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More Questions and Answers
Dark spots in tablets
Q: How do I determine the cause of dark spots in my tablets during production and prevent them from occurring?
A: John Norman, Natoli Engineering, says:
imageDark 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:
  • Punch lubrication in the powder
  • Scorched material
  • 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.
imagePunch lubrication in the powder
A common cause of dark spots is transfer of a contaminant to the powder, typically a tooling lubricant such as oil or grease (photo). Excess lubricant or lubricant of an inadequate viscosity increases the chance of spots on the tablet faces. Make sure the guide seals in the turret are in optimal condition to prevent punch-barrel lubricant from entering the compression zone. If your tablet press manufacturer didn't provide guide seals for the turret or if the problem persists, use dust cups to capture and contain excess lubricant. The cups also deflect dust inside the compression zone away from the punches.
It's critical to use lubrication on the punch heads and barrels to maintain satisfactory operating conditions. Adhering to the lubricant recommendations supplied by the tablet press manufacturer is strongly advised.
imageScorched 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).
imageTooling vendors offer lower-punch options that can mitigate this issue. Double-deep reliefs and narrow tip widths (Figure 1) 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 setup
Dark spots can also occur due to improper press setup or adjustment of the feeder. All rotary tablet presses operate on the overfill principle, in which the press initially overfills the dies. During the weight-adjustment phase, the extra fill is ejected from the die, swept away by a scraper, and recirculated back into the feeder. Recirculation causes the formulation's particles to break down, creating additional fines, so it's crucial to minimize the amount of recirculation.
Proper press setup plays a vital role in preventing dark spots from occurring. Confirm that you have positioned the feed frame at the correct distance above the highest point on the die table. If the feed frame height is not properly set, particles can lodge between the feeder and die table, leading to scorched material in the tablets.
Also, setting the feeder clearance at the correct location prevents contact between the feeder and the die table during press operation. Small amounts of ferrous metal from these components can scrape off upon contact and end up in your tablets as dark spots. Use metal detectors to detect foreign material.
Dark spots are a tableting defect that no manufacturer wants to encounter. Understanding the primary causes of dark spots can help you to implement proactive solutions, produce high-quality tablets, and reduce production downtime.

John Norman is mechanical drafter and designer, technical service support, at Natoli Engineering, St. Charles, MO. The company manufactures tablet press replacement parts, tablet compression tooling, and tablet presses and offers the largest selection of tablet compression accessories in the industry. For more information, contact the company.
October 28, 2019
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