Introduction of Plastic Pipe Sizing Method

When the pipe products are extruded from the die, the pipe extruder does not reach the normal temperature level. For example, the temperature of UPVC can reach about 180℃. In order to obtain good flatness, geometric shape and correct size of the pipe, sizing and cooling must be carried out in the sizing sleeve when the extrudate leaves the die. The pipes after sizing and initial cooling of the sizing sleeve enter the water tank for further cooling. The pipe is completely immersed in the water during the whole process of passing through the water tank, and the pipe has been completely finalized when leaving the water tank.

There are two methods for pipe sizing: outer diameter sizing and inner diameter sizing. The selection of sizing method depends not only on the advantages and disadvantages of the two, but also on the specification and standard of the pipe. If the outer diameter of the pipe has tolerance, the outer diameter shall be used for sizing; If the inner diameter of the pipe has tolerance, use the inner diameter sizing. The outer diameter sizing is to make the outer wall of the pipe contact the inner wall of the sizing sleeve. In order to achieve this goal, internal pressurization or vacuum pumping on the outer wall of the pipe is commonly used. Therefore, external diameter sizing is divided into internal pressure method and vacuum method.

For the internal pressure method, a certain pressure of compressed air shall be introduced into the pipe. In order to ensure the pressure in the pipe, the plug can be used to prevent air leakage. It should be noted that the compressed air is preheated, because the cold air will reduce the temperature of the mandrel and cause the inner wall of the tube to be not smooth. According to relevant introduction, the internal pressure method of outer diameter sizing can prevent compressed air from entering, and the production can be carried out normally depending on the pressure of atmospheric air. Practice has proved that when producing large pipes, compressed air can still be produced without being used.

The vacuum method for outer diameter sizing is actually a metal cylinder with many small holes punched in a certain area for vacuum pumping. The sizing device of vacuum method is relatively simple, and the nozzle can not be blocked, but a set of oil vacuum equipment is required. Due to the limited pressure generated, it is not suitable to extrude large-diameter pipes, so it is rarely used.

In this way, when the outer diameter of the pipe is subject to internal pressure, the tensile stress on the inner wall of the pipe is greater, while the compressive stress on the outer wall is small, and the stress distribution is uneven. When the inner diameter of the pipe is subject to internal pressure, the tensile stress and compressive stress on the inner wall of the pipe offset a part, and the stress on the whole pipe wall is more uniform.

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