Extruders in the domestic plastic industry are roughly divided into two categories, one is a single screw extruder,  and the other is a twin screw extruder, because these two devices have their own characteristics,  so they are widely used in the plastic industry. So what are the advantages of these two extruders,  and what are the differences between them?

01 Introduction and use of single screw extruder

(1) Introduction of single screw extruder

Single screw extruder As the name suggests, there is a screw inside the extruder cylinder. Generally in the effective length is divided into three sections, according to the diameter of the screw, pitch, screw depth to determine the effective length of the three sections, generally divided by one-third of each.

Section: Starting from the last thread of the feed port, it is called the conveying section. The material here is required not to plasticize, but to preheat, compressed compaction, the old extrusion theory in the past that the material here is loose body, and later proved that the material here is actually a solid plug, that is, the material here is a solid like a plug after extrusion, so as long as the completion of the conveying task is its function.

The second segment is called the compression segment. This section of the screw tank body volume gradually decreases from large, and the temperature to reach the degree of plasticization of the material, where compression is generated (from the delivery section three, here can be compressed to one, which is called the compression ratio of the screw - 3:1, some machines are different), and the plasticized material enters the third section.

Section 3: Measurement section. Here the material to maintain the plasticizing temperature, just like the metering pump as accurate, quantitative transport of melt materials to supply the head, at this time the temperature can not be lower than the plasticizing temperature, generally slightly higher.

(2) Application of single screw extruder

The single screw extruder is mainly used in the extrusion of pipe, sheet, plate and profile, and some modified materials are granulated.

(3) Maintenance of single screw extruder

① Because the electrical control system has high requirements for ambient temperature and dust prevention, the electrical system should be separated from the production site, and ventilation or ventilation fans should be installed, it is recommended to put the electric control cabinet in the simple room, keep the room clean and ventilated, so that the indoor temperature is not higher than 40 ° C.

② Single screw extruder does not allow empty operation, so as not to screw and machine plain wool. When the host starts idling, the value cannot exceed 100r/min. Start the host at a low speed, check whether there is any abnormal noise after starting the host, and then slowly increase the host speed to the process allowed range (can be adjusted to a better state is better). When the new machine is run in, the current load should be 60-70%, and the current in normal use is better not more than 90%. Note: If there is an abnormal sound during the operation of the extruder, it should be stopped immediately for inspection or repair.

③ Start the oil pump first, shut down after the oil pump; The pump keeps working during the entire production process, and the operation of the pump can not be stopped, so as to avoid the rise of the temperature of the barrel leading to the decomposition and carbonization of the materials in the barrel; The asbestos hood of the main motor fan needs to be cleaned regularly to avoid dust adhesion too much clogging the hood resulting in insufficient motor heat dissipation due to overheating and tripping.

④ Timely clean the dirt, tools and debris on the surface of the unit.

⑤ Prevent metal or other debris from falling into the hopper, so as not to damage the screw and barrel. In order to prevent iron debris from entering the barrel, magnetic absorbing parts or magnetic frames can be installed at the feeding port of the material into the barrel to prevent debris from falling into the material must be screened in advance.

Pay attention to the clean production environment, do not mix garbage impurities with materials to block the filter plate, affect the output of products, quality and increase the head resistance.

The reducer should use the lubricating oil specified in the machine manual, and add the oil according to the specified oil level height, the oil is too little, the lubrication is not enough, and the service life of the parts is reduced; Too much oil, heat, energy consumption, oil is easy to deteriorate, also make the lubrication failure, resulting in damage to parts. The sealing gasket (ring) should be replaced in time to ensure the amount of lubricating oil.

02 Introduction and use of twin screw extruder

(1) Introduction of twin-screw extruder

The twin-screw extruder contains the following systems, the screw system mainly completes the plasticizing and conveying process of the material, which has a great impact on the performance and quality of the finished product.

① Feeding system: including hopper, mixing motor, feeding motor. It can prevent material accumulation and facilitate its smooth entry into the feed port.

② External heating system: The main use of heating rods and cylinders for efficient heat increase of materials to promote plasticization.

③ Cooling system: The use of heat transfer system composed of heat transfer oil or water to reduce the body heat, so as to achieve effective control of the cylinder temperature.

④ Hydraulic net change system: The use of replaceable filter to intercept impurities, improve the degree of plasticization, to ensure the quality of the output of the material uniformity and stability.

⑤ Vacuum system: extract water and other low molecular volatiles in the material.

⑥ Electronic control system: responsible for monitoring and controlling the related equipment of the main auxiliary system.

7 Screw system: the most important part of the extruder, can be divided into conveying section, melting section (exhaust port), plasticizing section (vacuum port), discharge section.

(2) Segment of twin-screw extruder

The screw system of parallel twin screw extruder is generally divided into 4 sections: conveying section, melting section (exhaust port), plasticizing section (vacuum port), discharge section.

① Conveying section: the function is to transport materials and prevent feedbacks.

② Melting section: This section through heat conduction and friction shear, so that the material is fully melted and mixed.

③ Plasticizing section: the components of the material are further melted and mixed, with distribution and dispersion mixing function.

④ Discharge section: conveying and pressurizing, forming a certain pressure, making the material closer, while further mixing, to achieve the purpose of extrusion granulation.

(3) Conveying components

① Types are divided into: large leads and small leads.

② Lead use effect: the more leads, the more screw extrusion, the shorter the material residence time, the weaker the material mixing capacity.

(3) The usual use of small leads: gradually reduce the combination, used for conveying section, melting section, plasticizing section, pressurization, improve melting; Improve the homogenization degree of the mixture and the stability of the throughput.

(4) Shear element

① Direction classification: there are forward and reverse. Positive, promote material flow and realize its function; Reverse, also known as reverse rotation, has a reflux effect on the conveying of materials, extends the residence time of materials, enhances the plasticizing ability, and improves the mixing effect.

② Angle classification: generally has 30°, 45 °, 60 °, 90 ° points.

③ Action and effect: positive, increase the staggered Angle, reduce the conveying capacity, extend the residence time, improve the degree of plasticization. For dispersion, the larger the Angle, the more obvious the effect; Dispersion mixing is generally better at an Angle of 45°, followed by 30° and worst at 60°.

④ The effect of the number of heads

When the number of heads is less, the extrusion conveying capacity is larger, the torque is larger, the mixing effect is better, but the shearing effect is less;

In reverse, the fewer the number of heads, the smaller the extrusion conveying capacity and the better the mixing effect.

03

The difference between single and twin screw extruders

Let's understand what the difference is between these two extruders from the price, operability, screw and other aspects.

(1) The price of single screw extruder is lower

Single screw extruder has simple structure and low price;

Twin-screw structure is complex and the price is high. Twin-screw extruders are about 1 times more expensive than single-screw extruders. Individual will be higher, specific depending on the manufacturer, model.

(2) Single screw formula is simple

For operations, the difference is actually not that great. However, the process of single screw extruder, the formula is relatively simple, and the process, process and formula of twin screw extruder will be more complex.

(3) Twin screw can be freely combined screw

Twin-screw threads can be combined according to the material situation and the operator's mood. Single screw is more rigid, can not combine screw.

(4) Double screw efficiency is higher

The double screw extruder has large output, fast extrusion speed and low energy consumption per unit output, while the single screw is worse. The efficiency of twin-screw extruders is mostly about 1 times that of single-screw extruders. Specific depends on the manufacturer, model.

(5) Twin screw mixing plasticizing ability is strong

Simply put, single screw is suitable for plasticizing polymer extrusion, suitable for extrusion processing of particles, such as: molding, blowing film, injection molding, etc., the use of a wide range of materials;

The twin-screw extruder has a good mixing plasticizing ability and is more suitable for the modification of plastics. And twin-screw extruders can produce two colors.

(6) Single screw for easy maintenance

In terms of maintenance, a single screw will be simpler than a twin screw, because the structure of a single screw is simple and easy to repair.

(7) Transport mechanism

The conveying of single screw extruder mainly relies on the friction generated between the material and the barrel, the heterodirectional twin screw extruder is a forward conveying, which has the effect of forcing the material forward, and the heterodirectional twin screw extruder also has a shearing effect on the material at the mesh of the two screws. (for twin screw extruders with different direction)

(8) Velocity field

The velocity distribution in a single screw extruder is relatively clear and easy to describe, while the situation in a different direction twin-screw extruder is quite complex and difficult to describe. This is mainly because the screw has a meshing zone, the complex flow in the meshing zone makes it have many advantages such as sufficient mixing, uniform heat transfer, strong melting ability, good exhaust performance, etc., but it is difficult to accurately analyze the flow state in the meshing zone. (for twin screw extruders with different direction)

(9) Application examples

Application examples of twin-screw extruder: Glass fiber reinforced, fuel resistance granulation (such as PA6, PA66, PET, PBT, PP, PC enhanced flame retardant, etc.), high filling material granulation (such as PE, PP filled 75% CaCO3), heat sensitive material granulation (such as PVC, XLPE cable material), concentrated color masterbatch (such as filled 50% color powder), Anti-static masterbatch, alloy, coloring, low-fill blend granulation, cable material granulation (such as sheath material, insulation material), XLPE pipe material granulation (such as masterbatch for hot water crosslinking), thermosetting plastic extrusion (such as phenolic resin, epoxy resin, powder coating), hot melt adhesive, PU reaction extrusion granulation (such as EVA hot melt adhesive, polyurethane), K resin, SBS devolatilization granulation, etc.

Single screw extruder application example: suitable for PP-R pipe, PE gas pipe, PEX cross-linked pipe, aluminum-plastic composite pipe, ABS pipe, PVC pipe, HDPE silicon core pipe and a variety of co-extruded composite pipe; Suitable for extrusion of PVC, PET, PS, PP, PC and other profiles and plates, and extrusion of other plastics such as silk, rod, etc. Adjusting the speed of the extruder and changing the structure of the extruder screw can be applied to the production of PVC, polyolefin and other plastic profiles.