In the process of extrusion production, gases need to be discharged from the melt. If these gases cannot be discharged, defects such as pores, bubbles and dull surface may appear on the surface or inside of the product, which may seriously affect the physical and mechanical properties, chemical properties and electrical properties of the product. A small amount of material will affect the discharge of volatile matter and affect the quality of products; A large amount of material will block the exhaust port and even cause shutdown.
Causes of “vacuum port emitting material”
There are generally two reasons for discharging materials. One is the unreasonable design of the screw, which leads to the backflow of materials at the exhaust port; Second, the design of the exhaust port is unreasonable, and the molten material is blocked when it passes through the exhaust port. How to observe whether the material in the screw flows back from the exhaust port? In most exhaust extruders, It can see the melt rotating forward in the screw. Generally, the degree of material filling in the screw groove shall not exceed 50%,; If it is exceeded, it will not only affect the exhaust effect, but also cause material emission from the exhaust port; When it is less than 50%, the screw can work normally.
Influencing factors of “vacuum port emitting material”
Multistage design is usually used in twin-screw extruders. The advantage of this design is that the outlet is at atmospheric pressure and the material will not flow out. However, when the feeding speed of the first screw is greater than that of the second screw, the melt in the extruder will reflux. At this time, it is necessary to reduce the feeding volume of the first stage or increase the feeding volume of the second stage.
The process is mainly determined by the speed of feeding, the speed of extrusion and the setting of heating temperature outside the feeding section.
The feeding speed is too fast, which causes the screw groove in the feeding section to be full of materials. The heat increased by the twin-screw shear and the heat generated by the friction between the materials cannot meet the heat required for the supply of materials, resulting in poor plasticization of molten materials, and the phenomenon of “material emission” when the materials pass through the vacuum.
Under the same molding temperature, too fast extrusion speed will lead to too short residence time of materials in the feeding section and compression section, and the shear heat generated by screw shear can not meet the heat required for complete plasticization of materials, and the phenomenon of “feeding” will also occur.
The power of the heating device outside the feeding section of the extruder is too low. Even if the temperature of the feeding section is set higher, the actual temperature that can be reached is still low. The temperature provided by the heating device is difficult to meet the heat required for the plasticization of the material, which will also lead to poor plasticization and “material emission”.
Exhaust port design
If the screw at the exhaust port is only partially filled and the exhaust port is still overflowing, there will be some problems in the design of the exhaust port.
The exhaust port should be wider than the rolling material flow to ensure that the exhaust port is not blocked by the melt. At the same time, the opening of the exhaust port should not be too large, which can reduce the residence time of the melt and the expansion time of the material flow. The expansion of melt flow is determined by the time it passes through the exhaust port. Long residence time and large expansion. The residence time is controlled by the screw speed and the size of the exhaust port. Increasing the screw speed can reduce the residence time. This is the reason why low-speed extrusion is more serious than high-speed extrusion. However, the larger the opening of the exhaust port, the longer the residence time. When the melt accumulates at the exhaust port, it will block the exhaust port. The solution is to change the opening of the exhaust port to meet the normal expansion of the melt at the exhaust port.
After the twin-screw extruder operates for a certain period of time, the screw and the barrel will experience certain wear, resulting in a larger gap between the screw and the barrel. In the case of high-speed shear, the vacuum hole caused by the excessive gap between the screw and the barrel of the extruder will have a “blowout” phenomenon. After using the machine for a long time, it will also cause more blockage of the screen, causing the pressure of the machine head to rise, thus causing the vacuum port to “overflow”.
Whether the exhaust screw emits material is directly related to the extrusion process, plastic properties, and the design of the screw and exhaust port. Therefore, when using the exhaust extruder for production and processing, manufacturers should have a detailed understanding of the processing process, resin properties, and equipment performance, so that the exhaust extruder can operate normally and stably.