Compaction dwell time

Compaction dwell time must be a consideration when scaling up. 

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. 

The TF-Mini and TF-156 compactors (Vector Corporation, Marion, Iowa, U.S.A.) were equipped with concavo-convex rolls and single flight screws. The roll speed of the TF-156 was scaled to achieve the same linear velocity 74.2 in./min as the TF-Mini model. This setting maintained a comparable dwell time for material in the compaction zone. The TF-156 roll force was scaled to 5.6 ton, which equaled a force per linear-inch approximately equal to the TF-Mini 3.1 ton/in. roll width. The authors established a feed screw speed to roll speed ratio of 1.3 1 for the trials. Table 7 gives the compactor equipment settings. 

Force measurements made without displacement values are still useful in identifying the dependency of tablet hardness (and other associated characteristics) on compaction force, and also the effect of the tablet press compaction speed on tablet strength (influence of dwell time/effective... 

The punch pressure required to form a compact for tableting indices measurements is measured at the end of a long dwell time, typically 1.5 minutes, during which the punches remain in fixed positions and stress relaxation within the compact brings about a decay in the applied load. The reported pressure or CS is calculated from the relaxed force and it is dependent on the compact SF. A sample s CS at a standard SF, such as 0.85, can be interpreted to indicate the ease (i.e., the magnitude of the pressure) of forming compacts under standardized conditions. 

Cohesiveness, compactibility, and lubrication can be evaluated on research instrumented tablet presses, as discussed later. Such measurements should give a degree of assurance that the material will compress and eject properly. Only highspeed compaction sunulators or other equipment that controls dwell time, however, will give any indication of potential problems with high-speed presses. 

Compaction force vs. time curves to obtain a peak force on which to base the tablet property relationships and to mcnswe functional dwell time (the time over which the maximum force is applied, most often defined as the time where the compaction force is at 90% of the maximum value)... 

Some of these measurements can also be performed on compaction simulators, which are single-stroke presses specifically designed to evaluate individual materials and/or full formulations [ 12]. The simulation of short dwell times and of many different profiles for punch movement in real time are the advantages of this type of measurement. Recent work with a compaction simulator has even included a thermodynamic analysis of compaction [13,14]. 

Dwell time (which is most often defined as the time where the compaction force is at 90% of the maximum value)... 

In an effort to overcome the dwell time/scale-up issues. Metropolitan Computer Corporation (MCC) has developed a single-station development tablet press that reproduces the compaction event time of manufacturing-scale tablet presses. The Presster (see Fig. 6) can be set up to match the rate, roll wheel configuration, and tooling of any manufacturing-scale tablet press. This enables the formu-lator to eliminate compaction rate as a variable upon scale-up. It does require that early in development the formulation scientist identify the tablet press that the commercialized product will run on. This is in keeping with the theme of this chapter—begin a development project with the end in mind. 

The dies cracked due to the inordinate amount of force required to compact the tablet, coupled with a poorly designed tablet shape (kids/fun shapes). As the tablet press speed was increased, the dwell time of the compaction event decreased, thus increasing the amount of peak force required for compaction. 

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. 

Schwartz [23] suggests special consideration for the compression process. Press speed for material that compact by plastic deformation, overmixing of lubricant in the force feeder, heat buildup on long compressions runs, material abrasiveness, and tooling care are important variables for consideration. Dwell time and compression and ejection forces are other variables identiLed for monitoring process. 

A small rotary press is most likely used when the initial formulation and process is developed at small scale. However, a large rotary press, used in a production area, may have significant differences in the number of stations, dwell time, and compression speed compared with smaller compression machines. Thus, early formulation design should consider the performance requirements of commercial production. Compaction simulators provide a useful tool able to reproduce the punch speeds of production machines and require only small quantities of powder blends for testing.86 The simulators can play an important role in formulation and process development and can also facilitate the technical transfer from development to commercialization. 

The newest development for compaction simulation is a mechanical tableting machine simulator which operates with a cam. Thus it is called Stylcam (Figure 9) [59], The cam is positioned on the lower compression wheel and allows the simulation of different tableting machines and their dwell times due to different acceleration of the punches. It was introduced in 2005. 

Force measurements made without displacement values are still useful in identifying the dependency of tablet hardness (and other associated characteristics) on compaction force, and also the effect of the tablet press compaction speed on tablet strength (influence of dwell time/effective contact time). Dwell time dependency is an important scale-up factor for the tableting process, and evaluation of the sensitivity of a formulation to dwell time at small scale is useful, although the actual commercial dwell time is not always achievable on instrumented, small-scale tablet presses. 

Akande, O.F. Eord, J.E. Rowe, P.H. Rubinstein, M.H. The effects of lag-time and dwell-time on the compaction properties of 1 1 paracetamol/microcrystalline cellulose mixtures prepared by precompression and main compression. J. Pharm. Pharmacol. 1998, 50, 19-28. 

Many investigators have reported the effect of dwell times and strain rate sensitivity on the compaction of... 

When the compression displacement, dwell time, and decompression displacement profiles are coupled together as command inputs to the control system on a compaction simulator, a comp>ete compression cycle is formed. The command displacement input. 

Contemporary applications of die pressing are no longer limited to pharmaceutical applications. Mechanically operated presses are used whenever accurate product shapes are required such as in powder metallurgy or in the ceramic industry, e.g. for the manufacture of tiles. Hydraulically actuated die presses find their major field of application in waste and refuse compaction because of their ability to incorporate into the production cycle a certain dwell-time under pressure. 

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