Tablet press operation

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. 

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. 

Speed-related data for a number of commonly used tablet presses operating at maximum output (adapted from Armstrong 1989). 

The importance of the Ryshkewitch-Duckworth equation is that it suggests that porosity is a measure of the outcome of the tableting process. Tablets of the same porosity have the same mechanical properties even though they might have been made on vastly different tablet presses operating at different speeds. 

Tableting, pressing, mol ding, and extrusion operations are commonly used to produce agglomerates of well-defined shape, dimensions, and uniformity in which the properties of each item are important and output is measured in pieces per hour (see Ceramics, ceramics processing Pharmaceuticals Metallurgy, powderp tallurgy Plastics processing).

Roll presses can produce large quantities of materials at low cost, but the product is fess uniform than that from molding or tableting presses. The introduction of the proper quantity of material into each of the rapidly rotating pockets in the rolls is the most difficult problem in the briquetting operation. Various types of feeders have helped to overcome much of this difficulty. 

Strategy Since the tablet press is still in operation, an experiment is devised... 

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.

The manufacture of this system utilizes essentially the same process as the one inherent in the manufacture of OROS. The complicating factors are that a bilayer tablet press is required and the tablet must be oriented during the laser drilling operation to ensure that the drug-containing layer is directed toward the laser beam. 

To describe the procedure for validation of the tablet press to ensure that it meets installation, operational, and performance qualification requirements... 

A second flow measurement to be considered during scale-up is the ability of the granulate/powder to fill the dies. This can most efficiently be monitored by punch force variability and individual core weight measurements. Acceptable weight control (<3% RSD) and force (<5% RSD) may be masked at slower compression speeds typically used for development or when a tablet press is not fully tooled. Production operations will tend to run at the high end of any validated range, so flow must be consistent from batch to batch. 

Considering the processing fundamentals of this dry process, almost all of the issues associated with the scale-up of more traditional film-coating processes are eliminated, and the key objective is to feed tablets directly from the tablet press into the electrostatic coating operation, and thence in to packaging. Thus, once the process is defined in terms of coating one tablet, it is replicated an appropriate number of times in order to coat all of the tablets in the batch. 

The equipment used to perfonn the compaction operation is a tablet press. The small-scale equipment from which one might scale up would include ... 

The most common rotary production tablet presses to which one would scale up the compaction operation include ... 

In general, one should be aware that tablet press instrumentation involves the use of strain gauges or piezoelectric transducers to provide a voltage signal proportional to the force applied for the compaction operation. Let us say we can measure forces, such as those applied to the granulation by the punches, that applied to the die wall, that required for tablet ejection, etc. With tlie use of other transducers we can also measure distance. With the measurement of force and distance, we can calculate work, energy, etc. 

It is the authors experience that with a formulation compressed without a forced feeder in R D, the scale-up in a different country, but by gravity feed and without a forced feeder, was perfect. In a second country, however, all production presses operated with forced feeders, and the target tablet hardness was not achieved. It was possible to conclude that press speed was not the cause of the problem but it was much later (and based on laboratory experiments) that it was possible to conclude that the lubricant was ovennixed on the tablet press, resulting in a softer tablet. Fortunately, the drug was very soluble and no dissolution problem resulted, but slower dissolution could be a problem with a drug of low solubility. 

Tablet weight uniformity will provide a measure of the efficiency of the powder flow, tlie force feeder, or the automated weight control system. Weight monitoring by the press operator is usually the weight of a 5- or 10-tablet sample. Uniformity of individual tablets, which after all does relate to dose, will be more informative. The powder flow may not be sufficiently good, even with forced feeders.

After an exhaustive investigation, it was determined that the cause of the nonuniform tablets was segregation in the tote/overhead feeding system used in the manufacturing operation. The laboratory- and intermediate-size batches were hand-scooped into the hopper of the tablet press. The overhead-feed duct acted as a classifier. The differences in cohesion and adhesion of the two actives, coupled with the length and angle of the ductwork, fostered segregation. 

During compression, 1000 tablets were randomly selected for use by quality control. Inspection of the batch records revealed that all 30 batches were compressed on the same model press operating at approximately the same speed. All presses were fed by overhead delivery systems of the same design, thus tableting equipment will not be a source of variability from batch to batch. 

Material transfer operations between process steps such as charging the final lubricated blend into the tablet press hopper have the potential to impact the blend content uniformity when drag and excipients are not evenly distributed within the powder... 

Preparation and Characterization of Tablet Coatings Tablet cores were prepared by compressing the drug without any excipient using a hydraulic press operated at llOMPa. A stainless steel die with a diameter of 1.2cm was used to produce 400-mg drug tablet cores. Asymmetric-membrane tablet coatings were applied... 

Weight variation can arise as a result of a poorly prepared or operated tablet press. To solve this problem, one should inspect the press performance. Attention must be taken when dealing with a new die table on a load tablet press. In such a case, operation of the tablet press must regard the up-and-down motion of the punches within 76.2 pm of the setting without neglecting the conditions of the pressure rolls and cams. 

Schwartz, J.B. The instrumented tablet press uses in research and production. In Automation of Pharmaceutical Operations Fraud, D.J., Ed. Pharmaceutical Technology Publications Springfield, OR, 1983 229-239. 

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