Patent Publication Number: US-10786963-B2

Title: Tablet manufacture

Description:
RELATED APPLICATIONS 
     This application is a national phase entry of International Patent Application No. PCT/GB2016/053309, filed Oct. 21, 2016, which claims the benefit under 35 U.S.C. § 119(b) to Great Britain Patent Application No. 1518842.8, filed Oct. 23, 2015 and Great Britain Patent Application No. 1605939.6, filed Apr. 7, 2016, the entire contents of which are hereby incorporated by reference. 
     The present invention relates to improvements in tablet manufacturing. In particular, the invention relates to a device for small scale/bespoke manufacture of tablets. 
     The large-scale production of tablets typically involves the use of tablet punches which operate to compact a volume of powder located in a die. The powder in the die is held between opposing punches which move together by a predetermined distance of travel to produce a tablet of controlled thickness within a die of known geometry. This is such that the formed tablet has a known or determinable density according to the die geometry and volume of powder used but there is no direct control of the force applied to the tablet during the compaction process. 
     The required volume of powder and/or degree and force of compression to achieve a desired result will vary depending on the formulation being used. An iterative approach to tablet production and testing is generally needed in order to converge on a satisfactory tablet formulation and corresponding compaction process. Research into tablet formulations and production processes requires relatively small scale production and testing of tablets. 
     Known bespoke tablet machines for small scale tablet production and testing of tablets are known. One such device comprises a linearly actuated press member which is connected to a load cell. The press member can be used to compact powder held in a tablet chamber of a die assembly, and the tablet press can measure and record the forces detected by the load sensor during the compaction process. The same press member may also be used to eject a tablet from the die assembly, again with the tablet press measuring and recording the forces during the operation (the ‘ejection’ forces). This information about the tablet properties is of great value in correctly configuring machines for mass production of tablets, or for fine tuning formulations. 
     The known tablet press does, however, suffer certain drawbacks. 
     For example, before the tablet can be ejected a part of the die assembly has to be moved from a first position, in which it forms a solid floor of the tablet chamber, to a second position where it where it provides an opening for the ejection of a tablet. This operation, which minimises the required complexity of the tablet press, is typically performed by hand, which can complicate the operation and potentially contaminate the tablet. 
     The compaction of the powder in the chamber also creates some adhesion between the tablet/powder and the interior of the die assembly. This results in dissipation of compression forces at the internal side walls of the die during the compression process, leading to inaccuracies in the measurement of forces applied to the powder during compression. The magnitude of the error/inaccuracy is not determined in known devices, and will vary depending on the particular powder composition and compaction parameters. Without this information, a compensation factor cannot be reliably applied. The missing information would also be useful in fully understanding the behaviour of the powder during compaction. 
     Adhesion can also occur between the formed tablet and the floor of the die assembly. This force required to overcome this adhesion (the detachment force) can be fairly high, perhaps 30 kg or more, potentially causing difficulty to an operator. In addition, useful information concerning the detachment force for different tablet formulations and different compaction conditions is lost due to the manual operation. 
     It is an aim of the present invention to overcome the abovementioned drawbacks, while maintaining a relatively simple but precisely controllable tablet forming operation. 
     According to a first aspect of the invention there is provided a tablet press as defined in the appended claim  1 . Further beneficial features of the tablet press are recited in the dependent claims. 
     The invention provides a tablet press with biasing means associated with the compaction mechanism ensure that play/slop within the mechanism is minimised. This helps to ensure that the position of the press member can be precisely and reliably controlled simply by controlling the motor to a defined angular position. Indeed, tolerance on the positional control has been found to be of the magnitude of 0.1 μm. This avoids the need for separate position sensors to measure the position of the press member directly. 
     At least one further position of the press member may correspond to a further pre-set angular position of the motor stored in the controller or be determined based on the detection of a pre-set load by the load cell. 
     The compaction mechanism may comprise a fixed frame and support columns movable vertically relative to the fixed frame. Telescoping guards may be provided surrounding upper sections of the support columns that extend beyond the base to avoid contaminants, such as powder, entering the mechanism. 
     The mechanism may further comprise a ball screw which moves a horizontal member attached between a pair of support columns when a threaded rod of the ball screw is rotated by the motor. The resilient means, for example coil springs, may then be provided between the horizontal member and a part of the fixed frame and/or a block may be provided on the horizontal member to engage one or more limit switches attached to the frame. 
     The tablet press may also comprise a second load cell located below the die assembly. The provision of a second load cell below the die assembly allows for a comparison of the forces experienced at the top and bottom of a column of powder in the die during compression to provide additional information about the behaviour of the powder during tablet formation. 
     The second load cell may extend upwards from the base of the tablet press and the die assembly may comprise a die floor with a vertically movable portion which rests, in use, on top of the second load cell. 
     The die floor may comprise a slider block with an opening spaced from the vertically movable portion such that the die floor can be moved to position either the opening or the vertically movable portion beneath a chamber of the die, for example to allow ejection of a tablet, once formed, from the die. 
     The vertically movable portion may comprise a disc, held in place within the remainder of the die floor by a ball spring screw. 
     At least one further position of the press member may be determined based on the detection of a pre-set load by the second load cell. 
     The controller of the tablet press may monitor a limit value during a compaction operation and stop the motor to hold the mechanism at a fixed location for a defined period of time when the limit value is reached. 
     The period of time may be pre-set as an absolute value or may be a function of the speed of the motor during the compaction operation. This allows simulation of the operation of a number of different commercial scale tablet presses with different operational characteristics 
     The limit value may be a position of the press indicative of the end of a desired compaction operation, which may be achieved by the controller moving the motor to a set angular position. 
     Alternatively, the limit value is a desired/target pressure at the end of a desired compaction operation and/or a desired intermediate value during a desired compaction operation. 
     Where an intermediate value is used, the controller may be configured to re-start the compaction operation after the period of time, possibly with the motor running at a second, different, speed. The second speed may be pre-set by a user prior to the compaction operation or may be determined based on load cell readings and positional information during the time period. The positional information may be obtained from a monitored angular position of the motor. 
     The motor may be stopped by the controller maintaining the motor in an energised state with the motor speed set to zero. 
     A further aspect of the present invention provides a pivotally mounted die assembly as described in appended claim  27 . Further beneficial features are recited in the associated dependent claims. 
     The die assembly comprises a pivoting part mounted at a pivot to a supporting frame. The pivoting part comprises a die assembly with a chamber for forming a tablet. The chamber has an open end for receiving a linearly operating punch of a tablet press along an axis extending through the chamber, and a sliding floor component opposite the open end, which is movable substantially at right angles to the axis between first and second positions restraining means are provided to selectively stop rotation of the pivoting part at first and second defined angular positions relative to the supporting frame. 
     Providing a pivoting mounting allows the die assembly to be rotated between two set positions, so that the same linearly operating punch can engage with the die assembly from two different directions. In particular, a linearly operating punch of a tablet press capable of measuring compaction and ejection forces can be used to also move the sliding floor component once a tablet has been formed, and to measure forces involved in this operation. This beneficially provides information about the detachment force for a particular tablet formed in the die assembly. 
     The restraining means may comprise a fixed stop provided on the supporting frame, which may comprise two abutment surfaces for abutment with the pivoting part. For example, abutment surfaces may be provided to contact an upper surface of the pivoting part in two specific orientations. 
     The restraining means may further comprise one, two or more selectively engageable stop members. The or each selectively engageable stop member may also be provided on the supporting frame, and may comprises a blocking portion which is movable between a disengaged position and an engaged position, eg via a pivot to the surrounding frame. The or each selectively engageable stop member may also comprise a lever joined to the blocking portion for actuating the blocking portion. 
     The blocking portion of the or each selectively engageable stop member, in its engaged position, may provide an abutment surface such that the pivoting part of the assembly can be held between the fixed stop and the blocking part of a selectively engageable stop member. 
     One or more notches for receiving the blocking part of a selectively engageable stop member are provided on the pivoting part, for example in an upper surface or and end surface of the pivoting part. The engagement of said blocking part with a notch may hold the pivoting part at a further defined angular position. 
     Alternatively, the or each selectively engageable stop member may be provided on the pivoting part to engage with a notch, or one of a plurality of notches, provided on the supporting frame. 
     The restraining means may alternatively comprise a stepper motor controlled to hold the pivoting part at said first and second defined angular positions. The stepper motor may be additionally controlled to hold the pivoting part in at least one further angular position. 
     The at least one further angular position may be between said first and second angular positions. 
     The first and second angular positions are spaced apart by 90°. The third angular position may be, for example, at approximately 30°, 45° or 60° from the second angular position, either towards or away from the first angular position. 
     The pivot between the pivoting part and the supporting frame may intersect with the axis extending through the chamber, and with the sliding floor component. 
     The pivoting part may comprise a sloping floor or channel along which a tablet formed in the chamber can pass. 
     The sliding floor component may comprise an opening which is offset from the chamber when the sliding floor component is in its first position and is aligned with the chamber when the sliding floor component is in its second position. The sloping floor or channel may be part of a base plate below the chamber and the sliding floor component, or the sliding floor component comprises the sloping floor or channel. 
     The die assembly may be permanently fixed via the pivot to the supporting frame or the pivoting part may comprise a mounting part, such as a tray, for receiving the die assembly. 
     According to a further aspect of the invention there is provided a pivoting die mounting apparatus as described in appended claim  47 . Further beneficial features are recited in the associated dependent claims. 
     The pivoting die mounting apparatus comprises a supporting frame and a pivoting part, for receiving a die assembly, mounted at a pivot to the supporting frame. The pivoting part is open at one end so as not to obstruct access to an end of the die assembly, and restraining means are provided to selectively stop rotation of the pivoting part at first and second defined angular positions relative to the supporting frame. 
     The benefits of the pivoting mounting are as previously described. 
     The restraining means may comprise a fixed stop provided on the supporting frame, which may comprise two abutment surfaces for abutment with the pivoting part. For example, abutment surfaces may be provided to contact an upper surface of the pivoting part in two specific orientations. 
     The restraining means may further comprise one, two or more selectively engageable stop members. The or each selectively engageable stop member may also be provided on the supporting frame, and may comprises a blocking portion which is movable between a disengaged position and an engaged position, eg via a pivot to the surrounding frame. The or each selectively engageable stop member may also comprise a lever joined to the blocking portion for actuating the blocking portion. 
     The blocking portion of the or each selectively engageable stop member, in its engaged position, may provide an abutment surface such that the pivoting part of the assembly can be held between the fixed stop and the blocking part of a selectively engageable stop member. 
     One or more notches for receiving the blocking part of a selectively engageable stop member are provided on the pivoting part, for example in an upper surface or and end surface of the pivoting part. The engagement of said blocking part with a notch may hold the pivoting part at a further defined angular position. 
     Alternatively, the or each selectively engageable stop member may be provided on the pivoting part to engage with a notch, or one of a plurality of notches, provided on the supporting frame. 
     The restraining means may alternatively comprise a stepper motor controlled to hold the pivoting part at said first and second defined angular positions. The stepper motor may be additionally controlled to hold the pivoting part in at least one further angular position. 
     The at least one further angular position may be between said first and second angular positions. 
     The first and second angular positions are spaced apart by 90°. The third angular position may be at, for example, approximately 30°, 45° or 60° from the second angular position, either towards or away from the first angular position. 
     The pivoting part may comprise a sloping floor or channel along which a tablet formed in the chamber can pass. 
     The invention also provides a method of forming a tablet as described in the appended claim  62 . Further beneficial method steps are recited in the associated dependent claims. 
     The method comprises the following sequence of steps:
         A) actuating the punch of a tablet press to compress powder contained in the chamber of a die assembly against a sliding floor component of the die assembly;   B) withdrawing the punch from the chamber;   C) rotating the die assembly from a first set angular position to a second set angular position; and   D) actuating the punch of the tablet press to move the sliding floor component from a first position to a second position.       

     In the first angular position the tablet press, for example a vertically operating tablet press, can measure compaction forces applied by the punch in forming a tablet. In the second angular position, the same tablet press can measure the detachment force between the formed tablet and the sliding floor of the die assembly. 
     The method may comprise the additional steps of:
         E) rotating the die assembly from said second set angular position to said first set angular position; and   F) actuating the punch of the tablet press to eject a tablet from the chamber;   wherein steps E) and F) are performed in sequence after step D).       

     The additional steps allow the ejection force of a tablet to also be measured by the same tablet press. 
     The method may comprise the additional steps of:
         G) rotating the die assembly to a third angular position; and   H) placing powder into the open end of the tablet forming chamber;   wherein steps G) and H) are performed in sequence before step A).       

     These additional steps allow for the rotation of the die assembly to provide clearance between an upper end of the die assembly and the punch of the tablet press to facilitate filling of the chamber 
     The third angular position is between the first and second angular positions. The first and second angular positions are spaced apart by 90°, for example the first angular position may be a horizontal alignment of the die assembly, with the chamber of the die assembly vertically oriented to receive the punch from above, and the second angular position may align the die assembly vertically, allowing the slider block to be pushed at right angles to the chamber. The third angular position may be, for example, at approximately 30°, 45° or 60° from the second angular position, either towards or away from the first angular position. 
     The method may be performed using the pivotally mounted die assembly as previously described, or using the pivoting die mounting apparatus as previously described and a die assembly comprising a tablet forming chamber having an open end for receiving a linearly operating punch of the tablet press and a sliding floor component opposite the open end. 
     The various aspects of the invention variously provide improvements in the four main steps of tablet formation, namely filling the die, compacting the tablet, releasing the tablet and ejecting the tablet. As such, an improvement in the tabletting process is provided by each aspect individually, and an overall improvement is provided by the aspects in combination. 
     Wherever practicable, any of the essential or preferable features defined in relation to any one aspect of the invention may be applied to any further aspect. Accordingly, the invention may comprise various alternative configurations of the features defined above. 
    
    
     
       Practicable embodiments of the invention are described in further detail below by way of example only with reference to the accompanying drawings, of which: 
         FIG. 1  shows a schematic front view of a tablet press; 
         FIG. 2  shows a perspective view of a die assembly for use with the tablet press of  FIG. 1 ; 
         FIG. 3  shows a perspective view of a die mounting apparatus in accordance with the present invention in a first configuration; 
         FIG. 4  shows a perspective view of the die mounting apparatus of  FIG. 3  in a second configuration; 
         FIG. 5  shows a schematic view of a modified die mounting apparatus in accordance with the present invention in a third configuration; 
         FIG. 6  shows a schematic view of the modified die mounting apparatus from  FIG. 5  in the first configuration shown in  FIG. 3 ; 
         FIG. 7  shows a schematic view of the modified die mounting apparatus from  FIG. 5  in the second configuration shown in  FIG. 4 ; 
         FIG. 8  shows a schematic view of a modified die assembly in a first configuration; and 
         FIG. 9  shows a schematic view of the modified die assembly of  FIG. 8  in a second configuration; 
         FIG. 10  shows a front view of the compaction mechanism of a tablet press according to the invention; 
         FIG. 11  shows a perspective view of the mechanism from  FIG. 10 ; 
         FIG. 12  shows a cross sectional view of a modified die assembly located on a lower load cell assembly; 
         FIG. 13  shows a flow diagram of tablet press operation according to one embodiment of the invention; 
         FIG. 14  shows a flow diagram providing detail from a section of the flow diagram of  FIG. 13 ; 
         FIG. 15  shows an example plot of a compaction operation; 
         FIG. 16  shows a perspective view of alternative die mounting apparatus in a first configuration; 
         FIG. 17  shows a cross-sectional view of the die mounting apparatus from  FIG. 16 ; 
         FIG. 18  shows a perspective view of alternative die mounting apparatus in a second configuration; and 
         FIG. 19  shows a cross-sectional view of the die mounting apparatus from  FIG. 18 . 
     
    
    
     In  FIG. 1  there is shown a tablet press  10  having a base  12 , which comprises a base housing  14 . A lower region of the base  12  has feet  16  arranged to support the weight of the tablet press  10  on a suitable surface, such as a desk top  18 , for use. 
     In the upper surface of the housing  14  there are provided a plurality of openings, through which spacer arms, in the form of support pillars  20 , extend. The support pillars  20  have a lower end which is located within the base housing  14  and an opposing upper end which protrudes above the base housing  14 . The support pillars  20  are arranged generally vertically when the feet  16  are on a horizontal surface  18 . 
     At the upper end of the support pillars  20 , there is provided a support member  22  which extends between the support pillars  20  and which is arranged generally perpendicular to the longitudinal axes of the support pillars  20 . Mounted to the support member  22 , there is provided a press member, which is referred to herein as punch  24 . The punch  24  depends from the support member  22  at a location between, and typically equidistant from, the support pillars  20 . The punch  24  is elongate in form and extends towards the base  12  in a direction which is generally parallel with the support pillars  20 . 
     The punch  24  is generally cylindrical in shape although other shapes are possible including oval, square or other shapes to which tablets are conventionally formed. The punch  24  has a free end  25  which is blunt. The free end  25  defines in part the shape of a tablet formed by the tablet press  10  in use. Accordingly, the free end may be flat or curved in a desired tablet profile. In this regard, it may be possible to provide the punch  24  with interchangeable end sections to suit different tablet shapes. In such embodiments, the die shape will typically be interchangeable to correspond with the punch shape. 
     The support member  22  comprises a load sensor in the form of a load cell  26  arranged intermediate the punch  24  and the remainder of the support member. The punch  24 , at its fixed end, may be mounted at or on the load cell  26 , which may itself be mounted in a correspondingly shaped recess or formation in the support member. 
     The support pillars  20  terminate at their lower ends within the base housing  14 . Mounted within the base housing  14  is an electric motor assembly  28 , which, in this embodiment, comprises a conventional brushed DC motor. However, it will be understood that other types of motor may be used, such as, for example, brushless DC motors, including stepper motors. An electric motor is in many ways preferred as a suitable drive means for the tablet press due to the range of travel required by the support pillars  20 . However, it should be noted that other forms of electromechanical drive or actuator could be considered provided they can allow for suitable linear displacement of the support pillars  20  in use. 
     The motor assembly  28  is shown schematically in  FIG. 1  in cooperation with the support pillars  20 , and drives the support pillars  20  to generate the compression force for the tablet press  10 . 
     In this embodiment, the motor assembly  28  further comprises a linear servo amplifier which powers the motor. A digital encoder is also provided for the control of the motor. In this embodiment the encoder is an integral part of the motor assembly  28  within the base housing  14 . Thus, in use, the angular position of the motor is determinable and digitally controllable as will be described in further detail below. 
     A user interface  30  is provided, for example on a panel of the base housing  14 , and comprises a display screen  32  and a plurality of keys  32  in the form of a keypad. The keys allow for alphanumeric character entry by a user in a conventional manner. 
     In the upper portion of the base housing  14 , there is provided a die assembly  36  comprising a die member  38  and a die floor or base  40 . The die member and die floor are held in position against a plate  42  on the base  12  by retaining formations  44 . 
     A force path can be defined between the motor assembly  28 , the support pillars  20 , the support member  22 , including the load cell  26 , and punch  24 . The tablet press  10  and die assembly  36  are arranged so that the force acts along a working axis  46  that extends through the punch  24  and through the centre of the bore of the die member  38 . Accordingly, a load applied by the motor can be communicated to the punch  24  such that the punch  24  applies a load to powder in the die. Any reaction to the applied load experience by the punch  24  can be recorded by the load cell  26 . The motor  28  and load cell  26  are typically arranged to allow for a load of up to approximately 500 kg or 4900 N, although in some cases components suitable for loads of up to 50 kN will be required. The loads applied by the punch  24  during both the compaction and ejection operations can therefore be measured and recorded by the tablet press  10 . 
     Although not shown in  FIG. 1 , the compaction mechanism may also comprise a second/lower load cell, located below the die assembly  36 , as will be described in detail later. 
     An example of a die assembly  36  for use in conjunction with the base  12  of the tablet press  10  is illustrated in greater detail in  FIG. 2 . 
     The die assembly  36  comprises a die member  38  in its upper portion, shaped to define the die in which a tablet is formed in use. The die member  38  has an open ended funnel formation  37  leading to an upstanding wall  39  which is generally tubular or toroidal in shape and has a central opening or bore  41  into which powder can be inserted. The funnel  37  has an upwardly facing open mouth which tapers towards a narrow opening which leads into the bore  41  of the die member  38 . The powder is held within the bore  41  in the die member  38  and supported by the floor or base  40  to create a column of powder within the upstanding wall portion  39  of the die member  38 . 
     The die member  38  comprises a mounting portion  43  which is mounted to a pair of side walls  44 , which extend at right angles to a base plate  42 . The combined side walls  44 , base plate  42  and the mounting portion  43  of the die assembly  36  therefore provide an opening with a rectangular cross section to partially enclose and restrain an intermediate member  40  which forms a base or floor to the die assembly  36 . As illustrated, the base  40  comprises a slider block or drawer member having an opening  45  therein in the form of a cylindrical hole. 
     The slider block  40  can be actuated in forward and reverse directions between positions in which the opening  45  is respectively aligned with and offset with the bore  41  of the die member  38 . The opening is a close fit about the slider block  40  in order to constrain the slider block  40  to a linear motion only. This close fit causes friction between each of the base plate  42 , the side walls  44  and the mounting portion  43 , and the slider block  40 . 
     When forming a tablet, the slider block  40  is positioned as shown in  FIG. 2 , with the opening  45  offset from the bore  41  of the die member  38 . The desired powder is then measured and poured into the funnel  37  of the die member  38 , where is prevented from passing straight through the bore  41  by the tightly fitting slider block  40 . This will be referred to as the first, or compaction, position. 
     With the die assembly  36  located in the tablet press  10  such that the bore  41  of the die member  38  aligned with the working axis  46 , the punch  24  can be actuated to compress the powder to a predetermined degree, measured and controlled by the tablet press  10 , before being withdrawn to leave the compressed powder in a chamber defined by the defined by the tubular upstanding wall  39  of the die assembly  36 . 
     In order to eject a tablet from the die assembly  36 , the slider bock  40  must first be actuated to a second, ejection, position, in which the opening  45  is beneath the bore  41  of the die member  38 . This allows ejection of the tablet out of the bore  41  of the die member  38  and into the opening  45 , for example by a further actuation of the punch  24 . 
     The base plate  42  of the assembly is substantially planar in form and devoid of any opening. As such, the ejected tablet will be held in a cavity defined by the opening  45  and the base plate  42 . A subsequent reverse sliding actuation of the slider block  40  back to the first position exposes the opening  45  and allows removal of the tablet by an operator, as well as preparing the die assembly  36  for the next compaction operation. 
     The base plate  42  has a greater length than the die mounting portion  43  of the die member  38  to allow the opening  45  to slide out from beneath the die member  38  for removal of the tablet without risk of the tablet falling through the opening  45 . 
     The tablet press  10  of  FIG. 1  is typically used for small batch manufacturing of bespoke tablets, or for testing and assessment of new tablet formulations rather than for mass production. 
     Typically, the action of moving the slider block  40  between the first and second positions has been achieved manually. However, a significant amount of force can be required to move the slider block  40 . Aside from the friction that naturally arises from the tight fit between the slider bock  40  and the surrounding parts  42 , 43 , 44  of the die assembly, the powder making up the tablet also has a tendency to stick to the flat surface of the slider block during the compaction operation. As a result, forces of up to 50 kg (or 490N) are required to overcome the resistance and move the slider bock  40  from the first, compaction, position to the second, ejection, position. The size of the required force can be difficult for operators to apply by hand, causing discomfort and delaying the manufacture of tablets. 
     Perhaps more importantly, the current approach also provides no precise information about the forces involved in this operation. Information about the detachment force required for different formulations and different compaction pressures is of great interest and importance for future tabletting operations, particularly when the suitability of new formulations for mass production is being assessed. 
     One solution to these problems would be to include a further actuator and sensor arrangement, acting at right angles to the punch  24  of the tablet press  10 , specifically for the purpose of moving the slider block  40  during the ejection operation. However, this solution would require additional motors and other components, and further control architecture within the press  10 . A simpler solution is proposed by the present invention. 
     A die mounting apparatus  100 , for restraining a die assembly  36  in position, is illustrated in  FIG. 3 . The mounting assembly comprises a generally U shaped surrounding frame  102 , having a base  104  and two upstanding wall portions  106 . Channels  108  are provided centrally in each of the upstanding wall portions  106  to support a smaller pivoting U shaped frame, or tray  110 , which in turn receives a die assembly  36  as shown in  FIG. 2 . The smaller U shaped tray  110  is aligned with and fits within the surrounding frame  102 . A pair of rods  112  extend from the sides of the tray  110  and are received in the channels  108  in the upstanding wall portions  106  of the surrounding frame  102 , and are supported in place by bearings  114 . This provides the pivot between the smaller U shaped frame and the surrounding frame  102   
     A fixed stop  116  is provided on one of the upstanding wall portions to limit the rotation of the tray  110  within the surrounding frame  102  to ninety degrees. As shown in  FIG. 3 , the fixed stop  116  projects inwardly of the upstanding wall portion  106  so that a lower surface  118  of the fixed stop  116  engages with an upper surface  121  of a sidewall  120  of the smaller tray  110  to prevent rotation of the tray beyond the horizontal position shown in  FIG. 3 . 
     A first selectively engageable stop member  122 , comprising a blocking portion  122   a  and a lever  122   b , is pivotally mounted to a lower part of the upstanding wall portion  106  which comprises the fixed stop  116 . As shown in  FIG. 3 , the first selectively engageable stop member  122  is in its engaged position with the lever  122   b  pivoted in line with the upstanding side wall  106  so that the blocking portion  122   a  extends inwardly of the surrounding frame  102  below the tray  110 . The tray  110  is therefore held in the desired horizontal position between the blocking portion  122   a  of the first selectively engageable stop member  122  and the lower surface  118  of the fixed stop  116 . 
     A second selectively engageable stop member  124  is also shown, in a position, behind the die assembly  36 . The second selectively engageable stop member  124  similarly comprises a blocking portion  124   a  and a lever  124   b , and is pivotally mounted to the upstanding wall portion  106  which comprises the fixed stop  116 . In the disengaged position as illustrated, the blocking portion  124   a  can be seen extending vertically upwards from the upstanding wall portion  106  so as not to extend inwardly of the surrounding frame  102 . It should also be clear that the pivot between the second selectively engageable stop member  124  and the upstanding side wall  106  is perpendicular to the pivot between the first selectively engageable stop member  122  and the upstanding side wall  106 . 
     The die mounting apparatus  100  containing the die assembly  36  is received, in use, in a tablet press  10  such as that shown in  FIG. 1 .  FIG. 3  shows the die mounting apparatus in a compaction configuration, with the smaller U shaped tray  110  held horizontally. The die mounting apparatus  100  will be located so that the axis  46  along which the punch  24  will move is precisely aligned with the centre of the chamber defined by the tubular upstanding wall  39  of the die assembly  36  so that the punch  24  can reliably compress powder received in the die assembly  36 . The die assembly may be permanently fixed to, or formed integrally with, the tray  110 , or may be removably attached thereto. 
     The axis  126  about which the tray  110  rotates passes through the slider block  40  and intersects the working axis  46  at right angles. The significance of this will be clear with reference to  FIG. 4 , which shows the same die mounting apparatus  100  in a second configuration. 
     In  FIG. 4  the tray  110  has been rotated about the axis  126  through ninety degrees to a vertical position. The intersection of the pivoting axis  126  with the working axis  46  of the tablet press  10 , described above, means that the working axis  46  is now aligned with the centre of the slider block  40 . As a result, the punch  24  of a tablet press  10  of the type shown in  FIG. 1  can be used to apply a force to the slider block  40 . The configuration shown in  FIG. 4  can therefore be referred to as a sliding configuration. 
     To move the die mounting apparatus  100  from the compaction configuration of  FIG. 3  to the sliding configuration of  FIG. 4  requires only a few simple steps. Firstly, the first selectively engageable stop member  122  is pivoted to its disengaged position, withdrawing the blocking portion  22   a  as shown in  FIG. 4  so that the tray  110  is able to pivot away from the lower surface  118  of the fixed stop  116 . With the second selectively engageable stop member  124  still in its disengaged position, as shown in  FIG. 3 , the tray  110  is then pivoted until the upper surface  121  of its sidewall  120  engages with a vertical surface  128  of the fixed stop  116 . Finally, the second selectively engageable stop member  124  is pivoted into its engaged position, extending its blocking portion  124   a  inwardly of the surrounding frame  102  behind the U shaped tray  110  as shown in  FIG. 4 . The tray  110  is therefore held in a vertical position between the blocking portion  124   a  of the second selectively engageable stop member  124  and the vertical surface  128  of the fixed stop  116 . 
     Significantly, the pivoting action of the die mounting apparatus  100  permits repeatable movement of the die assembly  36  between a horizontal position, where the working axis  46  of a tablet press aligns with the chamber for forming a tablet, and a vertical position in which the working axis  46  aligns with the end of the slider block  40 . 
     The thickness of the slider block  40  is greater than the diameter of the end  25  of the punch  24  of the tablet press. The punch  24  can therefore be used to form and eject the tablet as before (with the die mounting apparatus  100  in the compaction configuration of  FIG. 3 ), and additionally to move the slider block  40  between its first, compaction position and its second, ejection, position. The tablet press  10  is already capable of measuring and recording the load applied by the punch  24  during tablet formation and ejection, so by using the same punch to move the slider block  40  the forces involved in this operation (the detachment forces) can also be readily, and centrally, measured and stored. 
     The invention provides a further benefit over a static die mounting. In order to minimise the time taken to form and eject a tablet, it is desirable to minimise the stroke of the punch  24  of the tablet press by minimising the distance between the free end  25  of the punch  24  and the top of the die assembly  36 . The result of this is that there is insufficient clearance between the free end  25  of the punch  24  and the top of the funnel  37  to allow powder to be poured into the die member  38  with the die assembly  36  in position within the press. Instead, the die assembly  36  would have to be removed from the press  10  for filling, and then reinserted and correctly aligned with the working axis  46  between every operation. 
     The pivoting action of the die mounting apparatus  100  allows for the funnel  37  to be pivoted away from the working axis  46  to give sufficient clearance for filling. The U shaped tray  110  can be pivoted to an angle between the extreme positions shown in  FIGS. 3 and 4  so that the bore  41  of the die member is clear of the punch  24 , but the angle of the funnel  37  does not pass beyond the horizontal. 
       FIG. 5  shows a schematic view of a modification allowing the tray  110  of the die mounting apparatus  100  to be held at an angle of approximately 45 degrees for a filling operation. Two notches  130 , 132  cut in upper surface  121  of the sidewall  120  allow the blocking portions  122   a , 124   a  of the existing selectively engageable stop members  122 , 124  to engage with the sidewall  120  and hold the tray  110  at a fixed angle between the compaction and sliding positions.  FIGS. 6 and 7  show the same modification with the tray  110 , respectively, in the horizontal compaction position of  FIG. 3  and the vertical sliding position of  FIG. 4 . In neither case does the presence of notches  130 , 132  affect the interaction between the upper surface  121  of the sidewall  120  and the fixed stop  116 . 
     The blocking portions  122   a , 124   a  need extend no further inwardly of the surrounding frame than the thickness of the sidewall, allowing freedom of the positioning of the notches  130 , 132  without impacting on the space for the die assembly  36  within the tray  110 . Alternative positioning of the notches  130 , 132  along the upper surface  121  or along an end of the sidewall  120  would allow fixing of the tray at different angles. Indeed, the use of notches  130 , 132  rather than simple stops allows a single selectively engageable stop member  122 , 124  to hold the tray  110  at a particular angle. As such, it would be possible to alter the spacing of notches so that they could not simultaneously be engaged by the two selectively engageable stop members  122 , 124 . This would result in two fixed positions between the compaction and sliding positions. 
     Accurate and precise positioning of the die assembly  136  in the positions shown in  FIGS. 3 and 4  is important for effective operation and to avoid damage to the end  25  of the punch  24  during operation of the tablet press  10 . Precise positioning is less important for the filling operation, where a user could simply hold the die assembly  36  in place while filling. As a result, notches such as those shown in  FIGS. 5 to 7  need not necessarily be provided in order for the invention to provide the described benefit. 
     A further alternative die mounting apparatus  300  is illustrated in  FIGS. 16 to 19 . The alternative mounting apparatus  300  is similar to the earlier described apparatus  100 , comprising a similar generally U shaped surrounding frame  302  supporting a smaller pivoting U shaped frame, or tray  310  on bearings  314 . The smaller U shaped tray  310  receives a die assembly  36 , and provides a similar pivoting arrangement as described in relation to  FIGS. 3 and 4  above. In each of  FIGS. 16 to 19  a removable punch  24   a  is shown received within the bore  41  of the die assembly  36  to close the bore  41  and avoid possible contamination when it is not in use. 
     The main difference between the alternative die mounting apparatus  300  of  FIGS. 16 to 19  and the apparatus  100  of  FIGS. 3 and 4 , is that the alternative die mounting apparatus  300  comprises only a single selectively engageable stop member  322 . 
       FIG. 16  shows the same ‘sliding configuration’ as  FIG. 4 . The single selectively engageable stop member  322  has a blocking portion  322   a , which is largely concealed in  FIG. 16 , and a lever  322   b . A central portion  323  of the selectively engageable stop member  322  is mounted at a pivot  325  to the base  304  of the surrounding frame  302  adjacent one of two upstanding wall portions  306 . The base  304  of the surrounding frame  302  extends beyond one end of the upstanding wall portions  306 , allowing the pivot  325  to be aligned with one end of the upstanding wall portions  306 . The pivot  325  is provided by a bolt in the illustrated example. 
     As shown in  FIG. 16 , the blocking portion  322   a  extends along the inside of one upstanding wall portion  306  and engages with an upper surface  321  of a sidewall  320  of the smaller tray  310 . This can be seen more clearly in the cross sectional view of  FIG. 17 . The fixed stop  316  in this embodiment can also be seen at the opposite end of the surrounding frame  302 , and the smaller U shaped tray  310  is held in a vertical position between a vertical face  328  of the fixed stop  316  and the blocking portion  322   a.    
     The central portion  323  of the selectively engageable stop member  322  has a curved outer surface so that it can pivot away from the position shown in  FIG. 16  through up to  90   o  so that the blocking portion  322   a  extends across at right angles between the two upstanding wall portions  306  and the lever  322   b  abuts the end of its adjacent upstanding wall portion  306 . The tray  310  is then free to pivot away from the vertical face  328  of the fixed stop  316 , past the blocking portion  322   a , into a horizontal position similar to that shown in  FIG. 3 . The selectively engageable stop member  322  can then be rotated back to the position described in  FIG. 16 , to arrive at the configuration shown in  FIG. 18 . 
       FIG. 18  shows the alternative die mounting apparatus  300  with the tray  310  locked in the horizontal ‘compaction configuration’. The blocking portion  322   a  of the selectively engageable stop member  322  is again obscured from view by the tray  310  and surrounding frame  302 , and its position will be better understood with reference to the cross-sectional view of  FIG. 19 . 
     As shown in  FIG. 19 , the underside of the tray  310  is engaged with a horizontal surface  318  of the fixed stop  316  at one end of the supporting fame  302 , and with an upper surface of the blocking portion  322   a  at the other. The tray  310  cannot, therefore, rotate from this position until the selectively engageable stop member  322  is rotated out of this position as described above. A chamfer  311  at the end of the tray  310  helps to provide sufficient clearance for the tray  310  to rotate past the blocking portion  322   a  when disengaged without compromising the compact overall size of the apparatus  300 . 
     As with the earlier apparatus  100 , the axis about which the tray  310  rotates passes through the slider block  40  of the die assembly  36  and will be positioned to intersect the working axis  46  of a tablet press at right angles in use. Therefore, the punch  24  of a press will align with the slider block  40  of the die assembly in the sliding configuration of  FIG. 16  and with its bore  41  in the compaction configuration of  FIG. 18  without any movement of the assembly  300 . 
     It will be understood that the apparatus  300  of  FIGS. 16 to 19  provides the same benefits as the earlier described apparatus  100  of  FIGS. 3 and 4 , including significantly, the repeatable movement of the die assembly  36  between a horizontal position and a vertical position, but with only a single selectively engageable stop member  322 . 
     Although described in relation to a manually operated system, the invention also provides benefits in more automated tablet manufacture, with a separate automated weighing/measuring of powder and automated delivery into the die member  38 . In an automated system, the pivoting action between the tray  110 , 310  and the surrounding frame  102 , 302  could be powered by a motor, and the selectively engageable stop members  122 , 124 , 322  could likewise be automated. Alternatively, all stop members  116 , 122 , 124 , 316 , 322  could be omitted and the entire operation of pivoting and fixing the tray  110 , 310  in desired positions could be achieved using a stepper motor with suitable control architecture. 
       FIGS. 8 and 9  schematically show a modified die assembly  36 ′ which would be of particular use in an automated system as described above, but would also find use in a manually operated system. The modified die assembly  36 ′ is similar in many ways to the known die assembly  36  described in relation to  FIGS. 1 and 2 . 
       FIG. 8  shows the die assembly  36 ′ in the compaction position. The mounting portion  43  of the die member  38  is joined to a base plate  42 ′ by a side wall  44  at the rear of the die assembly  36 ′ as shown. The other, opposite, side wall  44  has been omitted in  FIG. 8  so as not to obscure the slider block  40 . The bore  41  of the die member  38  and the opening  45  of the slider block  40  are illustrated in broken lines. Significantly, the base plate  42 ′ is not flat, but instead incorporates a slope from a position below the bore  41  to one end of the die member  36 ′. The base plate  42 ′ also has a flat area which together with the mounting portion maintain the slider block horizontal to provide a flat base for forming a tablet. 
       FIG. 9  shows the same modified die assembly  36 ′ in an ejection position. With the slider block oved to align the opening  45  with the bore  41  in the die member  38 , a tablet ejected from the bore  41  will pass through the opening  45  and down the sloping surface of the base plate  42 ′ where it can be collected in a suitable container. 
     As an alternative to the sloping surface illustrated in  FIGS. 8 and 9 , it should be understood that a sloping channel could be cut through a base plate to retain a larger flat upper surface to keep the slider block  40  horizontal. The sloping surface or channel could even be provided in one end of the slider block, provided that sufficient clearance was allowed for the tablet to clear the lower end of the bore  41  in the die member  38  when the slider block was moved to the ejection position. 
     Further detail of the compaction mechanism is shown in  FIGS. 10 and 11 . In  FIG. 1  a large part of the mechanism was obscured by the housing  14  of the tablet press  10 , so in  FIGS. 10 and 11  the mechanism is shown in isolation. 
     The lower part of the mechanism, generally indicated  200 , comprises an upper plate  202  and a lower plate  204  which are separated by four static pillars  206  to form a fixed frame. Each of the two support pillars  20  of the mechanism, as can be better seen in  FIG. 11 , extends through apertures in the upper and lower plates  202 , 204  between a pair of the static pillars  206 . 
     A yoke  208  is attached to both support pillars  20  between the upper and lower plates  202 , 204 . The interior of the yoke  208  provides a ball screw with a threaded rod  210  which is mounted on thrust bearings in the upper and lower plates  202 , 204  and passes through the centre of the yoke  208 . A driven gear  212  is provided on one end of the threaded rod  210  below the lower plate  204  so that the threaded rod can be driven in rotation by a drive gear (not shown) connected to a motor  214  which is secured to the lower plate  204 . The ball screw arrangement provided by the yoke  208  and the threaded rod  210  provides a low friction linear actuator so that the yoke  208 , and therefore the support pillars  20  can be moved vertically with great precision. 
     On the sides of the mechanism, outside the support pillars  20 , additional support rods  218  are provided. The support rods  218  are fixed to extensions of the yoke  208  at their upper ends, while their lower ends pass through holes in extensions of the lower plate  204 . Collars  220 , of a larger diameter than the support rods  218 , are provided at both ends of each support rod  218  to form location sites for a pair of compression springs  222 . The compression springs  222  ensure that a biasing force is constantly present on the yoke  208  so that the ball screw is properly and consistently seated. This helps to avoid the possibility of any remaining play/slop within the ball screw causing inconsistencies in the precise position of the yoke  208 . 
     The various features described above allow for great precision and confidence of relative position of the yoke  208 , and therefore of the support pillars  20 , for any given rotational position of the motor  214 . As a result, reliable operation of the tablet press can be achieved simply through appropriate control of the motor  214  without the need for dedicated sensors to monitor the position of the punch  24   
     As illustrated in  FIG. 10 , the mechanism is at the upper end of its movement, and the lower ends of the support pillars  20  and of the spring support rods  218  can just be seen extending below the lower plate  204 . A block  224 , which is fixed to the yoke  208 , is shown with its upper end adjacent a microswitch  226  provided on the upper plate  202 . Microswitch  226  acts as a limit switch at the upper end of the movement of the mechanism during calibration of the tablet press  10  and/or as a failsafe during use. A similar microswitch  228 , provided on the lower plate, serves the same purpose at the lower end of the movement. Although not illustrated, it will be understood that the lower ends of the support pillars  20  and of the spring support rods  218  will extend noticeable further through the lower plate  204  when the mechanism is at the lower end of its movement range. 
       FIGS. 10 and 11  also show additional detail of the upper part of the mechanism. The upper ends of the support pillars  20 , above the upper plate  202  and flange  230 , are housed within telescoping pillar guards  232  to shield the support pillars, and the remainder of the mechanism from powder and any other debris. The upper load cell  26  is housed within the support member  22 . The second, lower, load cell assembly  234 , located below a modified die assembly  236 , is also shown in  FIGS. 10 and 11 . 
       FIG. 12  shows some details of the modified die assembly  236  and the lower load cell assembly  234 . The load cell assembly  234  comprises a compression load cell  238  within a housing  239 . A head of the load cell  238  extends through the upper end of the housing  239  and is located below the bore  241  of the die assembly  236  and, in use, in line with the with the working axis  46  of the tablet press  10 . 
     The die assembly  236  shown in  FIG. 12  is similar in many respects to the die assembly  36  previously shown in  FIG. 2 . A slider block  240  provides a movable floor/base with an opening  245  to receive an ejected tablet once formed. One difference is that the slider block  240  comprises a region, in the form of a small disc  246 , that is vertically movable relative to the remainder. The disc  246  is located directly beneath the bore  241  of the die assembly  236  when in the compaction position as shown, and is held in place within the remainder of the slider block  240  by a ball spring screw  248 . Also in contrast with the die assembly of  FIG. 2 , the base plate  242  extends only beneath one end of the slider block  240  of the die assembly in  FIG. 12  such that the disc  246  of the slider block  240  can rest directly on the head of the compression load cell  238 . As a result, the lower load cell  238  provides a direct measurement of the reaction force at the lower end of a column of powder within the bore  241  during compression. 
     The tablet press  10  of the present invention is designed to form small batches of tablets, often from bespoke or unusual formulations of powder. As described above, these formulations are received in the die assembly  36 , 236  to provide effectively a column of powder, often with unknown compaction characteristics, aligned with the working axis  46  of the tablet press  10 . The mechanical interactions of powders during compaction are complex, and it is quite possible that interactions within the column of powder will result in discrepancy between the force experienced at the upper end of the column and at its lower end. In particular, during formation of a tablet there will be frictional losses within the powder and lateral pressure on the sidewalls of the die due to compression of the powder. These can differ significantly between different powder compositions. The die wall pressure provides useful information about a tablet composition, but is extremely difficult to measure directly. 
     By providing first and second load cells  26 , 238  as described above, separate readings can be obtained for the opposite ends of the column of powder within the die assembly  236 . The difference between the readings quantifies the losses occurring within the die assembly  236 . This information can be used to calculate the die wall pressure for any particular powder composition in a way that is being compressed. 
     The operation and control of the tablet press by a controller will now be described. The tablet press  10  comprises one or more processors, typically in the form of a microchip, and a data store or memory for controlling actuation of the punch by the motor  28  in accordance with user inputs. 
     The tablet press further comprises means for establishing a data connection with a separate computing means. In this embodiment, the tablet press  10  is connected by a lead  50  to a laptop  54 . Additionally, or alternatively, a wireless data link may be established in different embodiments by providing the tablet press with conventional wireless data transfer hardware, such as may be required for data transmission/reception by radio using, for example Wi-Fi, GSM, 3G, Bluetooth or other communication standards. 
     Whilst a laptop  54  is shown in  FIG. 1 , the reader will appreciate that numerous forms or alternative computational equipment exist which could be substituted, such as, for example, a desktop personal computer, PDA, mobile/cell phone, computer tablet or similar. 
     The operating system for the tablet press comprises two parts. The processor in the tablet press  10  itself is provided with machine-readable code in the form of firmware. The PC  54  is provided with software that controls the display of an on-screen user interface  32 . 
     Reference will now be made to the flow chart of  FIG. 13 , which shows an example of a tablet compaction routine in the firmware. 
     After switching the tablet press on at  52 , the firmware enters a machine start-up sequence at which point the tablet press waits until the PC software is started. 
     The tablet press then initialises by actuating the motor  28  such that the punch is moved to a fully retracted position, as determined by the first microswitch  226 . This position serves as the datum position for the machine. Any settings stored in the memory from a previous instance of use are retrieved from the memory. 
     Once the tablet press firmware establishes data communication with the PC, tablet pressing parameters can be set at  55  or altered using the user interface  30  on the tablet press  10  or an interface on the PC  54 . The parameters that are required for entry or upload by a user comprise the following:
         a. Compaction mode: Either fixed thickness or fixed load modes are available. In fixed thickness mode, the contents of the die will be compacted until the die reaches a specified position. In fixed load mode, the compaction continues until a specified load is applied to the punch (as determined by the load cell  26  and/or  238 );   b. Target thickness or load: The desired tablet thickness or maximum load, depending on the mode set in (a) above;   c. Compaction speed;   d. Die diameter: This is for information and is shown on the header of exported reports, but, in this embodiment, has no bearing on the compaction itself;   e. Die thickness: The total thickness of the die, which is used to calculate positions during the compaction routine.       

     Before a compaction can be started, the position of the bottom of the die is established by the firmware at stage  60 . The insertion of different dies into the press may change this parameter. The determination of the location of the floor of the die relative to the datum position at  60  is achieved by placing the empty die in the machine and starting the ‘new size’ procedure. The firmware controls actuation of the punch  24  downwards until it touches the die floor member  40 . The distance of travel and/or position of the die floor  40  relative to the datum position is stored. The punch  24  then retracts out of the die  38 . 
     The die is now loaded with powder by a user. This may be achieved by removing the die  38  or die assembly  36  and inserting powder therein using a suitable dispensing device. Alternatively, this may be achieved in-situ. Once the die and powder therein is correctly positioned in the tablet press  10 , the compaction stage can begin. 
     The compaction is started from the PC. The firmware is able to calculate a number of positions at point  62  in  FIG. 13 , comprising:
         i. Stop position: this is used in ‘fixed thickness mode’, and is defined as the bottom-of-die reference position minus the target thickness set at stage  55 ;   ii. Compaction speed position: this is the position at which the punch switches from full speed movement to compaction speed, as set in stage  55  above, and is defined as a predetermined distance above or below the top of the die, such as for example 5 mm below the top of the die in this example;   iii. Return position: The position the punch returns to after the compaction, defined as a predetermined distance above or below the top of the die, such as for example 2 mm above the top of the die in this example.       

     At  64 , a tablet description (identifier) can be input by the user via the PC interface. This is shown on exported reports. 
     The determined parameters are sent back to the PC by the tablet press firmware at  66 , along with an indication that the compaction is starting. 
     The firmware then controls operation of the motor  28 , 214  in conjunction with the digital encoder such that the punch  24  moves downwards at full speed until the compaction speed position (as calculated at stage  62 ) is reached. This position is determined by control loop  68 , at which point the firmware controls the change in operation of the motor  28  to operate the punch at the compaction speed, which is constant for the compaction phase of the process. 
     At  70  the punch  24  continues its downward movement such that it comes into contact with powder in the die. The change to compaction speed also triggers a signal from the tablet press to the PC such that the PC software will start plotting a graph of load against position for the punch. The load reading is taken from the load cell  26  and/or  238  and the position is determined by the angular position of the motor in accordance with the digital encoder. 
     Further downward movement of the punch compacts the powder in the die until either: the required position (calculated in (i) above) is reached  74   a , when in the ‘fixed thickness’ mode; or, the required load (set in b above) is reached  74   b , when in the ‘fixed load’ mode. In either mode, the compaction will be aborted if the load cell is overloaded. 
     The punch then stops. The punch may be held for a predetermined period at this position. In particular, the user may provide a desired dwell time at  75  for a particular operation of the tablet press. The dwell time may be input as an absolute value, or as a function of the set compaction speed. 
     The ability to control the dwell time of the tablet press  10  is significant because different mass production tabletting machines will hold the powder under compression for different lengths of time during a tablet forming operation, and it is important that the tablet press  10  of the present invention can simulate the operation of various different mass production machines. Furthermore, typical mass production machines operate in such a way that the compaction speed of the punches and their dwell time (the time while the powder is held under maximum compression) will vary with the speed of the machine. The invention allows for the dwell time to be similarly varied along with the compaction speed to simulate the operation of such machines. This all improves the capability of the tablet press  10  to accurately predict how a particular powder will behave in real world applications. For example, tests may show that a particular powder performs only if the dwell time is sufficiently large. This information could be used to provide a list of recommended tabletting machines and/or maximum operational speeds for manufacture. 
     When the dwell time at  75  is reached, the motor is controlled to retract the punch at compaction speed for a predetermined distance, such as, for example 2 mm. Graph plotting then ends. The motor then actuates the punch in the retraction direct at full speed to the datum position at  76 . 
     The user is given the option to eject the tablet from the die at  78 . If this is manually declined by the user, the routine ends and the firmware returns to a ready condition for a further compaction. 
     During ejection, the punch initially runs downward at full speed at stage  80 , until the compaction speed position is reached. The punch then continues at compaction speed at stage  82 . This motor control sequence is similar to that of the compaction itself and is not repeated here for conciseness. 
     The ejection process continues until the controller determines that the punch end  25  has reached the location of the bottom of the die (i.e. the location at which the floor member  40  was previously present). Once the bottom of the die is reached, the punch reverses to the return position. During the ejection process the load cell  26  and/or  238  may continue to measure the forces in order to determine the ejection force of a tablet. Similar assessments can be made during the process of moving the floor member  40  if this step is performed using the punch  24  of the tablet press. 
     The tablet press and associated firmware now return to a ready condition in which the tablet press is able to start the next compaction, or for settings to be altered. 
     Whilst the above embodiments make use of both on-board firmware and external computer software, it is to be noted that the tablet pressing process can be carried out entirely under the control of the machine firmware if necessary. The user may enter the necessary data using the keys  34  in response to simple prompts on display screen  32 . However, it is felt that the combined use of basic firmware and more advanced software running on a connected computer offers useful functionality that would otherwise add expense to a stand-alone tablet press device. However, any, or any combination, of on-board and remote or external data processing is envisaged as being possible based on the foregoing description. Any reference to a ‘controller’ herein may refer to one or more processors arranged either onboard the tablet press or in communication therewith to achieve the desired control function. 
       FIG. 13  shows only an overview of the ‘fixed load’ compaction mode in the region designated  72 . The additional control steps provided when the tablet press  10  is operating in the ‘fixed load’ mode allow for a pause to be provided within a compression operation and for a change in compaction speed, and are illustrated in  FIG. 14 . 
     At  86  the controller checks if the user has set an intermediate load value for the compaction operation, and if so the controller checks to see if this load has been reached at  88 . Once the limit has been reached, movement of the punch  24  is stopped/paused for a time period  90 . After this pause, the controller goes on to check, at  92 , whether or not a second compaction speed, which may be higher or lower than the compaction speed of step  70 , is required to complete the compaction operation. The punch is then moved downwards again, at the desired compaction speed, until the required load at  74   b  is reached as described in relation to  FIG. 13 . 
     The invention also relies on motor control to hold the punch  24  stationary during the pause  90  and/or dwell time  75 . In either case the motor remains energised, but set at zero speed, to provide a braking force and avoid the punch  24  moving under the force from the compressed powder/tablet. This can be achieved by, for example, by shorting the two ends of the motor winding together. This type of control reduces overrun/coasting of the motor to a stop, so allows the motor to be stopped quickly as well as ensuring that the resulting static position can be precisely maintained. 
     An example of a compression profile which can be achieved using the control of  FIG. 14  is shown in  FIG. 15 . In the illustrated example, a compression is initially performed at a first compaction speed  70 , paused for a set time  90 , and then continued at a second, faster, compaction speed  92 . As previously noted, the interactions within powders during are complex, and the tablet press of the invention will often be used with new or bespoke compositions. Including a pause during the formation of a tablet, can beneficially provide a time for powder to settle after an initial load has been applied, but before full compaction is achieved. 
     For example, a predetermined change of speed can allow the initial compaction to be performed more gently, if a known composition requires, before the main compaction is performed more quickly to minimise production time. Alternatively, the initial compaction may be performed quickly, to settle the powder before a more gentle main compaction is used. In either case, the duration of the pause can be set at any desired value, including zero. 
     Alternatively, the second compaction speed and/or the duration of the pause may be determined using feedback from the load cell  26  and/or  238  once the intermediate limit value  88  has been reached. The reaction forces from a column of powder may change as the partly compacted powder is held static under pressure, possibly as a result of particles settling or moving, or because of the absence of dynamic friction. By pausing the compaction and continuing to monitor the readings from the load cell  26  and/or  238  important information about the behaviour of a particular powder, can be determined and, if appropriate, the remainder of the compaction operation can be automatically modified. This is of particular use when working with an unknown bespoke powder composition for the first time. For example, a first compaction speed can be set based on a suspected similar composition, and a relatively low intermediate load could be set. The behaviour of the composition can then be compared with that of the known composition under equivalent conditions to determine whether or not the compaction speed is appropriate for the new composition or should be changed. 
     It should be understood that the control features discussed above allow for a great deal of control over the compaction operation. The mechanism of the tablet press  10  and the operation of the motor described above also help to ensure that accurate and precise control of the movement of the punch  24  is achievable, simply through motor control, and that the various limit values or positions can be achieved and held precisely.