搜索
News
Location:
Homepage
/
/
/
Several common ultra high vacuum sealing technologies

Several common ultra high vacuum sealing technologies

  • Categories:Industry Trends
  • Author:
  • Origin:
  • Time of issue:2021-08-12
  • Views:0

(Summary description)As we all know, vacuum is inseparable from good sealing. Vacuum sealing technology is an important means to ensure the vacuum degree of the system. Common vacuum sealing technologies include rubber seal, magnetic fluid seal, adhesive seal and metal seal, etc. Let's learn about these sealing methods and their characteristics.

Several common ultra high vacuum sealing technologies

(Summary description)As we all know, vacuum is inseparable from good sealing. Vacuum sealing technology is an important means to ensure the vacuum degree of the system. Common vacuum sealing technologies include rubber seal, magnetic fluid seal, adhesive seal and metal seal, etc. Let's learn about these sealing methods and their characteristics.

  • Categories:Industry Trends
  • Author:
  • Origin:
  • Time of issue:2021-08-12
  • Views:0
Information
As we all know, vacuum is inseparable from good sealing. Vacuum sealing technology is an important means to ensure the vacuum degree of the system. Common vacuum sealing technologies include rubber seal, magnetic fluid seal, adhesive seal and metal seal, etc. Let's learn about these sealing methods and their characteristics.
Vacuum rubber seal
In the mid-19th century, the first O-ring appeared. Today, only more than one hundred years have passed, but the O-ring has been comprehensively developed and widely used in the sealing of various vacuum systems because of its simple structure, convenient loading and unloading, reliable sealing, small dynamic friction resistance and no periodic adjustment.
There are two types of rubber used in vacuum systems: vulcanized rubber made of natural latex, synthetic rubber (including butyl, chlorine based and nitrile butadiene rubber), silicone rubber and fluorine rubber.
In order to solve the problem of vacuum rubber sealing, besides the correct sealing structure design, reasonable selection of sealing materials is also the key. Several main factors affecting vacuum sealing are: heat resistance, compression deformation resistance, air leakage rate, air permeability, air outlet rate, and sublimation (weight loss) of rubber.
·Heat resistance. In the vacuum system, it is often necessary to degass the system or components, which is generally completed by baking. In this way, the rubber seal is required to have certain heat resistance to ensure the smooth baking degassing. Generally, when the baking temperature is below 120 ℃ and the vacuum degree is 10 ^ - 5pa, butyl or nitrile rubber can be used; If a higher baking temperature is required, and it works in an ultra-high vacuum environment, fluororubber should be used.
·Compression deformation resistance. In the vacuum system, a large number of vacuum seals work in a compressed state. In order to make the seal have sealing reliability and maintain a certain sealing life, the vacuum sealing rubber should have a small compression deformation value (preferably less than 35%), and a relatively slow compression stress relaxation degree (i.e., a large compression stress relaxation coefficient), so as to ensure a high working life of the vacuum seal.
·Air leakage rate. According to experience and calculation, in the vacuum system, when the pumping rate of the vacuum pump is 8000L / s, it is necessary to maintain 5 × The air leakage rate of rubber shall not be greater than 5 under a vacuum of 10 ^ - 7pa. twenty-five × 10^-3Pa·cm^3/s。 Table 1 below shows the air leakage rate of various rubbers.
·Air permeability. Different rubbers have different air permeability at different temperatures, which is determined by their internal structures. NBR has low gas permeability due to methyl group; Moreover, because of the polar groups of the nitrile group, NBR has low permeability to non-polar gases. Therefore, the higher the acrylonitrile content of NBR, the lower its air permeability. It is worth mentioning that temperature has a great influence on the air permeability of rubber. The higher the temperature, the greater the air permeability. In addition, the gas permeability of different gases in different rubbers is also different. In the same gas, the order of gas permeability is: natural rubber > styrene butadiene rubber > nitrile butadiene rubber > neoprene rubber > butyl rubber. Table 2 below shows the gas permeability of various rubbers to different gases.
·Outgassing rate. Rubber outgassing rate is defined as the outgassing amount per unit area of rubber in unit time at a certain temperature. In vacuum sealing, it is generally required to be 10 ^ - 4 ~ 10 ^ - 5pa · L / s. According to the experimental data, according to the size of the outgassing rate, various rubbers can be arranged as follows: chlorohydrin rubber > vinyl silicone rubber > Natural Rubber > nitrile butadiene rubber > neoprene rubber > fluororubber.
·Sublimation (weight loss) of rubber. The weight loss of rubber under certain vacuum and temperature is called sublimation. In vacuum sealing, the sublimation value of the sealing material is required to be small. In order to make the rubber seal have a relatively stable relationship in the corresponding vacuum system to ensure the maintenance of the given vacuum, the sublimation value is generally required to be less than 10%. According to the size of vacuum sublimation value, various rubbers can be arranged as follows: natural rubber > nitrile butadiene rubber > neoprene rubber > chlorohydrin rubber > ethylene silicone rubber > fluororubber. Table 3 shows the weight loss of various rubbers in vacuum.
In high vacuum system, the main influence factors of rubber sealing elements on the limit pressure of vacuum system are the leakage rate and outgassing rate of materials.
Magnetic fluid seal
Magnetic fluid was invented in the 1960s. Magnetic fluid seal is a mature technology, which plays an increasingly important role in vacuum sealing.
In principle, magnetic liquid (sometimes called magnetic fluid or ferromagnetic liquid) is a solid-liquid mixed colloidal liquid composed of magnetic nanoparticles, which are uniformly dispersed into the liquid and mixed with it after special treatment. It has both fluidity and magnetism of liquid. Magnetic fluid sealing technology is realized by using the response characteristics of magnetic fluid to magnetic field. When we inject the well-made magnetic fluid into the gap of the magnetic circuit composed of high-performance permanent magnets, pole shoes with good magnetic permeability and shafts, under the action of the magnetic field, the magnetic fluid will form several liquid O-rings in the gap, the number of which is equal to the number of designed protrusions. When the magnetic fluid is affected by the pressure difference, it will move slightly in the non-uniform magnetic field, generating a magnetic force against the pressure difference, so as to achieve a new balance, Which plays a sealing role.
The magnetic fluid seal has the following advantages because only the contact seal between the conductive magnet and the liquid is provided inside the seal:
·Tight sealing. Ester based magnetic fluids can provide tight and stable dynamic and static seals for media (atmosphere or inert gas).
·Non measurable leakage rate. Basically, it is difficult to detect leakage, so people usually call the magnetic fluid seal "zero leakage".
·High reliability. When the magnetic fluid seal is under positive pressure and produces instantaneous overpressure breakdown, once the pressure is reduced to the extent that the seal can bear, the sealing effect can still be maintained, and the reliability of use is quite high.
·Basically no pollution. Since the seal itself does not have mechanical wear and the saturated vapor pressure of the magnetic fluid is extremely low, it will not cause pollution even when used in a high vacuum state.
·High speed performance. In the rotating state, magnetic fluid has the potential of high-speed motion because of its small internal friction and small power loss.
·Low friction, low wear and low heat. In the seal with bearing, in addition to the small mechanical wear of the bearing during rotation, the internal magnetic core assembly and the rotating shaft are not in direct contact, so the friction is low, the wear is small, the heat is small, and the required operating power is small.
·Good repairability. In use, if the magnetic fluid seal fails for some reasons, as long as the internal components function normally, the magnetic fluid seal can be repaired in a short time on site.
·No directional seal. If the pressure bearing direction needs to be changed, the magnetic fluid seal can be completed without adding any components.
Due to many advantages of MHD seal, its application fields have involved domestic and imported crystal growth equipment, diffusion furnace, vacuum brazing furnace, vacuum heat treatment furnace, coating machine and other vacuum equipment.
Adhesive seal
In 1985, the Institute of chemistry and the Institute of electronics of the Chinese Academy of Sciences developed a silicone high vacuum microporous sealant and transferred it to Nanchang xerography factory for trial production. This sealant is a colorless, transparent and low viscosity liquid, which has strong wettability to metals, ceramics, glass and plastics. It can be used to seal the micro leaks in high vacuum devices and high vacuum systems with good effect. The temperature range is 350 ℃ ~ - 196 ℃.
From the perspective of application scenarios, there can be the following: ① various parts are bonded and fixed in the vacuum chamber; ② Vacuum sealing or leakage stoppage, this vacuum glue has certain strength after curing, and can be used for vacuum sealing at the thread, connection and vacuum sealing between glass, ceramic and metal, sealing between the electrode of feedthrough / aviation plug and flange, connection and vacuum sealing of various vacuum tubes such as X-ray tube and laser tube, and connection and sealing between vacuum tube and flange; In addition, it can quickly plug the leakage holes of various vacuum systems or components. For general vacuum leakage, it can be directly applied to plug the leakage. After the glue hardens, it can be directly used for leak detection or normal use; ③ Vacuum adhesive is cured with curing agent, which has no irritating smell and volatile substances such as formaldehyde. Therefore, it can also be used in non vacuum applications with high odor requirements.
It can be said that for general vacuum leakage, the adhesive sealing method is very convenient for sealing and plugging, even for ultra-high vacuum, and it can also play a connecting role between some parts.
Metal seal
Metal seals have a long history of development and appear earlier than rubber seals. In contrast, rubber has a large outgassing rate and permeability, can not be baked at high temperature, and is not radiation resistant, so its application is limited to a certain extent. Metal seal makes up for the above shortcomings of rubber seal, so it is widely used in ultra-high vacuum environment.
In principle, metal materials, like rubber, have certain elasticity and ductility. When the metal seal ring is elastically deformed by external pressure, under the action of elastic restoring force, the seal ring has a tendency to restore its original shape, which fills the gap of the sealing surface and plays a sealing role.
Several metal sealing methods often used in ultra-high vacuum are described below.
1. Metal indium wire seal
The Mohs hardness of metallic indium is only 1. 2, much smaller than metal copper 2. 5-3 and aluminum 2-2. 9 and a melting point of 156. 6℃。 Its good ductility makes it very conducive to vacuum sealing connection. When sealing, a section of indium wire with a suitable length is spread on the flange surface, and both ends of the indium wire are overlapped. It is not necessary to process into a standard sealing ring in advance. Therefore, it is often used in occasions where the flange size is large and other metal sealing rings are not easy to process.
According to the size of flange, the diameter of indium wire can be selected as 1 ~ 2mm. However, due to its low melting point, the baking temperature cannot be higher than 150 ℃. And indium has good low-temperature performance. Indium wire seals are often used in low-temperature vacuum seals. However, the indium wire is easy to flow after being pressed, so the flange needs to be made into a step or groove type, as shown in Fig. 2, to prevent the indium wire from flowing into the vacuum chamber.
2. All metal quick release seal
Compared with the conventional bolt fastening flange, which requires multiple sets of bolts, the clamp has only two screws for fastening, which can complete the installation more quickly, so it is called quick release clamp. All metal quick release seal mainly consists of quick release clamp, flat flange and sealing ring, as shown in the figure below.
The axial clamping force of the quick release clamp is small. Generally, pure aluminum is used as the material of the sealing ring. In order to reduce the deformation of the clamp, the clamp is made of stainless steel with high rigidity. The quick release clamp is composed of multi segment clamps. With the increase of flange diameter, the number of clamps increases. And with the increase of flange diameter, under the same screw tightening force, the clamping force of the clamp on the axial direction becomes smaller. Therefore, for nominal caliber less than φ 160 flange, all metal quick release seal has good sealing performance.
The all metal quick release seal does not need axial screws to fix, so it saves axial space greatly, and has the characteristics of small number of screws and fast installation. It is often used in occasions where the installation space is small and fast installation is required. For example, the high-energy accelerator has a certain dose of radiation just after shutdown. In order to protect the personal safety of staff, it is necessary to complete the installation quickly. The all metal quick release seal has formed a trial standard in the Institute of high energy physics of the Chinese Academy of Sciences, and has been widely used in the vacuum sealing connection of multiple accelerators.
3. All metal quick release and indium wire combined seal
In order to better install the indium wire before installation, the indium wire sealing flange often needs to make a shoulder ring or groove on the flange, as shown in the figure above. Moreover, due to the soft indium wire, a large operating space is required during installation. The all metal quick release seal is often prone to small leakage rate due to insufficient axial clamping force.
The combined seal described in this section inherits the advantages of the soft indium wire and the fact that the all metal quick release seal can be installed in a narrow space. Adding indium wire outside the sealing surface of the all metal seal ring can solve the problem of air leakage caused by insufficient clamping force of the all metal quick release seal.
This sealing structure is equivalent to extending the gas leakage path and greatly reducing the flow conductivity of the leakage channel. In addition, the indium wire is located outside the metal seal ring, which can reduce the risk that the indium wire will melt and flow into the vacuum chamber due to high temperature, and improve the reliability of the system.
4. Spring accumulator helicoflex seal
The spring energized seal, or helicoflex, is a pressure auxiliary sealing device composed of metal aluminum, copper, silver, stainless steel or other polymeric material jackets and energy storage springs.
When the spring energy storage sealing ring is installed in the sealing groove, the spring is compressed, which makes the sealing surface of the jacket close to the sealing groove, thereby forming a seal. The spring provides elastic force to the sealing jacket and makes up for material wear and offset or eccentricity of mating parts. The system pressure will also assist the sealing jacket in energy storage, forming a constant and lasting preload through the spring elastic force and the system pressure, so as to achieve effective sealing.
Spring energy storage seal ring is used for sealing and application in ultra-high vacuum, nuclear devices, aerospace, petroleum, low temperature, chemical, metallurgy, power machinery, steam vessels and other equipment.
Keyword:

Scan the QR code to read on your phone

Recommended News

Changfeng Rubber And Plastic Products

Address: No. 6, Songshan Road, Qinghe Industrial Park

Tel: 0086-319-8717206 8717211

Fax: 0086-319-8030768

Website: http://en.hbcfxs.cn/

Email: hbchangfeng@163.com

COPYRIGHT © 2021 Hebei Changfeng rubber and plastic products Co., Ltd
gotop