What is the difference between single screw and twin screw extruder
Twin-screw extruders are the most common machinery in the modified plastics industry. They are widely used in the polymer processing industry and other production and processing fields.
There are many types of twin-screw extruders, which can be divided into two categories: intermeshing and non-intermeshing. Intermeshing twin-screw extruders can be divided into two categories: co-rotating and counter-rotating.
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1. Working principle of co-rotating twin-screw extruder and single-screw extruder
The structure and function of the co-rotating twin-screw extruder are very similar to the single-screw extruder, but there are big differences in the working principle. Mainly manifested in the following aspects:
1) Forced delivery.
The meshing co-rotating twin screws, where the two screws move in opposite directions, one screw pulls the material into the meshing gap, while the other screw pushes the material out of the gap, resulting in the material from a screw groove.
Turning to another screw groove, the material is forced to be conveyed in the direction of the machine head in the shape of “∞” along the screw.
2) Homogenization and mixing.
The co-rotating twin-screw has a small gap at the meshing position, and the speed of the screw edge and the screw groove are opposite, and the relative speed is high.
Therefore, the meshing zone has a high shear speed and a large shear force. The mixing effect is much better than that of a single Screw extruder and counter-rotating twin screw extruder.
3) Self-cleaning.
Co-rotating twin-screw extruder, due to the opposite direction of the speed of the screw edge and the screw groove in the meshing zone, the relative speed is high, so it has a fairly high shear speed, which can scrape off any accumulated material adhering to the screw.
Good self-cleaning effect, so that the residence time of the material is very short, and it is not easy to cause local degradation and deterioration.
4) Plasticization of materials.
The size of the screw gap has a great influence on the plasticization quality of the material.
The smaller the gap, the greater the shearing force, but the amount of material passing through is reduced;
the larger the gap, the amount of material passing through increases, but the shearing force decreases.
5) Compression of materials.
The co-rotating twin-screw extruder has many more ways to compress materials, and the overall effect is good.
6) Feeding method.
The co-rotating twin-screw extruder requires uniform and quantitative feeding, and adopts the metering and starvation feeding method.
7) Exhaust.
Because it is hungry feeding, a large-lead screw conveying element can be used to make the screw groove in an under-filled state and in a zero-pressure state, so that an exhaust section can be set.
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2. Application of twin screw extruder
The twin-screw extruder can be divided into counter-rotating twin-screw and co-rotating twin-screw according to the rotation direction of the two screws.
The intermeshing co-rotating twin-screw extruder is widely used in the physical and chemical modification of matrix resin, such as filling, strengthening, toughening, reactive extrusion, etc.
For the blending based on the dispersion and mixing of various raw materials, the same-rotating twin-screw extruder should be selected.
The speed vectors of the same-rotating twin-screws shown in the above figure are opposite to each other, so the twin-screws rotating in the same direction are opposite to each other.
Compared with the rotating twin screw, its shearing effect is much greater, which is not suitable for the mixing and molding of PVC.
Commonly used in blended and filled plastics are powdered calcium carbonate, talc, and titanium dioxide. The general filling amount is 30% to 60%, and the filling masterbatch is up to 80%.
Since the powdered filler contains a large amount of air, the air will be separated during mixing in the kneading zone of the screw, causing the material to flow backwards into the screw groove, affecting the forward conveying of the powder, and ultimately reducing the amount of extrusion.
Application examples of twin-screw extruders:
glass fiber reinforced, fuel-resistant pelletizing (such as PA6, PA66, PET, PBT, PP, PC reinforced flame retardant, etc.);
high-filling pelletizing (such as PE, PP filled with 75% CaCO3.);
heat-sensitive material granulation (such as PVC, XLPE cable material);
dense color masterbatch (such as filling with 50% toner);
anti-static masterbatch, alloy, coloring, low filling blending granulation; cable material granulation (Such as sheath material, insulating material);
granulation of XLPE pipe material (such as masterbatch used for hot water crosslinking);
mixing and extrusion of thermosetting plastics (such as phenolic resin, epoxy resin, powder coating);
hot melt adhesive , PU reactive extrusion granulation (such as EVA hot melt adhesive, polyurethane); K resin, SBS devolatization granulation, etc.
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3. Advantages and disadvantages
Due to the complexity of the structure design and processing of the twin-screw extruder, the theoretical development is not perfect, but the combined design of the same-rotating twin-screw extruder can arrange and combine threaded elements and barrel elements of various geometric shapes.
According to experience-theory-practice-inspection, the design can be optimized to meet the needs of different process formulations according to different mixing requirements, so that the co-rotating twin-screw extruder has strong adaptability. Mainly manifested in the following aspects:
1) Mixing performance: Because the two screws are meshed with each other, and various threaded elements can be added as needed, such as reverse threaded elements, meshing disc elements, toothed disc elements, etc., to precisely adjust the shearing/mixing, thereby The mixing intensity and mixing quality (whether distributed mixing or dispersive mixing) can be effectively controlled. This is beyond the reach of a single screw extruder.
2) Processing flexibility: co-rotating twin-screw extruders usually use metered starvation feeding. The output depends on the feeding amount and is independent of the screw speed.
It is an independently controllable variable, so it can flexibly handle multiple feeds on one machine. Various processing functions, such as melting, mixing, venting, reaction, etc.
3) Controllability of process parameters: narrow residence time distribution of co-rotating twin-screw extruders, better convective heat transfer and precisely controlled temperature curve, can obtain better shear-time-temperature history during extrusion , Input mechanical energy in a wider range, so as to get better product quality stability.
4) High efficiency of process production: The positive displacement conveying effect of the screw can handle more kinds of raw materials and mixed formulas, and the downtime is short.
5) Higher economy: Due to high process flexibility and productivity, a wider range of end products can be produced, and a highly consistent product quality can be provided. The screw speed can also be adjusted to compensate for screw wear.
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4. Difference between a counter-rotating twin-screw extruder and a single-screw extruder
What is the input principle of the counter-rotating twin screw extruder?
In the counter-rotating twin-screw extruder, the material is conveyed by a positive displacement similar to that in a gear pump.
The counter-rotating twin-screw extruder has a low speed, low shearing heat of the material, and the material is not easy to decompose.
Therefore, it does not require high thermal stability of the material. It is especially suitable for the molding and processing of heat-sensitive materials (such as PVC).
Compared with the same-rotating twin-screw extruder, the counter-rotating twin-screw extruder has higher conveying efficiency, better venting effect and better melting effect.
Although the dispersion and mixing effect of the material is worse than that of the same-rotating twin screw extruder, However, the ability to establish a stable head pressure is stronger, so it is more suitable for direct extrusion of products.
In addition, a counter-rotating twin-screw extruder is used instead of a single-screw extruder, which can eliminate the pre-plasticization and granulation process and directly extrude and process the PVC powder, which reduces the cost of the product.
The counter-rotating twin-screw extruder is mainly used for extruding PVC pipes. Profiles, plates and pellets. Parallel counter-rotating twin-screw extruders have also been applied in large HDHDPE pipe production lines.
The counter-rotating conical twin-screw extruder is traditionally used for the extrusion of polyvinyl chloride products.
But in recent years, there have been examples of the counter-rotating conical twin-screw extruder applied to the extrusion of polyolefin pipes in China.
Practice shows that, Basic success has been achieved. The purpose of this approach is to save energy.
A 65-body 132umum conical twin-screw extruder with a drive motor power of only 37kW.
When extruding polyethylene pipes, the pipe specifications and output can be equivalent to a 75mm or even 80mm single-screw extruder.
A 75mm or 80mm single-screw extruder has a driving power of at least 90kW.
There are already existing mature experience in extruding products with parallel counter-rotating twin-screw extruders in foreign countries.
In fact, the use of counter-rotating conical twin-screw extruders in other ways in China is actually due to equipment cost considerations.
Parallel counter-rotating twin-screw extruders The manufacturing cost is higher than the counter-rotating conical twin-screw extruder.
This method of use is certainly not a simple alternative. After all, the melt flow properties of polyethylene melt and polyvinyl chloride are significantly different.
Therefore, the counter-rotating conical twin-screw extruder used to extrude polyvinyl chloride products requires
The screw is redesigned before it can be used to extrude polyethylene; and the head should be modified accordingly.
In recent years, some people in China have tried to use co-rotating twin-screw extruders (including conical twin and flat twin) to extrude products, and some progress has been made.
However, the author believes that from the perspective of energy saving, co-rotating and counter-rotating twin screws are not significantly.
However, it is more difficult for a co-rotating twin-screw extruder to establish a stable head pressure. It is not as simple and easy as directly using a counter-rotating twin-screw.
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5. Differences between the counter-rotating twin-screw extruder and the single-screw extruder
1) Their transport mechanisms are different. The material conveying in the single-screw extruder is a drag flow, the solid conveying process is friction drag, and the melt conveying process is viscous drag.
The friction factor between the solid material and the metal surface and the viscosity of the melt material are very large. To a certain extent, it determines the conveying capacity of the single-screw extruder.
The material conveying in the counter-rotating twin-screw extruder is positive displacement conveying. As the screw rotates, the material is
The intermeshing threads are forced forward, and its positive displacement conveying ability depends on the closeness of the screw edge of one screw to the screw groove of the other screw.
A counter-rotating twin-screw extruder with close meshing can achieve the greatest positive displacement conveying.
2) The velocity fields of the two are different. The speed distribution in the single-screw extruder is relatively clear and easy to describe, while the situation in the twin-screw extruder is quite complicated and difficult to describe.
This is mainly due to the meshing zone in the twin-screw extruder.
The complex flow that occurs in the meshing zone makes the twin-screw extruder have many advantages such as sufficient mixing, uniform heat transfer, strong melting ability, and good exhaust performance.
But it is difficult to be accurate Analyze the flow state in the meshing zone.