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Twin Screw Extruder Main Components And Working Principle

Dec 15, 2017

Extruders have gained momentum since the advent of plastics, one of three major synthetic materials. It  is widely used in agriculture, building materials, packaging,  machinery, electronics, automobile, household appliances, petrochemical  and defense, extruding machine and people's daily life by using plastic  instead of steel, plastic forming non-ferrous metal and plastic molding  etc. Field, plastic is one of the most important raw materials for human activities. As extrusion molding is the most important form of plastic processing,  the development of plastic extrusion technology and equipment is of  great significance.

Twin-screw  extruder is an important equipment in plastic processing machinery. It  has been applied not only to the extrusion molding and mixing processing  of polymer materials, but also has widened its use to food, feed,  electrode, explosives, building materials, Packaging, pulp, ceramics and other fields. Extruder high-speed, high-yield, allows investors to lower input for greater output and high returns. However,  the high speed screw extruder speed has also brought a series of  difficulties need to be overcome: If the material in the screw to reduce  the residence time will lead to uneven plasticizing material mixing,  material over-shear may cause rapid heating material and heat Decomposition,  extrusion stability control difficulties will cause the extrudate  geometry to fluctuate, the related auxiliary devices and control system  accuracy must be increased, the screw and barrel wear increased need to  wear high wear-resistant material and ultra-high, reducer And bearings in the case of high-speed operation how to improve its life and other issues need to be resolved.

Extruder  working process of the electrical automation control also continues to  evolve, the traditional electrical control is used separately  stand-alone automation instruments to achieve, and now has developed  using the human-computer interface technology, computer technology,  inverter technology consisting of touch screen, PLC, Temperature control module, frequency control and other components of the electrical control system.

Extruder composition

Extruder mainly by the screw, barrel, heating and cooling system, transmission and control system components.

Screw and barrel

Screw  is the most important plastic machine equipment components, which is  directly related to plastics machine plasticizing effect and yield. Screw  in the barrel rotation work is under high temperature and high pressure  under the big twist, because it is in the rotation to promote strong  material forward, at the same time, it also has to withstand strong  friction and plastic decomposition of corrosive gas erosion, and  therefore Screw material must have high mechanical strength, to withstand the  huge torque and deformation under high temperature and pressure  conditions.

During  the rotation of the screw rod, the plastic is sheared and plasticized  mainly by the screw edges, and the plastic is pushed forward. As a  result, the screw edges are subject to enormous shearing stress and  frictional force. Due to the long-time working in harsh conditions, As the edge becomes smaller, the gap between the cylinder and the  cylinder increases. As a result, the amount of plastic extrusion  decreases. In severe cases, plastic reflow occurs, and the plasticizing  effect is reduced, resulting in serious decline of the crystal grains  and productivity.

The  process of melt extrusion is to pre-mix good material from the feed  port into the extruder barrel, the first paragraph of the barrel for the  feed section, the material will not melt at this stage, with the screw  drive, the material was brought into the second The  section is the compression section, the section is the heating stage,  the material begins to melt, the friction between the material  increases, forming a high-viscosity, continue with the screw drive into  the high shear of the third section for the homogenization section, it  is very effective separation of pigment aggregation Body, to achieve the purpose of full decentralization. At  present, the extruder used in powder coating equipment used in  twin-screw extruder, single-screw extruder and star screw extruder,  although the type of extruder, the internal structure varies, but the  design purpose Is consistent, that is, to maximize the dispersion of the material, so  the quality of the extruder directly determines the degree of  dispersion of the material.

The  working principle of the screw pump: Screw around its own axis while  rotating along the inner surface of the sleeve, forming a sealed  chamber. Each revolution of the screw, the liquid in the sealed cavity is  pushed forward by one pitch. As the screw rotates continuously, the  liquid spirally presses from one sealed cavity to the other sealed  cavity, and finally the pump body is extruded.

Heating the cooling system

Extruder  heating and cooling system is to ensure the normal operation of the  extruder, and maintain the extruder has a stable process temperature. Extruder  barrel heating is to make the barrel heated to a certain temperature,  cooling is to make high temperature barrel temperature down. Extruder  extrusion plastic production process, the barrel with heating and  cooling devices alternately work, so that the barrel temperature is  constant in a plastic extrusion process temperature range, to ensure the  normal extrusion of the extruder Products forming the smooth progress. The  melting of plastics relies mainly on the heat conduction of the barrel,  so the extruder must have sufficient heating device power. Barrel heating, resistance heating can be used, induction heating or heating medium heating. Heating control with a bit of control and proportional control. Bit control is relatively simple, is the switch control, always full power heating or cutting, temperature fluctuations. Proportional control is based on the actual temperature and set the  temperature difference to automatically select the heating power, so the  thermal inertia is relatively small, small temperature fluctuations.

Barrel  cooling can be air-cooled and water-cooled, air-cooled method is to use  electric fans to blow the cylinder cooling parts, so that the cold to  take away some of the cylinder heat, in order to achieve the purpose of  cooling the barrel. Air-cooled barrel is characterized by slow cooling barrel. Cooling water circulating cooling barrel faster, but easy to use for a  long time scale fouling, so if you want to use water-cooled way, you  should use the treated demineralized water.

Drive system

Extruder  drive system for the extruder to provide 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 enough torque; (2)  the screw can Start from low speed, and then transferred to the required speed, and  should be constant torque state; (3) smooth operation, speed does not  fluctuate.

Host

An extruder host by the extrusion, transmission, heating and cooling system composed of three parts. The extrusion system is mainly composed of screw and barrel, which is a key part of the extruder. Drive system is to drive the screw, to ensure that the screw in the work process with the required torque and speed. Heating and cooling system to ensure that the material and extrusion system in the molding process temperature control.

Auxiliary machine

The composition of the auxiliary equipment for extrusion equipment depends on the type of product. In general, the auxiliary machine by the dart stereotypes devices,  cooling devices, traction devices, cutting devices and products such as  coiling or stacking devices and other components.

Control System

Extruder  control system's main role is to achieve in the extrusion process on  the screw speed, barrel temperature and melt pressure control of process  parameters. At present, the instrument control system, PLC control system as the main choice. The  functions of the two are respectively as follows: the instrument  control system in the temperature control can realize the switch two  control, and the smart meter can be used to realize the simple ratio  control, while the PLC control system can realize PID  (ratio-integral-differential control) control through the analog  communication; The  former pressure control shows the melt pressure, while the latter shows  the melt pressure and closed-loop control; the former test function  only shows the function, while the latter can realize the serial  communication of the test unit. Extruder control system mainly by the electrical, instrumentation and implementing agencies, its main role is:

(1) Control the main motor and auxiliary motor drag motor to meet the  process requirements of the required speed and power, and to ensure that  the main and auxiliary functions can be coordinated operation.

(2) control the main, auxiliary engine temperature, pressure, flow and product quality.

(3) to achieve automatic control of the entire unit.

(4) Data collection and processing, to achieve closed-loop control.

The main technical parameters of twin-screw extruder:

(1) Nominal screw diameter. Screw nominal diameter refers to the screw diameter, in units of mm. For  variable diameter (or tapered) screw, the screw diameter is a variable,  usually with the minimum diameter and maximum diameter such as: 65/130.  The larger the diameter of the twin screw, the greater the machine's ability to process.

(2) aspect ratio of the screw. The aspect ratio of the screw is the ratio of the effective length of the screw to the outer diameter. General monobloc twin-screw extruder aspect ratio is between 7-18. For combined twin-screw extruders, the aspect ratio is variable. From the development perspective, the aspect ratio has gradually increased.

(3) screw the steering. Screw steering has the same direction and different points. Generally with the same direction of rotation of the twin-screw  extruder used for mixing, non-rotating extruder and more for extrusion  products.

(4) Screw speed range. Screw speed range refers to the minimum speed of the screw to the maximum speed (allowed value) between the range. Co-rotating twin-screw extruder can be high-speed rotation, non-rotating extruder generally only 0-40r / min speed.

(5) Drive power. Drive power is the power of the motor that drives the screw in kw.

(6) Production. Output refers to the amount of material extrusion per hour, in units of kg / h.

Twin-screw extruder process principle:

(1)  Turn on the power According to the process temperature control  instrument transferred to a certain temperature for heating, until the  temperature is reached, to keep the material in the barrel for 3 hours  to soften, and then the temperature control instrument transferred to  the process temperature control.

(2)  Turn on the host When turning on the host, pay attention to the current  of the motor. If the current is too large, you can not adjust the speed  at this moment. To check whether the material in the cartridge is  softened or if there is no fault on the extruder, Wait until normal, the speed will be transferred to the required speed of production products.

(3) open auxiliary feeder, traction machine, cutting machine.

Extruder system

The  working process is as follows: the set value in the touch screen is  transmitted to the PLC through the communication line, meanwhile, the  current temperature on the barrel and the die is transmitted to the  temperature control module for processing by the thermocouple and then  sent to the PLC by the temperature control module. When  the temperature value and the setting are planted in the PLC for  numerical comparison, if the current value is lower than the set value,  the PLC will issue a command to make the corresponding thermoelectric  device in the PLC work electrically and make the contactor work  electrically. Due to the contactor's closed energization, the heater is energized and heated. During  the heating process, the thermocouple continuously sends the current  temperature to the temperature control module, and the temperature  control module continuously sends the numerical value to the PLC for  numerical comparison. After  a period of heating, if the current temperature is higher than the set  value, PLC will issue a directive, making the PLC internal relay,  contactor power loss does not work. Due to the contactor is cut off, the heater is powered off without heating. On  the cooling fan, PLC at the same time issued a directive so that the  corresponding internal relay PLC work, the fan energized work. By fan cooling, the barrel of the current temperature is reduced. When  the barrel temperature reaches the set value, the PLC will issue a  command that the internal relay will lose power and will not work, and  the intermediate relay coil will lose power and will not work. As the middle of the relay off, the cooling fan to stop working power.

PID controller parameter tuning is the core of control system design. It  is based on the characteristics of the controlled process to determine  the PID controller proportional coefficient, integral time and  differential time size. PID controller parameter tuning many ways, summed up in two categories: First, the theoretical calculation of tuning. It  is mainly based on the mathematical model of the system, through  theoretical calculations to determine the controller parameters. Calculated  data obtained by this method may not be directly used, but also must be  adjusted and modified by engineering practice. Second,  the engineering tuning method, which relies mainly on engineering  experience, is directly carried out in the control system test, and the  method is simple, easy to grasp, is widely used in engineering practice.  PID controller  parameters of the engineering tuning methods, there are critical ratio  method, the reaction curve method and attenuation method. The  three methods have their own characteristics, the common ground is  through the test, and then adjust the controller parameters in  accordance with engineering experience formula. However,  no matter which method is adopted, the controller parameters need to be  adjusted and perfected in the actual operation. Now generally used is the critical ratio method. The  method of tuning the parameters of the PID controller is as follows:  (1) Firstly, select a sampling period short enough for the system to  work; (2) Only add the proportional control until the system has a  critical oscillation of the input step response, Make  a note of the ratio amplification factor and the critical oscillation  period at this time; (3) Calculate the parameters of the PID controller  through the formula under a certain degree of control.

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