1. What is Twin Screw Extruder

The twin-screw extruder is developed on the basis of the single-screw extruder. Due to its good feeding performance, mixing and plasticizing performance, venting performance, extrusion stability, etc., it has been widely used in the molding and processing of extruded products such as food and plastics.

twin screw extruder (1)

2. Twin Screw Extruder Types

According to the rotation direction of the screw, twin-screw extruders can be divided into co-rotating twin-screw extruders and counter-rotating twin-screw extruders.

The meshing state of the two screw threads of the co-rotating twin-screw extruder is divided into full meshing type, partial meshing type and non-meshing type.

The meshing state of the two screw threads of the counter-rotating twin-screw extruder is divided into full meshing type and non-meshing type.

According to whether the axis of the screw is parallel, it can be divided into an intermeshing counter-rotating twin-screw extruder with parallel axes and an intermeshing counter-rotating conical twin-screw extruder.

(1) Intermeshing co-rotating twin-screw extruder:

Because the speed of the co-rotating twin screws at the meshing point is opposite, one screw must pull the material into the meshing gap, while the other screw pushes the material out of the gap. As a result, the material is transferred from one screw to another. Advance in the shape of “∞”.

Because the gap in the meshing zone is very small, and the speeds of the thread and the screw groove at the meshing point are opposite, it has a high shear speed and a good self-cleaning effect, that is, it can scrape off any accumulated material adhering to the screw. The residence time of the material is very short, so the intermeshing co-rotating twin-screw extruder is mainly used for mixing and pelletizing.

(2). Intermeshing counter-rotating twin-screw extruder. In the intermeshing counter-rotating twin-screw extruder, the two screws are symmetrical. Due to the different rotation directions, the path of the material on one screw is changed by the other. The screw edges of the root screw are blocked and cannot form a “∞”-shaped movement.

In the solid conveying part, the material is conveyed forward in the form of a closed “C”-shaped small chamber.

However, in the design, a certain amount of clearance is left between the outer diameter of one screw and the root diameter of the other screw so that the material can pass through.

When the material passes through the radial gap between the two screws, it is subjected to strong shearing, stirring and calendering effects, so the material is plasticized better, and at the same time it obtains the compression ratio by gradually reducing the pitch, which is mostly used for processing products.

(3) Non-intermeshing counter-rotating twin-screw extruder: Its application is less than intermeshing type, and its working mechanism is different from intermeshing type, but similar to single-screw extruder, which conveys materials by friction and viscous drag.

In addition to moving in the direction of the machine head, the material also has a variety of flow patterns.

Since the two screws are not meshed, the radial gap between them is large, and there is a large leakage flow;

Because the relative positions of the screw edges of the two screws are staggered, that is, the material pressure on the thrust surface of one screw is greater than the material pressure on the drag surface of the other screw, so that flow occurs, that is, the material flows from the screw with higher pressure to the other. The flow of the drag surface of the root screw;

At the same time, as the screw rotates, the material is obstructed at A, resulting in flow and various other flow forms, so it has certain applications in mixing, exhausting, and devolatilizing.

(4) Compared with the parallel meshing counter-rotating twin-screw extruder, the volume of a series of C-shaped chambers formed by two screws and barrels gradually decreases from the feeding section to the discharging section. Larger powdery materials can be added in the feeding section. As the screw becomes smaller, the materials are compressed and melted.

In the discharging section, due to the small diameter of the screw and the small circumferential speed of the screw, the shear rate of the material here is low, and the friction heat generated is also small. It is suitable for processing heat-sensitive materials, so it is mainly used for processing PVC powder and direct processing. Into products.

3. Twin Screw Extruder Applications

1. Glass fiber reinforced, fuel resistant pelletizing (such as: PA6, PA66, PET, PBT, PP. PC reinforced flame retardant, etc.)

2. High filling material granulation (such as: PE, PP filled with 75% CaCO.)

3. Granulation of heat-sensitive materials (such as PVC, XLPE cable materials)

4. Dense masterbatch (such as: filled with 50% toner)

5. Anti-static masterbatch, alloy, coloring, low filling blending and granulation

6. Granulation of cable material (such as sheath material, insulating material)

7. Granulation of XLPE pipe material (such as masterbatch for hot water crosslinking)

8. Mixing and extrusion of thermosetting plastics (such as phenolic resin, epoxy resin, powder coating)

9. Hot melt adhesive and PU reactive extrusion granulation (such as: EVA hot melt adhesive, polyurethane)

10. K resin and SBS devolatilization granulation

4. Twin Screw Extruder Working Principle

From the point of view of the movement principle, the twin-screw extruder has different meshing, non-meshing and non-meshing types.

1. Co-rotating twin-screw extruder
This type of extruder has low speed and high speed. The former is mainly used for profile extrusion, while the latter is used for special polymer processing operations.

(1) Closely meshing extruder. The low-speed extruder has a tightly meshing screw geometry, in which the spiral edge shape of one screw closely matches the spiral edge shape of the other screw, that is, the conjugate screw shape.

(2) Self-cleaning extruder. The high-speed co-rotating extruder has a closely matched spiral shape. This kind of screw can be designed to have a relatively small screw gap, so that the screw has a closed self-cleaning effect.

This kind of twin-screw extruder is called a compact self-cleaning co-rotating twin-screw extruder.

2. Counter-rotating intermeshing twin-screw extruder
The gap between the two screw grooves of the tightly meshing counter-rotating twin-screw extruder is very small (much smaller than the gap in the co-rotating twin-screw extruder), so it can achieve positive conveying characteristics.

3. Non-intermeshing twin-screw extruder
The center distance between the two screws of the non-intermeshing twin-screw extruder is greater than the sum of the radii of the two screws.

twin screw extruder diagram

5. Twin Screw Extruder Opreation

1. The operation of instruments and meters in the extrusion production line  

1.  Inverter: Modern extruders are all controlled by inverter. For first-time users, as long as they learn the basic functions, they are all qualified before leaving the factory, so there is no need to move it before they are unfamiliar.

2.  Host current meter: The host current is used to observe the current when the host is running. This meter itself cannot be adjusted, but the host current can be adjusted by controlling the screw speed.

If the host current is too large, the screw speed should be increased. , Or reduce the amount of feed. In order to ensure the normal service life of the unit, during the first three months of use of the equipment, the host current cannot run at the highest (full load) current, which can reach 80% of the host current.

For example, the current of the main motor whose main motor is 45KW is 84.7A, and the current cannot exceed 68A during the running-in period. (Increasing the speed refers to increasing the speed of the motor. On the premise of the feed volume being constant, increasing the speed can reduce the current.

For example: if your feed volume is 20Hz, your speed is 200 rpm, and the inside of the barrel When the material is full, the current becomes larger, then the feeding volume remains the same, continues at 20Hz, and the speed increases to 300 rpm, then the conveying volume of the machine itself becomes larger, the load becomes smaller, and the corresponding current becomes smaller.)

3. Melt pressure gauge for die head: This table is used to observe the melt pressure in the extruder head. It cannot be adjusted by itself, but can be adjusted by the speed of the feeder. If the melt pressure of the die head is found to be too high, reduce The feeding speed of the slow feeder.

4. Gearbox oil pump pressure gauge: the normal display is 0.2-0.3. If the oil gauge does not display normally or does not display, check the oil circuit, whether the oil filter should be cleaned, or whether the oil circuit is blocked.

5.  Screw speed adjustment table: When it is turned on, the screw speed is always slow first and then fast, using a slow acceleration method.

2. Preparation before driving  

1. The plastic used for extrusion production should meet the required drying requirements, and further drying is required if necessary.

2.  Start the running equipment, from slow to fast, check whether it is running normally and observe the working status of the instrument.

3.  Turn on the heater to heat up the machine head body. When the temperature of each part reaches the set requirement value, it should be kept for about 40 minutes to make the temperature inside and outside the machine consistent.

4. Sometimes it is necessary to change the porous plate, filter screen, and add enough lubricating oil. The porous plate must be cleaned after use before it can be used again.

3, drive   

1. Before the plastic is extruded, no one is allowed to be directly in front of the die.

2.  Start driving at a low speed, idling, check whether the screw is abnormal and the motor current and other instruments are overloaded, whether the pressure gauge is normal (the machine idling time cannot be too long, the shorter the better, in order to prevent friction between the screws and the screw and The friction and scratches of the barrel or the screw caused by the barrel, the low-speed operation time of the screw should not exceed three minutes)

3.  Gradually add a small amount of material. When the plastic is extruded to the die, the screw speed can reach the normal operating speed before a large amount of material can be added.

4. After the plastic is extruded, the extrudate needs to be slowly led to the cooling and traction equipment, and these equipment should be turned on in advance,

Note: Do not drive cold or drive cold, because the extruder has not been “hot through” at that time (that is, the plastic in the extruder barrel has not fully reached the specified temperature, and the melt viscosity of the plastic is very high). Driving, especially the old extruder, may twist the screw. New extruders are generally equipped with torque overload protection devices.

A reliable and most effective method is to turn the coupling part by hand when the temperature rises to the required temperature value before driving, three turns to the left, After three turns to the right, it can be turned on after smooth rotation. If it cannot rotate smoothly, it just fails to turn on.

4, parking  

1.  Stop feeding, squeeze the plastic in the extruder as much as possible, turn off the power supply of the barrel and the head for the next operation.

2. Turn off the power of each auxiliary machine while turning off the power of the main engine.

3.  ①Stop normally. When the extruder is shut down without failure, it can be stopped in the following order: close the hopper outlet gate; adjust the feeder speed to zero, press the stop switch button; close the vacuum system, and gradually reduce the screw speed (if you need to change For new materials, the remaining material in the barrel should be drained as much as possible until the material is basically empty), after the main engine speed is adjusted to zero, press the main motor stop button; cut off the multi-power switch on the main control cabinet to cut off the power supply Main power switch; close each water inlet valve; clean the machine.

② Emergency stop. Once an emergency occurs during the operation of the twin-screw extruder unit, press the emergency stop button immediately (or press the stop button of the faulty part, the main engine will usually be stopped); after the emergency stop, remember to immediately turn on the main motor and the feeding motor Adjust the speed control knob to the zero position and shut down other auxiliary systems. After the fault is resolved, restart the car in the normal driving sequence.

③The dismantling steps of the twin-screw extruder. It has the same content as the single screw extruder. If the machine stops after extruding heat-sensitive materials such as PVC or POM, it is usually necessary to use polyolefin plastic to squeeze the material in the barrel to prevent the material from overheating and decomposing due to too long heating time before the next start-up.

Dismantle the car for cleaning if necessary. When disassembling the car, loosen the head flange while it is hot, remove the filter plate and filter screen, and then start the screw operation, extrude the remaining material in the barrel, pull out the screw while it is hot, and then use a copper rod and a copper shovel to quickly clean it Residual materials in the head, screw and barrel.

The cleaned parts should be returned in time. If stored, they should be coated with a little anti-rust oil. When the screw is placed, it should be hung.

6. Advantages of Twin Screw Extruder

What are the advantages of twin screw extruder?

1. Wear condition

Due to the convenient opening, the degree of wear of the threaded elements and the inner liner of the barrel can be found at any time, so that effective maintenance or replacement can be carried out. It will not be discovered when there is a problem with the extruded product, causing unnecessary waste.

2. Reduce manufacturing cost

When manufacturing masterbatch, it is often necessary to change the color. If it is necessary to change the product, open the open processing area within a few minutes. In addition, the mixing process can be analyzed by observing the melt profile on the entire screw. When changing the color of an ordinary twin-screw extruder, a large amount of cleaning material is needed to clean the machine, which is time-consuming, energy-consuming, and waste of raw materials. The split twin-screw extruder can solve this problem. When changing the color, it only takes a few minutes to quickly open the barrel for manual cleaning, so that no or less cleaning materials can be used, which saves costs.

3. Improve labor efficiency

During equipment maintenance, the heating and cooling system of the ordinary twin-screw extruder is often removed first, and then the screw is drawn out as a whole. The split twin screw is not needed. Just loosen a few bolts and turn the worm gear box handle device to lift the upper part of the barrel to open the entire barrel and then perform maintenance. This not only shortens the maintenance time, but also reduces the labor intensity.
High torque, high speed

The development trend of twin-screw extruders in the world is toward high torque, high speed, and low energy consumption. The effect of high speed is high productivity. The split twin-screw extruder belongs to this category, and its speed can reach 500 rpm. Therefore, it has unique advantages in processing high-viscosity, heat-sensitive materials.

In terms of the core technology of high speed and high torque, only German and Japanese related manufacturers have the core technology of asymmetric and symmetric high torque gearboxes, and the speed can reach more than 1800 rpm. However, the only domestic masters of this core technology are Sichuan Zhongzhuang Technology Co., Ltd. is also one of the main choices of domestic high-end material processing manufacturers, and belongs to the domestic independent innovation national encouragement project

4. Wide range of applications

Wide range of applications, suitable for processing a variety of materials

5. High output, high quality

It has other advantages of ordinary twin-screw extruder, and can realize high output, high quality and high efficiency.

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