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Flashing during tablet compression, part 2
Q: What causes protrusions around the edge of tablets after compression, and how can I prevent this from occurring?
A: Mike Beyl, Wilson Tool International, says:
In part 1 of this series, I explained that the protrusions that form along the tablet band during compression are called flashing, discussed flashing's impacts and causes, and recommended investing in a setting tool that can reduce tablet-press setup time and achieve a more consistent alignment of the upper punch in the die bore. In this installment of this series, I'll discuss how proper tablet and tooling design can help minimize flashing.
Making subtle tablet design changes can optimize production and increase tooling life. These changes can be so slight as to be unnoticeable to the eye, preserving a tablet's look and marketability while maintaining proper weight and thickness for purity and safety.
As part 1 of this article indicated, the clearance between the upper punch and the die affects both air release and the amount of flashing. The Tableting Specification Manual (TSM) specifies clearance values  that are optimized for air release, which can increase tablet integrity during high-speed production but may allow more flashing than desirable. In most cases, the TSM clearance values also prevent tip binding.
On the other hand, the European Union (EU) standard, International Organization for Standardization (ISO) 18084.2011 , uses the tighter ISO F7 clearance values , which are based on the tablet length for oblong tablets and the diameter for round tablets. If your tablet isn't prone to capping and the formulation doesn't cause filming problems on the die bore, then selecting an EU-style tip clearance can reduce flashing and provide better tool alignment, which can reduce excessive edge wear and extend the life of the punch tips.
Cup depth and land
The punch cup is the cavity at the end of the punch tip that forms the shape of the tablet face. Land is the flat surface around the perimeter of the punch tip between the punch cup and the tip's outside diameter. The dimensions of the punch cup and land not only determine the tablet shape but also can greatly influence the tooling performance and tablet quality. Often, simply increasing the land can increase tool life and decrease flashing.
Nutraceuticals are typically more difficult to compress than pharmaceuticals. This is due to the nutraceutical formulas, such as a multivitamins, that may have 25 or more active ingredients or that have more-natural ingredients that don't respond well to compression. These formulas may have particle variations as well as flow issues that then dispose the tablets to capping or lamination. This, combined with pressure from the nutrition industry for more natural products, limits the amount and number of excipients or fillers added to the formulas to aid in compression. A pharmaceutical product, on the other hand, may have only 3 to 4 active ingredients and 4 to 5 excipients and/or fillers to aid in compression. Before choosing an alternative tool steel or coating, nutraceutical manufacturers dealing with flashing from excessive tooling wear should first consider decreasing the cup depth and increasing the land width. A shallower cup can reduce the amount of compression force required to make a suitable tablet, and increased land makes for a stronger tool.
Excessive compression force
Some machine operators may apply more force than needed to compress a good tablet. This practice increases the pressure in the punch cup, which can lead to increased wear or even fractures to the punch tips. This additional wear increases the clearance between the punch tip and the die, causing more flashing.
Using an overload setting that's no more than 20 percent higher than the average compression force required to make a suitable tablet is acceptable as long as the co-efficient of variation (CV) is not wildly high. Typically a CV value of less than 5 is considered good. The overload settings will alert the operator when compression-force variations are higher than expected, thus providing the opportunity to correct the variation and reducing the likelihood of increased wear.
Shaped tablet punches don't wear evenly around the perimeter of the tip. Since the roller applies torque to the punch head during compression, the punch tip is naturally rotated in the direction of turret rotation to be snug against opposing corners of the die wall. As a result, tablet flashing for shaped tablets is uneven. To help reduce uneven flashing, use an interchangeable setting tool to align the dies and apply pre-torque by rotating the setting tool within the turret guideway in the direction of turret rotation.
Discuss your options with a tooling company when initially designing a tablet and take advantage of their many years of experience as well as the large cross-section of customer experiences upon which they draw. A perfect-looking tablet may not lend itself to ease of production in a rotary tablet press, and an experienced tooling engineer can help you achieve the tablet design you want while minimizing flashing and maximizing productivity and safety.
In part 3 of this series, I will discuss the importance of tool maintenance in reducing flashing. Tool maintenance is just as important as other aspects of tableting but is sometimes overlooked, with companies varying greatly in their maintenance practices. Part 3 will cover creating a standard operating procedure and a preventive maintenance schedule with the assistance of a tooling supplier to help detect, repair, and prevent flashing.
Mike Beyl is Western tablet tooling specialist at Wilson Tool International, White Bear Lake, MN. Wilson Tool's tableting division provides compression tooling, including standard punches and dies, accessories, and custom-designed tool solutions, to the pharmaceutical, nutraceutical, industrial compression, confectionary, and other industries.
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