How Screw Structure of Extruder Affects Finished Product Quality

1. Quality Impacts of Three-section Screw Configuration

1.1 Feeding & Conveying Section (large pitch, deep screw channel)

• Structural feature: Largest channel volume with wide pitch for continuous raw material feeding and air venting.
• Quality influence:
   
  ✅ Proper channel depth ensures stable feeding and consistent unit weight of finished products;
   
  ⚠️ Too small pitch or shallow groove causes bridging and intermittent feeding, resulting in irregular hollow granules and disordered internal pores from trapped residual air.

1.2 Compression & Melting Section (gradually reduced pitch + kneading blocks)

• Structural feature: Decreasing channel volume along material flow, fitted with kneading elements to generate frictional heat and shear force for starch gelatinization and protein denaturation.
• Quality influence:
  • Appropriate number of kneading blocks delivers uniform thermal-shear treatment, reaching targeted gelatinization rate (75%~92% for most feed) with controllable crispness or compactness;
  • Excessive shear (overmuch kneading pieces + short pitch) triggers local overheating, leading to over-degraded sticky products, excessive expansion and easy breakage; high-protein formulas suffer protein scorching and nutrition loss;
  • Insufficient shear leads to incomplete gelatinization, hard-textured end products that collapse quickly after water immersion and unqualified floating rate for aquatic feed.

1.3 Metering & Homogenizing Section (fixed small pitch, constant volume)

• Structural feature: Stable channel volume to stabilize extrusion pressure and feed rate before die head.
• Quality influence:
   
  ✅ Sufficient metering length ensures steady extrusion pressure for uniform dimension, even wall thickness and consistent expansion ratio;
   
  ⚠️ Undersized section length brings fluctuating pressure and uneven extrudate with rough surface and irregular internal pores.

2. Effects of Key Screw Parameters on Product Quality

2.1 Pitch & Channel Depth

• Large pitch: High conveying efficiency and low shear, ideal for high-fiber forage to avoid overheat and scorching;
• Small pitch: Strong compression and high shear for high-starch snack with high expansion rate; abrupt pitch variation causes instant pressure spike, triggering crack and random hole on products.

2.2 Length-Diameter Ratio (L/D)

• Short L/D (12:1~16:1): Short material residence time with mild thermal treatment, suitable for crispy snack to prevent over-expansion collapse;
• Long L/D (＞20:1): Full thermal exposure and high gelatinization, widely used for sinking aquatic feed with high density; overlong barrel causes residual material carbonization and black speck impurity.

2.3 Single-Screw vs Twin-Screw Design

• Intermeshed Twin-screw: Self-wiping function eliminates material stagnation, superior mixing performance with minor color difference, applied in premium pet food and filled extruded snacks;
• Single-screw: Low cost and mild shear for low-starch coarse extrusion, yet prone to material coking on barrel inner wall with occasional black particles in final products.

2.4 Screw Material & Wear Resistance

3. Screw Selection Guide for Typical Products