Cup and Tablet - Depth and Consistency
For a tablet produced by a tablet press, the cup depth is the distance measured between the highest point of the punch tip and the lowest area of a cup cavity. The cup depth is an important indicator of wear of the tooling equipment and it is important therefore to understand how cup depth can affect the tablets.
Two useful guidelines or standards on tablet compression tooling have been published. The first appeared in 1971 as the American Pharmacist Associations tablet Specification Manual, commonly referred to as the TSM. The TSM defines the allowable dimensions of the tooling that is used in the Americas. The second is the ISO 18084 International Tooling Standard, which defines the EU tooling requirements. In the TSM manual, the cup depth tolerance is set at +/- 0.003 inch. This tolerance applies to new punch tools and not in-process tools. The ISO 18084 does not refer to cup depth or its allowable tolerance or range.
Two punches are used to press each tablet, and the overall shape of the tablet is governed by the combination of both upper and lower punches. There are three measurements that can characterise each punch. These are the overall punch length (i.e., the distance between the foot of the punch and the highest point of the punch tip), the working length (i.e., the distance between the foot of the punch and the lowest point of the cup), and the cup depth as described above. The overall punch length is the sum of the working length and the cup depth. Therefore it is not necessary to measure all three dimensions of an individual punch, as one of the dimensions can be deduced from the other two.
According to the TSM manual, the total working length tolerance should be 0.002 inch, i.e., the total allowable deviation for each lower and upper punch set should be 0.002 inch. Unfortunately the TSM does not specify a tolerance for the overall punch length which is most common reference dimension.
The cup depth tolerance in the TSM was developed for tooling vendors as a way of identifying out of specification tooling rather than as a helpful measure for tablet manufacturers. Once tools are used in production, the TSM cup depth tolerance is less useful, and it is rather more important to consider an in-process dimensional range, which would help maintain the consistency of several products that the company may produce. For example, consider the case of a flat faced bevel edge tablet that has 0.21875 inch in diameter, composed of Schedule II API, which requires a 0.010-inch cup depth. If it were simply necessary to adhere to the TSM tolerance, the cup depth could deviate by 20% from the original value. Contrast this with an over-the-counter tablet having a 0.070-inch cup depth, the allowable deviation would less than 3%. By following these guidelines tablets containing controlled substances would have less dimensional control than regular over-the-counter tablets. This is clearly not acceptable.
Establishing a Limit
As new tools enter the production and become in-process tools, tablet manufacturers should set an allowable standard deviation limit to overcome the deficiency of the TSM tolerance, to ensure equipment life and consistency of tablet quality. A reasonable limit would be a 15% decrease to the cup depth from the original cup depth specification of the tooling. By implementing such a limit, it would be possible to maintain an adequate control of product dimensions irrespective of the original cup depth.
The set limit would also allow tablet manufacturers to maintain the tools and polish these without having to worry about affecting the tooling life or service. With such an in-house standard, a benefit may be a significant reduction of operating costs as less maintenance would be needed to measure individual cup depths. Aside from this, highly potent tablets requiring shallow cups would be more consistent and closer to their specification than is apparent from the currently published guidelines.