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.
It is also known that the actions which generally occur during the mixing process may be summarized as follows:
Incorporation of the ingredients in the polymer matrix;
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 WO2017-093849, 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 allow mixing of elastomeric materials without alteration of their properties or only limited alteration thereof, 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 herein disclosed subject matter.
Therefore, a machine for mixing elastomeric materials according to the invention 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 outside adapted to keep it connected with the external environment so as to ensure that its internal pressure remains at substantially atmospheric values and in that it comprises detection means arranged and configured to detect the presence of mixture inside the discharge chamber and to emit at least one signal for indicating the presence or absence of the mixture inside the discharge chamber; based on the signal indicating the presence or absence of mixture, the machine is configured to perform, during a mixing step, at least one reversal by the drive unit of the sense of rotation of the rotors and therefore the sense of advancing movement of the mixture in the axial direction from/towards the mixing chamber and towards/from the discharge chamber, so as to keep mixing active only inside the mixing chamber, the internal pressure of which is at substantially atmospheric values.
With this configuration, the ingredients being mixed are kept substantially always inside the mixing chamber which, being open towards the outside 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 said mixture; 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.
Furthermore, advantageously, the machine does not require doors for closing the discharge mouth and against which pushing the mixture, so that the discharge mouth may be kept open during mixing, making it easier to maintain the ambient pressure inside the mixing chamber and therefore the quality of the mixture, resulting at the same time in simplification of the structure and configuration of the machine.
Preferably, the detection means are arranged inside the discharge chamber in the proximity of the zone for connection to the mixing chamber.
The mixing chamber and the discharge chamber are preferably frustoconical and axially connected together.
According to a preferred embodiment, a loading opening for loading the ingredients to be mixed is provided, said opening being preferably one of said at least one connection opening for connecting the mixing chamber to the outside environment.
The drive unit may comprise at least one motor with a shaft for moving one of the two rotors and a transmission designed to reverse the sense of rotation of the drive shaft and connected to the other one of the two rotors.
The machine preferably comprises control means for controlling and actuating the moving parts of the machine, designed to perform automatic operation thereof; said control means may in particular be configured to send automatically to the drive unit a command for reversing the sense of rotation of the rotors in response to a signal indicating the presence or absence of mixture inside the discharge chamber, emitted by the detection means during the mixing step. Said reversal command is preferably sent immediately after the emission of a signal indicating the presence (or eventually the absence) of mixture inside the discharge chamber and/or after a predefined time interval Δt following the emission of a signal indicating the absence (or eventually the presence) of mixture inside the discharge chamber.
The control means may also be configured to switch an operating mode of the machine from a mixing mode to a discharge mode, where a sense of rotation of the rotors is maintained so as to cause the movement of the mixture from the mixing chamber towards the discharge chamber and the axial discharging thereof through the discharge mouth.
Preferably, 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 disclosure relates furthermore to a mixing process for mixing elastomeric materials by means of a machine comprising the steps of:
Advantageously, during the mixing step, the mixing chamber is kept connected to the external environment by means of at least one opening towards the outside so as to ensure that its internal pressure is kept at substantially atmospheric values, and mixing of the mixture is kept active only inside the mixing chamber, by performing at least one reversal by the drive unit of the sense of rotation of the rotors and therefore of the sense of advancing movement in the axial direction of the mixture from/towards the mixing chamber and towards/from the discharge chamber, in response to a signal indicating the presence or absence of the mixture inside the discharge chamber, emitted by the detection means arranged and configured to detect the presence of mixture inside the discharge chamber.
Preferably, the mixing step comprises the steps of:
Preferably, the mixture discharging step comprises the steps of:
According to a preferred embodiment, during the mixing step, control and actuating means send automatically to the drive unit a command for reversing the sense of rotation of the rotors in response to a signal, emitted by the detection means, for indicating the presence or absence of mixture inside the discharge chamber; and/or switch an operating mode of the machine from a mixing mode to a discharge mode so as to perform said discharge step, preferably after a predetermined mixing time.
The reversal of the sense of rotation of the rotors may be started after a predefined time interval Δt following the emission of the signal indicating the absence or presence of mixture inside the discharge chamber.
Further details may be obtained from the following description of a non-limiting 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 (
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, rotating in the opposite direction to the first rotor; the two rotors 131, 132 are therefore always counter-rotating.
Conventionally a positive sense of rotation RPM+ of the rotors is assumed, such as to cause an advancing movement of the mixture from upstream P to downstream A (
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 (
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 obtained since the absence of means for closing the discharge opening helps keep 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.
According to the invention it is envisaged that means 300 for detecting the presence of mixture inside the said chamber 120 are arranged at the inlet of the discharge chamber 120, namely in the zone axially close to the mixing chamber 110; preferably said means are realized by means of a pressure sensor 310, for example connected to display means (not shown) for displaying the current value of the pressure detected.
A pressure value threshold found to be suitable for the emission of the signal indicating the presence of material inside the discharge chamber may be for example between 1 and 5 bar.
It will be clear that the person skilled in the art may select alternative and suitable detection means (for example optical means) suitable for detecting the presence of mixture inside the discharge chamber.
Preferably, the machine is provided with means 500 for controlling and actuating the moving parts of the machine, which are designed to perform the automatic operation thereof; the means 300 for detecting the presence of the mixture inside the discharge chamber in particular are connected to the control and actuating means 500. This configuration is preferred, but it will be clear to persons skilled in the art that operation of the machine described below may also be manually controlled in response to the signals emitted by the detection means.
According to the invention, the means 300 for detecting the presence of mixture inside the discharge chamber are designed, when mixture is present inside the discharge chamber 120, to emit a first logic signal 310a, for indicating the presence of material inside the discharge chamber, designed to cause the reversal of the sense of rotation of the two rotors 131,132, said reversal being able to be performed by means of the control unit 500 or manually by an operator.
The same detection means 300 are preferably designed to emit also a second logic signal 310b when the discharge chamber is emptied by the action of the rotors following the prior reversal of movement from RPM+ to RPM−.
With reference to
The reversals of the sense of rotation of the rotors are such as to keep the discharge chamber substantially empty of mixture and impart to the materials being processed the range of movement necessary for the incorporation and distribution/homogenization actions, needed to produce the desired mixture.
Once the mixture is obtained (tn), the sensor 300 for detecting the presence of the mixture inside the discharge chamber 120 is deactivated, interrupting the cycle for reversal of the rotations and keeping them constant, in the positive sense RPM+, such as to cause advancing in the axial direction of the mixture from the mixing chamber 110 towards the discharge chamber 120 inside which the mixture is subject to the thrust of the rotors until it exits completely in the axial direction from the mouth 121 and with the end of the cycle (Fc). It is clear that the signal emitted by the sensor may alternatively simply be ignored or disabled during the discharge step.
The switching to discharge mode is preferably performed automatically by the control unit 500, for example after a predefined mixing time, but may also be performed manually for example following an evaluation by the operator of the mixing state of the mixture.
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.
In the aforementioned working conditions, it has also been established in tests that an optimum degree of mixing is obtained, while ensuring the high quality of the mixture.
Preferably, one or more of the renewed reversals of the sense of rotation is started after a predefined time interval Δt following the emission of a signal by the sensor 310, in particular following a signal 310b indicating the absence of mixture. The time interval Δt≥0 may be chosen depending on the type of mixture being processed, a value Δt≥0, and in particular a value greater than time necessary for the mixture to reach the rear wall 125, being in particular recommended.
For the mixing of some types of mixture it is in fact advantageous to have prolonged pushing (Δt>>0) of the mixture against the rear wall 125, which results in a so-called “backflow” reaction, which tends to impart a component of the movement in the axial direction from upstream to downstream, opposite to the direction of movement from downstream to upstream, determined by the rotation of the feeder screws.
The movement range of the mixture thus obtained is therefore composed of three movements, i.e.:
1st movement: circumferential, generated by the rotation of the rotors;
2nd movement: main axial flow generated by the form of the feeder screws;
3rd movement: backflow, generated by the resistance of the wall 125 which, opposing the main axial flow, tends to cause the mixture being formed to flow back in the downstream direction.
The pushing of the mixture in the upstream direction and towards the rear wall in any case results in an important technical effect: any mixing material (rubber or additional ingredients, in particular in the form of pellets) left inside the rear part of the mixing chamber during rotation of the feeder screws with pushing of the mixture in the downstream direction 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 top externally directed opening 110a,123 of 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 internal pressure and the axial thrust from upstream to downstream, in order to favour execution of the discharging action.
The present invention relates furthermore to a process for mixing rubber and/or plastic-based mixtures by means of a mixing machine, the process comprising the following steps:
a) providing a mixing machine comprising at least one mixing chamber and a discharge chamber communicating with each other in an axial direction of movement of the mixture from the mixing chamber to the discharge chamber, and
b) a pair of inter-penetrating and counter-rotating conical rollers 131,132 extending inside said chambers;
c) feeding to the machine ingredients to be mixed in order to obtain a mixture;
d) starting rotation of the rotors with a respective rotation sense RPM+ so as to cause a movement of the mixture in the axial direction from upstream P to downstream A namely towards the chamber for discharging the mixture;
e) detection by sensor means inserted inside the discharge chamber of the presence of material inside the said discharge chamber;
f) sending of a corresponding first logic signal 310a to a control unit 500;
g) reversal, by the control unit 500, of the sense of rotation RPM− of the two rotors 131,132;
h) reversal of the movement of the mixture which moves from downstream A to upstream P;
i) emptying of the discharge chamber 120;
j) emission, by the detection sensor 310, of a second different logic signal 310b indicating the absence of mixture and discharge chamber 120 empty;
k) reversal of the sense RPM+ of rotation of the rotors with renewed reversal of the movement of the mixture from upstream P to downstream A in the direction of the discharge chamber 120;
l) repetition of steps a) to k) until mixing has been completed;
m) optionally, deactivation of the sensor for detecting the presence of material inside the discharge chamber;
n) maintaining the sense RPM+ of rotation of the rotors able to cause the movement of the mixture towards the discharge chamber with a pushing force so as to favour the discharging of the mixture;
o) discharging of the mixture through the discharge mouth of the machine;
p) restoration of the initial conditions for a new cycle for mixing of a new batch of ingredients.
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 upstream to downstream, 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 downstream to upstream.
40 kg of silicone rubber and 480 g of peroxide 1.2 phr, 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. The machine was configured with the following parameters: Threshold Pcs=5 bar, Δt=180″ (seconds)
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.
50 kg of silicone rubber 100 phr were mixed with 500 g of blue pigment 1 phr.
The temperature of the rubber, Temp-rubber, was measured at 25° C. The machine was configured with the Threshold Pcs=2 bar, Δt=60″
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 and the process according to the invention it is possible to perform processing of the mixture at a low pressure, substantially ambient pressure, and at very low temperatures and/or without an undesirable increase in the temperature of the mixture, 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 and/or complete mixing of all the ingredients.
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 |
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102020000003461 | Feb 2020 | IT | national |
Filing Document | Filing Date | Country | Kind |
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PCT/IB2021/051372 | 2/18/2021 | WO |