PET Twin-screw Extruder Production Line: Model Selection Determines Production Efficiency Core
In the field of PET (polyethylene terephthalate) material processing, especially in the modified granulation process of recycled PET (rPET) or native PET, the configuration of a production line with twin-screw extruder as the core is crucial. The different models of twin-screw extruders and their corresponding pelletizing systems directly determine the production capacity, energy consumption, product quality (such as intrinsic viscosity, yellowing index, impurity content), and final particle morphology (underwater cutting vs. air-cooled die cutting) of the production line. Understanding the differences in models is of great significance for optimizing PET processing.
Basic composition of production line and PET processing characteristics
A typical PET granulator production line includes:
1. Pre treatment system: PET (especially rPET) needs to be strictly pre dried (crystallization dryer) to reduce moisture (<50ppm) and prevent hydrolysis degradation from causing a decrease in intrinsic viscosity (IV value). High crystallinity PET slices still require crystallization treatment.
2. Feeding system: The weight loss feeder ensures accurate and stable measurement of raw materials (PET slices, additives, masterbatch), which is the basis for formula consistency.
3. Core: Twin screw extruder: responsible for core tasks such as melting, mixing, exhaust, homogenization, and pressure building. Its co rotating meshing design is particularly suitable for the shear sensitivity and efficient mixing/stripping requirements of PET.
4. Melt filtration: precision screen changer (continuous or backwash type) is particularly critical for rPET processing, which can efficiently filter impurities, gel and unmelted particles in the melt.
5. Granulator unit: determines the particle shape (cylindrical, hemispherical) and appearance quality. Common types:
Underwater strip cutting machine: After the melt is cooled into strips in a water tank, it is cut into uniform particles, easy to dry, and has strong versatility.
Air cooled die surface hot cutting granulator: The melt die mouth is directly hot cut, centrifugal air cooled, with high efficiency and no wastewater, but strict requirements for PET melt strength and cooling control.
6. Auxiliary systems: cooling, drying, screening, dust removal, packaging.
The characteristics of PET require special requirements for equipment: high temperature processing (270-290 ° C), shear sensitivity (to avoid excessive shear causing molecular chain breakage and IV decrease), easy hydrolysis, and the need for efficient devolatilization (to remove moisture and small molecule volatiles).
Key parameters of twin-screw extruder models and their impact on PET production efficiency
Choosing the matching twin-screw extruder model is the core of production line efficiency:
1. Screw diameter (D):
Parameter range: commonly 35mm, 52mm, 65mm, 75mm, 90mm, 120mm+.
Impact on production efficiency:
Capacity: The most direct impact. As D increases, the theoretical yield approximately increases in a square relationship. For example, the production capacity of a 65mm machine can be increased by about 50% -80% compared to a 52mm machine.
Specific torque/specific power: Large diameter models have greater torque at the same speed, and have stronger ability to handle PET formulations with high filling, high viscosity, or requiring high dispersion mixing (such as high content fiberglass reinforcement).
Investment and operating costs: As D increases, equipment procurement costs, energy consumption, and land occupation space significantly increase.
2. Length to diameter ratio (L/D):
Parameter range: The commonly used L/D for PET processing ranges from 40:1 to 52:1 or higher.
Impact on production efficiency:
Plasticization and mixing effect: The larger the L/D ratio, the longer the material stays in the barrel, and the more complete the thermal process and shear mixing it undergoes. This is crucial for applications that require deep plasticization, sufficient devolatilization (multi exhaust port design), complex blending (such as PET/PC alloy), or high filling modification, which can improve product uniformity and performance.
Devouring efficiency: The aspect ratio provides more space to set up an efficient exhaust section (atmospheric pressure or vacuum), effectively removing moisture, monomers, and oligomers, preventing PET foaming, yellowing, and IV decrease. High L/D is particularly advantageous for rPET processing.
Flexibility: High L/D allows for the setting of more functional blocks (feeding port, side feeding, liquid injection, exhaust port), providing ample room for process adjustment. However, excessively long L/D may increase the risk of melt heat retention.
3. Drive power&specific torque (M ᵢ/a ³):
Meaning of torque ratio: The core indicator for measuring the mechanical strength of a twin-screw extruder, which is the maximum torque that a unit barrel volume can withstand. Modern high-performance machines can reach 11-15 Nm/cm ³ or even higher.
Impact on production efficiency:
Processing capability: High specific torque means that greater shear force can be applied at the same screw speed, which can handle higher viscosity melts, higher filling ratios (such as fiberglass, mineral powder), or formulations that require high dispersion force (such as carbon black masterbatch), while allowing operation at higher speeds to increase production capacity.
Energy efficiency: High specific torque design typically means a more efficient transmission system, which may have better specific energy consumption (kWh/kg) when achieving the same output and mixing effect.
Stability and lifespan: Adequate power and torque reserves are the foundation for dealing with fluctuations in raw materials (such as changes in rPET impurity content), ensuring continuous and stable operation, reducing downtime, and extending equipment lifespan. Insufficient torque can easily cause the host to overload and stop, or abnormal wear of the screw/barrel.
4. Screw configuration design:
Parameter range: Combination screw (composed of conveying, kneading, reverse threading and other components), with flexible and versatile design.
Impact on production efficiency:
Melting efficiency: Optimized compression section and kneading block design ensure fast and uniform melting of PET, reducing thermal mechanical degradation.
Mixing dispersion/distribution effect: The angle, quantity, and width of the kneading blocks determine the strength of dispersion mixing (breaking agglomeration) and the distribution mixing effect (uniform composition). Customized configurations are required for different additives (flame retardants, toughening agents, fillers).
Pressure building ability&temperature control: It affects the pressure and temperature stability of the molten material entering the mold head, and is crucial for the uniformity of the final particle size and avoiding mold dripping.
Self cleaning: The well engaged co rotating twin-screw has excellent self-cleaning ability, reduces material residue degradation, and is very important for maintaining PET color and quality.
5. Selection of matching granulator:
Underwater strip cutting: Wide applicability, precise particle size (such as 3x3mm), mature drying system (centrifugal dehydration+fluidized bed drying). Suitable for the production of high-quality and universal PET particles. The material of the cutting blade and the temperature control of the cooling water in the granulator affect the appearance of the particles and the amount of dust.
Air cooled die surface hot cutting granules: short process, high efficiency, and no wastewater treatment. The particles are close to spherical shape. But the requirements for PET melt strength, viscosity, die design (anti drip material), and cooling air speed/temperature control are extremely high. Particles may require post crystallization treatment. More suitable for scenarios with large quantities and less stringent requirements for particle shape.
Match models to optimize PET production efficiency
High capacity native PET/basic modification: Select models with large diameter (D ≥ 75mm), moderate L/D (40-44), and high specific torque, paired with an efficient water-based strip granulator to ensure high yield, stability, and uniform particle size.
High quality/high filling/fiberglass reinforced PET: High L/D (≥ 48) and high specific torque models are required to provide sufficient mixing, dispersion, and exhaust capabilities, and optimize screw configuration to reduce fiber breakage. The granulator needs to consider enhancing particle hardness to reduce tool wear.
Deep processing of recycled PET (rPET): Twin screw extruder is key. It is necessary to choose a model with high L/D (≥ 48-52), multi-stage high-efficiency exhaust (atmospheric pressure+vacuum), high specific torque, and equipped with strong melt filtration (automatic screen changing) to fully remove impurities, degrade and regulate (SSP or thickening agent), and ensure the purity of the melt. The selection of granulator should consider the influence of impurities on the cutting tools.
Small batch/high flexibility R&D/special grades: Small and medium-sized (D=35-52mm), high L/D (44-52) laboratory or pilot models are more suitable for quick adjustment of processes and formulas.
In the PET pellet production line, twin-screw extruders are not necessarily "bigger is better". Accurately matching its diameter (D), aspect ratio (L/D), specific torque, and screw configuration with the process requirements (production capacity, mixing strength, devolatilization requirements, melt filtration load) of the target product (native/regenerated, basic/modified, filling/enhanced), and synergistically selecting the most suitable granulator type (underwater cutting/air-cooled hot cutting), is the core of maximizing production line efficiency, ensuring PET particle quality (viscosity, color, purity, morphology), and controlling comprehensive costs. A deep understanding of the process behind the model parameters is the only way to achieve efficient and high-quality production of PET.
