Issue link: https://www.e-digitaleditions.com/i/238663
NUTRA_32A-32L_Nutra 12/31/13 9:50 AM Page 32D 32D January 2014 Tablets & Capsules Figure 1 Multivitamins contain a variety of particle sizes and materials that are coarse, hard, and granular—which can prematurely wear out compression tooling. Mottled, textured, or granular materials can make any embossing difficult to read. made from an optimal material. There are thousands of varieties of steel available, yet only a few meet the complex design and functional requirements of nutraceutical tablet tooling. Alternative materials, such as tungsten carbide [1], and enhancement coatings and treatments are worthy of consideration. Cold work tool steels containing high levels of carbon and chromium are commonly used for nutraceutical tooling. Tungsten carbide is often used for the dies to minimize wear and deformation of the die bores. It offers high compressive strength with extremely high wear resistance and lasts longer than conventional die steels. These properties help to reduce ring formation in the die bores. For some formulations, however, tungsten carbide is not adequate. For especially abrasive formulations, a powder metallurgy grade steel [2] may be the right choice. It's also important to design the tablet shape carefully. It's best to avoid small, delicate, and complex shapes, logos, and embossing—particularly in vulnerable, highstress areas of the tablet (Figure 2). Coatings: Pros and cons Our company has several coatings whose beneficial characteristics can be matched to the demands of a specific formulation: They increase corrosion- and wear-resistance and prevent sticky formulations from adhering to punch-tip faces. The most popular coating for tablet tooling is hard chromium, owing to its low cost and anti-corrosion, antiwear, and anti-stick properties. However, hard chromium applied by electroplating has several disadvantages. Hydrogen embrittlement. When hard chromium is applied to tooling, a certain amount of hydrogen penetrates the substrate, which makes the steel brittle. That is undesirable in a component subject to high cyclic loading. De-embrittlement—a process in which electroplated tools are "baked" at high temperatures for several hours shortly after plating—reduces, but does not totally eliminate, hydrogen embrittlement. Indeed, tooling coated with hard chromium can withstand a maximum of 80 percent of the designed safe working load of an uncoated tool. Micro-cracking. Microscopic cracks or micro-cracks may develop during the plating process when the internal stress exceeds the tensile strength of the chromium, which is hard and brittle. Micro-cracks are problematic because they impart a porous route to the substrate that will, over time, allow aggressive tablet formulas or cleaning solutions to attack the steel beneath the coating. Environmental concerns. Hard chromium itself is not an environmental hazard, but there are environmental risks associated with the plating process. The main concern is that the chromic acid solution used in the process generates a waste product that contains hexavalent chromium, a tightly controlled substance that must be disposed of correctly. To address these issues, our company developed a chromium-rich coating [3] applied via magnetron-sputtered physical vapor deposition (PVD). With magnetron sputtering, the chromium is vaporized by bombarding its atoms with argon gas particles. This process yields a very smooth (anti-stick) and dense coating, and incurs none of the drawbacks associated with electroplated hard chromium. Another option is to apply a resilient surface coating [4] via electron beam technology. This coating has a very smooth finish and a hardness value (Vicker's pyramid number) of 3,000, which is very high. This provides excellent wear resistance with formulations that contain hard, abrasive, sharp-edged minerals that, when repeatedly compressed, can scrape away or penetrate the surface of the tool and lead to the erosion of punch-tip detail, such as logo embossing and other identification—and eventually result in tablet weight variation and sticking. The coating is applied only to the punch tip, because the compression rollers have a much lower hardness; wear or damage would occur if the entire punch were coated. Figure 2 A tungsten carbide die custom-designed to produce a triceratops-shaped vitamin tablet. Care must be taken with such designs to minimize fragile contours vulnerable to breakage during packaging and shipping.