[Science Stickers] What are the common problems with the extrusion quantity of plastic pipes?

Large wall thickness error of pipe circumferential section

①The accuracy of the concentricity between the die and the mandrel in the forming die is poor after assembly, which makes the gap between the two parts of the melt flow channel uneven. The concentricity accuracy of the two parts should be adjusted.

②After a period of pipe extrusion production work, there will be an out-of-tolerance phenomenon in the wall thickness of the circumferential section. This is due to the loosening of the adjusting screw that adjusts the gap between the die and the mandrel. Pay attention to the tightening of the adjusting screw.

The longitudinal section wall thickness of the pipe has a large error

① The running traction speed of the tube billet is unstable, and the transmission system of the tractor should be overhauled to ensure the smooth operation of the tractor.

②The large fluctuation of the process temperature of the barrel makes the amount of extruded melt material unstable, and the unstable screw speed also makes the amount of extruded melt material inconsistent. As a result, the longitudinal wall thickness of the pipe is uneven. Process temperature fluctuation is the influence of the temperature control heating system, and the unstable screw speed is the influence of the power supply and transmission system, which should be overhauled.

The pipe is brittle

①The plasticization quality of the raw materials does not meet the process requirements (including uneven plasticization of the raw materials), and the melt temperature after the plasticization of the raw materials is low. The plasticizing temperature of the raw materials should be increased appropriately (that is, the temperature of the barrel should be increased), and the screw should be replaced if necessary.

② If there is too much moisture or volatile matter in the raw materials, the raw materials should be dried.

③The compression ratio of the molding die is too small, so 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 tube blank has a more obvious longitudinal melt fusion line, and the strength of the tube is reduced, and the mold structure should be revised.

⑤ Excessive proportion of the filler in the raw material is also a factor that makes the pipe brittle, and the raw material formula should be modified.

The outer surface of the pipe is rough and dull

①The temperature control of the die part in the forming mold is unreasonable, and the process temperature that is too high or too low will affect the quality of the outer surface of the tube. The temperature of the die should be adjusted appropriately.

②The inner surface of the die is rough or there is residual material. The mold should be disassembled in time and the working surface of the die should be polished.

The inner surface of the pipe is rough

①The length of the straight part of the mandrel in the forming mold is insufficient or the temperature is 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, so it should be cooled appropriately. When extruding PVC material, the temperature of heat transfer oil for screw cooling should be controlled at about 90℃.

③The compression of the mold is relatively small, so that the inner surface of the tube has a longitudinal melting line. The mold structure should be improved to increase the compression ratio.

④ The temperature of the mandrel of the large-size mold should be controlled at about 150°C (when using PVC raw materials), which can improve the quality of the inner surface of the pipe.

⑤ Note that high moisture or volatile content in the raw materials will also affect the internal surface quality of the pipe. Raw materials should be dried if necessary.

Streaks or scratches on the surface of the pipe

①Scratch or hang material on the surface of the die in the forming die. The working face of the die should be trimmed to remove the residual material.

②The small round holes of the vacuum sizing sleeve are unreasonably distributed or the hole size is not uniform, and there are tiny streaks. The arrangement of the vacuum holes of the sizing sleeve should be improved.

temperature

Temperature is one of the important conditions for the smooth progress of extrusion molding. Starting from powder or granular solid materials, high-temperature products are extruded from the die and undergo a complex temperature change process. Strictly speaking, the extrusion molding temperature should refer to the temperature of the plastic melt, but the temperature largely depends on the temperature of the barrel and screw. A small part of it comes from the frictional heat generated during mixing in the barrel, so it is often The barrel temperature is used to approximate the molding temperature.

Since 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 the plastic in the barrel smoothly, so as to efficiently produce high-quality parts, the key issue is to control well. The temperature of each section of the barrel and the adjustment of the barrel temperature are realized by the heating and cooling system and temperature control system of the extruder.

The temperature of the die must be controlled below the thermal decomposition temperature of the plastic, and the temperature at the die can be slightly lower than the temperature of the die, but the plastic melt should be guaranteed to have good fluidity.

In addition, the temperature fluctuation and temperature difference during the molding process will cause defects such as residual stress, uneven strength at each point, and dull and dull surface of the plastic part. There are many factors that produce such fluctuations and temperature differences, such as unstable heating and cooling systems, changes in screw speed, etc., but the quality of screw design and selection has the greatest impact

pressure

During the extrusion process, due to the resistance of the material flow, the change of the depth of the screw groove, and the obstruction of the filter screen, the filter plate and the die, etc., 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 uniform 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 density of the product. However, excessive head pressure will affect the output.

Like temperature, pressure changes with time will also produce periodic fluctuations. This fluctuation also has an adverse effect on the quality of plastic parts. Changes in screw speed and 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 device.

Extrusion rate

The extrusion rate (also known as the extrusion speed) is the mass (in kg/h) or length (in m/min) of the plastic extruded by the extruder die per unit time. The size of the extrusion speed characterizes 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, screw speed, heating and cooling system structure, and plastic characteristics. Both theory and practice have proved that the extrusion rate increases with the increase of screw diameter, spiral groove depth, homogenization section length and screw speed, and increases with the increase of the melt pressure at the end of the screw and the gap between the screw and the barrel. In the case that 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 geometric shape and dimensional accuracy of the plastic parts. Therefore, in addition to the correct determination of the screw structure and size parameters, the screw speed should be strictly controlled, the extrusion temperature should be strictly controlled, and the extrusion pressure and melt viscosity changes caused by temperature changes should be avoided, which will lead to fluctuations in the extrusion speed.

Traction speed

Extrusion molding mainly produces continuous plastic parts, so a traction device must be installed. The plastic parts extruded from the machine head and die will undergo stretch orientation under the action of traction. The higher the degree of tensile orientation, the greater the tensile strength of the plastic part along the orientation direction, but the greater the length shrinkage after cooling. Generally, the pulling speed can be comparable to the extrusion speed. The ratio of traction speed to extrusion speed is called traction ratio, and its value must be greater than 1.

This article comes from the Internet, only for learning and communication, no commercial purpose.

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