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Causes and Remedies for Dark Spots in Tablet Manufacturing

There are many reasons that these spots can appear and determining the cause is dependent on multiple factors.

There are number of possible reasons which are either related to formulation or with machine setting some of these includes
  1. If formulation contains Calcium Phosphate (DCP/TCP) then there are chances of appearance of black particles during compression due to its abrasive nature
  2. If drug is too hygroscopic then there are chances of formation of black spots due to friction of material in between turret and feed frame
  3. Too much fines in the product which enters in between feed frame and turret and causes friction and produces black spots
  4. Due to melting of drug (If drug have low melting) due to energy generation during compression
  5. Improper setting of feed frame

Dark spots are common defects that arise during tablet manufacturing, and determining their primary cause can be difficult. The first step in troubleshooting dark spots is to determine where the spots are located on the tablets.

  • Split a tablet to determine whether the spots occur throughout the tablet or just on the surface. If dark spots are present in the tablet's interior, the cause is likely an issue in a process upstream from the tablet press, such as blending.
  • If you observe the dark spots only on the upper or lower tablet faces, three factors could be causing the dark spots:
  1. Punch lubrication in the powder
  2. Scorched material
  3. Improper tablet press setup
  • Dark spots only on the upper tablet face are likely the result of punch lubricant or metal shavings or flakes scraped from the bottom of the feeder due to improper setup. Spots on the tablet's lower face or around the tablet band are often scorched formulation from within the die bore or from the punch tips.
You can mitigate these problems using the following troubleshooting methods.

The Location of the Dark Spot
  • 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 of 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.

Scorched Material
  • Another common cause of dark spots on tablets is scorched formulation that flakes off during press operation and finds its way into the powder fill. Scorching occurs when small particles of formulation migrate to the area between the punch tip and the die bore, generating additional friction and heat that results in product buildup on the wall of the die bore. The material flakes off during punch movement, appearing as specks around the tablet's perimeter (photo).
  • Tooling vendors offer lower-punch options that can mitigate this issue. Double-deep reliefs and narrow tip widths help 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 a formulation contains excessive fines. Using special tip sizes and tooling tolerances allows for greater control over the amount of clearance between these critical components.

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, the formulation is "damaged", and results in additional fines within the material. Minimizing the amount of product that gets recirculated is crucial.
  • Proper press set-up plays a vital role in preventing dark specks from occurring in tablets. Confirm that the feed frame is positioned at the correct distance above the highest point on the die table. If not properly set, particles can become lodged in-between the feeder and die table, leading to scorched material on the tablets. Also, setting the feeder clearance at the correct location prevents contact between the feeder and the die table during press operation. 
  • The result of an incorrectly set feed frame is depicted in Image. Small amounts of ferrous metal from these components could scrape off when contact is made and end up on your tablets as dark spots. Metal detectors should be utilized to detect foreign material.

Other Less Common Issues
  • There are instances where dark spots are a result of something far less common, but should be considered during the troubleshooting process. Poorly maintained or worn processing equipment can result in contamination. For example, sieves utilize mesh screens to control particle size distribution. The screens may deteriorate leading to contamination of the formulation, and must be inspected regularly. 
  • Blenders, used for processing powder, require bearings for general mechanical operation. If a bearing or seal is damaged, its lubricant may leak into the formulation, thus resulting in a contaminant that may be seen throughout the tablet. Ensure that all processing equipment is in good mechanical order prior to use to significantly reduce the likelihood of dark spots. If you're unable to ascertain the source of the problem, contact your tooling vendor for further assistance. In some cases, sending sample tablets and/or tooling for further analysis is an option as well.

Dark spots are a tabletting defect that no manufacturer wants to encounter. Understanding the primary causes of dark spots, whether as a result of lubrication in the powder, scorched material, or improper set-up, can help you to implement proactive solutions to produce high-quality tablets and reduce production downtime.

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