Is The Overload Set Point Really Important?
Fred Rowley, Watson Pharmaceuticals, Inc.
Introduction
The overload set point is one of the most misunderstood concepts associated with tablet press operation. It is often mistakenly associated with tablet hardness, compression force and even tablet weight. Although considered an important machine parameter by compressing professionals, it is not in fact a process parameter. It generally does not appear on a batch record or in many operating SOP’s. Nevertheless, failing to set itcorrectly maylead to a costly and even dangerous outcome. This article discusses the importance of understanding the overload set point, how it is correctly usedand what it does and does not do.
- Background and Discussion
What exactly is the overload set point? What is its purpose? Why is it important?
Why is it so misunderstood?
Let us begin to explore the answer by asking a basic question: is it possible to place too much force on a set of punches in a tablet press? What happens when this event occurs?
An overload condition or event occurs whentoo much material is placed in the die at one time, and amazingly,some of the material many not be the powder we are compressing.For example, suppose a bolt has shaken loose from a feed frame and entered into a die? The computer senses the difference, makes a decision and rejects the tablet. Product quality is preserved, but what potential effect does this upset condition have on the tooling? The tip of the tooling may fracture, or completely break off or the entire tool may fail. One protectsthe tooling against this potentially fatal situation by correctly setting the overload set point.
- Understanding How a Tablet Is Made Is Key To Understanding What May Go Wrong
Powder is actually “squeezed” into a tablet by the action of one upper and one lower punch sliding along ever closing cam tracks and meeting together at a predetermined point in a die between the two main pressure rolls. The predetermined point may be altered and the width of the “squeeze” may change by moving the lower pressure roll upward or downward. We call this result tablet thickness. The force observed we call the “compressing force”. Figure 1 demonstrates this concept:
The illustration demonstrates what happens when the die is filled under a normal compressing cycle. When an upset condition might occur, such as when an entire tablet sticks to the punch and fails to be ejected from the die, we have a potential for an overload condition. In this situation we have the die filled again followed by a double compressing event of the stuck tablet combined with the new material.
This results in a pressure far exceeding a normal level, and probably exceeding the maximum allowable for on the punch tip. Other similar events might also occur: a scraper bar comes lose and center itself over the die under the two punches, a bolt shakes loose off the press and enters the die cavity, a tablet jumps into a feed frame is torn apart by the feeder wheels and pieces enter multiple dies.
- The Overload Event
In Figure 2 below we see what happens when there is an overload event such as described above. Somehow the press senses an excessive pressure and reacts. The reaction is in fact a separation of the two main pressure rolls by momentarily moving the lower pressure roll downwards away from the other. Doing this relieves the pressure situation and protects the punch tips.
How does the press sense this situation? How do we determine what is the maximum allowable tip pressure, where is this information located and how do we set this information on our tablet press?
- Determining The Maximum Allowable Tip Pressure and Setting This Value on The Tablet Press
The tablet actually compares an input pressure value to actual pressure value and makes a decision to accept the value or back the lower pressure roll away. This may be done in more than one way. Some systems are hydraulic, others act on a “carriage” value or “support” value through a spring. For example, on older presses this is done by adjusting a large spring at the back of the press. Some presses like the Manesty series, have a simple dial adjustment.All computerized presses accept a simple input value.
So the overload set point has no relation to tablet weight, hardness or thickness except in the rare case that the overload is triggered. In this situation the tablet will be thicker and softer than the other tablets because the compressing process was stopped along the way when the lower pressure roll assembly pulled away from the upper.
The overload system is simply like the relief valve one sees on a pressure cooker. There are various sources for the overload value we seek. Most tablet press manuals provide established calculations for many standard round tablets. In some manuals detailed rules accounting for various pressure discounts for letters, numbers and bisects are also provided. In most cases, all other type of tablets, such as capsule and oval shapes, are calculated on a case by case basis.
In this illustration we see standardized round tablet diameters with corresponding maximum tip pressures. The arrow selects a ¼” round, standard concave tablet and the value indicated is 2 tons. For the same diameter tablet the value of a deep concave tablet would be 1 ¾ tons. Later we will take the 2 ton value and demonstrate how one sets this value up on a tablet press.
Manuals are not the only source of this value. Most drawings generated after 1999 that accompany newly received tooling have the same values printed somewhere on the face. For all non round tablets the values are calculated on a punch by punch basis since even upper and lower punches with the same shape may vary somewhat if the combination of letters, numbers and bisects are different. If you have two punches with different values you should use the lower number as the setting for both.
- Setting The Overload Set point Value on The Tablet Press
The overload may be set in more than one way, depending upon which type of press you are using. Remember that the overload value is a one time setting or input to the press for any number of batches being processed. It does not change until the next product in placed on the tablet press. With tablet presses having computerized set up, this is a numerical value for input in a field. For older presses, a physical adjustment of a spring or counterbalance is required. For example, for the Kikusui Libra one would input the overload value by manually moving the dial shown in Figure 4.
- Summary
The overload set point is a safety value that all tablet presses use to protect the tips of the punches. It is not a critical processing parameter but it is a very critical machine setting. It is not related to compressing force or tablet attributes and it has no influence on tablet quality. The overload values do not change using different types of tablet presses but it will vary from punch to punch depending upon the combination of letters, numbers and bisects used. Understanding and using the overload system correctly is one of many key factors involved in the overall tablet compressing operation.
About the Author
Fred Rowley is director of manufacturing technical support at Watson Laboratories, and previously held senior manufacturing and technical positions at Stayner Pharmaceuticals, Shaklee Corp. Syntex Laboratories, Alza Corp. and Wider Nutrition International. He has a B.S. degree in biochemistry from the University of Santo Thomas, Manila,