Manufacturers use tablet presses to produce a variety of products, such as pharmaceuticals, nutraceuticals, veterinary products, confectionary products, and industrial tablets. Each industry faces unique challenges depending on the properties inherent to the formulations that industry uses. A variety of steel types and coatings, tool-design modifications and options, and tablet designs can work together to increase tooling life and improve tablet quality.
Abrasive wear types and related issues
The most common challenge with nutraceutical tableting is tooling wear due to the abrasive nature of commonly used formulation ingredients, such as calcium, magnesium, and zinc. This abrasion leads to degradation of the punch cup finish, which can result in sticking and poor tablet quality.
The usual first step to addressing this issue is to use a wear-resistant grade of tooling steel. Common wear-resistant steels for punches include D2, K340, DC-53, and M2, and for extremely abrasive products, PM-3V, PM-9V, and PM-10V. The alloy chemistry and increased hardness of these steels make them more resistant to abrasion than other steels. However, wear-resistant steel grades can’t support higher compression forces due to their lower impact toughness.
Over time, abrasion can cause the land (or flat surface) around the perimeter of the punch cup to erode, compromising the punch tip’s mechanical integrity and making it more susceptible to damage and fracture. Eroded land can lead to the formation of J-hooks, potentially resulting in capping. You can address this issue by using one of the premium, abrasion-resistant grades of steel mentioned above. However, this may only delay erosion of the punch tip land.
You can remove J-hooks and restore punch tip land with minimal time and equipment using a deburring stone followed by a quick buffing (photo). This will reestablish the punch tip’s mechanical integrity.
Tablet dies are also subject to abrasion. The abrasive nature of nutraceutical formulations, combined with the high compression forces often used for many nutraceutical products, can lead to wear rings in the die bores (photo). These rings can allow formulation to extrude between the die walls and punch tips during compression, causing the outside edges of the punch tips to become rounded and resulting in tablet flashing. This can also accelerate wear around the perimeter of the punch tip. Die wall wear rings can also cause excessive ejection forces, which can prematurely wear the lower punch heads and accelerate head pitting.
Dark spots or visual defects aren’t uncommon in tablet compression, and they have many causes. Dark spots on the tablet faces, or even mixed throughout the tablet, can indicate excessive lubrication. A feeder pan set with too much clearance can trap formulation particles between the feed frame and the die table. These trapped particles act as an abrasive wearing material for the feeder pan and die table. A feeder pan with too little clearance will contact the die table, resulting in the two components scraping against one another. Both scenarios generate friction and heat, as well as removes flakes of metal, which can contribute to discoloration and dark spots in tablets.
Certain ingredients, such as magnesium, magnesium oxide, and talc, have a propensity to scorch and turn dark when they reach a specific temperature. When granules of formulation migrate between the punch tip and the die bore, they cause friction and generate heat, which can darken the granules and adhere them to the die bore wall. Particles of this discolored material can later break off and enter the formulation during tableting.
A few modifications to the tooling can help:
- First, use a narrow tip width on the lower punch. This decreases the punch tip’s surface area that is in contact with the die bore, reducing friction and heat generation.
- Second, add a double-depth relief (Figure 3). This provides a better scraping action for the lower punch to clean the die bore and also provides more clearance for the formulation to exit the die’s bottom.
- Third, use tapered die bores, which can help reduce ejection forces, another source of friction and heat. However, you should engineer the taper’s depth to be 20 to 30 percent greater than the upper punch’s penetration.
Generally, nutraceutical tablets contain multiple active ingredients that are a significantly higher percentage of their total weight than the percentage of excipients when compared to pharmaceutical tablets. They also tend to be larger than pharmaceutical tablets and are often more challenging to compress. You can address these unique properties at the tablet-design stage with some forethought and communication with your tooling vendor.
Large size. The large size of many nutraceutical tablets can be intimidating to some customers. Such large tablets give the impression that they are more difficult to swallow. A tablet design with areas that appear sharp will only reinforce that idea. For large, high-weight products, consider tablet designs such as a modified oval. The rounded edges and punch cup configuration make even large tablets appear easier to swallow and more appealing.
Higher compression forces. Nutraceutical tablets often require higher compression forces compared to pharmaceutical tablets, so you should design your nutraceutical tablets to support those high forces. Tablet design features such as excessively deep cups and minimal punch tip land decrease the force rating of the tooling. Keeping cup depths moderate and applying sufficient land ensures that tooling will have a high compression-force rating.
Abrasive formulations. A wide land increases the tooling’s wear life against abrasion. Moderate cup depths also help ensure a more uniform density distribution throughout the tablet compared to the density gradients, or soft spots, that can occur when compressing using deep punch cups. A shallow curve in a punch cup equates to a reduced velocity for the formulation as it flows over the punch faces during the filling and compression stages.