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the two most important processes of plastic molding are injection molding and extrusion molding. screw is the key component in the whole molding process, so the importance of screw has been emphasized. we know that there are various kinds of single screw, such as separation, mixing and compression decompression single screw. among them, the compression and decompression screw is divided into energy transfer (et) screw and wave shaped screw.
like ordinary single screw, corrugated screw is still divided into solid conveying section, compression section and metering section, but the difference is that from the compression section, the thread has an eccentric distance, and the screw groove is wavy when it is unfolded. during extrusion and plasticization of unplasticized polyvinyl chloride (upvc) materials, in addition to spiral conveying and certain thermal process, they also undergo intermittent action of wave peaks and troughs to promote the materials to receive periodic compression - decompression action, during which the molecules produce stress relaxation. in this way, the temperature of the material will not rise continuously due to continuous shearing and non rubbing during the plasticizing process of upvc, so as to degrade, which greatly improves the quality of upvc products.
after years of efforts, it has reached the practical stage and should be vigorously promoted and applied. south china university of technology mainly studies double waveform screw, exhaust or extruder screw, while the background chemical college focuses on the research of corrugated screw for injection molding. this paper focuses on this kind of corrugated injection screw.
in recent years, with the increasing output of extruders and driving power, the screw speed has also increased. in order to effectively improve the ability of bearing one-way torque, it is very important to analyze and study the failure and stress causes of extruder screw mandrel. why to obtain better plasticizing effect and production capacity? large diameter screw and ultra deep screw groove are the development trend of extruders in recent years. the greater the torque the shaft bears, the more the mandrel breaks during the operation of the extruder. therefore, it is of great significance to study the causes of the failure of the extruder screw mandrel for improving the production capacity and product quality of the extruder.
in plastic processing industry, reasonable assembly structure and processing precision of components shall be selected; the correct design of screw structure and shape, the adoption of new materials and the treatment of screw surface are effective measures to reduce screw surface wear and corrosion and improve productivity.
in addition to the extruder screw, the barrel and nozzle are also very important in the processing and production of the extruder plasticizing device. next, simply accept the structural characteristics of these two parts.
the extruder barrel is a straight barrel, which is closely matched with the screw. the radial clearance between the screw and the barrel is generally 0.2m (0.008in); the clearance of extruder with screw diameter no more than 40mm is about 0.15mm (0.006in). the barrel is usually made of bimetal by center casting. when the plasticizing device requires exhaust, an exhaust port shall be opened on the barrel. open the feeding port at the feeding end of the barrel. the feeding mouth is connected with the hopper, and the solid plastic particles are added to the extruder through the hopper.
the nozzle of the extruder is pushed onto the main casting sleeve. the tip of the nozzle is usually rounded, and the radius of the arc is slightly smaller than the radius of the matching main casting sleeve to achieve good sealing. the radius should be large enough to avoid excessive wear. in the american standard, the radius of magic spray is. 0.5 in and 0.75 in; in european union standards, sneezing radii are 10mm, 15mm, 20mm and 30mm. the nozzle can be either a straight through type or a switch device
set. straight through nozzles are generally recommended for the processing of heat sensitive integrators and high viscosity polymers, such as hard pvc, thermosetting resins, and elastomers. figure 1.12 shows the structure of a straight through nozzle.
1. the screw barrel shall be straight.
2. the screw barrel shall be designed to withstand the working pressure of the extruder. these pressures may be as high as 70-140 mpa (10000-20000 psi), even higher in injection molding.
3. the inner wall of the screw barrel should be more wear-resistant than the screw barrel of the extruder, because the screw barrel is easier to replace than the screw barrel, and the reprocessing is cheaper than the screw barrel.
4. the support at the rear end of the screw barrel shall be sliding support, which is allowed to expand when the screw barrel is heated. the rigid support will cause the bending of the screw barrel, which may cause serious damage to the screw and the screw barrel.
5. in order to improve group conveying, the feeding section of the screw can be grooved. the slotting and digestion section shall ensure good cooling capacity to take away the heat generated by high friction and prevent the melting of materials accumulated in the tank. in addition, the trench shall be sloped and the depth gradually becomes shallow to minimize the chance of material hanging in the structure grains.
6. if there is no air outlet, it shall be offset, and the side wall of the opening shall be tangent to the inner wall of the cylinder to reduce the chance of the plastic melt hanging on the front of the air outlet, as shown in figure 2.20. compared with the traditional symmetrical exhaust port design, it is easy to block the material at the bottom of the exhaust port. the opening of the exhaust port can be designed to tilt downward to avoid cooling the material flowing into the extruder screw.
1、 the assembly clearance between the extruder screw and its supporting transmission shaft is too large, and the two shaft centerlines are not concentric when rotating, resulting in too large error.
2、 extruder screw is bent and deformed, and the straightness of the shaft centerline exceeds the tolerance.
3、 the perpendicularity between the end face of the extruder barrel and the end face of the connecting flange of the base and the centerline of the barrel can not be guaranteed, and the error is large.
4、 when the extruder screw is assembled in the extruder barrel, the concentricity error of the two parts is large.
corrosive, abrasive and high temperature resistant resin may damage the screw and barrel unconsciously. for example, some engineering materials, thermoplastic elastomers and biopolymer materials sometimes form a corrosive environment. in addition, reinforcing materials (such as glass fiber, glass ball, etc.), some fillers and additives are all abrasive.
in order to avoid affecting the production efficiency and product quality, the processor must closely monitor the hazards that these materials may cause to the equipment. in order to maintain high processing standards, it is necessary to establish a preventive maintenance program (hereinafter referred to as "pm program"). when necessary, the barrel screw should be checked and measured, and the screw should be pulled out for reinstallation or replacement, because minimal wear will also affect the product quality.
to protect the equipment from the damage of corrosive and abrasive resins, the best way is to select appropriate materials to manufacture screws and barrels. the processing cost of wear resistant barrel and screw manufactured to cope with corrosive, abrasive and high temperature resistant melts is about 3 to 4 times higher than that of standard polyolefin screw. this is because the cost of wear resistant alloys is expensive and difficult to process. however, they can provide longer service life and can be repaired at longer intervals.
1. in the process of extrusion production, because the process temperature is too low or metal foreign matters are mixed with materials in the extruder barrel, the working rotating torque of the screw will increase suddenly. this torque force exceeds the strength limit of the screw, which will cause the screw to break.
2. if there are fillers such as calcium carbonate or glass fiber in the material, the abrasion of extruder screw and extruder barrel will be aggravated.
3. the extruder screw rotates in the extruder barrel and extrudes materials. long time friction between the screw, barrel and materials will inevitably cause wear of the barrel and screw. the inner diameter of the barrel will gradually increase and the outer diameter of the screw will gradually decrease. in this way, the gap between the screw and the barrel will gradually increase with their wear
4. due to improper operation and unstable process temperature control, a large amount of hci gas is often released when extruded materials are decomposed, such as pvc, which aggravates the corrosion of parts.
5. the heat treatment hardness of the working surface of the extruder screw or barrel does not reach the required value, which will accelerate the wear of the two parts.
6. improper material selection of screw and extruder barrel will result in insufficient working strength of the two parts and shorten their working life.
the output value of extrusion directly depends on the screw speed, so precision cutting and controlling the screw speed of extruder are very important. the measured and flashed crown rod speed shall be accurate to 0.1 r/min at least, and 0.1 r/min or less at best. in other words, the sensitivity and resolution of screw speed measurement should be 0.1 r/min or very good. assuming the full range is 200r/min, the measurement sensitivity is 0.05% of the full range. when the extruder operates at low speed, the measurement of rotating speed is particularly important. for example, if the extruder operates at 5r/min and the resolution of screw speed is 1r/min. when the screw speed changes within 20%, there will be no change in measurement control data. when the resolution is 0.1 r/min and the screw speed is changed within%, the measured flash value of the screw will not change.
another major question that needs to be considered is the speed scheduling of the extruder and the tractor. the speed dispatching accuracy of a typical brush type current appropriate motor with speed measurement response dispatching is 1% of the full scale. assuming that the maximum speed is 100r/min, when the screw speed is 10r/min, the change value can reach plus or minus 1r/min, that is, plus or minus 10%, and the total change can reach 20%. this is not allowed for most extrusion processes. brushless dc motor or digital pulse brush dc motor with digital driving skills can greatly improve the speed regulation accuracy, which can reach 0.01% of the full range or very good. that is to say, even at a very low speed, this driving method can ensure that the screw speed is very stable.
obviously, the dispatching of traction speed has the same main meaning. assuming that the output value of extrusion is stable and the traction speed is changing, the scale of finished products will also change. therefore, it is recommended that the speed scheduling accuracy of the extruder and the traction device should reach 0.01% or higher of the full scale. because the digital drive is not expensive compared with the old brush dc motor with or without tachometer in price, there is no reason not to use the digital drive device.
the screw needs to be cooled when extruding materials, mainly to ensure smooth production and prevent materials from decomposing due to too high plasticizing temperature. for example, cooling in the feeding section of the screw is to prevent the temperature of this section of the screw from being too high, and to prevent the materials from being attached to the screw thread groove due to the high temperature and rotating with the screw, which affects the forward movement of the material delivery, resulting in the failure of normal production due to insufficient material supply. the homogenizing section of the screw is cooled because the molten materials in this section generate more heat due to greater extrusion, friction and shearing, and the temperature rises faster. in order to prevent material decomposition, cooling is required.
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