- March 27, 2012
- Case Study
FEM-based tablet hardness and punch strength simulations in Optimus software revealed the best possible design trade-off between durability and ease of swallowing.
March 27, 2012 - Asahi, a Japanese manufacturer of alcoholic beverages, soft drinks and food supplements, uses Optimus to perfect the tablet shape to make sure that dietary supplements are easy to swallow. Sensory evaluations and response surface modeling (RSM) allowed Asahi to identify the most suitable tablet shape, both for compliance and usability objectives. However, making tablets rounder as the outcome suggested, also reduces the durability of both the tablets and the punching machines producing the tablets. Based on FEM-based tablet hardness and punch strength simulations in ANSYS, Optimus’ RSM techniques revealed the best possible design trade-off between durability and ease of swallowing. Making tablets easy to swallow Tablets are the most common form for the delivery of oral medicines and food supplements today, and they come in many different shapes, colors and flavors. Since senior citizens find it difficult to swallow tablets, it has become an important requirement to develop tablets that are easy to swallow. Therefore, Hideaki Sato, Research Laboratories for Fundamental Technology of Food at Asahi, set up research that studies the correlation between each of three factors (tablet diameter, radius of curvature, and thickness), and ease of swallowing. Tablets with a single radius of curvature as well as tablets with two radii of curvature were investigated. The response surface generated from the sensory evaluation results showed that a smaller tablet diameter is not necessarily better for swallowing. Considering the difficulty to change tablet diameters because of regulations, the conclusion was that the most appropriate solution would be to reduce the radius of curvature (i.e. making the tablet rounder). However, changing the tablet shape in this way weakens the tablet’s hardness and deteriorates the durability of the tablet press. Simulating tablet punching dynamics The tablet punch machine is in operation all day to compress powder instantaneously by means of extremely high mechanical pressure loads. The tablet press punches undergo high stress levels during the process, which sometimes cause mechanical failure. When reducing tablet curvature, the punch head shape will become sharper and the loads capacity of the punch head will drop to a lower level. To overcome punch head breakdown, Optimus’ response surface modeling (RSM) capabilities were applied to evaluate the relationship between punch/tablet shape and allowable mechanical loads. Simulations with ANSYS provided Optimus with the input data for the RSM evaluations. Typically, Optimus enables a design of experiments (DOE) approach to deliver the most valuable input for the subsequent RSM phase. DOE defines a virtual experiment plan so that the acquired amount of valuable information is maximized with a minimum simulation effort. Based on the results of the virtual experiments, Optimus applies an interpolation method to create a Response Surface Model. FEM-based tablet stress simulation in itself was very challenging because tablets consist of compressed formations of powdered substances. Unlike mechanical structures, both the shape and Young’s modulus can be quite different depending on the pressure loads. In collaboration with Cybernet Systems – the Japanese distributor of both ANSYS and Optimus – Sato established a revolutionary method for predicting tablet stresses. The Young’s modulus of the tablet material clearly could not considered to be constant. Therefore, they derived a realistic variable Young’s modulus by transcribing the reaction force of the punch head to the tablet, serving as an alternative approach to define the tablet’s local Young’s modulus. This approach was validated to be consistent with experimental results and led to suitable simulation results with practical accuracy. The local Young’s modulus inside the tablet gradually reduces toward the tablet edge, whereas density and risk for cracks increase toward the edge. The researchers also performed ANSYS simulations to predict the load capacity of the punch head. A 2D axisymmetric model was prepared and the contact element between the punch and the tablet was defined to represent the punch head sliding slightly over the tablet during the powder compression phase. In these simulations, calculations were repeated until the stress value inside the tablet reached the allowable stress value in order to identify the load capacity. Using Optimus RSM to improve tablet design The stress simulation results delivered by ANSYS were further analyzed using Optimus’ RSM. The researchers defined the tablet diameter (D), the radii of curvature in 2 directions (R1 & R2) and the ratio between both radii (S = R1/R2) as design factors. They specified the allowable load as the design objective to be maximized. The response surface was calculated with the least squares method, using a second-order polynomial. This mathematical procedure finds the best-fitting surface to a given set of data points by minimizing the sum of the squares of surface point residuals. Optimus’ response surfaces showed that the double-radius tablet type had a lower allowable load than the single-radius one. Moreover, the allowable load of the double-radius tablet type increased as both radii of curvature R1 & R2 increased. From the analysis of the response surface model, the researchers found that any design factor raised the allowable load as it increased. The diameter (D) had most impact, followed by the ratio of the radii (S), while both individual radii (R1 & R2) exhibited nearly equal influence. The response surfaces indicated that a tablet with a large radius of curvature R1 (to ensure tablet hardness) and a smaller radius of curvature R2 (to improve ease of swallowing), represented a tradeoff that best meets the global design targets. As Optimus’ Response Surface Modeling reveals inherent trends in the design space – which are often non-linear – this technique is well suited to assist engineers in making better-informed decisions faster, allowing them to design more durable and consumer-friendly products.Learn More
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