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tablet press toolingUnderstanding the importance of

punch length and cup depth

Dale Natoli

Natoli Engineering

Manufacturing tablets to a uniform hardness, weight, andthickness requires tablet press punches of consistent length. Thisarticle describes how to understand punch length, how to mea-sure it correctly, and how wear affects length and tablet consis-tency. The article also addresses the importance of specifyingcup-depth tolerances.

sing rotary tablet press punches of consistent length iscritical because their length directly relates to the unifor-mity of the hardness, weight, and thickness of com-pressed tablets. If you don’t understand how length andcup depth affect tablet quality, you may attribute tabletdefects to the wrong source. Thus, it’s important to estab-lish a punch inspection and maintenance program that

Reprinted from Tablets & Capsules July 2006 www.tabletscapsules.com

will enable you to verify that all punch lengths and cupsare within dimensional tolerances.

Working lengthPunches are engineered and manufactured to consis-

tent lengths. The working length of a punch is the dis-tance from the head flat to the lowest measurable area ofthe punch cup. See Figure 1. The figure also illustratesthe cup depth and the overall length, which is the dis-tance from the head flat to punch tip. The punch tipcomprises the cup and the land, as shown in Figure 2.

Understanding the working length leads to consistentoverall tablet hardness, weight, and thickness, so itshould be considered the most important dimension in atool inspection program. If the working length varies,then tablet hardness, weight, and thickness will also vary.

Working length consistency is the key. The workinglength of punches is engineered to a standard range of0.002 inch. This means that, within a set of punches, thedifference between the working lengths of the longest andshortest punch is no more than 0.002 inch. It is recom-mended that you periodically inspect the punches to ensureworking lengths do not exceed that tolerance (or the rangeyour company specifies). When inspecting to confirm auniform working length, it is important that you inspect theupper punches independent of the lower punches.

It’s also important to measure correctly. It is incorrectto calculate the working length by subtracting the cup

depth from the overall length, because that method canproduce results showing that some tools are out of speci-fication when in fact they are not. The tolerances of theoverall length and cup depth are greater than that of theoverall length. Therefore, they cannot be used to calcu-late the working length, with its much tighter tolerance.The working length of the punches should be measuredfor deviation from punch to punch rather than from a cal-culated number. You should measure working lengthusing a digital indicator mounted on a steel post fixed toa granite base. See the photo on page 40. This is basicmeasuring equipment that costs less than $500 and pro-vides the same accuracy as elaborate systems that costmore than $50,000.

Whatever equip-ment you use, measur-ing the working lengthof punches that haveembossing or a bisectat the lowest area ofthe punch cup is morecomplicated. In thosecases, it’s important tomeasure from thedeepest area of the cupthat is accessible withthe point of the indica-tor (Figure 3). Onceyou identify the lowestarea of the cup, be sureyou consistently mea-sure from there as youcheck the entire set.Otherwise, you mayhave to “hunt andpeck” for the lowestarea of the cup.

Figure 1

Cup depth, working length, overall length, and head flat of atablet press punch

Cut depth

Workinglength

Head flat

Overalllength

Figure 2

Land and punch cup

Land

Cup

Figure 3

Measure working length from thedeepest area of the cup that isaccessible with the point of the

digital indicator.

Most reputable tooling manufacturers can provide aworking-length matching report when they deliver a newset of punches. The matching report pairs each upperpunch with a lower punch, from longest to shortest, andnumbers them accordingly. Matched punch sets createthe best possible consistency in tablet hardness andthickness, and a matching report offers helpful guidanceduring press setup.

The length of the lower punch is more critical thanthat of the upper punch. That’s because the length of thelower punch largely determines how uniformly product(granulation) fills the die. (Product flow characteristicsand lower punch binding also strongly influence the uni-formity of die filling.) Deviations in the amount of prod-uct allowed into the die affect tablet hardness andweight.

Cup depthThe cup depth is the distance from the tip edge of the

punch to the lowest theoretical point of the cup. Somecup configurations have a varying depth, such as thoseused to manufacture tablets with scalloped edges. Thecup determines the configuration and appearance of thetablet faces.

The area between the two tablet faces created by thedie is called the tablet sidewall (Figure 4). It may also bereferred to as the tablet gate or the tablet band. Althoughthe sidewall is generally not inspected or measured, it iscritical to tablet appearance and manufacturing. Ideally,the sidewall width will be well proportioned with theoverall tablet thickness. This is important because a tabletwith an excessively thick sidewall appears to be thickeroverall, creating the perception that the tablet will be

uncomfortable to swallow. An excessively thick sidewallalso requires the tablet press to exert greater force toeject the tablet from the die.

The width of the sidewall depends on the tablet hard-ness, weight, and thickness in relation to the cup depth.As the punch tip wears, cup depth decreases and sidewallthickness increases. Thus, if you compare two tablets ofequal hardness and weight—the first tablet made with ashallow-cup punch and the second tablet made with adeep-cup punch—you’ll notice that the first has a thickersidewall. However, the shallow-cup tablet will be measur-ably thinner overall than the deep-cup tablet. But again,its wider sidewall makes it visually undesirable and unfa-vorable for manufacturing.

The wide sidewall of a shallow-cup tablet can alsocause difficulties during film coating because the tabletmay erode at the sharp corner where the shallow-cupradius and the vertical sidewall meet (Figure 5).

Most tablet press punches have a cup-depth toleranceof ±0.003 inch, which is published in the TabletingSpecification Manual [1]. This 0.003-inch tolerance iswidely accepted by the tablet compression industry andis used by tooling manufacturers worldwide. But whilethe published tolerance is adequate for most applications,it may be too liberal if you manufacture small-diametertablets or too conservative if you manufacture large-diam-eter tablets.

To understand this point, consider a small-diameter,flat-faced, beveled-edge tablet. Let’s assume that thistablet is a Schedule II drug and that it requires a cupdepth of approximately 0.010 inch. If you adhere to thepublished cup-depth tolerance, the cup depth couldrange from 0.007 to 0.013 inch. That means that youcould create a total deviation equal to 60 percent of thedesired cup depth.

Compare that to what occurs when manufacturing alarger modified-capsule tablet (such as a nutritional sup-

Figure 4

The cup depth is the distance from the tip edge of the punchto the lowest point of the cup. The cup determines the con-figuration and appearance of the tablet faces. The areabetween the two tablet faces is called the tablet sidewall.

Upper punch

Sidewallthickness

Overall tabletthickness

Punch cup/Tablet face

Lower punch

Figure 5

The wide sidewall of a shallow-cup tablet may create asharp corner where the radius and the vertical sidewallmeet. The corner can erode easily during film coating.

Area of tablet susceptible to erosion during coating

plement). For this tablet, let’s say the deepest part of thecup may be 0.060 inch. Following the published stan-dard, the acceptable range would be 0.057 to 0.063 inch.But because the cup is deeper than the one used to makethe small tablet, the percentage range of deviation is sub-stantially less: 10 percent. That is a considerable differ-ence, the importance of which is magnified by the factthat this small-diameter tablet is a Schedule II pharma-ceutical product.

To eliminate excessive cup-depth deviation, considerspecifying the tolerance as a percentage of the desiredcup depth. For example, specifying a tolerance range of20 percent of the desired cup depth for the first cup(0.010 inch deep) changes the range from 0.007 to 0.013inch to 0.009 to 0.011 inch. Cup depth inspection is sim-ple and uses the same basic measuring instruments usedto inspect the working length: a digital indicatormounted on a steel post fixed to a granite base.

Overall lengthThe overall length is the least important dimension of

the punch. It is the distance from the punch tip to thehead flat. The overall length is a reference dimension thatcomprises two or more critical dimensions, the workinglength and the cup depth. Since both working length andcup depth are manufactured to a specific tolerance, thereis no need to assign a tolerance to the overall length. Aslong as the working length and the cup depth are con-firmed to be within the acceptable range, then the overalllength will be consistent and you don’t need to inspect it.The overall length of the lower punch is critical when itcomes to setting the punch height for uniform tablettake-off, which is important to minimize the potential fortablet damage.

However, if your company’s standard operating proce-dures require inspection of the overall length, use thesame basic equipment that you used to inspect the work-ing length and cup depth. Of course, you must establish atolerance range and a pass-fail policy.

A word about punch wearWith normal use, punches show the most wear at their

tips, which reduces the cup depth. It follows, then, thattip wear also reduces the overall length of the punch,although it does not affect the critical working length.Any wear of the head flat (not as common as punch-tipwear) will further reduce the overall length, as well asreduce the working length. Head flat wear does not affectcup depth.

Normal maintenance of the cup face, tip, and head canalso affect punch length. Polishing the face using fineabrasives, hard felt bobs, or stiff brushes can alter criticallengths. Likewise, if you use hard cotton bobs and/or stiffbrushes with abrasive compounds to remove productadhered to the face or to remove surface discoloration orpits (typically the result of compressing abrasive prod-ucts), you will eventually deepen the punch cup andthereby possibly alter the critical working length.

Wear can also oc-cur at the land (Figure2), which is the nar-row flat area located atthe perimeter of thepunch tip. The land issubject to abrasionduring compressionand is commonly thefirst area of the punchto wear. When theland wears, the tip be-comes very thin, evenrazor sharp, some-times causing a condi-tion referred to as J-hooking (Figure 6). J-hooks normally occur on the upperpunch tip and are a common cause of tablet capping andlamination. Polishing the punch using a soft cotton wheeland a polishing compound will restore the land. Whilepolishing and restoring the land will prolong the usefullife of your punches, it will eventually reduce the cupdepth and overall length.

ConclusionUniform tool length is critical for maintaining tablet

consistency and smooth press operations. The mostimportant dimension of the punch related to tablet qual-ity is the working length, followed by the cup depth, andthen the overall length. To achieve the highest level oftablet uniformity, most reputable manufacturers of tabletcompression tooling can provide a punch-length match-ing report. Setting up the tablet press in the sequence ofthe supplier’s matching report will provide the best sce-nario for tablet consistency. T&C

Reference1. Tableting Specification Manual, 7th edition. American

Pharmacists Association, Washington, DC, 2006. Copiesare available for purchase at Tablets & Capsules’ website:www.tabletscapsules.com.

Dale Natoli is vice president of Natoli Engineering, 28 Re-search Park Circle, St. Charles, MO 63304. Tel. 636 9268900, fax 636 926 8910. Website: www.natoli.com. The com-pany manufactures punches, dies, and other parts, accessories,and supplies for tablet manufacturing and tablet press opera-tion. Dale Natoli has more than 30 years’ experience in tabletcompression tooling. He was chairman of the group that spear-headed the third edition of the IPT Manual, known today as theTableting Specification Manual.

Figure 6

When the land of the punch tipwears, it becomes very thin and

may form a J-hook.

J-hook