Twin-screw extruders fall into two categories depending on whether the two screws are intermeshed: intermeshing twin screw extruder and non-intermeshing twin screw extruder.
In intermeshing twin screw extruder, tightly interlocked screws provide strong mechanical interaction, enabling powerful shear and compression forces. Intermeshing co-rotating twin screw extruder excel in mixing, while counter-rotating twin screw extruder enhance solids conveying and exhaust capabilities. Non-intermeshing extruders have independently rotating screws, resulting in weaker shear and conveying forces.
Twin screw extruders, particularly intermeshing ones, are widely used due to their versatility, efficient mixing, and superior material handling, making them a preferred choice in various applications. In this article, let’s dive into the performance advantages of intermeshing twin-screw extruders to understand why they are so widely used.
Advantages of intermeshing twin screw extruder
Intermeshing twin-screw extruders can be divided into intermeshing co-rotating twin-screw extruders and intermeshing counter-rotating twin-screw extruders according to the direction of screw rotation.
The geometric characteristics of intermeshing counter-rotating twin-screw extruders include screws that intermesh while rotating in opposite directions, with each screw often featuring alternating ribs and grooves.
The geometric feature of the intermeshing co-rotating twin screw extruder is that the two screws are closely meshed, and the width of the screw rib is smaller than the width of the screw groove, and the longitudinal direction must be open. During operation, the two screws rotate in the same direction.
They mainly have the following performance characteristics:
1. Excellent feeding performance
Intermeshing twin-screw extruders are versatile machines designed to accommodate various feeding methods, allowing flexibility in processing different materials. Due to its positive displacement conveying capability, the added materials can be granular, powdery, pasty, high molecular weight or high viscosity, or irregularly shaped flakes, glass fibers, etc. At the same time, the rotation speed and feeding amount can be controlled separately, and multi-channel feeding can be carried out according to process requirements.
In terms of the following feeding methods, intermeshing twin-screw extruders have more advantages than other ordinary twin-screw extruders:
- Force Feeding: Force feeding involves using external devices such as screws, rollers, or other mechanisms to apply force to the material, ensuring a consistent and controlled flow into the extruder. This method is particularly useful for materials with poor flow properties.
- Liquid Injection: Some intermeshing twin-screw extruders are equipped to accept liquid injection feeding methods. Liquids, such as additives or reactive components, can be injected directly into the extruder to achieve precise mixing and distribution.
- Metered Feeding: Metered feeding involves controlling the feed rate based on specific parameters, such as the extruder’s screw speed or other processing conditions. This method allows for precise adjustment of the material flow.
Intermeshing co-rotating twin-screw extruders are often favored for their efficient self-wiping action, strong conveying capacity and effective mixing capabilities. Its tightly meshed screw contributes to a cleaner extrusion process. However, intermeshing counter-rotating twin-screw extruders excel in solids delivery and venting, making them better than intermeshing counter-rotating twin-screw extruders in processing materials with these concerns.
2. Excellent mixing performance
The material flows in a spiral shape “∞”, and it has no calendering effect. It can operate at high speed and produce a high shear rate, giving it good distribution and mixing capabilities. At the same time, kneading discs are widely used in intermeshing co-rotating twin-screw extruders, where materials are sheared, diverted and refluxed to improve distribution and dispersion mixing capabilities.
While intermeshing counter-rotating extruders can achieve good mixing, their mixing efficiency may not be as high as co-rotating designs. The opposing rotation creates a shearing effect, contributing to mixing, but the degree of intermeshing may be lower.
3. Excellent self-cleaning performance
Since the two screws of the intermeshing co-rotating twin-screw extruder move in opposite directions in the meshing area, they scrape each other and clean the meshing position, thereby preventing the material from being retained and adhered to the screw surface, and preventing degradation caused by excessive residence time; This superior self-cleaning property also continuously provides a fresh surface for mixing and exhausting. Compared to co-rotating designs, intermeshing counter-rotating extruders may result in less self-wiping and self-cleaning properties may be less pronounced.
4. Excellent exhaust performance
In the intermeshing co-rotating twin-screw extruder, the self-cleaning function continuously updates the material on the inner surface of the twin-screw, improves the exhaust efficiency, and is conducive to heat transfer. And due to the longitudinal opening, the actual exhaust space is much larger than the exhaust port, and a variety of exhaust methods can be used. But compared with counter-rotating twin-screw extruders, it may have limitations in venting performance, especially for materials with relatively high gas content.
In an intermeshing counter-rotating twin-screw extruder, the screws rotate in opposite directions to enhance the exhaust capacity. Because when the screws rotate in opposite directions, a shearing effect will occur between them. This shearing effect is conducive to better exhaust or degassing of the material, and the opposite rotation enhances the escape of gas from the material being processed. This makes intermeshing counter-rotating twin-screw extruders more suitable for materials with high gas or volatile content.
5. The residence time and its distribution are controllable
Both intermeshing co-rotating and counter-rotating twin-screw extruders provide some degree of control over residence time and distribution. By changing screw combination, screw speed and other factors, the residence time of materials in the screw can be changed from seconds to tens of minutes to meet different processing requirements.
6. Large production capacity and low specific energy consumption
The table below compares the performance of several common extruders. Each of these twin-screw extruders has its own characteristics.
Main performance comparisons of three twin-screw extruders and single-screw extruders
|Non-intermeshing counter-rotating twin screws
|Intermeshing counter-rotating twin screws
|Intermeshing co-rotating twin screws
|Positive displacement conveying capacity
|Distributed mixing performance
|Dispersion and mixing performance
However, the intermeshing co-rotating twin-screw extruder has flexible building block combination design, excellent dispersion and distribution mixing capabilities, excellent self-cleaning effect, narrow residence time distribution, good exhaust performance and higher production. It has become the first choice equipment for reactive extrusion and has been widely used.
Applications of intermeshing twin screw extruder
Because of the unique performance of the intermeshing co-rotating twin-screw extruder, its applications are mainly focused on two aspects:
Applications in polymer compounding include blend modification, filling, fiber reinforcement and alloying. By mixing with various additives or two or more polymers, physical and chemical modifications can be achieved to achieve high performance and alloying to obtain new polymers.
Applications in reactive extrusion include monomer polymerization and reactive modification of polymers. Reactive extrusion is a new type of polymer reactive processing technology that uses a twin-screw extruder as a continuous processing reactor to perform monomer polymerization or polymer modification and extrusion molding.
Intermeshing twin screw extruder’s advantages in reactive extrusion
Compared with traditional processing, reactive extrusion technology integrates the continuous synthesis reaction of monomer raw materials and the melt processing of polymers, achieving the integration of monomers-polymers-products in a short screw extrusion residence time. The chemical synthesis process can produce high-performance products that cannot be matched by traditional processes and reduce the processing cycle.
Currently, a variety of reactive extrusions can be completed on intermeshing co-rotating twin-screw extruders, such as ionic polymerization of polystyrene, grafting reaction of polyethylene and organosilane, etc. Therefore it has become a technological hotspot for polymer synthesis and processing.