Sticking is one of the most common problems in tablet compression. Powder particles stay on the punch tip face, tablets get surface defects, embossing becomes blurred, and rejects and cleaning stops increase. Here is where sticking comes from and how to eliminate it.
How to recognise sticking
- dull spots, dents or “picking” appear on the tablet surface;
- letters and logo embossing fill with powder and become blurred;
- a film builds up on the punches with every hour of operation;
- ejection force and rejects increase over time.
Main causes
1. Formulation and powder properties
High granule moisture, hygroscopic or low-melting components, too little lubricant or its uneven distribution are the most common causes of sticking. Particle size and the amount of fines also matter.
2. Condition of the punch tip face
Scratches, micro-wear and corrosion on the tip face hold on to powder particles. Even new tooling can cause sticking if it is not properly polished.
3. Tablet geometry and embossing
Deep or fine embossing, sharp letter corners, small “islands” inside characters (as in A, O, R) and a poorly designed break line create areas where powder sticks most.
4. Process parameters and environment
Insufficient compression force, high turret speed, and high room humidity and temperature also promote sticking.
How to eliminate sticking
- Check the formulation: granule moisture, lubricant content and blending method, particle size distribution.
- Polish the tooling: regular polishing of the tip faces restores smoothness and often solves the problem.
- Change the punch coating: chrome plating, titanium nitride (TiN), chromium nitride (CrN) and other coatings reduce adhesion.
- Optimise the embossing: change the font, depth and wall angle, increase radii, remove fine details.
- Adjust the parameters: compression force, turret speed, room conditions.
Prevention
A proper tooling care routine — cleaning after every product change, inspection, polishing and correct storage — greatly reduces the risk of sticking and extends the life of punches and dies.
AT Engineering engineers will help identify the cause of sticking and find a solution: coating, polishing or new tooling with optimised embossing. We also supply tablet press spare parts and carry out tablet press modernisation. Contact us — send photos of the tablets and punches and we will propose a solution.
Photo at the top: Robin Müller, Wikimedia Commons, licence CC BY-SA 3.0.
