Learn how to prevent common causes of product loss.
Finding an ideal balance between high speed and precision on a modern tablet press-to optimize the ratio between bulk product introduced into the feed hopper and the acceptable tablets that exit the discharge chute (i.e., yield)-is likely to rank number one on any end user’s priority list. Paradoxically, the attainment of idealized yields often remains the most elusive goal in the compression suite. This article sheds light on factors that affect yield and how to improve them.
Reaching an optimal yield state is predicated on getting the most bulk powder into the die cavities between punches and, ultimately, into finished tablets. It is not just about making the press look clean, with minimal dust and mess. One can easily reduce the mess made on a press by simply using maximum vacuum, running the press slowly, or indiscriminately scraping excess powder away from the die cavities. These examples, however, will not improve final yields, as the goal is to put the maximum (but desired) amount of powder into the cavities, rather than directing it elsewhere.
Most modern pharmaceutical manufacturers using state-of-the-art tablet presses will look to achieve final yields as high as 99% for single-layer tablets. Multi-layer yields are more challenging, given the number of different variables involved, with realistic figures often dipping closer to 85%. Multi-layer yields can be as high as 92%, however, on a press with excellent first-layer sampling features; sampling frequency, and the time it takes to cycle through a sample, can be precisely adjusted on select presses, resulting in significantly improved yields.
The best first step in attaining optimal, repeatable results is systematic training. Operators and set-up technicians should, ideally, participate in and successfully complete a formal, comprehensive training program, which are offered by equipment vendors. The best programs will teach about all of the contributing factors discussed in the following section, all of which have a strong influence on product yields.
Hopper seals. Perhaps somewhat obvious, but often overlooked, poor hopper seals can cause a surprising amount of product loss, especially over the course of an entire batch. Product can escape either from the top seal, where the hopper is connected to an overhead feed system, and/or from the bottom, where it connects to the feeder assembly. The following are some recommendations for hopper seals:
Feeder positioning. The correct positioning of an induced feeder can prove a fickle undertaking, which is probably why it is so often a major cause of product loss. Some degree of space must exist between the base of the feeder and the die table surface passing beneath it, but the optimal setting of this gap will vary across products. Leveling the feeder is also a significant concern, as an uneven feeder will invariably lead to premature wearing of other components. The following are recommendations for feeder positioning:
Feeder seals. Just as important as a feeder’s positioning is the installation, condition, and efficacy of the seals used beneath the feeder, in its base, to ensure that product is effectively contained and that only the desired amount reaches the die cavities. Worn seals lead to measurable product loss. There are many types of seals in use, but the best mitigate the effects of heat while also offering easy installation and removal. The following are recommendations for feeder seals:
Fill cam sizing. Fill cams play an integral role in making tablets. The filling process is similar from one press to the next, in that the fill cam allows the lower punches to be pulled down farther than they really need to go, thereby ensuring enough powder is in the die cavity to start (i.e., over-filling). A dosing cam will then “dose out” (i.e., push) a quantity from the die, such that the amount of product left in the cavity just prior to the upper punch making its entry is exactly how much is necessary for reaching target tablet weights. It is important that set-up technicians do not default to choosing fill cams with a filling range larger than necessary for a particular tablet weight. The following are recommendations for fill cam sizing:
Tooling tolerances. The use of new, or at least well-maintained, tooling should be a cornerstone guideline. Sloppy tolerances on inferior or worn tools will contribute to product loss, sometimes in a fashion that is hard to detect. The following are recommendations for tooling tolerances:
Condition and positioning of product scrapers. Modifying the position of scrapers can certainly affect product yields. The scrapers that follow the feeder, just prior to upper punch penetration of the die, are there to ensure product stays in the cavity where it belongs. Other scrapers are intended to direct any excess powder into a recirculation channel (if the press in question is so equipped), such that it can ultimately be returned to the entry side of the induced feeder. The following are recommendations for condition and positioning of product scrapers:
Penetration depth. The depth to which the upper punch penetrates a die presents opportunities for effectively containing product (thereby improving yields), but it’s another case of finding the “sweet spot.” Generally, if penetration depth is too shallow, it is likely powder will be blown out of the dies as the punch penetrates. If the penetration is too deep, the risk of entrapping too much air increases, resulting in another undesirable compression phenomenon known as “capping” (in which the finished tablets split into two pieces). The following are recommendations for upper punch penetration depth:
Recirculation features. Most modern tablet presses offer some form of recirculation features, which are specifically intended to guide any excess powder around the inner circumference of the turret and ultimately return it to the induced feeder. This end goal is accomplished through the use of strategically placed scrapers, a channel and, in many cases, a “plough.” It is common, however, for the adjustment of these parts to be off by just enough to negatively affect final yields. The following are recommendations for using recirculation features::
Vacuum settings. Virtually all tablet presses make use of some system for vacuuming excess powder from the die table surface. Excessive vacuum, however, can actually pull good product away from the die cavities, or elsewhere, resulting in unwanted losses. The following are recommendations for vacuum settings:
Start-up reject settings. Computer-controlled presses generally allow the user to configure a parameter for rejecting a certain number of tablets every time the machine is started, in certain modes. These features are offered due to the risk of producing out-of-specification, “transitional” tablets as the machine ramps up to operating speed and to help mitigate the risk of those tablets finding their way into the good tablet channel. The following are recommendations for start-up reject settings:
Many tablet manufacturers are compressing products that contain increasingly expensive active ingredients or, at a minimum, are implementing overall efficiency programs. It is thus of paramount importance to embrace good manufacturing practices, with an eye towards maximizing product yields.
Newer presses generally have features that will help prevent product loss, but this fact should never absolve the end user from maintaining a press to the highest standards and regularly implementing good usage habits to fully understand what impact those features have on final tablet quality and overall efficiency.
Take advantage of the resources available from a tablet press vendor and regularly request new or revised SOPs for operation, cleaning, and maintenance. Participate in training seminars whenever possible. Your yields may depend on it.
Vol. 41, No. 5
Pages: 66–68, 71
When referring to this article, please cite it as M. Bundenthal, " Optimizing Yields on Modern Tablet Presses," Pharmaceutical Technology 41 (5) 2017.
About the author
Matt Bundenthal is direct sales and communications manager at Fette Compacting America, www.fetteamerica.com.