The screw is the heart of the extruder and is a key component of the extruder. The performance of the screw determines the productivity, plasticizing quality, dispersion of the filler, melt temperature, power consumption, etc. of the extruder. . It is the most important part of the extruder, which can directly affect the application range and production efficiency of the extruder. Through the rotation of the screw, the extreme pressure of the plastic is generated. The plastic can move, pressurize and obtain part of the heat from the friction in the cylinder. The plastic is mixed and plasticized during the movement of the cylinder, and the viscous flow state The melt is shaped while being extruded to flow through the die to obtain the desired shape. Like the barrel, the screw is also made of a high strength, heat resistant and corrosion resistant alloy.

Due to the variety of plastics, their properties are also different. Therefore, in actual operation, in order to adapt to different plastic processing needs, the types of screws required are different, and the structures are also different. In order to maximize the efficiency of the plastics to maximize transportation, extrusion, mixing and plasticization. The picture shows several more common screws.

The basic parameters indicating the characteristics of the screw include the following: diameter, aspect ratio, compression ratio, pitch, groove depth, helix angle, screw and barrel clearance, and the like.

The most common screw diameter D is approximately 45 to 150 mm. As the screw diameter increases, the processing capacity of the extruder increases accordingly, and the productivity of the extruder is proportional to the square of the screw diameter D. The ratio of the effective length to the diameter of the working part of the screw (referred to as the aspect ratio, expressed as L/D) is usually 18~25. Large L/D can improve the material temperature distribution, facilitate the mixing and plasticization of plastics, and reduce leakage and backflow. Improve the production capacity of the extruder, the L/D large screw has strong adaptability and can be used for extrusion of various plastics; however, when the L/D is too large, the plastics will be degraded by the heating time and the screw The increase in self-weight, the free end deflection drooping, easy to cause material and screw scratches, and manufacturing and processing difficulties; increase the power consumption of the extruder.

A screw that is too short may cause poor plasticization of the kneading.

The half of the difference between the inner diameter of the barrel and the diameter of the screw is called the gap δ, which can affect the productivity of the extruder. As the δ increases, the productivity decreases. Usually, the control δ is preferably about 0.1 to 0.6 mm. δ is small, the material is subjected to a large shearing effect, which is beneficial to plasticization, but δ is too small, strong shearing action easily causes thermal mechanical degradation of the material, and at the same time, the screw is easily hugged or rubbed against the wall of the cylinder, and When δ is too small, there is almost no leakage or backflow of the material, which affects the mixing of the melt to some extent.

The helix angle Φ is the angle between the thread and the cross section of the screw. As the Φ increases, the productivity of the extruder increases, but the shearing effect and the pressing force on the plastic are reduced. Usually, the helix angle is between 10° and 30 degrees. Between °, along the direction of the change of the length of the screw, often using an equidistant screw, taking the pitch equal to the diameter, the value of Φ is about 17 ° 41 '

The greater the compression ratio, the greater the compression ratio that the plastic receives. When the groove is shallow, it can produce a higher shear rate for the plastic, which is beneficial to the heat transfer between the wall of the barrel and the material. The higher the material mixing and plasticizing efficiency, the lower the productivity; on the contrary, the groove is deep. The situation is just the opposite. Therefore, heat-sensitive materials (such as polyvinyl chloride) should be used with deep screw screws; for plastics with low melt viscosity and high thermal stability (such as polyamide), shallow screw screws should be used.

1. Segmentation of the screw

When the material moves forward along the screw, it undergoes changes in temperature, pressure, viscosity, etc., and this change is different within the full length of the screw. According to the changing characteristics of the material, the screw can be divided into adding (sending) material segments and compressing. Segment and homogenization segment.

1. Three states of plastics and plastics

Plastics have two types of thermosetting and thermoplastic. After thermosetting plastics are molded and cured, they cannot be heated and melted. The thermoplastic molded article can be reheated and melted into other articles.

As the temperature changes, the thermoplastic produces a three-state change of glass state, high elastic state and viscous flow state. Repeated changes with temperature, the three states produce repeated changes.

a. Different characteristics of the polymer melt in the tri-state:

Glass state - plastic is a rigid solid; thermal motion energy is small, intermolecular force is large, deformation is mainly contributed by the deformation of the bond angle; after the external force is removed, the deformation is instantaneously restored, belonging to the general deformation.

Highly elastic state - plastic is a rubber-like substance; deformation is contributed by the orientation of the macromolecular stretch caused by the orientation of the segment, and the deformation value is large; after the external force is removed, the deformation can be recovered but time-dependent, which is a high elastic deformation.

Viscous flow state - plastic appears as a highly viscous melt; thermal energy further intensifies the relative slip motion of chain molecules; deformation is irreversible and belongs to plastic deformation

b. Plastic processing and plastic tri-state:

Machinable in plastic glass. Stretchable in high elastic state, such as wire drawing, pipe extrusion, blow molding and thermoforming. In the viscous flow state, it can be coated, rotomolded and injection molded.

When the temperature is higher than the viscous flow state, the plastic will thermally decompose, and when the temperature is lower than the glass state, the plastic will be embrittled. When the plastic temperature is higher than the viscous flow state or lower than the glass state, the thermoplastic tends to be severely deteriorated and destroyed, so the two temperature zones should be avoided when processing or using the plastic product.

2, three-stage screw

There are three physical states of plastic in the extruder - the change of the glass state, the high elastic state and the viscous flow state, and each state requires different screw structures.

c. In order to meet the requirements of different states, the screw of the extruder is usually divided into three sections:

Feeding section L1 (also known as solid conveying section)

Melting section L2 (called compression section)

Homogenization section L3 (weighing section)

This is what is commonly referred to as a three-stage screw. The extrusion process of plastic in these three sections is different.

The function of the feeding section is to feed the material supplied by the hopper to the compression section, and 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, from not far from the hopper to 75% of the total length of the screw cup.

In general, the extruded crystalline polymer is the longest, the hard amorphous polymer is the second, and the soft amorphous polymer is the shortest. Since the feeding section does not have to produce compression, the volume of the groove can be kept constant, and the size of the helix angle has a great influence on the delivery capacity of the section, which actually affects the productivity of the extruder. Generally, the spiral angle of the powdery material is about 30 degrees, and the productivity is the highest. The spiral angle of the square material should be about 15 degrees, and the spherical material should be selected to be about 17 degrees.

The main parameters of the feeding section screw:

The helix angle is generally 17 ° ~ 20 °.

The groove depth H1 is calculated by determining the geometrical compression ratio ε of the screw after determining the depth of the groove of the homogenization section.

The length of the feed section L1 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, so that the material is converted from solid to melt, and the air in the material is excluded; in order to adapt to push the gas in the material back to the feeding section, the compacted material and the material are reduced in volume. Small features, this section of the screw should produce greater shear and compression of the plastic. For this reason, the volume of the groove is usually gradually reduced, and the degree of reduction is determined by the compression ratio of the plastic (the specific gravity of the product / the apparent specific gravity of the plastic). In addition to the compression ratio of plastics, the compression ratio is also related to the shape of the plastic. The specific gravity of the powder is small, and the entrained air is much larger, requiring a larger compression ratio (up to 4 to 5), while the pellets are only 2.5 to 3.

The length of the compression section is mainly related to properties such as the melting point of the plastic. Plastics with a wide melting temperature range, such as polyvinyl chloride starting to melt above 150 °C, the longest compression section, up to 100% of the full length of the screw (gradient type), and a narrow range of melting temperature (low density polyethylene 105~120 °C) , high-density polyethylene 125~135 ° C), etc., the compression section is 45~50% of the full length of the screw; most polymers such as polyamide with a narrow melting temperature range, the compression section even has only one pitch length.

The main parameters of the melting section screw:

The compression ratio ε: generally refers to the geometric compression ratio, which is the ratio of the first groove volume of the screw feed section to the last groove volume of the homogenization section.

ε=(Ds-H1)H1/(Ds-H3)≈H1/H3

Where H1 is the depth of the first groove of the feed section

H3 - the depth of the last groove of the homogenization section

The length of the melt section L2 is determined by an empirical formula:

For amorphous polymer L2 = 55% ~ 65% L

For crystalline polymer L2 = (1 ~ 4) Ds

The function of the homogenization section (metering section) is to feed the molten material into a machine head at a constant volume (quantitative pressure) to form it in the die. The volume of the groove of the homogenization section is the same as that of the feed section. In order to prevent the material from being decomposed due to being stuck at the dead end of the screw head end face, the screw head is often designed to be tapered or semi-circular; some of the studded sweat homogenization section is a completely smooth surface called a torpedo head, but also engraved Upper groove or milled into a pattern.

The torpedo head has the function of stirring and controlling the material, eliminating the phenomenon of pulsation (pulsation) during the flow, and increasing the thickness of the material layer, improving the heating condition, and further improving the plasticizing efficiency of the screw with increasing pressure of the material. This paragraph can be 20 to 25% of the total length of the screw.
Important parameters of the homogenizing section screw:

The 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% to 25%) L

d. According to the melt transport theory, there are four forms of melt flow in the screw homogenization section, and the flow of molten material in the screw groove is a combination of these four flows:

Positive flow - the flow of plastic melt in the direction of the head along the direction of the groove between the barrel and the screw.

Countercurrent - The flow direction is opposite to the positive flow, caused by pressure gradients caused by resistance such as the machine head, perforated plates, and filter plates.

Crossflow - The flow of the melt along a direction perpendicular to the wall of the thread affects the mixing and heat exchange of the melt during extrusion.

Leakage - due to the pressure gradient at the screw-to-barrel gap, in the axial direction of the screw.

2. Structure of ordinary screw

Conventional full-thread three-stage screws can be divided into three forms according to their thread lift and groove depth changes:

(1) Isometric deepening screw

The speed of the isotropic deepening screw from the depth of the groove can be divided into two forms:

1 Isometric gradient screw: The depth of the last groove from the beginning of the feed section to the homogenization section is a gradually shallower screw. On the longer melt section, the groove depth is gradually shallower.

2 Isometric abrupt screw: the screw depth of the feeding section and the homogenizing section is constant, and the screw depth at the melting section is suddenly shallower.

(2) Isometric variable pitch screw

The iso-depth variable pitch screw means that the depth of the groove is constant, and the pitch is narrow from the width of the first groove of the feeding section to the end of the homogenization section.

The characteristics of the constant-depth pitch screw are that the screw cross-sectional area at the feeding port position is large due to the depth of the screw groove, and there is sufficient strength, which is advantageous for increasing the rotation speed, thereby improving productivity. However, the screw processing is difficult, the flow rate of the melt is large, the homogenization is poor, and it is less used.

(3) deepening variable pitch screw

The variable depth variable pitch screw means that the depth of the groove and the angle of the thread are gradually changed from the beginning of the feeding section to the end of the homogenization, that is, the screw whose taper is gradually narrowed from the width and the depth of the groove is gradually shallower. The screw has the characteristics of the first two kinds of screws, but the machining is difficult and less used.

3. Screw material

The screw is a key component of the extruder. As the material of the screw, it must have the characteristics of high temperature resistance, wear resistance, corrosion resistance and high strength. At the same time, it should have the characteristics of good cutting performance, small residual stress after heat treatment and small thermal deformation.

For the material of the extruder screw, there are the following requirements:

1 High mechanical properties. It must have sufficient strength to adapt to high temperature and high pressure working conditions and improve the service life of the screw.

2 Mechanical processing performance is good. Have better machinability and heat treatment properties.

3 Good corrosion and wear resistance.

4 Easy to take.

4. New screw

Problems with conventional full-spiral three-stage screws:

1 The molten section has a solid bed and a molten pool co-located in a screw groove, the molten pool is continuously widened, and the solid bed is gradually narrowed, thereby reducing the contact area of ​​the solid bed on the barrel wall and reducing the direct transfer of the barrel wall to the solid. The heat of the bed reduces the melting efficiency, resulting in a low extrusion amount;

2 pressure fluctuations, temperature fluctuations and large fluctuations in production;