Die Swell in Melt Spinning | Flow Instabilities Part 1

What is Die Swell?

When a polymer is pushed through a spinneret (a tool with small holes) under pressure, part of the energy helps the polymer flow, while some is stored as elastic energy. After the polymer exits the spinneret, the stored elastic energy is released, causing the polymer to expand or “swell.”

In simple terms: Imagine squeezing toothpaste out of a tube—it expands a little as it leaves the nozzle because of the pressure inside. Similarly, polymers swell when they exit the spinneret.

How is it measured?
Extrudate swell is the ratio of: [pmath]({Diameter} {of} {swollen} {extrudate})/({Diameter} {of} {spinneret} {hole})[/pmath]

[pmath]{Die} {Swell} {Ratio}=({Diameter} {of} {Swollen} {Fluid} {at} {Exit})/({Diameter} {of} {Spinneret} {Hole})[/pmath]

[pmath]{Die} {Swell} {%}=((({Diameter} {of} {Swollen} {Fluid} {at} {Exit})-({Diameter} {of} {Spinneret} {Hole}))/({Diameter} {of} {Spinneret} {Hole}))*100[/pmath]

Why is it important?

• High die swell can cause poor spinning or defects like filament fusion when holes are too close.
• To minimize it, flow conditions must reduce the polymer’s elastic energy.

Factors Affecting Die Swell

1. Energy Stored During Flow:
   • The more elastic energy stored, the higher the swelling.
2. Polymer Elasticity:
   • High elasticity means the polymer chains don’t relax easily, causing more swelling.

How to Reduce Die Swell

1. Increase Relaxation Time:
   • Use a longer spinneret capillary (higher L/D ratio) to give the polymer more time to release elastic energy.
   • Especially important for polymers with high molecular weights, like HDPE or high-performance polyesters.
2. Control Temperature and Additives:
   • Increase the spinning temperature to lower polymer viscosity and help chains relax.
   • Use plasticizers or lower molecular weight polymers to reduce elasticity.
3. Reduce Shear Rates:
   • Lower flow rates or use a spinneret with larger holes to reduce deformation and energy storage.
4. Polymer Concentration (Solution Spinning):
   • In solutions, lower polymer concentration to reduce chain interactions and elasticity.
5. Apply Tensile Forces:
   • Stretch the extruded polymer in the spinning zone to direct relaxation in the spinning direction, reducing lateral swelling.

Key Takeaways

• Relaxation time: Longer inside the spinneret means less swelling.
• Flow rate: Slower flow reduces energy storage and swelling.
• Temperature and additives: Help polymers relax better.
• Spinneret design: Higher L/D ratios are better for high molecular weight polymers.
• Stretching: Applying downward force during spinning reduces lateral swell.

These adjustments ensure smooth extrusion and high-quality fibres.