Extruder is a type of plastic machinery, originated in the 18th century.

The extruder head can be divided into right-angle head and bevel head according to the direction of material flow in the head and the angle between the center line of the screw.

The screw extruder relies on the pressure and shear force generated by the rotation of the screw to fully plasticize and evenly mix the material and form it through the die. Plastic extruders can be basically classified into twin-screw extruders, single-screw extruders, and the less common multi-screw extruders and screwless extruders.

Development History

The extruder originated in the 18th century. The manual piston extruder for seamless lead pipes made by Joseph Bramah (England) in 1795 is considered to be the world’s first extruder. From then on, during the first 50 years of the 19th century, extruders were basically only used for the production of lead pipes, macaroni and other food processing, brick and ceramic industries. In the development process as a manufacturing method, the first clear record was the patent applied by R. Brooman in 1845 for the production of Gutebo rubber wires using an extruder. H. Bewlgy of Gutebo subsequently improved the extruder and used it to coat the copper wire of the first submarine cable between Dover and Calais in 1851. In 1879, the British M. Gray obtained the first patent for an Archimedean spiral screw extruder. In the following 25 years, the extrusion method gradually became important, and the previous manual extruders were quickly replaced by electric extruders. In 1935, German machinery manufacturer Paul Troestar produced an extruder for thermoplastics. In 1939 they developed the plastic extruder to its present stage, the modern single-screw extruder.

Mechanics

Principle of single screw extruder

A single screw is generally divided into three sections in terms of effective length. The effective length of the three sections is determined by the screw diameter, pitch and depth, and is generally divided into three sections with each section occupying one third.

The last thread of the material inlet is called the conveying section: the material here must not be plasticized, but must be preheated and pressed. In the past, the old extrusion theory believed that the material here was a loose body. Later, it was proved that the movement of the material here is similar to a solid piston. Therefore, as long as the conveying task is completed, it is its function.

The second section is called the compression section. At this time, the volume of the screw groove gradually decreases, and the temperature must reach the degree of plasticization of the material. The compression occurs here from the conveying section three, and is compressed to one here. This is called the compression ratio of the screw – 3:1. Some machines also have changes. The plasticized material enters the third section.

The third section is the metering section, where the material is kept at the plasticizing temperature. The melt is delivered accurately and quantitatively like a metering pump to supply the die. At this time, the temperature cannot be lower than the plasticizing temperature and is generally slightly higher.

Plastic Extruder Energy Saving

The energy saving of the extruder can be divided into two parts: one is the power part and the other is the heating part.

Energy saving in the power part: Most of them use frequency converters. The energy saving method is to save the surplus energy of the motor. For example, the actual power of the motor is 50Hz, but you only need 30Hz in production. The excess energy is wasted. The frequency converter changes the power output of the motor to achieve energy saving.

Energy saving in heating part: Energy saving in heating part mostly adopts electromagnetic heater, and the energy saving rate is about 30%~70% of the old resistance coil.

work process

The plastic material enters the extruder from the hopper and is transported forward by the rotation of the screw. During the forward movement, the material is heated by the barrel and subjected to shearing and compression by the screw, causing the material to melt, thus achieving a change between the glassy state, the highly elastic state and the viscous flow state.

Under pressure, the material in a viscous flow state passes through a die with a certain shape, and then becomes a continuum with a cross-section similar to the die shape. It is then cooled and shaped into a glassy state, thus obtaining the desired processed product.

Constitute

In general, the most basic and common extruder is the single-screw extruder, which mainly includes six parts: transmission, feeding device, barrel, screw, die head and die.

Transmission part

The transmission part is usually composed of a motor, a reduction gearbox and bearings. During the extrusion process, the screw speed must be stable and cannot change with the change of the screw load, so as to keep the quality of the obtained product uniform. However, in different occasions, the screw speed must be variable to meet the requirement that one device can extrude different plastics or different products. Therefore, this part generally uses AC commutator motors, DC motors and other devices to achieve stepless speed change, and the general screw speed is 10~100 rpm.

The function of the transmission system is to drive the screw and provide the torque and speed required by the screw during the extrusion process. It is usually composed of a motor, a reducer, and bearings. Under the premise of basically the same structure, the manufacturing cost of the reducer is roughly proportional to its size and weight. Because the reducer has a large size and weight, it means that more materials are consumed during manufacturing, and the bearings used are also relatively large, which increases the manufacturing cost.

For extruders with the same screw diameter, high-speed and high-efficiency extruders consume more energy than conventional extruders. The motor power is doubled, and the reducer frame size must be increased accordingly. However, high screw speed means low reduction ratio. For reducers of the same size, the gear module of a low reduction ratio is increased compared to a large reduction ratio, and the reducer’s ability to bear load is also increased. Therefore, the increase in the volume and weight of the reducer 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 has a smaller number, while the ordinary extruder has a larger number. In terms of unit output, the motor power of the high-speed and high-efficiency extruder is small and the reducer weight is small, which means that the unit output machine manufacturing cost of the high-speed and high-efficiency extruder is lower than that of the ordinary extruder.

Feeding device

The feeding material is usually granular, but it can also be in the form of strips or powders. The loading equipment usually uses a conical hopper, the volume of which is required to provide at least one hour’s supply. There is a cut-off device at the bottom of the hopper to adjust and cut off the material flow, and a sight hole and a calibrated metering device are installed on the side of the hopper. Some hoppers may also have a decompression device or a heating device to prevent the raw materials from absorbing moisture from the air, or some barrels may also have a stirrer that can automatically load or add materials.

1. Hopper

The hopper is generally made in a symmetrical form. There is a window on the side of the hopper to observe the material level and feeding situation. There is an opening and closing door at the bottom of the hopper to stop and adjust the feeding amount. A cover is added on the top of the hopper to prevent dust, moisture and impurities from falling in. When selecting the hopper material, it is best to use light, corrosion-resistant and easy-to-process materials, generally aluminum plates and stainless steel plates. The volume of the hopper depends on the size of the extruder and the feeding method. It is generally the extrusion volume of the extruder for 1 to 1.5 hours.

2. Loading

There are two ways of feeding: manual feeding and automatic feeding. Automatic feeding mainly includes spring feeding, air blast feeding, vacuum feeding, conveyor belt feeding, etc. Generally, small extruders use manual feeding, and large extruders use automatic feeding.

3. Classification of feeding methods

① Gravity feeding:

Principle: The material enters the barrel by its own weight, including manual loading, spring loading, and air blast loading.

Features: Simple structure, low cost. But it is easy to cause uneven feeding, thus affecting the quality of the product. It is only suitable for small-sized extruders.

② Mandatory feeding:

Principle – A device that can exert external pressure on the material is installed in the hopper to force the material into the extruder barrel.

Features: It can overcome the “bridging” phenomenon and make the feeding uniform. The feeding screw is driven by the extruder screw through the transmission chain, so that its speed is adapted to the screw speed. The overload protection device can be activated when the feeding port is blocked, thus avoiding damage to the feeding device.

Barrel

It is usually a metal barrel, made of alloy steel or composite steel pipe lined with alloy steel. Its basic characteristics are high heat and pressure resistance, strong wear resistance and corrosion resistance. Generally, the length of the barrel is 15 to 30 times its diameter, and its length is based on the principle that the material can be fully heated and plasticized evenly. 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 the plastic. The outside of the barrel is equipped with electric heaters, temperature automatic control devices and cooling systems for heating by resistance, inductance and other methods.

1. There are three types of barrel structures:

(1) Integral barrel

Processing method: Processed on the whole material.

Advantages: It is easy to ensure high manufacturing accuracy and assembly accuracy, which can simplify the assembly work, the barrel is heated evenly, and it has many applications.

Disadvantages: Due to the large length of the barrel, the processing requirements are high and the requirements for processing equipment are also very strict. The inner surface of the barrel is difficult to repair after wear.

(2) Combined material

Processing method: The barrel is processed into several sections, and then the sections are connected with flanges or other forms.

Advantages: Simple processing, easy to change the aspect ratio, mostly used in situations where the aspect ratio of the screw needs to be changed.

Disadvantages: High processing accuracy is required. Due to the large number of sections, it is difficult to ensure the coaxiality of each section. The flange connection destroys the uniformity of barrel heating, increases heat loss, and the setting and maintenance of the heating and cooling system are also difficult.

(3) Bimetallic barrel

Processing method: insert or cast a layer of alloy steel material inside the matrix of ordinary carbon steel or cast steel. It can not only meet the material requirements of the barrel, but also save precious metal materials.

① Bushing type barrel: The barrel is equipped with a replaceable alloy steel bushing. It saves precious metals, and the bushing is replaceable, which increases the service life of the barrel. However, its design, manufacturing and assembly are relatively complex.

② Casting barrel: A layer of alloy about 2mm thick is centrifugally cast on the inner wall of the barrel, and then the required inner diameter of the barrel is obtained by grinding. The alloy layer is well combined with the base of the barrel, and the combination along the axial length of the barrel is relatively uniform, with no tendency to peel off and cracking, and it has sliding performance, high wear resistance and long service life.

(4) IKV barrel

1. Longitudinal grooves are opened on the inner wall of the barrel feeding section

In order to improve the solid conveying rate, solid conveying theory knows that one method is to increase the friction coefficient of the barrel surface, and another method is to increase the area of ​​the cross section perpendicular to the screw axis at the feeding port. The two methods are concretely implemented by opening longitudinal grooves on the inner wall of the barrel feeding section and making a section of the barrel inner wall near the feeding port into a cone.

2 Forced cooling of the feeding section barrel

In order to increase the solid conveying capacity, there is another method, which is to cool the feeding section barrel, the purpose is to keep the temperature of the conveyed material below the softening point or melting point, avoid the formation of melt film, and maintain the solid friction properties of the material.

After adopting the above method, the conveying efficiency is increased from 0.3 to 0.6, and the extrusion volume is less sensitive to the change of the die head pressure.

Screw

The screw is the heart of the extruder and a key component of the extruder. The performance of the screw determines the productivity, plasticizing quality, dispersion of fillers, melt temperature, power consumption, etc. of an extruder. It is the most important component of the extruder and can directly affect the application range and production efficiency of the extruder. The rotation of the screw exerts extreme pressure on the plastic, so that the plastic can move, increase pressure, and obtain some heat from friction in the barrel. The plastic is mixed and plasticized during the movement in the barrel. When the viscous melt is extruded and flows through the die, it obtains the desired shape and is formed. Like the barrel, the screw is also made of high-strength, heat-resistant and corrosion-resistant alloys.

Since there are many types of plastics, their properties are also different. Therefore, in actual operation, in order to meet the different plastic processing needs, different types of screws are required, and the structures are also different. In order to maximize the efficiency of plastic transportation, extrusion, mixing and plasticization.

The basic parameters that represent the characteristics of the screw include the following: diameter, aspect ratio, compression ratio, pitch, screw groove depth, helix angle, clearance between the screw and the barrel, etc.

The most common screw diameter D is about 45~150 mm. As the screw diameter increases, the processing capacity of the extruder also increases accordingly, and the productivity of the extruder is proportional to the square of the screw diameter D. The ratio of the effective length of the working part of the screw to the diameter (abbreviated as aspect ratio, expressed as L/D) is usually 18~25. A large L/D can improve the temperature distribution of the material, facilitate the mixing and plasticization of plastics, and reduce leakage and backflow. Improve the production capacity of the extruder. The screw with a large L/D has strong adaptability and can be used for the extrusion of a variety of plastics; but when the L/D is too large, the plastic will be heated for a longer time and degraded. At the same time, due to the increase in the weight of the screw, the free end will bend and sag, which is easy to cause scratches between the material cylinder and the screw, and make manufacturing and processing difficult; increase the power consumption of the extruder. A screw that is too short is likely to cause poor plasticization of mixing.

Half the difference between the inner diameter of the barrel and the diameter of the screw is called the gap δ, which can affect the production capacity of the extruder. As δ increases, the productivity decreases. It is usually appropriate to control δ at about 0.1 to 0.6 mm. When δ is small, the material is subjected to a greater shearing effect, which is conducive to plasticization. However, when δ is too small, the strong shearing effect is likely to cause the material to undergo thermomechanical degradation, and the screw is likely to be held or rubbed against the barrel wall. Moreover, when δ is too small, there is almost no leakage and backflow of the material, which affects the mixing of the melt to a certain extent.

The helix angle Φ is the angle between the thread and the cross section of the screw. As Φ increases, the production capacity of the extruder increases, but the shearing effect and extrusion force on the plastic decrease. Usually the helix angle is between 10° and 30°, and changes along the direction of the screw length. Equidistant screws are often used, with the pitch equal to the diameter. The value of Φ is about 17°41′

The greater the compression ratio, the greater the extrusion ratio of the plastic. When the screw groove is shallow, it can produce a higher shear rate on the plastic, which is conducive to heat transfer between the barrel wall and the material. The higher the material mixing and plasticizing efficiency, the lower the productivity; on the contrary, when the screw groove is deep, the situation is just the opposite. Therefore, heat-sensitive materials (such as polyvinyl chloride ) are suitable for deep screw grooves; while plastics with low melt viscosity and high thermal stability (such as polyamide ) are suitable for shallow screw grooves.

1. Screw segmentation

When the material moves forward along the screw, it experiences changes in temperature, pressure, viscosity, etc. Such changes are different over the entire length of the screw. According to the changing characteristics of the material, the screw can be divided into the feeding (delivery) section, compression section and homogenization section.

①. Plastics and their three states

Plastics are divided into two categories: thermosetting and thermoplastic. After thermosetting plastics are solidified, they cannot be heated and melted to form other products. However, thermoplastic products can be heated and melted to form other products.

As the temperature changes, thermoplastics undergo changes into three states: glassy state, highly elastic state and viscous flow state. As the temperature changes repeatedly, the three states undergo repeated changes.

a. Different characteristics of polymer melts in the three states:

Glassy state – plastic appears as a rigid solid; the thermal kinetic energy is small, the intermolecular force is large, and the deformation is mainly contributed by the deformation of the bond angle ; the deformation recovers instantly after the external force is removed, which is a general elastic deformation.

Highly elastic state – the plastic appears to be a rubber- like substance; the deformation is contributed by the stretching of the macromolecular conformation caused by the chain segment orientation, and the deformation value is large; after the external force is removed, the deformation can be recovered but is time-dependent, which is a highly elastic deformation.

Viscous flow state – the plastic is a highly viscous melt; the heat energy further stimulates the relative slip motion of the chain molecules; the deformation is irreversible and belongs to plastic deformation

b. Plastic processing and three states of plastics:

Plastics can be cut in the glassy state. They can be stretched in the highly elastic state, such as wire drawing, tube extrusion, blow molding and thermoforming. They can be coated, rotomolded and injected in the viscous state.

When the temperature is higher than the viscous flow state, the plastic will undergo thermal decomposition, and when the temperature is lower than the glassy state, the plastic will become brittle. When the plastic temperature is higher than the viscous flow state or lower than the glassy state, the thermoplastic tends to deteriorate and be damaged seriously, so these two temperature ranges should be avoided when processing or using plastic products.

②. three-stage screw

Plastics exist in three physical states in the extruder: glassy state, highly elastic state and viscous flow state. Each state has different requirements for the screw structure.

c. In order to meet the requirements of different states, the screw of the extruder is usually divided into three sections:

Feeding section L1 (also called solid conveying section)

Melting section L2 (called compression section)

Homogenization section L3 (weighing and measuring section)

This is what is commonly called a three-stage screw. The extrusion process of plastic in these three stages is different.

The function of the feeding section is to send the material supplied by the hopper to the compression section. The plastic generally remains in a solid state during the movement and partially melts due to heat. The length of the feeding section varies with the type of plastic and can start from not far from the hopper to 75% of the total length of the screw cup.

Generally speaking, the extrusion of crystalline polymer is the longest, followed by hard amorphous polymer, and the shortest for soft amorphous polymer. Since the feeding section does not necessarily produce compression, the volume of its screw groove can remain unchanged. The size of the helix angle has a greater impact on the feeding capacity of this section, and actually affects the productivity of the extruder. Usually, the helix angle of powdery materials is about 30 degrees, which has the highest productivity. The helix angle of block materials should be about 15 degrees, and the helix angle of spherical materials should be about 17 degrees.

The main parameters of the screw in the 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 homogenizing section is determined.

The length of the feeding section L1 is determined by the empirical formula :

For amorphous polymers, L 1 = (10% to 20%) L

For crystalline polymers, L 1 = (60% to 65%) L

The function of the compression section (migration section) is to compact the material, convert the material from solid to molten, 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, compacting the material and reducing the volume when the material melts, the screw in this section should produce a greater shearing effect and compression on the plastic. To this end, the screw groove volume is usually gradually reduced, and the degree of reduction is determined by the compression rate of the plastic (specific gravity of the product/apparent specific gravity of the plastic). The compression ratio is related to the morphology of the plastic in addition to the compression rate of the plastic. The powder has a small specific gravity and carries more air, so a larger compression ratio (up to 4~5) is required, while the granular material is only 2.5~3.

The length of the compression section is mainly related to the melting point and other properties of the plastic. For plastics with a wide melting temperature range, such as polyvinyl chloride, which begins to melt above 150°C, the compression section is the longest, which can reach 100% of the total length of the screw (gradient type). For polyethylene with a narrow melting temperature range ( low-density polyethylene 105~120°C, high-density polyethylene 125~135°C), the compression section is 45~50% of the total length of the screw; for most polymers with a very narrow melting temperature range, such as polyamide, the compression section may even be only one pitch long.

Main parameters of the melting section screw:

Compression ratio ε: generally refers to the geometric compression ratio, which is the ratio of the volume of the first screw groove in the feeding section of the screw to the volume of the last screw groove in the homogenizing section.

ε=(D s -H 1 )H1/(D s -H 3 )≈H 1 /H 3

Where, H1 is the depth of the first screw groove in the feeding section.

H3 – the depth of the last groove in the homogenization section

The melting section length L2 is determined by the empirical formula:

For non-crystalline polymers, L2 = 55% to 65% L

For crystalline polymers L 2 = (1 to 4) Ds

The function of the homogenization section (metering section) is to feed the molten material into the die head at a constant volume (quantity) and pressure to form it in the die. The screw groove volume of the homogenization section is constant as that of the feeding section. In order to avoid the material from being trapped in the dead corner of the end face of the screw head and causing decomposition, the screw head is often designed to be conical or semicircular ; the homogenization section of some screws is a rod with a completely smooth surface called a torpedo head, but there are also grooves or milled patterns. The torpedo head has the function of stirring and controlling the material, eliminating the pulsation (pulse) phenomenon during flow, and with the increase of the material pressure, the thickness of the material layer is reduced, the heating condition is improved, and the plasticizing efficiency of the screw is further improved. This section can be 20-25% of the total length of the screw.

Important parameters of the homogenizing section screw:

The screw groove depth H3 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 melt conveying theory, there are four forms of melt flow in the screw homogenization section, and the flow of molten material in the screw groove is a combination of these four flows:

Positive flow – the plastic melt flows between the barrel and the screw along the direction of the screw groove toward the die head.

Backflow – the flow direction is opposite to the forward flow, caused by the pressure gradient caused by the resistance of the machine head, porous plate, filter plate, etc.

Cross flow – the flow of the melt in a direction perpendicular to the thread wall, which affects the mixing and heat exchange of the melt during extrusion.

Leakage flow – backflow formed in the gap between the screw and the barrel due to the pressure gradient, along the axial direction of the screw.

2. Structure of ordinary screw

Conventional full-thread three-stage screws can be divided into three types according to the changes in their thread lift and screw groove depth:

(1) Equidistant variable depth screw

The speed of the change in the depth of the screw groove of the equidistant variable depth screw can be divided into two forms:

① Equidistant Gradient Screw: The depth of the last screw groove from the feeding section to the homogenizing section gradually becomes shallower. In the longer melting section, the depth of the screw groove gradually becomes shallower.

② Equidistant sudden change screw: the screw groove depth in the feeding section and homogenization section remains unchanged, and the screw groove depth in the melting section suddenly becomes shallower

(2) Constant depth variable pitch screw

An equal-depth variable-pitch screw means that the depth of the screw groove remains unchanged, and the pitch gradually becomes narrower from the first screw groove in the feeding section to the end of the homogenizing section.

The characteristic of the constant depth variable pitch screw is that due to the constant depth of the screw groove, the cross-sectional area of ​​the screw at the feeding port is larger, and it has sufficient strength, which is conducive to increasing the speed and thus improving 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 variable pitch screw

The variable depth and variable pitch screw refers to a screw whose screw groove depth and thread lead angle gradually change from the feeding section to the homogenization end, that is, the thread lead gradually narrows from wide to narrow, and the screw groove depth gradually shallows from deep to shallow. This screw has the characteristics of the previous two screws, but it is more difficult to machine and is rarely used.

3. Screw material

The screw is a key component of the extruder. The material of the screw must have the characteristics of high temperature resistance, wear resistance, corrosion resistance, high strength, etc. It should also have the characteristics of good cutting performance, small residual stress after heat treatment, and small thermal deformation.

There are several specific requirements for the material of the extruder screw:

①　 High mechanical properties. It must have sufficient strength to adapt to high temperature and high pressure working conditions and increase the service life of the screw.

② Good machining performance. It should have good cutting performance and heat treatment performance.

③ Good corrosion resistance and wear resistance.

④ It is easy to obtain materials.

4. New screw

Problems with conventional full-screw three-stage screws:

① In the melting section, the solid bed and the molten pool coexist in the same screw groove. The molten pool continues to widen and the solid bed gradually narrows, thereby reducing the contact area between the solid bed and the barrel wall, reducing the heat directly transferred from the barrel wall to the solid bed, reducing the melting efficiency, and resulting in low extrusion volume;

② Large pressure fluctuations, temperature fluctuations and output fluctuations;

③ It cannot adapt well to the processing of some special plastics for mixing, coloring and other processes.

Common solutions to this kind of problems :

Increase the aspect ratio; increase the screw speed; increase the screw groove depth in the homogenizing section;

In order to overcome the shortcomings of conventional screws, people have created some new screws, mainly including:

①Separation type screw

Adding a secondary thread in the compression section overcomes the disadvantage of the solid bed and the melt coexisting in the same screw groove in the conventional screw, separates the molten material from the unmelted material as early as possible, and thus promotes the melting of the unmelted material.

This type of screw has high plasticizing efficiency and good plasticizing quality. Since there is no solid bed disintegration, the output fluctuation, pressure fluctuation and temperature fluctuation are relatively small. It also has the advantages of good exhaust performance and low energy consumption, and is widely used.

②Barrier screw

A screw in which a barrier section is set at a certain position of an ordinary screw to prevent unmelted solids from passing through and to promote the melting of solids.

This type of screw converts mechanical energy into thermal energy and performs heat exchange through shearing and eddy current mixing, so that the material is melted and homogenized, and the radial temperature difference is small. The output and quality are better than those of conventional screws.

③ Pin screw

When the material flow passes through the pins, the pins divide the solid material or the material that is not completely melted into many small material flows. These material flows merge again at the wider position between the two rows of pins. After multiple merging and separation, the plasticization quality of the material is improved.

The pins are arranged in the melting zone, and the arrangement shapes include herringbone, ring, etc. The pin shapes include cylindrical, diamond, square, etc.

Since the pins divide and divert the molten material multiple times, the mixing, homogenization and dispersion of the materials and additives are increased. In addition, since the solid fragments continuously absorb heat from the melt during the melting process, it is possible to reduce the melt temperature, so low-temperature extrusion can be achieved.

④Combination 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 elements, screws with various characteristics can be obtained to meet the processing requirements of different materials and different parts, and to find the best working conditions.

This type of screw has strong adaptability and is easy to obtain the best working conditions. It solves the contradiction between universality and special use to a certain extent, so it is increasingly widely used. However, the design is complex, and the assembly and disassembly of the combined components is troublesome, and it is difficult to implement it on a screw with a smaller diameter.

Die head and die

The die and the mouth die are usually a whole, and are usually collectively referred to as the die; but there are also cases where the die and the mouth die are separated. The function of the die is to convert the plastic melt in rotation into parallel linear motion, so that the plastic is further plasticized and uniform, and the melt is uniformly and smoothly introduced into the mouth die, and the necessary molding pressure is also applied to make the plastic easy to mold and the resulting product dense. The mouth die is a channel with a certain cross-sectional shape. The plastic melt obtains the desired shape when flowing in the mouth die, and is cooled and hardened by the shaping device and cooling system outside the mouth die to form. The components of the die and the mouth die include filter screen, porous plate, diverter (sometimes it is combined with the core into one component), core, mouth die and machine neck.

The porous plate in the die head can align the die head and the barrel, support the filter (to filter out unmelted impurities in the melt) and generate back pressure on the melt. There are also correction and adjustment devices ( positioning screws ) in the die head, which can adjust and correct the concentricity, size and shape of the core and the die. When producing pipes or blown films, compressed air can be introduced through the machine neck and the core. According to whether there is an angle between the direction of material flow and the center line of the screw, the die head can be divided into a right-angle die head (also called a T-type die head) and an angle die head (right angle or other angles). The right-angle die head is mainly used for extruding tubes, sheets and other profiles, while the angle die head is mostly used for extruding films, cable wrappings and blow-molded products.

Auxiliary Equipment

The auxiliary machines of plastic extruders mainly include pay-off device, straightening device, preheating device, cooling device, traction device, meter counter, spark tester, and take-up device. The auxiliary equipment used for the extruders varies with their uses, such as cutters, dryers, printing devices, etc.

Straightening device

The most common type of plastic extrusion waste is eccentricity, and various types of bending of the core are one of the important reasons for insulation eccentricity. In sheath extrusion, scratches on the sheath surface are often caused by the bending of the cable core. Therefore, straightening devices in various extrusion units are indispensable. The main types of straightening devices are: drum type (divided into horizontal and vertical types); pulley type (divided into single pulley and pulley group ); winch type, which has multiple functions such as dragging, straightening, and stabilizing tension; pressure wheel type (divided into horizontal and vertical types), etc.

Preheating device

Cable core preheating is necessary for both insulation extrusion and sheath extrusion. For the insulation layer, especially the thin layer insulation, the existence of pores is not allowed. The surface moisture and oil can be completely removed by high temperature preheating of the core before extrusion. For sheath extrusion, its main function is to dry the cable core to prevent the possibility of pores in the sheath due to moisture (or moisture in the wrapping cushion layer ). Preheating can also prevent the residual internal pressure of the plastic due to sudden cooling during extrusion. In the process of extruding plastic, preheating can eliminate the large temperature difference formed when the cold wire enters the high-temperature die and contacts the plastic at the die mouth, avoid the fluctuation of the extrusion pressure caused by the fluctuation of the plastic temperature, thereby stabilizing the extrusion volume and ensuring the extrusion quality. Electric heating core preheating devices are used in extrusion units, which are required to have sufficient capacity and ensure rapid heating, so that the core preheating and cable core drying efficiency are high. The preheating temperature is restricted by the pay-off speed, and is generally similar to the die temperature.

Cooling device

After the formed plastic extrusion layer leaves the die, it should be cooled and shaped immediately, otherwise it will deform under the action of gravity. The cooling method usually adopts water cooling, and it is divided into rapid cooling and slow cooling according to the water temperature. Rapid cooling is direct cooling with cold water. Rapid cooling is beneficial to the shaping of the plastic extrusion layer, but for crystalline polymers, due to the sudden cooling, it is easy to leave internal stress in the extrusion layer structure, resulting in cracking during use. Generally, PVC plastic layer adopts rapid cooling. Slow cooling is to reduce the internal stress of the product. Water of different temperatures is placed in the cooling water tank in sections to gradually cool down the product and shape it. Slow cooling is adopted for the extrusion of PE and PP, that is, it is cooled through three stages of hot water, warm water and cold water.

Classification

Plastic extruders are divided into twin-screw extruders and single-screw extruders

The difference between the two extruders:

Single screw machine and twin screw machine: one is one screw, the other is two screws. Both are driven by one motor. The power varies with the screw. The power of 50 cone twin is about 20kW, and that of 65 is about 37kW. The output depends on the material and the screw. The output of 50 cone twin is about 100-150 kg/h, and that of 65 cone twin is about 200-280kg/h. The output of single screw is only half.

Extruders can be divided into single-screw, twin-screw and multi-screw extruders according to the number of screws. Today, single-screw extruders are the most widely used and suitable for extrusion processing of general materials. Twin-screw extruders have the characteristics of less heat generated by friction, more uniform shearing of materials, larger conveying capacity of screws, more stable extrusion volume, longer retention of materials in the barrel, and uniform mixing.

Single screw extruders play an important role as both plasticizing and granulating machines and molding and processing machines. In recent years, single screw extruders have made great progress. The large single screw extruder for granulation produced in Germany has a screw diameter of 700mm and an output of 36t/h.

The main sign of the development of single screw extruder lies in the development of its key component – the screw. In recent years, people have conducted a lot of theoretical and experimental research on screws. There are nearly 100 types of screws now, the most common ones are separation type, shear type, barrier type, split type and corrugated type.

From the perspective of single screw development, although single screw extruders are relatively complete, with the continuous development of polymer materials and plastic products, more distinctive new screws and special single screw extruders will emerge. In general, single screw extruders are developing towards high speed, high efficiency and specialization.

Twin-screw extruders have good feeding characteristics and are suitable for powder processing. They have better mixing, exhaust, reaction and self-cleaning functions than single-screw extruders. They are more superior when processing plastics and blends with poor thermal stability. In recent years, the twin-screw extruders have made great progress. Various forms of twin-screw extruders have been serialized and commercialized, and there are many manufacturers. They are roughly classified as follows:

⑴ According to the relative position of the two axes, there are parallel and conical;

⑵ According to the meshing process of the two screws, there are meshing type and non-meshing type;

⑶ According to the rotation direction of the two screws, there are same direction and different direction, and the different direction can be divided into inward and outward;

⑷ According to the screw rotation speed, there are high speed and low speed;

⑸ According to the structure of the screw and barrel, there are integral and combined types.

Based on the twin-screw extruder, some manufacturers have developed multi-screw extruders such as planetary extruders to make it easier to process blends with poor thermal stability.

Operation Points

When making different plastic products, the operating points of the extruder are different, but there are also similarities. The following briefly introduces the common operating steps and the operating points of the extruder that should be paid attention to when extruding various products.

1. Preparation before start

⑴ For plastics used for extrusion molding, the raw materials should meet the required drying requirements and further drying is required if necessary. The raw materials should be sieved to remove agglomerates and mechanical impurities.

⑵ Check whether the water, electricity and gas systems in the equipment are normal, ensure that the water and gas lines are unobstructed and leak-free, whether the electrical system is normal, whether the heating system, temperature control and various instruments are working reliably; run the auxiliary machine at low speed and empty to observe whether the equipment is running normally; start the vacuum pump of the calibrating table to observe whether it is working normally; add oil and lubricate the smooth parts of various equipment. If any fault is found, eliminate it in time.

⑶ Install the machine head and the shaping sleeve. Select the machine head specifications according to the product type and size. Install the machine head in the following order.

2. Start

⑴ After the temperature is constant, the machine can be started. Before starting, the bolts of the die head and the extruder flange should be tightened again to eliminate the difference in thermal expansion between the bolts and the die head. The order of tightening the die head bolts is to tighten them diagonally and apply force evenly. When tightening the die head flange nut, the tightness should be consistent around the edges, otherwise the material will escape.

⑵ To start the machine, first press the “Ready to Start” button, then the “Start” button, and then slowly rotate the screw speed adjustment knob to start the screw speed slowly. Then gradually speed up and add a small amount of material at the same time. When adding materials, pay close attention to the changes in the indications of the main machine ammeter and various indicator heads. The screw torque cannot exceed the red mark (generally 65%~75% of the torque meter ). Before the plastic profile is extruded, no one may stand in front of the die to prevent injuries caused by bolt breakage or bubbling due to moisture in the raw material. After the plastic is extruded from the die of the die head, the extrudate needs to be slowly cooled and led to the traction device and the shaping die, and these devices are started. Then, according to the indication value of the control instrument (see Figure 1) and the requirements for the extruded product. Make corresponding adjustments to each part to make the entire extrusion operation reach a normal state. And add enough material as needed. The twin-screw extruder uses a metering feeder to feed evenly and at a constant speed.

⑶ When the material is discharged from the die evenly and well plasticized, it can be pulled into the shaping sleeve. The degree of plasticization needs to be judged by experience. Generally, it can be judged according to the appearance of the extruded material, that is, the surface is glossy, free of impurities, foaming, charring and discoloration. The extruded material is squeezed by hand to a certain degree without burrs or cracks, and has a certain elasticity. At this time, it means that the material is well plasticized. If the plasticization is not good, the screw speed, barrel and head temperature can be appropriately adjusted until the requirements are met.

⑷ During the extrusion production process, various process parameters should be checked regularly according to process requirements to see if they are normal, and the process record sheet should be filled in. The quality of profile products should be checked according to quality inspection standards, and solutions should be taken in a timely manner if problems are found.

3. Halt

⑴Stop adding materials, squeeze out all the plastic in the extruder, and when the screw is exposed, turn off the power of the barrel and die, and stop heating.

⑵ Turn off the power supply of the extruder and auxiliary machines to stop the screw and auxiliary machines.

⑶Open the connecting flange of the machine head and disassemble the machine head. Clean the porous plate and various parts of the machine head. To prevent damage to the inner surface of the machine head, the residual material in the machine head should be cleaned with steel and steel sheets, and then the plastic adhering to the machine head should be removed with sandpaper, polished, and coated with engine oil or silicone oil to prevent rust.

⑷ Cleaning of the screw and barrel: After removing the die head, restart the main machine, add the parking material (or crushed material), clean the screw and barrel. At this time, the screw should be operated at a low speed ( about sr /min) to reduce wear. After the parking material is crushed into powder and completely extruded, the residual granular material and powder can be repeatedly blown out from the feeding port and the exhaust port with compressed air until there is no residual material in the barrel. Then reduce the screw speed to zero, stop the extruder, and turn off the main power supply and the main cold water valve.

5. Safety issues that should be paid attention to during extrusion include: electrical, thermal, mechanical rotation and loading and unloading of heavy parts. The extruder workshop must be equipped with lifting equipment to install and disassemble heavy parts such as the die head and screw to ensure safe production.

Maintenance

The screw extrusion system is maintained in two ways: daily maintenance and regular maintenance:

⑴ Daily maintenance is a regular routine work, which does not occupy the equipment operation time and is usually completed during the start-up period. The focus is on cleaning the machine, lubricating the moving parts, tightening the loose threaded parts, and timely checking and adjusting the motor, control instrument, various working parts and pipelines, etc.

(2) Regular maintenance is usually carried out after the extruder has been running continuously for 2500-5000 hours and then stopped. The machine needs to be disassembled to inspect, measure and identify the wear of the main components, replace the parts that have reached the prescribed wear limit, and repair the damaged parts.

⑶ Idle operation is not allowed to avoid roughening of the screw and barrel.

⑷ If abnormal noise occurs during the operation of the extruder, stop the machine immediately for inspection or repair.

⑸ Prevent metal or other debris from falling into the hopper to avoid damaging the screw and barrel. To prevent iron debris from entering the barrel, a magnetic component or magnetic frame can be installed at the material feeding port where the material enters the barrel. To prevent debris from falling into the hopper, the material must be screened in advance.

⑹ Pay attention to the cleanliness of the production environment, and do not allow garbage and impurities to mix into the materials to block the filter plate, affecting the product output and quality and increasing the resistance of the machine head.

⑺When the extruder needs to be stopped for a long time, anti-rust grease should be applied to the working surfaces of the screw, barrel, die, etc. Small screws should be hung in the air or placed in a special wooden box and leveled with wooden blocks to prevent the screw from deformation or damage.

⑻ Regularly calibrate the temperature control instrument to check the correctness of its adjustment and the sensitivity of its control.

⑼ The maintenance of the reducer of the extruder is the same as that of the general standard reducer. It is mainly to check the wear and failure of gears, bearings, etc. The reducer should use the lubricating oil specified in the machine manual and add oil according to the specified oil level. Too little oil will lead to insufficient lubrication and reduce the service life of parts; too much oil will cause high heat, high energy consumption, and easy deterioration of oil, which will also make the lubrication ineffective and cause damage to parts. The sealing gasket of the oil leaking part of the reducer should be replaced in time to ensure the amount of lubricating oil.

⑽ The inner wall of the cooling water pipe attached to the extruder is prone to scale and the outer surface is prone to corrosion and rust. Careful inspection should be carried out during maintenance. Excessive scale will block the pipe and fail to achieve the cooling effect. Severe rust will cause water leakage. Therefore, descaling and anti-corrosion and cooling measures must be taken during maintenance.

⑾For the DC motor that drives the screw, the brush wear and contact should be checked, and the insulation resistance of the motor should be measured regularly to see if it is above the specified value. In addition, the connecting wires and other parts should be checked for rust and protective measures should be taken.

⑿ Designate a person to be responsible for equipment maintenance. Keep detailed records of each maintenance and repair and include them in the factory equipment management file.

Technological Innovation

China’s conventional extruders and production lines are gradually becoming popular in the international market with their excellent cost-effectiveness. At the same time, China continues to innovate in the field of advanced extrusion technology and has developed a variety of new extrusion products.

Melt gear pump

Precision extrusion molding can eliminate the need for subsequent processing methods, better meet the needs of product applications, and at the same time achieve the purpose of reducing material costs and improving product quality. Nowadays, to meet the needs of precision direct extrusion of plastic products, a variety of mature technologies have been introduced to the market, and polymer melt gear pumps are one of the important means. This technology has been widely used in production lines such as chemical fibers, films, profiles, pipes, plates, cables, composite extrusion, and granulation.

After years of systematic research on melt gear pumps, the Rubber and Plastic Machinery Research Institute of Beijing University of Chemical Technology has successfully completed the series development and research of plastic melt gear pumps. It is now able to design and manufacture plastic melt gear pump products such as 28/28 ( center distance / tooth width ), 56/56, 70/70, 90/90, etc. The maximum inlet and outlet pressure difference can reach 30MPa, which can meet the requirements of different production volumes and has been applied in practice with good results. The Rubber and Plastic Machinery Research Institute of Beijing University of Chemical Technology has designed and developed the 115 integrated gear pump extruder through in-depth research on the integrated gear pump extruder.

Gear pumps are also very beneficial to the precision molding of the rubber industry. In order to meet the domestic demand for rubber melt gear pumps, Beijing University of Chemical Technology also cooperated with Beijing Institute of Aeronautical Manufacturing Engineering and Hangzhou Chaoyang Rubber Co., Ltd. to jointly develop two types of rubber melt gear pump extruder units, XCP150/100 and XCP120/90. This unit has ideal working characteristics, ensuring that the extrusion volume is linearly related to the speed of the gear pump, which can achieve precise control of production and improve the dimensional accuracy of the product.

Multi-layer co-extrusion technology matures and develops

Multi-layer composite technology uses materials with medium and high barrier properties to composite with other packaging materials, combining the high barrier properties of barrier materials with the cheapness of other materials or special mechanical, thermal and other properties to achieve specific functional needs. The structural design of co-extruded composite films gradually requires the ability to systematically achieve the ideal state of integrating functions, technologies, costs, environmental protection, safety, and secondary processing, thereby maximizing the number of composite layers and becoming one of the technologies pursued by suppliers.

The key technologies used in the seven-layer composite film co-extrusion blow molding unit include: two short and one long screw and variable pitch screw plasticizing extrusion system, engineering analysis software to optimize the design of vibration -induced plasticizing device, flat valve plus die head and inclined valve plus die head, internal cooling technology and double air inlet negative pressure cooling technology, multi-component weightless metering feeding, online film thickness precision control system, computer centralized automatic control system and bus control (CANOPEN) technology, etc.

As the number of layers increases, the film production technology that adapts to special functions is also one of the hot spots of market development. Guangdong Shicheng Company has designed and manufactured a PP environmentally friendly wood grain film casting production line with a width of 3150mm. The production capacity of this production line exceeds 800kg/h. The screw is designed as a high-speed shearing, mixing, and high-efficiency plasticizing screw. Customers can directly use high-filled calcium carbonate powder and inorganic pigment toner, thereby saving expensive raw material costs. In addition to the production of PP environmentally friendly wood grain film, the entire line can also flexibly switch to produce other products to expand customer product categories. During the trial production process of Shicheng Company, not only beautiful PP wood grain film was produced, but also CPP film, PP stationery film and PP stationery sheet were produced.

Innovative triple screw compounding technology

Parallel co-rotating twin-screw extruders are used in compounding and granulation production lines. After more than 20 years of rapid development, the technology has become quite mature. On the other hand, traditional intermeshing disc and reciprocating screw extruders are adapted to the needs of high-filling compounding, and the degree of industrialization is constantly improving.

Chaotic mixing of high performance polymer packaging materials

The project “R& D of key equipment and technology for high-performance polymer packaging material molding based on chaotic mixing” jointly carried out by Huilong Company and South China University of Technology has passed the scientific and technological achievement appraisal of Guangdong Provincial Department of Science and Technology. The project’s “Chaotic Mixing Low-Energy Extruder with Multi-Flute Convection Screw” has a nominal power ratio of 0.17 kW/kg/h, which is only half of the national standard of 0.32 kW/kg/h, with significant energy savings. The overall technology and products have reached the international advanced level.

1 The rheological modeling method is used in combination with the micro- rheological model that controls the morphological evolution of polymer materials. The flow field in the extrusion processing of polymer materials and the morphological evolution of blends and nanocomposites are modeled, simulated and analyzed. In particular, the theoretical study of melting, mixing and melt flow in the extruder reveals the mechanism of how to improve melting and mixing performance and reduce energy consumption.

2 Based on the above theoretical research, the chaotic mixing type low-energy extruder developed is significantly different in principle from commonly used extruders: the latter undergoes the classic Maddock melting process and shear mixing, and its melting and mixing effects are poor; the former produces dispersed melting and chaotic mixing, and the shear heat generated by the material is less than the heat energy required for its melting, which can prevent the material from overheating and wasting energy during the melting and mixing process, and has a significant energy-saving effect. The on-site inspection by the Guangdong Provincial Technical Supervision Mechanical Product Quality Supervision and Inspection Station shows that the nominal specific power (i.e. unit consumption ) of the extruder is 0.17 kW/kg/h, which is 0.15 kW/kg/h lower than the specified value [0.32 kW/kg/h] of the national mechanical industry standard JB/T 8061-96. Comparison with the extruders of two companies (Davis Standard Company of the United States and Sumitomo Heavy Industries Modern Company of Japan) representing the highest level of extrusion compounding in the world today shows that the extruder developed in this achievement has the highest extrusion output and is equipped with the lowest motor power. The extruder also has the advantages of low extrusion melt temperature (10-20°C lower) and strong material adaptability.

⑶ On the basis of the above-mentioned macroscopic flow field simulation and microscopic morphological evolution theoretical research, combined with the developed chaotic mixing low-energy consumption extruder, the morphological evolution, dispersion state and macroscopic properties of polymer blends (especially those with a viscosity ratio much greater than 1) and nanocomposites (especially those based on non-polar materials such as polyolefins ) were systematically studied. It was confirmed that the chaotic mixing extruder can improve processing performance and reduce processing energy consumption. In particular, the stretching and folding effects it provides are conducive to the formation of highly dispersed, flaky, intercalated or peeled morphologies, which to a certain extent solves the problem of easy agglomeration of nanoparticles in polymer material processing and greatly improves the barrier and mechanical properties of packaging products.

(4) The chaotic mixing type low-energy consumption extruder is used to produce EVA pre-coated film by extrusion. Compared with the conventional method of dissolving EVA in toxic solvents and coating it on the substrate, it eliminates the emission of toxic organic solvents and its pollution to the environment and harm to the human body; in addition, it greatly improves the adhesion performance between the extruded film and the composite substrate, realizing a “green composite process without adhesion promoter “.

Failure Analysis

Extruder is a common plastic machinery equipment. In the daily operation of the extruder, various faults may occur in the extruder, affecting the normal production of plastic machinery. Below we analyze the extruder faults.

Common failures and solutions of rod extruder

Abnormal noise

(1) If it occurs in the reducer, it may be caused by bearing damage or poor lubrication, or gear wear, improper installation and adjustment, or poor meshing. It can be solved by replacing bearings, improving lubrication, replacing gears, or adjusting gear meshing.

(2) If the noise is a sharp scraping sound, you should consider the possibility that the barrel position is skewed, causing the shaft head and the transmission sleeve to scrape. This can be solved by adjusting the barrel.

(3) If the barrel makes noise, it may be that the screw is bent and swept or the set temperature is too low, causing excessive friction of solid particles. This can be solved by straightening the screw or increasing the set temperature.

Abnormal vibration

If this happens at the reducer, it is due to wear of the bearings and gears, which can be solved by replacing the bearings or gears; if it happens at the barrel, it is due to hard foreign matter mixed in the material, and the cleanliness of the material needs to be checked.

The main causes and solutions for screw extruder wear

The main reason for screw extruder wear

The normal wear of the screw and barrel of the screw extruder mainly occurs in the feeding area and the metering area. The main cause of wear is the dry friction between the chip particles and the metal surface. The wear decreases when the chip is heated and softened.

Abnormal wear of the screw and barrel occurs when the screw is entangled and stuck by foreign objects. The entanglement means that the screw is locked by condensed materials. If the screw extruder lacks good protection devices, the strong driving force may break the screw. The jamming will produce unusually large resistance, causing serious damage to the screw surface and serious scratches on the barrel. The scratches on the barrel are difficult to repair. The barrel is designed to ensure that the service life is longer than the screw. For the normal wear of the barrel, it is generally not repaired. The method of repairing the screw thread is often used to restore the radial clearance between the inner hole of the barrel and the outer diameter of the screw.

Solutions to screw wear

The local damage of the screw thread is repaired by surfacing special anti-wear and anti-corrosion alloy. Generally, inert gas shielded welding and plasma argon arc welding are used. Metal spraying technology can also be used for repair. First, the outer cylindrical surface of the worn screw is ground to a depth of about 1.5 mm, and then the alloy layer is surfacing to a sufficient size to ensure sufficient

The machining allowance is removed, and finally the outer circle of the screw and the side surfaces of the threads are ground until the outer dimensions of the screw are the original dimensions.

Ring blockage at the screw inlet

This failure is mainly caused by cooling water interruption or insufficient flow. The cooling system needs to be checked and the cooling water flow and pressure adjusted to the specified requirements.

Conclusion

(1) The natural life of the extruder is relatively long, and its service life mainly depends on the wear of the cylinder and the reducer. Selecting an extruder and a reducer with excellent design, material selection and manufacturing directly affects the life of the extruder.

Considering the performance, although the equipment investment increases, the service life is extended. Considering the overall economic benefits, it is more reasonable.

(2) The normal use of the screw extruder can give full play to the efficiency of the machine and keep it in good working condition. It is necessary to maintain it carefully and persistently to extend the service life of the machine.

(3) The main faults of screw extruders are abnormal wear, foreign matter stuck, material blockage, wear or damage of transmission parts, poor lubrication or oil leakage, etc. In order to avoid failures, the drying, mixing and feeding operations as well as the setting of process temperature must be strictly managed, and daily maintenance, care and inspection must be carried out in strict accordance with the requirements of the “Equipment Point Inspection Benchmark”.

Troubleshooting

Extruder feed roller wear

Since the extruder is made of metal with high hardness, it is subjected to vibration, impact and other composite forces during production and operation, resulting in gaps between parts and causing wear. Traditional repair methods include surfacing, thermal spraying, brush bridging, etc., but all of these methods have certain disadvantages: surfacing will cause the surface of the parts to reach a very high temperature, causing deformation or cracks of the parts, affecting dimensional accuracy and normal use, and even causing breakage in severe cases; although brush bridging has no thermal impact, the thickness of the bridging layer cannot be too thick, causing serious pollution, and its application is also greatly limited. Western countries have mostly applied polymer composite materials to the above problems. Its comprehensive performance and the characteristics of being machined at any time can meet the use requirements and accuracy after repair, and can also reduce the impact and vibration of the equipment during operation and extend its service life. Because the material is a “variable” relationship, when external force impacts the material, the material will deform and absorb the external force, and expand and contract with the expansion and contraction of the bearing or other components, always keeping a tight fit with the components to reduce the chance of wear. In response to the wear of large extruders, “molds” or “matching parts” can also be used to repair damaged equipment on-site, avoiding the overall disassembly of the equipment, maximizing the matching dimensions of the parts, and meeting the production and operation requirements of the equipment.

The processing size of the bushing of the extruder feeding section does not match

When the extruder bushing is made of 38CrMoAlA, due to machining reasons (the positioning keyway and the mating part are not on the same axis), there is a gap between it and the side plate (material 40Cr or 45). When the machine is turned on, the recoil of the rubber causes the rubber to leak. The temperature does not exceed 100℃. The company has previously used other products to repair it, but it can only be used for 1~2 days. Using polymer materials to repair it can solve this problem well.

The thread of the side cover of the extruder feeding section is damaged ( slipped thread )

During the process of pre-tightening the bolts of the extruder, the bolts deform due to tensile stress. Its recovery stress makes it tightly connected to the sealing part it is connected to. As time goes by, part of the tensile deformation becomes permanent deformation, and the recovery stress decreases, resulting in stress relaxation and torque drop, which leads to bolt loosening, causing thread slippage and wear. In severe cases, it may even cause damage to the internal threads of the fastened parts. The use of Meijiahua polymer materials for repair has the yield that metal does not have, which ensures the recovery stress after repair and the use effect of the parts. At the same time, the non-metallic nature of the material itself makes its astringency far greater than that of metal, eliminating the re-damage caused by looseness and ensuring the safe and continuous production of the enterprise.

Precautions

In order to protect the box of the twin-screw extruder transmission box, the following points must be achieved:

1. After the first 500 hours of use, replace the lubricating oil once;

⒉ This extruder is lubricated with original medium-pressure gear oil of grade 150 ;

3. The oil level should not be lower than the center line of the oil mark during normal operation. If it is lower than the center line, please refill it quickly.

4. Change the oil every 3,000 hours of use;

5. When changing the oil, the box and the oil filter should be cleaned. When changing the oil, clarify the lubricating oil used this time, put the clean lubricating oil into the box again to clean the box, then drain it and fill it with new lubricating oil;

6. The lubricating oil filter should be cleaned regularly every month during normal use and every week during the running-in period. To clean it, find the oil filter, open it, and remove the dirt inside;

Common Problems

Common problems and solutions of screw extruders In the use of mechanical equipment, it is inevitable that problems of one kind or another will occur. Therefore, it is very important to find problems in time and find solutions in production. Here we analyze some common problems of screw extruders and explore the best solutions.

Common problems of extruder and solutions

1. If the screw does not take in material, it will leak material and the screw will heat up.

1.1 Causes and solutions

1.1.1 The reamer is seriously worn. The gap between the reamer and the inner wall of the mud cylinder is too large, or the spiral angle of the reamer blade is incorrect. When replacing the reamer, you should pay attention to keeping the gap at 3-5mm, and the reamer blade should be made according to the designed angle.

1.1.2 The surface of the reamer blades is too rough, and the friction between the mud and the blades is too great. Therefore, when surfacing the reamer, do not weld in groups and replace them all at once. Instead, replace them step by step and in batches to ensure smooth brick production.

1.1.3 The mud cylinder wall bushing is seriously worn. The gap between the reamer blade and the mud cylinder wall is too large, resulting in too much rotation of the mud in the mud cylinder, and the mud cannot come out or go in. At this time, a new mud cylinder bushing should be replaced, or several ribs parallel or inclined to the axial direction should be installed on the inner wall of the bushing to replace the worn-out riveting grooves, to prevent the mud from rotating ineffectively and increase its effective extrusion.

1..1.4 The gap between the mud press plate and the auger is too large and the mud cannot be pressed down. At this time, the knife plate should be adjusted or repaired by welding or updated so that the gap between it and the auger blade is less than 10mm.

Mud strip movement bending

2.1 Causes and solutions

2.1.1 The mud strips are bent to one side. This is because the center lines of the machine mouth, core tool, mud cylinder and spiral auger are not aligned or the roller bed is installed tilted. This can be solved by adjusting the position.

2.1.2 The mud strips are S-shaped. This is because the compression length of the machine neck is not enough, the tops of the main blade and the auxiliary blade of the first section spiral auger are not aligned, or the auxiliary blade of the first section auger is severely worn and becomes smaller, resulting in only the semicircle of the main blade pushing out mud during operation. At this time, the first section auger should be replaced and the auxiliary blade should be welded.

Overload

3.1 Causes and solutions

3.1.1 The clay is too dry. In this case, the too dry clay should be taken out first. It is best to remove the machine mouth and machine head, start the machine to drain the dry material in the clay tank, and then install and run it, and appropriately increase the molding moisture within the specified range.

3.1.2 If the machine is shut down for a long time, the remaining material in the mud tank will become dry and hard, which will not only cause serious overload, but sometimes even make it impossible to start. In order to prevent this from happening, when the machine is shut down for more than 8 hours, do not close the dead machine port for water supply. For the two-stage vacuum extruder, the mud in the upper mud tank should also be properly supplied with water to keep it moist. If the machine is shut down for more than 2 days, try to empty the mud before shutting down.

3.2 Notes: When overloaded, the motor load remains high and the clutch slips. At this time, do not force start the machine to avoid damage to related parts or even major accidents such as squeezing the mud cylinder or machine head.

Machine “head swing”

4.1 Causes and solutions

4.1.1 “Head swing” is a common problem of screw extruders. This is because the auger shaft is a long cantilever shaft with poor stability. When the bearing is loose, the main shaft is bent, and the auxiliary blade of the first section of the auger is too small, this situation will be aggravated. If the mud cylinder is not installed correctly, the gap between the spiral auger blade and the mud cylinder wall is not the same, and the outer edge of the spiral auger blade is seriously out of round, the main shaft will be unevenly stressed and swing. This should be corrected in time, and the anchor bolts and connecting bolts should be tightened frequently to prevent it from swinging.

Spiral lines appear after mud strips are squeezed out

5.1 Causes and solutions

5.1.1 Due to the spiral action of the auger, the mud material in the mud cylinder section moves at different speeds. The mud material near the axis moves faster, while the mud material at the edge moves slower. An interface is formed between the mud flows with different speeds. Water and air are concentrated in the gaps on this surface, resulting in stratification. The main reasons are high plasticity of the mud material, excessive molding water content, too high spindle speed, and mud return on the inner wall of the mud cylinder. The treatment methods are as follows:

(1) Raw materials of different properties should be fully mixed and aged as much as possible to allow moisture to fully penetrate into the raw material particles and reduce the moisture on the surface of the raw mud.

(2) Appropriately reduce the molding moisture to increase the friction between the mud layers.