What are “co-rotating” and “counter-rotating”?
The two screws in the co-rotating extruder rotate clockwise or counterclockwise, and the threads of the two screws are in the same direction.
In the counter-rotating extruder, the left screw rotates clockwise, the right screw rotates counterclockwise (internal rotation) and vice versa. The threads of the two screws are also different.
The difference between “co-rotating” and “counter-rotating”
In the process of installation, there is no need to distinguish between the same twin screws and the same two screws.
The two screws of the counter-rotating twin-screw extruder are different and cannot be installed incorrectly, otherwise the material may not be added or even the screw will be damaged.
Shear stress is different
In a co-rotating twin screw extruder, during each screw rotation, the polymer is transferred from one screw to another and moves along the “∞” mode.
The polymerization flow is towed, so in this respect it is similar to the flow in a single screw extruder. However, when moving between screws, the polymer passes through a longer path and withstands a higher shear force. There is a reverse rotational motion in the screw gap, so there is a high shear stress in this area.
Therefore, the co-rotating twin screw extruder has a strong ability to disperse and mix plastics, the residence time of the material in the barrel is short, and the self-cleaning ability of the screw barrel can be obtained.
In a counter-rotating twin screw extruder, the polymer basically does not flow from one screw to another as it rotates in the same direction.
There is a co-rotating motion in the screw gap, so it does not produce high shear stress in this area like co-rotating extrusion.
The extruder has the advantages of low rotational speed, low shear calorific value, and the material is not easy to decompose, so it does not require high thermal stability, so it is especially suitable for the forming and processing of heat-sensitive materials (such as PVC).
Velocity distribution has different characteristics
The velocity distributions of co-rotating and counter-rotating twin screw extruder have different characteristics. In the meshing area of the counter-rotating twin screw extruder, the polymer has a strong movement trend in the cross section because the velocity directions of the two screw surfaces are the same. Therefore, the maximum speed in the counter-rotating twin screw extruder is located in the meshing area.
In the co-rotating extruder, the maximum speed is always at the tip of the two screws.
Compared with the reverse rotary extruder, the co-rotating extruder is expected to provide better polymer mixing. Therefore, the popularity of co-rotating extruder in compounding, devolatilization and chemical reaction applications.
The solids transport capacity of counter-rotating is limited
Near the hopper, the solid particles are distributed above the screw and transported freely along the screw.
However, the particles are mainly transported at the bottom of the barrel, and only a small part of them are transported at the upper part of the screw. The particles are collected in the bottom area of the barrel, near the propelling thread of the screw, and the particles are then heated by the barrel and dragged into the gap between the screws. In solid transportation, raw materials are transported forward in the shape of a similar closed “C” chamber. The material output efficiency of the counter-rotating extruder is lower, the pressure of the molten material in the barrel is lower and the pressure stability is poor.
In the co-rotating twin-screw extruder, because the speed direction of the screw edge and the screw groove in the meshing area is opposite, and the relative speed is large, it has a quite high shear speed, can scrape off any accumulated material adhered to the screw, and has a very good self-cleaning effect. as a result, the residence time of the material is very short, and it is not easy to cause local degradation and deterioration.
The application of co-rotating twin-screw extruder is different from that of counter-rotating twin-screw extruder
The counter-rotating extruder can better feed polymers into the extruder, especially in the case of polymers in the form of powder or polymers showing sliding properties, and provide particles in the extruder with shorter and less varying polymer residence time. Counter-rotating twin-screw extruders similar to gear pumps provide maximum positive displacement, so they are the preferred machines for profile extrusion, mainly from thermosensitive materials. And because its reducer bearing has larger installation space and can bear the greater axial force of screw work, it is widely used in the extrusion of rigid PVC products, but the manufacture of this kind of extruder is more difficult, so now when the conical diameter is larger than 80mm, parallel twin-screw extruder will be used instead of production.
The co-rotating twin screw extruder has the advantages of high conveying efficiency, strong ability of dispersion and mixing, good self-cleaning performance, uniform residence time distribution of materials in the machine and good adaptability.
It is more suitable for other applications, such as compounding, mixing, devolatilization and chemical reactions.
For example, blending modification between different plastics, between plastics and rubber, blending of various additives with plastics, glass fiber, carbon fiber reinforced plastics, continuous mixing of polymer modification and so on.