GB2335375A

GB2335375A - Two-roll mixing mill

Info

Publication number: GB2335375A
Application number: GB9805741A
Authority: GB
Inventors: John R Dare
Original assignee: J R DARE Ltd
Current assignee: J R DARE Ltd
Priority date: 1998-03-19
Filing date: 1998-03-19
Publication date: 1999-09-22
Anticipated expiration: 2018-03-19
Also published as: GB9805741D0; GB2335375B

Abstract

A two-roll mixing mill for the mixing of rubber and plastics compounds (4) comprises two axially parallel rollers (1) and (2) rotatably mounted in the mill such that there is a nip (3) therebetween. The mill is provided with a drive mechanism which allows the rollers (1) and (2) to be driven in either the opposite or same directions. With the rollers rotating in opposite directions, the compound forms a sheet around one of the rollers and a rolling bank (6) on top of the nip. One of the rollers may be intermittently reversed as required and this causes the sheet to be released from the roller around which it is located and assist with the mixing process by changing the compound in the rolling bank.

Description

2335375 1 TWO-ROLL MIXING MILL The present invention relates to a two-roll

mixing mill for use in the mixing of rubber and plastics compounds.

The mixing of batches of rubber and plastics compounds using a two-roll mixing mill is a long established procedure. Briefly, the mill comprises two axially parallel (generally horizontal) rollers between which is a nip. The mill is provided with a drive mechanism which is adapted to rotate the rollers in opposite directions at the same or different rotational speeds to create a friction ratio.

In use, the rubber or plastics formulation to be mixed is fed into the nip. The formulation becomes "wrapped" in the form of a sheet (or "band") around one of the rollers and also forms a "reservoir" (known as the "rolling bank") from which formulation is continuously drawn into the nip and into which formulation is replenished from the sheet on the roller thus resulting in mixing.

The existing two-roll mills do however suffer a disadvantage in that they are not particularly effective for providing mixing along the axial direction of the nip (i.e. across the width of the sheet). In order to overcome this problem, it is conventional practice to make an incision in the sheet so that it may be removed from the roller. The sheet is then generally folded or rolled and then fed back into the nip. This folding and/or rolling operation ensures more thorough mixing of the formulation when the latter is fed back into the nip. However the need to make an incision in the sheet is a considerable disadvantage associated with the existing two-roll mixers.

It is therefore an object of the present invention to obviate or mitigate the abovementioned disadvantages.

2 According to a first aspect of the present invention there is provided a two-roll mixing mill for the mixing of rubber and plastics compounds, the mill comprising two axially parallel rollers rotatably mounted in the mill such that there is a nip therebetween wherein the mill is provided with a drive mechanism which allows the rollers to be driven in either the opposite or the same directions.

According to a second aspect of the present invention there is provided a method of mixing a rubber or plastics formulation using a two-roll mixing mill in which the rollers rotate in opposite directions so that the compound forms a sheet around one of the rollers wherein one of the rollers is intermittently reversed as required to release the sheet from the roller around which it is located.

For the purposes of mixing a rubber or plastics formulation, the mill of the invention is operated in a conventional manner such that the two rollers rotate in opposite directions at the same or different speeds. As such, the formulation forms a sheet (or "band") around one of the rollers as well as a reservoir (or "rolling bank") as described above. However, as discussed, there is still only limited mixing along the length of the nip so there is still a need for some "reorientation" of the sheet and further mixing.

We have found, and this forms the basis of the invention, that the need to make an incision for the purpose of removing the sheet from the mill may be avoided simply by reversing one (the "first") of the two rollers so that both rollers rotate in the same direction. This has the effect of shearing the sheet at the nip. With the rollers continuing to rotate in the same direction, it is found that the sheet is released from the roller around which it was "banded" and, at a certain point in the rotation of the rollers, is supported by both rollers above the nip. At this point, rotation of the rollers in the same direction may be continued so that the sheet is fed off the rollers for collection. Such collection may, for example, be for the purpose of folding and/or 3 rolling the sheet prior to feeding the sheet into the nip between the rollers (again rotating in the opposite direction) for further mixing as described above or for collection of the sheet at the end of the mixing operation.

Alternatively, at the aforementioned point, the rotation of the first roller may be reversed (so that the rollers again rotate in opposite directions). This has the effect of drawing the formulation back into the nip for further mixing and avoids the need for collection and folding and/or rolling of the sheet.

In summary, therefore, the mill of the invention is operated in accordance with the following sequence of steps:

(a) feed rubber/plastics formulation to be mixed into the nip between the rollers rotating in opposite directions at different rotational speeds; (b) reverse direction of a first one of the two rollers so that the two rollers rotate in the same direction and the sheet is supported by the two rollers above the nip; either (c)(i) immediately reversing the direction of the first roller so that the two rollers again rotate in opposite directions to draw the sheet back into the nip to continue mixing; or (c)(ii) allowing the two rollers to continue rotating in the same direction, collecting the sheet, reversing the direction of the first roller, and feeding the (possibly rolled and/or folded) sheet back into the nip; and 4 (d) repeating steps (a)-(c) as often as required to provide the required degree of mixing.

The mill may be such that he reversals (b) and (c) may be effected selectively by an operator of the mill, e.g. by provision on a switch or the like on the mill. However we do not preclude the possibility that these reversals are effected automatically at predetermined time intervals. Generally the reversal (b) is effected for 1-2 seconds.

The two rollers are preferably independently driven by respective motors. Preferably both rollers may be driven at variable speeds, e.g. at a rotational speed up to 25 rpm. Conveniently the rollers are inverter driven. For preference, each roller is selectively rotatable in either direction.

The rollers may have provision for heating and/or cooling as required.

The invention will be further described by way of example only with reference to Figs. 1 to 4 of the accompanying drawings which schematically illustrate operation of a two-roll mixing mill in accordance with the invention.

Fig. 1 illustrates (in schematic traverse section a two roll mixing mill having two independently driven rollers 1 and 2 positioned so that a nip 3 is defined therebetween. The rollers 1 and 2 are preferably inverter driven and at least the roller 1 may be rotated either clockwise or anticlockwise.

The mill is shown in operation for the mixing of a rubber or plastics formulation 4 for which purpose the rollers 1 and 2 are rotated in opposite directions, (roller 1 being rotated clockwise and roller 2 anticlockwise) at the same or different rotational speeds as well known for two-roll mixing mills.

In Fig. 1, the formulation 4 is shown as being banded (in the form of a sheet 5) around the circumference of roller 2 with there being a "bank" 6 of the formulation in the region of the nip 3.

When required, the rotation of roller 1 is reversed. This has the effect of shearing the sheet 5 at the nip 3 so that the portion of the sheet in the nip forms two edges 7 and 8 (see Fig. 2). By maintaining rotation of the rollers in the same direction (e.g. for a period of 1-2 seconds) "rolling banC is removed from the nip and the sheet becomes supported by the two rollers 1 and 2 above the nip 3 (see Fig. 3). At this point, rotation of the roller 1 may again be reversed (Fig. 4) so that the sheet is drawn back into the nip 3. It will at this point be appreciated that the portions of the sheet previously sheared at the nip 3 (i.e. the edges 7 and 8) are now the last to re-enter the nip and a new "rolling bank" is established so that further mixing is facilitated.

Alternatively, from the configuration shown in Fig. 3, rotation of the two rollers 1 and 2 in the same direction may be continued so that the sheet 5 is fed off the two rollers. This may be for the purpose of collecting the sheet at the end of the mixing operation. Alternatively, the sheet may be rolled and/or folded and (with both rollers 1 and 2 again rotating in opposite directions) re-fed into the nip for further mixing.

From the configuration shown in Fig. 4, the compound may be come banded around either roller 1 or 2. For this reason, it is desirable for rollers 1 and 2 to be selectively reversible.

6

Claims

1. A two-roll mixing mill for the mixing of rubber and plastics compounds, the mill comprising two axially parallel rollers rotatably mounted in the mill such that there is a nip therebetween wherein the mill is provided with a drive mechanism which allows the rollers to be driven in either the opposite or the same directions.

2.

A mill as claimed in claim 1 wherein the rollers are inverter driven.

3. A mill as claimed in claim 1 or 2 wherein the rollers are independently driven.

4. A mill as claimed in any one of claims 1 to 3 wherein the rollers may be driven at variable speeds.

5. either direction.

A mill as claimed in any one of claims 1 to 4 wherein each roller is rotatable in

6. A method of mixing a rubber or plastics formulation using a two-roll mixing mill in which the rollers rotate in opposite directions so that the compound forms a sheet around one of the rollers wherein one of the rollers is intermittently reversed as required to release the sheet from the roller around which it is located.

A method of mixing a rubber or plastics formulation using a two-roll mixings mill in which the rollers rotate in opposite directions so that the compound forms a sheet around one of the rollers and a rolling bank on top of the nip, wherein one of the rollers is intermittently reversed as required to release the sheet from the roller around which it is located and assist with the mixing process by changing the compound in the bank.