The function of the transmission system is to drive the screw and supply the torque and rotation speed required by the screw in the extrusion process, which is usually composed of a motor, a reducer and a bearing. On the premise of basically the same structure, the manufacturing cost of the reducer is roughly proportional to its overall size and weight. Because of the large shape and weight of the reducer, it means that more materials are consumed and the bearings used are larger, which increases the manufacturing cost.
For extruders with the same screw diameter, high-speed and high-efficiency extruders consume more energy than conventional extruders, so it is necessary to double the motor power and correspondingly increase the number of reducer bases. But high screw speed means low reduction ratio. Compared with the reducer with large reduction ratio, the gear module of the reducer with small reduction ratio increases and the bearing capacity of the reducer increases. Therefore, the increase of reducer volume and weight is not linearly proportional to the increase of motor power. If the amount of extrusion is taken 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 and reducer weight of high-speed and high-efficiency extruder are small, which means that the manufacturing cost of high-speed and high-efficiency extruder is lower than that of ordinary extruder. Generally, granular materials are used for feeding, but belt materials or powder materials can also be used. Conical hopper is usually used in feeding equipment, and its volume is required to provide at least one hour's consumption. There is a cutoff device at the bottom of the hopper to adjust and cut off the flow of materials, and an observation hole and a calibration metering device are installed on the side of the hopper. Some hoppers may be equipped with decompression devices or heating devices to prevent raw materials from absorbing water from the air, or some barrels may be equipped with stirrers, which can automatically feed or feed materials.
1. box
Hoppers are usually made in a symmetrical form. There is a window on the side of the hopper to observe the material level and feeding situation, and there is a door at the bottom of the hopper to stop and adjust the feeding amount. Cover the hopper to prevent dust, moisture and impurities from falling in. When selecting hopper materials, it is best to use lightweight, corrosion-resistant and easy-to-process materials, generally using aluminum plates and stainless steel plates. The volume of the hopper depends on the size of the extruder and the feeding method. Generally, it is the extrusion capacity of extruder1~1.5h. ..
Step 2 feed
There are two feeding methods: manual feeding and automatic feeding. Automatic feeding mainly includes spring feeding, blast feeding, vacuum feeding and conveyor belt feeding. Generally speaking, small extruders use manual feeding and large extruders use automatic feeding.
3. Classification of feeding methods
① Gravity feeding:
Principle-Materials enter the barrel by their own weight, including manual feeding, spring feeding and blast feeding.
Features-simple structure and low cost. But it is easy to cause uneven feeding, thus affecting the quality of parts. It is only suitable for small extruders.
② Forced feeding:
Principle-Install a device that can exert external pressure on the material in the hopper to force the material into the barrel of the extruder.
Features-can overcome the phenomenon of "bridging" and make the feeding uniform. The feeding screw is driven by the extruder screw through the transmission chain, so its rotation speed is suitable for the screw rotation speed. The overload protection device can be started when the charging port is blocked to avoid damage to the charging device. Generally, it is a metal barrel made of alloy steel or composite steel pipe lined with alloy steel. Its basic characteristics are high temperature resistance, pressure resistance, firmness, wear resistance and corrosion resistance. Generally, the length of a barrel is 15~30 times of its diameter, and its length is based on the principle that materials are fully heated and plasticized uniformly. The barrel should have sufficient thickness and rigidity. The inside should be smooth, but some barrels are engraved with various grooves to increase the friction with plastic. An electric heater, an automatic temperature control device and a cooling system with resistors and inductors are attached outside the barrel.
1. There are three types of barrels in structure:
(1) integrated ink cartridge
Processing method-the whole material is processed.
Advantages-it is easy to ensure high manufacturing accuracy and assembly accuracy, which can simplify the assembly work, and the cylinder is evenly heated and has many uses.
Disadvantages-due to the long barrel, high processing requirements and strict requirements for processing equipment. It is difficult to repair the inner surface of the barrel after wear.
(2) comprehensive material list
Processing method-the cylinder is processed into several sections, and then the sections are connected by flanges or other forms.
Advantages-simple processing, easy to change the length-diameter ratio, mostly used to change the length-diameter ratio of screws.
Disadvantages-the machining accuracy is very high, because there are many sections, the coaxiality of each section is difficult to ensure, the flange connection destroys the heating uniformity of the cylinder, increases the heat loss, and it is difficult to set up and maintain the heating and cooling system.
(3) Bimetallic barrel
Processing method-insert or cast a layer of alloy steel material into the matrix of ordinary carbon steel or cast steel. It can not only meet the material requirements of gun barrel, but also save precious metal materials.
① Bushing body: The cylinder is equipped with replaceable alloy steel bushing. Precious metals are saved, the bushing can be replaced, and the service life of the cylinder is prolonged. But its design, manufacture and assembly are complicated.
(2) Casting barrel: centrifugally casting a layer of alloy with a thickness of about 2mm on the inner wall of the barrel, and then grinding to obtain the required inner diameter of the barrel. The alloy layer is well combined with the base of the barrel, evenly combined along the axial length of the barrel, and has no tendency of peeling and cracking. It also has excellent sliding performance, high wear resistance and long service life.
(4)IKV cylinder
1) The inner wall of the cylinder charging section is provided with a longitudinal groove.
In order to improve the solid conveying rate, according to the solid conveying theory, one method is to increase the friction coefficient of the barrel surface, and the other method is to increase the cross-sectional area of the material at the feed inlet perpendicular to the screw axis. The embodiment of these two methods is to open a longitudinal groove on the inner wall of the charging part of the injection cylinder, and gradually taper the inner wall of the injection cylinder part near the charging port.
2) Forced cooling of the barrel in the feeding section
There is another way to improve the solid conveying capacity. It cools the charging barrel in the charging section to keep the temperature of the conveyed material below the softening point or melting point, avoid the appearance of molten film and maintain the solid friction performance of the material.
After adopting the above method, the conveying efficiency is improved from 0.3 to 0.6, and the extrusion quantity is not sensitive to the change of die pressure. Screw is the heart and key component of extruder. The performance of screw determines the productivity, plasticizing quality, dispersion of additives, melt temperature and power consumption of extruder. It is the most important part of the extruder, which directly affects the application scope and production efficiency of the extruder. Only when the rotation of the screw exerts extreme pressure on the plastic, can the plastic move, pressurize and get some heat from friction in the barrel, and can the plastic be mixed and plasticized during the movement of the barrel. When the viscous melt is extruded and flows through the die, it can obtain the required shape and form. Screw, like barrel, is made of alloy with high strength, heat resistance and corrosion resistance.
Because there are many kinds of plastics, their properties are different. Therefore, in practice, in order to meet the needs of different plastic processing, different types of screws are needed, and their structures are also different. So as to maximize the transportation, extrusion, mixing and plasticization of plastics. The picture shows several common screws.
The basic parameters representing the characteristics of screw are as follows: diameter, length-diameter ratio, compression ratio, pitch, depth of screw groove, screw angle, clearance between screw and barrel, etc.
The most common screw diameter D is about 45 ~150 mm. With the increase of the screw diameter, the processing capacity of the extruder is also improved. The productivity of the extruder is directly proportional to the square of the screw diameter D. The ratio of the effective length of the screw to the diameter of the working part (L/D for short) is usually 18~25. Large L/D can improve the temperature distribution of materials, which is beneficial to the mixing and plasticization of plastics and reduce leakage and countercurrent. Improve the production capacity of extruder, and the screw with large length-diameter ratio has strong adaptability and can be used to extrude various plastics. But if the L/D is too large, the plastic part will be degraded due to the increase of heating time. At the same time, due to the increase of the weight of the screw, the free end will flex and sag, which is easy to cause scratches between the material and the screw, and it is difficult to manufacture and process. And the power consumption of the extruder increases. Too short screw is easy to cause poor plasticization during mixing.
Half of the difference between the inner diameter of the barrel and the diameter of the screw is called gap δ, which will affect the production capacity of the extruder. With the increase of δ, productivity decreases. Generally, it is appropriate to control the δ at about 0.1-0.6 mm. If the δ is small, the material will be subjected to greater shear, which is beneficial to plasticization. However, if δ is too small, strong shearing action will easily lead to thermo-mechanical degradation of the material, and at the same time, the screw will be held or rubbed against the barrel wall. Moreover, if δ is too small, the material will hardly leak and flow backwards, which will affect the mixing of melt to some extent.
The helix angle φ is the included angle between the thread and the cross section of the screw. With the increase of φ, the productivity of extruder increases, but the shearing effect and extrusion force on plastics decrease. Usually, the screw angle is between10 ~ 30, which varies along the length of the screw. Commonly used equidistant screw, the pitch is equal to the diameter. The value of φ is about 17 4 1'.
The greater the compression ratio, the greater the extrusion ratio of plastic. When the spiral groove is shallow, it can produce a higher shear rate to the plastic, which is beneficial to the heat transfer between the cylinder wall and the material. The higher the efficiency of material mixing and plasticizing, the lower the productivity. Conversely, when the spiral groove is deep. The situation is just the opposite. Therefore, heat-sensitive materials (such as PVC) should use deep spiral groove screws; For plastics with low melt viscosity and high thermal stability (such as polyamide), shallow spiral groove screws should be used.
1. Screw segment
When the material moves forward along the screw, it undergoes changes in temperature, pressure and viscosity, which are different throughout the length of the screw. According to the changing characteristics of materials, screws can be divided into feeding section, compression section and homogenization section.
(1), plastic and plastic.
There are two kinds of plastics: thermosetting plastics and thermoplastic plastics. Thermosetting plastics can't be heated and melted after molding and curing. Products formed from thermoplastics can be heated and melted to form other products.
With the change of temperature, thermoplastics change in three States: glassy state, high elastic state and viscous state, and with the repeated change of temperature, the three States change repeatedly.
A. Different characteristics of polymer melts in three states:
Glassy-plastic looks like a hard solid; Thermal motion energy is small, intermolecular force is large, and deformation is mainly contributed by bond angle deformation; The deformation instantly recovers after the external force is removed, which belongs to general elastic deformation.
High elastic state-plastic behaves as a rubber-like substance; The deformation is caused by the conformation stretching of macromolecules caused by segment orientation, and the deformation value is large; The deformation can be recovered after the external force is removed, but it is time-dependent and belongs to high elastic deformation.
Viscous flow-plastic appears as a highly viscous melt; Thermal energy further intensifies the relative sliding motion of chain molecules; Deformation is irreversible and belongs to plastic deformation.
B. Plastic processing and plastic three states:
Plastics can be processed in the glass state. In the state of high elasticity, it can be stretched, such as stretching textiles, pipe extrusion, blow molding and thermoforming. Viscous fluids can be coated, rotational molded and injection molded.
When the temperature is higher than the viscous fluid state, the plastic will decompose thermally, and when the temperature is lower than the glassy state, the plastic will become brittle. When the plastic temperature is higher than the viscous fluid state or lower than the glassy state trend, thermoplastics tend to deteriorate and destroy seriously, so these two temperature regions should be avoided when processing or using plastic products.
② Three-stage screw
There are three physical states of plastics in the extruder: glassy state, high elastic state and viscous state, and each state requires a different screw structure.
C. In order to meet the requirements of different states, the screw of extruder is usually divided into three sections:
Feeding section L 1 (also called solid conveying section)
L2 melting part (called compression part)
Homogenization section L3 (called metering section)
This is the so-called triple helix. The extrusion process of plastics in these three stages is different.
The function of the charging part is to send the materials supplied by the hopper to the compression part. Plastics usually remain solid during movement and partially melt due to heating. The length of the feeding section varies with the type of plastic, ranging from not far from the hopper to 75% of the total length of the spiral cup.
Generally speaking, extruded crystalline polymers are the longest, followed by hard amorphous polymers, and soft amorphous polymers are the shortest. Because the feeding section does not have to be compressed, the volume of its spiral groove can be kept constant, and the spiral angle has a great influence on the feeding capacity of this section, which actually affects the productivity of the extruder. Usually, the spiral angle of powdery materials is about 30 degrees, and the productivity is the highest. The helix angle of square material should be about 15 degrees, and that of spherical material should be about 17 degrees.
Main parameters of screw in feeding section:
The helix angle ψ is generally 17 ~ 20.
The screw groove depth H 1 is calculated by the geometric compression ratio ε of the screw after the screw groove depth in the homogenization section is determined.
The length of feed section L 1 is determined by empirical formula:
For amorphous polymer l 1 = (10% ~ 20%) l
For crystalline polymer L 1 = (60% ~ 65%) L
The function of compression section (migration section) is to compact the material, make the material change from solid state to molten state, and remove the air in the material; In order to adapt to the characteristics of pushing the gas in the material back to the feeding section when the material melts, compacting the material and reducing the volume, the screw in this section should have a great shearing and compression effect on the plastic. Therefore, the volume of spiral groove is usually gradually reduced, and the degree of reduction is determined by the compression ratio of plastic (product specific gravity/apparent specific gravity of plastic). The compression ratio is not only related to the compression ratio of plastic, but also related to the shape of plastic. Powder has a small specific gravity and more entrained air, which requires a larger compression ratio (up to 4~5), while granular materials are only 2.5~3.
The length of the compressed part is mainly related to the melting point and other characteristics of plastic. Plastics with a wide melting temperature range, such as PVC, begin to melt above 150℃, and the compression section is the longest, which can reach 100% of the total length of the screw (gradual type). Polyethylene with narrow melting temperature range (low density polyethylene 105~ 120℃, high density polyethylene 125 ~) for most polymers with narrow melting temperature range, such as polyamide, the compression section is even only one pitch long.
Main parameters of screw in melting section:
Compression ratio ε: generally refers to the geometric compression ratio, that is, the ratio of the volume of the first spiral groove in the screw feeding section to the volume of the last spiral groove in the homogenizing section.
ε=(Ds-h 1)h 1/(Ds-H3)≈h 1/H3
Where, h1-the depth of the first spiral groove in the feeding section.
H3—— the depth of the last spiral groove in the homogenization section.
The melting section length L2 is determined by empirical formula:
For amorphous polymers, L2 = 55% ~ 65% L.
For crystalline polymer L2 = (1 ~ 4) ds
The function of the homogenization section (metering section) is to send the molten material into the mold at a constant volume (quantity) and pressure, so that it can be formed in the mold. The volume of the spiral groove in the homogenization section is as constant as that in the feeding section. In order to avoid the decomposition caused by the material staying in the dead angle of the end face of the screw head, the screw head is always designed to be conical or semicircular; Some homogeneous sections of spiral sweat are rods with completely smooth surfaces, which are called torpedo heads, but some are engraved with grooves or milled into patterns. Torpedo head has the function of stirring and controlling the material, eliminating the pulsation (pulse) phenomenon when flowing. With the increase of material pressure, the thickness of material layer decreases, the heating conditions improve, and the plasticizing efficiency of screw can be further improved. This part can be 20-25% of the total length of the screw.
Important parameters of screw in homogenization section:
The depth H3 of the spiral groove is determined by the empirical formula H3 = (0.02 ~ 0.06) ds.
The length L3 is determined by the following formula: L3 = (20% ~ 25%) l.
D. According to the theory of melt transport, there are four forms of melt flow in the homogenization section of the screw, and the flow of molten material in the screw groove is the combination of these four flows:
Downstream-the plastic melt flows between the barrel and the screw along the screw groove towards the head.
Countercurrent-the flow direction is opposite to that of the downstream, which is caused by the pressure gradient caused by the resistance of the nose, perforated plate and filter plate.
Cross-flow-the melt flows in the direction perpendicular to the thread wall, which affects the mixing and heat exchange of the melt during extrusion.
Leakage-Backflow formed by the gap between the screw and the barrel along the axial direction of the screw due to pressure gradient.
2. The structure of ordinary screws
According to the change of thread lift and thread groove depth, the conventional full-thread three-stage screw can be divided into three forms:
(1) equidistant deepening screw
The change speed of equidistant variable depth spiral from the depth of spiral groove can be divided into two forms:
① equidistant gradual spiral: the depth from the feeding section to the last spiral groove in the homogenizing section gradually becomes shallow. In the longer melting section, the depth of the spiral groove gradually becomes shallow.
(2) Equal-axis abrupt change screw: that is, the screw groove depth of the feeding section and the homogenizing section is unchanged, and the screw groove depth of the melting section suddenly becomes shallow.
(2) constant depth variable pitch screw
Constant depth variable pitch screw means that the depth of the screw groove is constant, and the pitch gradually narrows from the first screw groove in the feeding section to the end of the homogenizing section.
The characteristics of constant depth variable pitch screw are equal depth of spiral groove, large cross-sectional area of screw at feed inlet and sufficient strength, which is beneficial to improve rotation speed and productivity. However, the screw is difficult to process, the melt backflow is large, and the homogenization effect is poor, so it is rarely used.
(3) variable depth and pitch screws
Variable depth and pitch screw refers to the screw whose groove depth and thread lift angle gradually change from the feeding section to the homogenizing end, that is, the thread lift gradually narrows from wide to shallow. This kind of screw has the characteristics of the first two kinds of screws, but it is difficult to process and is rarely used.
3. Screw material
Screw is the key component of extruder, and the material as screw must have the characteristics of high temperature resistance, wear resistance, corrosion resistance, high strength, good cutting performance, small residual stress after heat treatment and small thermal deformation.
For the material of extruder screw, there are the following specific requirements:
① High mechanical properties. It should have enough strength to adapt to the working conditions of high temperature and high pressure and improve the service life of the screw.
② Good machinability. It should have good machinability and heat treatment performance.
③ Good corrosion resistance and wear resistance.
(4) easy to obtain materials.
4. New screws
Problems existing in the traditional three-stage screw with full spiral ribs;
(1) In the melting section, the solid bed and the molten pool live together in the spiral groove, and the molten pool keeps widening and the solid bed gradually narrows, thus reducing the contact area between the solid bed and the cylinder wall, reducing the heat directly transferred from the cylinder wall to the solid bed, reducing the melting efficiency and leading to low extrusion;
② Pressure fluctuation, temperature fluctuation and output fluctuation are large;
③ It can't adapt to the processing of some special plastics, such as mixing and coloring.
Common solutions to such problems:
Increase the length-diameter ratio; Increase the screw speed; Increasing the depth of the spiral groove in the homogenization section;
In order to overcome the shortcomings of conventional screws, people have created some new screws, mainly including:
① Split screw
The auxiliary thread is added in the compression section, which overcomes the shortcoming that the solid bed and the melt are stored in one spiral groove in the traditional screw, so that the molten material and the unmelted material can be separated as soon as possible, thus promoting the melting of the unmelted material.
The screw has high plasticizing efficiency and good plasticizing quality. Because there is no disintegration of solid bed, the fluctuation of output, pressure and temperature is relatively small, and it has the advantages of good exhaust performance and low energy consumption, so it is widely used.
② Stop screw
A screw in which a blocking section is provided at a specific part of a common screw to prevent unmelted solids from passing through and promote solid melting.
Through the mixing action of shearing and vortex, this kind of screw converts mechanical energy into heat energy for heat exchange, which makes the material melt evenly, with small radial temperature difference, and the output and quality are better than those of the conventional screw.
③ Pin screw
When the material flow passes through the pin, the pin divides the solid material or the incompletely melted material into many fine material flows, and these material flows meet at a wide position between the two rows of pins. After many encounters and separations, the plasticizing quality of the material was improved.
The needles are arranged in the melting zone in herringbone and ring shape. Needles are cylindrical, diamond and square.
Because the pin divides and distributes the melt for many times, the mixing and homogenization of materials and the dispersion of additives are increased. In addition, because the solid chips continuously absorb heat from the melt during the melting process, the melt temperature can be reduced, so low-temperature extrusion can be obtained.
④ Combination screw
It consists of a screw body with a feeding section and various screw elements with different functions, such as conveying elements, mixing elements and shearing elements. By changing the type, quantity and combination order of these parts, screws with various characteristics can be obtained to meet the processing requirements of different materials and parts and find out the best working conditions.
This kind of screw has strong adaptability, easy to obtain the best working conditions, and solves the contradiction between universality and specialization to a certain extent, so it is widely used. However, the design is complicated, and the assembly and disassembly of the combined parts are troublesome, so it is difficult to realize it on a small diameter screw. Mold and die are usually a whole, which is commonly called die. However, there are also cases where the organic head and the die are separated. The function of the mold is to transform the rotating plastic melt into parallel linear motion, so that the plastic can be further plasticized evenly, the melt can be introduced into the mold evenly and smoothly, and the necessary molding pressure is given, so that the plastic can be easily molded and the obtained product is compact. The mold is a channel with a certain cross-sectional shape. When the plastic melt flows in the mold, it obtains the required shape and is cooled and hardened by the setting device and cooling system outside the mold. Die head and its components include filter screen, piercing wrench, diverter (sometimes it is combined with die core to form a part), die core, die head and neck.
The perforated plate in the mold can center and position the mold and the barrel, support the filter screen (filter insoluble impurities in the melt) and generate back pressure on the melt. There is also a correction and adjustment device (positioning screw) in the die head, which can adjust and correct the concentricity, size and shape of the die core and die. When producing pipes or blown films, compressed air can be introduced through the neck and the mold core. According to whether there is an angle between the material flow direction and the center line of the screw, the die can be divided into right-angle die (also known as T-head) and angle die (right angle or other angle). Right-angle dies are mainly used to extrude pipes, sheets and other profiles, while angle dies are mostly used to extrude films, cable coatings and blow-molded products.