Screw
is the most important part of extruder, can be said to be the heart of the
extruder, it can directly affect the application of the extruder and production
efficiency. The performance of screw determines the productivity, plasticizing
quality, dispersion of filler, melt temperature and power consumption of an
extruder. Through the screw rotation, the polymer plastic can be extruded, and
the plastic can move, pressurize and obtain some heat from friction in the
cylinder. During the movement of the cylinder, the plastic can be mixed and
plasticized.
1.
The
screw structure of polymer plasticizing and mixing extruder.
During processing,
when the material moves forward along the screw, it experiences changes in
temperature, pressure, viscosity, etc., which are different within the full
length range of the screw. There are three physical states of polymer plastic in extruder: glass state,
high elastic state and viscous flow state. In order to adapt to the
requirements of different states and according to the changing characteristics
of the material, the screw can be divided into three sections: feed section,
compress section and homogenize section.
The function of the feeding section is
to feed the material supplied by the hopper to the compression section. During
the movement of the plastic, it generally remains a solid state and partially
melts due to heat. The length of the feeding section varies with the type of
plastic. Compression section (transfer section) is the role of physical
material pressure, so that the material from solid into molten, well out of the
air in the material; In order to adapt to the characteristics of reducing the
volume when pushing the gas in the material back to the feeding section,
pressing the material and melting the material, the screw in this section
should produce greater shearing effect and compression of the plastic.
The
function of the homogenizing section (metering section) is to feed the molten
material to the machine head at constant volume (volume) and pressure so that
it is formed in the mouth mold. The volume of the spiral groove in the
homogenizing section is the same as that in the feeding section.
In order to avoid
material retention in the end of the screw head dead corner, causing decomposition,
screw head is often designed into a cone or semicircle; Some screw
homogenization section is - the surface is completely smooth body called
torpedo head, but there are also engraved grooves or milling patterns. The
torpedo head has the function of stirring and controlling the material,
eliminating pulsating (pulsating) phenomenon in the flow, reducing the
thickness of the material layer with increasing the pressure of the material,
improving the heating condition, and further improving the screw plasticizing
efficiency.
According to melt transport theory, melt flows in the screw
homogenization section in four forms, and the flow of molten material in the
screw groove is a combination of these four flows: positive flow -- plastic
melt flows between the cylinder and the screw in the direction of the screw
groove toward the machine head. Counter-current flow direction is opposite to
the positive flow, which is caused by the pressure gradient caused by the
resistance of the nose, porous plate and filter plate. The flow of the melt in
a direction perpendicular to the thread wall affects the mixing and heat
exchange of the melt during extrusion. Leakage flow - a backflow formed by the
pressure gradient between the screw and the cylinder, along the axial direction
of the screw. Different flow patterns have important effects on the mixing
uniformity of polymers.
Screw diameter and
the determination of structural form, mainly according to the production of
product output, specifications, types of processed materials and various
structural screw characteristics to determine. Generally, the feeding section
of the screw has a deeper groove and the groove depth is unchanged, compression
section (melt section) of the screw groove depth along the direction of
discharge from the deep to shallow, metering section (homogenized section) of
the screw has a shallow groove and the groove depth is unchanged.
2.
The
screw material.
Screw is the key
part of extruder, as the material of screw must have high temperature resistance,
wear resistance, corrosion resistance, high strength and other characteristics,
at the same time should have good cutting performance, heat treatment residual
stress, small thermal deformation and other characteristics. For extruder screw
material, there are specific requirements as follows:
(1)
High
mechanical properties. To have enough strength, to adapt to high temperature,
high pressure working conditions, improve the service life of the screw.
(2)
Good
machining performance. Good machining performance and heat treatment
performance.
(3)
Good
corrosion and wear resistance.
(4)
Easy
to draw.
