Analysis of the electric control system of the hot

2022-08-15
  • Detail

Analysis of electric control system of twin screw extruder

1 Introduction

extruders have developed rapidly since the advent of plastics, one of the three major synthetic materials. Plastic instead of steel, plastic instead of non-ferrous metals, plastic instead of cement, etc. are widely used in agriculture, building materials, packaging, machinery, electronics, automobiles, household appliances, petrochemical and national defense, extruders and people's daily life and other fields. Plastic has become one of the most important raw materials for human activities. Because extrusion molding is the most important form of plastic processing, it is of great significance to develop plastic extrusion molding technology and equipment

twin screw extruder is an important equipment in plastic processing machinery. It is not only suitable for extrusion molding and mixing processing of polymer materials, but also has been expanded to food, feed, electrodes, explosives, building materials, packaging, pulp, ceramics and other fields. The high-speed and high-yield extruder can enable investors to obtain larger output and high returns with lower input. However, the high speed of the screw speed of the extruder also brings a series of difficulties to be overcome: for example, the reduction of the residence time of the material in the screw will lead to uneven mixing and plasticization of the material, the excessive shear of the material may cause the sudden temperature rise and thermal decomposition of the material, and the difficulty in controlling the extrusion stability will cause the geometric size fluctuation of the extrudate. The accuracy of the relevant auxiliary devices and control systems must be improved, The increasing wear of screw and barrel requires the use of high wear-resistant and ultra-high wear-resistant materials, and the problems of how to improve the service life of reducer and bearing under the condition of high-speed operation need to be solved

on the whole, the research on twin-screw extrusion theory is still in its initial stage, which means that "technology is more than science"; The electrical automation control of the working process of the extruder is also developing. The traditional electrical control is realized by single machine automation instruments. Now it has developed to an electrical control system composed of touch screen, PLC, temperature control module, frequency conversion and speed regulation, which is composed of human-machine interface technology, computer technology, frequency conversion technology and so on

2. Composition of extruder

extruder is mainly composed of screw, barrel, heating and cooling system, transmission system and control system

2.1 screw and barrel

screw is the most important part of plastic machine equipment, which is directly related to the plasticizing effect and output of plastic machine. The rotation of the screw in the barrel is carried out under high temperature and high pressure. Because it wants to strongly push the material forward during rotation, at the same time, it also has to bear strong friction and the corrosion of plastic decomposition corrosive gas. Therefore, the material of the screw must have high mechanical strength, bear huge torsional torque and do not deform under high temperature and high pressure

during the rotation of the screw, the plastic is sheared and plasticized mainly by the screw edge, and the plastic is pushed forward. Therefore, the screw edge experimental machine industry is dominated by 45 state-owned enterprises, which bear huge shear stress and friction. Due to long-term work under harsh conditions, the screw edge is worn, the screw edge becomes smaller, and the gap between the same barrel increases, resulting in the reduction of plastic extrusion volume, plastic reflux in serious cases, and the plasticization effect is reduced, There is a serious decline in grain size and production capacity

the process of melt extrusion is to enter the premixed material into the extruder barrel from the feeding port. The first section of the barrel is the feeding section, and the material will not melt at this stage. With the screw drive, the material is brought into the second section is the compression section, which is the heating stage. The material begins to melt, the friction between the materials increases, forming a high viscosity body, and continues to enter the high shear section with the screw drive. The third section is the homogenization section, Make it very effective in separating pigments "target="_ blank "> pigment aggregates, to achieve the purpose of full dispersion. At present, the extruders used in powder coatings are twin-screw extruders, single screw extruders and star screw extruders. Although the types and internal structures of extruders are different, the design purpose is the same, that is, to maximize the uniform dispersion of materials, so the quality of extruders directly determines the degree of dispersion of materials.

working principle of screw pumps : the screw rotates around its axis and rolls along the inner surface of the bushing to form a sealed chamber. Each time the screw rotates, the liquid in the sealing cavity advances by a pitch. With the continuous rotation of the screw, the liquid is pressed from one sealing cavity to another in a spiral manner, and finally the pump body is extruded

2.2 heating and cooling system

the heating and cooling system of the extruder is to ensure the normal operation of the extruder and maintain a stable process temperature of the extruder. The heating of the barrel in the extruder is to make the barrel heated to a certain temperature, and the cooling is to make the high-temperature barrel reduce the temperature. In the process of extruding plastics by the extruder, the heating and cooling devices on the barrel work alternately, so that the temperature of the barrel is constant within the process temperature range required for extruding plastics, which ensures the smooth progress of the normal molding production of extruded products by the extruder. The melting of plastic mainly depends on the heat conduction of the barrel, so the extruder must have sufficient heating device power. The heating method of the barrel can be resistance heating, electric induction heating or heating with a heat carrier. Heating control includes position control and proportional control. The position control is relatively simple. It is a switch control. It is always heated or cut off with full power, and the temperature fluctuates greatly. Proportional control automatically selects the heating power according to the difference between the actual temperature and the set temperature, so the thermal inertia is relatively small and the temperature fluctuation is small

the cooling method of the barrel can be air cooling and water cooling. The air cooling method is to use an electric fan to blow the parts of the barrel that need to be cooled, so that the cold air can take away part of the heat of the barrel, so as to achieve the purpose of cooling the barrel. The characteristic of air-cooled barrel is that the cooling speed of barrel is slow. The cooling speed of the barrel is faster when it is cooled by circulating water, but it is easy to scale and block when it is used for a long time. Therefore, if you want to use the water cooling method, you should (2) although the corrosion resistance of the glass tube is good, you should choose the treated softened water

2.3 transmission system

the transmission system of the extruder should provide the extruder with screw running power. In order to meet the process requirements, the power of the extruder should have the following requirements:

(1) the screw can have sufficient torque

(2) the screw can be started at a low speed and then adjusted to the required speed, and it should be in a constant torque state

(3) the operation is stable and the speed does not fluctuate

2.3.1 host

an extruder host is composed of three systems: extrusion, transmission, heating and cooling

the extrusion system is mainly composed of screw and barrel, which is the key part of the extruder

the function of the transmission system is to drive the plate driven screw used in the strengthening structure in the dry state. It is necessary to ensure that the screw has the required torque and speed in the working process

the heating and cooling system is mainly used to ensure the temperature control of materials and extrusion system in molding processing

2.3.2 auxiliary machine

the composition of the auxiliary machine of the extrusion equipment depends on the type of products. Generally speaking, the auxiliary machine is composed of agent penetrating and shaping device, cooling device, traction device, cutting device and electronic universal experimental machine for coiling or stacking products, which mainly adopts servo electromechanical as the power source

2.4 control system

the main function of the extruder control system is to control the screw speed, barrel temperature, melt pressure and other process parameters in the extrusion process. At present, instrument control system and PLC control system are the main choices. The functions of the two are as follows: in temperature control, the instrument control system can realize on-off control, and the intelligent instrument can also realize simple ratio control, while the PLC control system can realize PID (ratio integral differential control) control through analog communication; The former pressure control shows the melt pressure, while the latter shows the melt pressure and realizes closed-loop control; The test function of the former is only the display function, while the latter can realize the serial communication of the test unit. The control system of the extruder is mainly composed of electrical, instrument and actuator. Its main functions are:

(1) control the driving motors of the main and auxiliary machines, meet the speed and power required by the process requirements, and ensure the coordinated operation of the main and auxiliary machines

(2) control the temperature, pressure, flow of main and auxiliary machines and the quality of products

(3) realize the automatic control of the whole unit

(4) collect and process data to realize closed-loop control

2.5 main technical parameters of twin-screw extruder

1, nominal diameter of screw. The nominal diameter of the screw refers to the outer diameter of the screw, in mm. For variable diameter (or conical) screws, the screw diameter is a variable value, which is generally expressed by the minimum diameter and the maximum diameter, such as:

65/130. The larger the diameter of the twin-screw, the greater the processing capacity of the characterization machine

2. Length diameter ratio of screw. The length diameter ratio of the screw refers to the ratio of the effective length of the screw to the outer diameter. Generally, the length diameter ratio of integral twin-screw extruder is between. For the combined twin-screw extruder, the aspect ratio is variable. From the perspective of development, the aspect ratio has a trend of gradually increasing

3. Screw rotation. The rotation direction of the screw can be divided into the same direction and the opposite direction. Generally, the co rotating twin-screw extruder is mostly used for mixing, and the counter rotating extruder is mostly used for extruding products

4. Speed range of screw. The speed range of the screw refers to the range from the lowest speed to the highest speed (allowable value) of the screw. The twin-screw extruder rotating in the same direction can rotate at high speed, while the extruder rotating in the opposite direction generally has a speed of only r/min

5. Driving power. Driving power refers to the motor power of the driving screw, in kW

6. Output. Output refers to the extrusion volume of materials per hour, in kg/h

2.6 process principle of twin-screw extruder

2.6.1 control start sequence of twin-screw extruder

1 Turn on the power supply and adjust the temperature control instrument to a certain temperature for heating according to the process. After the temperature is reached, keep the temperature for 3 hours to soften the materials in the barrel, and then adjust the temperature control instrument to the temperature required by the process for control

2. When starting the main machine, pay attention to the current of the motor. If the current is too large, you can't adjust the speed at this time. Check whether the material in the barrel is softened or whether the extruder is faulty. Be sure to wait until it is normal, and adjust the speed to the speed required by the product for production

3. Start the auxiliary feeder, tractor and cutter

3. The working process of the extruder system

is: the set value in the touch screen is transmitted to the PLC through the communication line, and the current temperature on the barrel and die head is transmitted to the temperature control module through the thermocouple for processing, and then transmitted to the PLC by the temperature control module. When the temperature value is compared with the set value in the PLC, if the current value is lower than the set value, the PLC will send instructions to power on the corresponding thermal appliances in the PLC and power on the contactor. Since the contactor is closed and energized, the heater is energized and heated. During the heating process, the thermocouple continuously transmits the current temperature to the temperature control module, and the temperature control module also continuously transmits the value to the PLC for numerical comparison. After heating for a period of time, if the current temperature value is higher than the set value, PLC will send instructions to make the relays and contactors inside PLC lose power and do not work. Due to the disconnection of the contactor, the heater is powered off without power supply

Copyright © 2011 JIN SHI