Rotary tablet machines

Muller FX, Augsburger LL. The role of the displacement-time waveform in the determination of Heckel behaviour under dynamic conditions in a compaction simulator and a fully instrumented rotary tablet machine. J pharm pharmacol 1994 46 468-475. 

Figure 12.8. Operation and specifications of rotary tabletting machines (Carstensen, 1984). (a) Action of the punches of a rotary tabletting machine, (b) Specifications of a Sharpies Model 328 Stokes-Pennwalt Co.), (c) Specifications of a Manesty Rotapress Mk 11 Manesty Machines Ltd. and Thomas Engineering Inc.).

Tableting conditions for DC and WG A rotary tableting machine (KIKUSUI) is used with 12 punches (punch size 10mm diameter, 12mm radius, compression force 98,147, and 196MPa tableting speed 20,40, and 60min-1).

The first machines for the production of tablets were simple hydraulic or manually operated presses. Later, eccentric and rotary tableting machines were developed. Today, eccentric tableting machines are only used for research, in early development, or for special applications. 

The newest development for production is a machine which fills the dies by centrifugal force [34], It is a special rotary tableting machine. Another innovation is a special machine which operates by ultrasound [37]. 

Rotary tableting machines are commonly used for tablet production. The principle of all rotary machines is the same, with one exception, which will be discussed separately. According to Konkel and Mielck [39], the information gained with eccentric and rotary machines complement each other. 

Rotary tableting machines work with a number of punch and die sets which move in a circle. The dies are fixed in a round die table and the die table circulates. Together with the dies the lower and upper punches circulate on tracks. The lower punches close the dies. The densification process is bilateral since both punches pass the compression wheels and the force is evolving on the upper as well as on the lower side of the powder bed. The produced tablets show the same hardness on the upper and lower surfaces. 

FIGURE 4 Operation of rotary tableting machine with precompression. (Courtesy of Fette.)... 

FIGURE 5 Example rotary tableting machine left, machine view right, detail view into compression chamber. (Courtesy of Kilian Synthesis 500.)... 

In order to increase the production rate of rotary tableting machines, doublesided rotary machines were build which possess two pairs of compression wheels and two filling shoes. Thus during one rotation double the number of tablets are produced compared with a one-sided rotary machine. If these machines are equipped with precompression wheels one machine contains four pairs of wheels. 

Special Rotary Machines As already mentioned one special rotary machine works slightly different—called IMA Comprima (Figure 6) [34], In this machine the material is filled by centrifugal force from the side directly into the die. The upper punch closes the die at the top and the lower punch closes the die at the bottom. However, when the given volume in the die is filled by the powder, both punches move downward until the die is completely closed. Then the compression process starts and the dies pass the compression wheels. After compression the tablet is ejected by the upper punch at the bottom of the die, contrary to all other rotary tableting machines which eject the tablet at the top of the die. 

High-Speed Rotary Tableting Machines High-speed rotary machines work with the same principles as all other rotary machines. They possess a huge number of punch and die sets and often two filling stations. Another possibility is to use punch and die sets which are able to produce several tablets simultaneously. Special tooling can be used for this purpose however this is not the subject of this chapter. As... 

Thus the simulation of tableting machines needs much effort and a real simulation is almost impossible because of the hydraulic control. Further the filling process of rotary tableting machines cannot be simulated since die filling is usually processed by a filling shoe moving forth and back. 

More recently mechanical compaction simulators have been developed. The first was the linear mechanical rotary table ting machine simulator Presster (Figure 8), which was introduced in 1998 [55,56], It can mimic the mechanics of different rotary tableting machines and is called a linear rotary tableting machine replicator. The name Presster was combined from press and tester. 

A single pair of punches moves linearly forth and back on a lower and an upper punch track. For tableting the punches pass the compression wheels which are equivalent in dimensions to those of rotary tableting machines used in practice. These compression wheels are exchangeable. Different machines are simulated by exchanging them. 

The machine speed can be varied and different tableting machines are simulated by using similar dwell times between 5 and 80 ms. Special tests exhibited that rotary tableting machines can be simulated with a precision of 1-5% [57, 58], One major disadvantage of the Presster is that it works with a moving Filing shoe, and thus the filling process of rotary machine cannot be simulated. 

With the Stylcam different dwell times are obtained by adjusting the speed of the compression wheels. Precompression is simulated by compressing a tablet twice. Thus the time interval between the precompression and main compression is freely adjustable. One further advantage of the Stylcam is that it works with a fixed filling shoe, as on a conventional rotary tableting machine. However, data on the precision of this instrument are not yet available. 

Using mechanical compaction simulators allows us to simulate the tableting process of rotary tableting machines to a greater extent than when using hydraulical compaction simulators. Thus they will be mainly used in formulation development and scale-up. 

For measurement strain gages are mostly used. These strain gages consist of constantan. They are applied in eccentric tableting machines at the upper or lower punch holder and in rotary tableting machines at the machine frame or the compression roll pin. Alternatively piezoelectric crystals can be used which have to be placed inside the punches [40],... 

The most often measured force is the upper punch force. For the eccentric machine it is the force which controls densification for rotary tableting machines upper and lower punch forces have ideally the same values. Schmidt et al. [67] measured force with a single punch of a rotary tableting machine. Ejection force is visible as a small lower punch signal which occurs shortly after the end of one compaction cycle. It is measured by lower punch instrumentation but needs more resolution. A review of force measurement is given by Bauer-Brandl [68]. 

A measurement of displacement on a rotary tableting machine was presented in 1987 by Schmidt and Tenter [72]. Another possibility was presented by Matz and co-workes [73, 74], Meanwhile touchless measurement systems for recording displacement were developed [73,74], For all measurements of displacement, correc-... 

The most basic analysis is the presentation of force versus time or displacement versus time. These curves are different for eccentric and rotary tableting machines. The data given in Figures 12 and 13 are valid for the contact time of the compaction cycle of one single tablet. Due to the eccentric-driven movement of the punches, the force-time curve can be described by a sharp peak at the maximum force evolving at the punches and the displacement-time curve can be described with a sharp peak at the minimum height of the powder bed. For curves of eccentric tableting... 

FIGURE 13 Force -time and displacement-time profile for rotary tableting machine. 

Some basic parameters can be directly read from the curves. For the force values upper and lower punch forces and ejection forces should be mentioned, and for the time values contact time should be mentioned. Deduced parameters such as pressure and normalized contact time can be calculated and further statistical data are often used for characterization (Table 2). Due to the different shapes of force-time curves from eccentric tableting machines compared with those from rotary tableting machines, some parameters can only be calculated from eccentric machine data and some can only be calculated from rotary machine data. 

TABLE 3 Parameters Calculated for Eccentric and Rotary Tableting Machines ... 

Only eccentric machine data allow us to calculate the R value (maximum upper punch force/maximum lower punch force), which is an indication of friction. They also allow us to calculate the time difference between the maximum upper punch force and the maximum lower punch force. Only dwell time and the minimum force during the dwell time can be calculated for rotary tableting machine data. The rise time of rotary machines is defined as the time during the compression phase, and peak offset time is defined as the time difference between maximum pressure and vertical alignment of the punches. Further the inflection points during the compression and decompression phases are mostly only calculated for rotary machine data. 

In addition, for force-time diagrams different methods to characterize the tableting process were developed. These methods can be divided in those applicable to force-time curves from eccentric and rotary tableting machines [90] and those applicable only to data from eccentric or rotary tableting machines (Tables 3-5). 

A low product yield is caused by loss of material during fast production processes. On rotary tableting machines this problem is solved by slightly lowering the lower punches before the compression event starts. 

Konkel, P., and Mielck, J. B. (1997), Associations of parameters characterizing the time course of the tabletting process on a reciprocating and on a rotary tabletting machine for high-speed compression, Eur. J. Pharm. Biopharm., 44, 289-301. 

Picker, K. M. (2000), Three-dimensional modeling to determine properties of tableting materials on rotary machines using a rotary tableting machine simulator, Ear. J. Pharm. Biopharm., 50(2), 293-300. 

Lamey, K. (2000), Correlations between compaction simulators and rotary tableting machines, paper presented at the Compaction Simulator s User Meeting, Lough-bourough, UK. 

Vogel, P. J., and Schmidt, P. C. (1993), Force-time curves of a modern rotary tablet machine. Part 2. Influence of compression force and tableting speed of the deformation mechanisms of pharmaceutical substances, Drug Dev. Ind. Pharm., 19,1917-1930. 

Palmieri, G. F., Joiris, E., Bonacucina, G., Cespi, M., and Mercuri, A. (2005), Differences between eccentric and rotary tablet machines in the evaluation of powder densification behaviour, Int. I. Pharm., 298(1), 164-175. 

Press-coated tablets were produced on modified rotary tablet machines using special tools with hollow punches, where first an inner tablet was formed and then the outer shell was compressed in the same die ( tablet-within-a-tablet ). ... 

Munoz-Ruiz, A. Jiminez-Castellanos, M.R. Cunningham, J.C. Katdare, A.V. Theoretical estimation of dwell and consolidation times in rotary tablet machines. Drug Dev. 

In 1966, a U.S. patent was granted to Knoechel and co-workers for force measurement on a tablet press. This patent was followed by two seminal articles in Journal of Pharmaceutical Sciences on the practical applications of instrumented rotary tablet machines. A number of other patents related to press instrumentation and control followed from 1973 on ward. ... 

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