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COVER STORY
Natoli Engineering examines the causes and solutions for dark spots in tablet manufacturing.
H John Norman — mechanical drafter and designer for technical service support at Natoli Engineering
Image 1: Punch lubrication has fallen onto the die table and mixed with the formulation during filling. The result is a dark spot on the tablet.
Image 2: Upper and lower punch diagram with excessive tip wear which results in dark spots around the outer edge of the tablet.
as production ever been halted due to dark spots on your tablets? If you answered yes, you’re not alone. Dark spots are a common defect that arise during the tablet manufacturing process and determining their primary cause can be difficult. In this article, we’ll discuss how to determine if the dark spots in your tablets are a result of one of these common causes: - Punch lubrication in the powder - Scorched material - Improper tablet press set-up The first step in troubleshooting the cause of dark spots is to evaluate where the spots are located on the tablets. If the dark spots are only seen on the upper tablet face, it is likely the defect is a result contaminated material, e.g. punch lubricant or scraped material from the bottom of the feeder. When spots are present on the lower face of the tablet or around the tablet band, it is often scorched material from within the die bore or from the punch tips. Additionally, a tablet should be split to verify if the spots are throughout the tablet. If there are dark spots present in the interior of the tablet, it is likely the result of an upstream process issue during powder preparation, such as blending. If the dark specks are only observed on the tablet faces (upper or lower), one of the following troubleshooting methods can mitigate the problem. 1: PUNCH LUBRICATION IN THE POWDER? One of the most common causes of dark spots are the result of a contaminant, typically tooling lubricant such as oil or grease, being transferred to the powder (see image 1). It is critical to have lubrication on the punch heads and barrels to maintain satisfactory operating conditions. Adhering to the recommendations for
lubricant as supplied by the tablet press manufacturer is strongly advised. If the lubricant is of an inadequate viscosity or is used in excess, the chances of spots within the tablets increase. Guide seals in the turret should be in optimal condition to restrict punch barrel lubricants from entering the compression zone. If guide seals are not supplied in your turret or if the problem persists, dust cups must be utilised to capture and contain excess lubricant. The cups also deflect dust from within the compression zone away from the punches. 2: SCORCHED MATERIAL? Another common cause of dark spots on tablets is a result of scorched material that flakes off during manufacturing and finds its way into the powder fill. Scorching occurs when small particles migrate in-between the punch tip and the die bore thus generating additional friction and heat resulting in product buildup. The material flakes off during punch movement and can then be seen around the perimeter of the tablet (see image 2). Tooling vendors offer lower punch options to mitigate this issue. Double-deep reliefs and narrow tip widths, as depicted in image 3, are designed to clean out the die bore during the downstroke of the lower punch following tablet ejection. Although more common on worn tooling, new tools also run the risk of forming dark spots if the formulation contains excessive fines. Utilising special tip sizes and tolerances on the tooling allows for greater control over the amount of clearance between these critical components. 3: IMPROPER TABLET PRESS SET-UP? Dark spots may also occur due to the improper set-up or adjustment of the feeder. All rotary tablet
presses operate on the overfill principle where the dies are initially overfilled. During the weight adjustment phase, the extra fill is ejected from the die, swept away by a scraper, and recirculated back into the feeder. When the product is recirculated,
Proper press setup plays a vital role in preventing dark specks from occurring in tablets