Title of Invention	METHOD FOR CONTROLLING A TABLET PRESS AND SUCH A PRESS
Abstract	A method for controlling a tablet press comprises the steps of consecutively supplying material into each die of a die table 1, subject- ing the material to a pre-compression and a main-compression, whereby the main-compression is performed under substantially constant compression force and variable resulting tablet thickness of the individual tablets, and measuring a weight value representative of the weight of the material fed into the die. The quantity of material supplied to each die is regulated on the basis of a deviation between a previously measured weight value and a first set value. The weight value is measured during pre-compression of the material located in each die. A rotary tablet press is also disclosed.

Title of Invention

METHOD FOR CONTROLLING A TABLET PRESS AND SUCH A PRESS

Abstract

A method for controlling a tablet press comprises the steps of consecutively supplying material into each die of a die table 1, subject- ing the material to a pre-compression and a main-compression, whereby the main-compression is performed under substantially constant compression force and variable resulting tablet thickness of the individual tablets, and measuring a weight value representative of the weight of the material fed into the die. The quantity of material supplied to each die is regulated on the basis of a deviation between a previously measured weight value and a first set value. The weight value is measured during pre-compression of the material located in each die. A rotary tablet press is also disclosed.

Full Text	Method for controlling a tablet press and such a press The present invention relates to a method for controlling a tab¬let press, whereby powder or granular material is compressed in dies ar¬ranged circumferentially in a rotary die table by means of reciprocating punches, said method comprising the steps: consecutively supplying a quantity of material to be compressed into each die, subjecting the quantity of materia! located in each die to a pre- compression and subsequently a main-compression, whereby the main-compression is performed under substan¬tially constant compression force and variable resulting tablet thickness of the individual tablets, measuring a weight value of a parameter representative of the weight of the quantity of material fed into the die, regulating the quantity of material supplied to each die on the basis of a deviation between a previously measured weight value and a first set value. EP 1 584 454 A2 (Courtoy N V) describes a method for control¬ling a rotary tablet press, whereby, during the main-compression, a hardness value of the tablet resulting from the compression is meas¬ured, and whereby the degree of compression that the quantity of mate¬rial located in each die is subjected to during main-compression is regu¬lated on the basis of a deviation between a previously measured hard¬ness value and a set value for the hardness. In this way, the mean tab¬let hardness of the produced tablets may be maintained within certain desired limits, although the resulting hardness of the individual tablets will vary slightly, which is indeed satisfactorily in most applications. DE 198 28 004 B4 describes a method for obtaining a constant compression force in a tablet press having computer controlled and by means of step motors adjustable compression rollers, whereby one of the compression rollers during each single compression of a tablet is po¬sitioned by positive and negative displacements, so that a predeter¬mined maximum compression force is maintained constant during a de¬fined time. However, the document is siient about, how the amount of the necessary displacement of a compression roller is determined, and how it is determined, whether the displacement should be positive or negative. Because the necessary displacement is dependent on powder properties, it is necessary to determine the compression behaviour of each specific powder to be compressed, and this is in practice a mayor disadvantage. In any case, the real-time control of the position of the compression rollers by means of step motors would be a very expensive solution. Furthermore, an industrial pellet press is known, whereby the main-compression Is performed under substantially constant compres¬sion force and variable resulting pellet thickness of the individual pellets, and whereby the weight of the produced pellets is controlled by regulat¬ing the quantity of material supplied to each die on the basis of a meas¬ured value corresponding to the pellet thickness resulting from the main-compression of a previously produced pellet. However, although the hardness of the resulting pellets is very consistent, the weight con¬trol is not accurate enough for a pharmaceutical tablet press. The object of the present invention is to provide a method for controlling a tablet press, whereby consistent tablet properties in terms of weight as well as hardness may be obtained. In view of this object, the method according to the invention is characterized by measuring the weight value during pre-compression of the quantity of material located in each die. By measuring the weight value during pre-compression, a more accurate measurement and consequently a more accurate weight con¬trol may be obtained. The measurement of a weight value during main- compression at constant compression force is less accurate than a measurement of a weight value during pre-compression, because the powder or granular material has already been compressed during pre- compression. Consequently, according to the invention, the weight may be controlled very accurately, and at the same time the density and thereby the hardness of the individual tablets may be maintained con¬sistent. This is very advantageous, especially if applied to a pharmaceu¬tical tablet press, because in such a press controlling both weight and hardness is of great importance. A consistent tablet hardness means consistent disintegration and dissolution of the tablets when swallowed, so that a consistent release profile and hence bioavailability of the pro¬duced tablets may be obtained. In an embodiment, the weight value corresponds substantially to a thickness of a tablet during pre-compression of said tablet under substantially constant compression force. At pre-compression, the com¬pression force is relatively small, and therefore the measurement of a value corresponding to the thickness of a tablet gives a rather accurate measurement of the weight of the tablet. Because both the pre- compression and the main-compression of each tablet is performed un¬der substantially constant compression force, the resulting density and therefore also the hardness of the individual tablets will be even more constant. A more constant tablet hardness means more constant disin¬tegration and dissolution of the tablet when swallowed, so that a sub¬stantially constant release profile and hence bioavailability of the pro¬duced tablets may be obtained. In an embodiment, the compression force of the pre- compression is maintained substantially constant by means of a pre- compression piston arranged displaceably in a gas cylinder, whereby the gas cylinder is supplied with compressed gas, and whereby the gas pressure in the gas cylinder is maintained substantially constant by means of a pressure regulator. By providing a suitable gas volume in the gas cylinder or in a separate vessel connected with this, the dis¬placements of the piston will in practice hardly change the gas pressure in the gas cylinder, and consequently, the compression force will be maintained substantially constant in real time when the piston is moving without any response time of a computer control loop implicated. In an embodiment, the powder or granular material is com¬pressed in the die between opposed first and second punches, each punch having first and second ends, whereby said first punch ends are received in the die, and said second punch ends, during pre- compression, interact with first and second pre-compression rollers, re¬spectively, whereby, during the pre-compression, the first pre- compression roller is displaced in the axial direction of the punches and the second pre-compression roller is fixed in said direction, and whereby the first pre-compression roller is carried by the pre-compression piston. In an embodiment, the weight value corresponds substantially to a pre-compression displacement value representative of a displace¬ment of the first pre-compression roller during pre-compression. In an embodiment, the weight value corresponds substantially to the maximum compression force exerted by a punch on a tablet dur¬ing pre-compression of said tablet to a predetermined tablet thickness. In an embodiment, the compression force of the main- compression is maintained substantially constant by means of a main- compression piston arranged displaceably in a gas cylinder, whereby the gas cylinder is supplied with compressed gas, and whereby the gas pressure in the gas cylinder is maintained substantially constant by means of a pressure regulator. By providing a suitable gas volume in the gas cylinder or in a separate vessel connected with this, the dis¬placements of the piston will in practice hardly change the gas pressure in the gas cylinder, and consequently, the compression force will be maintained substantially constant in real time when the piston is moving without any response time of a computer control loop implicated. In an embodiment, the movement of the main-compression pis¬ton is stopped by a dampening force after each main-compression. Thereby, the rotational speed of the die table may be increased without increasing noise and vibrations. In an embodiment, the dampening force is produced by a chamber containing compressed gas. The dampening force may thereby be varied by varying the pressure of the compressed gas. In an embodiment, the chamber containing compressed gas is a hollow ring of elastic material located between the main-compression piston and an abutment. In an embodiment, the dampening force is provided by a dampening piston arranged in a cylinder containing compressed gas. Thereby, the dampening force may be varied continuously by varying the pressure of the compressed gas, for instance by means of a pres¬sure regulator connected with the cylinder. In an embodiment, the dampening force is provided by a spring element. In an embodiment, the dampening force is provided by an elas¬tic O-ring located between the main-compression piston and an abut¬ment. In an embodiment, the dampening force is provided by an elas¬tic ring having rectangular cross-section and being located between the main-compression piston and an abutment. In an embodiment, the powder or granular material is com¬pressed in the die between opposed first and second punches, each punch having first and second ends, whereby said first punch ends are received in the die, and said second punch ends, during main- compression, interact with first and second main-compression rollers, respectively, whereby, during the main-compression, the first main- compression roller is displaced in the axial direction of the punches and the second main-compression roller is fixed in said direction, and whereby the first main-compression roller is carried by the main- compression piston. In this way, without having to decrease the rota¬tional speed of the die table, the dwell time of the tablets during main- compression may be increased, when compared to a prior art tablet press having fixed position of the main-compression rollers during com¬pression. Increased dwell time may be advantageous in order to obtain greater tablet hardness. Furthermore, the formulation of the powder or granular material to be compressed may be reworked in order to im¬prove the flowability of the material, whereby the lower compressibility that is the consequence of an improved flowability is compensated for by the increased dwell time. It is noted that flowability is inversely pro¬portional to compressibility. The improved flowability of the material is an advantage during handling of the material upstream the die table of the tablet press. In addition, a lower risk of tablet capping or tablet laminating may be obtained: an increase in dwell time will give more plastic defor¬mation, because plastic deformation is time dependent. This plastic de¬formation will in turn increase the tablet strength, so that it can better withstand the elastic recovery after ejection of the tablet. By increasing the plastic deformation the ratio between plastic deformation and brittle fracture will become higher, Consequently, because too much deforma¬tion by brittle fracture might give capping and laminating problems, in¬creasing the plastic deformation will give lesser capping or laminating problems, Increasing the dwell time may give a better deaeration of the powder bed and a better, more uniform particle rearrangement at com¬pression. This in turn will give less stress concentrations in the tablet. Less stress concentrations in the tablets will result in less tablets break¬ing in processing equipment downstream the tablet press, such as a tablet coater. This will in turn give less batch rejections. Less stress concentrations will also give less tablets breaking in packaging equip¬ment, like blister lines, and this will lead to lesser machine downtime and a higher productivity. Alternatively to increasing the dwell time, the rotational speed of the die table may be increased to arrive at the same dwell time as for the above mentioned prior art tablet press. Thereby, the production output rate may be increased. The dwell time is the time during which the compression force is at its maximum. In a prior art tablet press having fixed position of the main-compression rollers during compression, the dwell time is conse¬quently the time during which the flat end part of the second punch end rolls on the periphery of the main-compression roller and is therefore limited by the diameter of the flat end part. On the contrary, in a tablet press as described above, whereby the first main-compression roller is displaced during compression, the dwell time starts when the compres¬sion force balances the gas pressure in the gas cylinder, and the piston starts to move, which is before the flat end part of the second punch end starts rolling on the periphery of the main-compression roller. Cor¬respondingly, the dwell time ends, when the piston stops moving and hits the abutment, after the flat end part of the second punch end has stopped rolling on the periphery of the main-compression roller. There¬fore, in this case, the dwell time is not limited by the diameter of the flat end part. In case of problematic tablet formulations, an increased dwell time or a faster rotational speed of the die table may be an advantage. In an embodiment, a main-compression displacement value representative of a displacement of the first main-compression roller during main-compression is measured, and the position of the second main-compression roller in said direction is regulated on the basis of a deviation between a previously measured main-compression displace¬ment value and a second set value. The more the first main- compression roller is displaced during compression, the larger dwell time is obtained, provided that the rotational speed of the die table is maintained constant. By regulating the position of the second main- compression roller, it is possible to regulate the resulting displacement of the first main-compression roller and thereby to regulate the dwell time. Thereby, the above-mentioned tablet properties dependent on the dwell time may be controlled, so that more predictable results are pos¬sible. In an embodiment, said position regulation of the second main- compression roller is based on a mean value of several single measured main-compression displacement values. Thereby, fluctuations of the measured main-compression displacement value will not cause the con¬trol loop to overreact; instead, corrections to the position of the second main-compression roller will be based on progressive deviations regis¬tered by the control loop. In an embodiment, the position of the second main- compression roller is maintained constant as long as said mean value of the main-compression displacement value falls within preset correction tolerance limits. This will further prevent a possible tendency of the con¬trol loop to overreact, as corrections will only be performed when a measured value falls outside the preset iimits. In an embodiment, the position of the second main- compression roller is regulated so that the resulting main-compression displacement value is maintained substantially constant. Thereby, the dwell time will also be maintained substantially constant, whereby the above-mentioned tablet properties dependent on the dwell time will be substantially constant. The present invention further relates to a rotary tablet press comprising a housing and a rotary die table having a number of dies ar¬ranged circumferentially, each die being associated with first and second punches, each punch having first and second ends, said first punch ends being receivable in the die and arranged for compression of a powder or granular material in the die, the housing comprising a feeding device for the supply of mate¬rial to be compressed into the dies, a tablet discharge device for re¬moval of compressed material rn the form of tablets, and at least one pre-compression station and at least one main- compression station, each said compression station being provided with first and second compression rollers adapted to interact with the second punch ends, respectively, in order to perform compression of material located in the dies by reciprocation of the punches, the first main-compression compression roller of the main- compression station being supported by means of a main-compression piston arranged displaceably in a gas cylinder, the gas cylinder being connected to a supply of compressed gas, and a pressure regulator be¬ing adapted to maintain the gas pressure in the gas cylinder substan¬tially constant, the housing comprising a weight transducer for measuring a weight value of a parameter representative of the weight of a quantity of material fed into the die, a powder quantity regulator being provided for regulation of the quantity of material supplied to each die by the feeding device on the basis of a deviation between a previously measured weight value and a first set value. The rotary tablet press according to the invention is character¬ized in that the weight transducer is comprised by the pre-compression station. Thereby, the above-mentioned advantages may be achieved. In an embodiment, the first compression roller of the pre- compression station is supported by means of a piston arranged dis- piaceabfy in a gas cylinder, whereby the gas cylinder is connected to a supply of compressed gas, and whereby a pressure regulator is adapted to maintain the gas pressure in the gas cylinder substantially constant. Because both the pre-compression and the main-compression of each tablet is performed under substantially constant compression force, the resulting density and therefore also the hardness of the individual tab¬lets will be even more constant. Thereby, a substantially constant re¬lease profile and hence bioavailability of the produced tablets may be obtained. In an embodiment, the weight transducer of the pre- compression station has the form of a displacement transducer for measuring a pre-compression displacement value representative of a displacement of the piston in the gas cylinder. At pre-compression, the compression force is relatively small, and therefore the measurement of a value corresponding to the thickness of a tablet gives a rather accu¬rate measurement of the weight of the tablet. In an embodiment, a dampening element is provided between the piston of the main-compression station and an abutment. Thereby, the above-mentioned advantages may be achieved. In an embodiment, the dampening element has the form of a chamber containing compressed gas. Thereby, the above-mentioned advantages may be achieved. In an embodiment, the chamber containing compressed gas is a hollow ring of elastic material. Thereby, the above-mentioned advan¬tages may be achieved. In an embodiment, the dampening element has the form of a dampening piston arranged in a cylinder containing compressed gas. Thereby, the above-mentioned advantages may be achieved. In an embodiment, the dampening element has the form of a spring element. Thereby, the above-mentioned advantages may be achieved. In an embodiment, the dampening element has the form of an elastic O-ring. Thereby, the above-mentioned advantages may be achieved, In an embodiment, the dampening element has the form of an elastic ring having rectangular cross-section. In an embodiment, the main-compression station comprises a displacement transducer for measuring a main-compression displacement value representative of a displacement of the piston in the gas cylinder, and a position regulator for regulation of the position of the second main-compression roller on the basis of a deviation between a previ¬ously measured main-compression displacement value and a second set value. Thereby, the above-mentioned advantages may be achieved. In an embodiment, said position regulator is adapted to regu¬late the position of the second main-compression roller on the basis of a mean value of several single measured main-compression displacement values. Thereby, the above-mentioned advantages may be achieved. In an embodiment, said position regulator is adapted to main¬tain the position of the second main-compression roller constant as long as said mean value of the main-compression displacement value falls within preset correction tolerance limits. Thereby, the above-mentioned advantages may be achieved. In an embodiment, the first main-compression roller of the main-compression station is located above the rotary die table. This is advantageous, if the space below the rotary die table is limited. In an embodiment, the pressure regulator is adapted to main¬tain the gas pressure in the gas cylinder at or below 30 bars. Thereby, a simpler and consequently cheaper pressure regulator may be employed. In an embodiment, the total weight of the first main- compression roller, the main-compression piston, a yoke carrying the first main-compression roller and supplementary parts displaceable with the main-compression piston is less than 30 kg. Thereby, the rotational speed of the die table may be further increased without increasing noise and vibrations. The invention will now be explained in more detail below by means of examples of embodiments with reference to the very sche¬matic drawing, in which Fig. 1 shows in diagrammatic form an embodiment of a rotary tablet press with a control system according to the invention. Fig. 2 is a side view of a part of the feeding device of Fig. 1, Fig. 3 is a side view of the pre-compression station of Fig. 1, Fig. 4 is a side view of the main-compression station of Fig. 1 and Fig. 5 is a side view of another embodiment of the main- compression station in Fig. 4. Fig. 1 shows in diagrammatic form an embodiment of a rotary tablet press with a control system according to the invention. The tablet press has a rotary die table 1 for compression of a feedstock in the form of powder or granular material into tablets, compacts or the like. The press is of a type suitable for use in the pharmaceutical industry, but the press according to the invention may as well be a so-called industrial press employed in the production of a variety of different products, such as vitamins, pet food, detergents, explosives, ceramics, batteries, balls, bearings, nuclear fuels, etc. The tablet press is provided with a feeding device in the form of a well-known double rotary feeder with two not shown rotary paddles lo¬cated in a feeder housing and driven by means of separate drive motors providing for independent speed setting of the paddles. The feeder hous¬ing is open against the die table so that the paddles may ensure proper filling of the dies with feedstock. Other feeding systems may also be em¬ployed, such as a so-called gravity feeder or a vibration feeder. Fig. 2 shows a fill depth adjusting device 2 which in this descrip¬tion will be considered as a part of the feeding device. The rotary feeder itself is not shown in Fig. 2> The fill depth adjusting device 2 comprises a vertically displaceable cam 3 determining the vertical position of lower punches 4 at the feeding device, thereby determining the fill depth of the die. The fill depth determines in a manner known per se the quantity of material left in the dies for compression. The lower punches 4 have first ends 6 received in corresponding dies 7 of the die table 1 and second ends 8 sliding on the vertically displaceable cam 3. Upper punches 5 are maintained outside the dies 7 at this stage in order to permit filling of the dies. The vertical position of the cam 3 is adjusted by means of a linear actuator 9 in accordance with a fill depth signal received from a powder quantity regulator shown in Fig. 1. Fig. 3 shows a pre-compression station 10 comprising a lower compression roller 11 and an upper compression roller 12. The upper compression roller 12 is suspended in a piston 13 vertically displaceable within a gas cylinder 14, The gas pressure in the gas cylinder 14 is maintained constant by means of a not shown regulation system. In particular, the regulation system comprises a not shown air reservoir that is so large that limited displacements of the piston 13 within the gas cylinder 14 will, in practise, not affect the pressure in the gas cylin¬der 14. The air reservoir may, for instance, have a volume of 1 litre and the total system including the gas cylinder 14 may then, for instance, have a total volume of 1.5 litres. The vertical position of the piston 13 is measured by means of a displacement transducer 15, such as a LVDT (Linear Variable Differential Transformer). When an upper punch 5 passes under the centre of the upper compression roller 12, the dis¬placement transducer 15 measures a displacement substantially corre¬sponding to the thickness of the tablet after the pre-compression. Be¬cause the compression is being performed with a constant force being applied to the upper punch 5 by means of the piston 13, the displace¬ment measured by the displacement transducer 15 corresponds to the weight of the tablet compressed and thereby constitutes a weight value. At each pre-compression of a tablet, the displacement measured by the displacement transducer 15 is transferred in the form of a displacement signal to the powder quantity regulator and the control unit; see Fig. 1. Because the displacement at pre-compression is greater than at main- compression, a better sensitivity of the control loop is obtained by measuring the displacement at pre-compression instead of at main- compression. In the control unit, the displacement signal supplied for each tablet produced is compared with predetermined rejection tolerance lim¬its defining the maximum acceptable deviation from a desired tablet weight. If the displacement signal for a tablet falls outside the rejection tolerance limits, a rejection signal is sent from the control unit to a re¬jection device associated with a tablet discharge device, and the tablet is separated from the remaining tablets, when it reaches the rejection device, see Fig, 1. In the powder quantity regulator, a rigid or floating mean value of the displacement signal for several consecutive tablets is compared with a first set value that corresponds to a calibrated desired tablet weight and is received from the control unit. If the deviation falls out¬side preset first correction tolerance limits, the fill depth signal supplied to the feeding device is corrected correspondingly. Said correction toler¬ance limits may be calculated automatically by a general control system on the basis of user defined acceptable deviations, for instance in the form of percentage values, from the desired tablet weight. From the tablet discharge device the tablets are fed to an automatic testing device, for example a Kraemer Electronic Tablet Tester, in which the weight and hardness of a number of sample tablets are determined periodically, and whereby corresponding weight and hardness signals are transferred to the control unit, see Fig. 1. In the control unit, the weight signal received from the automatic testing de¬vice is compared with the desired tablet weight, and on the basis of the deviation between these values, a bottom roller height signal is gener¬ated and transferred to the pre-compression station. In the pre- compression station, the bottom roller height signal is fed into a linear actuator 16, which adjusts the height of the bottom compression roller 11 correspondingly; see Fig. 3. As a result, the powder quantity regula¬tor registers the change and adapts thereto. In an alternative embodi¬ment, the vertical position of the air cylinder 14 could be adjusted by means of a linear actuator. Thereby, the powder quantity regulation loop is re-calibrated on the basis of the actual tablet weights of the sampled tablets measured by the automatic testing device. It should be noted that said re-calibration could also be performed by adjustment of the first set value supplied to the powder quantity regulator by the con¬trol unit or by adjustment of the otherwise constant air pressure in the air cylinder 14. Furthermore, instead of using an automatic testing de¬vice, a number of sample tablets may be tested manually, and a meas¬ured weight and possibly hardness may then be entered in the general control system. Alternatively, the pre-compression station 10 may have a fixed distance between the lower compression roller 11 and the upper com¬pression roller 12, and the displacement transducer 15 may then be re¬placed by a strain gauge provided on the shaft of one of the compression rollers 11, 12 and by means of which a force signal is supplied to the powder quantity regulator and the control unit. The force signal then constitutes the weight value representative of the weight of the quantity of material fed into the die. Fig, 4 shows a main-compression station 17 comprising a lower compression roller IS and an upper compression roller 20. The upper compression roller 20 is suspended in a piston 21 vertically displaceable within a gas cylinder 22. The gas pressure in the gas cylinder 22 is maintained constant by means of a not shown regulation system. As ex¬plained above regarding the pre-compression station, also the not shown regulation system of the main-compression station may comprise an air reservoir in order to maintain constant pressure. Because the compression is being performed with a constant force being applied to the upper punch 5 by means of the piston 21, the resulting hardness of the individual tablets will be substantially constant. Thereby, a substan¬tially constant release profile and hence bioavailability of the produced tablets may be obtained. The vertical position of the piston 21 is measured by means of a displacement transducer 23, such as a LVDT (Linear Variable Differential Transformer). When an upper punch 5 passes under the centre of the upper compression roller 20, the displacement transducer 23 measures a displacement substantially corresponding to the thickness of the tablet after the main-compression. The displacement measured by the dis¬placement transducer 23 is also representative of the dwell time, that is, the period of time during which rie tablet is compressed by the maxi¬mum constant compression force. At each main-compression of a tablet, the displacement measured by the displacement transducer 23 is trans¬ferred in the form of a displacement signal to the dwell time regulator and the control unit; see Fig. 1. In the dwell time regulator, a rigid or floating mean value of the displacement signal for several consecutive tablets is compared with a second set value that corresponds to a calibrated desired tablet dwell time and is received from the control unit. If the deviation falls outside preset second correction tolerance limits, a bottom roller height signal is generated and transferred to the main-compression station 17. In the main-compression station 17, the bottom roller height signal is fed into a linear actuator 19, which adjusts the height of the bottom compres¬sion roller 18 correspondingly; see Fig. 4. In this way, the dwell time during main-compression of the individual tablets may be maintained substantially constant, and consequently the above-mentioned tablet properties dependent on the dwell time will alsc be substantially con¬stant. The dwell time regulation may counteract the tendency of the dwell time to change as a result of changing compacting properties of the material compressed in the. die. Changing compacting properties may be the result of a change in the humidity, the temperature, and the mean particle size over a batch, etc. However, according to the inven¬tion, the dwell time regulation may be omitted, and satisfying tablet properties may nevertheless be obtained. In the control unit, the hardness signal received from the auto¬matic testing device is compared with the desired tablet hardness, and on the basis of the deviation between these values, a re-calibration may be performed by adjustment of the second set value supplied to the dwell time regulator by the control unit. Alternatively, said re-calibration could be performed by adjustment of the otherwise constant air pres¬sure in the air cylinder 22 of the main-compression station 17. As it is seen in Fig. 4, an elastic ring 24 having rectangular cross-section is located between the main-compression piston 21 and an abutment 24 in the form of an inwardly directed, lower shoulder 25 of the gas cylinder 22. The abutment 24 provides a dampening force for the piston 21, whereby the rotational speed of the die table may be in¬creased without increasing noise and vibrations from the piston 21 and cylinder 22. This is especially advantageous in pharmaceutical tablet presses that generally run much faster than industrial presses. The dampening abutment 24 may have any other suitable configuration, for instance, a chamber containing compressed gas, a hollow ring of elastic material, a spring element, or an elastic O-ring. Furthermore, the dampening force for the piston 21 may be provided by a dampening pis¬ton 26 arranged in a cylinder 27 containing compressed gas, as it may be seen in Fig. 5, In order to further increase the rotational speed of the die table, the piston 21 may be made of a light weight material, such as titanium, for instance. Preferably the total weight of the piston 21 and other moving parts associated therewith does not exceed 40 kilos and preferably does not exceed 30 kilos. In addition, high-speed piston seals may be used for the piston 21 in order to further improve the rotational speed of the die table. Obviously, the invention is equally applicable to so-called sin¬gle-sided, double-sided or multi-sided tablet presses. For instance, in a double-sided press for the production of tablets having two layers, a first layer production section and a second layer production section, ar¬ranged along opposite sides of the die table, each has both a pre- compression station and a main-compression station. In this case, how¬ever, the first layer is compressed to a fixed thickness at main- compression in order to better be able to regulate the quantity of the second material supplied to each die. A substantially constant hardness of the entire tablet is obtained by performing the main-compression of the second layer under substantially constant compression force and variable resulting tablet thickness of the individual tablets, in the same way as explained above for a single-sided press. Similarly, in a press for production of tablets having more than two layers, the main- compression is performed under substantially constant compression force and variable resulting tablet thickness only for the last layer of the tablet. The other layers are compressed to a fixed thickness at main- compression. In a double-sided press for the production of single layer tab¬lets, two similar production sections each corresponding to that of a sin¬gle-sided press are provided, arranged along opposite sides of the die table, and each has both a pre-compression station, a main- compression station, a feeding device, and a tablet discharge device. Each production section is provided with both a powder quantity regula¬tor and a dwell time regulator. PATENT CLAIMS r A method for controlling a tablet press, whereby powder or material is compressed In dies arranged circumferentially in a nuiar! n ary cth talkie by means of reciprocating punches, said method com- pf $ifig tt e steps: \|icmsecutively supplying a quantity of material to be compressed o !»ch die, j subjecting the quantity of material located in each die to a pre- ctifoaresfiion and subsequently a main-compression, jnea?uring a weight value of a parameter representative of the vi\|\|jgfit o\|f the I quantity of material fed into the dier egujating the quantity of material supplied to each die on the Sid of deviation between a previously measured weight value and a set jraluq, and jnea^urinfl the weight value during pre-compression of the qflarsjtityj of material located in each die, \|:hairacterized in that the main-compression is per- f .1 ' (ting tjiblet thickness of the individual tablets, j whereby the powder or granular material is compressed in the li etvysen Opposed first and second punches, each punch having first d \|^ec\|n4 ejnds, whereby said first punch ends are received in the die, secjond punch ends, during main-compression, interact with iial direction of the punches and the second main-compression ■ isjixedj in said direction, and whereby the first main-compression ■ is carried by the main-compression piston, thereby a main-compression displacement value representa- [' a displacement of the first main-compression roller during main- mi >re^sion; is measured,' and the position of the second main- mprH5'0'1 roller in said direction is regulated on the basis of a devia- betfveerj a previously measured main-compression displacement (u^rj^ ^econd set value, hereby said position regulation of the second main- p\|res{ ion palfer js based an a mean value of several single measured compression displacement values, wherpby the position of the second main-compression roller is constant as long as said mean value of the msin- ^ras^iop displacement value falls within preset correction tolerance l": jyherjeby the position of the second main-compression roller Is rJfciriatefi s that the resulting main-compression displacement value is ni nnjtair^ ad substantially constant, \l. Almethod for controlling a tablet press according to claim 1, W jerebyi the jweight value corresponds substantially to a thickness of a tii >let d\|\|ringj pre-compression of said tablet under substantially con- idnt copprejssion force, ;P I J U v t'i[e SI s id 13. AI met hod for controlling a tablet press according to claim l wijereby the compression force of the pre-compression is main- nsubstantially constant by means of a pre-compression piston ar- id ijispl^ceably in a gas cylinder, whereby the gas cylinder is sup- vyip cajmpressed gas, and whereby the gas pressure in the gas linger jis maintained substantially constant by means of a pressure aftoj'. fl. Amethod for controlling a tablet press according to claim 3, lenebyj the pqwder or granular material is compressed in the die be¬an opposed first and second punches, each punch having first and eaid whereby said first punch ends are received in the die, and Sfiscj>n4 punch encjs, during pre-compression, interact with first and id j re-qompr^ssion rollers, respectively, whereby, during the pre- m;>res;ion, the fjrst pre-compression roller Is displaced in the axial di- r qtun cf th^ punches and the second pre-compression roller is fixed in sis jd dir^itforj, andj whereby the first pre-compression roller is carried by e i>re-j;ompressi(j>n piston. :S. A method for controlling a tablet press according to claim 4, leieby thp weight value corresponds substantially to a pre- ;C m jne^5ionjdi?pla\|cement value representative of a displacement of the impression roller during pre-compression. i. A'method for controlling a tablet press according to claim 1, the I weight value corresponds substantially to the maximum on; force exerted by a punch on a tablet during pre- on pf said tablet to a predetermined tablet thickness. . A :method for controlling a tablet pressaccording to any one feceding claims, whereby the compression force of the main- ion lis maintained substantially constant by means of a main- ion piston arranged displaceably in a gas cylinder, whereby the dep i is supplied with compressed gas, and whereby the gas in the gas cylinder is maintained substantially constant by ! a pressure regulator. i. A 'method for controlling a tabiet press according to claim 7, the movement of the ma in-compression piston is stopped by a ng fqrcp after each main-compression. . A method for controlling a tablet press according to claim a, the\| dampening force is produced by a chamber containing d ^jas. ; 10, A method for controlling a tablet press according to claim 9, i ' reby c vr e £1 W rihg wf fc n> sd I eby tie tw W1 :b Vie t rig itq H the; chamber containing compressed gas is a hollow ring of aterial: located between the main-compression piston and an utjtierit. j ': U. A method for controlling a tablet press according to claim 8, jeijeby the; dampening force is provided by a dampening piston ar- a cylinder containing compressed gas. L2. >jk method for controlling a tablet press according to claim 8, the dampening force is provided by a spring element. 13,- A method for controlling a tablet press according to claim 8, thejdampening force is provided by an elastic O-ring located eenj the main-compression piston and an abutment. fi pre- ie\|ayl V. c le m ressi \|>res\|M di eby VI' c re imp L4. A method for controlling a tablet press according to claim 8, eby the dampening force is provided by an elastic ring having rec- Mlar cross-section and being located between the main-compression n arid ari abutment. 5. /j rotary tablet press comprising a housing and a rotary die havling ^ number of dies arranged circumferentiaily, each die being ip^iated with first and second punches, each punch having first and o\|d end?, said first punch ends being receivable in the die and ar- r#^sd fjir compression of a powder or granular material in the die, \|he Housing comprising a feeding device far the supply of mate¬ria tjo bf; compressed into the dies, a tablet discharge device for re- rnlpv^l of compressed material in the form of tablets, and \it lebst one pre-compression station' and at least one main- i i n It p: rppresjiion Ration, each said compression station being provided with second compression rollers adapted to interact with the second ndi er dsf respectively, in order to perform compression of material 3tjsd i\|i the dies by reciprocation of the punches/ he housing comprising a weight transducer for measuring a light vi slue jof a parameter representative of the weight of a quantity mater al ffd into the die, the weight -transducer being comprised by f rebornpjression station, U ppjvder quantity regulator being provided for regulation of the anjtity of material supplied to each die by the feeding device on the ofdeviation between a previously measured weight value and a st set I'aM, :ha\|racterized in that the first ma in-compression com- 5S\|iion! roller .of the main-compression station being supported by 1 is q ' a main-compression piston arranged displaceably in a gas cyl- ; thje g'ais cylinder being connected to a supply of compressed gas, 3 pr »£$qreregu)ator being adapted to maintain the gas pressure in whifpejn the main-compression station comprises 3 displacement transducer for measuring a main-compression spfecenent value representative of a displacement of the piston in the s cylinder, .and \t portion regulator for regulation of the position of the second rffeiri-cajnpFejsslon roller on the basis of a deviation between a previ- n^asuiW ma rn-compression displacement value and a second set snd vtherein said position regulator is adapted to regulate the posi- ;l{fi ft tH» ?^ond main-compression roller on the basis of a mean value r^ single measured main-compression displacement values, and Wherein said position regulator is adapted to maintain the posi- Df tie sjacond main-compression roller constant as long as said ysjue pf the main-compression displacement value fails within se t cq Teptfion toleran ce I im its, 1.6. A rotary tablet press according to claim 15, wherein the life romprespion roller of the pre-compressfon statfon is supported by jpiis oi a piston arranged displaceably in a gas cylinder, whereby the i i!y}fr¥ ier is connected to a supply of compressed gas, and whereby a $s? ure regulator is adapted to maintain the gas pressure in the gas inter substantially constant. .7. A rotary tablet press according to claim 16, wherein the tj^it tjanstjueer of the pre-compression station has the form of a dis- j«ment transducer for measuring a pre-compression displacement rej resejntative of a displacement of the piston in the gas cylinder. :.8. A rptary tablet press according to any one of the claims 15 w wejn a dampening element is provided between the piston of r]nair\|-con^presslon station and an abutment. amber c; m;>eni si rmeni L9. A rotary tablet press according to claim 18, wherein the mbenhg ejerpent has the form of a chamber containing compressed p. f rotary tablet press according to claim 19, wherein the containing compressed gas is a hojlow ring of elastic material. II.; A rotary tablet press according to claim 18, wherein the ig ejerpent has the form of a dampening piston arranged in a dfijrjjderMont^inlng compressed gas. 22, jv rotary tablet press according to daim 18, wherein the rripeni igejement has the form of g spring element. 23. f rotary tablet press according to claim IS, wherein the ig e\|ement has the form of an elastic o-ring. >4. ^ rotary tablet press according to claim 18, wherein the d^rienijbg element has the form of an elastic ring having rectangular is^-seetiqn. : j ; :5. A rotary tablet press according to any; one of the claims 15 24,; wherein the first main-compression roller of the main- f res? ion station is located above the rotary die table. ;»6, 4 rotary tablet press according to any one of the claims 15 wierein the pressure regulator is adapted to maintain the gas iff s$ ur® in jtje gas cylinder at or below 30 bars. to IV ^ rotary tablet press according to any one of the claims 15 it, wherein the total weight of the first main-compression roller, the ti ilnj-conpne^slon piston, a yoke carrying the first main-compression \|eir ariil supplementary parts displaceable with the main-compression to^> ts \|es?\|than 30 kg.

Full Text

Method for controlling a tablet press and such a press
The present invention relates to a method for controlling a tab¬let press, whereby powder or granular material is compressed in dies ar¬ranged circumferentially in a rotary die table by means of reciprocating punches, said method comprising the steps:
consecutively supplying a quantity of material to be compressed into each die,
subjecting the quantity of materia! located in each die to a pre- compression and subsequently a main-compression,
whereby the main-compression is performed under substan¬tially constant compression force and variable resulting tablet thickness of the individual tablets,
measuring a weight value of a parameter representative of the weight of the quantity of material fed into the die,
regulating the quantity of material supplied to each die on the basis of a deviation between a previously measured weight value and a first set value.
EP 1 584 454 A2 (Courtoy N V) describes a method for control¬ling a rotary tablet press, whereby, during the main-compression, a hardness value of the tablet resulting from the compression is meas¬ured, and whereby the degree of compression that the quantity of mate¬rial located in each die is subjected to during main-compression is regu¬lated on the basis of a deviation between a previously measured hard¬ness value and a set value for the hardness. In this way, the mean tab¬let hardness of the produced tablets may be maintained within certain desired limits, although the resulting hardness of the individual tablets will vary slightly, which is indeed satisfactorily in most applications.
DE 198 28 004 B4 describes a method for obtaining a constant compression force in a tablet press having computer controlled and by means of step motors adjustable compression rollers, whereby one of the compression rollers during each single compression of a tablet is po¬sitioned by positive and negative displacements, so that a predeter¬mined maximum compression force is maintained constant during a de¬fined time. However, the document is siient about, how the amount of the necessary displacement of a compression roller is determined, and how it is determined, whether the displacement should be positive or negative. Because the necessary displacement is dependent on powder properties, it is necessary to determine the compression behaviour of each specific powder to be compressed, and this is in practice a mayor disadvantage. In any case, the real-time control of the position of the compression rollers by means of step motors would be a very expensive solution.
Furthermore, an industrial pellet press is known, whereby the main-compression Is performed under substantially constant compres¬sion force and variable resulting pellet thickness of the individual pellets, and whereby the weight of the produced pellets is controlled by regulat¬ing the quantity of material supplied to each die on the basis of a meas¬ured value corresponding to the pellet thickness resulting from the main-compression of a previously produced pellet. However, although the hardness of the resulting pellets is very consistent, the weight con¬trol is not accurate enough for a pharmaceutical tablet press.
The object of the present invention is to provide a method for controlling a tablet press, whereby consistent tablet properties in terms of weight as well as hardness may be obtained.
In view of this object, the method according to the invention is characterized by measuring the weight value during pre-compression of the quantity of material located in each die.
By measuring the weight value during pre-compression, a more accurate measurement and consequently a more accurate weight con¬trol may be obtained. The measurement of a weight value during main- compression at constant compression force is less accurate than a measurement of a weight value during pre-compression, because the powder or granular material has already been compressed during pre- compression. Consequently, according to the invention, the weight may be controlled very accurately, and at the same time the density and thereby the hardness of the individual tablets may be maintained con¬sistent.
This is very advantageous, especially if applied to a pharmaceu¬tical tablet press, because in such a press controlling both weight and hardness is of great importance. A consistent tablet hardness means consistent disintegration and dissolution of the tablets when swallowed, so that a consistent release profile and hence bioavailability of the pro¬duced tablets may be obtained.
In an embodiment, the weight value corresponds substantially to a thickness of a tablet during pre-compression of said tablet under substantially constant compression force. At pre-compression, the com¬pression force is relatively small, and therefore the measurement of a value corresponding to the thickness of a tablet gives a rather accurate measurement of the weight of the tablet. Because both the pre- compression and the main-compression of each tablet is performed un¬der substantially constant compression force, the resulting density and therefore also the hardness of the individual tablets will be even more constant. A more constant tablet hardness means more constant disin¬tegration and dissolution of the tablet when swallowed, so that a sub¬stantially constant release profile and hence bioavailability of the pro¬duced tablets may be obtained.
In an embodiment, the compression force of the pre- compression is maintained substantially constant by means of a pre- compression piston arranged displaceably in a gas cylinder, whereby the gas cylinder is supplied with compressed gas, and whereby the gas pressure in the gas cylinder is maintained substantially constant by means of a pressure regulator. By providing a suitable gas volume in the gas cylinder or in a separate vessel connected with this, the dis¬placements of the piston will in practice hardly change the gas pressure in the gas cylinder, and consequently, the compression force will be maintained substantially constant in real time when the piston is moving without any response time of a computer control loop implicated.
In an embodiment, the powder or granular material is com¬pressed in the die between opposed first and second punches, each punch having first and second ends, whereby said first punch ends are received in the die, and said second punch ends, during pre- compression, interact with first and second pre-compression rollers, re¬spectively, whereby, during the pre-compression, the first pre- compression roller is displaced in the axial direction of the punches and the second pre-compression roller is fixed in said direction, and whereby the first pre-compression roller is carried by the pre-compression piston.
In an embodiment, the weight value corresponds substantially to a pre-compression displacement value representative of a displace¬ment of the first pre-compression roller during pre-compression.
In an embodiment, the weight value corresponds substantially to the maximum compression force exerted by a punch on a tablet dur¬ing pre-compression of said tablet to a predetermined tablet thickness.
In an embodiment, the compression force of the main- compression is maintained substantially constant by means of a main- compression piston arranged displaceably in a gas cylinder, whereby the gas cylinder is supplied with compressed gas, and whereby the gas pressure in the gas cylinder is maintained substantially constant by means of a pressure regulator. By providing a suitable gas volume in the gas cylinder or in a separate vessel connected with this, the dis¬placements of the piston will in practice hardly change the gas pressure in the gas cylinder, and consequently, the compression force will be maintained substantially constant in real time when the piston is moving without any response time of a computer control loop implicated.
In an embodiment, the movement of the main-compression pis¬ton is stopped by a dampening force after each main-compression. Thereby, the rotational speed of the die table may be increased without increasing noise and vibrations.
In an embodiment, the dampening force is produced by a chamber containing compressed gas. The dampening force may thereby be varied by varying the pressure of the compressed gas.
In an embodiment, the chamber containing compressed gas is a hollow ring of elastic material located between the main-compression piston and an abutment.
In an embodiment, the dampening force is provided by a dampening piston arranged in a cylinder containing compressed gas.
Thereby, the dampening force may be varied continuously by varying the pressure of the compressed gas, for instance by means of a pres¬sure regulator connected with the cylinder.
In an embodiment, the dampening force is provided by a spring
element.
In an embodiment, the dampening force is provided by an elas¬tic O-ring located between the main-compression piston and an abut¬ment.
In an embodiment, the dampening force is provided by an elas¬tic ring having rectangular cross-section and being located between the main-compression piston and an abutment.
In an embodiment, the powder or granular material is com¬pressed in the die between opposed first and second punches, each punch having first and second ends, whereby said first punch ends are received in the die, and said second punch ends, during main- compression, interact with first and second main-compression rollers, respectively, whereby, during the main-compression, the first main- compression roller is displaced in the axial direction of the punches and the second main-compression roller is fixed in said direction, and whereby the first main-compression roller is carried by the main- compression piston. In this way, without having to decrease the rota¬tional speed of the die table, the dwell time of the tablets during main- compression may be increased, when compared to a prior art tablet press having fixed position of the main-compression rollers during com¬pression. Increased dwell time may be advantageous in order to obtain greater tablet hardness. Furthermore, the formulation of the powder or granular material to be compressed may be reworked in order to im¬prove the flowability of the material, whereby the lower compressibility that is the consequence of an improved flowability is compensated for by the increased dwell time. It is noted that flowability is inversely pro¬portional to compressibility. The improved flowability of the material is an advantage during handling of the material upstream the die table of the tablet press.
In addition, a lower risk of tablet capping or tablet laminating may be obtained: an increase in dwell time will give more plastic defor¬mation, because plastic deformation is time dependent. This plastic de¬formation will in turn increase the tablet strength, so that it can better withstand the elastic recovery after ejection of the tablet. By increasing the plastic deformation the ratio between plastic deformation and brittle fracture will become higher, Consequently, because too much deforma¬tion by brittle fracture might give capping and laminating problems, in¬creasing the plastic deformation will give lesser capping or laminating problems,
Increasing the dwell time may give a better deaeration of the powder bed and a better, more uniform particle rearrangement at com¬pression. This in turn will give less stress concentrations in the tablet. Less stress concentrations in the tablets will result in less tablets break¬ing in processing equipment downstream the tablet press, such as a tablet coater. This will in turn give less batch rejections. Less stress concentrations will also give less tablets breaking in packaging equip¬ment, like blister lines, and this will lead to lesser machine downtime and a higher productivity.
Alternatively to increasing the dwell time, the rotational speed of the die table may be increased to arrive at the same dwell time as for the above mentioned prior art tablet press. Thereby, the production output rate may be increased.
The dwell time is the time during which the compression force is at its maximum. In a prior art tablet press having fixed position of the main-compression rollers during compression, the dwell time is conse¬quently the time during which the flat end part of the second punch end rolls on the periphery of the main-compression roller and is therefore limited by the diameter of the flat end part. On the contrary, in a tablet press as described above, whereby the first main-compression roller is displaced during compression, the dwell time starts when the compres¬sion force balances the gas pressure in the gas cylinder, and the piston starts to move, which is before the flat end part of the second punch end starts rolling on the periphery of the main-compression roller. Cor¬respondingly, the dwell time ends, when the piston stops moving and hits the abutment, after the flat end part of the second punch end has stopped rolling on the periphery of the main-compression roller. There¬fore, in this case, the dwell time is not limited by the diameter of the flat end part.
In case of problematic tablet formulations, an increased dwell time or a faster rotational speed of the die table may be an advantage.
In an embodiment, a main-compression displacement value representative of a displacement of the first main-compression roller during main-compression is measured, and the position of the second main-compression roller in said direction is regulated on the basis of a deviation between a previously measured main-compression displace¬ment value and a second set value. The more the first main- compression roller is displaced during compression, the larger dwell time is obtained, provided that the rotational speed of the die table is maintained constant. By regulating the position of the second main- compression roller, it is possible to regulate the resulting displacement of the first main-compression roller and thereby to regulate the dwell time. Thereby, the above-mentioned tablet properties dependent on the dwell time may be controlled, so that more predictable results are pos¬sible.
In an embodiment, said position regulation of the second main- compression roller is based on a mean value of several single measured main-compression displacement values. Thereby, fluctuations of the measured main-compression displacement value will not cause the con¬trol loop to overreact; instead, corrections to the position of the second main-compression roller will be based on progressive deviations regis¬tered by the control loop.
In an embodiment, the position of the second main- compression roller is maintained constant as long as said mean value of the main-compression displacement value falls within preset correction tolerance limits. This will further prevent a possible tendency of the con¬trol loop to overreact, as corrections will only be performed when a measured value falls outside the preset iimits.
In an embodiment, the position of the second main- compression roller is regulated so that the resulting main-compression displacement value is maintained substantially constant. Thereby, the dwell time will also be maintained substantially constant, whereby the above-mentioned tablet properties dependent on the dwell time will be substantially constant.
The present invention further relates to a rotary tablet press comprising a housing and a rotary die table having a number of dies ar¬ranged circumferentially, each die being associated with first and second punches, each punch having first and second ends, said first punch ends being receivable in the die and arranged for compression of a powder or granular material in the die,
the housing comprising a feeding device for the supply of mate¬rial to be compressed into the dies, a tablet discharge device for re¬moval of compressed material rn the form of tablets, and
at least one pre-compression station and at least one main- compression station, each said compression station being provided with first and second compression rollers adapted to interact with the second punch ends, respectively, in order to perform compression of material located in the dies by reciprocation of the punches,
the first main-compression compression roller of the main- compression station being supported by means of a main-compression piston arranged displaceably in a gas cylinder, the gas cylinder being connected to a supply of compressed gas, and a pressure regulator be¬ing adapted to maintain the gas pressure in the gas cylinder substan¬tially constant,
the housing comprising a weight transducer for measuring a weight value of a parameter representative of the weight of a quantity of material fed into the die,
a powder quantity regulator being provided for regulation of the quantity of material supplied to each die by the feeding device on the basis of a deviation between a previously measured weight value and a first set value.
The rotary tablet press according to the invention is character¬ized in that the weight transducer is comprised by the pre-compression
station. Thereby, the above-mentioned advantages may be achieved.
In an embodiment, the first compression roller of the pre- compression station is supported by means of a piston arranged dis- piaceabfy in a gas cylinder, whereby the gas cylinder is connected to a supply of compressed gas, and whereby a pressure regulator is adapted to maintain the gas pressure in the gas cylinder substantially constant. Because both the pre-compression and the main-compression of each tablet is performed under substantially constant compression force, the resulting density and therefore also the hardness of the individual tab¬lets will be even more constant. Thereby, a substantially constant re¬lease profile and hence bioavailability of the produced tablets may be obtained.
In an embodiment, the weight transducer of the pre- compression station has the form of a displacement transducer for measuring a pre-compression displacement value representative of a displacement of the piston in the gas cylinder. At pre-compression, the compression force is relatively small, and therefore the measurement of a value corresponding to the thickness of a tablet gives a rather accu¬rate measurement of the weight of the tablet.
In an embodiment, a dampening element is provided between the piston of the main-compression station and an abutment. Thereby, the above-mentioned advantages may be achieved.
In an embodiment, the dampening element has the form of a chamber containing compressed gas. Thereby, the above-mentioned advantages may be achieved.
In an embodiment, the chamber containing compressed gas is a hollow ring of elastic material. Thereby, the above-mentioned advan¬tages may be achieved.
In an embodiment, the dampening element has the form of a dampening piston arranged in a cylinder containing compressed gas. Thereby, the above-mentioned advantages may be achieved.
In an embodiment, the dampening element has the form of a spring element. Thereby, the above-mentioned advantages may be achieved.
In an embodiment, the dampening element has the form of an elastic O-ring. Thereby, the above-mentioned advantages may be achieved,
In an embodiment, the dampening element has the form of an elastic ring having rectangular cross-section.
In an embodiment, the main-compression station comprises a displacement transducer for measuring a main-compression displacement value representative of a displacement of the piston in the gas cylinder, and
a position regulator for regulation of the position of the second main-compression roller on the basis of a deviation between a previ¬ously measured main-compression displacement value and a second set value. Thereby, the above-mentioned advantages may be achieved.
In an embodiment, said position regulator is adapted to regu¬late the position of the second main-compression roller on the basis of a mean value of several single measured main-compression displacement values. Thereby, the above-mentioned advantages may be achieved.
In an embodiment, said position regulator is adapted to main¬tain the position of the second main-compression roller constant as long as said mean value of the main-compression displacement value falls within preset correction tolerance limits. Thereby, the above-mentioned advantages may be achieved.
In an embodiment, the first main-compression roller of the main-compression station is located above the rotary die table. This is advantageous, if the space below the rotary die table is limited.
In an embodiment, the pressure regulator is adapted to main¬tain the gas pressure in the gas cylinder at or below 30 bars. Thereby, a simpler and consequently cheaper pressure regulator may be employed.
In an embodiment, the total weight of the first main- compression roller, the main-compression piston, a yoke carrying the first main-compression roller and supplementary parts displaceable with the main-compression piston is less than 30 kg. Thereby, the rotational speed of the die table may be further increased without increasing noise and vibrations.
The invention will now be explained in more detail below by means of examples of embodiments with reference to the very sche¬matic drawing, in which
Fig. 1 shows in diagrammatic form an embodiment of a rotary tablet press with a control system according to the invention.
Fig. 2 is a side view of a part of the feeding device of Fig. 1, Fig. 3 is a side view of the pre-compression station of Fig. 1, Fig. 4 is a side view of the main-compression station of Fig. 1
and
Fig. 5 is a side view of another embodiment of the main- compression station in Fig. 4.
Fig. 1 shows in diagrammatic form an embodiment of a rotary tablet press with a control system according to the invention. The tablet press has a rotary die table 1 for compression of a feedstock in the form of powder or granular material into tablets, compacts or the like. The press is of a type suitable for use in the pharmaceutical industry, but the press according to the invention may as well be a so-called industrial press employed in the production of a variety of different products, such as vitamins, pet food, detergents, explosives, ceramics, batteries, balls, bearings, nuclear fuels, etc.
The tablet press is provided with a feeding device in the form of a well-known double rotary feeder with two not shown rotary paddles lo¬cated in a feeder housing and driven by means of separate drive motors providing for independent speed setting of the paddles. The feeder hous¬ing is open against the die table so that the paddles may ensure proper filling of the dies with feedstock. Other feeding systems may also be em¬ployed, such as a so-called gravity feeder or a vibration feeder.
Fig. 2 shows a fill depth adjusting device 2 which in this descrip¬tion will be considered as a part of the feeding device. The rotary feeder itself is not shown in Fig. 2> The fill depth adjusting device 2 comprises a vertically displaceable cam 3 determining the vertical position of lower punches 4 at the feeding device, thereby determining the fill depth of the die. The fill depth determines in a manner known per se the quantity of material left in the dies for compression. The lower punches 4 have first ends 6 received in corresponding dies 7 of the die table 1 and second ends 8 sliding on the vertically displaceable cam 3. Upper punches 5 are maintained outside the dies 7 at this stage in order to permit filling of the dies. The vertical position of the cam 3 is adjusted by means of a linear actuator 9 in accordance with a fill depth signal received from a powder quantity regulator shown in Fig. 1.
Fig. 3 shows a pre-compression station 10 comprising a lower compression roller 11 and an upper compression roller 12. The upper compression roller 12 is suspended in a piston 13 vertically displaceable within a gas cylinder 14, The gas pressure in the gas cylinder 14 is maintained constant by means of a not shown regulation system. In particular, the regulation system comprises a not shown air reservoir that is so large that limited displacements of the piston 13 within the gas cylinder 14 will, in practise, not affect the pressure in the gas cylin¬der 14. The air reservoir may, for instance, have a volume of 1 litre and the total system including the gas cylinder 14 may then, for instance, have a total volume of 1.5 litres. The vertical position of the piston 13 is measured by means of a displacement transducer 15, such as a LVDT (Linear Variable Differential Transformer). When an upper punch 5 passes under the centre of the upper compression roller 12, the dis¬placement transducer 15 measures a displacement substantially corre¬sponding to the thickness of the tablet after the pre-compression. Be¬cause the compression is being performed with a constant force being applied to the upper punch 5 by means of the piston 13, the displace¬ment measured by the displacement transducer 15 corresponds to the weight of the tablet compressed and thereby constitutes a weight value. At each pre-compression of a tablet, the displacement measured by the displacement transducer 15 is transferred in the form of a displacement signal to the powder quantity regulator and the control unit; see Fig. 1. Because the displacement at pre-compression is greater than at main- compression, a better sensitivity of the control loop is obtained by measuring the displacement at pre-compression instead of at main- compression.
In the control unit, the displacement signal supplied for each tablet produced is compared with predetermined rejection tolerance lim¬its defining the maximum acceptable deviation from a desired tablet weight. If the displacement signal for a tablet falls outside the rejection tolerance limits, a rejection signal is sent from the control unit to a re¬jection device associated with a tablet discharge device, and the tablet is separated from the remaining tablets, when it reaches the rejection device, see Fig, 1.
In the powder quantity regulator, a rigid or floating mean value of the displacement signal for several consecutive tablets is compared with a first set value that corresponds to a calibrated desired tablet weight and is received from the control unit. If the deviation falls out¬side preset first correction tolerance limits, the fill depth signal supplied to the feeding device is corrected correspondingly. Said correction toler¬ance limits may be calculated automatically by a general control system on the basis of user defined acceptable deviations, for instance in the form of percentage values, from the desired tablet weight.
From the tablet discharge device the tablets are fed to an automatic testing device, for example a Kraemer Electronic Tablet Tester, in which the weight and hardness of a number of sample tablets are determined periodically, and whereby corresponding weight and hardness signals are transferred to the control unit, see Fig. 1. In the control unit, the weight signal received from the automatic testing de¬vice is compared with the desired tablet weight, and on the basis of the deviation between these values, a bottom roller height signal is gener¬ated and transferred to the pre-compression station. In the pre- compression station, the bottom roller height signal is fed into a linear actuator 16, which adjusts the height of the bottom compression roller 11 correspondingly; see Fig. 3. As a result, the powder quantity regula¬tor registers the change and adapts thereto. In an alternative embodi¬ment, the vertical position of the air cylinder 14 could be adjusted by means of a linear actuator. Thereby, the powder quantity regulation loop is re-calibrated on the basis of the actual tablet weights of the sampled tablets measured by the automatic testing device. It should be noted that said re-calibration could also be performed by adjustment of the first set value supplied to the powder quantity regulator by the con¬trol unit or by adjustment of the otherwise constant air pressure in the air cylinder 14. Furthermore, instead of using an automatic testing de¬vice, a number of sample tablets may be tested manually, and a meas¬ured weight and possibly hardness may then be entered in the general control system.
Alternatively, the pre-compression station 10 may have a fixed distance between the lower compression roller 11 and the upper com¬pression roller 12, and the displacement transducer 15 may then be re¬placed by a strain gauge provided on the shaft of one of the compression rollers 11, 12 and by means of which a force signal is supplied to the powder quantity regulator and the control unit. The force signal then constitutes the weight value representative of the weight of the quantity of material fed into the die.
Fig, 4 shows a main-compression station 17 comprising a lower compression roller IS and an upper compression roller 20. The upper compression roller 20 is suspended in a piston 21 vertically displaceable within a gas cylinder 22. The gas pressure in the gas cylinder 22 is maintained constant by means of a not shown regulation system. As ex¬plained above regarding the pre-compression station, also the not shown regulation system of the main-compression station may comprise an air reservoir in order to maintain constant pressure. Because the compression is being performed with a constant force being applied to the upper punch 5 by means of the piston 21, the resulting hardness of the individual tablets will be substantially constant. Thereby, a substan¬tially constant release profile and hence bioavailability of the produced tablets may be obtained.
The vertical position of the piston 21 is measured by means of a displacement transducer 23, such as a LVDT (Linear Variable Differential Transformer). When an upper punch 5 passes under the centre of the upper compression roller 20, the displacement transducer 23 measures a displacement substantially corresponding to the thickness of the tablet after the main-compression. The displacement measured by the dis¬placement transducer 23 is also representative of the dwell time, that is, the period of time during which rie tablet is compressed by the maxi¬mum constant compression force. At each main-compression of a tablet, the displacement measured by the displacement transducer 23 is trans¬ferred in the form of a displacement signal to the dwell time regulator and the control unit; see Fig. 1.
In the dwell time regulator, a rigid or floating mean value of the displacement signal for several consecutive tablets is compared with a second set value that corresponds to a calibrated desired tablet dwell time and is received from the control unit. If the deviation falls outside preset second correction tolerance limits, a bottom roller height signal is generated and transferred to the main-compression station 17. In the main-compression station 17, the bottom roller height signal is fed into a linear actuator 19, which adjusts the height of the bottom compres¬sion roller 18 correspondingly; see Fig. 4. In this way, the dwell time during main-compression of the individual tablets may be maintained substantially constant, and consequently the above-mentioned tablet properties dependent on the dwell time will alsc be substantially con¬stant.
The dwell time regulation may counteract the tendency of the dwell time to change as a result of changing compacting properties of the material compressed in the. die. Changing compacting properties may be the result of a change in the humidity, the temperature, and the mean particle size over a batch, etc. However, according to the inven¬tion, the dwell time regulation may be omitted, and satisfying tablet properties may nevertheless be obtained.
In the control unit, the hardness signal received from the auto¬matic testing device is compared with the desired tablet hardness, and on the basis of the deviation between these values, a re-calibration may be performed by adjustment of the second set value supplied to the dwell time regulator by the control unit. Alternatively, said re-calibration could be performed by adjustment of the otherwise constant air pres¬sure in the air cylinder 22 of the main-compression station 17.
As it is seen in Fig. 4, an elastic ring 24 having rectangular cross-section is located between the main-compression piston 21 and an abutment 24 in the form of an inwardly directed, lower shoulder 25 of the gas cylinder 22. The abutment 24 provides a dampening force for the piston 21, whereby the rotational speed of the die table may be in¬creased without increasing noise and vibrations from the piston 21 and cylinder 22. This is especially advantageous in pharmaceutical tablet presses that generally run much faster than industrial presses. The dampening abutment 24 may have any other suitable configuration, for instance, a chamber containing compressed gas, a hollow ring of elastic material, a spring element, or an elastic O-ring. Furthermore, the dampening force for the piston 21 may be provided by a dampening pis¬ton 26 arranged in a cylinder 27 containing compressed gas, as it may be seen in Fig. 5, In order to further increase the rotational speed of the die table, the piston 21 may be made of a light weight material, such as titanium, for instance. Preferably the total weight of the piston 21 and other moving parts associated therewith does not exceed 40 kilos and preferably does not exceed 30 kilos. In addition, high-speed piston seals may be used for the piston 21 in order to further improve the rotational speed of the die table.
Obviously, the invention is equally applicable to so-called sin¬gle-sided, double-sided or multi-sided tablet presses. For instance, in a double-sided press for the production of tablets having two layers, a first layer production section and a second layer production section, ar¬ranged along opposite sides of the die table, each has both a pre- compression station and a main-compression station. In this case, how¬ever, the first layer is compressed to a fixed thickness at main- compression in order to better be able to regulate the quantity of the second material supplied to each die. A substantially constant hardness of the entire tablet is obtained by performing the main-compression of the second layer under substantially constant compression force and variable resulting tablet thickness of the individual tablets, in the same way as explained above for a single-sided press. Similarly, in a press for production of tablets having more than two layers, the main- compression is performed under substantially constant compression force and variable resulting tablet thickness only for the last layer of the tablet. The other layers are compressed to a fixed thickness at main- compression.
In a double-sided press for the production of single layer tab¬lets, two similar production sections each corresponding to that of a sin¬gle-sided press are provided, arranged along opposite sides of the die table, and each has both a pre-compression station, a main- compression station, a feeding device, and a tablet discharge device. Each production section is provided with both a powder quantity regula¬tor and a dwell time regulator.

PATENT CLAIMS r A method for controlling a tablet press, whereby powder or material is compressed In dies arranged circumferentially in a
nuiar!
n ary cth talkie by means of reciprocating punches, said method com- pf $ifig tt e steps:
|icmsecutively supplying a quantity of material to be compressed o !»ch die,
j subjecting the quantity of material located in each die to a pre- ctifoaresfiion and subsequently a main-compression,
jnea?uring a weight value of a parameter representative of the vi||jgfit o|f the I quantity of material fed into the dier
egujating the quantity of material supplied to each die on the Sid of deviation between a previously measured weight value and a set jraluq, and
jnea^urinfl the weight value during pre-compression of the qflarsjtityj of material located in each die,
|:hairacterized in that the main-compression is per- f .1 '
(ting tjiblet thickness of the individual tablets,
j whereby the powder or granular material is compressed in the li etvysen Opposed first and second punches, each punch having first d |^ec|n4 ejnds, whereby said first punch ends are received in the die, secjond punch ends, during main-compression, interact with iial direction of the punches and the second main-compression
■ isjixedj in said direction, and whereby the first main-compression
■ is carried by the main-compression piston, thereby a main-compression displacement value representa-
[' a
displacement of the first main-compression roller during main- mi >re^sion; is measured,' and the position of the second main- mprH5'0'1 roller in said direction is regulated on the basis of a devia- betfveerj a previously measured main-compression displacement (u^rj^ ^econd set value,

hereby said position regulation of the second main- p|res{ ion palfer js based an a mean value of several single measured compression displacement values,
wherpby the position of the second main-compression roller is constant as long as said mean value of the msin- ^ras^iop displacement value falls within preset correction tolerance
l":
jyherjeby the position of the second main-compression roller Is rJfciriatefi s that the resulting main-compression displacement value is ni nnjtair^ ad substantially constant,
\l. Almethod for controlling a tablet press according to claim 1, W jerebyi the jweight value corresponds substantially to a thickness of a tii >let d||ringj pre-compression of said tablet under substantially con- idnt copprejssion force,
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id
13. AI met hod for controlling a tablet press according to claim l wijereby the compression force of the pre-compression is main- nsubstantially constant by means of a pre-compression piston ar- id ijispl^ceably in a gas cylinder, whereby the gas cylinder is sup- vyip cajmpressed gas, and whereby the gas pressure in the gas linger jis maintained substantially constant by means of a pressure
aftoj'.
fl. Amethod for controlling a tablet press according to claim 3, lenebyj the pqwder or granular material is compressed in the die be¬an opposed first and second punches, each punch having first and eaid whereby said first punch ends are received in the die, and
Sfiscj>n4 punch encjs, during pre-compression, interact with first and id j re-qompr^ssion rollers, respectively, whereby, during the pre- m;>res;ion, the fjrst pre-compression roller Is displaced in the axial di- r qtun cf th^ punches and the second pre-compression roller is fixed in sis jd dir^itforj, andj whereby the first pre-compression roller is carried by e i>re-j;ompressi(j>n piston.
:S. A method for controlling a tablet press according to claim 4, leieby thp weight value corresponds substantially to a pre- ;C m jne^5ionjdi?pla|cement value representative of a displacement of the

impression roller during pre-compression. i. A'method for controlling a tablet press according to claim 1, the I weight value corresponds substantially to the maximum on; force exerted by a punch on a tablet during pre- on pf said tablet to a predetermined tablet thickness. . A :method for controlling a tablet pressaccording to any one feceding claims, whereby the compression force of the main- ion lis maintained substantially constant by means of a main- ion piston arranged displaceably in a gas cylinder, whereby the dep i is supplied with compressed gas, and whereby the gas in the gas cylinder is maintained substantially constant by ! a pressure regulator.
i. A 'method for controlling a tabiet press according to claim 7, the movement of the ma in-compression piston is stopped by a ng fqrcp after each main-compression. . A method for controlling a tablet press according to claim a, the| dampening force is produced by a chamber containing d ^jas. ;
10, A method for controlling a tablet press according to claim 9,
i '
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the; chamber containing compressed gas is a hollow ring of aterial: located between the main-compression piston and an
utjtierit. j ':
U. A method for controlling a tablet press according to claim 8, jeijeby the; dampening force is provided by a dampening piston ar- a cylinder containing compressed gas. L2. >jk method for controlling a tablet press according to claim 8, the dampening force is provided by a spring element. 13,- A method for controlling a tablet press according to claim 8, thejdampening force is provided by an elastic O-ring located eenj the main-compression piston and an abutment.
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L4. A method for controlling a tablet press according to claim 8, eby the dampening force is provided by an elastic ring having rec- Mlar cross-section and being located between the main-compression n arid ari abutment.

5. /j rotary tablet press comprising a housing and a rotary die havling ^ number of dies arranged circumferentiaily, each die being ip^iated with first and second punches, each punch having first and o|d end?, said first punch ends being receivable in the die and ar- r#^sd fjir compression of a powder or granular material in the die,
|he Housing comprising a feeding device far the supply of mate¬ria tjo bf; compressed into the dies, a tablet discharge device for re- rnlpv^l of compressed material in the form of tablets, and
\it lebst one pre-compression station' and at least one main-
i i
n
It
p: rppresjiion Ration, each said compression station being provided with
second compression rollers adapted to interact with the second ndi er dsf respectively, in order to perform compression of material 3tjsd i|i the dies by reciprocation of the punches/
he housing comprising a weight transducer for measuring a light vi slue jof a parameter representative of the weight of a quantity mater al ffd into the die, the weight -transducer being comprised by f rebornpjression station,
U ppjvder quantity regulator being provided for regulation of the anjtity of material supplied to each die by the feeding device on the ofdeviation between a previously measured weight value and a st set I'aM,
:ha|racterized in that the first ma in-compression com- 5S|iion! roller .of the main-compression station being supported by
1
is q ' a main-compression piston arranged displaceably in a gas cyl- ; thje g'ais cylinder being connected to a supply of compressed gas, 3 pr »£$qreregu)ator being adapted to maintain the gas pressure in whifpejn the main-compression station comprises 3 displacement transducer for measuring a main-compression spfecenent value representative of a displacement of the piston in the s cylinder, .and
\t portion regulator for regulation of the position of the second rffeiri-cajnpFejsslon roller on the basis of a deviation between a previ- n^asuiW ma rn-compression displacement value and a second set
snd
vtherein said position regulator is adapted to regulate the posi- ;l{fi ft tH» ?^ond main-compression roller on the basis of a mean value r^ single measured main-compression displacement values, and Wherein said position regulator is adapted to maintain the posi- Df tie sjacond main-compression roller constant as long as said ysjue pf the main-compression displacement value fails within se t cq Teptfion toleran ce I im its,
1.6. A rotary tablet press according to claim 15, wherein the life romprespion roller of the pre-compressfon statfon is supported by jpiis oi a piston arranged displaceably in a gas cylinder, whereby the i i!y}fr¥ ier is connected to a supply of compressed gas, and whereby a $s? ure regulator is adapted to maintain the gas pressure in the gas inter substantially constant.
.7. A rotary tablet press according to claim 16, wherein the tj^it tjanstjueer of the pre-compression station has the form of a dis- j«ment transducer for measuring a pre-compression displacement rej resejntative of a displacement of the piston in the gas cylinder.
:.8. A rptary tablet press according to any one of the claims 15 w wejn a dampening element is provided between the piston of r]nair|-con^presslon station and an abutment.
amber
c; m;>eni
si rmeni
L9. A rotary tablet press according to claim 18, wherein the mbenhg ejerpent has the form of a chamber containing compressed
p. f rotary tablet press according to claim 19, wherein the containing compressed gas is a hojlow ring of elastic material. II.; A rotary tablet press according to claim 18, wherein the ig ejerpent has the form of a dampening piston arranged in a dfijrjjderMont^inlng compressed gas.
22, jv rotary tablet press according to daim 18, wherein the rripeni igejement has the form of g spring element.
23. f rotary tablet press according to claim IS, wherein the ig e|ement has the form of an elastic o-ring. >4. ^ rotary tablet press according to claim 18, wherein the

d^rienijbg element has the form of an elastic ring having rectangular is^-seetiqn. :
j ; :5. A rotary tablet press according to any; one of the claims 15 24,; wherein the first main-compression roller of the main- f res? ion station is located above the rotary die table.
;»6, 4 rotary tablet press according to any one of the claims 15 wierein the pressure regulator is adapted to maintain the gas iff s$ ur® in jtje gas cylinder at or below 30 bars.
to
IV
^ rotary tablet press according to any one of the claims 15 it, wherein the total weight of the first main-compression roller, the ti ilnj-conpne^slon piston, a yoke carrying the first main-compression |eir ariil supplementary parts displaceable with the main-compression to^> ts |es?|than 30 kg.

Documents:

6869-CHENP-2008 CORRESPONDENCE OTHERS 05-2-2014.pdf

6869-CHENP-2008 AMENDED CLAIMS 20-11-2014.pdf

6869-CHENP-2008 EXAMINATION REPORT REPLY RECEIVED. 20-11-2014.pdf

6869-CHENP-2008 FORM-3 20-11-2014.pdf

6869-chenp-2008 abstract.jpg

6869-chenp-2008 abstract.pdf

6869-chenp-2008 claims.pdf

6869-chenp-2008 correspondence others.pdf

6869-chenp-2008 correspondence-others.pdf

6869-chenp-2008 description (complete).pdf

6869-chenp-2008 drawings.pdf

6869-chenp-2008 form-1.pdf

6869-chenp-2008 form-18.pdf

6869-chenp-2008 form-3.pdf

6869-chenp-2008 form-5.pdf

6869-chenp-2008 pct.pdf

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Patent Number

264755

Indian Patent Application Number

6869/CHENP/2008

PG Journal Number

04/2015

Publication Date

23-Jan-2015

Grant Date

20-Jan-2015

Date of Filing

15-Dec-2008

Name of Patentee

COURTOY NV

Applicant Address

BERGENSESTEENWEG 186, B-1500 HALLE,

Inventors:

#	Inventor's Name	Inventor's Address
1	VOGELEER, JAN,	BRIEL 2, B-2880 BORNEM,
2	BOECKX, JURGEN	BEUKENDREEF 17, B-1820 STEENOKKERZEEL,
3	VAN DEN MOOTER, IVO.,	BAECKELMANSSTRAAT 93, B-2830 WILLEBROEK,
4	HAPPAERTS, WOUTER	LANGE ZANDSTRAAT, 48A, B-1800 WALEM,

PCT International Classification Number

B30B 11/00

PCT International Application Number

PCT/IB06/01261

PCT International Filing date

2006-05-15

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA