Single Screw Extruder VS Twin Screw Extruder

Introduction about single screw extruder vs twin screw extruder

The single screw extruder vs twin screw extruder is an enduring theme in the plastics base industry! The twin-screw extruder is developed based on the single-screw extruder. Due to its excellent feeding performance, mixing plasticizing performance, exhaust performance, and extrusion stability, it has been widely used in the molding processing of extruded products. So, what are the advantages and differences between single-screw and twin-screw extruders?

Introduction and Usage of Single-Screw Extruders

(1) Introduction to Single-Screw Extruders

As the name suggests, a single-screw extruder is one in which there is one screw in the barrel. It is usually divided into three sections in terms of effective length, based on the screw diameter, pitch, and depth. This is generally divided into thirds.

The first section: starting from the last thread of the feed port, is called the conveying section. The material here should not be plasticized, but should be preheated and compressed. It was previously thought that the material here was loose, but later proved to be a solid plug. In other words, the material here is like a solid plug after being compressed, so its function is simply to complete the conveying task.

The second section: the compression section. The volume of the screw groove here gradually decreases, and the temperature must reach the plasticization degree of the material. The compressed material (from volume 3 in the conveying section, compressed to volume 1 here, this is called the screw compression ratio—3:1, some machines are different) enters the third section after it is plasticized.

The third section: the metering section. The material here maintains the plasticizing temperature, just like the metering pump, it accurately and quantitatively delivers the molten material to supply the machine head. The temperature here cannot be lower than the plasticizing temperature, usually a bit higher.

(2) Application of Single-Screw Extruders

Single-screw extruders are mainly used for the extrusion of pipes, sheets, plates, and profiles, and some modified material granulation.

(3) Maintenance of Single-Screw Extruders

① As the electrical control system has high requirements for environmental temperature and dust prevention, the electrical system should be isolated from the production site and ventilation or exhaust fans should be installed. It is recommended to place the electrical control cabinet in a simple room to keep the room clean, ventilated, and the room temperature not higher than 40℃.

② Single-screw extruders are not allowed to run empty to prevent the screw and barrel from rolling hair. The host is not allowed to exceed 100r/min when starting to idle; start the host at a low speed, and after starting the host, check for any abnormal noise before slowly increasing the host speed to within the allowable range of the process (adjusting to the best state is preferred). During the running-in of the new machine, the current load should be 60%-70%, and the current should not exceed 90% during normal use. Note: If there is an abnormal sound when the extruder is running, stop immediately for inspection or repair.

③ Turn on the oil pump first when starting up, and turn off the oil pump after shutting down; the water pump must remain operational throughout the production process and the operation of the water pump cannot be stopped to prevent the barrel temperature from rising and causing the material in the barrel to decompose and carbonize; the asbestos air hood of the main motor fan needs to be regularly cleaned to prevent excessive dust from blocking the air hood and causing the motor to overheat and trip.

④ Clean the dust, tools, and debris on the surface of the machine group in time.

⑤ Prevent metal or other debris from falling into the hopper to avoid damaging the screw and barrel. To prevent iron impurities from entering the barrel, a magnetic component or magnetic rack can be installed at the material inlet to the barrel, and to prevent debris from falling in, the material must be screened in advance.

⑥ Pay attention to the cleanliness of the production environment, and do not allow garbage and impurities to mix into the material and block the filter plate, affecting the product output and increasing the head resistance.

⑦ The gearbox should use the lubricating oil specified in the machine manual and add oil to the specified oil level. If there is too little oil, the lubrication is insufficient, and the life of the parts is reduced; if there is too much oil, there is a lot of heat, a lot of energy is consumed, and the oil is easy to deteriorate, which also causes the lubrication to fail and damage the parts. The sealing gasket (ring) of the gearbox oil leakage part should be replaced in time to ensure the amount of lubricating oil.

Introduction and Uses of Twin Screw Extruder

Figure 2 is a schematic diagram of a twin-screw extruder suitable for large-diameter PVC-U double-wall corrugated pipes.

In the figure, the labels are: 1- machine head connector, 2- twin screw, 3- heating coil, 4- machine barrel, 5- distribution box, 6- coupler, 7- frequency conversion motor, 8- machine frame, 9- double hopper, 31- cast copper heating coil, 32- cast aluminum heating coil, 41- pressure gauge.

(1) Introduction to the Twin Screw Extruder

There are many types of twin screw extruders, which can be divided into meshing and non-meshing types. The meshing twin screw extruder can be further divided into co-rotating and counter-rotating types.

The twin screw extruder contains the following multiple systems, the screw system mainly completes the plasticization and transportation of the material, which greatly affects the performance quality of the finished product.

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

② External heating system: Mainly uses heating rods and barrel to efficiently heat the material, promoting plasticization.

③ Cooling system: Uses a heat exchange system composed of heat-conducting oil or water to reduce the heat of the machine body, thereby effectively controlling the temperature of the barrel.

④ Hydraulic screen changing system: Uses replaceable filter screens to intercept impurities, improve the degree of plasticization, and ensure the quality uniformity and stability of the output material.

⑤ Vacuum system: Extracts moisture and other low-molecular volatiles from the material.

⑥ Electric control system: Responsible for monitoring and controlling the related equipment of the main and auxiliary material systems.

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

（2）Segmentation of Twin Screw Extruder

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

① Conveying section: The function is to convey materials and prevent backflow.

② Melting section: This section makes the material fully melted and mixed through heat conduction and friction shearing.

③ Plasticizing section: It makes the components of the material further melt and mix, with the function of distributive and dispersive mixing.

④ Discharge section: Conveying and pressurizing, forming a certain pressure, making the material more compact, and further mixing, to achieve the purpose of extrusion granulation.

（3）Conveying Elements

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

② Use effect of lead: The more the lead, the more the extrusion of the screw increases, the shorter the residence time of the material, and the weaker the mixing ability of the material.

③ The usual usage method of small lead: gradually decreasing in combination, used for conveying section, melting section, plasticizing section, to increase pressure, improve melting; improve the homogeneity of the mixture and the stability of the conveyance volume.

（4）Cutting Components

① Directional Classification: There are forward and reverse directions. Forward direction promotes material flow and its functionality; reverse direction, also known as anti-rotation, has a backflow effect on the material transport, prolonging the material’s dwell time, enhancing its plasticizing ability, and improving the mixing effect.

② Angle Classification: Generally, there are 30°, 45°, 60°, and 90°.

③ Role and Effect: When forwarding, increasing the stagger angle reduces the transport capacity, extends the dwell time, and enhances the degree of plasticization. In terms of dispersion, the larger the angle, the more noticeable the effect; the dispersion mix is generally best at an angle of 45°, followed by 30°, and the worst is at 60°.

④ The Effect of Number of Heads

When forwarding, the fewer the number of heads, the greater the extrusion transport capacity, the greater the torque, the better the mixing effect, but the less the shearing action; in reverse, the fewer the number of heads, the smaller the extrusion transport capacity, the better the mixing effect.

Single Extruder VS Twin Screw Extruder

Next, let’s specifically understand the differences between these two types of extruders in terms of price, operability, and screws.

Items	Single Screw	Twin Screw
Screw Length-to-diameter Ratio (L/D)	comparatively small	comparatively long
Screw Cross-section Thread Shape	Rectangular	Circular
Screw Cylinder Structure	Pinch pin on screw barrel	Smooth inner surface of the screw barrel
Screw Temperature Controls	Internal screw cooling for easy temperature control	The cooling seal inside the screw is relatively poor, and the temperature is not easy to control.
Variety and Convenience of Paint Changes	Convenient	General
Difficulty of Processing Special Powder Coatings Such As Fast-curing Coatings	Comparatively Easy	General
Difficulty of Equipment Maintenance	Relatively Difficult	Easy
Sensitivity to Metallic Foreign Bodies in Boredom	Sensitive, not easy to process recycled powder materials	Less sensitive

（1）Single Screw Extruder is More Affordable

The structure of a single screw extruder is simple, thus, it’s more affordable.

The structure of a twin screw extruder is complex and more expensive. A twin screw extruder is about twice as expensive as a single screw extruder. The price can be even higher depending on the manufacturer and model.

（2）Single Screw Extruder Has a Simpler Formulation

In terms of operation, there is not much difference between the two. However, the process and formulation of a single screw extruder are simpler, while those of a twin screw extruder are more complex.

（3）Twin Screw Extruder Allows for Free Combination of Screws

The threads of a twin screw extruder can be freely combined according to the material conditions and the operator’s preferences. The single screw extruder is more rigid and does not allow for combination of screws.

（4）Twin Screw Extruder is More Efficient

The twin screw extruder has a larger output, quicker extrusion speed, and lower energy consumption per unit output than the single screw extruder. The efficiency of the twin screw extruder is mostly about twice that of the single screw extruder, depending on the manufacturer and model.

（5）Strong Plasticizing Ability of Twin-Screw

Single screw is suitable for the plasticizing extrusion of polymers, suitable for the extrusion processing of granular materials, such as molding, blow molding, injection molding, etc., with a wide range of material uses.

The twin-screw extruder has a good plasticizing and mixing ability, which is more suitable for plastic modification. And the twin-screw extruder can produce dual colors.

（6）Single Screw is Easy to Maintain

In terms of maintenance, single screw is simpler than twin screw because the structure of single screw is simpler and easier to repair.

（7）Conveying Mechanism

The conveying of the single-screw extruder mainly relies on the friction force generated between the material and the machine barrel. The counter-rotating twin-screw extruder is forward conveying, forcibly pushing the material forward, and the counter-rotating twin-screw extruder also generates shearing effect on the material at the meshing place of the two screws. (Specific to counter-rotating twin-screw extruders)

（8）Speed Field

The speed distribution in the single-screw extruder is relatively clear and easy to describe, while the situation in the counter-rotating twin-screw extruder is quite complicated and difficult to describe. This is mainly due to its screw has a meshing area, the complex flow occurring in the meshing area 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 of the meshing area. (Specific to counter-rotating twin-screw extruders)

Application Examples

（1）Application Examples of Twin Screw Extruder

Applications include fiberglass reinforcement, flame retardant pelletization (such as PA6, PA66, PET, PBT, PP, PC enhanced flame retardant, etc.), high filling pelletization (such as PE, PP filled with 75% CaCO3), thermosensitive material pelletization (such as PVC, XLPE cable material), high concentration color masterbatch (such as filling 50% color powder), anti-static masterbatch, alloys, coloring, low filling blending pelletization, cable material pelletization (such as sheath material, insulation material), XLPE pipe material pelletization (such as masterbatch for heat water crosslinking), thermosetting plastic mixing extrusion (such as phenolic resin, epoxy resin, powder coating), hot melt adhesive, reactive extrusion pelletization (such as EVA hot melt adhesive, polyurethane), K resin, SBS volatilization pelletization, etc.

（2）Application Examples of Single Screw Extruder

It is suitable for PP-R pipes, PE gas pipes, PEX crosslinked pipes, aluminum-plastic composite pipes, ABS pipes, PVC pipes, HDPE silicon core pipes and various co-extruded composite pipes; it is suitable for extrusion of PVC, PET, PS, PP, PC profiles and sheets, and other various plastic extrusions such as silk, rods, etc.; by adjusting the speed of the extruder and changing the structure of the extrusion screw, it can be used to produce PVC, polyolefin and other various plastic profiles.

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