Nanjing Rubber & Plastics Machinery Plant

Parallel co-rotating twin screw extruder is normally made up of two-stage extruder. It can make wood flour dry and resin melting procession separate. Besides, it also can operate wood flour and plant fibers directly and then continually mix and extrude wood flour with the melting resin after wood flour is dried.

This kind of two-stage extruder can be called as “extruder for wood”. Although this kind of two-stage extruder can dry wood flour, it has certain requirement for water contention, normally 4% to 8 %( mass fraction). Another form is similar to this one. Wood flour is added into the main charging hole of extruder and the front part of the extruder is the dehydration equipment, and then add the plastic resin and additive through the side feed device so that the screw of extruder is relatively longer .

Parallel co-rotating twin screw extruder can be described as the “mixed-type ”equipment with high speed, high energy consumption in wood processing industry, and normally it is modular –screw, which can adjust the length-diameter ratio and configuration, and set the exhausting hole freely.

Nowadays the main operation of WPC is twin screw extruder, because it delivers materials relying on the principle of positive displacement and having no pressure returning ,easy feeding ,good exhausting affection. Thus the volatile components in wood flour can be fully ruled out. The meshing of screws with each other and the strong shearing can make the material have better mixing and plasticization affection. The wood flour cannot be charred because the less amount of wood flour is, the shorter time the material will stay in twin screw extruder. Thus, the flour can be used in WPC product for the twin screw extruder having a good material adding and mixing affection. Twin screw extruders have two types: parallel co-rotating twin screw extruder and parallel counter-rotating twin screw extruder.

The extruding formation can also be called as extrusion molding. It is a forming method that with the help of the extrusion affection of screw or plug screw, and the uniform plasticization melting plastic can pass the mouth model forcibly so that it can form series of products with constant cross-section.

Extrusion forming is mainly used to forming thermoplastics and the forming fashioned product includes the covering of tubes, rods, plates, wires, films, cable wires and aluminum profile with a variety of cross-section shapes. In exceptional circumstances, extruding formation can also be used to forming several kinds of thermoplastic plastics.

The general working process of plastic extruder is adding the graininess plastic into the hopper when extruding, then extruding the material in the barrel along with the rotation of screw and the electricity heating of the out wall of barrel, and lastly producing melt plastic.

With the continuous improvement of modern extrusion technology especially the usage of high-speed extruding of latest-type screw and latest material (such as cross linking polyethylene and so on), the high-precision control system is needed in order to make sure the quality of high-speed extruding and original material saving.

The main trend of plastic machinery such as twin screw extruder is with portfolio structure, specialization, serialization, standardization, micro-and large-scale, personalization, and intelligence. Users can do the “mix and match” based on their actual requirements. Besides, a lot of new type equipment and special purpose equipment have been developed, due to the requirement of technological principle of power, material saving and special application.

Conventional single screw extruder is not suitable for WPC forming operation, because it processes the weak ability for material transmission and plasticization. The transmission affection of single screw extruder mainly depends on the role of friction. The wood structure is too fluffy to feed, and the filling of wood structure make the polymer melt viscosity increase which makes extruding more difficult, so that the material has a longer residence time in the cylinder, and at the same time it cannot operate to the plant fiber with high water percentage for its poor exhausting affection .Therefore, conventional single screw extruder will be greatly restricted in the extruding process of wood-plastic composite materials.

The single screw extruder, which can be applied to the WPC forming operation, should use the specially designed screw. This kind of screw should has strong ability for material transmission and mixing material plasticization, and it can often mix and pelletize materials before extruding.

Although the screw of strong shearing cold feeding screw has a more complex configuration and higher manufacturing costs, it has widely used in manufacture for its good exhausting affection. The operation principle of strong shearing cold feeding screw is similar to that of cold feeding screw. But it has its own character.

The feeding parts of it use the large four-head screw configuration with angle shallow groove. This configuration has a lot of advantages: firstly, the strength and rigidity of the screw bottom have been improved;Secondly, the feeding ability of the large angle screw thread can be increased so that the imperfection that the shallow groove has a poor feeding ability can be made up and the transmission ability of rubber can be improved as well. In addition, multi-head screw thread can improve the feeding speed and is good for the stability and the feeding ability of various rubber materials in the extruding procession of rubber materials.

Pin barrel cold feeding extruders are the breakthrough of cold feeding extruders. It can be applied to 90% of the plastic material. Pin barrel cold feeding extruders use a streaming method rather than rather than rely on a specially designed screw or setting intense heat of the section in screw.

This streaming method can turn the plastic material into the shunt-type screw of cold feed extruder, which has a good quality of mixing and a good consistency of temperature.

Feed zone of a screw extruder, which uses forward transmission components with larger pitch, can gain a larger feed capability. Besides, feed zone will compact the powder filler by following measures in order to promote melting and plasticization of materials:

a. Part the modular screw (such as co-rotating twin screw and screw of disc extruders) in order to descend pitch change.

b.Reduce the pitch of non-modular screw gradually.

It is the fluctuation of feed that make the distribution and mixture of materials become particularly important. It is directly related to the consistency of the product.

As for parallel counter-rotating twin screw, the mixture capability of screw can be improved by several measures, such as enlarging the space between screw diameter and barrel wall on the solid transmission part and mixture part, setting reaction thread on pre-plasticization part of screw, setting the kneading disc and so on.

For the co-rotating twin screw extruder, the shear component of it is kneading disk.2 (diamond) disk kneaders can be used, which has strong transmission capacity but not very strong shear capacity. At the same time it can reduce the shear by reducing the thickness of kneading disc and increasing the wrong angle of kneading block.

Some factories of foreign counties have designed a non-mating-type mixing elements called “multiple processing units” in screws. These elements can be used with the retro directive component and form a mixing box. This can make the materials annular flow in part of the screw so that the capacity of distribution of mixed can be increased, and high temperature caused by the over shear of materials can be availed at the same time. Besides, temperature can be controlled through reducing shear by lower screw speed and liquid cooling of the screw in the high shear zone.

According to the research, although reducing the screw speed will reduce the production capacity of the co-rotating twin screw extruder,it can reduce the extrusion volatility so that make the surface of the extrusion products smoother.

Among single screw extruders, the pin screw extruder is suitable for the extrusion screw of wood-plastic composite extrusion system, because the pin can be cut on the material flow and repeatedly to form a new interface and then repeatedly re-oriented so that the heat of the liquid can be passed to the solid phase in time. Thus the screw pin has a good ability of mixing, dispersion and plasticizing. The screw can be extruded with low-temperature under the conditions of the same formula and the critical of the screw can be increased at the same time, so that the production will be increased .Besides, the appearance and performance of the extrusion products are better than ordinary single screw extruders.