Consistency’s the key: How pharma manufacturers can avoid punch wear

Natoli Engineering president, Dale Natoli, explains the importance of understanding punch length and cup depth in regard to punch wear and more.

Manufacturing tablets to a uniform hardness, weight, and thickness requires tablet press punches of consistent length. 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. Thus, 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

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. If the working length varies within a set of tools, then tablet hardness, weight, and thickness also will vary. The working length of punches is engineered to a standard range of 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). 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, 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 and using a digital indicator mouthed on a steel post fixed to a granite base. 

It's important to measure from the deepest accessible area of the cup with the tip of the indicator. 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. 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. 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). Although the sidewall is generally not inspected or measured, it is critical to tablet appearance and manufacturing. Ideally, the sidewall width will be well proportioned with the overall tablet thickness. A tablet with an excessively thick sidewall creates the perception that the tablet will be uncomfortable to swallow and requires 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.

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.

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 eliminate excessive cup-depth deviation, consider specifying the tolerance as a percentage of the desired cup depth. 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 a reference dimension that comprises two or more critical dimensions; the working length and the cup depth. 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. 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.

A Word About Punch Wear

With normal use, punches show the most wear at their tips, which reduces the cup depth. 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.

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.