During the extrusion process, gas should be discharged from the molten material. If these gases can not be discharged, the surface or interior of the products may have defects such as pores, bubbles and dark surface, which can seriously affect the physical and mechanical properties, chemical properties and electrical properties of the products. 1~2 air outlets are arranged between the feed port and the machine head to remove moisture and other volatiles in the melt extruding material. But the opening of the steel tube often occurs. The most common problem is to take the material from the vent. A small amount of material will affect the discharge of the volatiles, and the quality of the product is affected; a large amount of material will clog the exhaust port and even lead to a stop.
In general, there are two reasons for the material to be taken. One is that the design of the screw is unreasonable, which leads to the reverse flow of the material at the exhaust port; the two is the unreasonable design of the exhaust port, and it is “hung” when the material is passed through the vent. To find out the reason first, see whether the material in the screw is reflow from the exhaust port. In most of the exhaust extruders, we can see the melt in the screw moving forward in the screw. In general, the slot full of material is not more than 50%. If it is exceeded, it not only affects the exhaust effect, but also may cause the exhaust vent. When less than 50%, the screw can work normally. The material may be caused by unreasonable design of the exhaust port or shunt element.
Factors affecting material and Solutions
1. screw factors
Multi screw design is adopted in the exhaust screw. The advantage of this design is that the outlet is at normal pressure and the material does not flow out. An extruder with an exhaust port requires 2 orders of screw, and 2 exhaust ports require 3 stages of screw. Every 1 order has the normal pressure section, the compression section and the measurement section, the first order start section is the normal pressure feeding section, the second order is the normal pressure exhaust section, which is the position of the exhaust port. There are 2 main problems in the design of the exhaust extruder screw. First, when the exhaust section reaches the exhaust section, the material must be completely fused to discharge the volatile; the second, the second order screw must be more than the first order screw to make the slot at the beginning of the second order not full, so that the exhaust port can be kept at normal pressure. When the feed rate of the first stage screw is larger than that of the second order screw, the melt in the extruder will reverse flow. To solve this problem, we need to reduce the feed rate of first orders or increase the feeding volume of second orders.
2. process conditions
The simplest and fastest way to solve the problem is to change the technological conditions. Such as cooling, increasing friction and shear stress along the cylinder or screw, and increasing friction or viscosity along the surface of the cylinder to increase the throughput.
The first order feed volume can be reduced in the following ways:
(1) increase the temperature of the 2 and 3 areas of the steel tube.
(2) cooling the first order screw.
(3) feed the feed by starvation.
(4) adjust the temperature of the feeder (repeated experiments).
The following second methods can be used to increase the feed volume.
(1) reduce the temperature of the second order steel tube.
(2) increase the temperature of the second order screw.
(3) raise the temperature of the head of the machine.
(4) increase the gap between the head of the machine or reduce the resistance of the head.
(5) reduce the number of filter networks.
(6) the use of a larger gap filter.
If changing the processing conditions can not solve the problem, it is necessary to use other methods, such as redesigning the screw, reducing the head resistance, lengthening the screw and steel tube or installing a gear pump between the extruder and the head. Installing gear pumps can solve the problem of material handling, but it costs more than the new screw.
3. exhaust port
If the screw at the outlet is only partially filled and the exhaust port is still filling, the design of the exhaust port will have some problems. 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, so that the residence time of the melt and the expansion time of the material flow can be reduced.
Under normal operation, the slot is half full, and at this time the exhaust port is constant pressure. In fact, there is still pressure in the rolling stock. The pressure is about 0.21 to 0.35MPa or higher enough to expand the melt at the vent. When the vent is designed, the viscoelastic expansion of the material should be taken into account. Otherwise, the flow of part of the rolling material will be “hung up” and accumulated in the distributary components. The amount of material flow in the melt is determined by the time it passes through the exhaust port. The residence time is long and the expansion is large. 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 why the low speed extrusion is more serious than the high speed extrusion.
However, the larger the opening of the exhaust port is, the longer the residence time will be. When the melt is accumulated in the exhaust port, it will plug 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. If the rolling material flow is expanded 5 to 10mm, the depth of the exhaust port should be at least 5 to 10mm.
4. exhaust manifold
As an example of real life, a single screw extruder with a 150mm that has just been worn out of a single screw extruder has been heavily fed and unable to produce a qualified product. The operator wants to check the molten material in the slot at the outlet of the exhaust port, but it is impossible to see the screw at all. This shows that there is a big problem in the design of the shunt element, and the operator has to take off the shunt element to check the fullness of the slot. After checking, it was found that the screw groove was only 40% full, indicating that the screw design was reasonable.
The focus of the trouble is on the shunt element. Although it is a famous extruder manufacturer, there are serious defects in the design of its shunt components. When checking the shunting element, it is found that the exhaust port not only has a bottom incision which can make the material expand normally, but also has the mouth at the lower level. There is no need to design a bottom notch for the upper order screw, and it is extremely unfavorable to set this incision because it increases the retention time of the pool at the vent and expands the pool more, increasing the accumulation of the melt pressure at the vent.
Another problem is flat and rectangular exhaust ports, which are much more difficult to clean than straight ones. If the shunt element is reasonably designed, the exhaust port should be perpendicular to the radius of the cylinder. Through the above analysis, the shunt component can be repaired by the following two ways: first, fill the metal with the upper order bottom incision; second, the rectangular opening is changed to the radial, so that the operator can see the material in the screw. The improvement of the above way solves the problem of material handling and produces high quality products.
Conclusion:
Whether the exhaust screw or not, has a direct relationship with the extrusion process, the plastic performance and the design of the screw and the exhaust port, so the manufacturer is able to make a detailed understanding of the processing technology, the resin characteristics and the equipment performance in the production and processing of the exhaust extruder to make the exhaust extruder run normally and steadily.
