WO1994004340A1

WO1994004340A1 - Process and apparatus for producing strands of low density extruded material

Info

Publication number: WO1994004340A1
Application number: PCT/GB1993/001773
Authority: WO
Inventors: Michael Patrick Parker
Original assignee: Rothmans International Services Limited
Priority date: 1992-08-20
Filing date: 1993-08-20
Publication date: 1994-03-03
Also published as: GB9217719D0; AU4729093A; GB2284176A; GB9502449D0

Abstract

A die for the extrusion of a mix for expansion beyond the die is formed by the nip between cam-surfaced rollers (26', 27') at least one of which is grooved. Extrudate is delivered through over-size nozzle (24) to the nip. Upon pressure rise at the die being sensed, the rollers are rotated through 180° to clear any blockage and to re-form the die with another part of their circumference. Notches (63) on side faces (28') of the nozzle conforming to the roller surfaces (29', 30') assist in preventing back-leakage of the extrudate.

Description

PROCESS AND APPARATUS FOR PRODUCING STRANDS

OF LOW DENSITY EXTRUDED MATERIAL

This invention relates to a process and apparatus for producing strands of expanded extruded material. Such material would usually but not necessarily be vegetable based and can comprise a variety of mixes for use in the food and tobacco industries, for example in the production of a breakfast cereal or in the manufacture of a tobacco substitute material.

One existing method of producing stranded material is to first mix the ingredients in an extruder in which they are cooked and then extruded through a die. As the material leaves the die it expands and it is then later passed through a pair of rollers, of which at least one is grooved. The extrudate is compresed into the grooves, formed and then scraped out. Thus much, and sometimes all, the expansion gained in extrusion is lost in rolling. It is also known to cut the material into short lengths as it leaves the die and to then subsequently roll it in compression groove rollers, again to form the shaped and compressed product. An alternative method of producing stranded material is to pass an agglomerate or stiff mix directly through a pair of rollers of which at least one is grooved. This also produces a dense material.

A further known alternative method is to extrude a band of extrudate through a straight or curved, slotted die and then longitudinally slit the product with knives with or without prior stretching.

If a low density stranded material is required, for example as a breakfast cereal or as a tobacco substitute, a mix (which comprises, in the main, cereal or tobacco base as appropriate plus starch and sometimes specific cellulose, humectant and hydroscopic compounds as well as water) may be generated, heated and pressurised within a screw extruder and passed through a die. When a hot material of this type passes through an extruder,die from high pressure to atmospheric pressure some of the water present in the material "boils off" causing the material to expand and dry out. Typically an expansion factor of 4 is obtained. If a final product not thicker than 2 mm is required the die aperture cannot be wider than 0.5 mm. Unfortunately the particles in some mixes are too big to be forced through a 0.5 mm wide aperture. Particles more than 0.5 mm long can be orientated to pass through a 0.5 mm wide slotted die but they would choke a 0.5 mm x 0.5 mm hole. The present invention is intended to provide a process and apparatus for producing strands of low density extruded material which overcomes the difficulties referred to above. According to the present invention a process for producing strands of low density extruded material includes generating a mix of said material, heating and pressuring said mix within a screw extruder and passing the treated mix through a die defined by a grooved roller, and then allowing the material to expand.

There can be a pair of nipping rollers mounted directly at the discharge end of the extruder such that they in effect replace a normal, fixed, die.

In this invention, therefore, expansion occurs primarily and preferably almost exclusively after formation of a strand of material by a die defined by an annularly grooved roller. The roller, rotatable about an axis transverse to the direction of flow through the die, may be permanently rotated during operation, or intermittently: the intermittent operation may be regular or upon demand. The effect of its rotation, in any of these cases, is to break up any obstruction or blockage occurring in the aperture whether due to oversize particles or other causes. The die may be formed, as stated, by the nip between a pair of grooved rollers with lands of one roller registering with grooves of the other, by the nip between one grooved and one non-grooved roller or, less preferably, by the nip between a grooved roller and a stationary surface. When a pair of rollers with a non-grooved roller is used, the non-grooved roller may be plain (cylindrical) or may have a stepped cammed surface as will be described, interacting with a similarly cammed surface of the grooved roller. The extruded mix may pass from the extruder into the die through a nozzle with a slot which is of greater height than the die.

Thus the extrudate mix finds an easy passage through the small grooves in the rolling die with minimum change to mass and volume flow and product pressure as,it passes from the slot in the nozzle to the nip point between the rollers. The pressure reduction to induce expansion occurs after the roller nip point . With this process a pair of nipping rollers, especially if both grooved, can be driven at a speed such that their mean surface velocity is similar to or greater than the velocity of the material passing through the slotted nozzle. Ideally the velocity of the roller is such that the volume of the mix passes through them with the minimum of throttling or extension .

If any large particulate matter comes between the nipping points of the rollers after passing through the slotted nozzle it will either be crushed or cut and driven through the roller.

If desired the slotted nozzle may be relieved to accommodate the outside diameter of the grooved nipping rollers.

With a permanently rotating roller, if a slotted nozzle 1 mm high is used and the clearance through the die is 0.5 mm, then provided that the surface velocity of the rollers is twice (or more) than the velocity of the extruder mix into the die, little or no compression will take place between the rollers, but expansion will occur relative to the 0.5 mm roller grooves clearance rather than the 1 mm slot. Moreover, with the correct roller speed, sideways expansion of the extrudate is limited, preventing contamination of the roller bearings. Some mixes however are abrasive. This is especially true of reconstituted tobacco mixes, which may be contaminated by sand particles. In such a case continuous rotation of the roller die is undesirable because of the high wear that it sustains and in such cases we prefer intermittent motion only of the roller. This intermittent motion may be a regularly or irregularly timed stepping motion such that any actual or potential blockage of the die is cleared without reference to the actual conditions therein; but most preferably the pressure obtaining in the nozzle of the extruder will be monitored and rotation of the roller initiated when a rise in that pressure indicates that a problem is occurring. This monitoring is standard for extruders (for safety purposes) and the sensor provided conventionally for that can be used though it will be set to actuate the roller at a much lower pressure rise than the danger level.

In such a case the roller is preferably one of a pair each having surfaces with a step beyond the nip so as to allow free expansion of the extrudate. The process can also include combing the roller grooves and in order to minimise the combing effect and possible damage to the product it is preferred to pass the extrudate through rollers with slightly tapered grooves. If desired an appropriate fiberised filler can also be added to the mix as this tends to add strength to the final extruded material . A typical filler for use with a tobacco product could be finely shredded cereal or tobacco stem or other vegetable or synthetic fibre.

The process can also include cutting the strands transversely into predetermined lengths after emerging from the nipping rollers.

Also included within the invention are strands of expanded material produced by the process. The invention can be performed in various ways but embodiments will now be described by way of example and with reference to the accompanying drawings in which:

Figure 1 is a diagrammatic drawing of the apparatus used for producing strands of low density extruded material;

Figure 2 is a diagrammatic drawing showing the extrusion end of a screw extruder provided with a slotted die and grooved nipping rollers embodying the invention;

Figure 3 is an enlarged part view showing the cross section through the engaging surfaces of the nipping rollers;

Figure 4 is a cross sectional side elevation of the nipping roller assembly;

Figure 5 is a plan view from below on the line V-V shown in Figure 4;

Figure 6 is a cross sectional end elevation on the line VI-VI shown in Figure 1; and Figure 7 is a detail analogous to Figure 6 but of a second embodiment . As shown in Figure 1 the apparatus is for carrying out a process for producing strands of low density expanded material made from small particles, for example dust. The dust is supplied from a container 1 to a metered feed 2 which delivers it to a cleaner 4 to remove any unwanted materials, for example, sand. The unwanted materials are ejected at 5 in the direction of the arrow 6. The cleaned dust passes across a sieve 7 and particles which are fine enough pass downwardly into a fine dust hopper 8. The remaining material passes across the sieve and into a mill 9 where it is reduced to an appropriate size. It then enters the hopper 8. From the hopper 8 the fine dust is metered to a twin screw extruder 11. Cellulose binder may be added through a hopper 12 and starch from a hopper 13. At this point an appropriate fiberised filler, as indicated by reference numeral 14, can also be added. This filler can take the form of a tobacco product, for example tobacco stem and, when the final product is produced, tends to add strength. The materials are mixed in the first part 15 of the twin screw extruder 11 and water, as indicated at 16, hydroscopics, as indicated at 17, and plasticisers, as indicated at 18, may subsequently be admitted to the hopper for mixing with the other materials . The mix progresses down the twin screw extruder in the direction of the arrow 20 and emerges into a guide 19 which turns it through 90° from the horizontal to vertical and directs it through a slotted nozzle 21 which is shown in more detail in Figure 2. The slotted nozzle 21 has an entry portion 22 and an elongated compression slot 23. The lower end of the slot 23 is formed as a nozzle 24 and produces a flat layer of compressed mix which it delivers tangentially into a nip 25 between a pair of grooved intermeshing nipping rollers 26 and 27. The outer surface of the lower part of the slotted nozzle 21 is necked-in, as indicated by reference numeral 28, so that it fits closely between the rollers 26 and 27. The outer surfaces 29 and 30 of the rollers 26 and 27 are annularly grooved as shown in Figure 3. Thus each roller carries a series of slightly tapered relieving grooves 31, 32 respectively and the parallel axes of rotation 33, 34 are arranged so that the lands 35 of the roller 27 extend into the grooves 31.

Similarly the lands 36 between the grooves 31 of the roller 26 extend into the grooves 32 of the roller 27, as is most clearly shown in Figure 3.

In the construction being described the width of each groove at its open end is 0.75 mm and the depth of each groove from the land 35 or 36 to the base of the groove, as indicated by reference numerals 37, 38, is 0.5 mm. The overlap, or extension, of the lands into the grooves is zero to up to 0.3 mm dependent on the thickness of the final product required. The overall diameter of each of the rollers is 60 mm and the distance between the rollers is 120 mm minus the dimension of the intermeshing.

The height of the slotted nozzle is 1 mm: it is nearly as wide, axially of the rollers, as the rollers are long.

Any product in the roller grooves 30, 32 is combed out by combs 40, 41 and the taper on the grooves minimises the combing effect and possible damage to the product. As will be seen from the drawings the extrudate passes vertically through the rollers and into a space beneath them, indicated by reference numeral 42, where it can optionally be vacuum heated or cooled to assist the expansion which occurs after leaving the nip. The formed, expanded, extrudate then passes through steadier rollers 43 to a transverse cutter 44 where it is chopped into predetermined lengths . The cut strands then pass down a vibrating conveyor 45 and are discharged in the direction of arrow 46. The conveyor 45 has a sieve 47 through which dust or short material can pass and be delivered by an appropriate conveyor, indicated by broken line 48, back to the metered feed hopper 2. This arrangement can also be adapted so that oversized as well as undersized particles can be routed back to the feed hopper 2 with only the most suitably sized particles discharged in the direction of arrow 46.

In use the various materials are mixed in the twin screw extruder 11 and are passed through the necked-in nozzle 21 into the nip of the rollers 26 and 27. The height of the slotted nozzle is 1 mm and the clearance between the intermeshing roller grooves is 0.5 mm. The surface velocity of the rollers is arranged so that it is twice or more than the velocity of the extruder mix into the rollers and thus little or no compression takes place between the rollers, but expansion occurs relative to the 0.5 mm roller grooves rather than the 1 mm slotted nozzle. Moreover, with the correct roller speed, sideways expansion of the extrudate is limited, preventing contamination of the roller bearings. However, side plates (not shown) may bear against axial end surfaces of the rollers and labyrinth seal grooves may be formed in those end surfaces to seal with the side plates.

The combs 40, 41 comb out the material from the grooves and the strands are cut transversely by the transverse cutter 44 to provide suitable lengths for adding back to cut lamina. If required the strands, on leaving the rollers, can be connected together to form a bundle with or without any other tobacco for direct wrapping into a cigarette rod.

In Figures 4, 5 and 6 the construction of the grooved nipping rollers and their associated mounting are shown in more detail . In these figures the same reference numerals are used as in the previous figures and it will be seen that the rollers 26 and 27 are keyed to parallel shafts 50, 51 which are carried in bearings 52 located in a suitable mounting 53. The shaft 50 which carries the roller 26 is driven through a pinion 54 by a pinion 55 carried on a drive shaft 56 mounted in a drive motor 57 through suitable reduction gearing (not shown) . The pinion 54 drives a pinion 58 carried on the shaft 51 to rotate the slotted roller 27.

Alternatively the drive to each roller can be supplied by matched hydraulic motor or other matched primary drive means. Hydraulic means for opening the rollers for start up/clean down purposes, and closing them to achieve the best running setting and means for heating them can be provided.

The mounting 53 is secured to the end of the guide 19 and carries the slotted nozzle 21, the centre¬ line of which is indicated by reference numeral 61 in Figure 4 and which is shown in broken lines in Figure 6.

The rollers 26, 27 have been described as continuously rotating while extrudate has passed through the die formed by them. It has now been found that such continuous rotation is undesirable if the mix is abrasive. Grooved rollers are expensive to make and should be replaced as seldom as possible.

To deal with this, rotation of the roller defining the die may be intermittent. It may be step- wise, actuated by a regular or irregular timer: successive steps may be in the same or opposite directions. However it is now preferred that rotation should be initiated upon demand, as a result of a rise in pressure upstream of the die. Sensors are conventionally provided in extruders to detect pressure build-up, for safety reasons. We prefer to utilise one or more of these sensors, preferably that or those nearest the nozzle, or one or more specially-provided sensors in or near the nozzle, to actuate rotation of the roller(s) forming the die only when a pressure rise indicates a start of a blockage in the die. Such rotation may also be step-wise or may be continuous until the pressure falls acceptably. A preferred embodiment of roller die is seen in

Figure 7. Nozzle 21 is provided on extruder guide 19 as before. Rollers 26', 27' form a die-nip immediately in front of the nozzle slot 24 as before. Roller 26' is however a non-grooved roller, 27' being annularly grooved, with lands 36' and groove bases 38' essentially as before.

However neither roller is exactly cylindrical. Each has in its cylindrical surface 29', 30' a pair of axially-extending radial steps 60, 61. When the rollers are arrested a die is formed by the nip between the surfaces of maximum radius, by the grooves on the roller 27' and the surface of the roller 26' . The steps 60,61 allow free expansion of the extrudate beyond the nip. If a rise in pressure is detected by a sensor in the nozzle or extruder, diagrammatically shown at 62, the rollers 26', 27' are actuated to rotate (in either direction relative to the nozzle and in counter-rotation or in the same sense relative to each other) through 180°, so that a die is once again formed between the parts of maximum radius . Any blockage which had formed is cleared, and a new portion of the surfaces 29'30' is presented as the die. If when the rollers reach the 180° position the pressure has not dropped acceptably, a further 180° rotation is initiated. Figure 7 also illustrates a solution to the problem of leakage back past the rollers at the side of the nozzle. Side walls 28' of the nozzle are formed to conform to the surfaces 29', 30' of the rollers at their arrested position: on wall 28' ribs project into the grooves of the roller 27' . In each surface 28' are labyrinth seals formed by a plurality of notches 63 extending parallel to the axes 33, 34 of the rollers. Any extrudate passing back along the surface of the rollers under the pressure exerted at the die tends to be trapped by the notches and build up, and harden, in them. This is found to form a surprisingly effective seal and one which, due to the anchoring effect of the notches, is not dislodged in the intermittent movement of the rollers. Notches 63 are closed at the axial ends of the nozzle, for example by side plates (not shown) acting also as side plates for the rollers.

Strippers 64, 65 are air-knives rather than the mechanical devices 40, 41 of the first embodiment. The assembly including the roller die and nozzle can be provided as a complete rolling extrusion die assembly which merely requires attachment to the outlet end of the twin screw extruder. With this assembly the extrudate can easily be delivered to a steadier, as shown in Figure 1, or it can be used so that the strands on leaving the rolling die can be collected together to form a bundle as set forth above.

Claims

CLAIMS :

1. A process for the formation of strands of expanded extruded material in which expansion occurs after the material leaves a die characterized in that forming the die by a nip of a grooved rotatable roller (26,27,26' ,27' ) .

2. A process according to claim 1 in which the roller is intermittently rotated during the extrusion.

3. A process according to claim 2 in which the intermittent rotation is initiated by sensing a pressure rise upstream of the die.

4. A process according to any one of the preceding claims wherein the nip is formed between a pair of rollers (26,27,26' ,27' ) constrained to be rotatable together.

5. A process according to claim 4 wherein the rollers are one annularly-grooved roller (27') and one non-grooved roller (26') , each roller having steps (60,61) axially along its outer surface and adjacent each die-forming portion, the rotation of the rollers being to bring a next die-forming portion to the nip between them.

6. An extruder head assembly for the production of strands of expanded material including a die for forming the strands before expansion characterized in that the die is a nip of a grooved roller (26,27,26' ,27') .

7. An extruder head assembly according to claim 6 wherein the nip is between a pair of rollers.

8. An extruder head assembly according to claim 7 wherein a nozzle (21,24) directs the extrudate to the nip, and there are sealing means between the nozzle and the rollers.

9. An extruder head assembly according to claim 8 wherein the nozzle has an orifice (24) of a height greater than that of the die formed by the nip.

10. An extruder head assembly according to claim 8 or claim 9 wherein the sealing means comprise side walls (28') of the nozzle (21) conforming to adjacent surfaces of the rollers and labyrinth seal means (63) in said side walls.

11. An extruder head assembly according to any one of the preceding claims operatively linked with a pressure sensor (62) upstream of the die whereby the roller is actuated to rotate upon increase of pressure at the sensor.

12. An extruder head assembly according to any one of the preceding claims wherein said rotation is a part revolution whereby to present a new portion of the surface of the grooved roller as said die.