What are the segments of the screw?

When the material moves forward along the screw, it experiences changes in temperature, pressure, viscosity, etc. This change is different throughout the entire length of the screw. According to the changing characteristics of the material, the screw can be divided into a feeding (feeding) section, a compression section, and a compression section. section and homogenization section.
①. Plastics and three states of plastics
There are two categories of plastics: thermoset and thermoplastic. After thermoset plastic is molded and solidified, it cannot be heated and melted for molding. Products formed from thermoplastic plastics can be reheated and melted to form other products.
As the temperature changes, thermoplastic plastics produce three-state changes: glassy state, highly elastic state and viscous flow state. As the temperature changes repeatedly, the three states change repeatedly.
a. Different characteristics of polymer melt in three states:
Glassy state - the plastic appears as a rigid solid; the thermal kinetic energy is small, the intermolecular force is large, and the deformation is mainly contributed by bond angle deformation; the deformation recovers instantaneously after the external force is removed, which is a general elastic deformation.
Highly elastic state - the plastic appears as a rubber-like substance; the deformation is contributed by the conformational stretching of macromolecules caused by chain segment orientation, and the deformation value is large; the deformation can be restored after external force is removed but is time-dependent, which is a highly elastic deformation.
Viscous flow state - plastic appears as a highly viscous melt; thermal energy further intensifies the relative slip motion of chain molecules; the deformation is irreversible and belongs to plastic deformation
b. Plastic processing and three states of plastic:
Plastics can be machined when in the glassy state. In the high elastic state, it can be stretched and processed, such as wire drawing, extrusion, blow molding and thermoforming. In the viscous flow state, it can be processed by coating, rotational molding and injection molding.
When the temperature is higher than the viscous flow state, the plastic will thermally decompose, and when the temperature is lower than the glassy state, the plastic will become brittle. When the plastic temperature is higher than the viscous flow state or lower than the glassy state, the thermoplastic plastic tends to be seriously deteriorated and destroyed, so these two temperature areas should be avoided when processing or using plastic products.
②. Three-stage screw
There are three physical states of plastic in the extruder - glassy state, highly elastic state and viscous flow state. Each state has different requirements for the screw structure.
c. In order to adapt to the requirements of different states, the screw of the extruder is usually divided into three sections:
Feeding section L1 (also called solid conveying section)
Melting section L2 (called compression section)
Homogenization section L3 (called metering section)
This is commonly known as a three-stage screw. The extrusion process of plastic in these three stages is different.
The function of the feeding section is to send the material supplied from the hopper to the compression section. The plastic generally remains in a solid state during the movement and is partially melted due to heat. The length of the feeding section varies with the type of plastic, and can start not far from the hopper and end at 75% of the total length of the screw cup.
Generally speaking, extruded crystalline polymers are the longest, followed by hard amorphous polymers, and soft amorphous polymers are the shortest. Since the feeding section does not necessarily produce compression, the volume of the screw groove can remain unchanged. The size of the helix angle has a greater impact on the feeding capacity of this section, which actually affects the productivity of the extruder. Usually, the spiral angle of powdery materials is about 30 degrees, which has the highest productivity. The spiral angle of square materials should be about 15 degrees, and the spiral angle of spherical materials should be about 17 degrees.
Main parameters of the screw in the feeding section:
The helix angle ψ is generally 17°~20°.
The screw groove depth H1 is calculated based on the geometric compression ratio ε of the screw after determining the screw groove depth in the homogenization section.
The length L1 of the feeding section is determined by the empirical formula:
For amorphous polymer L1=(10%～20%)L
For crystalline polymer L1=(60%～65%)L
The function of the compression section (migration section) is to compact the material, convert the material from solid to molten, and eliminate the air in the material; in order to adapt to pushing the gas in the material back to the feeding section, compacting the material and reducing the volume of the material when it melts. Due to its small size, this section of the screw should produce greater shearing and compression of the plastic. For this reason, the volume of the screw channel is usually gradually reduced, and the degree of reduction is determined by the compression rate of the plastic (specific gravity of the product/apparent specific gravity of the plastic). The compression ratio is not only related to the compression rate of the plastic, but also to the shape of the plastic. The powder has a small specific gravity and a lot of entrained air, so it requires a larger compression ratio (up to 4~5), while the pellets are only 2.5~3.
The length of the compression section is mainly related to the melting point and other properties of the plastic. Plastics with a wide melting temperature range, such as polyvinyl chloride, which starts to melt above 150℃, have the longest compression section, reaching 100% of the total screw length (gradient type), and polyethylene with a narrow melting temperature range (low density polyethylene 105~120℃ , high-density polyethylene (125~135℃), etc., the compression section is 45~50% of the total length of the screw; for most polymers with a very narrow melting temperature range, such as polyamide, the compression section is even only one pitch in length.
Main parameters of the melting section screw:
Compression ratio ε: generally refers to the geometric compression ratio, which is the ratio of the volume of the first groove in the screw feeding section to the volume of the last groove in the homogenization section.
ε=(Ds-H1)H1/(Ds-H3)≈H1/H3
In the formula, H1 - the depth of the first screw groove in the feeding section
H3--Depth of the last groove in the homogenization section
The melting section length L2 is determined by the empirical formula:
For amorphous polymer L2=55%~65%L
For crystalline polymers L2=(1～4)Ds
The function of the homogenization section (measuring section) is to feed the molten material into the machine head at a constant volume (quantitative amount) and constant pressure to form it in the die. The channel volume of the homogenizing section is as constant as that of the feeding section. In order to prevent materials from being retained in the dead corner of the screw head and causing decomposition, the screw head is often designed into a cone or semicircle; the homogenizing section of some screws is a rod with a completely smooth surface called a torpedo head, but there are also engraved ones. Grooved or milled into patterns. The torpedo head has the function of stirring and controlling the material, eliminating the pulsation phenomenon during flow, and increasing the pressure of the material, reducing the thickness of the material layer, improving the heating condition, and further improving the plasticizing efficiency of the screw. This section can be 20 to 25% of the total length of the screw.
Important parameters of the screw in the homogenization section:
The screw groove depth H3 is determined by the empirical formula H3=(0.02～0.06)Ds
The length L3 is determined by the following formula L3=(20%～25%)L
d. According to the melt transport theory, there are four forms of melt flow in the screw homogenization section. The flow of molten material in the screw groove is a combination of these four flows:
Forward flow - the flow of plastic melt between the barrel and screw along the groove direction toward the machine head.
Counter flow - the flow direction is opposite to the forward flow, caused by the pressure gradient caused by the resistance of the machine head, porous plate, filter plate, etc.
Cross flow - the flow of melt along the direction perpendicular to the thread wall, affecting the mixing and heat exchange of the melt during the extrusion process.
Leakage flow - the backflow formed in the gap between the screw and barrel due to the pressure gradient, along the axial direction of the screw.
2. The structure of ordinary screw
Conventional fully threaded three-section screws can be divided into three forms according to changes in thread lift and groove depth:
(1) Equally spaced depth screw
The speed of the equidistant depth-changing screw from the groove depth can be divided into two forms:
① Equidistant gradient screw: A screw whose depth gradually becomes shallower from the feeding section to the last screw groove in the homogenizing section. Over the longer melting section, the groove depth gradually becomes shallower.
② Isometric mutation screw: that is, the screw groove depth in the feeding section and homogenization section remains unchanged, but the groove depth in the melting section suddenly becomes shallower
(2) Constant depth variable pitch screw
The constant depth variable pitch screw means that the depth of the screw groove remains unchanged, and the screw pitch gradually becomes narrower from the first screw groove in the feeding section to the end of the homogenization section.
The characteristic of the constant-depth variable pitch screw is that due to the constant depth of the screw groove, the cross-sectional area of the screw at the feeding port is larger and has sufficient strength, which is conducive to increasing the rotation speed, thereby improving productivity. However, screw processing is difficult, the flow of melt backflow is large, and the homogenization effect is poor, so it is rarely used.
(3) Variable depth and variable pitch screw
The variable depth and variable pitch screw refers to a screw whose groove depth and thread lift angle gradually change from the beginning of the feeding section to the end of the homogenization, that is, the thread lift gradually becomes narrower from wider, and the screw groove depth gradually becomes shallower from deeper. This screw has the characteristics of the previous two screws, but it is difficult to machine and is rarely used.
3.Screw material
The screw is a key component of the extruder. The screw material must have the characteristics of high temperature resistance, wear resistance, corrosion resistance, high strength, etc. It should also have good cutting performance, small residual stress after heat treatment, and small thermal deformation.
For the material of the extruder screw, there are the following specific requirements:
①High mechanical properties. It must have sufficient strength to adapt to high temperature and high pressure working conditions and increase the service life of the screw.
② Good mechanical processing performance. It must have better cutting performance and heat treatment performance.
③Good corrosion resistance and anti-wear properties.
④ Easy to obtain materials.