Table of Contents
How do you usually choose a screw extruder? It is necessary not only to analyze one’s own needs, but also to have a good understanding of the supplier and the extruder.
Most enterprises have basically been clear before purchasing a new extruder, need to buy twin screw extruder or single screw extruder, what kind of materials need to be produced, according to different product specifications, the amount of materials is different, you can refer to the “screw diameter and product specifications”, first select the diameter of the screw, and then further select the specifications and models of the extruder by the screw diameter.
After the type and specification of the extruder are determined, how to find an equipment manufacturer is also a problem that should be paid attention to, not to mention foreign brands, there are many extruder companies that have been built for a long time, have strong power and have many years of practical experience, and can choose from multiple perspectives such as product quality and after-sales service.
The rotational speed of the screw
This is the most critical factor affecting the productivity of an extruder. The screw speed is not only to improve the extrusion speed and extrusion volume of the material, but more importantly, to make the extruder achieve high output and get good plasticizing effect at the same time.
In the past, the main way to increase the output of the extruder was to increase the screw diameter. Although the screw diameter increases, the extruded material per unit time will increase. But the extruder is not a screw conveyor. In addition to extruding materials, the screw also extrudes, stirs, and shears the plastic to plasticize the plastic. Under the premise that the speed of the screw remains unchanged, the stirring and shearing effect of the screw with large diameter and large screw groove on the material is not as good as that of the screw with small diameter.
Therefore, modern extruders mainly increase their production capacity by increasing the speed of the screw. The screw speed of ordinary extruders, the traditional extruder is 60 to 90 revolutions (per minute, the same below). Now it has generally been increased to 100~120 rpm. The high-speed extruder reaches 150 to 180 rpm.
If the diameter of the screw remains the same and the speed of the screw is increased, the torque of the screw will increase, and when the torque reaches a certain level, the screw is in danger of being torsioned. However, by improving the material and production process of the screw, reasonably designing the screw structure, shortening the length of the feeding section, increasing the flow rate of the material, and reducing the extrusion resistance, the torque can be reduced and the bearing capacity of the screw can be improved. How to design the most reasonable screw and maximize the screw speed under the premise that the screw can withstand it requires professionals to obtain it through a large number of tests.
The screw structure is the main factor affecting the productivity of the extruder. If there is no reasonable screw structure, it is against the objective law to simply increase the screw speed to increase the extrusion capacity, and it will not succeed.
The design of the high-speed and high-efficiency screw is based on high rotational speed. The plasticizing effect of this screw will be worse at low speed, but the plasticizing effect will gradually improve after the screw speed is increased, and the best effect will be obtained when the design speed is reached. At this time, there is not only a higher production capacity, but also a qualified plasticizing effect.
The improvement of the barrel structure is mainly to improve the temperature control of the feed section and the setting of the feed trough. The full length of this independent feed section is a water jacket, and the temperature of the water jacket is controlled by advanced electronic control devices.
Whether the temperature of the water jacket is reasonable is very important for the stable operation and efficient extrusion of the extruder. If the temperature of the water jacket is too high, the raw material will be softened prematurely, and even the surface of the raw material particles will melt, which will weaken the friction between the raw material and the inner wall of the barrel, thereby reducing the extrusion thrust and extrusion capacity. However, the temperature should not be too low, the temperature of the barrel is too low, the screw rotation resistance is too large, and when the bearing capacity of the motor is exceeded, it will cause the motor to start difficult or the speed is unstable. Advanced sensors and control technology are used to monitor and control the water jacket of the extruder, so as to automatically control the temperature of the water jacket within the optimal range of process parameters.
Gearbox For Screw Extruder
Under the premise that the structure is basically the same, the manufacturing cost of the reducer is roughly proportional to its size and weight. Because the shape and weight of the reducer are large, it means that more materials are consumed during manufacturing, and the bearings used are also relatively large, which increases the manufacturing cost.
For the same screw diameter extruder, the high-speed and efficient extruder consumes more energy than the conventional extruder, the motor power is doubled, and the frame number of the reducer is increased accordingly. But a high screw speed means a low reduction ratio. For the same size reducer, the gear module of the low reduction ratio is increased compared with the large reduction ratio, and the load bearing capacity of the reducer is also increased. Therefore, the increase of the volume and weight of the reducer is not linearly proportional to the increase of the motor power. If the extrusion amount is used as the denominator and divided by the weight of the reducer, the number of high-speed and efficient extruders is small, and the number of ordinary extruders is large.
In terms of unit output, the motor power of the high-speed and high-efficiency extruder is small and the weight of the reducer is small, which means that the manufacturing cost per unit output of the high-speed and high-efficiency extruder is lower than that of the ordinary extruder.
For the same screw diameter, the high-speed and efficient extruder consumes more energy than the conventional extruder, and it is necessary to increase the motor power. The high-speed 65 extruder is equipped with a 55 kW to 75 kW motor. The high-speed 75 extruder is equipped with a motor from 90 kW to 100 kW. The high-speed 90 extruder is equipped with a motor from 150 kW to 200 kW. This is one to two times more powerful than the motor configured with a normal extruder.
During the normal use of the extruder, the motor drive system and the heating and cooling system are always working. The energy consumption of transmission parts such as motor and reducer accounts for 77% of the energy consumption of the whole machine; Heating and cooling account for 22.8% of the input energy consumption of the whole machine; Instrumentation electrical accounted for 0.8%.
An extruder with the same screw diameter is equipped with a larger motor, which may seem to be power-intensive, but if calculated in terms of output, a high-speed and efficient extruder is more energy-efficient than a conventional extruder. For example, an ordinary 90 extruder has a motor of 75 kilowatts, a capacity of 180 kilograms, and a consumption of 0.42 kilowatts of electricity per kilogram of material extruded. A high-speed and efficient 90 extruder, with a capacity of 600 kg and a motor of 150 kilowatts, consumes only 0.25 kWh of electricity per kilogram of extruded material, and the power consumption per unit extrusion is only 60% of the former, and the energy-saving effect is significant. And this is only a comparison of the energy consumption of the motor, if you take into account the power consumption of the heater and fan on the extruder, the difference in energy consumption is even greater. Extruders with large screw diameters need to be equipped with larger heaters, and the heat dissipation area is also increased. Therefore, the two extruders with the same capacity, the barrel of the new extruder with high rotation and high efficiency is smaller, and the energy consumption of the heater is less than that of the traditional large screw extruder, and it also saves a lot of electricity in heating.
In terms of heater power, the high-speed and efficient extruder does not increase the power of the heater due to the increase in capacity compared to the ordinary extruder with the same screw diameter. Because the heater of the extruder consumes electricity, mainly in the preheating stage, in normal production, the heat of material melting is mainly converted by consuming the electrical energy of the motor, and the conductivity of the heater is very low, and the power consumption is not large. This is even more noticeable in high-speed extruders.
When inverter technology was not yet widely used, the traditional extruder with large extrusion capacity generally used DC motor and DC motor controller. In the past, it was generally believed that the power characteristics of DC motors were better than those of AC motors, and the speed regulation range was relatively large, and they were more stable when running at low speeds. In addition, high-power inverters are more expensive, which also limits the application of inverters.
In recent years, the development of inverter technology has been relatively fast, and the vector inverter has realized sensorless control of motor speed and torque, and the low-frequency characteristics have made great progress, and the price has also dropped rapidly. The biggest advantage of inverters compared to DC motor controllers is energy saving. It makes the energy consumption proportional to the motor load, the heavy load increases the energy consumption, and the motor load decreases when the energy consumption is automatically reduced. The energy savings in long-term applications are significant.
Shock absorption measures
The high-speed extruder is prone to vibration, and excessive vibration is very harmful to the normal use of the equipment and the service life of the parts. Therefore, multiple measures must be taken to reduce the vibration of the extruder in order to increase the service life of the equipment.
The most vibrating part of the extruder is the motor shaft and the high-speed shaft of the reducer. First of all, the high-speed extruder should be equipped with a high-quality motor and reducer to avoid becoming a source of vibration due to the high-speed shaft vibration of the motor rotor and reducer. The second is to design a good drivetrain. Paying attention to improving the rigidity, weight and quality of the frame in all aspects of processing and assembly is also an important part of reducing the vibration of the extruder. A good extruder is not fixed with anchor bolts when used, and there is basically no vibration. This depends on the frame having sufficient rigidity and dead weight. In addition, it is necessary to strengthen the quality control of the processing and assembly of various components. For example, control the parallelism of the upper and lower planes of the frame and the perpendicularity of the mounting surface of the reducer and the plane of the frame during processing. During assembly, the shaft height of the motor and reducer should be carefully measured, and the reducer cushion block should be strictly prepared to make the motor shaft and the reducer input shaft concentric. and make the mounting surface of the reducer perpendicular to the plane of the frame.
The extrusion production operation is basically a black box, and the situation inside cannot be seen at all, only reflected through instrumentation. Therefore, precise, intelligent and easy-to-operate instrumentation will make us better understand its internal situation, so that production can achieve results faster and better.