Extrusion Process: Screw Design, Flow Mechanics and Die Geometry

Extrusion is the continuous polymer processing method used to manufacture pipes, films, sheets, profiles, and wire insulation. The single-screw extruder converts solid polymer pellets into a homogeneous melt via three functional screw zones: Feed Zone, Compression/Melting Zone, and Metering Zone. Understanding screw $L/D$ ratio, compression ratio, drag flow ($Q_{drag}$), pressure backflow ($Q_{pressure}$), and flight leakage flow ($Q_{leakage}$) governs throughput stability, melt temperature uniformity, and dimensional tolerances.

By completing this lesson, you will be able to:

• Analyze the 3 functional zones of a single-screw extruder ($L/D = 24:1 - 30:1$).
• Calculate net volumetric throughput ($Q_{net} = Q_{drag} - Q_{pressure} - Q_{leakage}$) combining drag flow, pressure backflow, and leakage flow.
• Derive flight-clearance leakage flow ($Q_{leakage}$) from radial clearance and geometry-supported 10 metering flight turns.
• Compare coat-hanger film dies, spiral mandrel blown film dies, and annular pipe dies.

mermaid
graph TD
    A["Hopper Solid Feed (Pellets / Flakes)"] --> B["Feed Zone (Solids Conveying, Constant Channel Depth Hf)"]
    B --> C["Compression Zone (Melting & Tapered Channel Depth Hf to Hm)"]
    C --> D["Metering Zone (Homogenization & Constant Shallow Channel Depth Hm)"]
    D --> E["Breaker Plate & Screen Pack (Contaminant Filter & Backpressure)"]
    E --> F["Extrusion Die (Shaping Orifice: Pipe / Sheet / Profile)"]

4.1 Screw-Channel Fluid Mechanics Model

The net volumetric flow rate $Q_{net}$ in the metering section equals drag flow $Q_{drag}$ minus pressure backflow $Q_{pressure}$ minus flight clearance leakage $Q_{leakage}$:

Explicit Input Parameter Table for Reproduction

Worked Numerical Example:

Problem: Calculate drag flow $Q_{drag}$, pressure backflow $Q_{pressure}$, flight leakage $Q_{leakage}$, net volumetric throughput $Q_{net}$, and HDPE mass output $dot{m}$ ($ho = 0.90 ext{ g/cm}^3$) using the parameter table above.

Solution:

1. Calculate Drag Flow ($Q_{drag}$):

1. Calculate Pressure Backflow ($Q_{pressure}$):

1. Calculate Flight Leakage Flow ($Q_{leakage}$):

Across 10 geometry-supported metering turns:

1. Calculate Net Throughput ($Q_{net}$) and HDPE Mass Output ($dot{m}$):

• HDPE Water Pipe Extrusion: Annular basket-die extrusion in Vadodara cluster. (Illustrative Indian industry scenario based on BIS certified pipe manufacturing).

• $L/D$ Ratio: Ratio of screw flighted length to outside diameter ($24:1$ to $30:1$ standard).
• Compression Ratio: Ratio of feed channel depth to metering channel depth ($2.5:1$ to $3.5:1$).

1. ISO 294-1:2017 — Plastics — Injection moulding of test specimens of thermoplastic materials, ISO.
2. Rauwendaal, C. (2014). Polymer Extrusion, 5th Ed., Hanser Publishers.