How Compression Ratio, Pitch & Channel Depth Impact Extrusion Performance — A Must-Read for Process Engineers

In extrusion, success lies in the details of screw geometry.

Three key parameters determine how well your extruder handles plastic melting, mixing, pressure building, and output:

1. Compression Ratio (CR)
2. Screw Pitch
3. Channel Depth (a.k.a. flight depth or groove depth)

This guide breaks down how each of these affects your output rate, melt quality, energy consumption, and product stability — and how to choose the right configuration for your material and application.

1️⃣ Compression Ratio (CR)

Definition:
The ratio between the channel depth in the feed zone and the channel depth in the metering zone.

Typical Values:

• Low CR: 1.2 – 1.6
• Medium CR: 1.7 – 2.4
• High CR: 2.5 – 3.5

🔧 High Compression Ratio (>2.5)

• Strong pressure buildup
• High shear → improves mixing
• Reduces voids and bubbles
• Used for rigid PVC, WPC, filled compounds

⚠️ Risk: Overheating or degradation of heat-sensitive polymers

🔧 Low Compression Ratio (<1.6)

• Gentle melting, low shear
• Better for LDPE, EVA, PLA, foamed products

Best For: Heat-sensitive, high-slip or low-viscosity polymers

🧠 Juyuan Insight:
If your product has surface defects or burn marks, reducing CR can reduce shear and improve melt stability.

2️⃣ Screw Pitch (Lead Distance)

Definition:
The axial distance between two adjacent flights on the same helix.

Impact:

• Controls residence time
• Affects conveying rate and pressure gradient
• Influences shear stress and output stability

🔧 Long Pitch

• Faster conveying
• Less residence time
• Used for high-throughput, low-viscosity materials

Risk: Poor melting or incomplete homogenization

🔧 Short Pitch

• Longer residence time → better melting
• Creates higher pressure buildup
• Used for crystalline materials, recycled plastics, PET, PVC

📌 Juyuan Tip: A variable pitch (gradually decreasing from feed to metering) is often used in barrier screws to balance throughput and melting efficiency.

3️⃣ Channel (Flight) Depth

Definition:
The radial depth between the screw root and the barrel inner surface.

Impact:

• Controls material volume per turn
• Affects melt pool size, shear rate, and energy input

🔧 Deep Channel

• High volume throughput
• Less pressure build-up
• Best for bulk conveyance in feed zone

🔧 Shallow Channel

• Stronger pressure
• Improved melt sealing and homogenization
• Common in metering zone

Channel Depth Design by Zone:

⚠️ Watch Out: Too shallow a channel in the metering zone may cause back pressure, increased torque, and overheating.

🎯 Putting It All Together: Design Example

Material: WPC (Wood Plastic Composite)
Recommended:

• CR: 2.8–3.2
• Pitch: short in metering zone
• Channel: deep feed, shallow metering

Goal: High-pressure melting with strong mixing for abrasive filler-loaded material.

Material: PLA Film
Recommended:

• CR: 1.4–1.6
• Pitch: long and constant
• Channel: medium depth

Goal: Gentle melting, avoid degradation

🔬 Modern Trends in Screw Geometry

• Variable pitch + variable depth: Tailored melting curves
• Barrier screws: Two-channel design for precise melt control
• Mixing sections (Maddock, Spiral): Improved melt quality without over-shear
• CFD design simulation: Predicts melt pressure, temperature, shear zones

At Zhoushan Juyuan Machinery, we use these techniques to design screws optimized for your polymer, throughput, and machine brand.

📞 Need Help Choosing the Right Screw Design?

Whether you’re processing PVC pipe, PET sheet, WPC profile, or PLA film — the right combination of compression ratio, pitch, and channel depth makes all the difference.

Zhoushan Juyuan Machinery Co., Ltd.
📧 Email: cxj.job@gmail.com
📱 WhatsApp: +86 13905808474
🌐 Website: https://juyuan-machinery.com

Send us your material and product specs — we’ll design a screw that maximizes performance and longevity.