How to choose a suitable extruder?

• First confirm what the product is going to process;

• Confirm the machine model according to the product output or power;

• After above, check the configuration, quality and manufacturer's ability of different brands of equipment.

1. According to the raw materials & final products:

1. For the extrusion granulation of PE, PP, PS, PC, ABS plastics, you can choose a single screw extruder, which is cheap and easy to operate and maintain. You can also choose a twin-screw extruder. The twin-screw extruder is more efficient, the mixing is more uniform, and the quality of the products produced is better.

The difference between single screw extruder and twin screw extruder:___________

2. The extrusion equipment for PVC is diverse. ①Single-screw extruder, since PVC is a heat-sensitive plastic, the screw speed of the extruder should not be too high. The PVC powder (sheet) should be granulated first, and then put into the production of products.

②Conical counter-rotating twin-screw extruder, the price of this type of extruder is appropriate, and it will not cause excessive friction heat for the extrusion of PVC powder, avoids the decomposition of raw materials, the material is stable, the output of finished products is high, and the performance is guaranteed. At present, when using PVC mixed powder to produce rigid PVC products, the first consideration is to use this extruder.

③ Parallel counter-rotating twin-screw extruder, which can directly extrude PVC powder to form pipes and special-shaped materials, and is also suitable for HDPE material to extrude pipes, and the output is also higher than the above two extruders. However, due to the high price and complicated maintenance, there are still few applications at present.

2. According to the machine performance

First, it's important to understand how the extruder works.

The function of the extruder is to use heat, pressure and shear to transform the solid plastic into a uniform melt and send the melt to the next process. The production of the melt involves mixing additives such as color masterbatches, blending resins, and regrinding. The finished melt must be homogeneous in concentration and temperature. The pressure must be large enough to extrude the viscous polymer. The extruder completes all the above processes through a barrel with a screw and a screw channel.

Plastic pellets enter the barrel through a hopper at one end of the barrel, and are then conveyed to the other end of the barrel by a screw. In order to have sufficient pressure, the depth of the threads on the screw decreases with increasing distance from the hopper. External heating and internal heat due to friction between the plastic and the screw soften and melt the plastic. Different polymers and different applications often have different design requirements for extruders. Many options involve venting, multiple feeding ports, cooling and heating of the melt, or no external heat source (adiabatic extruder), relative size changes in the gap between screw and barrel, and the number of screws. For example, twin-screw extruders allow for more thorough mixing of the melt than single-screw extruders. In tandem extrusion, the melt extruded by the first extruder is fed to the second extruder as the raw material, which is usually used to produce extruded polyethylene foam.

The characteristic dimensions of an extruder are the diameter (D) of the screw and the ratio (L/D) of the length (L) of the screw to the diameter (D).

An extruder usually consists of at least three stages. The first section, near the feeding hopper, is the feeding section. Its function allows material to enter the extruder at a relatively steady rate. In general, to avoid clogging of the feed channel, this part will be kept at a relatively low temperature. The second part is the compression section, where the melt is formed and the pressure is increased. The transition from the feed section to the compression section can be abrupt or gradual (gentle). The last partial metering section, next to the extruder outlet. The main function is that the material flowing out of the extruder is uniform. In this part, to ensure the uniformity of composition and temperature, the material should have sufficient residence time. At the end of the barrel, the plastic melt leaves the extruder through a head designed to the desired shape, through which the extruded melt stream passes.

Another important part is the drive mechanism of the extruder. It controls the rotational speed of the screw, which determines the output of the extruder. The power required is determined by the viscosity (flow resistance) of the polymer. The viscosity of polymers is dependent on temperature and flow rate and decreases with increasing temperature and shear force. The extruder is equipped with a filter screen, which can block impurities on the filter screen. To avoid downtime, the filter should be able to be replaced automatically. This is especially important when processing resins with impurities, such as regrind. The screw of the extruder is divided into feeding section, plasticizing and melting section. The temperature is adjusted according to the process parameters of the plastic particles. The model is divided into 45, 65, 75, 80, 90, 120... The length-diameter ratio of the screw is generally 40, 44, 48.... It can be adjusted according to the different raw materials depending on the specific application.

1. Screw rotary speed

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 also to make the extruder achieve a high output and obtain a good plasticizing effect.

In the past, to increase the output of the extruder, the main method was to increase the diameter of the screw. Although the material extruded per unit time will increase after the screw diameter is increased, the extruder is not a screw conveyor. In addition to extruding the material, the screw also extrudes, stirs and shears the plastic to plasticize the plastic. On the premise that the screw speed remains unchanged, the screw with large diameter and large groove does not have as good a stirring and shearing effect on the material as the screw with small diameter.

Therefore, modern extruders mainly increase production capacity and control quality by increasing the screw speed. The screw speed of the ordinary extruder, the traditional extruder is 60 to 90 revolutions (per minute, the same below). And now it has generally been raised to 100~120 rpm. The high speed extruder reached 150 to 180 rpm. The maximum speed of the Haisi extruder can reach 600-800rpm. If the screw diameter is unchanged and the screw speed is increased, the torque on the screw will increase. When the torque reaches a certain level, the screw is in danger of being twisted and broken. However, by improving the material and production process of the screw, rationally 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 through a lot of experiments.

In terms of energy consumption. Extruders with the same screw diameter, high-speed and high-efficiency extruders consume more energy than conventional extruders, and it is necessary to increase the motor power. The high-speed 65 extruder should be equipped with a motor of 55 kW to 75 kW. The high-speed 75 extruder is equipped with a motor of 90 kW to 100 kW. The high-speed 90 extruder should be equipped with a motor of 150 kW to 200 kW. This is one to two times larger than the motor power of the ordinary extruder.

The extruder with the same screw diameter is equipped with a larger motor, which seems to consume electricity, but if calculated according to the output, the high-speed and high-efficiency extruder is more energy-saving than the conventional extruder. For example, an ordinary 90 extruder, the motor is 75 kilowatts, the production capacity is 180 kilograms, and each kilogram of material extruded consumes 0.42 kWh of electricity. A high-speed and high-efficiency 90 extruder has a production capacity of 600 kilograms and a motor of 150 kilowatts. It only consumes 0.25 kWh of electricity per kilogram of material extruded, and the power consumption per unit of extrusion is only 60% of the former. The energy saving effect is remarkable. And this is only a comparison of the energy consumption of the motor. If the power consumption of the heaters and fans on the extruder is taken into account, the difference in energy consumption will be even greater. The extruder with large screw diameter needs to be equipped with a larger heater, and the heat dissipation area is also increased. Therefore, for the two extruders with the same production capacity, the high-speed and high-efficiency new extruder has a smaller barrel, and the heater consumes less energy than the traditional large-screw extruder, and saves a lot of electricity in heating.

In terms of heater power, the high-speed and high-efficiency extruder does not increase the heater power due to the increase in production capacity compared with 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 from the consumption of electrical energy of the motor, the conduction rate of the heater is very low, and the power consumption is not large. This is more pronounced in high speed extruders.

2. Screw structure

The screw structure is the main factor affecting the capacity of the extruder. If there is no reasonable screw structure, simply increasing the screw speed to increase the extrusion volume will go against the objective laws and will not succeed.

The design of the high-speed and high-efficiency screw is based on high rotational speed. The plasticizing effect of this kind of screw will be worse at low speed, but the plasticizing effect is gradually improved after the screw speed is increased, and the best effect is obtained when the design speed is reached. At this time, there is not only a high production capacity, but also a qualified plasticizing effect.

3. Barrel structure

The improvement of the barrel structure is mainly to improve the temperature control of the feeding section and set the feeding trough. The entire length of this independent feeding section is a water jacket, and the temperature of the water jacket is controlled by an advanced electronic control device.

Whether the temperature of the water jacket is reasonable is very important for the stable operation of the extruder and 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 be melted, which will weaken the friction between the raw material and the inner wall of the barrel, thereby reducing the extrusion thrust and extrusion volume. However, the temperature should not be too low. If the temperature of the barrel is too low, the rotation resistance of the screw will be too large. When the bearing capacity of the motor is exceeded, the motor will be difficult to start or the rotation speed will become unstable. Using advanced sensors and control technology, the water jacket of the extruder is monitored and controlled, so that the temperature of the water jacket is automatically controlled within the optimal process parameter range.

4. Gearbox

Under the premise that the structure is basically the same, the manufacturing cost of the reducer is roughly proportional to its overall size and weight. Because the shape and weight of the reducer are large, it means that a lot of materials are consumed during manufacturing, and the bearings used are also relatively large, which increases the manufacturing cost.

The extruder with the same screw diameter, the high-speed and high-efficiency extruder consumes more energy than the conventional extruder, the motor power is doubled, and the frame size of the reducer is correspondingly increased. But high screw speed means 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 weight of the reducer is not linearly proportional to the increase of the motor power. If the extrusion volume is used as the denominator and divided by the weight of the reducer, the number of high-speed and high-efficiency 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 machine manufacturing cost per unit of the high-speed and high-efficiency extruder is lower than that of the ordinary extruder.

5. Motor drive

When inverter technology has not been widely used, traditional extruders with large extrusion volume generally use DC motors and DC motor controllers. Because it is generally believed that DC motors have better power characteristics than AC motors, the speed regulation range is relatively large, and it is relatively stable when running at low speed. Another high-power inverter is more expensive, which also limits the application of the inverter.

In recent years, the frequency converter technology has developed rapidly. The vector type frequency converter has realized the sensorless control of the motor speed and torque. The low-frequency characteristics have made great progress, and the price has also dropped rapidly. Compared with the DC motor controller, the biggest advantage of the inverter is energy saving. It makes the energy consumption proportional to the motor load, the energy consumption is increased when the load is heavy, and the energy consumption is automatically reduced when the motor load decreases. The energy-saving benefits of this in long-term applications are very significant.

6. Vibration reduction 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 to increase the service life of the equipment.

The most prone to vibration 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 the vibration source 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 processing and assembly of the frame is also an important part of reducing the vibration of the extruder. A good extruder is used without anchor bolts and has basically no vibration. This depends on the frame having sufficient rigidity and self-weight. In addition, it is necessary to strengthen the quality control of the processing and assembly of each component. For example, control the parallelism of the upper and lower planes of the rack during processing, and the perpendicularity of the installation surface of the reducer to the plane of the rack. When assembling, carefully measure the shaft height of the motor and the reducer, and strictly prepare the reducer pads so that the motor shaft and the reducer input shaft are concentric. And make the installation surface of the reducer perpendicular to the plane of the frame.

7. Dashboard

The production operation of the extrusion is basically a black box type, and the situation inside cannot be seen at all, only reflected by the instrumentation. Therefore, sophisticated instrumentation will make us better understand its internal situation, so that production can achieve faster and better production results.

3. Choice of equipment manufacturers

After the type and specification of the extruder are determined, how to find the equipment manufacturer is also a problem that should be paid attention to. The domestic manufacturers with better quality are mainly distributed in Nanjing, Qingdao, Dalian, Shunde, Zhangjiagang. These manufacturers have been established for a long time, have strong forces and have many years of practical experience, reliable quality, long service life and good after-sales!