The main differences and features of single screw and parallel twin screw for PET granulation
In the PET recycling process, it is very important to choose the right type of extruder when granulating PET bottle flakes for re-production. Single screw extruder and twin screw extruder are two common granulation equipment, each with its own advantages and disadvantages, suitable for different application scenarios. The following is a detailed analysis of the two extruders and how to choose the right equipment to produce high-quality PET pellets.
I. Comparison between single screw extruder and twin screw extruder
1. Single screw extruder
(1) Working principle:
The single screw extruder has a simple processing process. The material is transported and plasticized through a single screw, which is suitable for the processing of a variety of plastic materials. It mainly consists of three parts: extrusion system, transmission system and heating and cooling system.
(2) Advantages:
Low equipment cost: The design and manufacturing cost of the single screw extruder is low, which is suitable for recycling plants with limited budgets.
Simple operation: Compared with the twin screw extruder, it is simple to operate and maintain.
Good stability: Due to its simple structure, the single-screw extruder has good stability during production, has little shear degradation effect on polymers, and is suitable for long-term production.
Low energy consumption: The single-screw extruder has low energy consumption and low operating costs.
Mature technology: It is the most mature and widely used type of extruder on the market.
(3) Disadvantages:
Poor mixing effect: Since the mixing of materials in the screw is relatively simple, the single-screw extruder has a weaker ability for some additives or fillers that require precise mixing and uniform dispersion.
Single raw material processing: The material conveying of the single-screw extruder mainly relies on friction, which limits the feeding performance. Powder, paste, glass fiber and inorganic filler are difficult to add.
Poor plasticizing effect: When processing complex materials (such as PET bottle flakes with more impurities), the single-screw extruder has a poor plasticizing effect, which may result in uneven particles or unstable quality.
2. Twin-screw extruder
(1) Working principle:
A twin-screw extruder consists of two screws that rotate against each other, and mix, plasticize and compress simultaneously in the extrusion chamber in a parallel or interlocking manner. The two screws usually rotate in the same direction or in opposite directions, depending on the specific equipment design and processing requirements. It is suitable for processing high-viscosity and complex materials and can provide stronger material handling capabilities.
(2) Advantages:
Good mixing effect: The twin-screw extruder has a high shear force on the material, so it is more efficient in mixing and plasticizing, especially when dealing with mixtures of different types of plastics, impurities or additives, it can ensure better mixing uniformity.
Good plasticizing effect: For high-viscosity materials such as PET, the twin-screw extruder provides better plasticizing effect and ensures the quality of PET particles.
Wide range of application: It can process a wider range of recycled materials, such as high-filling, high-viscosity, heat-sensitive materials, PET bottle flakes with more pollutants, and raw materials with more plastic impurities.
Good self-cleaning performance: The meshing design of the twin screws allows the material to be constantly turned and mixed during the conveying process, reducing the adhesion of the material to the screw surface.
Precise interval pressure and temperature control: The temperature control and pressure control system of the twin screw extruder are more precise, which can optimize the PET recycling process and reduce performance degradation.
(3) Disadvantages:
Higher equipment cost: The equipment cost and maintenance cost of the twin screw extruder are higher than those of the single screw extruder.
High operating cost: Because its production process requires high power, the energy consumption of the twin screw extruder is high.
Complex operation: Because of the more complex structure, the operator needs more operating experience and technical support.
II. Selection of single screw extruder and twin screw extruder
(1) Type and quality of recycled materials:
If the recycled PET flakes are relatively clean, have fewer pollutants, and do not require excessive mixing or plasticization, it is more appropriate to choose a single screw extruder.
If the recycled PET flakes contain more impurities, contain plastic miscellaneous materials (such as PE, PP, PVC, etc.), or need to process mixed recycled materials, it is better to choose a twin screw extruder.
(2) Production scale and equipment budget:
For small-scale production and limited budget, a single-screw extruder is a more economical choice. For recycling manufacturers that need large-scale production, although the cost of a twin-screw extruder is high, it can provide more stable production of high-quality plastic pellets.
(3) Plastic pellet quality requirements:
If the final PET pellets require higher quality, a twin-screw extruder can better meet these requirements. A single-screw extruder may have poor uniformity of pellets due to insufficient plasticization, affecting the final quality.
III. How to configure and produce high-quality PET pellets?
(1) Temperature control
For the recycling process of PET, temperature control is very critical. Too high temperature may cause PET degradation, while too low temperature may cause insufficient plasticization, affecting the quality of the final product.
When using a twin-screw extruder, due to its stronger plasticization ability, temperature control is more precise. The temperature can be precisely adjusted through multiple temperature control sections to ensure the best plasticization effect of PET.
(2) Vacuum degassing system
During the PET granulation process, a vacuum degassing system is usually required to remove moisture and volatiles. This link is crucial to the quality of PET pellets, especially removing moisture to avoid hydrolysis and degradation of PET and ensure the transparency and strength of the pellets.
Twin-screw extruders are usually equipped with a more efficient degassing system to ensure the complete removal of moisture.
(3) Appropriate additives and auxiliaries
In the PET recycling process, antioxidants, plasticizers, UV absorbers and other additives are often required. The mixing effect of the twin-screw extruder can ensure the uniform distribution of these additives and improve the quality of the final pellets.
(4) Granulation and cooling system
There are usually two types of pelletizing: underwater pelletizing and above-water pelletizing. Underwater pelletizing can ensure more uniform pellets and better cooling effect.
The cooling system should be optimized according to the production speed and pellet temperature to ensure rapid cooling of the pellets and avoid deformation during the cooling process.
IV. Summary
Single-screw extruders are suitable for processing relatively clean PET bottle flakes with less pollutants. The equipment cost is low and the operation is simple, but the mixing and plasticizing effects are poor.
Twin-screw extruders are suitable for processing PET bottle flakes containing more impurities. They can provide better mixing, plasticizing and degassing effects and are suitable for the production of high-quality PET granule resins, but the equipment cost is relatively high.
The final choice should be determined based on the characteristics of the recycled material, production requirements, equipment budget and quality requirements.
Kitech Machinery has professional technical team and advanced production technology. We will always be committed to producing cost-effective and high-quality plastic recycling machinery, helping global customers achieve green production. We will continue to contribute to the world's ecological environment and provide customers with higher quality products and services.