Pressing upper punch

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.

The actual compression process is a cycle of die fill, compaction by intervention of the upper punch using great pressure on the granulation material in the die, and upward movement of both punches to achieve ejection of the tablet from the die. Singe-punch presses have only one die-and-punch arrangement and the compression is quick, with Httle dwell time of the top punch in die. 

Pharmaceutical compressed tablets are prepared by placing an appropriate powder mix, or granulation, in a metal die on a tablet press. At the base of the die is a lower punch, and above the die is an upper punch. When the upper punch is forced down on the powder mix (single station press) or when the upper and lower... 

Figures 15 and 16 provide a summary of the compression cycles for rotary and single-punch tablet presses. The formation of the tablet compact in these two types of presses mainly differs in the compaction mechanism itself, as well as the much greater speeds achieved with rotary type presses. The single punch basically uses a hammering type of motion (i.e., the upper punch moves down while the lower punch remains stationary), while rotary presses make use of an accordion-type compression (i.e., both punches move toward each other). The former find their primary use as an R D tool, whereas the latter, having higher outputs, are used in most production operations.

All operations take place simultaneously in different stations. Sixteen stations were commonly used in earlier machines with outputs between 500 and 1000 TPM and tablet diameters up to 15 mm. Presses with outputs orders of magnitude greater than the above are now widely available. The dies are filled as they pass beneath a stationary feed frame, which may be fitted with paddles to aid material transfer. The die cavities are completely filled and excess ejected prior to compression. Compression involves the movement of both punches between compression rolls, in contrast to single station operations where only the upper punch effects compression. Ejection occurs as both punches are moved away from the die on cam tracks until the tablet is completely clear of the die, at which point it hits the edge of the feed frame and is knocked off the press. Tooling pressure may be exerted hydraulically, rather than through the use of mechanical camming actions, as is the case with machines produced by Courtoy. 

On all presses, the upper punch is set to come down to a specific point in the die cavity this position, which is set by the operator, controls tablet thickness. More specifically, this setting controls the compaction force (pressure) and, in turn, tablet hardness. 

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... 

Third, after the tablet is compressed, the upper punch is withdrawn from the die and lower punch moves upwards to eject the tablet. Successful ejection of tablets without chipping or sticking requires sufficient lubrication of the powder blend so there is minimum adhesion between the tablet and the die wall. Lower ejection forces are preferred during tablet production to avoid unnecessary mechanical wear on the tablet press. 

In this shaping process a powder is pressed between two punches in a pelleting press. The three different steps in the process are presented in Fig. 8.11. In step I the free-flowing powder is poured into the cylindrical hole. In step II the lower punch and the upper punch are moving towards each other and the powder is pressed at a pressure of about 102 - 5 x 103 atm and a pellet is formed. In step m the upper punch moves upwards and the lower punch pushes the pellet out of the hole. In the next step the pellet is moved away, the lower punch moves downwards and the hole is filled again. 

During the compression process on an eccentric press, there are other pressure ratios at the upper and lower punches. The pressure at the upper punch is usually higher than the pressure at the lower punch. A part of the pressure is lost in the material and in the resulting radial friction force against the die wall during the compression [1,2],... 

The equipment employed for tablet compression is generally categorized according to the number of compression stations and dislocation mode. Therefore, eccentric model presses have only one compression station (one die and one pair of punches, upper and lower) while rotary models have multiple compression stations (each station with one die and one pair of punches, upper and lower). The basic difference between the two types of compression equipment is that for eccentric models the compression force applied during compression is due to the upper punch whereas for rotary models it is mainly applied by the lower punch. 

Upper punch, lower punch, and die which accommodate one station in tablet press... 

Regarding the importance of compression tooling to the performance of the tablet press and the quality of the tablets, it is of paramount importance that punches and dies are handled with care. The first criterion is the identification of the tooling that is, punches and dies should be identified according to the standard and be designated by upper punch without key, upper punch with key, lower punch with key, lower punch without key, or die, the reference of the standard (e.g., TSM, EU, JN, ISO), and the punch or die diameter. Punches and dies should also have a marking that includes at least the manufacturer s identification, the number of the punch in the series, and/or the identification number. Upon... 

Online, force-loop-feedback systems are used for weight control on most production tablet presses. Upper and lower punch forces as well as mean punch force can be monitored by load cells for each station on the tablet press. Deviations between the target and actual compaction forces are measured, and adjustments to the punch filling depth are automatically made to bring the mean force back into the target range. In some cases, the speed of the force feeders on the press is adjusted by reducing the observed variability of the compression force data. 

Upper punch removal/dwell cams The upper punches are loaded and removed from the machine at this location. These cams typically reside directly above the material feeder. In many press designs, the upper punch dwell... 

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. 

Dust Extraction. Adequate dust extraction is necessary to maintain high-speed operation for extended periods of time. The entire compression area should be shrouded to minimize dust infiltration into other press areas. Effective dust extraction minimizes dust and oil contamination on the surface of the tablets, which could produce black specs. Insufficient dust extraction results in excessive material build-up on the lower and upper punches leading to tight punches. However, the proper balance of dust extraction without high levels of material loss must be determined. If the dust extraction level is too high material could be extracted from the die cavities and the recirculation channel. Furthermore, the dust extraction systems preferentially removes the fine particles. Therefore, if the granulation is a direct-compression blend where the active constituent is of fine particle size, minimum dust extraction levels combined with minimal recirculation may be necessary to prevent a loss of active constituents (resulting in possible low assay). 

Fig. 8 shows the output from force and displacement transducers fitted to both punches of an excentric tablet press. The upper punch describes an approximately sinusoidal path as it descends to penetrate the die (point A) and then rises after the compression event has taken place, leaving the die at point B. The lower punch remains motionless during the compression event and then rises to eject the tablet from the die (point C). 

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. ... 

The rate of consolidation in an excentric press is governed by the speed at which the upper punch moves into the die. This in turn is governed by the lengths of the excentric arms of the press and its speed of operation. In a rotary press, punch speed is governed by the diameter of the die table, the diameter of the pressure roll, the geometry of the punch head, and the speed of operation. Formulae for the calculation of punch speeds at any point of the compression cycle for both types of press have been derived. ... 

A rotary press pull-up cam can be instrumented to measure the upper punch pull-up force (the force required to pull up the upper punch from the die). Likewise, the lower punch pull-down force is measured on a bolt holding the pull-down cam. " It is useful in determining the smoothness of press operation (extent of lubrication, cleanliness of the machine, and long batch fatigue buildup). 

Fig. 14 shows a typical set of upper and lower compression profiles. One can see that the lower trace is smaller than the upper. On a single station press, only the upper punch is usually moving, and the difference is caused, mainly, by the friction of the compressed powder inside the die. 

On a rotary press, both punches are moving and therefore the friction of the punches inside the turret and the die causes the difference. The lower punch fits the turret with a larger standard clearance compared to the upper punch. In addition, the lower punch is never leaving the die, while the upper punch leaves and enters the die with each stroke. This results in a comparatively better alignment and lower friction. The close fit of the upper punch does not allow it to penetrate the die smoothly and this can cause the increase in friction. Thus, an excessive difference between the two peaks may indicate an underlubricated die or some punch misalignment problem. 

In addition, it was shown that the lower and upper punches may not move synchronously. Moreover, maximum force does not coincide in time with the minimum punch gap. These and other considerations (press deformation, contact time, etc.) make the effort of simulating a production press on a hydraulic compaction simulator rather impractical. That is why, to quote from a paper by Muller and Augsburger, " Although compaction simulator have been designed to mimic the displacement time behavior of any tablet press, they rarely have been used in that fashion. ... 

Eccentric tablet presses are single station tablet presses that use an eccentric shaft connected to a rotating wheel to control the displacement of the upper punch into the die. The Encychpedia of Pharmaceutical Technology provides a good summary of the construction and function of an eccentric tablet press <25). The details of its operation are not reported here since it is the only simulation of the compression cycle that is of interest to this chapter. In an eccentric press, the displacement profile of the upper punch is sinusoidal and the displacement rate is controlled by adjusting the rotation rate of the eccentric wheel. The lower punch remains stationary during the compression and acts only to enable uniform die filling prior to tablet formation and tablet ejection after tablet formation. 

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