Feed frame

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.

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. 

The Osaka machines (Fig. 8) utilize a vibratory feed mechanism [48,49]. In this machine, the capsule body passes under a feed frame, which holds the powder in... 

Minimize transfer steps. With each transfer step and movement of the bin or drum, the tendency for segregation increases. Ideally, the material would discharge directly from the blender into the tablet press feed frame with no additional handling. In-bin blending is as close to this as most firms can practically obtain and is the best one can ask for—so long as a well-mixed blend can be obtained within the bin in the first place. 

Compaction, also known as tableting, involves the compression of the blend into a unit dose. The mechanism for this type of processing has remained unchanged for quite some time. The main components of the compression cycle are pressure rolls, weight adjustment cam, ejection cam, and feed frame. The main considerations when scaling up is compression speed. Compression speed effects dwell time and feed rate. As you go from a small development compression machine to a high-speed production machine, the powder is processed much more rapidly. 

Compression is a critical step in the production of a tablet dosage form. The materials being compressed will need to have adequate flow and compression properties. The material should readily flow from the hopper onto the feed frame and into the dies. Inadequate flow can result in rat holing in the hopper and/ or segregation of the blend in the hopper/feed frame. This can cause tablet weight and content uniformity problems. As for the compressibility properties of the formulation, it should be examined on an instrumented tablet press. Factors to consider during compression are as follows ... 

First, the die is filled when it passes beneath a stationary feed frame and the lower punch is in the filling position. Tablet weight variation is dependent on uniform powder filling into the die cavity. Since the residence time of the die under the feed frame is very short, the formulation blend must flow easily and reproducibly. 

Some tablet presses are equipped with two sets of rollers a smaller roller located between the feed frame, and the main compression roller which allows a small degree of compression (e.g., precompression) to take place. Precompression can remove air from the powder bed in order to minimize tablet capping and lamination. Good com-pactibility of the powder blend is essential to tablet formation under the influence of compression forces and ensures that tablets will remain intact compact when the compression force is removed. 

FIGURE 2 Rotary tablet press machine (a) left-side view (b) black-side view. (1) Cabinet, (2) compression, (3) turret, (4) gear, (5) weight control assembly and lower cams, (6) plate cams, (7) guarding, (8) hopper system, (9) feed frame assembly, (10) take-off chute, (11) aspiration assembly, (12) electrical system. 

Gravity Feed Frame. Older machines typically employ gravity feed frames which rely on gravitational and... 

Material Recirculation. Material is recirculated from the center of the turret into the feed frame. Some press designs include recessed recirculation channels to minimize particle attrition and prevent excess material loss to the vacuum system. It is critical not to recirculate too much material because this can result in low product yields and can have a detrimental effect on the powder s physical properties, which could result in poor compressibility, uniformity, and final properties (e.g., reduced dissolution rate). 

The feed frame height should be maintained between 0.05 and 0.10 mm (0.003-0.004 in.). For very fine particle size granulation, the clearance should be reduced to 0.025-0.05mm (0.001-0.002 in.). 

Loss of mix quality in hopper and feed frame gives poor content uniformity Additional shearing of lubricant in feed frame lowers dissolution rates... 

Take-off force is monitored by mounting a strain gage to a cantilever beam on a press feed frame (in front of the take-off blade, Fig. 6). It is done to measure adhesion of tablets to lower punch face. Such adhesion is indicative of underlubricated granulation, poor tooling face design, die-wall binding, and tablet capping. ... 

Steady fiow will occur with fine powders if the target flow rate (feed rate through the system) is below the critical flow rate that occurs when the solids stress is balanced by the air pressure at the outlet. The target flow rate is often controlled by a feeder, such as at the inlet to a compression machine (press feed frame). The critical flow rate, and the flow properties tests used to determine it, is described in more later in this chapter. At target flow rates exceeding the critical flow rate, unsteady flow can occur by two different modes described below. 

A non-uniform feed density is not a primary concern. Since tablet presses operate as volumetric feeders, variation of the feed density into the press feed frame can result in tablet weight variation. A funnel flow bin will typically have a more non-uniform feed density than a mass flow bin, since the blend in the funnel flow bin will be subjected to different consolidation pressures depending upon where in the bin it is discharged from. For instance, the blend located at the bottom of the bin at the hopper walls, which is outside the flow channel, may be more consolidated and have a higher density than the blend within the flow channel. 

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. 

The powder hopper is positioned above a feed frame, a frame that retains a powder bed above the dies when the lower punch is in the filling position. As the lower punches pass below the feed frame, they descend within the die to their lowest possible position so the whole die cavity can be filled with powder. The powder is filled into the dies by the suction effect caused by their descent and gravity from the feed frame above. To optimise filling, the feed frame is designed so that the powder in contact with the die table and following the rotary action of the table is directed in a manner that makes it pass to and fro across the dies. Some machines... 

Following compression, the upper punches are removed by the upper punch cam track, and the lower punches pass over an ejection cam, a gentle ramp that moves the lower punches vertically within the die until the tablet is fully ejected. The tablets are removed from the punch tip by a scraper blade positioned on the edge of the feed frame, and the punch then descends to allow die filling to occur for the next cycle. 

The die passes under the feed frame containing the compression coat material. A shallow fill takes place. 

The lower punch moves down, and the die passes beneath a feed frame containing the coating material. The tablet core is now surrounded by the coating material. 

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