Double screw extruderStructure and type of
The extrusion system, transmission system, and heating and cooling system of a twin-screw extruder are functionally identical to those of a single screw extruder, but their specific structures are specifically designed based on the actual needs of twin-screw extruders.
Twin screw extruders can be classified into meshing and non meshing types based on the relative position of the two screws. Mesh type twin-screw
It can be divided into partial meshing and full meshing types according to the degree of meshing. As shown in Figure 3-11. Center distance of fully meshed twin-screw
A =r +R ， In the formula, r is the root radius of the screw, and R is the top radius of the screw. The center distance of partially meshed twin-screw A>r
+R. The center distance A of non meshing twin-screw is ≥ 2R.
According to the different situations of screw edge and pitch, the groove in the meshing area can be open or closed. The so-called openness refers to things
The material can pass through a certain location; Closure refers to the inability of materials to pass through. The situation of opening and closing is divided into longitudinal opening or closing, transverse opening or closing according to the spiral groove and transverse spiral groove.
Double screw extruderThe working principle of
Twin screw extruders, like single screw extruders, are responsible for conveying, plasticizing, mixing, and kneading polymers,
But there are many differences in working principle compared to a single screw.
Double screw extruderThe principle of conveying materials is different from single screw extruders and is not equivalent to plugs or teeth
Positive displacement conveying of wheel pump. The so-called positive displacement conveying refers to the conveying method in which a moving external surface replaces a portion of the liquid in the system. The conveying mechanism of twin-screw is related to its meshing, longitudinal and transverse opening, degree of opening, and rotation direction of the screw.
Non meshing twin-screw extruder has no positive displacement conveyor due to the inability of two screws to form a closed or semi closed chamber
The conveying mechanism of the material is similar to that of a single screw extruder, and the friction coefficient between the material and the barrel or screw, as well as the viscous force of the material, are the main factors controlling the conveying volume.
Meshing typeDouble screw extruderDepending on the specific structure, it exhibits varying degrees of positive displacement conveying characteristics.
Mesh counter rotating twin-screw extruder
The better the longitudinal and transverse closure of the spiral groove, or the better the conjugation, the stronger the positive displacement conveying characteristics. Fully engaged, complete in both longitudinal and transverse directions of the screw groove
Only fully enclosed can achieve positive displacement conveying of *. If there is a certain degree of openness in the longitudinal or transverse direction of the screw groove, it will be lost
Loss of certain positive displacement conveying capacity, because under the action of pressure gradient, materials will flow through these open channels, which will result in
Leakage flow. The loss of positive displacement conveying capacity is replaced by an improvement in mixing capacity or other performance (such as exhaust).
For a rectangular spiral groove with conjugate longitudinal and transverse closure, the material is enclosed by a lead spiral groove and the inner wall of the barrel
The formed shape is shown in Figure 3-14 (2) as a "C" - shaped space, which is called a C-shaped chamber. Along the screw and machine
The cylinder axis has two series of C-shaped chambers that are not connected to each other. When the screw rotates once, the material in the C-shaped chamber moves forward
One lead, therefore, its speed of movement along the screw axis is:
v Ⅰ=t·n (3 -15)
In the formula, vL represents the axial movement speed of the material;
T - Thread lead;
N - The rotational speed of the screw.