How much compression do you need to get a good seal when using a silicone gasket? This is the question that the silicone gasket compression percentage answers. Too little compression will lead to an ineffective seal but too much compression can also result in a gasket that performs poorly even in the short term.
What is the Silicone Gasket Compression Percentage?
The silicone gasket compression percentage is the difference between the original thickness of the gasket and the thickness of the compressed silicone gasket, divided by the gasket’s original thickness, i.e., before compression, expressed as a percentage. It is a simple measure of how much a silicone gasket is compressed when in use.
The silicone rubber gasket compression percentage can serve as a good guide that ensures you don’t apply too little compression when installing your rubber gasket. Perhaps more significantly, it can also serve as a guide that ensures you don’t apply too much compression on your seal.
The common assumption is that the tighter you can get your rubber gasket the better but this common mistake negatively affects the gasket’s mechanical properties and can cause poor long-term performance of a seal in some applications.
What are Silicone Gaskets?
Silicone gaskets are flexible seals that are installed and compressed between mating surfaces to keep substances from leaking into or out of the sealed space. They are also used to maintain pressure levels inside sealed spaces.
When compressed, a silicone gasket adapts to the unevenness of the mating surfaces, filling grooves and scratches. This creates a mechanical barrier that keeps substances from moving from one side of the mating surfaces to another.
Traditionally, gaskets were made using asbestos and natural rubber, but silicone gaskets have become increasingly popular because of properties such as:
- Low toxicity
- Excellent high and low-temperature performance compared to natural rubber gaskets
- Good insulation properties
- Good resistance to ozone, ultraviolet light, and many chemicals
- Safe for food and medical applications
- Low flammability
There are different types of silicone gaskets possessing a range of properties that make them suitable for different applications. Some types of silicone gaskets include:
- Die-cut gaskets: Die-cut silicone gaskets are produced using a die and a press, resulting in precise cuts and consistency from one gasket to the next.
- FDA-approved food-grade gaskets: These silicone gaskets are made from materials that are considered food-safe. They can withstand heat and aren’t degraded by cleaning products. These will be vital to you if you are a beverage or food manufacturer.
- Fluorosilicone gaskets: These gaskets offer superior resistance to petroleum-based products such as oils, fuels, and coolants compared to other silicone gaskets. These can provide a reliable seal in an automotive or aerospace application.
- Silicone foam gaskets: These gaskets are lightweight and possess good compression set properties. They are softer than sponge gaskets and can be used for sealing, insulation, and cushioning.
- Silicone sponge gaskets: Silicone sponge is considered waterproof and can withstand high temperatures. Its tear resistance and tensile strength exceed that of silicon foam and you can also use this gasket type for cushioning and sealing in the outdoors.
- Solid silicone gaskets: These gaskets are not as soft as their foam or sponge counterparts. They form a good seal at lower compression rates and have varieties with both low and mid-range durometers.
- Conductive rubber gasket: Electrically conductive rubber gaskets contain conductive particles. You can use these where you need a reliable seal along with shielding from electromagnetic interference (EMI) and radio frequency interference (RFI).
Compression in Gasket Design
How much a gasket can be compressed along its thickness is a factor that must be considered when a gasket is being designed. Leakages are more likely to occur if a gasket compresses too little or too much.
Role of Compression in Sealing
A certain amount of compression is necessary for a gasket to form a good seal between mating surfaces. When you compress a gasket, it undergoes some elastic and plastic deformation, forcing it to fill the irregularities that would permit the escape of a gas, liquid, or powder.
If the compression is too little, the gasket will not fill enough of the irregularities to completely prevent a leak. However, too much compression can also cause more permanent deformation, making it less likely that the gasket will rebound enough to form an adequate seal the next time you compress it.
Factors Influencing Gasket Compression
Several factors affect how much a gasket can be compressed. These include:
- How tightly the bolts are torqued
- The properties of the material used to form the gasket
- The design of the mating surfaces
To form good seals, the bolts of your flanges should neither be too tight nor too loose. If the bolts are not torqued enough, the gaskets are not deformed enough to form good seals between mating surfaces. However, the gaskets will undergo permanent deformation if the fasteners are over-torqued.
A softer silicone gasket can form a better seal since it requires less compressive force to conform to the imperfections on the mating surfaces. However, such soft materials would also struggle to maintain the seal under higher pressures.
Some flanges offer a large surface area for the gasket while others have less space available to them. Flanges with more surface area allow the use of wider gaskets which can be compressed more. These offer better sealing but they take up more space and don’t suit low-pressure applications.
Gaskets with smaller widths can be used where there’s less space but they don’t offer the best seals.
The Role of Compression Set
Compression set is a key factor when deciding whether a material will serve as a good gasket material. Compression set tests are ordinarily used as a measure of sealing performance of O-ring compounds. It can be used to quickly compare the expected durability of different materials when an application features cyclic compressive loads.
What is a Compression Set?
Broadly speaking, a compression set is a parameter that quantifies how good a material is at regaining its original shape after being compressed. In the case of a silicone gasket, compression sets can be used to express how much of its thickness has been permanently lost after being subjected to a compressive force.
The compression set of a material is important because it can tell you if it is resilient enough to be used reliably for a specific application. A lower compression set is preferred because it implies the material recovers more and will have greater durability in applications with cyclical compressive loads.
Compression Set VS. Silicone Gasket Compression Percentage
Compression sets A and B measure how much a gasket material can recover after it has been subjected to a compressive force and allowed to recover. Compression set A involves subjecting the material to a constant force while compression set B involves compressing the material to 75% of its initial thickness.
Gasket compression percentage is the percentage of the difference in thickness between the original gasket and the compressed gasket.
To calculate compression set B, the silicone gasket compression percentage is set to 25% by default. This means you can work backward using the compression percentage formula to identify the thickness under compression.
For example, suppose I had a 1.5mm thick piece of silicone rubber. In that case, I can use the compression percentage formula above to identify the thickness under compression needed for the test as shown below.
The result of the above operation, 1.125, is 75% of 1.5mm.
What Does a High Compression Set Mean in Practice?
Materials with a high compression set are less likely to regain a significant portion of their original dimensions after the compressive force is removed. This indicates poor resilience and such materials may develop gaps and leaks sooner if used as seals.
In applications that involve cyclic loading, a gasket will be subjected to different compressive stresses at different times. A lower compression set material will be compressed when compressive stress is high and regain most of its original thickness when the compressive stress lessens. It is less likely that a gap will be left between the mating surface and the gasket.
However, a higher compression set material will be compressed but will recover less of its original thickness when the compressive stress reduces. This means that when loading is cyclic, gaps may form faster between the mating surface and the gasket.
This does not mean that high-compression set gaskets have no use. They are well-suited for use in electronic equipment and other applications with permanent compression.
Factors Affecting Compression Percentage
It’s not enough to know a material’s ideal compression percentage. Several factors can limit how much compression you can apply including the thickness of the gasket, the surfaces of the metal parts being mated, and the joint’s operating environment.
Gasket Thickness
It is generally recommended that you use a thinner gasket if possible. Thinner gaskets are less likely to leak, less likely to suffer a blowout, cost less, and experience less creep relaxation.
However, a gasket compression percentage of 20% in a thicker gasket can seal more imperfections than the same compression percentage in a thinner gasket. Therefore, a thinner gasket may need a greater compression percentage to provide an adequate seal, and in some cases, even this isn’t enough.
Deciding the best gasket thickness is guided by many factors that revolve around the specific application. These include:
- Flange thickness
- Flange diameter
- Internal pressure on the gasket during operation
- Presence of damage to the flange
In some applications, tight spacing may require a specific final thickness. This means that the gasket would have to be a specific size when compressed. The chosen gasket thickness would have to factor in the compression percentage to avoid exceeding the maximum compressive stresses.
Flange Surface Finish
Flange surfaces commonly feature a smooth or serrated finish. The type of finish affects both the type of gasket material and the gasket compression percentage that is needed to get a good seal.
Compared to a smooth finish, a serrated finish offers better resistance to the gasket material. This friction between the gasket material and the flange surfaces makes it harder for the gasket to become dislocated and cause a leak even when the compression percentage is lower.
It is common practice to use serrated sealing surfaces in applications that feature high temperatures and high pressures. These types of finishes should be paired with soft gaskets such as silicone which can deform more easily and enter the grooves for superior sealing.
You can also pair a soft silicone gasket with a flange with a smooth finish in low-pressure, low-temperature applications. However, mating surfaces with a smooth finish are usually paired with hard or metallic gaskets.
Operational Temperatures
When silicone rubber is heated to a temperature of around 300°F (150°C) there may not be any significant difference in its compressive behavior. However, at around 392°F(200°C), silicon rubber will start to harden. A gasket may also swell and its shape may change.
This increased deformation at higher temperatures may be combined with poorer recovery. This means the material would have higher compression sets.
Very low temperatures can also result in lower compressibility of the silicone rubber. These changes in the compressibility of silicone rubber at certain temperatures may make the gasket less effective.
To ensure that sealing is not significantly affected by temperature variations, you can take steps such as:
- Knowing the temperatures your gasket will be exposed to during operation.
- Choosing the right gasket material, especially for high-temperature and low-temperature applications.
- Ensuring proper installation of the gasket including choosing the right compression percentage to accommodate temperature-induced expansion and contraction of metal parts.
Conclusion
When you install a new silicone gasket, one of the questions you are likely to ask is how much compression you need to form a tight seal. This is a question that can be partially answered using the silicone gasket compression percentage.
Although there is an ideal range of compression percentage values, there are other factors that dictate how much compression you should use to get a seal that withstands the test of time. These properties include the gasket material, the smoothness of the mating surfaces, operating temperatures, and more. This guide is a great place to start but it certainly helps to talk to an expert to discuss your specific gasket needs so you can make a better choice when you need a silicone gasket.
