APPARATUS AND METHOD FOR MIXING ELASTOMERIC MATERIALS

Abstract

A Machine for mixing elastomeric materials with a mixing unit, and a drive unit; the mixing unit has a mixing chamber arranged downstream of the drive unit and closed by a rear wall, a discharge chamber arranged downstream of the mixing chamber, with which it communicates and provided with an opening for discharging the mixture; a pair of inter-penetrating and counter-rotating conical rotors connected with the drive unit and having their vertices situated at the mouth of the discharge chamber. The rotors are rotated by the drive unit in a first sense (RPM+) 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, and in second sense of rotation, opposite to the first sense, to cause the mixture to be pushed towards the chamber and the discharge opening for discharging thereof.

Claims

1-15. (canceled)

16. A machine for mixing elastomeric materials, comprising a drive unit (20) a mixing unit (100) for mixing a mixture, comprising: a mixing chamber (110) arranged downstream of the drive unit (20) and closed towards upstream by a rear wall (125); a discharge chamber (120) arranged downstream of the mixing chamber (110), with which it communicates towards upstream, and provided with a discharge opening (121) for discharging the mixture; a pair of inter-penetrating and counter-rotating conical rotors (131,132), each rotor comprising a respective feeder screw (131a, 132a) mirror-inverted with respect to the other feeder screw; Wherein the conical rotors are respectively connected upstream to the drive unit (20), whereby the drive unit rotationally drives the conical rotors; Wherein the conical rotors have their vertices situated at the discharge opening (121) of the discharge chamber (120); wherein the mixing chamber (110) has at least one opening (110a) towards the exterior, designed to keep it connected with the external environment so as to ensure that its internal pressure remains at substantially atmospheric values, and wherein machine is configured so that: during a step for mixing a mixture of the elastomeric materials the rotors are rotated by means of the drive unit (20) only in a first sense of rotation (RPM+) such as to cause the mixture to be pushed towards the rear wall (125) of the mixing chamber, so as to keep mixing active only inside the mixing chamber at a substantially atmospheric pressure during the mixing step, and, during a subsequent mixture discharge step, the rotors are rotated only in a second sense of rotation, opposite to the first sense of rotation, such as to cause the mixture to be pushed towards the discharge chamber (120) and the discharge opening (121).

17. The machine according to claim 16, wherein the mixing chamber (110) and the discharge chamber (120) are frustoconical and axially connected together.

18. The machine according to claim 16, comprising a loading opening (123) for loading an ingredients to be mixed.

19. The machine according to claim 18, wherein said loading opening (123) is one of said at least one opening (110a) towards the exterior of the mixing chamber.

20. The machine according to claim 16, wherein said drive unit (20) comprises at least one motor (21) with a drive shaft (21a) arranged for rotationally moving one (131;132) of the two conical rotors and a transmission (22) designed to reverse the direction of rotation of the drive shaft (21a) and connected to the other conical rotor.

21. The machine according to claim 16, comprising control means (500) for controlling and actuating the moving parts of the machine, designed to perform automatic operation thereof.

22. The machine according to claim 21, wherein the control means (500) are configured to automatically send to the drive unit (20) a reversal command for reversing the sense of rotation of the conical rotors (132;132) upon completion of the mixing step.

23. The machine according to claim 16, comprising at least one cover movable into a closed or open position so as to close the mixing chamber (110) during axial discharging of the mixture.

24. The machine according claim 16, wherein the discharge opening (121) for discharging the mixture is always open towards the external environment, there being no means for closing the discharge opening.

25. A process for mixing elastomeric materials by means of a machine, wherein the machine comprises: a drive unit (20) a mixing unit (100) for mixing a mixture, comprising: a mixing chamber (110) arranged downstream of the drive unit (20) and closed towards upstream by a rear wall (125); a discharge chamber (120) arranged downstream of the mixing chamber (110), with which it communicates towards upstream, and provided with a discharge opening (121) for discharging the mixture; a pair of inter-penetrating and counter-rotating conical rotors (131,132), each rotor comprising a respective feeder screw (131a, 132a) mirror-inverted with respect to the other feeder screw; Wherein the conical rotors are respectively connected upstream to the drive unit (20), whereby the drive unit rotationally drives the conical rotors; Wherein the conical rotors have their vertices situated at the discharge opening (121) of the discharge chamber (120); wherein the mixing chamber (110) has at least one opening (110a) towards the exterior, designed to keep it connected with the external environment so as to ensure that its internal pressure remains at substantially atmospheric values, the method comprising the steps of: loading ingredients to be mixed into the mixing chamber; mixing the ingredients by the feeder screws (131a, 132a) to obtain a mixture, with rotation of the conical rotors (131,132) only in a first sense of rotation (RPM+) which causes the mixture to be pushed towards the rear wall (125) so as to keep mixing active only inside the mixing chamber at substantially atmospheric pressure; discharging the mixture, comprising reversal of the sense of rotation of the conical rotors which are made to rotate with a second sense of rotation (RPM-), opposite to the first sense of rotation, for a time period (t3-t2) such as to cause the mixture to be pushed towards the discharge chamber (120) and discharged through the discharge opening (121).

26. The process according to claim 25, characterized in that, during the mixing step, the rotation of the rotors in the first sense of rotation (RPM+) causes: a reaction on the rear wall (125) such as to impart a component of the movement of the mixture in the axial direction from upstream (P) to downstream (A); formation of a movement range designed to cause three separate movements of the mixture, respectively a circumferential movement (I), a main axial flow (II) and a secondary axial flow (III).

27. The process according to claim 25, wherein loading of the ingredients is performed via a loading opening (123;110a).

28. The process according to claim 27, wherein the loading opening is closed during the discharging step.

29. The process according to claim 25, wherein the mixing step comprises: starting rotation of the rotors (131,132) in a first sense of rotation (RPM+), take-up of the ingredients by the feeder screws (131a,132a) and starting of the mixing operation with pushing of the mixture towards the rear wall (125) situated upstream (P); maintaining the first sense of rotation (RPM+) for a mixing time period (t1) until completion of the mixing operation; and/or in that the discharging step comprises the steps of: sending, manually or by control means (500), a reversal command for reversing the sense of rotation (RPM+) of the two conical rotors (131,132); reversal of the sense of rotation of the conical rotors which are made to rotate with a second negative sense of rotation (RPM-) for a time period (t3-t2) such as to cause the mixture to be pushed towards the discharge chamber (120) and discharged through the discharge opening (121).

30. The process according to claim 29, characterized in that, during the mixing step, the opening (121) for discharging the mixture is always open towards the external environment.

31. The process according to claim 22 wherein the control means (500) are configured to automatically send to the drive unit (20) the reversal command after a predefined mixing time (t1).

32. Process according to claim 27, where the loading opening (123;110a) is one of said at least one opening (110) towards the exterior of the mixing chamber.

Description

[0053] 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:

[0054] FIG. 1: shows a side view of the machine according to the present invention;

[0055] FIG. 2: shows a view from above of the machine according to FIG. 1;

[0056] FIG. 3: shows a partially sectioned view from above of a machine according to FIG. 1 during mixing with an axial movement of the material towards the rear part;

[0057] FIG. 4: shows a partially sectioned view from above of the machine according to FIG. 3 during discharging with pushing and axial movement of the material towards the front part; and

[0058] FIG. 5: is a diagram illustrating the various operating steps of the machine according to the invention.

[0059] As shown in FIG. 1 and assuming solely for easier description and without a limiting meaning a reference axis with a longitudinal direction X-X corresponding to the lengthwise extension of the machine; as well as a front part A or downstream part, corresponding to the part where the mixture exits and a rear part P, or upstream part, opposite to the front part, an example of the machine according to the invention which, in its general configuration, falls within the general category of “dump extruders” comprises essentially: [0060] a support base 10 for the functional units; [0061] a mixing unit 100; [0062] a drive unit 20, comprising at least one motor 21, with its shaft 21a connected to a transmission 22 designed to reverse the sense of rotation of the drive shaft 21a as will emerge more clearly below.

[0063] The mixing unit 100 comprises: [0064] a mixing chamber 110, preferably frustoconical, arranged downstream of the drive unit 20; the mixing chamber comprises an upstream wall 125 which axially closes the chamber towards the rear part P; [0065] a discharge chamber 120 for discharging the mixture, which is in turn preferably frustoconical, arranged downstream of the mixing chamber 110 and provided with an opening 121 for discharging the mixture in the axial direction, arranged in the front part “A” of the machine and with the upstream part mechanically connected to the mixing chamber with which it communicates in the axial direction by means of a corresponding opening 122.

[0066] Preferably, the mixing chamber 110 has an opening 123 for loading the raw materials (ingredients) to be mixed; [0067] a pair of inter-penetrating conical rotors 131, 132, which are respectively connected upstream to the drive unit 20 and have their vertices at the mouth 121 of the discharge chamber 120; each rotor comprises a respective feeder screw 131a,132a mirror-inverted (with an opposite winding sense) with respect to the other one.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] It is feasible that the opening 110a and the opening 123 for loading the raw materials may coincide.

[0072] 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.

[0073] 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.

[0074] As shown (FIGS. 3,4) the rotors 131,132 have a feeder screw 131a,132a which respectively extend so as to cause, with a respective first sense of rotation (FIG. 3), a movement of the mixture from downstream to upstream and, with a respective reversed sense of rotation (FIG. 4), a movement of the mixture in the downstream direction towards the discharge opening 121. Control means 500 for controlling and actuating the moving parts of the machine may also be provided, said means being designed to ensure automatic operation of the machine.

[0075] With reference to the embodiment of the machine shown, it is possible to control operation thereof as follows: [0076] rotors 131,132 configured with feeder screws 131a,132a such that, when they are rotated in the senses as shown in FIG. 3 - conventionally in a positive sense RPM+ - they are able to cause a movement of the mixture in the axial direction from downstream “A” to upstream “P”; [0077] loading of the ingredients through the feeder mouth 123; [0078] starting rotation of the rotors in a first positive sense RPM+, maintained for a time period t1 (FIG. 5); [0079] the ingredients fed are taken up by the feeder screws 131a,132a of the rotors and, as a result of rotation thereof, start mixing, pushing the mixture towards the upstream wall 125; [0080] pushing against the wall 125 causes a reaction, a so-called backflow, which tends to impart a movement component in the axial direction from upstream to downstream namely in the opposite direction to the preferred direction from downstream to upstream.

[0081] The movement range obtained is preferably composed of three movements, i.e.: [0082] 1st movement: circumferential, generated by the rotation of the rotors; [0083] 2nd movement: main axial flow generated by the form of the feeder screws; [0084] 3rd movement: secondary axial flow or 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.

[0085] This movement range is that which is preferred in order to obtain mixing; [0086] once a satisfactory degree of mixing has been obtained: [0087] manual or automatic reversal by the control unit 500 of the sense of rotation of the two rotors 131,132; [0088] reversal of the sense of rotation of the rotors for a time period t3-t2 (FIG. 5); [0089] the reverse sense of rotation, conventionally negative sense RPM-, causes the mixture to be pushed towards the discharge chamber 120 from where it exits through the discharge mouth 121; [0090] once the discharge chamber 120 has been emptied the control unit emits a new signal for reversal of the sense of rotation of the rotors so that the machine is set to feed a new batch of ingredients and start a new mixing cycle.

[0091] 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.

[0092] 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.

[0093] 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.

EXPERIMENTAL TESTS

[0094] 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 FIGS. 1-4.

[0095] 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.

Test 1

[0096] 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.

[0097] A temperature of the rubber entering the mixing chamber (Temp-rubber In) was measured before loading, resulting in a temperature of about 25° C.

[0098] Table 1 shows the different operating steps performed by the machine at different time instants during the process.

TABLE-US-00001 Time Operating state T = 0 Rotors started with positive sense of rotation Loading of 10,000 g of silicone rubber; Loading of 120 g of peroxide; Mixing maintained with v+=10 RPM for 5' v+=+10 RPM T= 5' Reduction of speed of rotation v+= 5 RPM T= 15' Switching to discharge mode; Reversal of sense of rotation of feeder screws v-=10 RPM T= 17' End of discharging of mixture through discharge opening

Results

[0099] The temperature (Temp-mixture out) of the mixture extracted from the discharge chamber was measured at different points using a thermal probe.

[0100] The temperature, Temp-mixture out, was always less than 35° C., the limit established for passing the test.

[0101] 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%.

[0102] 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.

Test 2 - Colouring of Silicone Rubber with Pigment

[0103] 50,000 g of silicone rubber were mixed with 500 g of blue pigment in powder form.

[0104] The temperature of the rubber, Temp-rubber, was measured at 25° C.

[0105] Table 2 shows the different operating steps performed by the machine at different time instants during the process.

TABLE-US-00002 Time Operating state T = 0 Rotors started with positive sense of rotation Loading of 50,000 g of silicone rubber v+=+10 RPM T= 30ʺ Loading of 500 g of blue pigment in powder form Speed of rotation maintained v+=10 RPM T=10ʹ30ʺ Switching to discharge mode; Reversal of sense of rotation of feeder screws v-=10 RPM T=12ʹ30ʺ End of discharging of mixture through discharge opening

Results

[0106] The temperature (Temp-mixture out) of the mixture extracted from the discharge chamber was measured at different points using a thermal probe.

[0107] The temperature, Temp-mixture out, was always less than 35° C., the limit established for passing the test.

[0108] The homogeneity of the colour of the mixture was assessed visually. The colour was uniformly distributed without coloured zones.

[0109] 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.

[0110] 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.