WO1996035507A1

WO1996035507A1 - Internal mixers

Info

Publication number: WO1996035507A1
Application number: PCT/GB1996/001128
Authority: WO
Inventors: Christopher John Brown
Original assignee: Francis Shaw & Company (Manchester) Limited
Priority date: 1995-05-13
Filing date: 1996-05-13
Publication date: 1996-11-14
Also published as: EP0827420A1; JPH11504566A; AU5657696A; GB9509742D0; CZ358697A3; CA2220935A1

Abstract

An internal mixer comprising at least one rotor supported to rotate about a predetermined axis within a mixing chamber, at least one projection mounted on the or each rotor and extending towards an internal wall of the chamber, and means for rotating the or each rotor about its axis, the said at least one rotor defining projection edges of different geometries and being rotated such that each of the said edges is a leading edge in at least part of a mixing cycle, whereby material within the chamber is subjected to different mixing actions by the edges of different geometries.

Description

INTERNAL MTXERS

The present invention relates to internal mixers which are used for

mixing batches of compound incorporating different components which

must be mixed together to provide a homogenised mixture.

Various designs of internal mixers are currently in use in for

example the rubber and plastics industries to produce compounds for a

wide range of products, for example tyres, electrical insulation plastics

and the like. In such mixers, components such as rubber, fillers,

reinforcing agents, chemical additives and curatives are introduced into a

mixing chamber through an inlet passageway leading to an opening in a

wall of the chamber. At least one rotor turns within the chamber, the

rotor supporting projections which extend towards the internal wall of

the chamber.

The rotor has to perform a number of functions. Firstly bulk

materials, for example large pieces of rubber, must be drawn into the

chamber and divided into smaller pieces or at least re-shaped. Secondly,

the temperature of the material within the chamber must be increased by the application of stresses and strains. The temperature increase is

primarily a result of the stress applied to the material through contact

with the internal surfaces of the mixer, that is the surfaces of the chamber

walls and of the or each rotor. The major criteria in applying stress are firstly that the material is in contact with machine surfaces and secondly that there is relative movement between the material and the surfaces.

Thirdly, the separate components of the mixture must be distributed

throughout the volume of the chamber in order to achieve a fine

distribution of the various ingredients. For example, filler agglomerates

must be dispersed and broken down into a desired fine particle size and

then distributed through the bulk of the compound in the mixer.

Fourthly, the mixed compound must be plasticized in order to achieve the

requisite final rheological properties. Finally, all of the fully mixed

compound must be discharged from the mixer through an outlet which

can be opened in the chamber wall.

Given the different functions which the mixer must perform

during a single batch mixing operation, and given the changes in the

properties of the mixed material during the course of a batch mixing

operation, the design of internal mixers must be such that adequate

performance is achieved in all the phases of a mixing operation. As a

result it is always necessary to compromise on certain features of the

design of an internal mixer to ensure that the overall performance is

acceptable. By way of example, internal mixers with interlocking rotors

are widely used in the rubber industry. In such mixers, two rotors are

arranged side by side within a single chamber with their axes of rotation

parallel. The rotors comprise projections or nogs supported on generally J cylindrical shafts, the radially outer end of the nogs on one rotor

extending to a short distance from the adjacent inner wall of the cavity

and a short distance from the surface of the shaft of the other rotor. The

rotation of the two rotors is synchronised to ensure that the nogs on one

rotor do not contact the nogs on the other. With an interlocking rotor

mixer of this type, a single batch mixing operation involves three separate

phases. Firstly, the rotors serve to ingest the materials into the cavity, to

distribute them around the chamber, and initially to plasticate the

materials by means of the stressing action taking place between the

rotors. Secondly, as the material is heated and softens, it begins to flow

over the radially outer edges of the nogs, that is through the gap between

the nogs and the chamber walls. Because of the length and width of this

gap, the material is subjected to substantial extensional and shear stresses

and strains, thereby achieving good dispersion, while the disposition of

the nogs on the rotors provides a degree of circulatory flow within the

chamber, thereby assisting with the distributive mixing. Thirdly, the

rotors serve to propel the compound downwards and outwards through

the outlet when a discharge door is opened at the end of the mixing cycle.

The configuration of the nogs on the rotors of an internal mixer

with interlocking rotors has been evolved over many years. A design

which is currently in general use is described in European Patent

Specification 0 170 397. In that design the rotors are separated by a gap which is large enough to ensure that sufficient space is available to ingest

the material relatively easily, but which is small enough to ensure that

adequate extensional and shear stresses and strain rate can be applied to

the material in order to obtain dispersive mixing of its components. The

larger the rotor gap, the greater the ingestion but the less the dispersion.

Accordingly the selection of the rotor gap must be a compromise given the

competing requirements. It is also necessary to tailor the method of

operation of the machine to the particular stage of the mixing cycle, for

example to reflect the physical and chemical condition of the material.

This can involve the variables of speed of rotation, the pressure with

which material to be ingested is forced into the chamber opening, the

temperature of the mixture and the time for which the mixture is worked.

Of these variables, temperature control is generally the most significant.

It is therefore necessary to design internal mixers in such a way as to

ensure that the required levels of extensional and shear stresses, strains,

and temperature are achieved at all stages of the cycle whilst seeking

minimum cycle times. Given the changing rheological properties of the

mixture during the cycle, these factors cannot be readily optimised during

all parts of the cycle. Thus the design of the know internal mixers

represents a set of carefully established compromises.

Known internal mixers are conventionally driven in a single

direction during the normal mixing process. In mixers with two contra- rotating rotors, the leading edges of the two rotors are geometrically

substantially identical. Thus each rotor subjects the material to be mixed

to the same mixing action. It is believed that in some circumstances it has

been possible to reverse the direction of rotor movement to facilitate the

release of jammed rotors but it is not believed that internal mixers have

ever been operated in a normal mixing cycle which includes phases

during the process of mixing a single batch during which the direction of

rotation of the rotors is reversed.

It is an object of the present invention to provide an improved internal mixer.

According to the present invention, there is provided an internal

mixer comprising at least one rotor supported to rotate about a

predetermined axis within a mixing chamber, at least one projection

mounted on the or each rotor and extending towards an internal wall of

the chamber, and means for rotating the or each rotor about its axis, the

said at least one rotor defining projection edges of different geometries

and being rotated such that each of the said edges us a leading edge is at

least part of a mixing cycle, whereby material within the chamber is

subjected to different mixing actions by the edges of different geometries.

In one embodiment of the present invention, the rotating means rotates

the or each rotor in either direction about its axis, and the or each

projection is formed such that the shapes of its leading and trailing edges are different, whereby the mixing action to which material within the

chamber is subjected is a function of the direction of rotation of the rotor.

The invention also provides a method for operating an internal

mixer comprising at least one rotor supported to rotate about a

predetermined axis within a chamber, and at least one projection

mounted on the rotor and extending towards an internal wall of the

chamber, the or each projection, being formed such that the shapes of its

leading and trailing edges are different, wherein a batch of material is

delivered to the cavity, the or each rotor is rotated in a first direction to

subject the material to a first mixing programme in which the mixing

action is appropriate to its initial condition, and the direction of rotation

of the or each rotor is reversed at least once to subject the material to at

least one further mixing programme in which the mixing action is

appropriate to the condition of the material after completion of the first

mixing programme.

In an alternative embodiment of the present invention, in which at

least one rotor is provided defining at least two projections, the or each

rotor is rotated in only one direction during a mixing cycle, the leading

edge of at least one projection is relatively steep so as to grip material to

be ingested and force it into the chamber, and the leading edge of at least

one other projection is relatively less steep so as to encourage material to

flow over the radially outer surface of the projection. The present invention is based upon the realisation that in an

internal mixer it is the leading edge of the projection which largely

determines the mixing action to which material within the chamber is

subjected. Thus by arranging for the rotor to be reversible and reversing

the direction of rotation of the rotor during the mixing of a single batch,

improved performance can be achieved by designing the leading edge of

the projections when the rotor turns in one direction to be appropriate for

the condition of materials at one stage of the mixing cycle, and designing

the leading edge of the projections when the rotor turns in the opposite

direction to be appropriate for the condition of the material to be mixed

at another stage of the mixing cycle. Alternatively, in a double rotor

machine in which the rotors turn in only one direction during a complete

mixing cycle, one rotor can have a leading edge which ensures rapid

ingestion of material at the beginning of a mixing cycle and the other

rotor a leading edge appropriate to a later stage in the same mixing cycle.

In a machine with reversible rotor direction, the rotor direction

may be reversed two or more times during a single batch cycle, for

example to assist in material discharge, or to ensure good

dispersion/distribution of ingredients added to the compound during the

course of the mixing cycle. Thus the designer can produce an internal

mixer with characteristics that in the prior art would have required two

separate mixers used sequentially to process a single batch. A first edge of the or each projection which is the leading edge

when the rotor is rotated in one direction may be relatively steep so as to

grip material to be ingested and thereby force material into the chamber,

whereas a second edge of the or each projection which is the leading edge

when the rotor is rotating in the other direction is relatively less steep so

as to encourage material to flow over the radially outer surface of the

projection, thereby providing for optimal extension flow into the gap and

shear flow within the gap. The first edge is the leading edge during initial

material ingestion and plastication, and the second edge is the leading

edge during a subsequent stage in the mixing cycle. It would of course be

possible in a further stage of the mixing cycle to again reverse the

direction of rotation of the rotor.

The first leading edge may be inclined at an angle of less than 45°

to a radius drawn from the axis of rotation. The smaller this angle, the

steeper the surface of the projection advancing towards the material to be

mixed. That steep edge may be provided with an undercut so as to

positively grip introduced material and in addition could have sharp

corners so as to assist in breaking up initially introduced materials.

The or each projection may be provided with indentations and

depressions that are sloped in the helical, axial or circumferential

direction so as to improve material distribution. It will be appreciated

that in contrast to prior art mixers which during a mixing cycle rotated in only one direction it would be possible in accordance with the present

invention to provide projections defining spaces which are "dead", that is

to say which are not swept out by the flow of material within the mixer

when the rotor turns in one direction, providing those areas are swept out

when the direction of rotation is reversed.

The present invention is applicable to a variety of internal mixer

designs but is particularly applicable to mixers having two rotors located

with the nip between them beneath a material inlet and above a material

outlet, the rotor supporting interlocking projections shaped such that the

leading edges of the projections when the rotors are rotated to move the

projections downwards at the nip grip material introduced through the

inlet, and such that the leading edges of the projections when the rotors

are rotated to move the projections upwards at the nip force material

within the chamber against the internal wall of the chamber.

Embodiments of the present invention will now be described, by

way of example, with reference to the accompanying drawings, in which;

Figure 1 is a view from above of two rotors of a conventional

internal mixer in which such rotors are arranged side by side within a

mixing chamber;

Figure 2 is a sectional view on the line-to-to of Figure 1;

Figure 3 is a sectional view corresponding to that of Figure 2

showing the ingestion of material to be mixed; Figure 4 illustrates a later stage in the mixing process shown in

Figure 3;

Figure 5 illustrates the structure of an alternative internal mixer of

conventional form;

Figure 6 is a view similar to that of Figure 2 but showing

modifications made to the cross-section of a rotor in accordance with the

present invention; and

Figure 7 illustrates in cross-section the rotors of a reversible mixer

in which each rotor has been modified in the same manner as the rotor of

Figure 6;

Figures 8 and 9 schematically represent modifications which may

be made to a rotor in accordance with the present invention; and

Figure 10 illustrates in cross-section the rotors of a mixer in which

each rotor rotates in only one direction but the rotors have been modified

in different ways such that the rotors subject material within the mixer to

different mixing actions.

Referring to Figures 1 and 2, the illustrated conventional internal

mixer is manufactured by the applicants and is sold as the "Intermix"

internal mixer. The illustrated mixer comprises a casing defining an

internal wall 1 within which rotors 2 and 3 are supported. The rotor 2 is

supported on a shaft 4 and in use is rotated in the direction of arrow 5.

The rotor 3 is supported on shaft 4 and in use is rotated in the direction of arrow 6. The rotor 2 has a cylindrical outer surface from which

projections 7, 8 and 9 extend towards the wall 1. The rotor 3 supports

projections 10, 11 and 12 which also extend towards the wall 1. The

projections 7 and 10 define portions of a helix and the rotors are arranged

such that the projection 7 is received in the spacing between projections

11 and 12 as the two rotors are turned and the projection 10 is received

between the two projections 8 and 9 as the rotors turn. Thus the

projections of the two rotors are interlocked and such mixers are

generally referred to as having interlocking rotors. At the nip defined

between the two rotors there is a small clearance between the radially

outer surface of the projections on one rotor and the adjacent cylindrical

surface of the other rotor.

The projection 10 defines a leading edge 13 which subtends an

angle 14 of about 53° with a radius drawn from the axis of rotation of the

rotor. That same projection defines a trailing edge 15 which defines an

angle 16 relative to a radius. The angles 14 and 16 are substantially the

same. Similarly, the projection 11 defines a leading edge 17 defining an

angle 18 of about 47°, and a trailing edge 19 defining an angle 20 which is

substantially the same as the angle 18. Thus the shape of the leading edge

of each projection is substantially the same as the shape of the trailing

edge of each projection. The shape is not always identical and in some

forms of the Intermix internal mixer manufactured by the applicants there are minor differences between the shapes of the leading and trailing

edges as can be appreciated from the disclosure of European Patent

Specification 0 170 397. These differences between the shape of the

leading and trailing edges are relatively minor however and are generally

concerned with smoothing the flow of material over the projections in

regions adjacent the axial ends of the rotors.

The casing incorporates an inlet 21 in which a ram 22 is slidably

received. The casing also comprises an outlet beneath the nip between the

rotors. The outlet is now shown in Figures 1 and 2 but it will be

appreciated that it comprises a door which is normally fixed in position

and which can be opened to discharge a batch of mixed material from the

chamber.

Figure 3 illustrates the operation of the mixture of Figures 1 and 2

when a batch of raw polymer is ingested. The polymer is generally

supplied in relatively large pieces and must be broken down within the

mixer. The introduced material may be pressed down by the ram into the

nip of the rotors and the material is gradually drawn into the mixing

chamber through the narrow gap between the rotors. The gap between

the rotors and the degree to which the material to be ingested is gripped

by the rotors significantly affect the rate at which material can be

ingested. Referring to Figure 4, this illustrates the disposition of the material

being mixed shortly before the end of the processing of a batch of

material. It will be seen that ram has been lowered considerably as

compared with Fig. 3 and material now passes over the radially outer

surfaces of the projections. The material is also worked between the

rotors. The rheological conditions within the chamber are clearly very

different at the beginning of a batch processing as illustrated in Figure 3

and at the end of the same batch as illustrated in Figure 4. At the

beginning of the process, it would be advantageous to widen the gap

between the rotors so as to speed up ingestion and increase the usable

volume of the mixer. The gap cannot be too large however if the

components of the material to be mixed are to be adequately stressed and

distributed. At the end of the process, when the material to be mixed is

warm and relatively soft, it is desirable for the material to be forced

between the radially outer surfaces of the projections and the chamber

wall so as to ensure adequate extensional shear stresses and strain rates to

obtain dispersive mixing of the components. The pressure upstream of

the projections is partly a function of the steepness of the leading edge of

the projections. As the material softens, it is desirable that the proportion

of the material which is circulated between the helical projections rather

than forced over the radially outer surface of those projections tends to

decrease, and hence in later stages of the mixing process it would be desirable to make the leading edges of the projections less steep so as to

encourage more material to pass over the radially outer edges of the

projections thereby generating significant extensional stressing and shear

stressing of the material. The varying rheological properties of the

material to be mixed hence forces the mixer designer to adopt a whole

series of compromises when it comes to the configuration of the rotors.

The leading edges of the projections are not as steep as would be ideal for

the initial phases of a mixing process for ingestion and distribution but

are steeper than would be desirable for the final stages of that process for

dispersion.

The problems of detailed rotor designs associated with interlocking

rotor mixers of the type illustrated in Figures 1 to 4 are encountered in

other mixers, for example the Banbury-type mixer schematically

illustrated in Figure 5. In that mixer a rotor 23 turns about axis 24 within

a chamber 25. Material 26 to be mixed is trapped between the chamber

wall and the rotor and as a result the material is smeared against the

chamber wall. Given that the rheological properties of the material to be

mixed vary over time due to heating and plasticisation the profile of the

rotor is once again a compromise, the angle between the rotor surface

adjacent the chamber wall and the wall itself being sufficiently large to

ensure that a fresh batch of material can be ingested but sufficiently small

to ensure that the softened material is adequately stressed. Once again, in internal mixers of the type schematically illustrated in Figure 5 the rotor

is turned in one direction only and there is no significant difference

between the configuration of the rotor surface as between the upstream

and downstream sides of that surface.

It has been known to provide an internal mixer of the type

illustrated in Figures 1 to 4 with a reversible motor to enable the rotors to

be driven in reverse to assist in releasing a jammed mixer. No proposals

have been made however to drive the rotors of an internal mixer in both

directions during a normal mixing process cycle.

The present invention relies upon a realisation that a rotor can be

designed to provide a first set of conditions within a mixing chamber

when rotated in one direction and a second set of conditions when driven

in the opposite direction. By shaping the circumferentially spaced sides of

the projections in an appropriate manner, a rotor can be designed which

provides a performance equivalent to that which would be achieved by

using separate first and second mixers sequentially, the first mixer in the

sequence being designed to optimise performance in a first phase of a

batch mixing process and the second mixer being designed to optimise

performance for the completion of that process. All that is required to

put the invention into practice is a redesign of the circumferentially

spaced surfaces of the projections and the provision of a drive mechanism which is capable of driving the rotors in both directions over significant

periods of a mixing cycle.

Figure 6 illustrates a rotor modified in accordance with the present

invention, and Figure 7 illustrates in cross-section the two rotors of the

machine in which each rotor has the same configuration as that of Figure

6. The general structure of the rotor of Figure 6 is identical to that of the

rotor to the left hand side of Figures 2 to 4 but it will be seen that the

upstream edges of the projections when the rotor turns in the direction of

arrow 27 have been made more steep whereas the upstream edges of the

projections when the rotor is rotating in the direction of arrow 28 have

been made less steep. The shape of the surfaces before modification in

accordance with the present invention is indicated in Figure 6 by the

dotted lines 29. Given the modification of the rotor as shown in Figure 6

and equivalent modifications to the right hand rotor as shown in Figures

2 to 4, to produce the arrangement shown in Figure 7, and assuming

reversal of the direction of rotation of the rotors at appropriate times,

various benefits arise. Firstly, by increasing the steepness of the leading

edges of the projections when turning in the direction of arrow 27 the

material within the mixer can be more positively gripped by the rotors to

improve ingestion, transport of material within the chamber, and

discharge of material from the chamber. In particular, the speed of

ingestion is increased and the gap between the rotors may be increased to further improve the speed of ingestion and discharge and to increase the

usable volume of the mixer. Secondly, by decreasing the steepness of the

leading edges of the projections when turning in the direction of arrow 28

greater quantities of material are subjected to high stress and strain.

Figure 8 illustrates one simple modification to a projection which

can be made in accordance with the present invention. To simplify the

illustration, Figure 8 shows the projections having straightened out the

curvature due to the cylindrical outer surface of the main rotor body.

The full line in Figure 8 represents the configuration of the projection of a

conventional mixing machine of the type shown in Figures 2 to 4, whereas

the dotted line shows the configuration of a modified projection in

accordance with the present invention. The arrow 30 represents the

direction of movement of the projection during the initial phases of the

mixing process in which the relatively steep leading edge enables the

introduced material to be positively gripped and forced into the chamber

interior. The arrow 31 represents the direction of movement of the

projection after reversal of the mixer drive such that the relatively gently

sloping leading edge of the projection encourages material to travel up

that leading edge and over the upper portion of the projection during

which time it is subjected to high stress.

Figure 9 illustrates yet another conceivable configuration for a

modified projection for use in an embodiment of the present invention. Again the dotted line represents the configuration of the modified

projection, whereas the full line shows the standard configuration. It will

be seen that the leading edge (in the direction of arrow 30) is undercut so

as to provide a very secure "gripping" of material during the ingestion

phase. Again the leading edge (in the direction of arrow 31) is a relatively

gentle slope so as to provide extensional flow within the conveying and

diverging gaps and to encourage material to flow onto the radially outer

surface of the projection. It may be that the undercut configuration as

shown in Figure 9 would result in "dead" or stagnant space which would

not be adequately swept out if the rotor always turned in the direction of

arrow 31. This need not be a problem however providing during the final

stages of a mixing cycle the rotor turns in a direction which avoids any

dead spaces occurring. For example, during the initial ingestion phase

the projection could move in the direction of arrow 30, the rotor direction

could then be reversed until immediately before the completion of the

mixing cycle, and the discharge portion of the cycle could be effected after

a further reversal of the direction of rotation of the rotors such that

during the discharge phase the projection moves in the direction of arrow

30.

It will be appreciated that in addition to the control of the

direction of rotation of the rotors, a mixing cycle can also be controlled by

adjusting the speed of rotation, the ram pressure, coolant temperature and mixing cycle duration. The embodiment of the present invention

described in Figures 6 to 9, in proposing the reversal of the direction of

rotation of the rotors in association with a modification to the shape of the

rotor projections, adds a further variable which can be used to control the

mixing cycle. The rotors can be designed to optimise ingestion,

distribution and discharge operations while the rotors operate in one

direction (downwards at the nip) and to optimise dispersion and

plastication while the rotors turn in the opposite direction (upwards at the

nip).

The relatively steep edges of the projections may be given surface

features to improve the gripping of material during the ingestion phase.

As illustrated in Figure 9 undercuts may be formed but alternatives are

possible, for example the provision of relatively sharp corners on the

projection edges. In addition, the surfaces of the projections and of the

rotor between the projections may be provided with indentations and

depressions that are sloped in the helical, axial or circumferential

directions. Experimentation will be necessary to define the optimum

configuration.

In a double rotor mixer with contra-rotating rotors, during the

ingestion phase high pressure develops above the nip between the rotors

and beneath the ram. When the rotors are reversed, high pressure

develops beneath the nip in the region of the chamber discharge door. The discharge door is fixed in position and thus serves as a high pressure

ram to ensure that the material to be mixed is subjected to high stress.

The concept of rotor direction reversal with modification of the

projection surfaces provides an extra control variable which can be used

for example to achieve shorter cycle times and/or greater mixing capacity

as a result of the closer match between the machine configuration and

specific processing needs.

An alternative embodiment of the invention is illustrated in Figure

10. This embodiment comprises contra-rotating rotors 32 and 33 but the

rotors turn in only one direction as indicated by arrows 34. The leading

edges 35 and 36 of the projection on rotor 32 have been made relatively

more steep by changing their shape from that indicated by dotted lines.

The trailing edges 37 and 38 are of conventional shape, corresponding to

the shapes shown in Figures 3 and 4, in contrast, the leading edges 39, 40

of the projections on rotor 33 have been made relatively less steep by

changing their shape from that indicated by dotted lines. The trailing

edges 41 and 42 are of conventional shape.

In the embodiment of Figure 10, the relatively steep leading edges

35, 36 of the projections on rotor 32 ensure rapid ingestion of material

into the mixer. The leading edges 39, 40 of the projections on rotor 33

then ensure that the material is subjected to high stress and strain.

Claims

1. An internal mixer comprising at least one rotor supported to

rotate about a predetermined axis within a mixing chamber, at least one

projection mounted on the or each rotor and extending towards an

internal wall of the chamber, and means for rotating the or each rotor

about its axis, the said at least one rotor defining projection edges of

different geometries and being rotated such that each of the said edges is a

leading edge in at least part of a mixing cycle, whereby material within

the chamber is subjected to different mixing actions by the edges of

different geometries.

2. An internal mixer according to claim 1, wherein the or each

projection is formed such that the shapes of its leading and trailing edges

are different, and the rotating means rotates the or each rotor in either

direction about its axis, whereby the mixing action is a function of the

directions of rotation of the or each rotor.

3. An internal mixer according to claim 2, wherein the or a first edge

of the or each projection which is the leading edge when the rotor is

rotated in one direction is relatively steep so as to grip material to be

ingested and thereby force material into the chamber, and wherein a second edge of the or each projection which is the leading edge when the

rotor is rotated in the other direction is relatively less steep so as to

encourage material to flow over the radially outer surface of the

projection.

4. An internal mixer according to claim 3, wherein the first leading

edge is inclined at an angle of less than 45° to a radius drawn from the

axis of rotation.

5. An internal mixer according to claim 3 or 4, wherein the first

leading edge is undercut.

6. An internal mixer according to claim 3, 4 or 5, wherein the first

leading edge defines at least one sharp corner.

7. An internal mixer according to any one of claims 2 to 6,

comprising two rotors located with the nip between the rotors beneath a

material inlet and above a material outlet, the rotor supporting

interlocking projections shaped such that the leading edges of the

projections when the rotors are rotated to move the projections

downwards at the nip grip material introduced through the inlet, and

such that the leading edges of the projections when the rotors are rotated to move the projections upwards at the nip force material within the

chamber between the internal wall of the chamber and the radially outer

surface of the projection and/or between the rotors.

8. An internal mixer according to claim 1, comprising at least one

rotor defining at least two projections, the or each rotor being rotated in

only one direction during each mixing cycle, wherein the leading edge of

at least one projection is relatively steep so as to grip material to be

ingested and thereby force material into the chamber and the leading

edge of at least one other projection is relatively less steep so as to

encourage material to flow over the radially outer surface of the

projection.

9. A method for operating an internal mixer in accordance with any

one of claims 2 to 7, wherein a batch of material is subjected to a first

mixing action by rotating the rotor in a first direction to ingest and

plasticate the material, and the plasticated material is then subjected to at

least one further mixing action by reversing the direction of rotation at

least once.

10. An internal mixer substantially as hereinbefore described with

reference to Figures 6, 7, 8, 9 or 10 of the accompanying drawings.

11. A method for operating an internal mixer substantially as

hereinbefore described with reference to Figures 6, 7, 8, 9 or 10 of the

accompanying drawings.