GB2173414A - Mixer for rubber or plastics material - Google Patents

Abstract

The mixer comprises a mixing chamber (12) consisting of two intersecting parallel cylindrical portions (14, 16) in which two rotors (18, 20) are mounted for rotation about an axis A coaxial with the cylindrical portion (14, 16) in which the rotor is mounted. The rotors (18, 20) rotate in opposite directions in synchronism, at the same speed. Each rotor (18, 20) is of substantially circular cross-section at any section along its length, with its centre of cross-section C eccentric relative to the axis A of rotation, and may be cylindrical or helical (see Fig. 2) in configuration. The construction and arrangement of the rotors (10, 20) is such that the gap between them fluctuates to draw in material. The dimensions of the mixer are selected to obtain satisfactory performance: preferably the gap (X) between the rotors (18, 20) is at least twice the minimum distance (Y) between each rotor (18, 20) and the adjacent surface of the chamber 12. This construction may be applied to either batch mixers or continuous mixers and affords a more efficient mixing action than many heretofore known mixers. <IMAGE>

Description

SPECIFICATION Mixers This invention is concerned with mixers for rubber or plastics material and is especially concerned with mixers suitable for use in mixing synthetic rubber.

Various mixers, both batch mixers and continuous mixers, have been proposed for mixing rubber or plastics materials. Many of the mixers heretofore used, for example "Banbury" mixers manufactured by the applicant company, have rapidly produced well mixed batches of materials. However, although the mixing achieved has been good, it has not usually been perfect and generally a very small, unmixed residue remains which can spoil the mixture. This residue of unmixed material is that which has missed the "intensive mixing. portion" of the relevant mixing machine. Subsequent treatment can up-grade mixtures but breaking down of agglomerates is difficult and, in any event, furthei processing adds to the cost of producing the final end product.

Amongst mixing machines for rubber and plastics material, the best results are often obtained using a so-called "two roll mill" which merely comprises two large diameter heavy rolls into the nip between which the materials to be mixed are fed and dragged through the nip by rotation of the rolls, the material being subjected to mixing in the gap between the rolls. However, this type of machine has a number of disadvantages firstly, some of the materials initially introduced into the nip between the two rolls fall through the nip and require intervention on the part of an operator to ensure that all the materials intended to be mixed are in fact present. Secondly, it is difficult to ensure good cross-blending of the batch in a two roll mill without considerable involvement of the operator.Thirdly bale stock will not pass through the nip easily and often requires to be cut into pieces by an operator, again a time consuming operation. Finally, and very importantly, only small percentages of the metal surfaces of the two rolls are used to effect intensive. mixing-thus the mixing rate is low and, in order to process a significant quantity of material large, space-consuming, machines are required.

One of the various objects of the present invention is to provide an improved mixer for rubber and plastics materials.

There is hereinafter described, to illustrate the invention by way of example, a mixer for rubber and plastics materials, especially suitable for synthetic rubber, comprising a -housing in which is formed a mixing chamber having a generally figure "8"-shaped cross-section which comprises two intersecting, paral lel, cylindrical portions. Two rotors are mounted, one in each of the cylindrical portions, for rotation about an axis coaxial with the axis of the cylindrical portion in which it is mounted; the rotors are mounted for rotation in opposite directions at the same speed and in synchronism.Each rotor is of substantially circular cross-section and is mounted so that the centre of cross-section of each rotor is eccentric in relation to its axis of rotation: in other words the centre of gravity of a crosssection of the rotor at right angles to its axis of rotation at any point along the rotor is offset from the axis of rotation. Thus each of the rotors rotates eccentrically about its axis of rotation and the mixer is constructed so that the regions of the mixing chamber swept by each rotor intersect in a central region of the rotor. In the illustrative mixer the construction and arrangement of the rotors at any cross-section perpendicular to the axes of rotation is such that the gap between the rotors fluctuates between a maximum and minimum as the rotors rotate, along the whole length of the rotors.The rotors are so dimensioned that the part of the rotor surface which is at the maximum distance from the axis of rotation is close to the cylindrical wall of the cylindrical portion of the mixing chamber in which that rotor rotates: the actual spacing between this part of the rotor surface and the cylindrical wall is chosen according to the dimensions of the mixing chamber and rotor together with the use to which the mixer is to be put. Preferably the gap between the rotors is at least twice and preferably about three times, the minimum distance between the surface of each rotor and an adjacent cylindrical surface of the housing defining the cylindrical portion of the mixing chamber in which the rotor rotates.

The illustrative mixer described hereinafter in detail by way of example is a batch mixer having an inlet opening to the chamber in an upper part of the housing and an outlet opening in the lower part of the housing, both centrally disposed between the two cylindrical portions, the periphery of the rotors being arranged to travel downwardly in the central region of the chamber. The illustrative mixer comprises a door, pivotally mounted on the housing, for movement into and out of the outlet opening whereby to close the outlet opening for mixing and open the outlet opening for discharge of material from the chamber. The illustrative mixer likewise comprises a closure member, namely a ram, for closing the inlet opening and urging material to be mixed into the mixing chamber: other forms of closure member may be used if desired.

Whereas the illustrative mixer described hereinafter is a batch-type mixer, a mixer according to the invention may be a continuous mixer which preferably likewise has an inlet opening in an upper part of the housing and an outlet opening in a lower part of the housing, centrally disposed between the two cylin drical portions and with the periphery of the rotors being arranged to travel downwardly in the central region of the chamber. However, whereas, in the batch-type mixer, the inlet opening and the outlet opening extend substantially along the entire length of the mixing chamber, in an otherwise similar continuous mixer, the inlet and outlet openings preferably extend only along part of the mixing chamber, the inlet opening being disposed at one end of the mixing chamber and the outlet opening being at the other end.

The cross-section of the rotor of the illustrative machine is substantially circular at any point along the length of the rotor. However, the rotors may be of helical configuration, either double helical or single helical, or, of course, of cylindrical configuration. Single helical or cylindrical rotors are preferred for continuous mixers; double helical rotors may be preferable for batch mixers. Where the rotor comprises a single helix each rotor preferably undergoes a twist of between 1200 and 1800, suitably about 1500 about the axis of rotation.

Whereas the illustrative mixer comprises two rotors mounted for rotation about parallel axes which lie in a horizontal plane, the axes of rotation of the rotors may lie in any convenient plane, for example in a vertical plane with each axis horizontal. Preferably the inlet opening is at the opposite side of the plane in which the axes of rotation lie to the outlet opening, both inlet and outlet opening being generally centrally disposed between the axes of rotation and the rotors being arranged to rotate so that the rotor surfaces move in this central region from the inlet opening towards the outlet opening.

From one aspect the invention is directed to a mixer comprising a housing having a mixing chamber comprising two intersecting, parallel cylindrical portions, two rotors each mounted for rotation about an axis coaxial with the cylindrical portion in which it is mounted, the rotors being mounted for rotation in opposite directions in synchronism at the same speed and each rotor being of substantially circular cross-section, the centre of cross-section of each rotor eccentric in relation to its axis of rotation.

The above and other of the various objects and several features of the present invention will become more clear from the following description, to be read with reference to the accompanying drawings, of a mixer for rubber and plastics material embodying the invention.

It will be realised that this mixer has been selected for description to illustrate the invention by way of example and that the invention may reside in any novel features taken singularly or in combination.

In the accompanying drawings: Figure 1 is a view in cross-section at right angles to axes of rotation of the illustrative mixer; Figure 2 is a plan view showing rotors of the illustrative mixer; and Figures 3a to f are views of the mixing chamber in section on the same line as figure 1, showing rotors of the mixer at various points in their rotation The illustrative mixer is generally similar in construction and operation to a "Banbury" mixer supplied by the applicant, except for the configuration and operation of the rotors which, in the illustrative mixer intermesh but which do not intermesh in a "Banbury" mixer.

The illustrative mixer ccmprises a housing 10 in which is formed a mixing chamber 12 of generally figure "8' '-shaped cross-section comprising two intersecting parallel, cylindrical portions 14, 16. Two rotors 18,. 20 are mounted for rotation about parallel axes A which lie in a horizontal plane; the rotor 18 is mounted in the cylindrical portion 14 with its axis of rotation coaxial with the axis of the portion 14 and the rotor 20 is mounted in the cylindrical portion 16 with its axis coaxial with the cylindrical portion 16.

The rotors are rotated in opposite directions (as indicated by the arrows B) at the same speed and in synchronism. The rotors are driven by drive means (not shown) of known construction, including a motor driving a gear box by which the rotors are driven, the gears intermeshing so that the rotors 18, 20 rotate in synchronism. The speed of rotation of the rotors can be varied in known way to control the mixing action.

Each of the rotors 18, 20 is of substantially circular cross-section, the centre of cross-section C 6f each rotor being eccentric in relation to the axis A of rotation, that is, the centre C of each circular. or substantially circular crosssection of the rotor is off-set from the axis of rotation of the rotor. Whilst, as discussed previously, the rotors may have a variety of forms, the rotors 18, 20 of the illustrative mixer have portions F at one end portion of each rotor which undergo about 1500 of twist about the axis of rotation merging into portions G (at the other end of the rotor) which are cylindrical. Both rotors 18, 20, of course, having a substantially circular section at any cross-section transverse to the axis of rotation A with the centre C of that cross-section offset relative to the axis A. The rotors 18, 20 and housing 10 may be provided with passages for heat exchange fluid to heat or cool the rotors 18, 20 and housing 10 in known manner. The construction and arrangement of the rotors 18, 20 at any cross-section perpendicular to the axes A of rotation and the gearing of the rotors so that they rotate in synchronism is arranged to be such that the gap X between the rotors fluctuates between a maximum and minimum, the maximum gap occurring when the centre C of the cross-section of one rotor is vertically above its axis A of rotation and the centre C of the other rotor at that cross-section is vertically beneath its axis of rotation, and the minimum gap occurring when the centres C of cross-section (at the same cross-section) are in the same horizontal plane as the axes A of rotation.The gap between the rotors depends, of course, on the spacing (d) between the axes, the radius (r) of the circular section and the eccentricity (e) of the circular section. The minimum gap is equal to d-2r and the maximum gap can be calculated using Pythagorus theorem to be equal to (d2+4e2) -2r. Thus, the greater the eccentricity, the greater the fluctuation in the gap. As each rotor rotates the part of the rotor surface most distant from the axis A (the crest of the rotor) is closes to the cylindrical surface of the housing defining the cylindrical portions 14, 16 and is spaced from this surface by a distance Y. Substantially diametrically opposite this part of the rotor is a part of the rotor surface which is closest to the axis A and the distance between this part of the surface of the rotor and the adjacent surface of the housing 14, 16 is a maximum.

The gap X is preferably at least twice the distance Y, conveniently about three times the distance Y.

As the rotors 18, 20 rotate the gap X opens and closes (as previously discussed).

This action helps to pull material down between the rotors into the space under the rotors. The action of the rotors 18, 20 alternately lifts up the material as the crests of the rotors sweep close to the cylindrical surface providing the wall of the mixing chamber 12.

It is important that the rotors have a capacity to pull more material down between the rotors 18, 20 than they can lift up, otherwise most material will remain above the rotors and not mixed efficiently. Of course, the actual amount of material pulled down between the rotors, when a steady state.has been reached will be equal to the amount pulled up by the rotors but a major part of the material will normally be below the rotors and subjected to a certain amount of pressure by the rotors.

The illustrative mixer has an inlet opening 22 to the mixing chamber 12 extending along the whole length of the mixing chamber, the inlet opening 22 being formed in an upper part of the housing 10 centrally disposed between the two cylindrical portions 14, 16, of the mixing chamber. A feed assembly of the same general construction as commonly usedin heretofore known mixers is arranged to feed material to be mixed through the inlet opening 22 into the chamber 12. The feed assembly comprises a hopper 24, a throat portion 26 of which leads to the inlet opening 22. A closure member, provided by a ram 28 (sometimes referred to in known internal mixers as a "weight") is arranged to close the inlet opening 22.The ram 28 is mounted at a lower end portion of a piston rod 30 of a piston and cylinder arrangement, the cylinder 32 of which is mounted on an upper portion of the hopper 24 above the inlet opening 22.

The ram 28 slides in the opening 22 and throat portion 26 of the hopper 24 and prevents escape of material through the opening 22, during mixing, from the mixing chamber 12. The ram 28 may be raised to uppermost position by admission of pressurised fluid, namely air, to the cylinder; whilst held in this raised position-, materials to be mixed may be introduced through the hopper 24 and inlet opening 22 into the mixing chamber 12. Some of the materials to be mixed may be in the form of fairly large pieces of rubber stock and may not merely fall into the mixing chamber but can be pushed into the mixing chamber by the ram 28 (or such other means as may be provided in mixers generally similar to the illustrative mixer.When the materials to be mixed have been introduced the ram is lowered by admission of air under a controlled pressure to the cylinder 32, the air pressure together with the weight of the ram 28 urging the ram 28 downwardly to push the materials to be mixed into the mixing chamber 12.

When all of the material to be mixed has been drawn into the mixing chamber 12 the ram - will have reached a lowermost position as shown in figure 1. Depending on the configuration of the rotors, the materials to be mixed and other factors e.g. speed and temperature of rotors, it may be found that the rotor action draws material into the mixer with little or no assistance from the ram 28.

The illustrative mixer also comprises an outlet opening from the mixing chamber in a lower part of the housing 10 likewise centrally disposed between the two cylindrical portions 14, 16 of the mixing chamber 12. The illustrative mixer comprises a closure member namely a so-called drop door 36. The drop door 36 is of known construction generally as used in the Banbury mixer referred to above.

The drop door 36 is mounted for pivotal movement about an axle 37 journalled in the housing 10. The outlet opening 34 extends substantially the whole length of the mixing chamber 12. The drop door is shown in figure 1 in a closed position in which it is in tight engagement with walls of the outlet opening, thereby sealing the outlet opening against escape of material from the mixing chamber 12.

The drop door 36 is held in its closed position by locking means 38 comprising a cylinder 40 mounted on the housing 10 from which a piston rod 42 projects. The piston rod 42 is urged outwardly of the cylinder to engage a surface of the drop door 36 to urge the drop door 36 firmly into engagement with walls of the outlet opening 34 thus to seal the opening. When it is wished to open the drop door the piston rod 42 is withdrawn clear of the door. The door can then be pivotted about the axle 37 to move the door 36 completely clear of the outlet opening 34, by moving means (not shown). When it is desired to close the door the door is pivotted towards its closed condition by the same movement means.

As can be seen from figure 1 of the accom panying drawings, an upper portion of the door has a cusp-like configuration, the upper most surfaces of the door providing continua tions of the cylindrical surfaces of the cylindri cal portions 14, 16 which meet at an edge on a central plane of the mixing chamber. Like- wise, a lowermost surface of the ram 28 may have a similar configuration providing a contin uation of the cylindrical surfaces of the cylin drical portions 14, 16, but with a flat, down wardly-facing, central surface 29 parallel with the plane of the axes A: alternatively the low ermost surfaces of the ram 28 like-wise meet ing at an edge lying in the central plane of the mixing chamber 12, similar to the edge on the door.

In the operation of the illustrative mixer, as hereinbefore mentioned, material is introduced, through the inlet opening 22 into the mixing chamber 12 with the ram 28 held in an upper most position. The rotors are caused to rotate in the direction of the arrows B at suitable speeds, the direction of rotation of the rotors tending to drag the material introduced through the opening 22 through the gap be tween the rotors 18, 20 and into the mixing chamber. The drop door 36 is locked in its closed position (as shown in figure 1) during mixing.Material to be mixed is urged into the mixing chamber, likewise, by pressure exerted by the ram 28 which, as material is forced into the mixing chamber by the ram 28 and drawn into the chamber by action of the ro tors 18, 20 gradually descends under the pressure applied by the cylinder 32 and its own weight, until it reaches its lowermost po sition in which it is shown in figure 1. The temperature of the housing 10 and rotors 18, 20 is controlled during mixing by the heat ex change fluid referred to previously. The speed of rotation of the rotors 18, 20 can be fixed or variable to give the material a desired mix ing cycle. If necessary additional material to be mixed may be introduced after a first part of a batch has been given an initial mixing.

The illustrative mixer subjects the material to be mixed to a milling action -in the gap be tween the rotors 18, 20, similar to the action of the two roll mill mentioned previously. Material to be mixed is also sheared and smeared over the surfaces of the cylindrical portions 14, 16 of the mixing chamber, thus being subjected to a different type of mixing action, in a manner similar to that achieved by a Banbury mixer as referred to above.It can be seen that all of the internal surfaces of the mixing chamber and of the rotors contribute both to the mixing effort and to heat transfer (efficient heat transfer between the -rotors 18, 20 and housing 10 on the one hand and the material to be mixed on the other hand, is important in order to ensure most efficient mixing without over-heating the material to be mixed which in most instances will lead to degrading of the material). The temperature and time of mixing (and, where possible speed of the rotors) are controlled to achieve a desired mixing cycle.It will be appreciated that the rotors 14, 16 ensure that the whole of the interior of the mixing chamber 12 isswept by the rotors 18, 20: no "dead spots'' are present in the mixing chamber which militates against any material remaining in the chamber from one cycle of operation to the next which would lead to reduced quality of product. The construction and arrangement of the rotors 18, 20 in relation to the cylindrical portions 14, 16 of the mixing chamber 12 is such as to achieve a suitable balance between the gap X of the central nip between the rolls 18, 20 and the gaps Y between both rotors 18, 20 and the walls of the cylindrical portions 14, 16. The central gap, especially, is critical: it appears that the action of the rotors creates a certain amount of pressure in the bottom of the mixing chamber 12 below the gap between the rotors 18, 20.The rate at which the rotors 18, 20 tend to urge material through the gap X between the rotors is greater than the rate at which material is transferred to the upper part of the mixing chamber around the outside of the rotors 18, 20 leading to pressurisation of this region below the gap X. In the illustrative mixer the configuration of the rotors tends to urge the material D above- the gap of the rotors in the direction of the arrow towards the left end of the mixer (viewing Figure 2): as it approaches the left end the tendency for this material D to be pulled through the gap between the rotors increases. On the other hand material E between the rotors 18, 20 and surfaces of the mixing chamber 12 above the rotors-tends to be urged in the direction of the arrows towards the right end of the chamber. This action tends to achieve thorough blending of the material. The illustrative mixer provides intensive mixing to the material to be mixed by combining extensive and compressive mixing actions and provides an efficient mixing operation.

After material in the chamber 12 has been adequately mixed the drop door 36 is opened as hereinbefore described, and material is discharged from the mixing chamber 12 through the outlet opening 34. Rotation of the rotors 18, 20 in the direction indicated by the arrows B tends to urge the mixed material from the mixing chamber through the outlet opening 34.

As can be seen from the drawings, as the rotors rotate, material to be mixed will be forced from one of the cylindrical portions 14, 16 of the mixing chamber 12 to the other so that material does not remain in one of the cylindrical portions 14, 16 but material from both portions becomes blended. Where the rotors in a machine in accordance with the invention have a degree of helical twist there will, in addition to this blending of materials transversely of the mixing chamber, be a tendency for material to also be urged lengthwise of the mixing chamber, as discussed above, and an even more comprehensive blending of materials in the chamber will occur.Even where the invention is utilised in a continuous mixer with helical rotors, the mixer is preferably arranged so that although blending lengthwise as well as transversely of the mixing chamber 12 may occur there is little or no tendency for the mixing rotors to urge the material through the chamber, feeding pressure being generated by infeed pressure generated by whatever feeding means feeds material to be mixed to the mixing rotors, for example helically threaded portions at inlet end portions of the rotors which act merely to generate a suitable feed pressure (any such threaded portions being coaxial with the axis of rotation and, at the downstream end of the threaded portion, blending with the mixing rotor portion).

The rotors of mixers in accordance with the invention are relatively simple to machine and thus more rapidly produced and less expensive than rotors of commonly known machines. The configuration of the intermeshing rotors is such that, as well as sweeping the mixing chamber, they have no re-entrant portions or cavities in which material can lodge, this also militating against poor mixing.

Although the illustrative mixer is a batch mixer, as hereinbefore discussed the invention may equally be applied to continuous mixers and the construction of mixers embodying the invention may be varied using various known constructions provided that the mixing rotors have the configuration defined herein.

Claims (8)

1. A mixer comprising a housing in which is formed a mixing chamber comprising two intersecting, parallel cylindrical portions, two rotors each mounted for rotation about an axis coaxial with the one of the cylindrical portions in which it is mounted, the rotors being mounted for rotation in opposite directions at the same speed and in synchronism, each rotor being of substantially circular cross-section, the centre of cross-section of each rotor being eccentric in relation to its axis of rotation.

2. A mixer according to claim 1 wherein each rotor is cylindrical.

3. A mixer according to claim 1 wherein each rotor is helical.

4. A mixer according to any one of the preceding claims wherein the gap between the rotors is at least twice the minimum distance between the surface of each rotor and adjacent surface of the housing defining the mixing chamber.

5. A mixer according to any one of the preceding claims having an inlet opening to the chamber in an upper part of the housing and an outlet opening in a lower part of the housing, both disposed between the two cylindrical portions, the periphery of the rotors being arranged to travel downwardly in the central region of the chamber.

6. An internal mixer according to claim 5 comprising a door mounted for movement into and out of the outlet opening whereby to close the outlet opening for mixing and open the outlet opening for discharge of material from the chamber.

7. An internal mixer according to claim 6 comprising a closure member for closing the inlet opening.

8. A mixer for rubber and plastics material constructed arranged and adapted to operate substantially as herein before described with reference to the accompanying drawings.