How does a single screw extruder work?
The working principle of the single-screw extruder is to heat the thermoplastic raw material to melt it, make the material move forward through pressure, form it through the mold, and then cool and fix it into the final product. The components of the extruder can be divided into the following four parts according to its work principle:
- Extrusion system
- Transmission system
- Heating & cooling system
- Electrical control system
1.Single Scew Extruder’s Extrusion System
The extrusion system includes a screw, a barrel, a hopper, a head, and a die. The plastic raw material is plasticized into a uniform melt through the extrusion system, and continuously extruded through the screw, under the pressure established in this process.
Screw — The screw is like the “heart” of the extruder. It is directly related to the scope of application and productivity of the extruder. It is made of high-strength and corrosion-resistant alloy steel. Through the rotation of the screw, the raw material is moved and pressurized in the barrel, and part of the heat is obtained from friction.
The screw’s performance determines the productivity, plasticization quality, dispersion of additives, melt temperature, power consumption, and so on of an extruder.
Structural parameters of the normal screw:
Screw diameter D: D – Average diameter of the screw
Db – screw outer diameter
Ds – screw root diameter
Screw length L: L- the length of the effective working part of the screw
L1- feeding section length
L2 – length of the compression section
L3 – length of homogenization section
What is the screw L/D ratio?
The L/D ratio of a single screw extruder is the ratio of the length over the screw diameter.
Not only consider the length and diameter of the screw but also consider the type of screw, which must be selected according to the different raw material characteristics used and the requirements for the product.
Barrel — It is a metal cylinder made of heat-resistant, high-compressive strength, high-strength, wear-resistant, and corrosion-resistant alloy steel or composite steel pipe lined with alloy steel.
The barrel and screw are cooperated to realize the crushing and softening of plastics. Melt, plasticize, vent and compact, and deliver the melt to the molding system continuously and uniformly. Generally, the length of the barrel is 15 to 30 times its diameter. This makes the plastic fully heated and completely plasticized.
Hooper —The hopper stores enough materials for a certain period without manual feeding. The bottom is equipped with a cutting device for adjusting and cutting off materials. The side of the hopper is equipped with observation holes and calibration metering devices. Current hoppers usually have functions such as drying and weighing.
Machine Head and Die — The head is composed of an alloy steel inner sleeve and a carbon steel shell. The forming mold is at the head position. The function of the machine head is to transform the rotating plastic melt into parallel and linear motion and introduce it into the mold evenly and smoothly.
To ensure that the plastic flow path in the machine head is reasonable and eliminate the dead corners that tend to accumulate plastic, a shunt sleeve is usually installed in the machine head. A pressure equalizing ring is also provided to eliminate pressure fluctuations during plastic extrusion.
2. Single Screw Extruder’s Transmission System
The transmission system is usually composed of a motor, a reducer, and a V-belt drive or coupling. The function of the transmission system is mainly to drive the screw to rotate within a certain speed range. According to the requirements of the production process, it can ensure that the screw rotates evenly and stably under a certain torque force to complete the plastic melting and plasticization and be pushed out of the barrel.
Motor — During the extrusion process, the screw speed must be stable and not change with the load of the screw to maintain the uniform quality of the products obtained. However, in order to achieve the requirement that a set of equipment can extrude different plastics or different products, the screw must be variable speed. Therefore, AC commutator motors, DC motors, and other devices are generally used to achieve step-less speed change. Generally, the screw speed is 10~100 Rpm
Reducer — The purpose is to reduce the speed and increase the torque. The power of the motor when the motor is running at high speed is reduced by the gear with fewer teeth on the input shaft of the reducer to mesh with the large gear on the output shaft.
3. Single Screw Extruder’s Heating & Cooling System
When the single-screw extruder starts extrusion production, heating the barrel is to provide a certain amount of heat to the plastic in the barrel to promote the plasticization of the plastic under the action of the screw. The cooling of the barrel is to control the temperature rise of the plastic in the barrel due to the extrusion and friction of the screw and to limit the temperature of the barrel to a certain range to ensure smooth extrusion production.
Heating system — Usually uses electric heating, which is divided into resistance heating and induction heating. The heating device heats the plastic in the cylinder from the outside to increase the temperature to reach the temperature required for process operation.
Cooling System — The cooling system is used to ensure that the plastic is in a suitable temperature range during the extrusion process. Specifically, it is to eliminate the excess heat generated by the shear friction caused by the rotation of the screw and to avoid the high temperature which makes the plastic difficult to decompose, scorch, or deform.
There are two types of barrel cooling: water cooling and air cooling. Generally, small and medium extruders are more suitable for air cooling, while large extruders are mostly water-cooled or a combination of two cooling methods.
4. Single Screw Extruder’s Electrical Control System
The control system of the extruder adopts an instrument control and PLC programmable control system. Instrument control is the mainstream before the industrial technology innovation, but today with the rapid development of Industry 4.0 and artificial intelligence, there are more and more PLC programmable control systems to be accepted and used.
Comparing the two control systems, the PLC programmable control system is more suitable for the intelligent, automated, and digital control of the extrusion process of the extruder.
Use our PLC system as an example to illustrate the benefits of the PLC system. The advantage is that it is used for production management. Our experts can diagnose your machine via the Internet. We not only consider your current needs but also care about your possible future expansion needs.
- Controller (zone temperature control and motor contactor)
- Drive module
- Heating module (each with short circuit protection)
- Ampere module (fault diagnosis of the heater and Solid State Rela (SSR))
- Cooling module (control solenoid valve)
- Industrial PC (built-in WIFI)
- One button start/stop(according to the setting temperature, speed, etc.)
- Recipe handling (includes feeding capacity, temperature, and all-speed)
- Data collecting and centralized management can be sent by Ethernet
- Networking(with dosing and downstream machine)
- Process flow display on one page
- Alarm diagnosis and maintenance remind
Single Screw Extruder VS Twin Screw Extruder
Many people will discuss this issue, whether to choose a single screw extruder or a twin-screw extruder. At present, the single-screw extruder is widely used and is suitable for the extrusion processing of general materials. The twin-screw extruder has good feeding characteristics and is suitable for powder processing, and has better mixing and discharging than the single-screw extruder. Gas, reaction, and self-cleaning functions, characterized by its superiority when processing plastics and blends with poor thermal stability.
Need to consider the use of twin screw or single screw according to the raw materials and output requirements of the production.
This article is about the introduction of “Single screw extruder structure and main parts”. Through the above introduction, you can easily understand the structure of the single screw extruder and the structure and characteristics of some of the more important components. For selection What kind of extruder is suitable for your product needs to be analyzed in detail according to the characteristics of the raw materials, the size and appearance of the product, and even the output.
The single-screw extruder is relatively simple in structure, low in cost, easy to use and operate, and easy to maintain and repair the equipment. Therefore, single screw extruder is the first choice for many manufacturers of plastic products.