In the operation of mechanical equipment and sealing of various systems, sealing technology is a key link to ensure equipment stability and prevent medium leakage. Whether it is pipelines, pressure vessels, or core components of precision instruments in industrial production, they all need to rely on reasonable sealing solutions to solve leakage problems. This article will start from the basic understanding of sealing, systematically sort out the classification, material selection, characteristics and application scenarios of various sealing forms, and provide comprehensive reference for the practical application of sealing technology. ​

1、 Basic knowledge of sealing: leakage and sealing principles

Leakage is a common malfunction of mechanical equipment, which is caused by two core factors: first, during the machining process, there are inevitably defects, shape and size deviations on the surface of the parts, resulting in gaps at the joints of the parts; The second is that there is a pressure difference on both sides of the seal, and the working medium will leak along the gap. Therefore, reducing or eliminating gaps is the core idea to prevent leakage. The essence of sealing is to seal the gap between the joint surface, isolate the leakage channel, increase the leakage resistance, or set up working elements to balance the pressure difference in the channel, thereby achieving leak prevention effect. ​

For vacuum systems, in addition to the conventional leakage of the medium directly through the sealing surface, two special forms of leakage should also be considered: one is leakage, which refers to the leakage of the medium through the capillary tube of the sealing material under the action of pressure difference; The second is diffusion, which refers to the transfer of substances through sealed gaps or material capillaries under the action of concentration differences. These two forms of leakage pose higher requirements for the sealing accuracy of vacuum systems. ​

2、 Classification of Seals: Divided by Motion State and Structural Characteristics

There are various ways to classify seals, and the most crucial one is to divide them into two categories based on the motion state of the joint surface: static seals and dynamic seals. Different categories have further subdivisions to adapt to different application scenarios. ​

（1） Static seal: relative static sealing of the joint surface

Static sealing is used for joint surfaces of parts without relative motion, such as pipeline flanges, pressure vessel covers, etc. It is mainly divided into three categories:

Point sealing: Sealing is achieved through local point contact, suitable for scenarios with small sealing areas and low pressure. ​

Adhesive sealing: Fill gaps with sealant, adapt to irregular joint surfaces, and provide flexible sealing performance. ​

Contact sealing: relying on the comprehensive contact between the sealing element and the joint surface to achieve sealing, it is the mainstream form of static sealing. ​

According to work pressure, static seals are further divided into medium and low pressure static seals and high pressure static seals: sealing gaskets with softer materials and wider gaskets (such as rubber gaskets and asbestos gaskets) are commonly used in medium and low pressure scenarios; In high-pressure scenarios, metal gaskets with harder materials and narrow contact widths (such as copper gaskets and stainless steel gaskets) are selected to withstand the sealing pressure under high pressure. ​

（2） Dynamic seal: sealing of the joint surface in relative motion

Dynamic seals are used for mating surfaces of parts with relative motion, such as shafts, piston rods, etc. They can be subdivided according to their motion form and contact characteristics:

Divided by sports form:

Rotary seal: Suitable for the rotational motion of shaft components, such as centrifugal pump shafts and motor spindle seals. Common forms include packing seals, oil seals, and magnetic fluid seals. ​

Reciprocating seal: Suitable for the linear reciprocating motion of parts, such as hydraulic cylinder piston rods and cylinder piston seals, commonly used lip seals (such as Y-shaped and U-shaped rings). ​

Divided by contact characteristics:

Contact seal: The seal is in direct contact with the moving parts, with good sealing performance, but limited by friction and wear, and is suitable for scenarios with low linear speeds (such as packing seals for low-speed shafts). ​

Non contact seal: The seal does not have direct contact with moving parts, has low friction, and is suitable for high-speed scenarios, but its sealing performance is relatively poor (such as magnetic fluid seals and labyrinth seals). ​

3、 Sealing materials: characteristic requirements and common types

The performance of sealing materials directly determines the sealing effect, and the selection should be based on the characteristics of the medium, working temperature, pressure, and other conditions. The core requirements include good density and low leakage; Having appropriate mechanical strength and hardness; Excellent compressibility and resilience, with minimal permanent deformation; Strong stability at high and low temperatures; Good corrosion resistance and wear resistance; Good adhesion with the sealing surface; Convenient processing and controllable cost. ​

（1） Classification and characteristics of commonly used sealing materials

Rubber materials: the most widely used sealing materials, suitable for different types and scenarios:

Nitrile rubber: resistant to fuel oil and aromatic solvents, not resistant to ketones and esters, suitable for oil resistant sealing products (such as oil seals and O-rings). ​

Chloroprene rubber: resistant to oil, weather aging, ozone, suitable for door and window sealing strips, vacuum sealing products, and has good resistance to inorganic acids. ​

Natural rubber: excellent mechanical properties, high resilience, good cold resistance, not resistant to mineral oil, suitable for brake system seals and ordinary sealants. ​

Fluororubber: High temperature resistance (200-250 ℃), outstanding oil resistance, suitable for cylinder liner seals and high-temperature working condition seals. ​

Silicone rubber: resistant to high and low temperatures (-70-260 ℃), ozone, not resistant to oil, low mechanical strength, suitable for thermal mechanical sealing gaskets (such as lampshade liners), high temperature vacuum sealing. ​

EPDM rubber: resistant to aging, steam, and chemical corrosion, with low density, suitable for washing machine accessories, door and window sealing strips, and steam resistant film sheets. ​

Polyurethane rubber: good wear resistance and sealing performance, temperature resistance range -20-80 ℃, suitable for oil seals, O-rings, and diaphragms. ​

Chlorinated ether rubber: with comprehensive oil resistance, heat resistance, and ozone resistance, suitable for medium and low temperature oil seals and sealing rings. ​

Acrylic rubber: resistant to high temperature oil (175 ℃ long-term use), poor cold resistance, suitable for high temperature oil sealing in non severe cold areas. ​

Other sealing materials:

Graphite: resistant to high temperature and corrosion, suitable for high temperature and high pressure sealing (such as valve sealing, reactor sealing). ​

Polytetrafluoroethylene: resistant to chemical corrosion, high and low temperatures, with a low friction coefficient, suitable for sealing strongly corrosive media (such as chemical pipeline sealing). ​

Metal materials: such as copper, stainless steel, carbon steel, with high strength and high temperature resistance, suitable for high-pressure sealing (such as pressure vessel metal gaskets). ​

Asbestos and Cork: Low cost, suitable for medium and low pressure, low temperature scenarios (such as ordinary pipeline gaskets), but the application of asbestos is gradually decreasing due to environmental issues. ​

4、 Mainstream sealing forms: structure, characteristics, and applications

According to the sealing structure and working principle, common sealing forms can be divided into gasket sealing, sealant sealing, packing sealing, molded packing sealing, magnetic fluid sealing, high-pressure sealing, etc. Each form has clear applicable scenarios and operational requirements. ​

（1） Gasket sealing: a classic form of static sealing

Gasket sealing is widely used in pipeline, pressure vessel, and shell joint surfaces, achieving sealing by filling gaps with gaskets. It is divided into three types according to materials: non-metallic gaskets (rubber, asbestos, polytetrafluoroethylene), non-metallic metal composite gaskets (rubber coated metal gaskets), and metal gaskets (copper, stainless steel gaskets). ​

There are three main forms of leakage in gasket seals: interface leakage (leakage of medium along the gap between the gasket and the joint surface), permeation leakage (leakage of medium through the capillary tube of the gasket material), and destructive leakage (leakage caused by gasket damage), among which the first two are the main control objects. When selecting, the gasket material should be matched according to pressure and temperature. For example, metal gaskets should be selected for high-pressure scenarios, and polytetrafluoroethylene gaskets should be selected for strong corrosion scenarios. ​

（2） Sealing glue: a flexible seal that adapts to irregular gaps

Sealing glue is used to fill complex or inconvenient gaps during construction. It is divided into vulcanized rubber gaskets, non vulcanized sealing tapes, and paste/putty liquid sealing glue according to its form. Its core advantage is strong adaptability, which can cope with irregular joint surfaces. ​

1. Classification of sealant

Divided by base material:

Rubber type: based on polysulfide rubber, silicone rubber, polyurethane rubber, etc., with good elasticity and strong sealing, suitable for dynamic or vibration scenarios. ​

Resin type: based on epoxy resin and unsaturated polyester resin, with high strength and aging resistance, suitable for static high-strength sealing (such as equipment shell sealing). ​

Oil based type: based on vegetable oil and animal oil, with low cost, suitable for low-pressure and low-temperature scenarios (such as ordinary pipeline sealing). ​

Divided by vulcanization method:

Atmospheric vulcanization type: single component polyurethane and silicone rubber, vulcanized using moisture in the air, easy to apply. ​

Chemical vulcanization type: two-component polyurethane and epoxy resin, requiring mixed reaction vulcanization, suitable for high-strength sealing. ​

Heat conversion variant: PVC resin plasticizer, heat cured, suitable for high temperature scenarios. ​

Oxidation hardening type: Dry vegetable oil-based adhesive, relying on oxidation hardening, suitable for glass installation and joint sealing. ​

Solvent volatile type: butyl rubber, polyisobutene based rubber, cured after solvent evaporation, suitable for low-pressure sealing. ​

Non drying plastic type: polybutene, non oxidizing adhesive base adhesive, maintains plasticity for a long time, suitable for sealing removable parts (such as equipment maintenance ports). ​

2. Selection and construction of sealant

The selection should be based on the usage conditions (pressure, temperature, medium), sealing material, and construction process. For scenarios with high stress and large temperature differences, rubber sealant with good toughness should be selected; During construction, the selection method should be based on the form. Paste adhesive should be scraped with a scraper or injected with an injection gun, liquid adhesive should be brushed or sprayed with a brush, and film adhesive should be directly applied to ensure a uniform and bubble free adhesive layer. ​

（3） Packing seal: the traditional form of dynamic seal

Packing seal is the oldest dynamic sealing method, which achieves sealing by filling the packing in the leakage channel. It is widely used in centrifugal pumps, compressors, vacuum pump shafts, reciprocating compressor piston rods, valve stems and other scenarios. ​

1. Classification and characteristics of fillers

Twisted packing: Made of asbestos wire twisted together, with a simple structure, suitable for low-pressure and low-temperature scenarios (such as ordinary water pump sealing). ​

Braided filling material: Cotton, linen, asbestos fiber spun into threads, impregnated with lubricants or polytetrafluoroethylene, with improved sealing and wear resistance, suitable for medium and low pressure scenarios. ​

Plastic filler: formed by mold pressing, with high density and good sealing, suitable for medium and high pressure scenarios (such as compressor shafts). ​

Metal packing: Semi metal (metal non-metal combination) or all metal, high strength, high temperature and high pressure resistance, suitable for extreme working conditions (such as high-temperature valve stem). ​

Carbon fiber filler: a new type of high-performance filler that is self-lubricating, resistant to high and low temperatures, resistant to chemical corrosion, has good elasticity, and only has slight penetration leakage (which can be improved by impregnating polytetrafluoroethylene). It is suitable for high-end precision equipment (such as aerospace equipment sealing). ​

2. Selection and filling of fillers

Selection should consider equipment type, medium characteristics (corrosiveness, temperature), working pressure and speed, such as selecting PTFE impregnated packing for strong corrosion scenarios and metal or carbon fiber packing for high temperature scenarios; When filling, it is necessary to clean the packing chamber and check the surface of the shaft (without scratches or burrs). Fill in circles one by one (it is forbidden to fill multiple circles at the same time). The cut of each circle should be inclined at a 45 ° angle and staggered. Finally, pre press with a cover to ensure that the packing is tightly attached to the shaft without excessive friction. ​

（4） Molding packing seal: standardized and efficient sealing

Formed packing seal is a ring-shaped sealing ring processed by molding or turning. It is divided into extrusion type sealing rings (O-ring, square ring) and lip type sealing rings (V-shaped, U-shaped, Y-shaped ring) according to working characteristics. It is also divided into rubber, plastic, leather, and metal types according to materials. Its advantages are standardized structure, easy installation, and stable sealing performance. ​

1. Characteristics and Applications of Typical Molding Fillers

O-ring: The most classic extruded sealing ring, with a simple structure, light weight, self sealing effect, almost no leakage in static sealing, low frictional resistance in motion, standardized size, and low cost. It is widely used in hydraulic and pneumatic systems, static sealing and low-speed dynamic sealing scenarios (such as cylinder piston and valve sealing). ​

V-shaped sealing ring: a lip shaped sealing ring with good sealing performance and resistance to impact and vibration. It can be used multiple times to adjust the clamping force and is suitable for reciprocating motion (such as hydraulic cylinder piston rods). The disadvantage is that it has high frictional resistance. ​

Y-shaped sealing ring: lip shaped sealing ring, with short lips and thick bottom, overcomes the problem of unstable installation of U-shaped rings, has high strength and tear resistance, suitable for sealing pistons and piston rods, and is suitable for medium and high pressure scenarios. ​

J-type/L-type sealing ring: Low pressure sealing ring, working pressure ≤ 1MPa, J-type suitable for piston rod sealing, L-type suitable for low-pressure pneumatic/hydraulic equipment sealing. ​

（5） Magnetic fluid seal: high-end non-contact seal

Magnetic fluid sealing utilizes the magnetic properties of magnetic fluids to achieve sealing, suitable for high-speed and high vacuum scenarios (such as precision motors and vacuum equipment shafts). Its core advantages are non-contact, wear free, strong sealing, and suitability for high peripheral speeds (30-80m/s). ​

1. Composition and Characteristics of Magnetic Fluid

Magnetic fluid is composed of solid ferromagnetic particles (Fe ∝ O ₄, approximately 100 Å), surfactants (to prevent particle aggregation), and carrier liquids (water, oil, esters). It needs to meet the requirements of good stability (non aggregation, non precipitation), high saturation magnetization, low viscosity, and low saturation vapor pressure. The vapor pressure of the carrier liquid directly affects the sealing life and vacuum degree. ​

2. Working principle and limit conditions

When working, permanent magnets, pole shoes, and shafts form a magnetic circuit, and the magnetic field concentrates the magnetic fluid in the gap to form an "O" ring, blocking the leakage channel. The limit conditions include:

Evaporation: The evaporation of the carrier liquid will reduce the amount of magnetic fluid, and a low vapor pressure carrier liquid (such as a grease lubricant, suitable for 1.333 × 10 ⁻⁷ Pa ultra-high vacuum) should be selected. ​

Temperature rise: Temperature ≤ 105 ℃ (to avoid demagnetization of the magnet and excessive evaporation of the carrier liquid), cooling is required for overheating. ​

Weekly speed: generally ≤ 80m/s (considering heat dissipation and pressure resistance). ​

（6） High pressure seal: a special form of coping with extreme pressure

High pressure seals are used for high-pressure vessels and pipelines, and are divided into forced seals and self sealing seals according to their working principles, suitable for different pressure levels and working conditions. ​

1. Forced sealing

Relying on bolt pre tightening force to generate contact pressure between the sealing element and the joint surface, suitable for medium and high pressure scenarios, common forms:

Flat gasket sealing: The structure is simple, and the gasket is made of rubber, asbestos or metal, suitable for medium and low pressure (≤ 10MPa). ​

Kazari seal: metal gasket+pressure cover, good sealing performance, suitable for high pressure (10-30MPa). ​

Octagonal gasket sealing: Metal octagonal gasket with small contact area and concentrated pressure, suitable for high pressure (≥ 30MPa). ​

2. Self sealing

The higher the pressure, the greater the contact pressure between the sealing element and the joint surface, and the better the sealing performance. It is suitable for high pressure (≥ 20MPa) or pressure fluctuation scenarios. Common forms:

Semi self tightening seal: Double cone seal, relying on conical contact for self tightening, with a relatively simple structure, suitable for medium and high pressure. ​

Fully self tightening seal: wedge seal, C-ring seal, hollow metal O-ring seal, relying on the deformation and self tightening of the component itself, with excellent sealing performance. However, the structure is complex and the manufacturing difficulty is high, suitable for high pressure (≥ 50MPa) or ultra-high pressure scenarios. ​

5、 Selection and maintenance of seals: key to ensuring sealing effectiveness

The selection of seals should follow the principle of "matching working conditions and considering costs", with core considerations including:

Operating parameters: pressure (choose metal gasket and self sealing for high pressure, rubber gasket and packing seal for low pressure), temperature (choose fluororubber, metal and carbon fiber for high temperature, silicone rubber and natural rubber for low temperature), medium (choose polytetrafluoroethylene and fluororubber for strong corrosion, nitrile rubber for oil), motion state (choose gasket seal and sealant for static, packing seal and oil seal for rotation, lip seal ring for reciprocating). ​

Performance requirements: For high sealing requirements, contact seals and formed fillers are selected. For high-speed applications, non-contact seals (magnetic fluid seals) are selected. For detachable scenarios, non drying sealants and filler seals are selected. ​

Cost and maintenance: Standardized molding fillers and rubber pads (low cost, easy to replace) are selected for conventional scenarios, while carbon fiber and magnetic fluid seals (performance priority) are selected for high-end scenarios.
In terms of maintenance, it is necessary to regularly check the wear and aging of seals and replace expired seals in a timely manner; Clean the impurities on the sealing surface to avoid scratches; For packing sealing, regularly replenish or replace the packing, adjust the pressure of the gland (to avoid friction heating caused by excessive tightness and leakage caused by excessive looseness); For high-pressure seals, regularly verify the pre tightening force of bolts to ensure a tight fit of the sealing surface. ​
6、 Summary
Sealing technology is the foundation for stable operation of mechanical equipment and systems, and its core lies in matching sealing forms, materials, and structures according to working conditions. From basic gasket seals and packing seals, to efficient molded packing seals and high-end magnetic fluid seals, various sealing forms have their own adaptation scenarios, and need to be selected comprehensively based on pressure, temperature, medium, and motion state. Meanwhile, reasonable construction and regular maintenance are the key to ensuring the sealing effect. Only by comprehensively mastering the basic knowledge of sealing can the system achieve leak free and long-term stable operation.