Under Pressure: Breaking the Misconceptions of Maximum Compression Force

Understanding Maximum Compression Force

A quality tablet punch starts with a quality tool steel.? Many different types of tool steels are utilized in the tableting industry, each with unique and advantageous properties.? An industry standard for many years, AISI S7 is suitable for many applications due to its excellent shock resistance and thus good resistance to splitting/shearing stress and its ability to withstand relatively high compression forces.? Other tool steels such as AISI D2, DC-53, K340, and PM class steels offer excellent wear resistance due to their higher attainable hardness and increased carbide content.? Tool steels with a higher chromium content, such as AISI 440C and M340 are particularly useful when pressing corrosive or sticky products.? As a general rule, as the Rockwell hardness of a material increases the wear resistance increases but the impact toughness decreases. However, some of the PM class steels have both excellent wear resistance and relatively high impact toughness due to their unique chemical composition and distinctive forging/manufacturing process. Steel cleanliness (prevalence of voids and impurities) plays a valuable role as well as the heat treatment process used to harden the steel.? Quality tool vendors offer a variety of tool steels so that the optimal steel can be selected that exceeds the requirements for desired compression force and different granulations’ abrasive, sticky, or corrosive properties.????

The tip forces listed on Natoli Engineering tablet drawings are determined by considering multiple factors; including material properties (yield strength, impact toughness), cup area, cup configuration, and stress concentration factors (bisects and embossing).

When the max allowable compression is determined for a specific tablet a number of factors are analyzed. First, the mechanical properties of the punch material are taken into consideration. The two most important material properties used when determining compression force are yield strength and impact toughness. Yield strength is the maximum stress a material can withstand before experiencing plastic deformation. And impact toughness is the maximum energy that a material can absorb from an impulse or shock loading condition before fracture.? Material composition and Rockwell hardness affect these mechanical properties. For example, S7 tool steel has higher impact toughness, but a lower hardness and thus a lower wear resistance. D2 tool steel has a higher attainable hardness and thus greater wear resistance, but a reduced impact toughness. It is also important to note that this is the reason that Natoli Engineering lists different maximum compression forces for different materials on our tablet drawings because different materials have different mechanical properties. Next, when determining the maximum allowable compression force, the cup area, cup configuration, and stress concentration factors are analyzed. The larger the tablet the greater the cup area and cross-sectional area, so the higher the allowable compression force as there is a greater area over which the force is distributed. The basis for maximum compression force calculations comes from the relationship from stress (pressure) relative to force and area. The basic formula for stress is: stress=force/area. The maximum stress a punch tip can withstand is determined from the yield strength of the punch material, and the area over which the force is distributed (attained from the tablet geometry). This comparison of maximum stress to area is the basis for determining allowable compression force. However, some cup configurations have areas that experience higher levels of stress due to transitions in their geometric profile. This small area of higher stress results in a lower maximum compression force. For example, flat face bevel edge tablets experience an area of localized stress where the bevel meets the flat of the cup due to the sharp transition of the geometric profile. These localized stresses occur with varying intensity in all non-uniform cup profiles.? Lastly, many tablets have stress concentration factors that affect the maximum compression force. As discussed above, sharp transitions result in areas of localized stress and thus a lower allowable compression force. The most common features on tablets that result in stress concentrations are bisects and embossing. A bisect and/or embossing creates a sharp transition in the geometric profile of the cup at the small radius where the bisect or embossed character meets the larger cup radius. This is the reason why tablets with embossing and bisects have a lower maximum compression force on tablet drawings from Natoli Engineering.

With the advanced CAD and FEA software available here at Natoli Engineering it is possible to accurately model the tablet a customer requires. A 3D model of every tablet design requested is created with every tablet drawing that is made. These 3D models contain the exact geometric profile of the tablet, and these models can be exported directly to our FEA software for stress analysis. Using this software, it is possible to simulate the force experienced during compression using the material the punch will actually be made from. The result of this analysis lets us view and measure the resulting stress and strain. This also allows us to see areas of stress concentrations and allows us to measure just how much more stress is occurring in these areas relative to the rest of the cup profile. This data is then used to calculate maximum compression force. It is also important to note that the maximum compression force listed on the tablet drawings considers fatigue life of the tooling. Most of the time tablet compression tools are used to produce thousands if not millions of tablets. As the compression tooling undergoes repeated loading cycles during tablet production, stress events above the endurance limit will result in accumulating fatigue which will shorten the life of the tooling.

The maximum force the punch can withstand decrease over time due to wear on the tooling and the build-up of residual stress.

For small tablet tooling (below 4.76mm, the tip of the lower punch is likely to bend or buckle before the cup fractures. For this reason, the maximum compression force for small tablets is determined using the slender column bending formulas. The force attained from this calculation is the maximum force that can be applied before the punch tip bends and thus the maximum allowable compression force. This calculated bending force is then applied to the Rankine Gordon formula, where the critical load is given as 1/P=1/Pc+1/Pe where crushing and buckling loads are combined to result in a more accurate value compared to the standard method of determining tip force.