GB2231503A

GB2231503A - Continuous mixers

Info

Publication number: GB2231503A
Application number: GB8904621A
Authority: GB
Inventors: Craig Hodgkinson; Henry Ellwood
Original assignee: Farrel Ltd
Current assignee: Farrel Ltd
Priority date: 1989-03-01
Filing date: 1989-03-01
Publication date: 1990-11-21
Anticipated expiration: 2009-03-01
Also published as: GB2231503B; GB8904621D0

Abstract

A through food mixer comprises two rotors, each being without a helical twist, mounted for rotation parallel to each other in a mixing chamber (3). First and second feeding devices are provided for introducing material into the chamber. They are axially spaced along the rotor and comprise respective reciprocable plungers (36) which are synchronised in operation so that that of the second device strokes at a whole number multiple of that of the first. By introducing material to be mixed at separate points along the mixing path much better mixing can be achieved at lower temperatures. <IMAGE>

Description

MIXING MACHINES The present invention relates to a continuous mixing machine for elastomeric and polymeric materials. The invention is particularly, but not exclusively, concerned with continuous mixers of the mixing and venting type. Such a machine is described in United Kingdom Patent No.1,550,364 which belongs to the applicants.

Although the machine of this existing patent works well, it has been found that with certain materials the dispersion quality of one material in another or others could be improved.

According to the present invention there is provided a through-feed mixer suitable for use with plastics and rubbery materials comprising a mixing chamber which is generally '8' shaped, having two axially parallel portions of substantially circular cross section opening into one another along the length of the mixing chamber, two rotors one mounted in each portion of the mixing chamber and arranged for rotation in opposite directions, each rotor comprising mixing sections and having no helical twist, and the peripheral path of the mixing section of each rotor being closely adjacent the circular surfaces of the mixing chamber portions, a first feeding device adapted to supply material to be mixed to an inlet end portion of the mixing chamber under a substantially constant pushing force and a second feeding device adapted to supply material to the mixing chamber at a point intermediate the first feeding device and the exit from the machine.

In a preferred embodiment of the invention, the first and second feeding devices comprise reciprocating plungers which advantageously are synchronised in operation so that that of the second device strokes at a whole number multiple (perhaps 2 or 3) of that of the first. Each plunger is formed of a triangular prism around the base of which a ledge is disposed. The sloping sides of the triangular prism, perhaps at an angle of about 70 degrees, helps to prevent build-up of material in the middle of the corresponding material inlet and the ledge helps to prevent jamming of the material at that inlet, on the up stroke of the plunger.

Dams are disposed on the rotors intermediate their axial ends. There are preferably two sets of dams one on each respective rotor axially spaced from one another to cause the material being mixed to follow a "chicane" path as it passes along the rotors. Ploughs are disposed at one or both axial ends of the second feeding device to prevent material short circuiting the dams. Each plough has a complex form with inclined faces which direct material to either side of the plough and projects down between the loci of rotation of the rotors. A third feeding device for the introduction of oil may be provided intermediate the axial ends of the rotors leading into the mixing chamber.

In order that the invention may be more clearly understood, one embodiment thereof will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a plan view of one form of mixing machine according to the invention, Figure 2 shows a section along the line Y-Y of Figure 1, Figure 3 shows a section along the line X-X of Figure 1, and Figure 4 shows detail views of a part of the machine of Figure 1.

Referring to Figure 1, the apparatus is a through feed mixer and extruder which is adapted to receive rubbery or plastics material in powder or granular form, to mix the material thoroughly and to plasticise it by the input of work thereto and to supply the mixed and plasticised material for further processing.

It comprises two rotors 1 and 2 each being without helical twist and having mixing sections of a uniform cross-section throughout their length. The rotors are mounted for rotation parallel to each other in a mixing chamber 3. A first feeding device 4 is adapted to supply material to be mixed to an inlet end portion of the mixing chamber 3 by reciprocation of the plunger with a substantially constant downward force, and an extrusion device indicated diagrammatically at 5 receives mixed material from the chamber 4 and extrudes it for further processing.

The mixing chamber 3 which, in use, extends horizontally and which is generally '8' shaped, that is to say has two axially parallel portions of substantially circular cross section opening into one another along the length of the chamber 3. The two rotors 1 and 2 are mounted for rotation one in each portion of the chamber 3.

Referring to Figure 2, a circular vertical inlet passage 22 defined by a wall 20 is provided in an upper portion 24 of the body defining the mixing chamber 3 and leads to the inlet end portion of the chamber 3.

An outlet passage 78 shown dotted in Figure 1 is provided in a lower portion of the body and leads from an outlet end portion of the chamber 3 to a traverse extruder screw of the extrusion device 5 which is mounted under the rotors 1 and 2 and in a transverse chamber provided in the body. This is shown in more detail in figure 6 of U.K. Patent No. 1,550,364.

Referring to Figure 2, the feeding device 4 comprises a funnel 34 which is mounted on the upper portion 24 of the body portion and which leads to the inlet passage 22. Mounted in the funnel 34 is a rec iprocable plunger 36 which is mounted for up and down movement in a sleeve 38 between an upper position, where a head 40 of the plunger is close to the sleeve 38, and a lower position as shown in Figure 2 just clear of the mixing chamber 3. The plunger 36 is moved up and down in the operation of the apparatus by a hydraulic or pneumatic power device 39 and this device, the funnel 34 and the plunger 36 together provide means for supplying granular or powdered material in the operation of the apparatus under a substantially constant force to the inlet passage 22. The uppermost and lowermost positions of the plunger 36 are determined by limit switches 25 and 26 respectively.

The head 40 of the plunger 38 has a special shape comprising a rectangular base portion 41 surmounted by a triangular prism 42. The dimensions of the base of the triangular prism 42 is less than that of the base portion 41, so that a ledge 43 is defined around the triangular prism 42, and the base portion 41 is spaced from the wall of the passage 22. This latter spacing and the width of the ledge are indicated at 44 and 45 respectively on the inset detail figure on Figure 2.

This spacing and special shaping reduces any tendency of material to jam on entry to the mixing chamber or when the plunger is raised.

A second feeding device 50 is spaced from the first feeding device 4. This second feed device is similar in constuction to the first feeding device. Referring to Figure 3, a plunger 52 is reciprocally mounted in a hopper 54. The plunger is movable by means of a hydraulic or pneumatic power device 59 between upper and lower positions determined by upper and lower limit switches 55 and 56. A plunger park position limit switch 57 is also provided together with a locking mechanism 58 operative to lock the plunger in the parked position. The head of the plunger comprises a triangular prism 52 Surmounting a rectangular portion 51 and defining therewith a ledge 53. Portion 51 is spaced from the wall 60 of the inlet passage 62 to define a gap indicated at 63 in the inset detail figure on figure 3. The ledge width is indicated at 64.

Referring again to Figure 1, rotor dams 71 and 72 are disposed adjacent rotors 1 and 2 upstream and downstrem respectively of the second feeding device 50.

Ploughs 73 and 74 are also disposed upstream and downstream of the second feeding device 50. A third feeding device 80 is provided extending through the body of the mixer for introducing oil to the mix.

In use of the above described mixer, materials, such as a soft synthetic rubber and a hard natural rubber, to be mixed are fed to the mixing chamber 3 through the first feeding device 4 by means of the reciprocating plunger 36. As the material is fed axially along the rotors 1 and 2 of the mixer, work is done on the material raising the temperature of the material to approximately 1000C. Carbon black powder is added to the mix through the second feeding device 50 and oil through a third feeding device 80. Powder material is fed into the mixer through the second feeding device 50 by the reciprocation of the plunger 52.The stroke rate of the two plungers 36 and 52, governs the rate at which the respective materials are fed to the mixing chamber and the stroke rate of plunger 52 should be a whole number multiple of the stroke rate of plunger 36 in order to prevent blocking of the second feeding device 50. The steep sides of the triangular prism 42 and 52a (perhaps at an angle of say 70 degrees) prevent build-up of material in the inlet to the mixing chamber and the ledges 43 and 53 prevent jamming of material between the plungers and surrounding inlet walls.

To negotiate the dams 71 and 72 the material follow a chicane on its passage down the rotors of the mixer thus promoting a more thorough mixing action. The ploughs disposed at opposite ends respectively of the second feeding device 52 prevent material flowing directly along the rotors past that feeding device.

This is not essential, but it is preferred. The profile of each plough is shown in more detail in Figure 4.

Each plough is substantially V-shaped, the apex of the 'V' extending down between the rotors. Inclined faces 81 and 82 to opposite sides respectively of a central flat portion 83 direct material to those sides respectively. By introducing material to be mixed at separate point along the mixing path, a much better dispersion of the powder in the polymer can be achieved than would otherwise be possible. Furthermore, this can be achieved at lower temperatures.

It will be appreciated that the above embodiment has been described by way of example only and that many variations are possible without departing from the invention.

Claims

1. A through-feed mixer suitable for use with plastics and rubbery materials comprising a mixing chamber which is generally '8' shaped, having two axially parallel portions of substantially circular cross section opening into one another along the length of the mixing chamber, two rotors one mounted in each portion of the mixing chamber and arranged for rotation in opposite directions, each rotor comprising mixing sections and having no helical twist, and the peripheral path of the mixing section of each rotor being closely adjacent the circular surfaces of the mixing chamber portions, a first feeding device adapted to supply material to be mixed to an inlet end portion of the mixing chamber under a substantially constant pushing force and a second feeding device adapted to supply material to the mixing chamber at a point intermediate the first feeding device and the exit from the machine.

2. A through-feed mixer as claimed in claim 1, in which the first and second feeding devices comprise reciprocating plungers.

3. A through-feed mixer as claimed in claim 2, in which means are provided for synchronising the plungers of the first and second feeding devices so that that of the second device strokes at a whole number multiple of that of the first.

4. A through-feed mixer as claimed in claim 3, in which the whole number multiple is 2.

5. A through-feed mixer as claimed in claim 2, in which the whole number multiple is 3.

6. A through-feed mixer as claimed in any of claims 2 to 5, in which each plunger comprises a triangular prism around the base of which a ledge is disposed, the ledge being operative to inhibit jamming of material at the material inlet on the up stroke of the plunger.

7. A through-feed mixer as claimed in any preceding claim, in which claims are disposed on the rotors intermediate their axial ends.

8. A through-feed mixer as claimed in claim 7, in which two sets of dams are disposed on respective rotors and are axially spaced from one another so that in operation the material being mixed is caused to follow a "chicane" path as it passes along the rotors.

9. A through-feed mixer as claimed in claim 7 or 8, in which ploughs are disposed at one or both axial ends of the second feeding device to prevent material short circuiting the dams.

10. A through-feed mixer as claimed in claim 7, in which each plough has a complex form with inclined faces which direct material to either side of the plough and which projects down between the loci of rotation of the rotors.

11. A through-feed mixer as claimed in any preceding claim, in which a third feeding device is provided intermediate the axial ends of the rotors leading into the mixing chamber for the introduction of oil.

12. A through feed mixer substantially as hereinbefore described with reference to the accompanying drawings.