Evaluation System of Single-Screw Extruder Adaptability to Different Materials (PP/PS/PE/PLA)

I. Introduction: Material Processing Challenges in Single-Screw Extrusion

The single-screw plastic sheet extrusion thermoforming machine, as the core equipment for polymer material processing, its adaptability to different polymers directly determines product quality and production efficiency. PP (polypropylene), PS (polystyrene), PE (polyethylene), and PLA (polylactic acid) exhibit distinct rheological behaviors and thermal stability requirements during extrusion due to their significant differences in molecular structure and physical properties. Evaluating the processing adaptability of single-screw extruders requires constructing a scientific evaluation system from four dimensions: material characteristic analysis, equipment structure matching, process parameter coordination, and product quality correlation.

II. Fundamental Correlation Between Material Properties and Extrusion Processing

(A) Intrinsic Property Differences of Polymer Materials

(B) Core Impacts of Material Properties on Extrusion Process

• PP: The high crystallization rate leads to a large shrinkage rate when the melt is cooled, and attention should be paid to the screw compression ratio and the temperature gradient control of the cooling and shaping section; the shear thinning characteristics require the coordinated adjustment of the screw speed and back pressure to avoid melt rupture.
• PS: The rigid molecular chain makes melt elasticity prominent, prone to melt fracture during extrusion, requiring optimization of the screw mixing section structure to reduce elastic energy storage; poor thermal stability requires temperature control accuracy within ±2°C.
• PE: Low-viscosity melt is prone to leakage flow, requiring an increase in the screw length-diameter ratio (L/D ≥28) to extend residence time and ensure uniform plasticization; the difference in the melting range between HDPE and LDPE requires matching different screw groove depth designs.
• PLA: Strong hygroscopicity (water content needs to be <0.02%), requiring pre-drying treatment; low melt strength and sensitive to shear, extrusion should avoid high rotation speed (recommended <100rpm), and use a low compression ratio (1.8-2.2) screw to reduce shear heat.

III. Evaluation Indicators of Equipment Structure Adaptability to Materials

(A) Matching of Screw Geometric Parameters

1. Length-Diameter Ratio (L/D)

   • PP/PE processing requires L/D=25-30; insufficient L/D will lead to insufficient plasticization of crystalline polymers, causing crystal points in products;
   • PLA processing is suitable for L/D=20-24; an excessively long screw will increase melt residence time and aggravate thermo-oxidative degradation.

2. Compression Ratio

   • PP (compression ratio 2.5-3.0): High compression ratio promotes crystallization structure regularization and reduces sheet thickness tolerance;
   • PS (compression ratio 1.8-2.2): Low compression ratio reduces melt elastic accumulation and inhibits extrusion swell;
   • PLA (compression ratio 1.8-2.0): Avoid excessive compression ratio causing local overheating, combined with a gradient screw structure (screw groove depth decreases linearly from the feeding section to the homogenizing section).

3. Mixing Element Design

   • For the low-viscosity characteristics of PE, barrier-type mixing pins can be set in the screw homogenizing section to enhance melt tearing and distributive mixing;
   • PS/PLA processing requires shear-free or low-shear mixing structures (such as DIS screw elements) to prevent molecular chain fracture.

(B) Adaptability of Molding Dies and Auxiliary Machines

• Die Flow Channel Design:
  PP sheet extrusion is suitable for fish-tail dies (expansion angle 30°-45°) to reduce melt residence time in the flow channel; PLA requires streamlined dead-angle-free dies (surface roughness Ra <0.2μm) to avoid melt sticking and degradation.
• Cooling and Shaping System:
  PE sheets require quenching rolls (temperature 30-50°C) to inhibit excessive crystal growth; PLA requires medium-temperature cooling (60-80°C) to prevent internal stress cracking caused by rapid cooling.

IV. Evaluation Methods for Process Parameter Collaborative Optimization

(A) Temperature Control Strategies

1. Three-Zone Temperature Partition Principle
   • Feeding section: PP/PE needs to maintain the barrel temperature 10-20°C lower than the melting point (e.g., PP feeding section 150°C) to prevent material sticking to the wall; the PLA feeding section needs to be supplied with dry air (dew point < -40°C) to avoid moisture infiltration.
   • Compression section: The PS compression section temperature needs to be controlled at 200-220°C; exceeding 240°C will cause benzene ring oxidation and yellowing; the PLA compression section temperature upper limit is 175°C, and a forced air cooling device is required.
   • Homogenizing section: The PE homogenizing section temperature needs to be 20-30°C higher than the melting point (e.g., HDPE homogenizing section 150-160°C) to ensure melt fluidity; the PP homogenizing section temperature is 180-190°C, balancing plasticization and energy consumption.

(B) Pressure and Rotation Speed Regulation

• Back Pressure Setting:
  PP extrusion back pressure is 0.5-1.0MPa to improve melt compactness and reduce sheet bubbles; PLA back pressure ≤0.3MPa to avoid sudden increase in melt viscosity caused by high pressure.
• Screw Rotation Speed:
  PE (low density) can adopt high rotation speed (150-200rpm) to increase output; PLA rotation speed needs to be <80rpm, combined with a low-speed high-torque motor (torque fluctuation ≤5%).

© Tension and Stretching Ratio Control

• PP sheet tension ratio 1.2-1.5, stretching ratio 2-3, needing to match a synchronous speed-regulating tension machine;
• PLA sheet tension ratio ≤1.2 to avoid melt fracture caused by high tension speed, and the tension roll temperature needs to be maintained at 50-60°C to improve melt strength.

V. Correlation Evaluation Between Product Quality and Processing Adaptability

(A) Key Performance Testing Indicators

1. Physical Properties

   • Thickness uniformity: PP/PE sheet tolerance needs to be ≤±3%, PLA ≤±5% (limited by melt strength);
   • Mechanical properties: PS sheet tensile strength needs to be ≥40MPa, elongation at break ≥2%; if shear is excessive during extrusion, elongation at break will drop to below 1%.

2. Microstructure

   • PP sheet crystallinity should be controlled at 55%-65%; too high will lead to increased sheet brittleness;
   • The molecular chain orientation degree of PLA sheets needs to be <15%, which can be evaluated by observing birefringence phenomena with a polarizing microscope.

(B) Typical Defects and Equipment Adaptability Correlation

• Melt Fracture (common in PS/PLA):
  The cause may be too high screw shear rate (PS shear rate >1000s⁻¹) or too large die inlet angle, requiring reducing rotation speed and optimizing the die convergence section angle to 15°-20°.
• Sheet Yellowing (common in PS/PLA):
  Indicates thermal degradation occurrence, needing to check whether the homogenizing section temperature exceeds the PS upper limit of 240°C or whether there is a local overheating area in the PLA barrel (such as thermocouple failure).

VI. Extended Evaluation of Sustainable Processing Adaptability

For biobased materials like PLA, in addition to traditional processing indicators, attention should also be paid to:

• Energy Efficiency: PLA extrusion energy consumption is about 1.3-1.5 times that of PP, requiring evaluation of the matching between screw heating power and cooling efficiency (recommended heating power density ≤0.3kW/kg·h);
• Clean Production: Residual materials in equipment after PLA processing need to be cleaned with special cleaning agents (such as isopropyl alcohol) to avoid cross-contamination with other materials, evaluating the disassembly convenience of the screw (such as quick-release flange structure).

VII. Constructing a Multi-Dimensional Adaptability Evaluation Model

The evaluation of single-screw extruder adaptability to PP/PS/PE/PLA needs to take the logical chain of “material characteristics-equipment structure-process parameters-product quality” as the core, form a closed-loop evaluation system through customized screw geometric parameters (such as high compression ratio for PP, low-shear mixing section for PLA), collaborative optimization of temperature-pressure-rotation speed (such as low-temperature low-back pressure strategy for PS), combined with real-time monitoring of product microstructure and physical properties. For new materials (such as PLA), sustainable processing indicators should also be incorporated into the evaluation framework, promoting equipment upgrading from “single-material adaptation” to “multi-material compatibility + green production”. The establishment of this evaluation system not only provides theoretical support for sheet extrusion process optimization but also offers technical basis for modular design of single-screw extrusion equipment.