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[Science and Technology Post] What are the common extrusion problems of plastic pipe?

Update:20-12-2019
Summary:

Large circumferential wall thickness error ① The accura […]

Large circumferential wall thickness error
① The accuracy of the concentricity between the die and the mandrel in the forming mold is poor, which makes the gap of the melt flow path between the two parts uneven. The concentricity accuracy of the two parts should be adjusted.
② After a period of time during the pipe extrusion production work, the phenomenon of tolerance error in the wall thickness of the circumferential cross section appears. This is because the adjustment screw that adjusts the gap between the die and the mandrel appears loose. Pay attention to the tightening of the adjustment screw.

Large longitudinal wall thickness error
① The running speed of the pipe blank is unstable. The drive system of the tractor should be overhauled to ensure the smooth operation of the tractor.
② The process temperature of the barrel fluctuates greatly, which results in unstable extrusion melt volume. The unstable screw rotation speed also makes the extrusion melt volume inconsistent, resulting in uneven longitudinal wall thickness of the pipe. Process temperature fluctuation is the influence of temperature control heating system, and the unstable screw speed is the influence of power supply and transmission system. It should be overhauled.

The tube is brittle
① The plasticizing quality of the raw materials does not meet the process requirements (including the uneven plasticizing of the raw materials), and the melt temperature after the plasticizing of the raw materials is low. The plasticizing temperature of the raw materials should be appropriately increased (that is, the barrel temperature should be increased), and the screw should be replaced if necessary.
② There is too much water or volatile matter in the raw materials. Dry the raw materials.
③ The compression ratio of the molding mold is too small. The compression ratio of the mold to the melt molding should be appropriately increased.
④ The size of the straight section between the die and the mandrel is too small, so that the pipe blank has a more obvious longitudinal melt fusion line, the strength of the pipe is reduced, and the mold structure should be modified.
⑤ Too large a proportion of filling material in the raw material is also a factor that makes the pipe brittle, and the raw material formula should be modified.

Rough outer surface of the pipe
① The temperature control of the die part in the forming mold is not reasonable. Too high or too low process temperature will affect the outer surface quality of the tube. The die temperature should be adjusted appropriately.
② The inner surface of the die is rough or there is residual material. The mold should be dismantled in time to repair the working surface of the die.

The inner surface of the pipe is rough
① The length of the straight part of the mandrel in the forming mold is insufficient or the temperature is too low. The mold structure should be appropriately improved to extend the size of the straight section.
② The temperature of the screw is too high, and the temperature should be appropriately reduced. When extruding PVC material, the temperature of the heat-conducting oil for screw cooling should be controlled at about 90 ° C.
③ The compression of the mold is relatively small, so that the inner surface of the pipe has a longitudinal melting bond line. Should improve the mold structure and increase the compression ratio.
④ The core temperature of large-size molds should be controlled at about 150 ° C (when using PVC raw materials), which can improve the molding quality of the inner surface of the pipe.
⑤ Note that the high moisture or volatile content in the raw material will also affect the internal surface quality of the pipe. If necessary, the raw materials should be dried.

Streaks or scratches on the surface of the pipe
① The surface of the die in the forming mold is scratched or hanging. The working surface of the die should be repaired to remove the residual material.
② The small circular holes of the vacuum sizing sleeve are unreasonably distributed or the aperture specifications are not uniform, and tiny stripes appear. The arrangement of the vacuum holes of the sizing sleeve should be improved.

temperature
Temperature is one of the important conditions for smooth extrusion. Starting from powdery or granular solid materials, high-temperature products are extruded from the head and undergo a complex temperature change process. Strictly speaking, the extrusion molding temperature should refer to the temperature of the plastic melt, but this temperature largely depends on the temperature of the barrel and the screw. A small part comes from the frictional heat generated when mixing in the barrel, so often The molding temperature is approximated by the barrel temperature.
Because the temperature of the barrel and the plastic are different in each section of the screw, in order to make the process of conveying, melting, homogenizing and extruding plastic in the barrel smoothly, in order to efficiently produce high-quality parts, the key issue is to control The temperature of each section of the barrel and the barrel temperature are adjusted by the heating and cooling system and temperature control system of the extruder.
The head temperature must be controlled below the thermal decomposition temperature of the plastic, and the temperature at the die can be slightly lower than the head temperature, but the plastic melt should have good fluidity.
In addition, temperature fluctuations and temperature differences during the molding process will cause defects such as residual stress, uneven strength at various points, and a dull and matte surface. There are many factors that cause such fluctuations and temperature differences, such as unstable heating and cooling systems, changes in screw speed, etc., but the quality of the screw design and selection has the greatest impact.

pressure
During the extrusion process, due to the resistance of the flow, the change in the depth of the screw groove, and the obstruction of the filter screen, filter plate and die, a certain pressure is generated in the plastic along the axis of the barrel. This pressure is one of the important conditions for the plastic to become a homogeneous melt and obtain a dense plastic part.
Increasing the head pressure can improve the mixing uniformity and stability of the extruded melt, and increase the product density. However, excessive head pressure will affect the output.
As with temperature, pressure changes with time will also produce periodic fluctuations. Such fluctuations also have adverse effects on the quality of plastic parts. Changes in screw speed, instability of heating and cooling systems are all causes of pressure fluctuations. In order to reduce pressure fluctuations, the screw speed should be controlled reasonably to ensure the temperature control accuracy of the heating and cooling devices.

Extrusion rate
The extrusion rate (also known as extrusion speed) is the mass (in kg / h) or length (in m / min) of the plastic extruded from the extruder die per unit time. The size of the extrusion speed represents the level of extrusion production capacity.
There are many factors that affect the extrusion speed, such as the structure of the head, screw and barrel, the speed of the screw, the structure of the heating and cooling system, and the characteristics of the plastic. Both theory and practice have proved that the extrusion rate increases with the increase of the screw diameter, the depth of the spiral groove, the length of the homogenization section and the speed of the screw, and increases with the melt pressure at the end of the screw and the gap between the screw and the barrel. When the structure of the extruder and the type of plastic and the type of plastic parts have been determined, the extrusion rate is only related to the screw speed. Therefore, adjusting the screw speed is the main measure to control the extrusion rate.
The extrusion rate also fluctuates during the production process, which will affect the geometry and dimensional accuracy of the plastic part. Therefore, in addition to correctly determining the structure and size parameters of the screw, the screw speed should be strictly controlled, and the extrusion temperature should be strictly controlled to prevent changes in extrusion pressure and melt viscosity caused by temperature changes, which will cause fluctuations in extrusion speed.

Traction speed
Extrusion mainly produces continuous plastic parts, so traction devices must be provided. The plastic parts extruded from the die and die will be stretched and oriented under traction. The higher the degree of tensile orientation, the greater the tensile strength of the plastic part along the orientation direction, but the larger the length shrinkage after cooling. Generally, the traction speed can be comparable to the extrusion speed. The ratio of traction speed to extrusion speed is called the traction ratio, and its value must be greater than 1.