Moulding Quality Problems In PVC, PE, And PP Pipe Extrusion
Date:2022-5-28 Author:hcscrew
01 Dimensional errors in the circumferential section of the pipe
1. Poor concentricity accuracy of the mouth die and mandrel assembly in the forming die, so that the melt flow gap between the two parts is not uniform. It is necessary to adjust the concentricity accuracy of the two parts.
2. After a period of time, the circumferential cross-sectional wall thickness error occurs in pipe extrusion production work. It appears that the adjustment screws for the mouth die and mandrel gap are not tight enough. Pay attention to the tightening of the adjustment screws.
02 Dimensional errors in longitudinal and cross-sectional wall thickness measurements
1. As the running traction speed of the pipe billet is not stable, the transmission system of the traction machine needs to be overhauled to ensure the smooth operation of the traction machine.
2. During the barrel process, the temperature fluctuates greatly, resulting in a large amount of inconsistent extruded melt. Furthermore, an instabile screw speed results in an inconsistent amount of melt extruded. As a result, the longitudinal wall thickness of plastic pipe (PVC, PE, PP pipe) is uneven. Faulty temperature control heating systems cause process temperature fluctuations, and faulty power supply and transmission systems cause screw speed instability.
03 Pipes are brittle
1. There are problems with raw material plasticisation quality (for example, uneven raw material plasticisation), raw material plasticisation after low melt temperatures. It is ideal to improve the plasticizing temperature of raw materials (i.e. barrel temperature) and, if necessary, to replace the screw.
2. Drying the raw material if it contains excessive moisture or volatiles.
3. Compression ratio of the moulding is too small, so it needs to be improved.
4. It is possible to form the pipe billet with a more obvious longitudinal melt fusion line because the flat section between the mouth mould and the mandrel is too small. As a result, the strength of the pipe is reduced, and the mould structure has to be modified again.
5. There is too much filler material in raw material, making the pipe brittle. The raw material formula needs to be modified to reduce the amount of filler material in raw material.
04 Pipe’s outer surface is rough and lustreless
1. When controlling the mouth mould temperature, too high or too low temperatures will deteriorate the surface quality of the pipe. Adjusting the temperature accordingly is important.
2. The inner surface of this mouth mould is rough or contains residual material. Make sure that the mouth mould is disassembled and repaired in a timely manner.
05 Pipe inner surface is rough
1. In the forming mould, the flat part of the mandrel is not long enough, or the temperature is too low. In order to improve the straight section of the mould, the shape of the straight part should be extended.
2. The screw is too hot, and it needs to be cooled down properly so that it does not reach a dangerous temperature. For example, when using heat-conducting oil to cool the screw, the screw temperature should be adjusted to 90°C when extruding PVC material.
3. Since the compression ratio of the die is relatively small, the inner surface of the pipe has a longitudinal melt bonding line. The die structure should be improved in order to increase the compression ratio.
4. It is recommended to control the mandrel temperature of large-size moulds at about 150°C (when using PVC raw materials). This will improve the quality of the inner surface of the pipe.
5. Raw materials containing high moisture or volatile matter can also affect the quality of the inner surface of the pipe. If necessary, dry the raw material.
06 Pipe has small black or white dots and fish eyes
1. Raw materials contain too many impurities. Adding a strainer to the machine barrel would be helpful. If a strainer is fitted, it is possible that the strainer is damaged and should be replaced.
2. In the forming mould, there is a small amount of decomposition of raw material. It is necessary to clean the mould and remove the residuals.
3. This raw material contains fisheyes or small amounts of low molecular weight compounds that cannot be plasticised. If necessary, replace the raw material.
07 Scratched or streaked pipe surface
1. The forming mould surface may have scratches or hanging material. You should repair the working surface of the mouth mould and remove the residual material.
2. If the small holes of the vacuum sizing sleeve are not evenly distributed. Or, if the aperture size specs aren’t uniform, and tiny streaks appear, the way the holes are arranged needs to be improved.
Summary
The above is a brief description of common moulding quality problems in PVC, PP, and PE pipe extrusion. If you would like to know more about plastic pipe extrusion process skills and knowledge about related screw and barrel, contact us HUAHCEN Screw Barrel Factory.
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