John Norman, mechanical drafter and designer for technical service support at Natoli Engineering, examines the causes and solutions for dark spots in tablet manufacturing.
Can you remember a time when production came to a halt due to dark spots on the tablet post-compression? If you are quick to recall an instance, you’re sitting right alongside the majority of tabletting manufacturers. There are many reasons that these spots can appear and determining the cause is dependent on multiple factors.
One of the most common causes of dark spots is a result of a contaminant, typically a tooling lubricant such as oil or grease, being transferred to the powder. During tablet press operation, it is critical to have proper lubrication on the punch heads and barrels to maintain satisfactory operating conditions. Most modern tablet presses are equipped with oil seals to restrict punch barrel lubricants from entering the compression zone. Regular inspections of these seals should be performed. Uneven or extended wear and damage to the seals may cause lubricant to make its way into the compression zone. Additionally, if the upper punch lubricant is unsuitable for the application, whether an inadequate viscosity or used in excess, the chances of spots, depicted in image 1, within the tablets are increased.
Image 1: Punch lubrication has fallen onto the die table and mixed with the formulation during filling. The result is a dark spot in the final tablet.
Dust cups are an accessory that can be installed on most upper punch barrels or tips to keep excessive lubrication from finding its way onto the die table. Older rotary tablet presses do not have oil seals; therefore, the use of dust cups to deter lubrication from entering the compression zone is critical. Ask your tooling vendor about dust cups applicable for your upper punches and your machine. In addition to regular inspection of the oil seals, the paddle bearing seals inside of a force feeder should be performed to ensure grease from within the bearings is contained and not contaminating the formulation. If the grease is not properly contained, they need to be replaced.
Formulation can become overworked during standard operation of a rotary tablet press, resulting in dark specs within tablets. All rotary tablet presses operate on what is known as the overfill principle, meaning each die bore is initially overfilled and is set at the weight adjustment ramp to ensure each bore has the same amount. During the weight adjustment phase, the extra fill is swept away by a scraper bar, recirculated and reintroduced back into the feed frame or feeder pan. Minimising the amount of excess product that gets recirculated is crucial. When particles in the formulation undergo enough strain, they can begin to change appearance (colour) — reducing the amount of stress the formulation experiences before compression is essential.
Proper press set-up plays a vital role in dark specks from occurring in tablets. Confirm that the feed frame or feeder platform is positioned at an acceptable height above the highest point on the die table, with minimal clearance to keep particles from becoming lodged in-between the feeder and the die table. These particles can then become scorched and result in an undesirable discolouration of the formulation that will eventually become tableted. Setting the feeder clearance at the correct location is crucial to prevent the die table from contacting the feed frame or feeder pan during press operation. If set-up is flawed in any way, material from these components may scrape off and end up in the tablets.
Image 2: Excessive punch tip wear around the tip edge allows product to migrate between the tip and die bore. Keeping a sharp lower tip relief edge is crucial to help clean out the die bore with every press revolution.
Punch tip to die bore clearance can also play a role in the appearance of dark spots on or within a tablet. If an existing tooling set begins to produce tablets with discoloured specks over time, there is a chance the tooling has experienced significant wear to the punch tips and/or die bore. This wear allows particles to migrate in-between the tip and die bore and become scorched due to the friction generated by punch movement, as shown in image 2. Burned material from within the bore or from a punch tip can flake off during manufacturing and find its way into the formulation prior to final tablet compression. To help prevent product build-up within the die bore, tool vendors offer different punch tip options such as sharp lower tip relief and double deep tip relief, both designed to clean out the die bore on the downstroke of the lower punch following tablet ejection.
With brand new tooling, there is a possibility that the product formulation may contain very small particles, or fines, that run the risk of finding their way between the punch tip and die wall. Small particles (dust) can adhere (scorch) onto the periphery of the upper and/or lower punch tips, as seen in image 3. Increasing and controlling particle size within the formulation can help prevent the scorching from occurring. Alternatively, decreasing the amount of clearance between the punch tip and die bore may resolve this issue as well. Applying special tip sizes and tolerances to a tooling set allows for greater control over the amount of clearance between the punch tips and die bores.
Image 3: Dark specks in tablets can occur when scorched product build-up flakes from the upper and/or lower punch tips into the compression zone.
The key to troubleshooting the cause of dark specks in the defective tablets is to evaluate where the spots are. If only seen on the upper tablet face, it’s likely the defect is from punch lubricants and/or scraped material from the bottom of the feed frame or feeder pan. Spots that are present on the lower face and tablet band are likely from scorched material present within the die bore or on the punch tips. If spots occur throughout the tablet, it may be the result of an upstream process issue during powder preparation, such as blending.
Sometimes further examination is necessary to ascertain the exact cause. Sending samples to your full-service tooling vendor for further analysis is always an option. These discolored areas could be oil, grease, scorched material, or metal shavings from one of the tools or components of the tablet press. Metal detectors should be utilised to detect small amounts of ferrous metal and reject the affected tablets. Sample tablets containing the material would need to be evaluated extensively to pinpoint an exact source of the contamination. Contact your tooling vendor with any questions you may have regarding dark spots within your tablets, and they will help you troubleshoot the possible causes.
In conclusion, dark spots are an eyesore that no tablet manufacturer wants to see in their final product. Regular tooling and press inspections, proper press set-up, and replacement of worn components are all proactive measures that can be taken to prevent the black spots from occurring.