The Reason And Solution Of The Vacuum Of The Extruder

  The Reason and Solution of the Vacuum of the Extruder

  In the extrusion process, the need to remove the gas from the melt, such as the gas can not be discharged, the product surface or the internal surface may appear pores, bubbles and surface dull and other defects, can seriously affect the physical and mechanical properties of products, chemical properties and Electrical performance. 1 to 2 exhaust ports between the feed port and the head can remove moisture and other volatile components from the molten extruded material. But the cylinder opening is often the emergence of some of the most common problem is from the exhaust vent material, a small amount of material will affect the discharge of volatile, so that the quality of products affected; a large number of material will block the exhaust, and even lead to downtime The

  There are two reasons for the material, one is the screw design is unreasonable, leading to the exhaust at the back of the material; Second, the exhaust port design is unreasonable, in the molten material through the exhaust when the "caught." Find the reason First of all, from the exhaust port to see whether the material in the screw back, in most exhaust extruder, you can see the screw rotating forward movement of the melt. Under normal circumstances, the groove filled with the degree of material does not exceed 50%, if more than, not only affect the exhaust effect, but also may cause exhaust vent; less than 50%, the screw can work properly. The material may be due to unreasonable design of the vent or shunt element.

  1. screw factor

  Exhaust screw multi-use multi-level design, the design of the advantages of the exhaust at the atmospheric pressure, the material will not flow out. An extruder with an exhaust port requires a second-order screw, and two exhaust ports require a third-order screw. Each stage has an atmospheric pressure section, a compression section and a measurement section. The first stage start section is an atmospheric pressure feed section, and the second stage is the atmospheric pressure exhaust section, that is, the position where the exhaust port is located. There are two main problems with the design of the exhaust extruder screw: first, the material must be completely melted to the volatiles when the exhaust section is reached; the second, the second-order screw must be fed more than the first Type the screw so that the groove at the beginning of the second order is not full, thus keeping the atmospheric pressure at the vent. When the first-order screw feed volume than the second-order screw, the extruder in the melt will occur backflow. To solve this problem, you have to reduce the first order of the amount of feed or increase the second order of the amount of feed.

  2. Process conditions

  The easiest and quick way to solve the problem is to change the process conditions. Such as cooling, increasing the friction and shear stress along the steel cylinder or screw, and increasing the friction or viscosity along the surface of the cylinder to increase the amount of material.

  The first order of feeding can be reduced in the following ways:

  (1) Increase the temperature of steel cylinder 2 and zone 3.

  (2) cooling the first-order screw.

  (3) Feeding with hunger.

  (4) adjust the temperature of the feed bin (need to repeat the experiment).

  Increase the second order of the amount of feed can be used as follows:

  (1) to reduce the temperature of the second order cylinder.

  (2) raise the temperature of the second-order screw.

  (3) Raise the head temperature.

  (4) increase the nose gap or reduce the head resistance.

  (5) reduce the number of filters.

  (6) the use of large gaps in the filter.

  If you can not solve the problem by changing the processing conditions, you have to use other methods such as redesigning the screw, reducing the head resistance, lengthening the screw and the cylinder or installing a gear pump between the extruder and the head. The installation of the gear pump can solve the problem of feeding, but it is higher than the cost of the new screw.

  3. Exhaust port

  If the screw at the exhaust port is only partially filled, the exhaust port is still blank, then the design of the exhaust port there is a certain problem. The exhaust port should be wider than the rolling 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 melt retention time and the expansion time of the material.

  In normal operation, the groove is half filled, when the exhaust port is atmospheric pressure, in fact rolling the pressure is still in the melt, the pressure is about 0.21 ~ 0.35MPa or higher enough to make the exhaust at the melt Inflated. So that the exhaust port in the design should take into account the normal viscoelastic expansion of the material, otherwise, part of the rolling of the material will be "caught", stacked in the shunt element. The amount of melt flow is determined by how much time it passes through the exhaust port. Stay long, swell big. The dwell time is controlled by the screw speed and the size of the exhaust port, and the screw speed can be increased to reduce the dwell time. This is the reason why low-speed extrusion is more serious than high-speed extrusion.

  However, the greater the opening of the exhaust, the longer the residence time. When the melt is in the exhaust port, it will block the exhaust port, the solution is to change the opening of the exhaust to meet the melt in the exhaust at the normal expansion. If the rolling material expands 5 ~ 10mm, the depth of the exhaust port should be at least 5 ~ 10mm.

  4. The shunt of the exhaust port

  To give an example of real life, with 150mm just polished with a single-screw extruder extruded sheet extruded sheet seriously impersonate, can not produce qualified products. Operators would like to check the exhaust groove at the location of the melt, but simply can not see the screw, which shows the design of the diversion component there is a big problem, the operator had to remove the shunt element to check the filling of the groove degree. Check that the groove is only filled with 40%, indicating that the screw design is reasonable.

  Then the focus of the fault will be concentrated on the shunt element. Although this is a well-known extruder manufacturer, but the design of its diversion components there are serious flaws. When checking the shunt element, it is found that the exhaust port is provided with not only the bottom cutout which allows the material to expand normally, but also the same in the lower order. It is not necessary to design the cut at the end of the screw, and it is extremely unfavorable to set the cut because it increases the residence time of the bath at the exhaust and increases the pool more, increasing the melt at the vent Body pressure accumulation.

  Another problem is flat and rectangular exhaust, which is more difficult to clean than the straight mouth. If the diverter element is designed properly, the vent should be perpendicular to the barrel radius. Through the above analysis, the shunt element can be used in the following two ways to repair: First, the upper end of the incision to fill the metal; Second, the rectangular opening into radial, the operator can see the screw in the melt situation The The improvement of the above way to solve the problem of goods, and produce a high-quality products.

  in conclusion

  Whether the exhaust screw is out of material, and the extrusion process, plastic properties and the design of the screw and the exhaust has a direct relationship, so the manufacturer in the use of exhaust extruder for production and processing, to the processing technology, resin Characteristics and equipment performance for a detailed understanding of the exhaust extruder to normal operation.