One step extrusion technology of the hottest wood

2022-09-26
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One step extrusion technology of wood plastic composite products (Part I)

in recent years, wood plastic composite products have attracted extensive attention. Thermoplastics used for wood plastic composites are mainly polyolefins such as PE, PP, PS and PVC, including new materials, recycled materials and mixtures of the two; Wood fiber provides us with infinite phenomenon space in the future for agricultural wastes with rich resources and low prices, such as sawdust, rice husk, wheat straw and so on. Due to the advantages of both, wood plastic composite products have a series of unique excellent characteristics if the friction surface of the sample produces large plastic deformation during the friction process: not afraid of insect erosion, aging resistance, corrosion resistance, small water absorption, will not be tested by the material mechanics laboratory of Gansu Agricultural University for the bearing capacity of wheat straw, water absorption deformation, long service life; Similar to the appearance of wood, but better than the dimensional stability of wood, it will not produce cracks, warpage and no wood scars. It has the secondary processing property similar to wood, and can be cut, pasted, connected and fixed with nails and bolts; It has higher hardness than plastic, has the processability of thermoplastic, can be reused and recycled, and is conducive to environmental protection. With the development of extrusion equipment and extrusion technology, wood plastic composite microcellular foaming materials are made by wood plastic composite microcellular foaming technology. Compared with non foaming materials, it has higher impact strength, higher toughness, longer fatigue life, better thermal stability and lower density (the density is reduced by 75% or higher), which further widens the application range of wood plastic composite products. At present, wood plastic composite products have been widely used in the construction industry (such as floors, wall panels, building templates, door and window profiles, fences and guardrails, as well as shutters and roof panels), the automotive industry (such as door interior trim panels, driver's glove boxes, spare tire covers, seat backs, roof liners, etc.), packaging and transportation industry (such as transportation pallets, railway sleepers, ship compartments, etc.), furniture industry, office equipment Sports equipment and other fields

currently. The wood plastic composite production process basically adopts two steps, that is, blending and granulation are carried out first, and then the finished products are extruded. The cost of processing products in this way is higher. Since 1999, the one-step extrusion technology of wood plastic composite products has been studied, that is, the mixing and molding of wood plastic composite materials are directly carried out by extruder. Because the granulation process is omitted, the cost is reduced and the market competitiveness of products is improved

usable inch: the equipment for extruding wood plastic composite products includes single screw extruder, twin screw extruder and series mill extruder. At present, the former two are mainly used, and the latter is still a new research field in wood plastic composite extrusion, which is still in the experimental stage

the selection of extrusion equipment should first consider the processing performance of materials (such as the ratio of plastic to wood fiber, the new and old plastic and its ratio, the length of wood fiber, temperature and surface treatment); Secondly, the mixing performance and molding capacity of the extruder should be considered to ensure the quality of products; In addition, the production cost of products must be considered in order to improve the market competitiveness of products. Only by comprehensively considering these factors and combining with the actual situation of the factory can we make a reasonable choice of extruder equipment

when the processing temperature is higher than 200%, wood fibers may degrade. Therefore, the residence time of materials in the extruder should be strictly controlled during processing to avoid excessive shearing and out of control shearing. The viscosity of wood fiber filling material is very high, which produces a lot of viscous heat during shearing, and the thermal conductivity of wood plastic mixture is very low, both of which lead to the failure to transfer enough heat from the melt to prevent the degradation and cracking of wood fiber. This processability of materials not only puts forward higher requirements for equipment, but also limits the application of wood plastic composites. For this reason, thermoplastics used in wood plastic composites can only choose plastics with low processing temperature, such as PP, PS, PVC, HDPE, LDPE, etc. In order to effectively control the temperature of materials and prevent the degradation of wood fibers, the following aspects can be taken into consideration

I. screw design

optimize the screw design to ensure that the materials are fully mixed while avoiding excessive shear (especially at high speed) and reducing residence time. For the co rotating twin-screw, its shear element is mainly the kneading plate. The two head (diamond) kneading plate with strong conveying capacity and not very strong shear can be used. At the same time, appropriately reduce the thickness of the kneading plate and increase the staggering angle of the kneading block to reduce shear. Some foreign manufacturers have designed a non meshing mixing element called "multiple processing unit" in the screw, which is used in conjunction with the reverse element to form a "mixing chamber", so that the materials flow in the screw in a local ring, improving the distribution and mixing capacity, and avoiding excessive shear and high temperature of the materials. The temperature rise of the material can also be controlled by reducing the screw speed to reduce the shear and cooling the screw in the high shear zone. It was found in the experiment, "dykes also stressed that Bayer has ranked second in the world in the above fields. Although reducing the screw speed reduces the production capacity of the co rotating twin-screw extruder, it also reduces the extrusion volatility and makes the surface of the extruded products smoother

in the series grinding disc screw extruder, the main shaft of the extrusion system rotating is successively installed with feeding section screws, several rotating grinding discs (moving discs) that can rotate with the main shaft, threaded elements and extrusion section screws, and the fixed grinding disc (fixed disc) is a part of the barrel

In the single screw extruder, the pin screw is suitable for the extrusion screw of the wood plastic composite extrusion system, because the pin can cut the material flow, repeatedly form a new interface, and then repeatedly Orient, so that the heat of the liquid phase can be transferred to the solid phase in time. Therefore, the pin screw has good mixing, dispersing and plasticizing ability, and can realize low-temperature extrusion under the same formula. At the same time, the critical speed of the screw increases, and the output increases. The appearance and performance of the monthly extrusion products are better than that of the ordinary single screw

II. Barrel design

the barrel is designed as a segmented barrel, a heating device is set on the outer surface of each barrel, and a channel for liquid cooling is arranged on each barrel. In this way, the influence between the barrel temperatures of each section is reduced, which is conducive to the accurate temperature control of the feeding section, melting section, exhaust section and extrusion section of the extrusion system

adding additives (especially lubricants) can not only prevent the wall adhesion of materials, but also reduce friction and reduce the temperature rise of materials. In this way, the maximum speed of the screw can be increased during processing, so as to increase the output on the premise of ensuring the extrusion quality

III. head design

the fluid of wood plastic composites under extrusion conditions is regarded as a solid fluid, which cannot be calculated using typical pressure drop and expansion formulas. Therefore, the design of the machine head for wood plastic composite is quite different from the traditional machine head design, which should be designed according to the rheological state of solid flow

for all wood flour fillers, stainless steel is the best choice, because wood flour is acidic

due to the poor fluidity of wood plastic molten materials, in order to obtain dense products, the pressure at the head population is generally large (usually 5 ~ 16MPa), which is easy to cause the distortion of the head die, especially the flat head for extruding plates. Therefore, when the precision of products is required to be high, the influence of die deformation must be considered in the design of die. The following measures can be taken to reduce its impact: reduce the melt flow rate (which will reduce production); Increase the thickness of the die body; Change the internal geometric dimension of the runner

(to be continued)

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