Rotary tablet press forces

During tablet compression, the distance between the rollers remains constant unless a machine adjustment is made to change tablet hardness or thickness. Additionally, all tooling dimensions (tooling length and die cavity size) are constant within established standards. Under these conditions, for a specific material of uniform density, if the same volume of material is delivered to each die, the maximum measured compression force for each punch station is the same. If, on the other hand, different volumes of material are delivered to each die, the maximum measured compression force for each punch station is different. On this basis, adjustment of fill depth (fill volume) to maintain a constant compression force should result in a constant tablet weight. This concept is the general basis of all rotary tablet press force control systems. 

Subsequent to these successful pilot scale trials, the formulation was scaled-up to the production launch site. Once again, only those parameters recognized as critical were modified during manufacture. The compression parameters that were modified were speed and hardness. A total of four production-sized batches were manufactured, with the first batch being compressed on a 43 station Fette 2080 rotary tablet press, and three subsequent batches compressed on a 36 station Fette 2090 rotary tablet press. Three compression speeds and three compression forces were successfully evaluated... 

The pharmaceutical industry produces tablets almost exclusively on rotary tablet presses from pilot plant to commercial manufacture. The output from different tablet presses may range from a few thousand tablets per hour to more than 1 million tablets per hour. By design, the compression event occurs using three parts a die, lower punch, and upper punch. The dies and punches are mounted on a rotating turret. The shape of the die controls the shape of the tablets, while the distance between the lower and upper punch tips at the maximum compression force determines the thickness of the tablets. The tablet compression process is divided into three steps powder filling into the die, compression, and tablet ejection from the die.85... 

Oates, R. J., and Mitchell, A. G. (1994), A new method of estimating volume during powder compaction and the work of compaction on a rotary tablet press from measurements of applied vertical forces, J. Pharm. Pharmacol., 46, 270-275. 

It is suggested that four mechanisms are basically involved in the process of compression of particles deformation, densification, fragmentation, and attrition. The process of compression is briefly described as follows small solid particles are filled in a die cavity and a compression force is applied to it by means of punches and then the formed monolithic dosage form is ejected. The shape of the tablet is dictated by the shape of the die while the distance between the punch tips at the point of maximum compression governs the tablet thickness, and the punch tip configuration determines the profile of the tablet. The compression cycle in a conventional rotary tablet press will be described in detail in this chapter and is illustrated in Figure 1. 

Compaction simulators are single-station machines capable of mimicking the in-die compaction event that occurs on a rotary tablet press in real time. Simulators have been used to predict material behavior on scale-up and to evaluate various compaction parameters (punch force, ejection force, displacement, speed, etc.). Hydraulic compaction simulators (ESH) as well as mechanically driven machines (i.e., Presster and Stylecam ) are available for such studies. 

Tablet scrape-off occurs immediately after ejection. Fig. 2 illustrates a tablet stripper on a rotary tablet press. Typical forces during tablet scrape-off are 2N or less. Scrape-off forces of 6N or higher result in tablets sticking to the lower punches and subsequently picking or, under extreme circumstances, shearing the bottom of the tablet. Frequently, shearing of the lower portions of the tablet due to scrape-off problems is mistaken for capping. However, this can be easily distinguished by examining the lower punches and rotating the press manually.

Because the compression characteristics of powders are time-dependent (the exact extent of this dependency depends on the primary modes of deformation), the final tablet properties depend not only on maximum compression forces but also on the rate at which these forces (rate of deformation) are applied and removed. On a rotary tablet press, the rate of deformation is determined by the tangential velocity of the punch and the compression roller diameters. The tangential velocity of the punch is a product of the press speed and the die table circumference (i.e., die table rpm x 3.14 x pitch circle diameter). As the tangential velocity increases, the rates of compression and decompression increase while the overall compression time decreases. The roller diameter affects both the rate of compression and decompression. As the diameter increases, the rates of compression and decompression decrease. 

Although most rotary tablet presses operate by maintaining fixed roller positions during compression, some designs incorporate a compression compensator system in which the counterforce for compression is air pressure. This system compresses to a constant force and allows roller movement when the preset force is achieved. Under these conditions, potential exists to increase the time that the force is maintained near its peak value (approximately 90% of maximum). Compression to a constant force should theoretically provide a more uniform tablet hardness and more uniform dissolution profiles while allowing a greater variation in tablet thickness. 

Many modern rotary tablet presses use off-the-shelf load cells for force measurement. These load cells are highly accurate, durable, and easily replaced and calibrated. However, the final accuracy and repeatability of force measurement in the machine not only depend on the quality of the load cell, but also on the design of the compression assembly and the placement of the load cell within the assembly. 

It should be noted that this discussion assumes that only one punch is actively applying the force to the powder mass while the other is stationary and passive. This is true in the case of an excentric press, but with a rotary tablet press, both punches move and hence both exert forces on the powder bed. The force distribution so obtained is thus different from that shown in Fig. 7, and results in two low density zones near the faces of the tablet and a high density zone in approximately the centre of the powder mass. 

Williams, J.J. Force Measurement and Analysis Particularly Relating to Rotary Tablet Presses. US Patent 4,030,868, 1977. 

Mechanically powered compaction simulators are also available to replicate the compression event of tablet presses (3). These machines leverage the design of traditional rotary tablet presses where the punches are forced between a set of roll wheels to enable the compression event. The punch type and roll wheels can be changed to replicate the compression event of different press types. In some models, the fill station, compression station(s), and ejection station arc aligned in series on a linear track, where the punches and die travel along this track from station to station to complete the fUl-comprcss-ejection cycle. 

Precompression is often used to tamp or apply a small compression force prior to the main compact compression cycle. Rotary tablet presses are often equipped with a separate precompression station, which is positioned between the die-filling feed frame and the main compression station. Typically, precompression is used to improve the quality of tableted products, where it increases the strength of the compact and/or decreases the incidences of capping and lamination. The compact strength is enhanced through the increase in the effective contact time in which the powder particles are in contact under an applied force. During this extended contact time, stronger interparticulate bonds form and stress relaxation occurs. 

Compaction simulators are most commonly used to replicate the compaction process of a rotary tablet press. Manufacturing parameters, such as production speed, dwell lime, compression force, precompression force are often studied using a relatively small quantity of bulk powder. These parameters can significantly affect tablet crushing strength, friability, and disintegration time. The relationship among compaction pressure. 

Granulate Mixture I with Solution II, dry, sieve and mix in the components of III, then press into tablets on a rotary tablet press using a low compression force. 

Below is an example of pilot work in support of a scale-up to production. In this example, the final blend was compressed on a 16 station rotary Beta press using ovaloid tooling. A compression profile, as shown in Figure 9, was generated. The speed of the tablet press was fixed at 78 rpm. Forces ranging from 8 to 28 kN were evaluated. Lamination was observed at forces greater than 25 kN. The curve was relatively flat between 15 and 25 kN forces with hardness ranging from 175 to 215 N. 

The details of instrumentation for tablet presses is thoroughly described in several texts and review papers [1,2,9,10] and will not be repeated here. The concepts, however, are important. The first and most important result of the instrumentation results in a plot of force vs. time (see Fig. 1). This shows the maximum force (or pressure) used for compaction of a tablet and such plots become even more important on a rotary press, when this measurement can be made on each tablet produced. 

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. ... 

Tablets are produced using tablet presses. While these presses vary in their output, from approximately 3,000 tablets per hour to more than 1 million per hour for the fastest machines, the principle of manufacture remains the same. Powder is filled to a specified depth in a die and compressed between two punches. The compression force is ended by removal of the upper punch, and the lower punch then moves upward in the die to eject the tablet. Presses can be divided into two types, single punch (or eccentric) presses and rotary presses.

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