Punch sticking is a common issue in tablet compression. This phenomenon occurs when materials adhere to the punch surface during the compression process, leading to several consequences:

• • Reduced production efficiency: Frequent machine stoppages are required to clean the punch tips.
  • Increased operational costs: Due to cleaning, maintenance, or punch replacement.
  • Decreased tablet quality: Tablets become prone to breaking, chipping, or uneven surfaces.
  • Equipment damage: The punch tips may become scratched or worn, reducing equipment lifespan.

1. Mechanisms of Punch Sticking

Punch sticking mainly results from interactions between the material and the punch surface. The forces involved include:

• Adhesion forces between the material and punch surface:
  • Van der Waals forces: Influenced by contact area and surface roughness.
  • Capillary forces: Occur under high humidity, forming liquid bridges.
  • Electrostatic forces: Electrostatic charges in the powder attract it to the punch surface.
• Cohesion forces between particles: Depends on the nature of the material, compression force and compression time..
  • Material characteristics: Granules containing excipients with elastic deformation properties may stick to the punch surface.
  • Compression time: Extended compression times can allow powders to adhere more strongly to the punch.
  • Compression force: Insufficient force impacts particle bonding and increases sticking potential.

Punch sticking occurs when adhesion forces between granules and the punch surface exceed cohesion forces between the granules.

2. Types of Punch Sticking

Type I (Filming)

• • Occurs when the adhesion force between the API – punch > the API – excipients > API particles.
  • The API forms a thin film on the punch surface without further accumulation.
  • Minor impact, often resulting in dull tablet appearance.

Type II (Layer Accumulation)

• • Occurs when the adhesion force between the API – punch > API particles > the API – excipients
  • The API layer on the punch surface thickens over time.
  • Severe sticking that requires machine stoppage to clean the punch tips.

3. Causes of Punch Sticking

3.1. Formulation Factors

• Formulation composition
  • High moisture content or lack of lubricating/glidant excipients increases adhesion forces.
  • APIs with low melting points are prone to sticking as temperature rises.
  • A high proportion of fine powder increases sticking risk.

• Tablet design
  • Suboptimal tablet shapes or logos can promote sticking.
  • Design requirements for tablet shape may create technical challenges.

→ Solutions:

• • Optimize the formulation by using appropriate types and ratios of lubricants or glidants (e.g., magnesium stearate, stearic acid, sodium stearyl fumarate…)
  • Reduce raw material moisture content and control the compression environment.
  • Control particle size and shape.
  • Simplify fonts and avoid intricate designs.
  • Apply tapering techniques (designing punch engravings with gradual slopes instead of sharp angles) or reduce engraving depth to minimize sticking.

3.2. Manufacturing Process

• • Environmental conditions: High humidity and temperature can alter material properties.
  • Compression time and force: Short compression times or insufficient force reduce particle bonding.

→ Solutions:

• • Control humidity and temperature in the production area.
  • Increase compression force and optimize compression time.

3.3. Punch Material

• Worn, scratched, or non-coated punch surfaces exacerbate sticking.

→ Solutions:

• • Use wear-resistant steel punches (e.g., M340, 440C).
  • Apply anti-sticking coatings (e.g., chromium, chromium nitride) to punch surfaces.
  • Regularly polish punch surfaces to reduce friction.
  • Modify punch designs, such as transitioning from concave to flat tips if necessary.

4. Summary

5. Conclusion

Punch sticking is a complex issue but can be effectively controlled through optimized formulations, manufacturing processes, and tool design. Understanding the underlying causes and implementing synchronized solutions will enhance tablet quality, reduce costs, and ensure process stability.

6. References

TExplore more articles about punches and dies on the SEN Pharma website: Punch and die standards, Structure of punch and die in tablet press, Terminology of Punch and Die