PVC plastic is a multi-component plastic, according to different uses can be added to different additives, due to different components, PVC products show different physical and mechanical properties, for different applications. PVC plastic pipe in the plastic pipe accounted for a large proportion.
PVC pipe is divided into hard and soft two kinds
RPVC tube is the PVC resin and stabilizer, lubricants and other additives mixed, after granulation molding machine, can also be used powder extrusion molding. RPVC tubes are resistant to chemical corrosion and good insulation, mainly conveying a variety of fluids, as well as wire casing. RPVC Tauis cutting, welding, bonding, heating can be bent, so the installation is very convenient.
SPVC tube is made from PVC resin by adding a lot of plasticizer and a quantitative stabilizer, as well as other auxiliaries, after granulation and extrusion molding. SPVC tubing has excellent chemical stability, excellent electrical insulation and good softness and coloring, this kind of pipe is commonly used to replace the rubber pipe, used to transport liquid and corrosive media, but also used as cable casing and wire insulation pipe. In this paper, we will explain in detail the raw material selection, process flow, process control and the defects of RPVC and the reasons for improvement.
RPVC raw material selection and formula
In the hard tube production resin should choose the SG-5 type resin with lower polymerization degree, the higher the polymerization degree, the better the physical and mechanical properties and heat resistance, but the resin fluidity is poor, the processing brings some difficulty, so the general selection of viscosity is (1.7~1.8) X10-3pa·s SG-5 resin is advisable.
The rigid tube generally uses the lead stabilizer, its thermal stability is good, commonly used three salt base lead, but it itself is poor lubricity, usually and lubricity of good lead, barium soap.
Processing of hard tubes, lubricants selection and use is very important, not only considering the internal lubrication to reduce intermolecular force, so that the melt viscosity drop favorable formation, but also consider the external lubrication, to prevent the melt and hot metal adhesion, so that the surface of the product bright.
Internal lubrication generally used metal soap, external lubrication with low melting point wax.
Filling agent mainly with calcium carbonate and barium (barite powder), calcium carbonate to make the surface properties of tubes, barium can improve molding, make pipe easy to shape, the two can reduce costs, but too much will affect the performance of pipe, pressure pipe and corrosion-resistant tube best not add or less filler.
II. Process Flow
The RPVC tube is molded using SG-5 PVC resin and added stabilizer, lubricant, fillers, pigments, and so on, the raw materials are treated according to the formula after the appropriate kneading, if the squeeze tube using single-screw extruder, but also the kneading powder after the granulation, and then extrusion molding; if double screw extruder is used, it can be directly formed by powder material, The RPVC tubing process is shown in Fig. 3-1.
In addition, in the production can be different from the above-mentioned process, that is, the use of powder extrusion pipe directly without granulation, but should pay attention to two points:
1 One, Powder Direct extrusion molding is the best use of twin-screw extruder, because the powder and granular material compared to a less mixed shear molding process, so the use of twin-screw extruder can strengthen the shearing plasticizing, to achieve the desired effect;
2 second, because of granular material than powder dense, heating and heat conduction bad, so the processing temperature of powder material than the corresponding granular material processing temperature of 10 ℃ or so appropriate.
III. Process conditions and control
In the production process, because PVC is thermal sensitive materials, even if the addition of heat stabilizer can only improve the decomposition temperature, prolong the stability of time and it is not possible not to decompose, which requires the molding process temperature of PVC should be strictly controlled. Especially RPVC, because of its processing temperature and decomposition temperature is very close, often because of improper temperature control caused by decomposition phenomenon. Therefore, the extrusion temperature should be determined according to the formula, the characteristics of extruder, the structure of the head, the speed of the screw, the position of temperature measurement, the error of temperature measuring instrument and the depth of temperature measurement.
1. Temperature control
Temperature control is an important control factor in extrusion operation. The temperature of the control factors required for extrusion molding is barrel temperature, machine diameter temperature and die temperature. Temperature is too low, plastic bad, the appearance of the tube is not shiny, poor mechanical performance, product quality is not up to the requirements: temperature is too high, the material will decompose, product discoloration and other phenomena appear.
2. Screw speed
Screw speed increase, extrusion volume increase, which can increase production, but easy to produce undesirable phenomenon, resulting in pipe wall coarse, strength drop, at this time should adjust the head pressure, so that yield, quality to reach the best. The screw temperature control affects the material transportation rate, the material plasticizing, the melting quality and so on. Extrusion pipe need to pass cooling water, reduce screw temperature, is conducive to improve the quality of plastic, screw cooling water temperature in $number around.
3. Traction Speed
The adjustment of traction speed is very important in the extrusion operation, and the material is drawn by extrusion and melt-plasticizing, and then the driving speed should be matched with the extrusion speed when the extruder is extruded from the head to the setting device, the cooling device and the traction device. Generally in normal production, the traction speed should be faster than the extrusion speed of the pipe $number around.
4, compressed air and pressure
The compressed air can inflate the pipe tubing to keep the pipe in a certain roundness. Pressure should be appropriate in size. The pressure is too small, the pipe is not round, the pressure is too big, the core mold is cooled, the pipe wall appears crack, not smooth, the pipe quality drops. At the same time, pressure demands stability, such as pressure suddenly big and small, the pipe easily produce bamboo status.
5, sizing device, cooling device temperature
Extrusion of different plastic products, the use of different sizing and cooling methods, the cooling medium can be air, water or other types of liquids, the need to control temperature, its temperature is mainly related to production efficiency, product internal stress and so on.
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