Types of Screws Used in Blown Film Extrusion: Working, Functions and Selection Guide

Conventional Single-Flight Screw Blown is the most basic screw used in film extrusion that comes with only a single helical flight. It has a uniform pitch, and this screw is divided into 3 different zones—feed zone, compression zone, and metering zone.

Parameters and Dimensions of Screws

Screw Zones and Their Work

(A) Feed Zone:

The feed zone is 40–50% of the total screw length.

• Length: If length (L) = 30D, then feed zone length will be 12D–15D. D is the diameter of the screw.
• Function: When plastic material (granules) enters the screw, then with the help of screw rotation, this plastic material moves forward, i.e., towards the diameter, and with the help of heat generated from friction and external heat, this material melts.
• Material does not melt in the feed zone; only material moves forward.
• Flight Depth: 5–8 mm is the deepest section so that more material can be held.

Important Considerations:

The feed zone should be properly designed otherwise

• If the feed section is too short, the material will not be conveyed properly.
• If it is too long, then proper heating will not be available, and there may be a problem in melting the material, which may cause problems with melted particles.

(B) Compression Zone:

The compression zone is 25-30% of the total screw length.

• Length: If L = 30D, then the length of the compression section will be 7D–9D. Where D is the diameter of the screw.
• Function: In this section, the material starts melting, the plastic material starts becoming soft, and the volume of the material also starts decreasing. In this section, the depth of screw flight gradually decreases, due to which both pressure and shear force start increasing.
• Flight Depth: The flight depth of this section is 1.5–4 mm. Because in this the material does not have to be held much.
• Compression Ratio: 2.5:1 or 3:1.

Important Considerations:

• The compression zone is properly designed.
• If the compression zone is short, there can be issues of overheating or un-melted particles.
• A long compression zone is best for film uniformity.

(C) Metering Zone:

This section is 20–25% of the total screw length.

• Length: Suppose the screw length is L = 30D; then the length of this screw can be 5D–7D.
• Function: Material is completely melted in this section. Shear force is controlled in this section, due to which material is melted propulsively. Pressure buildup occurs with the help of which material is pushed forward to the die.
• Flight Depth: Flight depth of this section is less than 1.5 mm–3 mm.

Important Considerations:

• If the metering zone is small, then there can be issues of pressure fluctuations and inconsistent film thickness.
• If it is too long, then unnecessary shear heat can be generated.

2. Barrier Screw

• The barrier screw is an improved version of the screw with an additional barrier flight to allow easy separation of solid or unmelted material from molten material. In other words, it keeps the solid and molten material in separate zones by providing an additional barrier flight.
• This screw makes the melting process more uniform and the output more stable.
• It controls the shear heat and residence time, thus improving processing and avoiding many troubleshoots.

Read more about barrier screw 

Function

• Barrier screws are mostly used for high-performance processing.
• With its help, complex and multi-layer films are easily made.
• This screw provides better melt homogeneity, which leads to low gel formation.
• L/D Ratio: 28:1 to 32:1
• Barrier Flight Depth: 50% of primary flight depth

3. Mixing Screw

• Mixing screws are slightly different from normal screws because some advanced features are added to them, like Maddock, Dulmage, or pineapple mixers. This section is after the feed section, compression section, and melting section, which is called the mixing section. These sections mix and homogenize the material better.
• Mixing sections are fitted in the last part of the screws. These sections repeatedly cut and fold the material, which results in uniform mixing.

Function:

• Better mixing of additives and colors.
• Uniform melt temperature.
• Improved film quality and production.

Mixing Zone:

This zone is optional and is 5–10% of the total screw length.

• Length: Suppose your L = 30D; then the screw length will be 1.5D–3D. D denotes the diameter of the screw.

Function:

• This screw makes the melted material more uniform.
• This screw improves pigment dispersion and additive mixing.
• There are very rare chances of gels or unmelted particles coming in it because the material is melted and mixed very well.

Flight Design:

• Some designs have Maddock mixers or spiral mixing flights.

Important Considerations:

• If high-quality film is to be made, then mixing zones are beneficial.
• If mixing is excessive, then melt temperature may increase, which will cause degradation.

4. Venting Screw:

The venting screw has a vent hole through which gases and vapors escape. This screw removes the moisture or volatile compounds trapped in the material. Or we can say, in other words, there is an additional decompression zone in the screw where the pressure is less and from where the gases escape.

Advantage

• Better melt quality and film clarity
• Reduces defects like bubbles, voids, and degradation
• Higher mechanical strength in the final product
• Improves stability in high-output extrusion

Note: Blown film Extrusion Screws 

Conclusion

The selection of screw design in blown film extrusion directly impacts processing efficiency, film quality, and output rate. Each screw type has a specific function and advantage that makes it suitable for different applications.

• For general-purpose extrusion, a single-flight screw is best.
• For high output and better melting, the barrier screw is preferred.
• For pigment and additive dispersion, the Maddock Mixing Screw is ideal.
• For moisture and volatile removal, the venting screw is used.