PRODUCTION OF PVC PIPE
PVC PIPE is divided into soft pipeand hard pipe.
Rpvc pipe is PVC resin, stabilizers, lubricants and other additives mixed, after the granulation of extrusion molding machine, can also be used powder extrusion molding. RPVC pipe chemical corrosion resistance and good insulation, the main transportation of various fluids, and used as wire casing. Rpvc pipe easy cutting, welding, bonding, heating can be bent, so installation and use is very convenient.
SPVC pipe is made of PVC resin with a large amount of plasticizer, a certain amount of stabilizers, and other additives, after granulation extrusion molding. SPVC pipe has excellent chemical stability, excellent electrical insulation and good flexibility and coloring. This pipe is often used to replace the rubber tube, used to transport liquids and corrosive media, also used as cable sleeve and cable insulation pipe. In this paper, we will detail the raw material selection of RPVC, process flow, process control, as well as the defects of extrusion pipe and the reasons for improvement.
Raw Material Selection and formula of RPVC
Sg-5 resin with low polymerization degree should be used in the production of hard pipes. The higher the polymerization degree is, the better the physical and mechanical properties and heat resistance of the resin are, but the poor flowability of the resin brings certain difficulties to the processing Therefore, sg-5 resin with viscosity of (1.7 ~ 1.8)10-3Pa · s is suitable for general selection.
Hard Pipe is generally used lead stabilizer, its good thermal stability, common use of tribasic lead, but its own lubrication is poor, and usually lubricity and good lead, Barium soap and use.
The choice and use of lubricant is very important for hard pipes. It is important to consider both internal lubrication to reduce intermolecular force and reduce melt viscosity for good molding as well as external lubrication to prevent the melt from sticking to hot metal and to make the surface of the product bright.
General Internal Lubrication with metal soap, external lubrication with low melting point wax.
The filler mainly uses calcium carbonate and barium carbonate (Barite powder) . Calcium carbonate can improve the surface properties of the tube, Barium can improve the formability of the tube, and make the tube easy to shape. Both of them can reduce the cost, but too much of them can affect the properties of the Tube Pressure pipe and corrosion-resistant pipe is best not to add or less filler.
Rpvc pipe is molded with sg-5 PVC resin and added with stabilizers, lubricants, fillers and pigments. After proper treatment, the raw materials are kneaded according to the Formula The kneaded powder should be made into granules and then extruded. If twin-screw extruder is used, powder can be directly molded.
In addition, in the production process may be different from the above-mentioned process, that is, the use of powder material directly extruded pipe without granulation, but should pay attention to two points:
1) firstly, the twin-screw extruder should be used for the direct extrusion of powder, because there is less mixing and plasticizing process between powder and granule, so the twin-screw extruder can strengthen the shearing and plasticizing, and achieve the expected effect;
2) secondly, the processing temperature of granular material is about 10 °c lower than that of the corresponding granular material because of its denseness, poor heat transfer and poor heat transfer.
PROCESS CONDITIONS AND CONTROL
In the production process, because PVC is a heat sensitive material, even the addition of heat stabilizer can only increase the decomposition temperature and prolong the stabilization time, which requires the processing temperature of PVC should be strictly controlled. RPVC, in particular, due to its processing temperature and decomposition temperature is very close, often due to improper temperature control caused decomposition phenomenon. Therefore, the extrusion temperature should be determined according to the formula, extruder characteristics, head structure, screw speed, temperature measuring position, temperature measuring instrument error and temperature measuring depth.
1 Temperature Control
Temperature control is an important control factor in extrusion operation. The temperature controlling factors of extrusion molding are barrel temperature, machine diameter temperature and die temperature. Low temperature, poor plasticization, dull appearance of pipe, poor mechanical properties, product quality can not meet the requirements: High temperature, the material will decompose, product discoloration and other phenomena.
2 Screw Speed
With the increase of screw speed and extrusion quantity, the output can be increased, but it is easy to produce poor plasticization, which results in rough inner wall and strength decrease. The temperature control of the screw affects the conveying rate, the plasticization of the material and the melting quality. Cooling water is needed to extrude pipe, lower the temperature of screw rod, which can improve the quality of plastication. The temperature of cooling water is about 50 ~ 70 °C.
3 TRACTION SPEED
It is very important to adjust the drawing speed in re-extrusion operation. Generally in normal production, the traction speed should be faster than the extrusion speed of 1% ~ 10% .
4 COMPRESSED AIR AND STRESS
The Compressed air is able to blow up the pipe so that it maintains a certain roundness. It is required that the pressure should be appropriate. The pressure is too small, the pipe is not round, the pressure is too large, the Mandrel is cooled, pipe wall cracks, not smooth, pipe quality decline. At the same time the pressure needs to be stable, such as the pressure from big to small, easy to produce bamboo tube status.
The temperature of sizing device and cooling device
Different plastic products are extruded with different sizing and cooling methods. The cooling medium can be air, water or other liquids. The temperature needs to be controlled.