Views:7 Author:Site Editor Publish Time: 2017-05-15 Origin:Site
In the extrusion process, the need to expel gases from the melt, such gases can not be discharged, the surface or internal products may appear pores, bubbles and surface dull defects, seriously affect the product's physical and mechanical properties, chemical properties and electrical properties. The moisture and other volatile parts of the molten extrusion material can be removed from the 1 ~ 2 exhaust vents between the feeding nozzle and the nose. But the steel tube openings often appear some, the most common problem is from the exhaust vent, a small amount of material will affect the emission of volatile, so that the quality of products affected; A large amount of material will plug the exhaust vents, and even lead to downtime.
The riser generally has two reasons, one is the screw design unreasonable, causes the exhaust outlet material backflow, the second is the exhaust outlet design unreasonable, the melt material passes the vent port when the "hang". Find the reason first from the vent to see if the screw in the material flow backwards, in most of the exhaust extruder, can see the screw rotating in the forward movement of the melt. In general, the screw groove is filled with the degree of no more than 50% of the material, if exceeded, not only affect the exhaust effect, but also may cause the exhaust outlet riser; the screw can work normally when less than 50%. The riser may be due to unreasonable design of the vent or shunt element.
Influencing factors and solutions
1. Screw factor
The exhaust type screw adopts multi-order design, the advantage of this design is that the exhaust outlet is atmospheric pressure, the material does not flow out. An outlet extruder requires a 2-order screw, with 2 exhaust vents requiring a 3-order screw. Every 1 order has the atmospheric pressure section, the compression section and the metering section, the 1th Order starting section is the atmospheric pressure Feeding section, the 2nd order is the atmospheric pressure exhaust section, namely the exhaust outlet position. There are 2 main problems in the design of the screw rod of exhaust type extruder: First, when the exhaust is reached, the material must be completely melted to drain the volatile part; second, the 2nd-order screw feed must be greater than the 1th-order screw, so that the level of the first stage of the 2nd screw groove is not filled, so that the vent to maintain atmospheric pressure. When the feeding quantity of the 1th order screw is larger than the 2nd-order screw, the melt in the extruder will be backward. To solve this problem, we have to reduce the 1th order of feeding or increase the amount of the 2nd order of the feeding.
2. Process conditions
The simplest and fastest way to solve the riser is to change the process conditions. such as cooling, increasing the friction and shear stresses along the steel barrel or screw and increasing the friction or viscosity along the steel cylinder surface to increase the amount of material.
The 1th-order feed quantity can be decreased in several ways:
(1) To raise the temperature of the steel tube 2 and 3 districts.
(2) Cooling the first order screw.
(3) Feeding with hunger method.
(4) Adjusting the temperature of the hopper (need to experiment repeatedly).
The following methods can be used to increase the feeding quantity of the 2nd order:
(1) Lowering the temperature of the 2nd-order steel tube.
(2) The temperature of the 2nd order screw is elevated.
(3) lifting the nose temperature.
(4) Increase the nose clearance or reduce the nose resistance.
(5) Reduce the number of filters.
(6) The use of large gaps in the filtration network.
If changing the processing conditions still do not solve the problem, we have to adopt other methods, such as redesign screw, reduce the nose resistance, lengthened screw and steel tube or between extruder and the head of the installation of a gear pump. Installing gear pumps can solve the problem of the riser, but it is more costly than the new screw.
If the vent at the screw is only partially filled, the exhaust vents still take the material, then the design of the exhaust outlet has some problems. The vent should be wider than the rolling material flow to ensure that the vent is not blocked by the melt. At the same time, the opening of the exhaust outlet should not be too large, so that the melt retention time and the expansion time of the material flow can be reduced. In normal operation, the spiral groove is half full, when the vent is atmospheric pressure, in fact, the rolling melt is still stressed, pressure about 0.21 ~ 0.35MPa or higher, enough to make the vent at the melt expansion. This way the exhaust vents should be designed to take into account the normal viscoelastic expansion of the material, otherwise, the partial rolling flow will be "hang", stacked in the shunt components. The amount of molten material flow expands by the time it passes through the vent. Stay longer and swell. The residence time is controlled by the screw speed and the size of the exhaust outlet, and the speed of the screw can be reduced. This is the reason why the low speed extrusion is serious. However, the larger the exhaust vents, the longer the stay time. When the melt at the vent, it will plug the vent, the solution is to change the vent to meet the melt at the vent at the normal expansion. If the rolling material flow expands 5 ~ 10mm, then the vent depth should be at least 5 ~ 10mm.
4. Exhaust vent Elements
To cite a real life example, with the 150mm freshly polished with the exhaust outlet of the single screw extruder extrusion sheet Metal when serious material, unable to produce qualified products. The operator wants to check the vent in the screw groove melting material situation, but can not see the screw, this explains that the design of shunt components has a great problem, operators have to remove the shunt components to check the full extent of the screw groove. The check found that the screw groove is full of 40%, indicating that the screw design is reasonable.
Then the focus of the fault is on the shunt element. Although this is a well-known manufacturer of extruder, the design of its shunt element has a serious flaw. When examining the shunt element, it is found that the exhaust outlet not only has the bottom incision that can make the material swell normally, but also has this mouth in the lower order. Upper-order screw is absolutely unnecessary to design the bottom incision, and set this incision is extremely unfavorable, because it increases the melting pool at the vent time at the residence and increases the melting pool, increasing the melt pressure accumulation at the vent. Another problem is flat mouth and rectangular vents, which are harder to clean than straight mouth. If the shunt element is reasonably designed, the vent should be perpendicular to the cylinder radius. Through the above analysis, the shunt element can be repaired in two ways: first, the upper-order incision is filled with metal, and the second, the rectangular openings are converted into radial, which can make the operator see the melt material in the screw rod. The improvement of the above method solves the problem of the riser and produces the quality products.
The exhaust screw is the material, with the extrusion process, plastic performance and screw and vent design has a direct relationship, so manufacturers in the use of exhaust extruder production and processing, to the processing technology, resin characteristics and equipment performance in detail to understand, in order to make the exhaust type extruder normal and stable operation