Compressing Force: Myth vs Reality

Compressing force is a dynamic number recorded as the attempt to maintain two target values in close tolerance

By Fred A. Rowley, Solid Dosage Training, Inc.

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While recently reading several technical papers, one of which concerned itself with the effect of excipients, compressing force, film coating and storage on hypromelose performance, [1] it became apparent that there are many misconceptions on how a tablet press works. These papers specifically used the term compressing force, a term I find frequently misunderstood in technical conversation, both in academia and in industry. For example, during a recent discussion with several quality engineers, I heard each of them say that “during a compression run, the computer alters the compressing force to maintain tablet attributes.” This paper will explain what compressing force is and why the term is misused and misunderstood, so that future scientists can better understand and convey important data when compressing tablets. [2, 3, 4]

The reader may be shocked to learn that the list of scientific papers, articles and conversations that use compressing force and other conflicting solid dosage terms and references is almost endless. A recent question posed by a professional in the Journal of Validation Technology concerning compressing force, proves my point:

“The process validation of tablet compression processes is accomplished by testing quality attributes such as content uniformity, dissolution, moisture and other attributes from stratified sampling conducted throughout the process. Machine operating parameters may vary between lots; example parameters include precompressing force, main compressing force and turret speed. After appropriate set-up of operating parameters, in-process testing or machine control of tablet weight, thickness and hardness maintains compressing at target attributes by adjusting machine parameters.

“Question: How are ranges of operating parameters such as compressing forces and turret speeds handled in process validation? Are separate runs at high- and low-parameter extremes conducted to validate ranges of parameters? Or are multiple runs at the same parameter settings conducted to demonstrate repeatability? Any other options?” [5]

How can something as elementary as the force necessary to make a tablet be so misunderstood and misquoted? Many authors write about it, scientists wonder about it and manufacturing and quality units debate it. Unfortunately, even the professionals need to read the literature more closely to more fully understand what the various workers are trying to convey. [6]

Several decades ago, the term “compressing force” or “compression force” was not widely used. The terms “support pressure” or “tonnage” appeared far more frequently. At that time, tablet presses were less complex and these terms were easier to visualize and comprehend, since there wasn’t all that much to see. The primary press controls were both easy to see and understand.

In the 1970s, most presses had identical controls: thickness, pressure and hardness, all on the same labels. These were the only primary controls on the tablet presses of that era. There never was a “compressing force” control.

In addition, there was a spring-based device that set the maximum pressure each set of tooling tips could withstand. Infrequently, these pressure limiting devices had a gauge displaying what overload (pressure release) point had been set. On older presses, this was accomplished by compressing a spring and setting it to a specific force value.

l first noticed confusion over these terms when Manesty introduced the Rotopress and Mark II machines in the 1970s. These and other more advanced machines had a new gauge that was somewhere on the front control panel. It was frequently misread as “compressing force” and sometimes reported in the literature as such.

Where the disconnect between terms and actions may have come about was in the impression that somehow the force seen on that control panel dial was the force being used to manufacture tablets. [7] This was never the case. Other manufacturers, such as Kilian, Stokes and Fette also followed with different dials indicating pressure.

So why all of this confusion? Is compressing force real? What does it mean? Let us review the basic tablet press, the tablet compressing cycle and then discuss why this term requires clear understanding.

The Basic Tablet Compressing Cycle

As shown in the figure on page 31, going from left to right, powder is fed into the horizontal feeder from the powder hopper. [8] The powder floods a portion of the die table and then enters the die. The desired fill volume, called tablet weight, is adjusted with the first control, called the weight cam.

After the excess powder has been scraped off, the powder in the die is pressed together by the action of the upper and lower punches rolling over the pressure rolls. The desired tablet thickness is obtained by moving the lower pressure roll either away from or closer to the fixed upper pressure roll. Doing this moves the lower punches either away from or closer to the upper punches. This is tablet thickness.

Other than press speed, on many tablet presses these are the only controls (also called adjustments or set points) available to the operator. There is no “compressing force” control and many presses do not have any means to read this value.

On more advanced tablet presses, there are several second-tier controls called the pre-compression station, the upper-punch entry adjustment, the powder feeder speed and the pre-compressing tablet thickness control. None of these have any direct impact on what we call compressing force. Even on computer-adjusted tablet presses used today, there is no control named “compressing force.” There are three basic controls: press speed, die-fill volume (weight) and the distance between the punch tips (expressed as tablet thickness).

The research paper mentioned earlier contained a table showing “the influence of compression force and fillers on drug release (T50%) from HPMC SR tablets” which included a section listing values of “(T50%) for tablets manufactured at different compression forces (4kN, 10kN and 14kN).” How is it possible that one can list three different compression forces using a constant set of excipients at “different compressing forces”? In addition, on a basic tablet press, where would one make such an adjustment to achieve the forces of 4kN, 10kN and 14kN?

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