Smarter tablet design

20th June 2018

Top view and side view of the tablet land
Land wear from granulation
Tablet with blended land

Bill Turner and Kevin Queensen explain how introducing land into tablet design will strengthen the punch

The design of a tablet plays an integral role in the functionality and longevity of tablet compression tooling. Avoiding punch-tip wear when possible, and quickly identifying it when it does occur, will provide great benefits to the quality of tablets and to a manufacturer’s operation. Tablet defects, such as capping, lamination, and flashing, may be related to the punch tip’s land and edge wear.

The tablet land is the narrow, horizontal surface perpendicular to a tablet’s periphery, which creates a junction between that periphery and the cup. The three primary reasons for incorporating land into a tablet’s design are: to increase the strength of the punch tip’s edges; to increase the wear characteristics of the inner edge of the punch cup; and to reduce the severity of nicks on the punch’s edges.

Steps to strengthen the edge of the punch cup include reducing the slope of the cup, and more importantly, introducing an appropriate amount of land into the tablet’s design.

On one hand, excessive land can make it more challenging to coat a tablet. On the other hand, land adds strength and durability to the punch tip. Here, we provide some suggestions on how to reduce the issues related to coating the tablet and to increase the punch tip’s strength and durability.

Tablet coating
The edge of a tablet created by the junction of its face and its bellyband, or sidewall, is prone to erosion from the tumbling action in a coating pan. The presence of punch-tip land results in a small lip or ridge around the tablet’s periphery, and the junction of the land and the cup face can result in a sharp inside corner on the tablet. To avoid difficulties during the coating process, the land should have a generous blend radius at this junction, which will eliminate the sharp corner by adding a curved transition between the two surfaces.

Punch tip’s strength and durability
The cup determines the configuration and appearance of the tablet’s faces. With normal use, punches show the most wear at their tip edges, which reduces the cup’s depth. Most tablet punches have a cup-depth tolerance of ±.003in (.076mm). This tolerance is widely accepted by the tablet compression industry, and most tooling manufacturers worldwide use it. While the published tolerance is adequate for most applications, it can be too liberal when manufacturing small, shallow-cup tablets or too conservative when manufacturing large, deep-cup tablets.

Customers’ requirements and preferences force many tooling vendors to minimise land width, which is usually only approximately 5% of the cup’s depth. Knowledgeable tablet designers indicate that land width should be closer to 10% and occasionally as much as 25% of the cup’s depth, especially for difficult-to-compress tablets that require high forces. Such designers consider a punch tip with no land, i.e. a razor-sharp edge, to be unwise. Having sufficient land in your tablet’s design adds strength and wear resistance to the punch tips, both on the inside of the cup and on the outside of the tip, while creating a more robust tablet and enhancing tablet stability.

Also, it is common that a tooling manufacturer will suggest a wider land when a tablet requires a compound cup design. When a punch cup is excessively deep or the concavity of the cup is steep nearest the edge, as with some compound cups, the cup’s configuration creates nearly vertical surfaces, and the cup will be more susceptible to abrasion. Punches with compound cups incorporated into their designs can result in a steeper slope of the cup where it is adjacent to the inner edge of the land. That slope is greater than that of a single-radius cup, which reduces edge strength and the allowable tip force. This type of cup design isn’t as robust as a standard cup. Therefore, land becomes even more critical because it compensates for much of the strength lost due to the steeper slope.

Wear on the land and tip
Land is the area of the cup most susceptible to wear, and it should be inspected regularly. Identifying wear of the tip edge by visual inspection is quite challenging, especially without magnification. A traditional micrometer cannot properly check tip-edge wear. The quickest and most efficient method is to use a horizontal optical comparator, which is often simply called a comparator.

In operation, a comparator projects a magnified silhouette of a punch tip on a screen, usually at 10x magnification but sometimes at 20x or greater, which enables easy inspection for wear and even measurement against prescribed limits. A comparator is a valuable piece of equipment. In fact, it is the only efficient method to check punch-tip wear; unfortunately, it is not used often.

Generally, an increase in land width will strengthen the punch and reduce the likelihood of cup deformation or breakage while allowing for higher compression forces and extended tool life. An experienced tooling manufacturer can help navigate the design process to ensure a successful tableting project, and a CAD software program, such as Natoli Engineering’s TabletCAD, can provide convenient, real-time feedback on your tablet designs.

Bill Turner and Kevin Queensen are with Natoli Engineering





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