Issue link: https://www.e-digitaleditions.com/i/571156
Stress buildup, bent tips, and safety Note that the maximum compression force that vendors list on tablet and tool drawings account for the fatigue of the tooling, which could be used to make thousands if not millions of tablets. Fatigue is an important consideration because as compression tooling undergoes repeated loading cycles during tablet production, residual stresses build up. This phenomenon—known as fatigue stress—can shorten the life of the tooling. In fact, it's normal for the maximum force that a punch can withstand to decrease over time because of wear and the buildup of residual stress. This means that the tooling can withstand more force when it presses the first tablet than it can when pressing the 500,000 th . Ask your vendors if their tooling is designed and engineered for high-cycle loading. A final factor to consider when discussing maximum compression force: Bending of the lower punch-tip straights. Generally, it's a concern only when using tooling with tips 4 millimeters in diameter or smaller. When the tips are that small, cup geometry is unlikely to be the limit- ing factor when determining maximum compression force. Rather, it will be the propensity of the tips to bend under compressive load. To calculate the maximum compression force of these small-tipped tools, the tooling vendor should determine the compressive load required to bend the tips. After accounting for all these factors, the appropriate maximum compression force is determined by comparing the calculated stress and force results to the appropriate fac- tor of safety (FOS) for the intended application. The FOS is simply the ratio of the calculated stress to the yield strength of the material. An FOS of two means that the force applied generates a stress in the material that is one- half the yield strength of the material. The FOS value spec- ified depends on the industry and the application, and it can even vary within an industry from one vendor to the next. That's true of the tablet compression industry, where different vendors cite different maximum compression forces. The difference usually stems what value a particular vendor deems an acceptable FOS. T&C Kevin Queensen is a technical support engineer at Natoli Engineer- ing, 28 Research Park Circle, St. Charles, MO 63304. Tel. 636 926 8900. Website: www.natoli.com. He holds a BS in mechanical engineering, and his work focuses on designing specialty tablets and tooling. Tablets & Capsules September 2015 13 Manufacturing Plant Itapevi | São Paulo | Brazil Excellence in the Manufacture of Pharmaceutical Excipients Global company with more than 30 years of experience, present in over 100 countries and the leader in Latin America www.blanver.com MICROCEL® Microcrystalline Cellulose SOLUTAB® Croscarmellose Sodium EXPLOSOL® Sodium Starch Glycolate SORB-CEL® Effervescent Co-processed Compound TABULOSE SC® Colloidal Microcrystalline Cellulose om Technical Assistance for New Developments, Reformulations and Scale up Global Technical Support for Production and R&D Specialized Customer Service Just in Time Logistics Visit our booth: #3755 at SupplySide West Oct 5 - 9 Las Vegas As compression tooling undergoes repeated loading cycles during tablet production, residual stresses build up and must be taken into account.