Extrusion Screws: Barrier vs. Compression

In plastic extrusion, screws are crucial for melting, mixing, and pumping the polymer. Two primary screw designs are the barrier screw and the compression screw, each with distinct characteristics.

Barrier Screw

• Function: Features a dedicated barrier flight to physically separate the solid plastic bed from the melt pool, enhancing melting efficiency.
• Advantages:
  • Superior melting uniformity and rate.
  • Enhanced mixing, minimizing unmelted particles and color inconsistencies.
  • Increased throughput capacity.
  • Potential for reduced energy consumption.
• Disadvantages:
  • Higher initial cost.
  • Application-specific design, limiting versatility.
  • More complex geometry, posing cleaning challenges.

Compression Screw

• Function: Employs a gradual reduction in flight depth to increase pressure and temperature, facilitating plastic melting.
• Advantages:
  • Lower acquisition cost.
  • Broad applicability across various extrusion processes.
  • Simplified design, easing cleaning procedures.
• Disadvantages:
  • Potentially incomplete melting, leading to melt inconsistencies.
  • Suboptimal mixing, potentially causing color variations and unmelted particles.
  • Lower throughput capabilities.

Comparative Summary:

Barrier screws excel in melting efficiency, mixing, and output, making them suitable for demanding applications. However, their higher cost and limited versatility must be considered. Conversely, compression screws offer cost-effectiveness and versatility but may compromise on melt quality and throughput. The optimal choice depends on the specific requirements of the extrusion process, including material properties and desired output.

We are seeing a video of a barrier mixing screw for injection moulding.

Starting from the end.

1. Ring valve.

2. Metering section with mixing pins to even out temperature variations and act as a blender.

3. Spiral Maddox section. Material enters every alternate groove and cannot exit. It has to exit via the next groove, So material has to flow through a narrow gap.

4. The barrier section. The melted material on the barrel wall passes over the barrier and thus the melt is separated from the pellets.

5. First section is the feed section. The material from the hopper is heated before it gets to the barrier section.