Core extrusion principle: Raw materials go through screw conveying → frictional shearing → high-temperature & high-pressure gelatinization → instant pressure release & flash expansion at die head. Four core factors including shearing intensity, temperature-pressure condition, material residence time and screw structure directly determine the physical, chemical and sensory properties of extruded products.
1. Shearing Effect: Governs Gelatinization Level, Texture and Expansion Rate
Excessive Shearing
Small clearance between screw and barrel plus abundant shear screw elements lead to over-broken starch molecules and excessive protein denaturation. Finished goods feature tiny dense pores, elevated hardness and deteriorated crispness; for feed products, poor water stability and easy collapse in water.
Insufficient Shearing
Uneven raw material blending and incomplete starch gelatinization result in low expansion ratio and dense rigid texture. Puffed snacks have raw inner core, while pet feed gains reduced nutrient digestibility.
Twin-screw intermeshing design delivers uniform shear distribution, generating consistent gelatinization and narrower hardness deviation compared with single-screw extruder.
2. Temperature & Pressure Variation: Determines Pore Structure, Appearance and Nutrient Retention
In-barrel High-Temperature & High-Pressure Stage
- Over-high temperature/pressure: Premature moisture vaporization and massive loss of heat-sensitive nutrients (vitamins, active protein); over-expanded fragile goods with hollow structure and chipped edges.
- Under-low temperature/pressure: Inadequate starch gelatinization fails to provide enough expanding power during pressure drop, creating high-density under-expanded rigid snacks and unqualified floating rate for aquafeed.
Instant Die-head Pressure Drop (Key for Expansion Molding)
Sharp pressure decline at die triggers instantaneous vaporization of internal pressurized water inside material:
- Larger pressure difference brings high expansion rate and well-developed porous crisp texture; smaller difference yields compact fine-pore products.
- Fluctuated die pressure causes irregular dimension, surface blistering or cracking on extrudates.
3. Material Residence Time: Controls Gelatinization Uniformity and Color Consistency
Overlong Residence Time (Common dead zone defect of single-screw equipment)
Local overheating triggers material scorching, generating burnt particles, uneven color, oxidized grease and deteriorated flavor.
Insufficient Residence Time
Under-cooked raw core and abnormal water absorption occur, causing poor rehydration of puffed food and excessive sinking ratio of aquafeed.
Twin-screw forced conveying & self-wiping structure shortens and stabilizes residence time without accumulated scorched leftover, delivering superior color uniformity.
4. Screw Conveying Structure: Quality Difference between Single-screw & Twin-screw Extruder
表格
| Equipment Type | Core Working Feature | Product Quality Performance |
|---|---|---|
| Single-screw | Rely on material friction for feeding, weak blending & frequent material backflow | Cost-effective for basic extrusion; large batch-to-batch density fluctuation, poor homogeneity for complex-formula products |
| Twin-screw | Intermeshed forced feeding with multi-direction mixing & shearing | Wide formula adaption (multi-ingredient pet food, filled puffed snacks), uniform ingredient dispersion and stable extrusion indicators, preferred for high-end production |
5. Targeted Quality Impacts by Product Category
Puffed Snack Food (Rice cracker, crispy chip)
Process parameters from extrusion principle dominate expansion rate → bulkiness; shear & thermal condition → crispness and residue content; pressure stability → regular appearance.
Pet & Aquatic Extruded Feed
Shear and thermal pressure define starch gelatinization degree (core factor of feed digestibility); pressure-drop parameter controls floating/sinking property; process uniformity determines pellet water absorption and water resistance duration.
6. Practical Optimization Solutions
- Shear adjustment: Replace customized shear screw segments or fine-tune screw-barrel clearance
- Thermal-pressure control: Zoned segmented barrel temperature regulation and die hole cross-section modification
- Residence tuning: Modulate screw rotating speed or select proper machine model (twin-screw recommended for complicated feed formulas)