Hydraulic compaction simulators

Compaction properties of each material were determined with a standardized test performed on a custom-built hydraulic compaction simulator using 8 mm (0.3150 in.) round flat-faced punches. A linear saw-tooth upper punch position profile was selected with a punch velocity of 300 mm/sec for both punch extension and retraction. The lower punch position was at a fixed position within the die during the compaction event. The powder weight loaded into the die for each compression was calculated from the equation below so as to form a cylindrical tablet having a thickness-to-diameter ratio of 0.30 at a theoretical SF of 1.0. These dimensions are typical of commercially elegant tablets. 

However, hydraulical compaction simulators are still used in research for basic material characterization. They show the advantage of controlling speed exactly and of using low and high punch travel speeds, between 10 and 300 mm/s. Mostly a simple displacement profile is used for characterization (e.g., a saw tooth or a sine wave profile), and the evolving forces at the lower and upper punches are measured. Further the speed of the punches can be controlled separately and both punches move freely and independently from each other. Time intervals in which the punches stand still can be freely set. Thus lots of freedom for material characterization is possible and these compaction simulators are important tools. 

Using mechanical compaction simulators allows us to simulate the tableting process of rotary tableting machines to a greater extent than when using hydraulical compaction simulators. Thus they will be mainly used in formulation development and scale-up. 

However, for mechanical compaction simulators the movement of the punches is mechanically determined and, compared to hydraulic compaction simulators, not freely programmable. Thus, for basic material characterization and early formulation development, hydraulical compaction simulators can be advantageous. 

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. 

Compaction simulators (Fig. 20) were designed to mimic the compression cycle of any prescribed shape by using hydraulic control mechanisms that are driving a set of two punches (upper and lower) in and out of the die. All hydraulic compaction simulators are similar in design and construction. A compaction simulator consists of several main units the load frame (column supports and crossheads with punches), the hydraulic unit (pumps and actuators that move the crossheads), and the control unit (electronic console and computer). Usually, a simulator accepts F tooling only, but can be retrofitted to use standard IPX B tooling. Under computer control, the hydraulic actuators maintain load, position, and strain associated with each punch. 

The simulation can be achieved by one of the two procedures matching either the force (load control) or the movement of punches (position control) at any given moment of time. Thus, when running a simulator, one has a choice to mimic the force/time path (compression profile) or the motion of the punches (punch displacement curves). It is impossible to mimic both at the same time on any hydraulic compaction simulator. 

Because load control profiles are not practical, users of hydraulic compaction simulators overwhelmingly prefer to utilize punch displacement profiles in hope that, once the punches are forced to move in the same... 

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. ... 

To summarize, one can say that hydraulic compaction simulators are ideally suited for basic compaction research but are not very practical for simulation of production presses. 

FIGURE 1 Block diagram of a hydraulic compaction simulator. 

Hydraulic compaction simulators have the unique advantage of mimicking any compaction displacement profile. Common profiles for pharmaceutical studies include sawtooth (or linear), sinusoidal, and rotary tablet press waveforms. In some cases, the profile... 

The use of compaction simulators was first reported in 1976. Since then, a variety of simulators have been developed. Hydraulic simulators, as well as mechanical simulators, are available to characterize raw materials, drug substances, and formulations, as well as to predict material behavior on scale-up. The appeal of simulators is due to the fact that they purport to provide the same compaction profile as experienced on a tablet press while using only gram or even milligram quantities of powders. Compaction simulators can achieve high speeds, as would be experienced on a production tablet press, and can be instrumented to measure a variety of parameters, including upper and lower punch force, upper and lower punch displacement, ejection force, radial die wall force, take-off force, etc. Summaries on the uses of simulators and tablet press instrumentation can be found in (19,20). 

The first compaction simulators developed were hydraulic [41-45], The hydraulic system is electronically controlled. An example is given in Figure 7. Either compression force cycles or movement of the punches was freely adjustable. This allowed... 

FIGURE 7 Example hydraulically working compaction simulator left, machine view right, detail view into compression chamber. (ESH compaction simulator, Courtesy of Huxley Bertram.)... 

Bateman and coworkers report a comparative analysis of six different hydraulically powered compaction simulators manufactured by a variety of vendors (Table 1), In their round-robin study, they found that the compaction simulators were comparable when operated within a moderate compression stress range of 50-2(X)MPa. However, at higher pressures, correction factors needed to be applied because of elastic distortion and differences in loading characteristics of the hydraulic systems (5). These results are not surprising since like rotary tablet presses, compaction simulators are not perfectly rigid. Therefore, compaction simulators should be properly calibrated (including corrections for mechanical flexure and electronic noise) to ensure the collection of quality experimental data. 

U.S. EPA s rationale for the requirement of composite bottom liner option in the final doubleliner rule is based on the relative permeability of the two liner systems.13 The results of numerical simulations performed by U.S. EPA,10 which compared the performance of a composite bottom liner to that of a compacted soil bottom liner under various top liner leakage scenarios, showed that liquids passing through defects in the top FML enter the secondary LCRS above the bottom liners. The hydraulic conductivities of bottom liner systems greatly affect the amount of liquids detected, collected, and removed by the secondary LCRS. 

Singh and Kolay [32] have investigated the process simulation of ash-water interaction and its influence on various geotechnical properties (viz., compaction, consolidation, hydraulic conductivity and shear strength) of lagoon ash as a fill material for its bulk utilization. They have demonstrated that the activation of ash... 

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