UNDERSTANDING THE IMPORTANCE OF PUNCH LENGTH AND CUP DEPTH
Manufacturing tablets to a uniform hardness, weight, and thickness requires tablet press punches of consistent length. We will describe how to understand punch length, how to measure it correctly, and how wear affects length and tablet consistency. We will also address the importance of specifying cup-depth tolerances.
Using rotary tablet press punches of consistent length is critical as their length directly relates to the uniformity of the hardness, weight, and thickness of the compressed tablets. If not correctly understood, tablet defects could be attributed to the wrong source. This, it is important to establish a punch inspection and maintenance program that will verify all punch lengths and cups are within dimensional tolerances. Working Length Punches are engineered and manufactured to consistent lengths. The working length of a punch is measured as the distance from the head flat to the lowest measurable area of the punch cup (Figure 1). The figure also illustrates the cup depth and the overall length, which is the distance from the head flat to punch tip. The punch tip comprises the cup and the land, as shown in Figure 2. Understanding working length leads to consistent overall tablet hardness, weight, and thickness, so it should be considered the most important dimension in a tool inspection program. If the working length varies within a set of tools, then tablet hardness, weight, and thickness also will vary.
Working length consistency is the key. The working length of punches is engineered to a standard range of 0.002 inch (0.051 mm). This means that, within a set of punches, the difference between the working lengths of the longest and the shortest punch is no more that 0.002 inch (0.051 mm). Periodically inspect the punches to ensure working lengths do not exceed that tolerance (or the range your company specifies). When inspecting to confirm a uniform working length, it is important to inspect the upper punches independent of the lower punches. It is also important to measure correctly. Do not calculate the working length by subtracting the cup depth from the overall length, as that method can produce results showing some tools are out of specification when in fact they are not. The working length of the punches should be measured for deviation from punch to punch rather than from a calculated number. Working length should be measured using a digital indicator mouthed on a steel post fixed to a granite base. This is basic measuring equipment that costs less than $500 and provides similar accuracy as elaborate systems that cost more than $50,000. Whatever equipment used, measuring the working length of punches that have embossing or a bisect at the lowest area of the punch cup is more complicated. In those cases, it is important to measure from the deepest accessible area of the cup with the tip of the indicator (Figure 3). Once the lowest area of the cup is identified, be sure to measure consistently from there when checking the entire set. Most reputable tooling manufacturers can provide a working-length matching report when they deliver a new set of punches. The matching report pairs each upper punch with a lower punch, from the longest to shortest, and numbers them accordingly. Matched punch sets create the best possible consistency in tablet hardness and thickness, and a matching report offers helpful guidance during press setup. The length of the lower punch is more critical than that of the upper punch.
That’s because the length of the lower punch largely determines how uniformly product (granulation) fills and doses in the die. (Product flow characteristics and lower punch binding also strongly influence the uniformity of die filling.) Deviations in the amount of product allowed into the die affect tablet hardness and weight.
Cup Depth The cup depth is the distance from the tip edge of the punch to the lowest theoretical point of the cup. Some cup configurations have a varying depth, such as those used to manufacture tables with scalloped edges. The cup determines the configuration and appearance for the tablet faces. The area between the two tablet faces created by the die is called the tablet sidewall (Figure 4). It may also be referred to as the tablet gate of the tablet band. Although the sidewall is generally not inspected of measured, it is critical to tablet appearance and manufacturing. Ideally, the sidewall width will be well proportioned with the overall tablet thickness. This is important because a tablet with an excessively thick sidewall appears to be thicker overall, creating the perception that the tablet will be uncomfortable to swallow. An excessively thick sidewall also required the tablet press to exert greater force to eject the tablet from the die. The width of the sidewall depends on the tablet hardness, weight, and thickness in relation of the cup depth. As the punch tip wears, cup depth decreases and sidewall thickness increases. Thus, when comparing two tablets—made from a shallow-cup punch and a deep-cup— the shallow-cup punch has a thicker sidewall. However, the shallow-cup tablet will be measurably thinner overall than the deep-cup tablet. Its wider sidewall makes it visually undesirable and unfavourable for manufacturing. The wide sidewall of a shallow-cup tablet also can cause difficulties during film coating because the tablet may erode at the sharp corner where the shallow-cup radius and the vertical sidewall meet (Figure 5). Most tablet press punches have a cup-depth tolerance of 0.003 inch (0.076mm), which is published in the Tableting Specification Manual. This 0.003-inch tolerance is widely accepted by the tablet compression industry and is used by tooling manufactures worldwide. But while the published tolerance is adequate for most applications, it may be too liberal if manufacturing small-size tablets or too conservative if manufacturing large-size tablets. To understand this point, consider a small-diameter, flat faced, bevelededge tablet. Let’s assume that this tablet is a Schedule II drug and that is requires a cup depth of approximately 0.010 inch (0.254 mm). If you adhere to the published cup-depth tolerance, the cup depth could range from 0.007 to 0.013 inch (0.178 to 0.333 mm). That means that you could create a total deviation equal to 60 percent of the desired cup depth. Compare that to what occurs when manufacturing a larger modified capsule tablet (such as a nutritional supplement). For this tablet, let’s say the deepest part of the cup may be 0.060 inch (1.524 mm). Following the published standard, the acceptable range would be 0.057 to 0.063 inch (1.448 to 1.600 mm). Because the cup is deeper than the one used to make the small tablet, the percentage range of the deviation is substantially less: 10 percent. That is a considerable difference, the importance of which is magnified by that fact that this small-diameter tablet is a Schedule II pharmaceutical product. To eliminate excessive cup-depth deviation, consider specifying the tolerance as a percentage of the desired cup depth. For example, specifying a tolerance range of 20 percent of the desired cup depth for the first cup (0.010 inch deep) changes the range from 0.007 to 0.013 inch (0.1778 to 0.330 mm) to 0.009 to 0.011 inch (0.229 to 0.279 mm). Cup depth inspection is simple and uses the same basic measuring instruments used to inspect the working length: a digital indicator mounted on a steel post fixed to a granite base.
Overall Length The overall length is the least important length dimension of the punch. It is the distance from the punch tip to the head flat. The overall length is a reference dimension that comprises two or more critical dimensions; the working length and the cup depth. Since both working length and cup depth are manufactured to a specific tolerance, there is no need to assign a tolerance to the overall length. As long as the working length and the cup depth are confirmed to be within the acceptable range, then the overall length will be consistent, and inspection is unnecessary.
Note: the overall length of the lower punch is somewhat critical when it comes to setting the punch height for uniform tablet take-off, which is important to minimize the potential for tablet damage. If your company’s standard operating procedures require inspection of the overall length, use the same basic equipment used to inspect the working length and cup depth. Of course, a tolerance range and a pass-fail policy must be established.
A Word About Punch Wear With normal working length. Any wear of the head flat (not as common as punch-tip wear) will further reduce the overall length, as well as reduce the working length. Head flat wear does not affect cup depth. Normal maintenance of the cup face, tip, and head also can affect punch length. Polishing the face using fine abrasives, hard felt bobs, or stiff brushes can alter critical lengths. Likewise, if using hard felt bobs and/or stiff brushes with abrasive compounds to remove product adhered to the face or to remove surface discoloration or pits (typically the result of compressing abrasive products), eventually the punch cup will deepen and possibly alter the critical working length. Wear can also occur at the land (Figure 2), which is the narrow flat area located at the perimeter of the punch tip. The land is subject to abrasion during compression and is commonly the first area of the punch to wear. When the land wears, the tip edge becomes very thin, even razor sharp, sometimes causing a condition referred to as J-hooking (Figure 6).
J-hooks normally occur on the upper punch tip and are a common cause of tablet capping and lamination. Polishing the punch using a soft cotton wheel and a polishing compound will remove and J-hook and restore the land. While polishing and restoring the land will prolong the useful life of your punches, it will eventually reduce the cup depth and overall length.
Conclusion: Uniform tool length is critical for maintaining tablet consistency and smooth press operations. The most important dimension of the punch related to tablet quality is the working length, followed by the cup depth, and then the overall length. To achieve the highest level of tablet uniformity, most reputable manufacturers of tablet compression tooling provide a working-length matching report. Setting up the tablet press in the sequence of the supplier’s matching report will provide the best scenario for tablet consistency. use, punches show the most wear at their tips, which reduces the cup depth. It follows, then, that tip wear also reduces the overall length of the punch, although it does not affect the critical