The sticking defect occurs when the granules of a formulation become stuck to the face of the press punch. Picking, on the other hand, happens when the granules stick to the design embedded in the punch tip such as in lettering or logos. Both sticking and picking result in defective tablets. A visual inspection is usually carried out as an element of quality control, and may identify sticking and picking. Visual inspection, however, is time-consuming and can decrease the yield production – but many manufacturers have no option. As the batch reaches the compression stage, the operator must adjust the press to comply with the product’s characteristic designs. The tablet press setup and operation, tooling and maintenance can each affect the quality of the product. There are also times when the granules not fully dried. That is, they may be dry and hard on the outside but moist or wet on the inside. This can seriously affect the quality of the tablet as the poorly dried particles can break open during compression and stick to the surfaces of the punch press. If this happens, it is important to check the granule drying process.
Steps to Improve and Eliminate Sticking:
1.Adjusting the compression force. The compression force can affect the product in that increasing the force may cause sticking for over-granulated products and decreasing force may result in the particles sticking to the punch face than to each other.
2.Increase pressure. If picking is observed, one the punch pressure can be increased to encourage the particles to bond with each other rather than stick to the punch.
3.Pre-compression. Use of extended dwelling time through pre-compression or slowing the press speed makes the granules stick together rather than on the punch face.
4.Polishing. Polishing the punch face may help with the sticking problem.
5.Proper lubricant mix. It is important to mix the lubricant properly as over mixing can result to the lubricant being ineffective in preventing sticking or picking.
Sticking can happen anytime during production, but most often happens during the initial setup of the press. Although sometimes unpredictable, it can also be a frequent occurrence, and what may be an acceptable level of sticking for one product may not be for another. To determine the likelihood of sticking, it pays to know the product and formulation properties such as the moisture content and size distribution of the particles. Having said that, it is only when the formulation is in the tablet press that sticking will become apparent.
To prevent sticking during pressing, the hardness of the tablet can be increased by making it thinner. Another solution is to increase the dwell time to make sure that the wet granules stick to the other granules instead of the punch face. Sticking or picking may also occur when the powder blending is incomplete. When the lubricant in the formula is not blended well with the granules they stick to the punch face. If improving the blending does not solve the sticking or picking problem, polishing the cup surface may help. Other causes of the problem may be associated with the product itself, the upstream process, operation of the press or the tooling, or a combination of these factors.
Products that have high compressible formulations tend to turn out high quality tablets. In the event that sticking occurs with these formulations, experienced operators sometimes increase the compression pressure significantly. This shock method may work for limited products but is not recommended as a long term solution as it can overload the punches leading to damage of the tooling, and machine. Again, it must be emphasized that experience operators understand the limitations of the tablet press and would know the best approach to solving a sticking or picking problem.