In addition to the screw and barrel, these components are equally important when choosing an extruder!
How do people usually buy extruders? Not only do you need to analyze your own needs, but you also need to have a good understanding of the supplier and the extruder.
Most of the enterprises in the purchase of new extruder has been basically clear, need to buy twin-screw or single-screw, need to produce what kind of material, according to the product specifications, the amount of material is not the same, you can refer to the “screw diameter and product specifications size”, first choose the diameter of the screw, then by the screw diameter and then further choose the extruder The diameter of the screw is then used to select the extruder model.
After the type and specification of the extruder is determined, how to find the equipment manufacturer is also a problem that should be paid attention to, foreign brands since needless to say, there are also many domestic factories built a long time, strong and have many years of practical experience in the extruder enterprise, you can choose from the product quality, after-sales service and other multiple perspectives.
Rotational speed of the screw
This is the most critical factor affecting the capacity of an extruder. The screw speed is not only to increase the speed and volume of extrusion of the material, but more importantly to enable the extruder to achieve a high output with a good plasticization effect.
In the past, the main way to increase the output of an extruder was to increase the diameter of the screw. Although an increase in screw diameter increases the amount of material extruded per unit time. But an extruder is not a screw conveyor. In addition to extruding the material, the screw also extrudes, mixes and shears the plastic to plasticize it. Under the premise of the screw speed remains unchanged, the mixing and shearing effect of a large diameter screw with a large screw groove on the material is not as good as that of a small diameter screw.
Therefore, modern extruders increase their capacity mainly by increasing the screw speed. The screw speed of an ordinary extruder was traditionally 60 to 90 rpm (per minute, same below). This has now generally been increased to 100 to 120 rpm. Higher speed extruders reach 150 to 180 rpm.
If the diameter of the screw remains unchanged and the speed of the screw is increased, the torque on the screw will increase, and when the torque reaches a certain level, the screw is in danger of being twisted off. However, by improving the screw material and production process, designing the screw structure, shortening the length of the feed section, increasing the material flow rate and reducing the extrusion resistance, the torque can be reduced and the capacity of the screw can be increased. How to design the most reasonable screw to maximize the screw speed as long as the screw can withstand it is a matter for professionals to obtain through extensive testing.
The screw structure is the main factor affecting the extruder’s capacity. Without a reasonable screw structure, trying to simply increase the screw speed in order to increase the extrusion capacity goes against the objective rules and will not be successful.
High-speed, high-efficiency screws are designed for high rotational speeds. The plasticizing effect of these screws is less effective at low speeds, but gradually improves as the screw speed increases, achieving the best results at the design speed. This results in both high production capacity and satisfactory plasticization.
The main improvements to the barrel structure are the improved temperature control of the feed section and the installation of a feed trough. The full length of this independent feed section is a water jacket, with advanced electronic control devices for temperature control of the water jacket.
The temperature of the water jacket is very important for the stable work of the extruder and the efficient extrusion. 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 and weaken the friction between the raw material and the inner wall of the barrel, thus reducing the extrusion thrust and the extrusion volume. However, the temperature should not be too low, as a low temperature barrel will cause the screw to rotate with too much resistance, which will cause difficulties in starting the motor or unstable speed when the motor’s carrying capacity is exceeded. Using advanced sensor and control technology, the extruder water jacket is monitored and controlled so that the temperature of the water jacket is automatically controlled within the optimum process parameters.
The manufacturing cost of a gearbox is roughly proportional to its size and weight, provided that the structure is basically the same. The large size and weight of the gearbox means that more material is consumed in the manufacture, and the bearings used are also larger, increasing the manufacturing costs.
An extruder with the same screw diameter, a high speed and high efficiency extruder consumes more energy than a conventional extruder, the motor power is doubled and a corresponding increase in the seat number of the reducer is necessary. But a high screw speed means a low reduction ratio. For the same size reducer, a lower reduction ratio increases the gear module compared to a larger reduction ratio, and the capacity of the reducer to withstand the load increases. Therefore the increase in volume and weight of the gearbox is not linearly proportional to the increase in motor power. If the extrusion volume is used as the denominator and divided by the weight of the reducer, the high speed and high efficiency extruder will have a smaller number and the common extruder will have a larger number.
The low power of the motor and the low weight of the reducer per unit of output means that the manufacturing costs per unit of output are lower for high speed high efficiency extruders than for normal extruders.
For the same screw diameter extruder, a high speed and high efficiency extruder consumes more energy than a conventional extruder and it is necessary to increase the motor power. For a high speed 65 extruder, a 55 kW to 75 kW motor is required. For a high speed 75 extruder, a 90 kW to 100 kW motor is required. For a high speed 90 extruder, a motor of 150 kW to 200 kW is required. This is one to two times more powerful than 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 the drive part, such as the motor and gearbox, accounts for 77% of the energy consumption of the whole machine; heating and cooling accounts for 22.8% of the energy consumption of the input of the whole machine; the instrumentation and electrical accounts for 0.8%.
An extruder with the same screw diameter with a larger motor may appear to be power hungry, but when calculated by output, a high speed and efficient extruder is more energy efficient than a conventional extruder. For example a normal 90 extruder with a 75 kW motor and a capacity of 180 kg consumes 0.42 kWh of electricity per kg of material extruded. A high speed and high efficiency 90 extruder, with a capacity of 600 kg and a motor of 150 kW, consumes only 0.25 kW of electricity per kg of material extruded, which is only 60% of the power consumption per unit of extruded volume. And this is only comparing the energy consumption of the motor, if we then take into account the electricity consumption of devices such as heaters and fans on the extruder, the difference in energy consumption will be even greater. A larger screw diameter extruder has a larger heater and a larger heat sink. Therefore two extruders with the same capacity, the new high rpm extruder with a smaller barrel and a smaller heater consumes less energy than the traditional large screw extruder and saves a lot of electricity in terms of heating.
In terms of heater power, high speed and high efficiency extruders do not increase the power of the heater due to increased capacity compared to a normal extruder with the same screw diameter. This is because the heater in an extruder consumes power, mainly during the preheating phase, and during normal production the heat of melting material is mainly converted by consuming electrical energy from the motor, and the heater has a very low conduction rate and does not use much power. This is even more evident in high-speed extruders.
When frequency converter technology was not yet commonly used, traditional extruders with large extrusion volumes generally used DC motors and DC motor controllers. This is because DC motors were previously considered to have better power characteristics than AC motors, with a larger speed range and more stable operation in the lower speed range. Another high power inverter is more expensive, which also limits the application of inverters.
In recent years inverter technology has developed relatively quickly, vector type inverters have achieved sensorless control of motor speed and torque, low frequency characteristics have come a long way and prices have fallen relatively quickly. The biggest advantage of inverters compared to DC motor controllers is energy efficiency. It makes the energy consumption proportional to the motor load, increasing the energy consumption when the load is heavy and automatically adjusting it down when the motor load drops. The energy saving benefits of this in long-term applications are significant.
Vibration damping measures
High speed extruders are 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 improve the service life of the machine.
The most likely links of the extruder to generate vibration are the motor shaft and the high speed shaft of the reducer. Firstly, the high rpm extruder should be equipped with a high quality motor and reducer to avoid being a source of vibration due to the vibration of the motor rotor and the high speed shaft of the reducer. The second is to design a good transmission system. Paying attention to improving the rigidity, weight and quality of all aspects of processing and assembly of the frame is also an important part of reducing the vibration of the extruder. A good extruder is essentially vibration free even when it is in use without the use of ground bolts to secure it. This relies on the frame having sufficient rigidity and self-weight. In addition, we must strengthen the quality control of the processing and assembly of each component. For example, the parallelism of the upper and lower plane of the frame, the verticality of the mounting surface of the reducer and the plane of the frame should be controlled during processing. During assembly, the shaft height of the motor and reducer should be measured carefully, and the reducer gasket should be strictly formulated so that the motor shaft is concentric with the input shaft of the reducer. As well as making the reducer mounting surface and the rack plane perpendicular.
The production operation of an extrusion is basically a black box, where the situation inside cannot be seen at all and is only reflected by the instrumentation. This is why precise, intelligent and easy to operate instrumentation will give us a better understanding of the internal situation and enable production to achieve faster and better results.