Extruder Screw

The screw is the heart of the extruder and a key component of the extruder. The performance of the screw determines the productivity, plasticizing quality, dispersion of fillers, melt temperature, power consumption, etc. of an extruder. It is the most important component of the extruder, which can directly affect the application range and production efficiency of the extruder. The rotation of the screw exerts extreme pressure on the plastic, so that the plastic can move, pressurize, and obtain some heat from friction in the barrel. The plastic is mixed and plasticized during the movement in the barrel. When the viscous melt is extruded and flows through the die, it obtains the desired shape and is formed. Like the barrel, the screw is also made of high-strength, heat-resistant and corrosion-resistant alloys.
Because there are many types of plastics, their properties are also different. Therefore, in actual operation, in order to adapt to different plastic processing needs, different types of screws are required, and the structures are also different. So that the plastic can be transported, extruded, mixed and plasticized with the greatest efficiency.
The basic parameters that represent the characteristics of the screw include the following: diameter, aspect ratio, compression ratio, pitch, groove depth, helix angle, clearance between the screw and the barrel, etc.
The most common screw diameter D is about 45~150 mm. As the screw diameter increases, the processing capacity of the extruder also increases accordingly, and the productivity of the extruder is proportional to the square of the screw diameter D. The ratio of the effective length of the working part of the screw to the diameter (abbreviated as aspect ratio, expressed as L/D) is usually 18~25. A large L/D can improve the temperature distribution of the material, facilitate the mixing and plasticization of the plastic, and reduce leakage and backflow. Improve the production capacity of the extruder. The screw with a large L/D has strong adaptability and can be used for the extrusion of a variety of plastics; but when the L/D is too large, the plastic will be heated for a longer time and degraded. At the same time, due to the increase in the weight of the screw, the free end will bend and sag, which is easy to cause scratches between the barrel and the screw, and make manufacturing and processing difficult; increase the power consumption of the extruder. Too short a screw is likely to cause poor plasticization of mixing.
Half the difference between the inner diameter of the barrel and the diameter of the screw is called the gap δ, which can affect the production capacity of the extruder. As δ increases, the productivity decreases. It is usually appropriate to control δ at about 0.1 to 0.6 mm. When δ is small, the shearing effect on the material is large, which is conducive to plasticization. However, if δ is too small, the strong shearing effect is likely to cause thermomechanical degradation of the material, and the screw is likely to be held or rubbed against the barrel wall. Moreover, when δ is too small, there is almost no leakage and backflow of the material, which affects the mixing of the melt to a certain extent.
The helix angle Φ is the angle between the thread and the cross section of the screw. As Φ increases, the production capacity of the extruder increases, but the shearing effect and extrusion force on the plastic decrease. Usually, the helix angle is between 10° and 30°, and changes along the direction of the screw length. An equidistant screw is often used, and the pitch is equal to the diameter. The value of Φ is about 17°41′
The greater the compression ratio, the greater the extrusion ratio received by the plastic. When the screw groove is shallow, it can produce a higher shear rate on the plastic, which is beneficial to the heat transfer between the barrel wall and the material. The higher the material mixing and plasticizing efficiency, the lower the productivity. On the contrary, when the screw groove is deep, the situation is just the opposite. Therefore, deep screw groove screws are suitable for heat-sensitive materials (such as polyvinyl chloride); while shallow screw groove screws are suitable for plastics with low melt viscosity and high thermal stability (such as polyamide).
1. Segmentation of the screw
When the material moves forward along the screw, it experiences changes in temperature, pressure, viscosity, etc. This change is different within the entire length of the screw. According to the characteristics of the material changes, the screw can be divided into a feeding (feeding) section, a compression section, and a homogenization section.
①, Plastics and three states of plastics
Plastics are divided into two categories: thermosetting and thermoplastic. After thermosetting plastics are formed and solidified, they cannot be heated and melted. However, the products formed by thermoplastics can be heated and melted to form other products.
As the temperature changes, the thermoplastic plastic changes into three states: glassy state, highly elastic state and viscous flow state. As the temperature changes repeatedly, the three states change repeatedly.
a. Different characteristics of polymer melts in the three states:
Glassy state - plastic appears as a rigid solid; the thermal kinetic energy is small, the intermolecular force is large, and the deformation is mainly contributed by the bond angle deformation; the deformation recovers instantly after the external force is removed, which belongs to general elastic deformation.
Highly elastic state - plastic appears as a rubber-like substance; the deformation is contributed by the stretching of the macromolecular conformation caused by the orientation of the chain segments, and the deformation value is large; the deformation can be recovered after the external force is removed but is time-dependent, which belongs to highly elastic deformation.
Viscous flow state - plastic appears as a highly viscous melt; thermal energy further stimulates the relative slip motion of the chain molecules; the deformation is irreversible, which belongs to plastic deformation
b. Plastic processing and the three states of plastic:
Plastics can be cut in the glassy state. In the highly elastic state, they can be stretched, such as wire drawing, tube 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 undergo thermal decomposition, 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 deteriorate and be damaged seriously, so these two temperature areas should be avoided when processing or using plastic products.
②, three-stage screw
Plastics exist in three physical states in the extruder - the change process of glassy state, high elastic state and viscous flow state, and each state has different requirements for the screw structure.
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)
Homogenizing section L3 (called metering section)
This is what is usually called a three-stage screw. The extrusion process of plastics in these three sections is different.
The function of the feeding section is to send the material supplied by 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 be from not far from the hopper to 75% of the total length of the screw cup.
Generally speaking, the extrusion of crystalline polymers is the longest, followed by hard amorphous polymers, and the shortest for soft amorphous polymers. Since the feeding section does not necessarily produce compression, the volume of its screw groove can remain unchanged. The size of the helix angle has a greater impact on the feeding capacity of this section, and actually affects the productivity of the extruder. Usually, the helix angle of powdered materials is about 30 degrees, and the productivity is the highest. The helix angle of block materials should be about 15 degrees, and the helix angle of spherical materials should be about 17 degrees.