Tabletting press

Abstract

 In a tabletting press comprising a turret (2) rotatable with respect to a housing (1) and carrying rotatably with it a plurality of dies and a plurality of opposed pairs of punches (3; 3', 3") and a pair of rotatable pressure rolls (7; 7', 7") between which the opposed punches are passed to form tablets in successive dies, and respective curves (5 ; 5', 5") mounted, in use, non-rotatably with respect to the housing to control movement of the opposed pair of punches in less heavily loaded sectors of the tabletting cycle outside the influence of the compression rolls, an improved compression profile is achieved by interposing between at least one of the compression rolls and heads of associated punches under its influence a rotatable disc (11 ; 11', 11) one surface (15) of which is acted on by the associated compression roll and the other surface (17) of which contacts the heads of the punches to control the compression profile of the punches outside of the influence of the fixed curves. Conveniently the disc has an axis of rotation inclined with respect to the axis of the punches.

Description

[0001] The present invention relates to a tabletting press.

[0002] The final compression of a tablet in a rotary tablet press is conventionally accomplished by passing the heads of a pair of opposed punches between a pair of freely rotating pressure rollers. The tablet is formed in the die between the punch tips. The shape of the compression profile (path of the punch tips) is therefore controlled by the circular geometry of the rollers and the form of the punch heads. The latter are domed but have a small flat area at their centres which creates a short dwell, during which the punch tips maintain a constant separation. The flat also gives line contact between the punch head and roller throughout the period of highest loading.

[0003] Such a construction gives rise to a compression profile which approximates to two circular arcs joined by a short straight line. The mechanism of particle consolidation and hence tablet strength is known to be improved by any reduction in the speed at which the punch tips approach each other at compression. Equally significant is a reduction in the speed of separation after compression. Being aware of these facts, and in an effort to keep the relative punch tip speed as low as possible during the compression phase, tablet press manufacturers strive to fit pressure rolls of the largest possible diameter to their machines although in practice there is an upper limit to a maximum practical size for the pressure rolls. In addition to the use of the pressure rolls at final compression, fixed cams are used to control the punch tip path in the less heavily loaded sectors of the tabletting cycle with the aim being to raise the top punch swiftly after final compression to permit tablet ejection and with plenty of height to accommodate the feeder. In theory these cams could be designed to provide the optimum compression cycle but with present materials and lubricants these are not a reality at final compression because of the exceptionally high loads involved. Hence the functionally imperfect roller has retained its prominence thanks to its mechanical simplicity and ease of manufacture.

[0004] Another problem which the press manufacturer has to contend with is that of wear due to friction between the heads of the punches and the compression roll due to scrubbing as the punches are moved on a circumferential path as they rotate past the compression roll. In an attempt to reduce this it has been proposed to incline the compression roll so that the surface presented to the head of the punches also has a circumferential component of motion. This helps alleviate the wear problem and also has the effect of modifying the compression profile moving it away from the circular towards the elliptical but only slightly in both instances because the forces limit the extent to which the compression roll can be inclined to the vertical to only a few degrees. Furthermore inclining the compression roll is disadvantageous because it entails extensive modification to the existing design of tabletting press.

[0005] GB 1311353 addresses the problem of minimising friction due to scrub between the compression roll and the heads of the punches, and in one embodiment simply inclines the compression roll in a manner described above whilst, in a further embodiment, a cam track controlling the up and down movement of the punches throughout the full rotation of the turret is provided by an inclined disc which rotates with the turret thereby avoiding the scrubbing. As such it produces an elliptical compression profile but has the disadvantage that its fixed inclination dictates that the punch has a constant rate of rise and fall during the entire rotary movement of the turret.

[0006] It is an aim of the present invention to produce an improved compression profile at final compression in a manner which enables the conventional compression rolls to be utilised and in a manner which allows the path of the punches during the less heavily loaded sectors of the tabletting cycle to be controlled independently of the compression profile at final compression.

[0007] Accordingly, the present invention provides a tabletting press comprising a turret rotatable with respect to a housing and carrying rotatably with it a plurality of dies and a plurality of opposed pairs of punches, and a pair of rotatable pressure rolls between which the opposed punches are passed to form tablets in successive dies, and respective curves mounted, in use, non-rotatably with respect to the housing to control movement of the opposed pair of punches in less heavily loaded sectors of the tabletting cycle outside the influence of the compression rolls, the improvement comprising interposing between at least one of the compression rolls and heads of associated punches under its influence a rotatable disc one surface of which is acted on by the associated compression roll and the other surface of which contacts the heads of the punches to control the compression profile of the punches outside of the influence of the fixed curves.

[0008] More particularly, a respective rotatable disc is interposed between each compression roll and the heads of the associated punches. The rotatable discs are preferably freely rotatable.

[0009] For ease of reference the curves are referred to hereinafter as fixed curves, although that is not to say that they are not adjustable for example, for setting purposes and/or releasable from the housing to be carried by the turret.

[0010] The axis of the rotatable disc is disposed within the pitch circle diameter of the punches such that the surface of the disc which contacts the heads of the punches has a circumferential component of movement. This is provided to reduce the scrubbing referred to above. The axis of rotation may be vertical but that would call for a rim part of the disc which makes contact with the successive punch heads to be profiled to provide the desired punch movement for compression and retraction. Accordingly, it is much preferred to have the axis of rotation of the disc inclined with respect to the longitudinal axis of the punches. Where the disc axis is inclined, it has the advantage that a rim part in the form of a flat annular surface will have the effect of imposing an elliptical compression profile on the punch. The circumferential component will also be elliptical and closer to the circular path of the punches. In one embodiment said rim part and/or said one surface is/are normal to the axis of the punches at final compression. There may be chamfering or other profiling radially outwardly of said flat annular surface to smooth the movement of the punches between the influence of the disc and the fixed curves.

[0011] The angle of inclination of the disc will be chosen to provide a suitable compression profile whilst having regard to the provision of a circumferential component of movement. Machine construction may impose further constraints on the angle and diameter of the disc. We have found that a diameter of slightly greater than half the pitch circle diameter of the punches and with the disc is inclined at an angle of 20° to the punch axis provides particularly good results. A practical upper limit of inclination may be 60°.

[0012] By means of the interposed inclined disc the normal compression profile of the circular compression roll is modified to produce an advantageous compression profile which is flattened out compared with the conventional circular compression profile and yet which allows independent control of the punch path during the less heavily loaded sectors of the tabletting cycle under the influence of the fixed curves. The proposal is particularly advantageous since it requires limited modification to the existing designs of tablet press allowing the utilisation of the existing perpendicular compression rolls and at least a substantial part of the existing fixed curves.

[0013] By interposing the disc in this way it is only required to withstand light loads. Conveniently the mounting of the inclined disc allows it to float vertically to accommodate the normal adjustments for tablet thickness and upper punch penetration and/or overload deflection. The arrangement also has the advantage of minimising the relative radial movement between punch heads and disc face. By retaining the fixed curves, the clearance between the upper punch tips and the feeder is preserved, with all punch control functions other than final compression remaining as standard.

[0014] The present invention will now be described further by way of example only with reference to the accompanying drawings, in which:-

Figure 1 is a fragmentary cross sectional view of a tabletting machine showing the present invention applied to the top punches of a tabletting machine, and

Figure 2 illustrates the compression profile produced according to the invention, and

Figure 3 is a fragmentary sectional view showing application of the invention to be top and bottom punches of a tabletting machine.

[0015] Referring to the drawing of Figure 1, there is illustrated an upper fixed housing part 1 of a tabletting machine and the upper part of a rotary turret 2 in this instance rotatably journalled on a fixed spindle 1a. The turret carries a plurality of top punches 3 guided for sliding movement throughout the less heavily loaded portions of their rotary movement by fixed curves 5 which are secured to the upper housing part 1 in the illustrated embodiment. Also illustrated is a compression roll 7 journalled on shaft 9 carried in the illustrated embodiment in the upper fixed housing 1. Alternative mounting arrangements for the compression roll may be utilised, such as the well known beam mounting system. As is conventional practice, the axis of shaft 9 is perpendicular to the axial direction of the punches 3 and the compression roll is freely rotatable.

[0016] The apparatus further comprises a disc 11 jour- nailed for rotation on a shaft 13 which is inclined with respect to the axis of the compression roll and the axis of the punches. In the illustrated embodiment the disc axis is angled at 20° to the axis of the punches. The disc is freely rotatable and is axially slidable on the shaft 13. The disc 11 is positioned so as to take over controlling movement of the punches 3 from the fixed curves 5 on the lead in to and exit from the point of final compression. Thus a lower surface 17 of a rim part of the disc 11 contacts a head part 3a of each punch as are rotate past the disc. An upper surface 15 of the rim part of the disc 11 is contacted by the compression roll 7 and by this means the compression forces are transmitted through the rim of the disc to the compression rolls without imparting substituted compression forces to the bearing comprising the disc. The introduction of the disc 11 has the effect of imparting an elliptical compression profile to the punches 3a as denoted by curve 21 in Figure 2. Line 23 denotes the incoming path of the punches as dictated by the fixed curves 5. It will be understood that by utilising both the fixed curves and the inclined disc with backup compression roll a more advantageous compression profile can be achieved at final compression without effecting the punch movement during the other less heavily loaded sectors of the punch movement.

[0017] Figure 1 shows the rotatable disc applied to the top punches of a tabletting machine and the principle may also be applied to the bottom punches. In practice it is envisaged that rotatable discs will be provided for both top and bottom punches and Figure 3 shows that somewhat diagrammatically.

[0018] The same reference numerals with the addition of a ' have been used to denote parts corresponding to the parts denoted with reference to Figure 1. Housing or frame parts are shown generally as 1', and the rotatable turret as 2' rotatable on spindle 1a' journalled in the housing. A die disc 2a is carried by the turret and the opposed top and bottom punches are illustrated at 3' and 3" respectively. The section is taken at the point of final compression. The top and bottom compression rolls are shown at 7' and 7" respectively and the interposed top and bottom indirect discs of 11' and 11" respectively. Thus the movement profile of the top and bottom punches under the influence of the discs 11', 11" will be a mirror image.

Claims

1. A tabletting press comprising a turret (2) rotatable with respect to a housing (1) and carrying rotatably with it a plurality of dies and a plurality of opposed pairs of punches (3; 3', 3"), a pair of rotatable pressure rolls (7; 7', 7") between which the opposed punches are passed to form tablets in successive dies, and respective curves (5; 5', 5") mounted, in use, non-rotatably with respect to the housing to control movement of the opposed pair of punches in less heavily loaded sectors of the tabletting cycle outside the influence of the compression rolls, characterised in that the improvement comprises interposing between at least one of the compression rolls and heads of associated punches under its influence a rotatable disc (11; 11', 11") one surface (15) of which is acted on by the associated compression roll and the other surface (17) of which contacts the heads of the punches to control the compression profile of the punches outside of the influence of the fixed curves.

2. A tabletting press as claimed in claim 1 in which a respective rotatable disc (11', 11") is interposed between each compression roll (7', 7") and the heads of the associated punches.

3. A tabletting press as claimed in claim 1 or 2 in which the or each rotatable disc is freely rotatable.

4. A tabletting press as claimed in claim 1, 2 or 3 in which the axis of the or each rotatable disc is disposed within the pitch circle diameter of the punches such that the surface of the disc which contacts the heads of the punches has a circumferential component of movement.

5. A tabletting press as claimed in any one of claims 1 to 4 in which the axis of rotation of the disc inclined with respect to the longitudinal axis of the punches.

6. A tabletting press as claimed in claim 5 in which the rim part of the or each rotatable disc has a flat annular surface in a plane normal to the axis of the punches at the point of final compression.

7. A tabletting press as claimed in anyone of claims 1 to 6 in which the surface (17) of the rotatable disc which contacts the punches has chamfering or other profiling to its radially outer edge to smooth the movement of the punches between the influence of the disc at final compression and the fixed curves.

8. A tabletting press as claimed in claim 5 or 6 in which the angle of inclination of the axis of the rotatable disc is up to 60° relative to the punch axis.

9. A tabletting press as claimed in any one of the preceding claims in which the diameter of the rotatable disc is of the order of one half the pitch circle diameter of the punches.

Drawing