New

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

Drive part

The transmission part is usually composed of motor, gearbox and bearing. In the extrusion process, the screw speed must be stable, and it can not change with the change of screw load, so as to keep the quality of the products uniform and consistent. But in different situations, screw can be changed to meet the requirements of a device that can extrude different plastics or different products. Therefore, the AC commutator motor, DC motor and other devices are generally used in this part to achieve stepless speed change. The general screw speed is 10-100 rpm.

The function of the transmission system is to drive the screw, and supply the torque and speed required by the screw during extrusion, usually composed of motor, reducer and bearing. On the premise of the same structure, the manufacturing cost of the reducer is proportional to its overall size and weight. Because the reducer has a large shape and weight, it means that the material consumed in manufacturing is much, and the bearing used is also relatively large, which makes the manufacturing cost increase.

For extruders with the same screw diameter, the high-speed and high efficiency extruder consumes more energy than the conventional extruder, and the motor power is doubled, and the seat number of the reducer is necessary. But high screw speed means a low deceleration ratio. Compared with the large reduction ratio, the gear modulus increases and the capacity of the reducer to bear the load increases. Therefore, the increase of the volume weight of the reducer is not linear proportion to the increase of motor power. If the extrusion amount is used as denominator, divided by the weight of reducer, the number of high-speed and efficient extruders is small, and the number of ordinary extruders is large. In unit output, the motor power and weight of reducer are small, which means that the manufacturing cost of unit output machine of high-speed and high efficiency extruder is lower than that of ordinary extruder.

Feeding device

Generally, the feed is granular, but it can also be ribbon or powder. The loading equipment usually uses a conical hopper, and its volume requires a minimum of one hour of consumption. The hopper bottom is equipped with a cutting device to adjust and cut off the flow of material. The side of the hopper is equipped with a viewing hole and a calibration and metering device. Some hoppers may also have pressure reducing devices or heating devices to prevent the absorption of water from the air, or some drums may also have agitators to automatically feed or feed them.

1. hopper

Hopper is generally symmetrical. A window is opened on the side of the hopper to observe the feeding level and feeding condition. The bottom of the hopper has an opening and closing door to stop and adjust the feeding quantity. Cover the hopper to prevent dust, moisture and impurities from falling into the hopper. When choosing hopper material, it is better to use light, corrosion resistant and easy to process materials, generally aluminum plate and stainless steel plate. The volume of hopper depends on the size and feeding mode of extruder. Generally, the extrusion volume of extruder is 1-1.5H.

2. feeding

There are two feeding methods: manual feeding and automatic feeding. Automatic feeding mainly includes spring feeding, air blast feeding, vacuum feeding, conveyor belt feeding and other forms. Generally, small extruders are fed manually and large extruders are fed automatically.

3. classification of feeding mode

① Gravity feed:

Principle: materials enter the barrel by their own weight, including manual feeding, spring feeding and blast charging.

Features - simple structure and low cost. But it is easy to cause uneven feed, which affects the quality of the parts. It is only suitable for small extruders.

② Forced feeding:

Principle: a device is installed in the hopper to exert external pressure on the material, and the material is forced into the extruder barrel.

Characteristics: it can overcome the phenomenon of "bridge erection" and make the feeding uniform. The feeding screw is driven by the extruder screw through the transmission chain, so that its speed is suitable to the screw speed. It can start overload protection device when the feeding port is blocked, thus avoiding the damage of the charging device.

Barrel

Generally, it is a metal bucket, which is made of alloy steel or composite steel pipe lined with alloy steel. Its basic characteristics are high temperature resistance and pressure resistance, strong wear resistance and corrosion resistance. The length of the general cylinder is 15-30 times of its diameter, and its length is based on the principle of fully heating and plasticizing the material. The barrel shall have sufficient thickness and rigidity. The interior should be smooth, but some drums are engraved with grooves to increase friction with plastics. The electric heater, temperature control device and cooling system with resistance, inductance and other heating methods are attached outside the material barrel.

1. there are three types of barrel structure:

(1) Integral barrel

Processing method - processed on the whole material.

Advantages: it is easy to ensure high manufacturing accuracy and assembly accuracy, simplify assembly work, uniform heating of material barrel, and more application.

Disadvantages - because of the large barrel length and high processing requirements, the requirements for processing equipment are also strict. It is difficult to repair the inner surface of the barrel after it is worn.

(2) Combination material Brief

Processing method - the barrel is processed in several sections, and then each section is connected by flange or other means.

Advantages: simple processing, easy to change the ratio of length to diameter, and is mostly used in the case where the ratio of length to diameter of screw needs to be changed.

Disadvantages: the machining accuracy is very high. Because of the many sections, it is difficult to ensure the coaxiality of each section. The flange joint destroys the uniformity of the material barrel heating, increases the heat loss, and the setting and maintenance of the heating cooling system is also difficult

(3) Bimetal barrel

Processing method - insert or cast a layer of alloy steel material in the base of general carbon steel or cast steel. It can meet the material requirements of the material barrel and save precious metal materials.

① Liner type material barrel: the material barrel is equipped with replaceable alloy steel liner. The utility model can save precious metals, and the bushing can be replaced, and the service life of the material barrel is improved. But its design, manufacture and assembly are more complex.

② Casting barrel: centrifugal casting of a layer of alloy about 2mm thick on the inner wall of the barrel, and then the required inner diameter of the barrel can be obtained by grinding. The alloy layer is well combined with the base of the barrel, and the bonding along the axial length of the barrel is uniform, which has no tendency of peeling off and cracking, and also has sliding performance, high wear resistance and long service life.

(4) IKv material barrel

1. Longitudinal groove is set on the inner wall of the feeding section of the material barrel

In order to improve the solid transport rate, one method is to increase the friction coefficient of the surface of the barrel, and another method is to increase the area of the material at the feeding port passing through the cross-section perpendicular to the screw axis. The two methods are to set up longitudinal grooves on the inner wall of the feeding section and make the inner wall of the material barrel near the feeding port into a cone.

2. Forced cooling charging section material barrel

There is also a way to improve the solid transport. It is to cool the charging section material barrel, which aims to keep the temperature of the material transported below the softening point or melting point, so as to avoid the appearance of the molten film to maintain the solid friction property of the material.

The efficiency of the transportation was increased from 0.3 to 0.6 by the above method, and the sensitivity of extrusion volume to the head pressure change was small.

screw

Screw is the heart of extruder and the key part of extruder. The performance of screw determines the productivity, plasticizing quality, dispersion of filler, melt temperature and power consumption of an extruder. It is the most important part of extruder, which can directly affect the application scope and production efficiency of extruder. Through the action of screw rotation, plastic can be moved, pressurized and part of heat can be obtained from friction. During the process of plastic moving in the material barrel, the plastic can be mixed and plasticized. When the viscous melt is squeezed and flows through the mouth mold, the required shape is obtained and formed. Like the barrel, the screw is also made of high strength, heat-resistant and corrosion-resistant alloy.

Because of the variety of plastics, their properties are different. Therefore, in practice, in order to meet the different needs of plastic processing, the types of screw required are different, and the structure is different. In order to maximize the efficiency of plastic transportation, extrusion, mixing and plasticization.

The basic parameters that represent the screw characteristics include the following points: diameter, length to diameter ratio, compression ratio, pitch, groove depth, screw angle, screw and barrel clearance, etc.

The most common screw diameter D is about 45-150 mm. The productivity of extruder is proportional to the square of screw diameter D. The ratio of effective length to diameter of the working part of screw (l/d) is usually 18-25. Large l/d can improve the temperature distribution of materials, facilitate the mixing and plasticization of plastics, and reduce leakage and reverse flow. The large L / D screw has strong adaptability to improve the production capacity of extruder, and can be used for extrusion of many kinds of plastics; However, when l/d is too large, it will degrade the plastics industry due to the increase of heating time. At the same time, due to the increase of screw weight and the flexure and sagging of free end, it is easy to cause Scratch between material and screw, and make manufacturing difficult; The power consumption of extruder is increased. Too short screw, easy to cause mixing of the plasticization.

Half of the difference between the inner diameter of the barrel and the diameter of the screw δ， It can affect the production capacity of extruder, and δ Increase of, productivity reduction. Usually controlled δ It is better to be about 0.1-0.6 mm. δ Small, the material is subject to a larger shear action, which is conducive to plasticization, but δ Too small, strong shear action can easily cause thermal mechanical degradation of materials, and it is easy to make screw be locked or friction with the material barrel wall, and, δ Too little material leakage and reverse flow, to a certain extent, affect the mixing of melt.

Helix angle Φ Is the angle between the thread and the screw cross section, with Φ The production capacity of extruder is increased with increasing, but the shear effect and extrusion pressure on plastics are reduced, and the helix angle is usually between 10 ° To 30 ° The screw length is changed along the direction of the length of the screw. The constant distance screw is often used, and the pitch is equal to the diameter, Φ About 17 ° 41′

The greater the compression ratio, the greater the compression ratio the plastic receives. When the screw groove is shallow, it can produce high shear rate for plastics, which is conducive to the heat transfer between the material wall and the material. The higher the mixing and plasticizing efficiency of the material, the lower the productivity; On the contrary, when the groove is deep. The opposite is true. Therefore, deep screw should be used for heat sensitive materials (such as PVC); For plastics with low melt viscosity and high thermal stability (such as polyamide), shallow screw should be used.

1. screw segment

When the material moves forward along the screw, it experiences the changes of temperature, pressure and viscosity. This change is different in the whole length of the screw. According to the characteristics of material change, the screw can be divided into adding (feeding) material section, compression section and homogenizing section.

① , plastic and plastic three states

There are two kinds of Plastics: thermosetting and thermoplastic. After the thermosetting plastics are formed and solidified, they can not be heated and melted again. The products formed by thermoplastic can be heated and melted to form other products.

With the change of temperature, the thermoplastic has three states, namely glass state, high elastic state and viscous flow state, and the three states change repeatedly with the temperature.

a. The different characteristics of polymer melt in three states are as follows:

Glass state - plastic presents rigid solid; The thermal energy is small and the intermolecular force is large, and the deformation is mainly contributed by the bond angle deformation; The deformation is restored instantaneously after the external force is removed, which belongs to the general elastic deformation.

High elastic state - plastic is rubber like material; The deformation is caused by the orientation of the chain segment, which leads to the expansion of the conformation of the macromolecules, and the deformation value is large; The deformation can be restored but time-dependent after the external force is removed, which belongs to the high elastic deformation.

Viscous flow state - plastic presents high viscosity melt; The thermal energy further intensifies the relative slip motion of chain molecules; Irreversible deformation, which belongs to plastic deformation

b. Plastic processing and plastic three states:

Plastic glass can be cut and processed. It can be stretched in high elastic state, such as drawing textile, extrusion pipe, blow molding and hot forming. When the flow is viscous, it can be coated, rolled and injected.

When the temperature is higher than the viscous state, the plastic will produce thermal decomposition, and when the temperature is lower than the glass state, the plastic will produce embrittlement. When the temperature of plastic is higher than the viscous or glass state, the thermoplastic tends to deteriorate and destroy seriously. Therefore, the two temperature areas should be avoided when processing or using plastic products.

② , three-stage screw

There are three physical states in extruder, namely, glass state, high elastic state and viscous flow state. Each state requires 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 L1 (also known as solid conveying section)

Melting section L2 (called compression section)

Homogenization section L3 (called metering section)

This is what is commonly referred to as the three-stage screw. The extrusion process of plastics in these three segments is different.

The function of the feeding section is to send the material supplied by the hopper to the compression section. The plastic generally keeps solid state during the moving process, and partially melts due to heat. The length of the feeding section varies with the plastic type, from the distance of the hopper to 75% of the total length of the screw cup.

Generally speaking, the extruded crystalline polymer is the longest, the hard amorphous polymer is the second, and the soft amorphous polymer is the shortest. Because the feeding section does not have to produce compression, the volume of the screw groove can be kept unchanged. The size of the screw angle has a great influence on the feeding capacity of this section, and the actual impact on the productivity of the extruder. Generally, the screw angle of powder material is about 30 degrees, and the productivity is the highest. The square material should be about 15 degrees, because the spherical material should choose about 17 degrees.

Main parameters of screw in feeding section:

Helix angle ψ 17 for general °～ twenty °。

 The depth H1 of the screw groove is the geometric compression ratio of the screw after determining the groove depth of homogenization section ε To calculate.

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

For amorphous polymer l1= (10% ~ 20%) l

For crystalline polymer l1= (60% ～ 65%) l

The function of compression section (moving section) is to compact the material, make the material from solid to molten body, and remove the air in the material; In order to adapt to the characteristics of pushing the gas back to the feeding section, reducing the volume of compacted materials and materials during melting, the screw in this section should have a greater shear effect and compression on plastics. To this end, the volume of the screw groove is usually gradually reduced, and the reduction degree is determined by the compression rate of plastic (the specific gravity of the product / the apparent specific gravity of the plastic). The compression ratio is related to the compression rate of plastics, but also the shape of plastics. The proportion of powder is small and the air is more, so it needs a larger compression ratio (up to 4-5), while the aggregate is only 2.5-3.

The length of the compression section is mainly related to the melting point of plastic and other properties. Plastic with wide melting temperature range, such as PVC melting at 150 ℃ or above, the longest compression section can reach 100% of the total length of screw (gradient type), polyethylene with narrow melting temperature range (105-120 ℃ low density polyethylene, 125-135 ℃ high density polyethylene), etc., and the compression section is 45-50% of the total length of the screw; Most polymers with narrow melting temperature range, such as polyamide, have only one pitch length in compression section.

Main parameters of the screw in the melting section:

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

ε=( Ds-H1)H1/(Ds-H3)≈H1/H3

Where H1 - depth of the first slot in the feeding section

H3 - depth of the last groove in homogenization section

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

For amorphous polymers L2 = 55% - 65% L

For crystalline polymer l2= (1-4) ds

The function of homogenization section (metering section) is to send molten material into the head to form in the die at constant volume (Quantitative) pressure. The volume of screw groove in homogenization section is as constant as that of the feeding section. In order to avoid the material being stuck at the dead angle of the screw head end, the screw head is usually designed to be tapered or semi-circular; Some of the screws