Screw extruders are divided into single screw extruders […]
Screw extruders are divided into single screw extruders and multi-screw extruders. Single screw extruders are the most important type of extruder in the polymer industry. The main advantages are lower cost, simple design, robust and reliable, and a satisfactory performance cost ratio. The screw of a conventional plasticizing extruder has three different geometric segments, as shown in the figure below:
This geometry is also referred to as "single stage." "Single stage" means that the screw has only one compression section, although there are only three different sections, and the first section (closest to the feed port) is generally deep. The materials in this section are mostly in a solid state. This section is referred to as the feed section of the screw. The last slot (closest to the die) is usually shallow. Most of the materials in this section are in a molten state. This mast section is referred to as a metering section or an extrusion section. The second section connects the feed section and the metering section. This section is called a transition or compression section. In most cases, the groove depth (or screw height) decreases linearly from the feed section to the metering section, thus subjecting the material to compression in the channel. It will be demonstrated in the future that this compression is essential for proper operation of the extruder under most conditions.
The operation of a single screw extruder is quite simple. Material enters from the hopper. Usually the material flows into the extruder barrel by gravity from the hopper. Some materials are not easy to flow in a dry state, and special measures must be taken to prevent materials from hanging in the feed hopper. Once the material falls into the extruder, it is in the annular space between the extruder screw and the barrel, and is surrounded by the spiral snail and the passive slat. The machine is stationary and the mast rotates. Therefore, friction acts on the material as well as the barrel and screw surfaces. At least the material is in a solid state (below the melting point) and these friction forces are responsible for transporting the material forward.
When the material moves forward, it is heated by the heat generated by the friction and the heat conducted by the barrel heater. When the temperature of the material exceeds its melting point, a film is formed on the inner surface of the barrel, and the plasticizing section begins. It must be pointed out that the starting point of the plasticizing section is usually not the starting point of the compression section. The dividing line for each functional segment depends on polymer properties, extruder geometry, and operating conditions. Thus, the dividing line can be changed due to changes in operating conditions. However, the geometry of the screw is determined by design and does not change due to changes in operating conditions. As the material moves forward, the amount of solid material at each location will decrease due to melting. When all of the solid polymer disappears, the end of the plasticizing section is reached and the melt transport section begins. In the solids transport section, the melt is uniformly fed to the die.
As the polymer flows into the die, it assumes the shape of the die runner, and thus, as the polymer exits the die, its shape more or less conforms to the cross-sectional shape of the last portion of the die runner. Since the die creates flow resistance, pressure is required to force the material through the die. This pressure is often referred to as die pressure. The die pressure is determined by the shape of the die (especially the flow path), the melt temperature of the polymer, the flow rate through the die, and the rheological properties of the polymer melt. The die pressure is generated by the die and not by the extruder. The extruder simply generates enough pressure to force the material through the die. If the temperature of the polymer, the extrusion amount, the die and the die are the same, no difference will be caused regardless of whether the extruder is a gear pump single screw extruder or a double screw extruder, and the head force is the same. Thus the die pressure is caused by it.