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Tablets & Capsules SDD
Flashing during tablet compression, part 1
Q: What causes protrusions around the edge of tablets after compression, and how can I prevent this from occurring?
imageA: Mike Beyl, Wilson Tool International, says:
Protrusions that form along the tablet band during compression are called flashing. Flashing can occur regardless of the tablet shape and is most common when compressing materials that undergo plastic deformation under high pressure. The particle size of the formulation and the amount of pressure applied during compression also affect flashing. This article is the first in a three-part series that will discuss the impacts of flashing, examine its causes, and suggest some preventive measures to help minimize it. In this installment, I'll focus on flashing's impacts and causes.
Impact of flashing
Flashing occurs in the production of every tablet to some degree, due to the clearance between the punch tip and the side wall of the die. When a punch tip's outside edge becomes worn, the clearance increases, which increases the propensity for flashing to occur during compression. Flashing is one way some tablet manufacturers determine that tooling is nearing the end of its useful life. I would not recommend this method as best practice, however, because flashing:
  • Reduces yield and increases manufacturing costs;
  • Causes excess dust in bottles, creating a poor customer experience;
  • Creates raised sharp edges around the tablet band, again creating a poor customer experience during swallowing;
  • Is not aesthetically appealing to the customer; and
  • Requires more downstream processing to remove the flashing, which increases production costs.
Causes of flashing
Tablet press. The tablet press can introduce variations in the tablet compression process, and these variations can contribute to the amount of flashing that occurs. For example:
  • Worn guideways can cause excessive punch deflection and nonuniform clearances.
  • Turret misalignment can cause uneven wear on punch tips and nonuniform clearances.
  • Die-table runout can affect fill depth and cause tablet weight variations.
  • Wear on the flight control cam or punch holding plug or improper setup can increase tablet weight variations.
  • Scraper-blade wear can increase weight variations.
  • Tail-over-die wear can increase weight variations.
  • The feeder platform's height and wear on the feeder's bottom surface can affect weight variations.
Weight variation is directly proportional to force variation as is the pressure applied during tablet compression. Tablets compressed under higher than average pressure can exhibit more flashing. Generally speaking, batch runs with higher weight variations require a higher average compression force to help the low-weight tablets meet the targeted hardness values. Flashing is worse when you run the tablet press at high compression forces.
All of the above issues can have an effect on the tablet compression process, potentially causing nonuniform or excessive wear to punch tips and die bores.
In contrast, a well-maintained press can reduce the number of variances, minimizing flashing and tool wear as well as the need for extra downstream processing.
Tool loading. If your tablet press is well maintained and in good working order, the next factor to consider is how you are loading the tools into the tablet press. The clearance between an upper, shaped (not round) punch tip and die bore when made to TSM standards can be as much as 0.003 inch (0.076 millimeter), providing room for manual manipulation.
The manual manipulation occurs when the setup operator inserts the upper punch of a shaped tool into the die. Best practice includes manually turning the punch in the direction that the press rotates, clockwise or counter-clockwise, to align the die before locking it down. This human involvement in alignment has some inherent error associated with it, such as the lack of consistent torque from one punch to the next.
I recommend investing in a setting tool that's designed to reduce setup time and achieve a more consistent alignment. This tool can add a little to upfront costs, but over time, it can save thousands more by reducing tooling wear and preventing critical fracture from tools being repeatedly misaligned and developing excessive cross-edge wear. Make time to discuss this issue with your tooling vendor and see what setting tools the company has to offer.
In the next installment of this series, I'll discuss how proper tablet and tooling design can help minimize 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.
May 6, 2019
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