Main technical parameters of screw components for twin-screw extruder parts
(a) Screw element diameter
The size of the screw diameter reflects to a large extent the standard size of the extruder and the size of the production ability. The other series of parameters are also related to the screw diameter. For example, the screw speed must be determined, and the peripheral speed of the screw is determined. Together, the shear rate accepted by the material in the groove and the gap between the outer diameter of the screw and the inner wall of the barrel are determined. The shear rate accepted by the material; when the head pressure is constant, it basically determines the axial thrust of the screw, thus also determining the standard and quantity of the thrust bearing.
In general, the screw diameter required depends on how much extrusion is planned.
(b) Center distance
Although the user does not have to know, the center distance between the two screws of the twin-screw extruder is an important technical parameter. This is because it has decided on the following links and has therefore decided on the relevant planning parameters:
(1) The output shaft of the transmission system (connected to the screw) and the dimensions of the gears, thus determining the size of the useful torque that the screw can accept.
(2) The outer diameter and the root diameter of the screw element, thus determining the degree of meshing of the two screws, the depth of the groove and the larger theoretical extrusion amount of the twin-screw extruder.
(3) The size of the outer diameter of the thrust bearing is determined, so the larger head pressure should be determined.
Here, it should be pointed out that the twin screw extruder center distance between the two screws, the outer diameter and root diameter of the screw, the depth of the groove, the large theoretical load, and the large torque that the screw can accept are a kind of mutual influence and mutual restriction. . Because it is closely related to the development direction of the intermeshing twin-screw extruder and the planning goal to be reached.
(c) aspect ratio
It reflects the standards and performance of twin-screw extruders to a certain extent. Because twin-screw extruders are primarily used to modify polymers, sometimes requiring several feed ports and several vents, sometimes requiring longer material residence times, thus requiring longer screws. The aspect ratio is large, and it is easy to establish the required pressure distribution along the path. At the same screw speed, in addition to lengthening the residence time of the material, the quality of the plasticizing, mixing or reverberation process can be greatly improved.
(d) Screw element speed scale
This is a very important parameter. It reflects the extrusion ability and mixing ability of the twin-screw extruder to a certain extent. It depends on the requirements for the amount of extrusion, the nature of the material being processed and the mission of the extrusion process.
Intermeshing co-rotating twin-screw extruders are primarily used for compounding, requiring high shear rates and shear stresses, thus requiring higher screw speeds. In addition, at high rotational speeds, the specific energy consumption of the extrusion process is lower than at low screw speeds.
(e) Extrusion amount
The twin-screw extruder generally uses metering (starvation) feeding, so the amount of extrusion depends on the amount of feeding, and there is no connection with the screw speed. However, if the screw speed is high, the metering amount can be increased much.
The amount of extrusion has a different concept. If the head is not installed, it is a larger theoretical extrusion.
(f) Large head pressure
During the extrusion process, the melt at the end of the screw establishes a pressure that will generate an axial thrust to the screw. A change in the pressure gradient formed by the pressure spread along the path will also produce an axial thrust to the screw. It acts on the thrust bearing of the transmission system. Because the pressure of the twin-screw extruder can be relatively low and the axial thrust is relatively small, the number of bearings for the thrust bearing string is small. The larger head working pressure of a twin-screw extruder is generally less than 20 MPa.