Sticking is one of the most common problems of tablet making. It occurs when granules attach and stick to the faces of the punches instead of locking together to create a uniform tablet. 

Picking is a specific type of sticking in which particles stick within the letters and logos that are embossed or debossed on the faces of the compression tooling.

Regardless whether it’s sticking or picking, the result is a defective tablet. To complete the batch, you may have to visually inspect the tablets. This certainly will slow production and decrease yields, but there is no alternative. The formulation is completed; you can’t send it back down the hallway for reprocessing.

Sticking can happen at any time throughout a batch. It occurs most often at the initial setup of the tablet press, but it might just as easily appear randomly in a production run. It might also appear at regular, predictable times.


Difference between sticking and picking

Sticking occurs when granules attach themselves to the faces of tablet press punches. Picking is a more specific term that describes product sticking only within the letters, logos, or designs on the punch faces.


Remedy

  • Knowing the moisture content, particle size distribution, and other product properties will help you predict whether a product will compress without sticking.
  • The stronger compaction forces cause the granules to bind with the tablet and pull the stuck granules away from the punch face.
  • Changing compression settings, such as by increasing the force, reducing tablet thickness, or decreasing pre-compression thickness.
  • The choice of steel and the degree of polish on the punch will also affect picking. Type D2 and Type 440C steel contain more chromium than other steels, which reduces sticking and picking.
  • In some cases, changing the tool design and its surface finish is enough to stop sticking and picking.
  • Lubricating the right way may reduce the problems.
  • To prevent sticking at startup, some companies routinely distribute lubricant by hand before tableting. Thisputs a dusting on the press that prevents sticking at thestart.Some people think this is unacceptable.


Air entrapment, dwell time and tapered dies effect on Sticking and Picking

The act of compression can trap air in the concave cup of the punch face. The deeper the cup, the more likely it is to trap air. This trapped air creates a soft area on the very top of the tablet. In such cases, the granules don’t know whether to stick to each other or to stick within the punch cup. It is similar to making a tabletthat is too soft: The granules aren’t sure where or what tostick to.

  • The solution here is to make certain the punch dwell time(is the time that the punch head remains in contact with the compression roller) is correct and that air evacuation is adequate. 
  • The primary way to reduce entrapped air is to increase the force of the pre-compression stage so that there is less air to evacuate during final compression. 
  • The higher it is within the die, the easier and faster the air can escape during compression.
  • If those adjustments are not possible, consider using tapered dies to help get the air out. Talk with your tooling manufacturer. Tooling manufacturers are specialists and they can probably help you solve the problem by adding a taper.
  • Another possible solution is to specify a tablet shape that uses a compound radius. Doing so “flattens” the very top of the tablet, eliminating the air pocket. The change is slight but effective. Furthermore, it will not cause a noticeable change in the tablet shape or design.
  • Sometimes new punches are more likely to entrap air than used punches simply because of their tighter clearances. Tight clearances are good, but they can cause air to escape more slowly during compression. With the old tooling, air escapes more quickly so particle-to-particle bonding is more likely.