Many people are confused about the type of twin screw extruder. Today’s article introduces three common twin-screw extruders, hoping to help readers better understand the relevant industry knowledge.
Intermeshing co-rotating twin screw extruder
There are two kinds of Intermeshing corotating twin screw extruders include low speed and high speed. The former is mainly used for profile extrusion, while the latter is used for special polymer processing operations.
(1) tight meshing extruder
The low-speed extruder has a closely meshing screw geometry, in which the screw edge shape of one screw is closely matched with that of the other screw, that is, the conjugate screw shape.
(2) self-cleaning extruder
The high-speed co-directional extruder has a closely matched screw shape. This kind of screw can be designed to have a relatively small screw gap so that the screw has a closed self-cleaning effect, this kind of twin-screw extruder is called a tight self-cleaning co-rotating twin-screw extruder.
Structurally, the screw groove of the intermeshing co-rotating twin screw is a non-closed structure, which is conducive to the flow and mixing of the material. after the material is dragged from the feeding port to a screw, the material moves along the screw groove of the screw to the lower narrow gap under friction dragging.
After being compressed to a certain extent, the other screw is lifted and transported along the other screw groove under the friction drag on the surface of the barrel, and this process is repeated when the material advances to the upper narrow gap. It only moves a certain distance in the direction of the axis, so the material flows around the two screws in a spiral “∞” shape.
Intermeshing counter-rotating extruder
The intermeshing counter-rotating twin-screw extruder generally uses two screws of the same size but in opposite thread directions.
In order to strengthen the mixing effect, different screw numbers, variable pitches and mixing chambers are adopted on the screw.
The mixing chamber formed by the partial disconnection of the thread makes the sequentially advancing materials gather for a time in the conveying process, resulting in a complex mixed flow, which can effectively enhance the material mixing.
The gap between the two screw grooves of the closely meshing counter-rotating twin-screw extruder is very small, so the positive conveying characteristic can be achieved.
Intermeshing counter-rotating twin-screw granulator extruder is widely used in extrusion moulding and granulation. Intermeshing counter-rotating twin-screw extruder for PVC profile extrusion. The shear rate is limited to [γ] = 6~8S-1. The maximum linear speed V=7~8m/min of the screw is used in PVC pipe extrusion, so the maximum linear speed V=10~12m/min of the screw is mostly operated at a lower rotational speed.
Non-intermeshing counter-rotating twin screw extruder
The center distance between the two screws of the non-meshing twin screw extruder is greater than the sum of the radii of the two screws.
For the reverse rotary non-meshing twin screw extruder, for two parallel screws, at the small gap between the screw edges, the material has a larger pressure gradient and velocity gradient, so the material in the screw edge area is easier to mix, while the material in the screw groove area is difficult to mix.
In order to improve the mixing degree of the material, to improve the uniformity of the mixture and the adequacy of the chemical reaction, it is necessary to improve the mixing performance of the screw groove area.
The structure of the non-intermeshing counter-rotating twin screw extruder is similar to the single screw extruder’s but compared with the single screw structure, the material exchange between the screws is increased at the cost of reducing the displacement conveying characteristics.