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Other tool steels, such as AISI D2, DC-53, K340, and powder-metallurgy (PM) class steels offer excellent wear resistance because they can attain a higher hardness than other steel types and have a high carbide content. Tool steels with a high chromium content, such as AISI 440C and M340, are particularly useful when compressing corro- sive or sticky products. The general rule: As the Rockwell hardness of a material increases, wear resistance increases and impact toughness decreases. Some PM-class steels, however, exhibit both excellent wear resistance and rela- tively high impact toughness due to their unique chemical composition and the distinctive forging/manufacturing process used to make them. Reputable vendors of tablet tooling offer punches and dies made from a variety of tool steels, enabling the vendor to select the best steel, one that exceeds the requirements of the desired compression force and withstands the abrasive, sticky, or corrosive properties of the granulation to be tabletted. Steel strength Whatever the application and whichever tool steel you use, when calculating maximum tip force, the most impor- tant properties to take into account are tensile strength, compressive strength, yield strength, and impact toughness. Compressive, tensile, and yield strength depend on the chemical composition of the steel and its Rockwell hard- ness. Compressive strength indicates how well a material— in this case steel—resists deformation in pure compressive loading. Tensile strength refers to the maximum stress a material can undergo when pushed or stretched before it fails. Yield strength—the most important indicator—is the maximum amount of stress a material can withstand before plastic deformation occurs. Impact toughness describes the maximum amount of energy from an impulse or shock load- ing that a material can withstand. Because each type of tool steel has unique or distinctive mechanical properties, the maximum compression force of each also differs. Stress concentration During the compression phase of tabletting, forces are applied normally to all surfaces of the cup (Figure 1). These forces result in stress, which is a function of both the cup's area and its geometric profile. Basic stress relates to the force exerted and the area over which it is applied, but calculating maximum allowable compression force requires comparing maximum stress to the yield strength of a given material. In addition to force and area, stress concentration factors must be factored in. Stress concen- tration refers to small areas of significantly increased stress caused by sharp transitions in the geometric profile of the cup and/or punch face. Examples include bisects and the blend radius where the embossing meets the cup radius. Punches with stress concentrations have lower maximum compression force compared to tooling that makes plain tablets. A punch face's land—the flat edge at the perimeter of the punch cup where the cup radius ends—also plays a critical role in determining maximum allowable compres- sion force. Despite the small size of this feature, it's an important factor in calculating how much force a punch tip can withstand before it fails. In general, the larger the land of a tablet or tool, the greater the maximum allow- able compression force. In almost all cases, increasing the size of the land of a punch is one of the easiest and quick- est ways to increase its maximum compression force. Keep in mind, however, that as the tooling wears, the land erodes and becomes smaller. That's why new punches with unworn lands withstand more force than punches of the same design that have been used for many production runs. That fact also illustrates why proper tooling mainte- nance is so important: not only does it ensure quality tablets, but it also helps to maintain the mechanical integrity of the tooling itself. Predictive modeling Like other manufacturers, many tooling vendors have adopted finite element analysis (FEA) to better understand how different tooling designs affect performance. FEA is a powerful tool, enabling engineers to apply any combination of forces and/or pressures to a solid model and see the result, including the stresses, strains, and displacement, as Figure 1 Normal forces acting on round and oval punch-cup faces Tablets & Capsules September 2015 11