixture throughflow channel for the twin-screw assembly according to <FIG> follow a similar law of variation, as illustrated in <FIG>.

The screw and the twin-screw assembly in <FIG>, <FIG>, <FIG> are similar to those in <FIG>, <FIG>, but the variation of the cross-section in the transition sector <NUM> in this case has a decreasing quadratic progression (<NUM>) The screw of <FIG> has a cross-section of the throughflow channel which is constant (and maximum) over four <NUM>° rotations of the thread (three pitches) in the intake sector <NUM>, decreases over three rotations of the thread in the transition zone and is constant (minimal) over a <NUM>° rotation of the thread in the high-pressure zone <NUM>.

Advantageously, all the preferred examples shown, which are only some of the possible geometries, are able to keep the length of the threaded part of the screw within the ten - preferably eight - thread pitches.

In preferred embodiments of the extruders according to the invention it is envisaged that the extruded mixture filtration/output section <NUM> comprises a filter-holder plate <NUM> coupled to the connection flange <NUM> and closed by a shaped head <NUM>.

A "filter" (not shown here), generally consisting of a one or more metal meshes, is placed between the connection flange <NUM> and the filter-holder plate <NUM>; the mixture is forced by the thrust produced by the rotation of the screws to flow across this filter which retains any impurities which are larger than the mesh aperture.

Preferably, one or more pressure and temperature sensors <NUM> are placed in the flange <NUM> and allow constant monitoring of the pressure and temperature of the mixture being processed, so as to obtain full control over the filtration step.

<FIG> show cross-sections through the housing of the screws <NUM> and <NUM> inside the cylinder <NUM>.

It can be seen how the play δ between the crest of each screw and the cylinder <NUM> and the play σ between the crest of one screw and the core of the other screw are very small and in any case such as to ensure simultaneously a pumping action and the absence of contact between screws and cylinder.

It is therefore clear how the screws, the twin-screw assembly according to the invention and the extruder provided with this twin-screw assembly provide a solution to the problems of the prior art, resulting in:.

Claim 1:
Screw (<NUM>;<NUM>) adapted for use in a twin-screw assembly with intermeshing screws of an elastomer mixture extruder, comprising a threaded part with a thread which defines at least three different sectors (<NUM>,<NUM>, <NUM>) 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 (<NUM>), which is adapted to capture the mixture and to push it downstream along the longitudinal direction (X-X) and which has a cross-section (S) of the throughflow channel comprised between adjacent flanks of the thread which is constant over at least two pitches or a rotation through <NUM> degrees of the thread;
- a transition sector (<NUM>), 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 the thrusting pressure acting on the mixture in transit in the longitudinal direction (X-X);
- a high-pressure sector (<NUM>), downstream of the transition sector, which has a cross-section of the throughflow channel which is minimal, constant over at least one pitch and designed to cause the compression of the mixture so as to obtain a maximum pressure of the mixture;
characterized in that the thread of said threaded part is a single-start thread and in that the geometry of the screw in the high-pressure sector (<NUM>) has a crest width W = (<NUM>-<NUM>)D, constant over one or two pitches, where D is the external diameter of the screw.