Rotary tablet press punches

FIGURE 2 Rotary tablet press punch and die terminology. Source Adapted from Ref. 6. 

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. (1989), Calculation of punch displacement and work of powder compaction on a rotary tablet press,/. Pharm. Pharmacol., 41, 517-523. 

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.

Pharmaceutical tablets are generally produced on rotary tablet presses (Fig. 3), where upper and lower punches reside in the upper and lower turret, respectively. The dies are inserted in the die table and secured by die lock screws. The upper and lower turret and the die table are precisely aligned. The movement of the... 

Upper precompression and main compression rollers insertion depth adjustments Insertion depth for both precompression and main compression is adjusted in the upper cam section. The insertion depth determines the location of tablet formation in the die cavity relative to the top of the die table as shown in Fig. 5. It is measured as the distance at which the upper punch enters into the die at the tangent between the upper punch head and the compression roller. Insertion depth can be varied between 2 and 6 mm on most machines and is typically maintained between 3 and 4 mm. For precompression and main compression, the insertion depth should be maintained at approximately the same position. On most modern rotary tablet presses, the adjustments for precompression and main compression insertion depth are independent. However, on many older designs, the precompression roller is attached to the main compression roller assembly and its position is measured relative to the main compression roller position. In this way, the ratio of precompression to main compression remains constant as machine adjustments are made. 

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. 

Lower Punch Brakes. Most rotary tablet presses are equipped with lower-punch brakes that are Teflon tipped and spring loaded to apply constant pressure to the lower punches. Alternatively, some manufacturers apply pressure to a friction belt that provides resistance on the lower punches. The lower-punch brakes act as a retention system for holding the lower punches in place during press setup. More importantly, these systems help to minimize lower-punch chatter at high press speeds thus minimizing tablet weight variation. Some press manufacturers use the lower punch seals to retain the lower punches. 

Most high speed rotary tablet presses employ automatic lubrication systems during operation. Effective punch... 

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. 

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. 

Transducer output from a rotary tablet press differs in two aspects. Firstly, the lower punch plays an active role in the compression event and moves upwards as the upper punch moves downwards. The second difference is small but important. Because of the sinusoidal movement of the upper punch in an excentric press, the punch speed is only zero at the instant when it reverses direction (point E). Punches on a rotary press have a flat area on the punch head. As the punches pass under or over the pressure rolls, the flat area dictates that there is no punch movement. The period during which this occurs is called the dwell time, and though it only lasts a fraction of a second, it can have a major effect on the consolidation process. ... 

Oates, R.J. Mitchell, A.G. Comparison of calculated and experimentally determined punch displacement on a 125. rotary tablet press using both manesty and IPX punches. 

In rotary tablet presses, only the dies are fixed in the die table, whereas the punches are vertically mobile and are not fixed to a holder. They slide up and down in the turret bores, driven by cam tracks guiding the punch heads. The dwell time, i.e., the period during which the tablet is under full compression, is determined by the diameter of the flat part of the punch head. 

Fig. 1 General terminology of tooling (sets of upper and lower punches and dies) for rotary tablet presses.

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. 

The basic powder compression operation of a rotary tablet press is rather simple and is essentially the same from machine to machine (Fig. 14). The powders are compressed by passing the upper and lower punch between two rollers (or cams), thus mechanically driving punches towards one another to a specified target tip separation. The distance between the rollers can be adjusted to control the maximum degree of powder consolidation within the die (i.e., minimum tip separation), which has a direct influence on final tablet properties, such as thickness and crushing strength. 

All the formulations were devised on rotary tabletting presses that were fitted with 10-20 punches. 

Fig. 6.3-14 Left) upper and right) lower punches and the die insert for the multi-die station of a rotary tabletting press (courtesy Kilian, Koln, Germany)...

Tablet Press. The main components of a tablet compression machine (press) are the dies, which hold a measured volume of material to be compressed (granulation), the upper punches which exert pressure on the down stroke, and the lower punches which move upward after compaction to eject the tablets from the dies. Mechanical components deflver the necessary pressure. The granulation is fed from a hopper with a feed-frame on rotary-type presses and a feeding shoe on single-punch presses. A smooth and even flow ensures good weight and compression uniformity. Using the proper formulation, demixing in the hopper is minimized.

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