In plastic extrusion molding equipment, plastic extruder is usually called the main machine, and the subsequent equipment plastic extrusion molding machine is called the auxiliary machine. After more than 100 years of development, plastic extruder has evolved from the original single screw to twin screw, multi-screw, and even screwless models. Plastic extruder (main machine) can be matched with various plastic molding auxiliary machines such as pipes, films, holding materials, monofilaments, flat wires, strapping tapes, extruded nets, plates (sheets), profiles, granulation, cable coating, etc., to form various plastic extrusion molding production lines and produce various plastic products. Therefore, plastic extrusion molding machinery is one of the widely used machines in the plastic processing industry, whether now or in the future. It is composed of extrusion, transmission and heating and cooling systems.

Composition

The main machine of the plastic extruder is the extruder, which is composed of an extrusion system, a transmission system and a heating and cooling system.

Extrusion system

The extrusion system includes a screw, a barrel, a hopper, a die, and a mold. The plastic is plasticized into a uniform melt through the extrusion system, and is continuously extruded out of the die under the pressure established in this process.

¢ÅScrew: It is the most important component of the extruder. It is directly related to the application range and productivity of the extruder. It is made of high-strength and corrosion-resistant alloy steel.

¢ÆBarrel: It is a metal cylinder, generally made of alloy steel with high heat resistance, high compressive strength, strong wear resistance, corrosion resistance, or a composite steel pipe lined with alloy steel. The barrel cooperates with the screw to realize the crushing, softening, melting, plasticization, exhaust and compaction of the plastic, and continuously and evenly transport the rubber to the molding system. Generally, the length of the barrel is 15 to 30 times its diameter, so that the plastic can be fully heated and fully plasticized.

¢Ç Hopper: A cut-off device is installed at the bottom of the hopper to adjust and cut off the material flow, and a viewing hole and a calibration metering device are installed on the side of the hopper.

¢È Die and mold: The die is composed of an alloy steel inner sleeve and a carbon steel outer sleeve. A molding mold is installed in the die. The function of the die is to convert the rotating plastic melt into parallel linear motion, evenly and smoothly introduce it into the mold sleeve, and give the plastic the necessary molding pressure. The plastic is plasticized and compacted in the barrel, and flows into the die molding mold through the porous filter plate along a certain flow channel through the die neck. The mold core and the mold sleeve are properly matched to form an annular gap with a continuously decreasing cross-section, so that the plastic melt forms a continuous and dense tubular coating around the core line. In order to ensure the reasonable plastic flow channel in the die and eliminate the dead corners of accumulated plastic, a diversion sleeve is often installed. In order to eliminate pressure fluctuations during plastic extrusion, an equalizing ring is also set. The die is also equipped with a mold correction and adjustment device to facilitate the adjustment and correction of the concentricity of the mold core and the mold sleeve.

According to the angle between the die flow direction and the center line of the screw, the extruder divides the die into an angled die (angle 120o) and a right-angle die. The shell of the die is fixed to the body with bolts. The mold in the die has a core seat and is fixed to the die inlet port with a nut. The front of the core seat is equipped with a core. The center of the core and the core seat has a hole for passing the core wire. A pressure equalizing ring is installed at the front of the die to balance the pressure. The extrusion molding part consists of a die sleeve seat and a die sleeve. The position of the die sleeve can be adjusted by bolts through the support to adjust the relative position of the die sleeve to the die core, so as to facilitate the adjustment of the uniformity of the thickness of the extruded layer. A heating device and a temperature measuring device are installed on the outside of the die.

Transmission system

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 a bearing.

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 shape 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 number must be increased accordingly. However, high screw speed means low reduction ratio. For reducers of the same size, the gear module of low reduction ratio is increased compared with that of large reduction ratio, and the load-bearing capacity of the reducer 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 and 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 weight of the reducer 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.

Heating and cooling device

Heating and cooling are necessary conditions for the plastic extrusion process to proceed.

¢Å Extruders usually use electric heating, which is divided into resistance heating and induction heating. The heating plates are installed on the body, neck and head of the machine. The heating device heats the plastic in the barrel from the outside to raise its temperature to the temperature required for the process operation.

¢Æ The cooling device is set to ensure that the plastic is within the temperature range required by the process. Specifically, it is to eliminate the excess heat generated by the shear friction of the screw rotation to avoid excessive temperature causing the plastic to decompose, burn or become difficult to shape. Barrel cooling is divided into water cooling and air cooling. Generally, air cooling is more suitable for small and medium-sized extruders, while large ones mostly use water cooling or a combination of the two forms of cooling; screw cooling mainly uses central water cooling, the purpose is to increase the solid conveying rate of the material, stabilize the amount of glue, and improve the product quality at the same time; but the cooling at the hopper is to strengthen the conveying effect on the solid material, prevent the plastic particles from becoming sticky and blocking the material port due to temperature rise, and to ensure the normal operation of the transmission part.

Development process

Early application

The extruder originated in the 18th century. The manual piston extruder for making seamless lead pipes made by Joseh Bramah in England in 1795 is considered to be the world's first extruder. From then on, in the first 50 years of the 19th century, extruders were basically only used in the production of lead pipes, macaroni and other food processing, brick and ceramic industries.

Wide application

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 Company subsequently improved the extruder and used it to coat the copper wire of the first submarine cable between Dover and Calais in 1851. In the following 25 years, the extrusion method 1 3 gradually became important, and the manual extruders were rapidly replaced by electric extruders. The early mechanically operated plunger extruders produced thousands of kilometers of insulated wire and cable, thus firmly establishing the position of extrusion for the production of cables. The early extruders for producing cables, whether manual, mechanical or hydraulic, were all plunger-type. In this production process, the plunger presses the hot gutta-percha into the die through which the copper wire is passed, and the gutta-percha is squeezed out of the die, thus coating the copper wire to form an insulating layer.

Categories

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

Single-screw extruders occupy an important position as either plasticizing and granulating machinery or molding processing machinery. In recent years, single-screw extruders have made great progress.

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. On the basis of twin-screw extruders, multi-screw extruders have been developed to make it easier to process blends with poor thermal stability, such as Guanghua Plastic Extruder.

1. According to the number of screws, it is divided into single screw extruder, twin screw extruder and multi-screw extruder;

2. According to whether there is a screw in the extruder, it is divided into screw extruder and plunger extruder;

3. According to the running speed of the screw:

Ordinary extruder: the speed is below 100r/min;

High-speed extruder: the speed is 100~300r/min;

Ultra-high-speed extruder: the speed is 300~l500r/min.

4. According to the assembly structure of the extruder: there are integral extruders and separate extruders;

5. According to the spatial position of the screw in the extruder, it can be divided into horizontal extruders and vertical extruders;

6. According to whether the extruder is exhausted during the processing, it can be divided into exhaust extruders and non-exhaust extruders.

Forming principle

The extrusion method of the plastic extruder generally refers to melting the plastic at a high temperature of about 200 degrees, and the molten plastic forms the required shape when passing through the mold. Extrusion molding requires a deep understanding of the characteristics of plastics and rich experience in mold design, and is a molding method with high technical requirements.

Extrusion molding is a method of forming materials by heating and pressurizing in an extruder to continuously pass through the die in a flowing state, also known as "extrusion molding". Compared with other molding methods, it has the advantages of high efficiency and low unit cost.

The extrusion method is mainly used for the molding of thermoplastics, and can also be used for some thermosetting plastics. The extruded products are all continuous profiles, such as tubes, rods, wires, plates, films, wire and cable coatings, etc. In addition, it can also be used for mixing, plasticizing and granulating, coloring, blending, etc. of plastics.

The extruded products can be called "profiles", and because the cross-sectional shapes are mostly irregular, they are also called "special profiles".

Fault Analysis

Plastic 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. Let's analyze the extruder fault.

Unstable main engine current

1. Production reasons:

¢ÅUneven feeding.

¢ÆThe main motor bearing is damaged or poorly lubricated.

¢ÇA certain section of the heater fails and does not heat.

¢ÈThe screw adjustment pad is incorrect, or the phase is incorrect, and the components interfere.

2. Treatment method:

¢ÅCheck the feeder and eliminate the fault.

¢ÆRepair the main motor and replace the bearing if necessary.

¢ÇCheck whether each heater is working properly and replace the heater if necessary.

¢ÈCheck the adjustment pad and pull out the screw to check whether the screw has interference.

The main motor cannot start

1. Cause:

¢ÅThe start-up procedure is wrong.

¢ÆThe main motor thread has a problem, whether the fuse is burned.

¢ÇThe interlocking device related to the main motor is working

2. Treatment method:

¢ÅCheck the procedure and restart the car according to the correct start-up sequence.

¢ÆCheck the main motor circuit.

¢ÇCheck whether the lubricating oil pump is started and the status of the interlocking device related to the main motor. The oil pump is not turned on and the motor cannot be turned on.

¢ÈThe induction power of the inverter has not been fully discharged. Turn off the main power supply and wait for 5 minutes before starting.

¢ÉCheck whether the emergency button is reset.

The head of the machine is not unsmooth or blocked

1. Cause:

¢ÅA certain section of the heater does not work, and the material is poorly plasticized.

¢ÆThe operating temperature is set too low, or the molecular weight distribution of the plastic is wide and unstable.

¢Ç There may be foreign matter that is not easy to melt.

2. Treatment method:

¢Å Check the heater and replace it if necessary.

¢Æ Verify the set temperature of each section, and negotiate with the process engineer to increase the temperature setting value if necessary.

¢Ç Clean and check the extrusion system and the machine head.

The main electric starting current is too high

1. Cause:

¢Å Insufficient heating time and large torque.

¢Æ A certain section of the heater does not work.

2. Treatment method:

¢Å When starting, use the handwheel. If it is not easy, extend the heating time or check whether the heaters of each section are working normally.

The main motor makes an abnormal sound

1. Cause:

¢Å The main motor bearing is damaged.

¢Æ A certain thyristor in the thyristor rectifier circuit of the main motor is damaged.

2. Treatment method:

¢Å Replace the main motor bearing.

¢Æ Check the thyristor rectifier circuit and replace the thyristor components if necessary.

Features

Modularization and specialization

The modular production of plastic extruders can adapt to the special requirements of different users, shorten the research and development cycle of new products, and strive for a larger market share; and specialized production can arrange the fixed-point production and even global procurement of various system module components of extrusion molding equipment, which is very beneficial to ensure the quality of the whole period, reduce costs, and accelerate capital turnover.

High efficiency and multifunctionality

The high efficiency of plastic extruders is mainly reflected in high output, low energy consumption, and low manufacturing cost. In terms of function, screw plastic extruders are not only used for extrusion molding and mixing processing of polymer materials, but their uses have been expanded to food, feed, electrodes, explosives, building materials, packaging, pulp, ceramics and other fields.

Large-scale and precision

The large-scale plastic extruder can reduce production costs, which is more obvious in large twin-screw plastic granulation units, film blowing units, pipe extrusion units, etc. The major technical equipment required for national key construction services and one of the three key equipment for large-scale ethylene projects, the large-scale extrusion granulation unit, has long relied on imports, so it is necessary to accelerate the process of localization to meet the needs of petrochemical industry development.

Intelligence and networking

Plastic extruders in developed countries have generally adopted modern electronic and computer control technology, and the process parameters of the entire extrusion process, such as melt pressure and temperature, each section of the fuselage temperature, main screw and feeding screw speed, feeding amount, the ratio of various raw materials, the current and voltage of the motor, are tested online, and microcomputer closed-loop control is adopted. This is extremely beneficial to ensure the stability of process conditions and improve the accuracy of products.

Maintenance methods

¢± The plastic extruder equipment should be placed in a ventilated position to ensure the heat dissipation of the motor and extend its life; the machine should be well grounded.

¢² Check the tool screws regularly. After using the new machine for 1 hour, tighten the screws of the moving and fixed knives with tools to strengthen the fixation between the blade and the knife holder; lubricate the bearings regularly to ensure the lubrication between the bearings; to ensure the sharpness of the tool cut, check the tool frequently to ensure its sharpness and reduce unnecessary damage to other parts caused by blunt blades; check the belt regularly to see if it is loose and tighten it in time.

¢³ Restart - Before starting for the second time, remove the remaining debris in the machine room to reduce starting resistance. The inertia cover and pulley cover should be opened regularly to clear the ash outlet under the flange, because the powder discharged from the crusher chamber enters the shaft bearing. "}]}]},

¢´ Replacement of parts-When replacing the cutter, the gap between the moving cutter and the fixed cutter: 0.8MM is better for crushers above 20HP, and 0.5MM is better for crushers below 20HP. The thinner the recycled material, the larger the gap can be.

Domestic problems

Die head: In our country's plastic extruders, both imported and domestic die heads are used more, and imported ones are not necessarily better than domestic ones. Because the requirements for imported die heads are very high, they are specially designed according to the fluidity of the material when they are designed and circulated. Imported die heads use high-quality materials, and the quality of the products produced is also very good. Using imported die heads does not mean that high-quality films can be produced. High-quality products require the cooperation of the entire line. Now with the improvement of various industries, domestic die head processing equipment has made great progress in materials, heat treatment, and electroplating. In the past ten years, domestic equipment has basically used domestic die heads. In recent years, some products have begun to be equipped with imported die heads, but domestic die heads are still the main ones.

Cold roller: Many people think that the domestic machine can't start fast because there are problems with the die head and mechanical design. In fact, the roller is also a key problem. The thickness of domestic cast rollers is 18mm, and the thickness of foreign ones is 8mm. The cast film is cold-set suddenly. If it is too thick, it will be too slow to set and can't start fast. Imported machines start fast because they are thin. Now Shicheng is also making cast rollers with a thickness of about 9-10mm. I hope the production line can be accelerated.

Corona: The corona machine does not flatten the film when entering the corona, making the corona uneven. The film must be flattened before entering the corona. The hardness of the corona roller liquid silicone is best at 70 degrees. Now the domestic corona rollers are all around 65 degrees, which is not hard enough. Therefore, Shicheng's liquid rubber roller has reached 70 degrees. The main problem of the reverse corona is also on the rubber roller. The film is not flattened and the rubber roller is not hard enough.

Online synchronization: Many domestic cast film units are synchronized at about 5%. When starting up, the speed is slowly adjusted, and the thickness is uniform, but there is a lot of waste. In fact, the main problem is the electrical design and the selection of the reducer. Now Shicheng Company has improved it. The synchronization is about 0.1%, and it will also start faster and faster.

Operation procedures

Each type of product in the plastic extrusion production line has its operating characteristics. Only by having a detailed understanding of its operating characteristics can the efficiency of the machine be fully utilized. The extruder is one of the types of machines. Grasp the operating points of the extruder and use the extruder correctly and reasonably. The use of the screw extruder includes a series of links such as the installation, adjustment, commissioning, operation, maintenance and repair of the machine. Its use has the commonality of general machines, mainly in terms of the drive motor and the speed reduction device. However, the working system of the screw extruder, that is, the extrusion system, has its own unique characteristics. When using the screw extruder, special attention should be paid to its characteristics. The installation, adjustment and commissioning of the machine are generally clearly stipulated in the instruction manual of the extruder. Here are the key points of the operation, maintenance and care of the extruder: The operator must be familiar with the structural characteristics of the extruder he operates, especially the structural characteristics of the screw, the characteristics of the heating and cooling control instruments, the characteristics of the die head and the assembly conditions, so as to correctly grasp the extrusion process conditions and operate the machine correctly.

Use

The plastic extruder needs to go through three stages in the extrusion molding of plastics, namely, plasticization of raw materials, molding, and cooling. Plastic extruders are also divided into many different types. Extruders with screws are divided into single-screw extruders, twin-screw extruders, multi-screw extruders, etc.; extruders can be divided into continuous extrusion and non-continuous extrusion according to their functions. The output of the extruder is determined by the speed of the screw. The screw speed can affect the output of the extruder extruded plastic and the quality of the plasticization effect.

Working principle

The solid plastic is converted into a uniform melt by pressurization and shearing, and the melt is sent to the next process. The production of the melt involves mixing additives such as masterbatch, blending with resins, and re-grinding. The finished melt must be uniform in concentration and temperature. The pressure must be large enough to extrude the viscous polymer.

The plastic extruder completes all of the above processes through a barrel with a screw and a spiral path. Plastic pellets enter the barrel through a hopper at one end of the barrel and are then conveyed to the other end of the barrel by the screw. In order to have sufficient pressure, the depth of the screw threads decreases as the distance from the hopper increases. External heating and internal heat generated by friction between the plastic and the screw soften and melt the plastic. Different polymers and different applications often require different design requirements for plastic extruders. Many options involve discharge ports, multiple feed ports, special mixing devices along the screw, cooling and heating of the melt, or no external heat source (insulated plastic extruder), the relative size of the gap between the screw and the barrel, and the number of screws. For example, a twin-screw plastic extruder can make the melt more thoroughly mixed than a single-screw plastic extruder. Tandem extrusion is the use of the melt extruded from the first plastic extruder as raw material to supply the second plastic extruder, usually used to produce extruded polyethylene foam.

DL The characteristic dimensions of the plastic extruder are the diameter (D) of the screw and the ratio (L/D) of the length (L) of the screw to the diameter DL/D (D). Plastic extruders usually consist of at least three sections. The first section, close to the L/D) hopper, is the feeding section. Its function is to allow the material to enter the plastic extruder at a relatively stable rate. Generally, in order to avoid clogging of the feeding channel, this part will be kept at a relatively low temperature. The second section is the compression section, in which the melt is formed and the pressure increases. The transition from the feeding section to the compression section can be sudden or gradual (smooth). The last section is the metering section. Close to the plastic extruder outlet. The main function is to ensure that the material flowing out of the plastic extruder is uniform. In this section, to ensure the uniformity of composition and temperature, the material should have enough residence time.

At the end of the barrel, the plastic melt leaves the plastic extruder through a die, which is designed to an ideal shape, and the extruded melt stream passes through here.

Another important part is the drive mechanism of the plastic extruder. It controls the rotation speed of the screw, which determines the output of the plastic extruder. The required power is determined by the viscosity (flow resistance) of the polymer. The viscosity of the polymer depends on the temperature and flow rate, and decreases with the increase of temperature and shear force. Plastic extruders are equipped with filters that can block impurities on the filter. To avoid downtime, the filter should be able to be replaced automatically. This is particularly important when processing resins with impurities, such as recycled materials. The screw of the extruder is divided into feeding section, plasticizing section, and melting section. The temperature is based on the process parameters of the plastic particles. The model is divided into 20, 36, 52, 65, 75, 95, 120, and 135 according to the screw diameter. After the plastic particles are heated, the movement of the screw changes the original state. There are many types, depending on the specific application. The capacity of the frequency converter is proportional to the diameter of the screw, and is adjusted according to the different raw materials.