Sticking,Picking and Tablet Toolings

Dear Friends,

 It is quite sometime after that I meet you all in this forum with a different subject about tabletting issue of sticking.

Sticking, picking capping are some of the most common defects that one would have come across in their shop floor experience of tabletting processes.

It is nothing to wonder if I say that some of the products,(a constituent of a particular API or excipient) are characterised with the issues of sticking.

The consistency nature of these materials in exhibiting sticking is such that every time a particular product constituent of that API  is compressed in the machine their characteristic sticking issue is even considered as identification test for the presence of that API in that formulation.

Products have their own pattern of exhibiting sticking to the punch surfaces.

For some products, sticking starts in the beginning stage of compression and solves in the long run.

For some others, the initial stage of tabletting would be flawless and as the time goes the sticking issue starts appearing in between.

For some other products, the issue appears and resolves by itself and reappears after sometime 

Many time sticking issues are solved by the addition of lubricating materials in the blend by dividing into multiple lots. but again this may not be required for the entire batch as the sticking issue solves by itself in the long run.

Altogether the sticking picking issues are totally mysterious that one could not judge as to when it all started and how it got corrected in due course, but only can anticipate that the issue would crop up at any point during compression and to keep a vigil.

In general, in my own three decades of experience, I have understood that there are primarily three factors that influence sticking or picking in tablets.

• Nature of Materials
• Environmental conditions
• Tooling quality. 

While in the first case if sticking is owing API nature nothing can be done, but in latter cases, we often fail to give importance in making studies and create a conducive atmosphere to get a quality tablet.

More especially when it comes for tooling most of the people fail to understand and rather ignorant of the importance of procuring specific grade of punches and die combination to combat the sticking issues.

Sticking and picking issues generally arise from either formulation or tablet design inadequacies. 

Both emerge because of the differences in physical properties of the formulation excipients and drug substance. The process of blending the formulation components results in a powder that will either consolidate and compress.

Cohesive forces: 

In production of a solid dose tablet form

The formation of a compact is the result of chemical dispersive and mechanical forces joining particles to form a solid of measurable density and porosity. 

The cohesive forces binding the particles together can be assessed using a series of studies to determine whether problems with tablet compaction can be anticipated.

 Compaction profiles and strain rate studies are important aspects of development projects. These studies ensure that the probability of successful scale-up and technical transfer is high. Therefore, market supply is not impacted because of technical due diligence was overlooked.

Tablet particles are bound by cohesive forces. However, during the compression process, the cohesive forces binding the particles are challenged by the adhesive forces of the punch cup and the embossing within the punch cup. The first stage of a sticking problem occurs when the adhesive force of the punch cup can pull even a single particle away from the tablet. A recent publication indicates that this is principally caused by the physical properties of the active pharmaceutical ingredient (API), the mechanism and kinetics of which can be successfully modelled.

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1. Moisture content of blend.

2. Insufficient Glidant and Lubricant.

3. Punch tool design (Pre-picking) and embossing (especially 'island' numerics or fonts).

4. Imbalance in Cohesion and Adhesion

5. Granules to fines ratio in blend

6. Melting point of API / Excipients

7. Blend containing "Polyols" as major diluent tend to pick/stick.

In short punch, sticking problem is prevalent in tablet manufacturing especially ODT's.

Materials of tooling construction

If the sticking and picking have been discovered during the R&D stage, or if it is time to order the next set of punches for a product with known sticking and picking issues, another consideration is to have the punches made from speciality steel. 

It is widely accepted that punch steels with a high concentration of chromium in the alloy chemistry, usually from 16–18%, enhances the release of the compressed product.

A reputable tool vendor will have multiple grades of high chrome steel available to manufacture the punches from. There are also several other speciality sheets of steel available that can enhance the performance and service life of the punches and dies with regard to compressive strength, wear resistance and corrosion resistance.

As an alternative to using speciality steel, some may choose to have a coating applied to their standard steel punches to enhance the release characteristics of the punch faces. 

The most common coatings are Hard Chrome (Cr) and Chromium Nitride (CrN).

 There are several other coatings available as well, depending on the unique characteristics of the blend to be compressed. However, it should be noted that there is the possibility that the thin chrome coating layer may wear off with time, owing to the abrasion of the formulation and polishing. 

Thus, the better choice may be high chrome steel, without coating, for a long-term solution.

Discuss these issues with your tooling vendor early in the process to help reduce production issues and additional costs. Your tooling vendor should be able to explain the unique properties and advantages of the various steels and coatings available for your tooling.

 conclusion:

While most of the chemists in shop floor are do aware of nature of materials they handle and about the importance of establishing right environmental conditions to get rid of sticking issues but beyond that if problems still persist only little they are aware of the need to go for procuring right grade of tooling materials to combat the issues.

Unfortunately, people are not much trained in getting to know about the metallurgy of punches and dies. What are  different grade of punches and dies available in the market what are different coatings that can be given over and above our regular ONHS punches and how they help us to alleviate our technical issues etc?

It is high time that we should not fail to tap the technological advancement

To our benefit.

It is always advised that involving our tooling provider in our attempt to curb the issues relating to tabletting is the only best choice.

And it certain that doing so would not only help to mitigate the issue but also save a lot Time , Money, Energy and result in producing quality products,

SRIKANTH SANTHANARAMAN