Patent Application
20230089028
The present invention relates to a machine for mixing elastomeric materials with a mixing chamber operating at ambient pressure.
It is known in the technical sector relating to the production of rubber and/or plastic-based compounds that there exists the need to perform the mixing of elastomeric materials by means of which, using a suitable process, several raw materials (ingredients) which are heterogeneous (for example, rubbers, mineral fillers, resins, various additives) and different and separate from each other are converted into a homogeneous product - the so-called “compound” - which incorporates all the base components introduced at the start of the process, being homogeneous once mixing is completed.
It is also known that the actions which generally occur during the mixing process may be summarized as follows:
Dispersion, i.e. the transformation from agglomerates of particles into aggregates; this basically consists in the reduction of the size of the fillers (for example carbon black) introduced into the polymer matrix;
Distribution/homogenization of all the primary materials.
It is also known that all these actions depend on the movement range (speed and pressure) imparted to the materials being processed by the movement of the moving surfaces (cylinders, screws, rotors) of the mixing machines. In particular, it is known that, while the dispersion depends on the characteristics of the movement range, such as the cutting force and deformation gradient, the distribution of the various ingredients in the polymer matrix depends on the efficiency of the speed range, i.e. the possibility of moving the mixture without creating stagnation points or zones of the mixture where there are pressure peaks.
One of the main problems to be dealt with during the process of mixing highly viscous materials, however, consists in the need to control the temperature of the mixture, which must be kept within certain limits to prevent the triggering of undesirable degradation or pre-crosslinking reactions.
Higher temperatures arise more significantly in those technologies which involve mixing in so-called closed chambers, since processing takes place at pressures which in turn are relatively high.
The undesirable increase in temperature during mixing also occurs during the mixing performed by means of machines of the type which are generally known as “dump extruders”, namely so-called conical, inter-penetrating, counter-rotating twin-screw extruders, in which the discharge/outlet zone of the machine for discharge/outlet of the mixture must be:
Examples of such machines are for example known from US 2007/0159916. Mixing inside closed chambers, however, results in the uncontrolled and undesirable increase in the temperature of the mixture with the consequent drawbacks mentioned above.
A further example of closed-chamber mixing is known from WO2017-093849, which describes a process for the production of an elastomeric compound comprising:
In the process according to WO2017-093849, mixing is performed almost entirely inside the part of the twin-screw mixer chamber close to the closed outlet mouth, with the conical screws which rotate in a first sense of rotation so as to push the mixture against the door which closes the outlet mouth.
The pressure inside the twin-screw conical mixer is greater than the ambient pressure since the mixer is connected sealingly with the outlet of the batch mixer inside which the pressure is high owing to the action of the piston, the outlet mouth is closed and the temperature must be kept high (135-145° C.) so that at least 50% of the quantity of silane coupling agent reacts with the reinforcing filler inside the twin-screw conical mixer.
In order to improve mixing, the document propose inverting for brief periods the direction of rotation of the conical screws, without however the mixture leaving the mixing chamber part proximal to the closed outlet mouth.
In W02017-093849, in order to keep the temperature within the desired range, the temperature is measured inside the chamber and, based on the measurement, a speed of rotation of the conical screws is adjusted in the direction of advancing movement of the mixture towards the outlet mouth and against the closing door thereof.
The technical problem which is posed, therefore, is that of providing mixing machines of the type known generally as “dump extruders” which are able to solve or at least partially overcome the said problems of the prior art, allowing the mixing of elastomeric materials without alteration of their properties or only limited alteration thereof and allowing in particular the temperature of the mixture to be kept under control, preventing an undesirable increase thereof during mixing.
In connection with this problem, it is also required that this machine should have small dimensions, be easy and inexpensive to produce and assemble and be able to be easily installed also in any user location.
These results are obtained according to the present invention by a machine for the mixing of elastomer-based materials according to the present disclosure.
The machine comprises a mixing unit and a drive unit, the mixing unit comprising:
The machine according to the present invention is characterized in that the mixing chamber has at least one opening towards the external environment adapted to keep it connected with the outside so as to ensure that its internal pressure remains at substantially atmospheric values and in that, for mixing, the rotors are made to rotate by means of the drive unit only in a first sense of rotation able to cause the mixture to be pushed towards the rear wall of the mixing chamber, so as to keep mixing active only inside the mixing chamber during the mixing step. In order to discharge the mixture during the following discharge step, the rotors may be made to rotate with a second sense of rotation, opposite to the first sense, able to cause the mixture to be pushed towards the chamber and the discharge opening.
With this configuration, the ingredients being mixed are kept substantially always inside the mixing chamber which, being open towards the surrounding environment and therefore at a substantially atmospheric pressure, does not cause undesirable increases in the pressure and/or the temperature of the mixture, avoiding damaging effects on the mixture such as alteration of the chemico-physical characteristics of the fillers and/or pre-crosslinking of the mixture itself; an optimum degree of mixing is furthermore obtained. With the machine according to the present invention it is therefore possible to obtain in a simple manner a high quality of the mixture.
Advantageously, the machine does not require doors for closing the discharge mouth, which may be kept open during mixing, making it easier to maintain the ambient pressure inside the mixing chamber and resulting in a simplification of the structure and configuration of the machine.
Preferably, the mixing chamber and the discharge chamber are frustoconical and axially connected together.
The machine preferably comprises a loading opening for loading the ingredients to be mixed, which in particular may be one of said at least one opening for connecting to the outside environment the mixing chamber.
According to a preferred embodiment, the drive unit comprises at least one motor with a shaft for moving one of the two rotors and a transmission designed to cause reversal of the sense of rotation of the drive shaft and connected to the other one of the two rotors.
The machine may advantageously comprise control means for controlling and actuating the moving parts of the machine, designed to perform automatic operation thereof. Preferably said control means are configured to send automatically to the drive unit a command for reversing the sense of rotation of the rotors when mixing has been completed, in particular after a predefined mixing time.
According to a preferred aspect, a cover is movable into a closed or open position so as to close the mixing chamber during the axial discharging of the mixture.
The present invention relates furthermore to a mixing process for mixing elastomeric materials, which comprises the steps of:
Preferably, during the mixing step, the rotation of the rotors in the first sense of rotation produces:
Loading of the ingredients preferably occurs from a loading opening, which is preferably one of said at least one opening towards the outside of the mixing chamber. The loading opening may advantageously be closed during the discharge step.
According to a preferred embodiment, the mixing step comprises the following steps:
According to a preferred embodiment, the discharge step comprises the following steps:
Further details may be obtained from the following description of a nonlimiting example of embodiment of the subject of the present invention provided with reference to the attached drawings in which:
As shown in
The mixing unit 100 comprises:
Preferably, the mixing chamber 110 has an opening 123 for loading the raw materials (ingredients) to be mixed;
The two rotors 131, 132 are counter-rotating; in the example described, one 132 of the two rotors 131,132 maintains the direction of rotation of the motor 20, while the other rotor 131 receives the movement from the transmission 22, therefore always rotating in the opposite direction to the first rotor.
It is envisaged also that the two rotors may be each operated by an associated motor, independent of the other motor, but connected by synchronization means designed to ensure the correct rotation and prevent the feeder screws from colliding.
Advantageously, the mixing chamber 110 has at least one opening 110a in the radial direction, formed in the upwards directed part of its side surface and designed to keep the mixing chamber connected to the outside and therefore the pressure inside it at substantially atmospheric values.
It is feasible that the opening 110a and the opening 123 for loading the raw materials may coincide.
The discharge chamber has, instead, a radially closed surface and only a front opening 121 for discharging in the axial direction the mixture obtained. Advantageously, the front discharge opening 121 may be always open towards the outside or downstream devices, a door for closing the discharge chamber 120 not being necessary nor useful since the mixing always and only takes place inside the upstream mixing chamber 110 under atmospheric pressure.
A further simplification and improvement compared to the known machines is therefore obtained since the absence of means for closing the discharge opening helps keeping the mixing at atmospheric pressure inside the mixing chamber, improving the quality of the mixture obtained, and eliminates the need for complicated automatic systems for opening and closing the discharge chamber.
As shown (
With reference to the embodiment of the machine shown, it is possible to control operation thereof as follows:
The movement range obtained is preferably composed of three movements, i.e.:
This movement range is that which is preferred in order to obtain mixing;
With this operating cycle, the ingredients are kept always in the mixing state inside the mixing chamber 110 which, being open towards the outside and therefore at a substantially atmospheric pressure, does not cause undesirable increases in the temperature, avoiding damaging effects on the mixture such as alteration of the chemico-physical characteristics of the fillers and/or pre-crosslinking of the said mixture.
The pushing of the mixture in the upstream direction and towards the rear wall result in an important technical effect: any mixing material (rubber or additional ingredients, in particular in the form of pellets) fed to the rear part of the mixing chamber comes into contact with the mixture and is therefore incorporated in it, therefore resulting in complete incorporation of the ingredients in the mixture and leaving the machine clean.
Although not shown, it is also envisaged being able to provide the machine with a cover which can be moved so as to open/close the mixing chamber 110, so as to keep the opening open during mixing, in order to maintain a low pressure and low temperature, and instead closed during the discharge step, so as to produce an increase in the area of contact between the mixture and the temporary mixing chamber and therefore axial thrust from upstream to downstream, in order to favour execution of the discharging action.
The following experimental tests were carried out in a machine according to the invention with a structure and configuration as described above with reference to
A rotation with a speed “v+” having a positive sign indicates a positive sense of rotation of the feeder screws, corresponding to an advancing direction of the mixture from downstream to upstream, while a negative speed “v-” indicates an opposite sense of rotation of the feeder screws and a direction of advancing movement of the mixture from upstream to downstream.
10,000 g of silicone rubber and 120 g of peroxide, a crosslinking agent in pellet form, were fed to the mixing chamber for mixing thereof.
A temperature of the rubber entering the mixing chamber (Temp-rubber In) was measured before loading, resulting in a temperature of about 25° C.
Table 1 shows the different operating steps performed by the machine at different time instants during the process.
The temperature (Temp-mixture out) of the mixture extracted from the discharge chamber was measured at different points using a thermal probe.
The temperature, Temp-mixture out, was always less than 35° C., the limit established for passing the test.
The rheometric properties was measured on 10 samples of the mixture extracted. The variation coefficient (std variation/average) for 10 samples was less than 3%.
The mixing chamber was visually inspected and it was noted that no peroxide pellets remained inside the mixing chamber, the rear part of which was clean and free from pellets.
50,000 g of silicone rubber were mixed with 500 g of blue pigment in powder form.
The temperature of the rubber, Temp-rubber, was measured at 25° C.
Table 2 shows the different operating steps performed by the machine at different time instants during the process.
The temperature (Temp-mixture out) of the mixture extracted from the discharge chamber was measured at different points using a thermal probe.
The temperature, Temp-mixture out, was always less than 35° C., the limit established for passing the test.
The homogeneity of the colour of the mixture was assessed visually. The colour was uniformly distributed without coloured zones.
It is therefore clear how with the machine according to the invention it is possible to perform processing of the mixture at a low pressure, substantially ambient pressure, and with negligible increases in the temperature, while improving the quality of the mixture obtained; in addition the possibility of controlling and determining the direction of the flow of material is able to ensure a movement range suitable for obtaining satisfactory mixing, in particular of all the material fed to the chamber.
Although described in connection with a number of embodiments and a number of preferred examples of implementation of the invention, it is understood that the scope of protection of the present patent is determined solely by the claims below.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102020000003515 | Feb 2020 | IT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2021/051374 | 2/18/2021 | WO |
