Rough Going? Tips for Fighting Abrasive Wear of Punches and Dies

Manufacturers face several challenges when compressing nutraceutical tablets, many of which are unrefined natural ingredients that are abrasive, corrosive, and/or coarse. It’s thus important to determine which material will prolong the service life of compression tooling. As the components most in contact with the product, tablet punches and dies require robust metallurgy and design. 

While sticking—the adherence of granules to the punch tip face—can be difficult for pharmaceutical manufacturers to overcome, the ingredients they use are often formulated to improve tablet compression and binding without much physical impact on the tooling. In the nutraceutical industry, it is not so simple. 

Nutraceutical formulations can contain tens of active ingredients that tend to be bulkier than those used in pharmaceuticals, which makes production problematic. Formulation characteristics like wide variation in particle size, poor compressibility, incompatible ingredients, and moisture sensitivity can also lead to production and tooling issues. To avoid replacing your tooling prematurely and to minimize tablet defects, use punches and dies made from a suitable steel. Also consider adding a coating to the punch tips. 

Wear-Resistant Steel and Coatings 

In their natural states, vitamins and minerals can be hard and thus difficult to compress. In fact, some ingredients are just as hard as low-chromium steel and can wear punch faces and dies quickly. That can lead to weight variation, sticking, and other problems and cause you to scrap punches too soon. Wear also degrades embossing on the punch face, leading to poorly defined if not illegible tablet lettering. 

Most suppliers of tooling offer a variety of steels that resist abrasion, corrosion, and impregnation by hard granules. Deciding which to use can be overwhelming, but your supplier should offer guidance. One option is electro slag re-melted premium steel¹, which is refined to reduce non-metallic inclusions and make it more homogenous. Its high chromium content also improves wear resistance. 

Tungsten carbide² extends the life of tooling by reducing premature die wear and die bore deformation. It also offers high compressive strength. But tungsten carbide is also very brittle and using it to tablet nutraceuticals can lead to premature punch tip fractures. In addition, certain formulations react with tungsten carbide, and when it’s used in dies, it can leave black spots on the tablet walls. When tabletting especially abrasive formulations, a powder metallurgy grade of steel may be a better choice for punches and dies³. 

If wear-resistant steel alone can’t withstand the rigors of nutraceutical tablet production, consider adding a wear-resistant coating to the punch tip. One resilient surface coating⁴ is approximately five times harder (about 3,000 on the Vickers scale) than standard tablet tooling steels and can extend punch service life by more than 900 percent. It also boosts corrosion protection and helps prevent sticking. Because of its hardness, the coating is only applied to the punch tip to prevent excessive wear of the compression rollers, which are much softer than the coating.

But don’t settle for a trial-and-error approach. Specifying tooling that way is expensive and time consuming. Instead, consider using a predictive coating model⁵ that takes the guesswork out of tooling selection. It does so by taking into consideration the properties of the active ingredient(s) and excipients, as well as the effects of Van der Waals forces, capillary action, deformation mechanics, the compression environment, and the chemistries of different coatings. If sticking is the problem, for example, our proprietary predictive approach can identify which coating will best prevent your formulation’s particle from adhering to the punch face.

Design Details

Tablet design also affects susceptibility to wear, and a good design helps maintain high overall quality. If the tablet requires a punch with a deep concave face, it will wear quickly around the steepest areas because, during compression, the granules will tend to move laterally across the concave area and erode the tip face. That can lead to capping and delamination.

In such cases, consider using a flatter tablet profile or one specially designed to reduce the effects of abrasive wear. Using a compound radius is often helpful.

Also consider the gap between the periphery of the punch and the die wall. If abrasive particles get sandwiched there, they’ll accelerate wear of both the die bore and punch tip. That can cause wear rings to form in the die, and the concave face of the upper punch will gradually curl inward to form a claw, causing flashing.

References

1. HPG-P from I Holland, Nottingham, UK.

2. HPG-TC from I Holland.

3. HPG-MP and HPG-MD from I Holland.

4. PharmaCote RS from I Holland.

5. TSAR Predict from I Holland.