Narrow escape: How to decrease tooling wear using tapered dies

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Natoli explains how using tapered dies increases tablet quality and decreases tooling wear for manufacturers.

Utilising tapered dies is beneficial for both increasing tablet quality and decreasing tooling wear. Standard dies have straight bores throughout their entire height. Taper is an optional feature that increases the bore dimension at the top of the die, then tapers inward until reaching the nominal bore size. Tapered die bores offer a multitude of benefits, which include reducing tablet defects, such as capping and lamination. 

Capping & Lamination

Capping occurs when the upper or lower cup portion of a tablet separates horizontally from the main body of the tablet. It can partially or completely separate to form a “cap.” Lamination is the separation of a tablet horizontally into two or more layers. One of the most common causes of capping and lamination is the compression of low-density formulations.

A tapered die bore provides additional space for air contained within low density formulations to exhaust as the upper tip first enters the die at the beginning of the compression cycle. Tapered dies are also effective in combating capping and lamination when pre-compression isn’t available or is insufficient to solve the issue. A tapered die bore allows air to be exhausted at a faster rate when compared to a die bore without taper. The taper effectively increases the clearance around the upper punch tip, which provides a wider pathway for the air to escape.

Decompression/expansion related stress is a critical factor in capping and lamination defects. The taper allows the tablet to expand gradually throughout the ejection cycle instead of the rapid expansion that would take place in a tablet that is ejected from a straight-bore die. This gradual expansion reduces internal stresses within the tablet that are frequently associated with capping and lamination defects.

While tapered dies were designed as a solution for capping and lamination, they can also be used to prevent other common tablet defects. 

Excessive Ejection Force

As the compression force is removed, tablets begin to expand. This expansion results in the tablet exerting pressure on the die walls. Taper aids in tablet ejection by allowing the tablet to expand gradually throughout the ejection cycle. This reduces the kinetic friction between the band of the tablet and die wall, thus lowering the force necessary to move the tablet through and out of the die bore.

In general, tablet expansion is proportional to tablet size. Larger tablets tend to expand more than smaller tablets, so the amount of taper can be customised based on the size of the tablet.

As the ejection cycle begins, a break-away force will be needed to overcome the static friction caused by expansion. This initial break-away force is greater than the force needed to continue to eject the tablet. Taper depth can also be customised based on band thickness and upper punch penetration to help minimise this break-away force.

Scorched Belly Band

A scorched belly band of a tablet is identified by vertical striations or dark spots on the band of the tablet.  A scorched, or slightly burned, tablet belly band is typically caused by high ejection forces related to a marginal formulation.

Friction generated during ejection results in heat generation. One or more formulation components that have a low melting point may be subject to discolouration caused by the high temperatures generated during ejection.  These discoloured particles can adhere to the die wall, then flake off during subsequent filling and compression cycles and can then be observed as dark spots on and within later compressed tablets. Ejecting the tablet through the taper reduces friction and heat associated with this phase, which helps reduce the occurrence of these dark visual defects as well.

Tooling Wear

High ejection force may cause excessive wear on the lower punch head and the ejection cam of the press. However, tapering the die bore can help reduce ejection forces and therefore, the associated tooling wear. After compression takes place, tablets begin to undergo elastic recovery (expansion). Die taper allows the tablet to begin expanding gradually as soon as the ejection cycle begins, which reduces the related forces the tablet is exerting on the die wall. Therefore, reduced ejection force will equate to less wear to lower punch heads and the ejection cam.

Die bore taper can also be used to correct some minor turret wear-related alignment issues. Wear in punch guides and die pockets can cause slight misalignment between the punch tips and die bores. The taper can act as a “funnel” to help guide the upper tip into the die bore possibly preventing some of the misalignment related damage such as j-hook on the upper punches, which is a contributing cause of capping and lamination.

Taper Attributes

The TSM (Tableting Specification Manual, 7th ed.) standard taper amount is 0.003” (0.08mm) wide by .188" (4.76mm) deep, which is considered to be suitable for the majority of tableting situations. However, as mentioned above, the taper dimensions can be customised for specific situations or formulations. 

Dies with a tapered bore can be one-sided or tapered on both sides. Two-sided taper can prolong the working life of the die due to the ability to flip the die over for use on both sides. One-sided taper is recommended to compare the efficacy of taper vs non-tapered die bores in R&D applications. Die taper is a frequently used tooling option to address common tableting defects and excessive tool wear. Consult with your tooling vendor for additional advice and guidance regarding the benefits of tapered dies.