Flexible foam fabrications are often a good choice for gasket material. There are many different types of foam but most are lightweight, can be cut into various shapes, and may be purchased pre-laminated with pressure sensitive adhesive (PSA) for ease of installation. Most importantly, most foams compress readily and do an excellent job of sealing out, (or keeping in,) fluids, gases, dusts and other contaminants.
For a fabricated foam gasket to perform as needed it must be matched to the application. That demands a good appreciation of the following:
- How gaskets work
- Typical gasket applications
- Key performance characteristics
- Environmental factors
- Foam gasketing materials available from Merryweather.
How Gaskets Work
If surfaces like pipe flanges and cabinet doors could be made perfectly flat there would be no gaps for fluid to leak through. In the real world though, surfaces are far from flat. Sheet metal panels inevitably have undulations that cause varying gaps as a door is closed and pipe flanges have machining marks that create leak paths. And even if the surfaces were smooth, alignment tolerances would still create gaps of varying widths.
A gasket is a piece of compressible material fitted between two surfaces that will be clamped together. It's job is to prevent leaks from or through the joint. Being soft, it squashes down more where the gap is smaller while remaining thicker for wider spaces. Its softness also lets it flow into small indentations on the two surfaces, blocking even microscopic leak paths.
The skill in designing a gasket lies in matching the material thickness to the clamping load and material properties. A hard material needs high clamping forces to compress it more where the gap is smallest. At the same time, the gasket must be thick enough to seal the widest gap.
A good rule of thumb is to use the thinnest, softest material that will stand up to the conditions. These include the temperatures the gasket will see, it's environment, the nature of the media being sealed against, and the pressure differential.
Typical Gasket Applications
Some gaskets keep things out, others keep things in. A good example of the former is a gasket around the door of an electrical enclosure. It's job is to keep out dust and moisture, so avoiding faults like short circuits or sticking relays. It may also perform an insulation role by keeping hot ambient air away from sensitive electronics.
For examples of gaskets keeping things in, consider pipes and HVAC ducting. Pipes transport fluids and must be joined at various locations. Every joint needs a gasket to prevent fluid escaping, which would be wasteful and possibly dangerous. HVAC ducting performs a similar role, and gaskets are used both to seal ducts and to prevent expensively heated or cooled air from leaking out around dampers.
A third use of gaskets is to fill gaps resulting from manufacturing tolerances that would otherwise look unsightly and/or cause parts to rattle. Appliances, electronics, and sometimes automobiles, all use gaskets for this purpose.
Key Performance Characteristics
Three properties to consider when selecting gasket material are softness, compression set resistance, and temperature range. Softness, or compressibility, is how much force it takes to achieve a given percentage reduction in thickness. For foam this is sometimes referred to as Indentation Force Deflection (IFD).
Compression set resistance is a measure of elasticity or springiness. It's more important for a gasket that sees a cycling load, as when a door is opened and closed repeatedly, than for one subject only to static loads. The compression set resistance of foam depends on the manufacturing process and the material it's made from.
Many elastomeric gasket materials don't perform well at elevated temperatures. Most foams retain their properties up to around 70°C but if a gasket is likely to experience higher temperatures the material should be chosen accordingly. For example, silicone foam material can be used when temperatures are as high as 204°C (400°F).
When considering temperature, it's essential to take into account the use of cleaning media like steam that may be much hotter than what the gasket usually sees.
Other factors to consider are exposure to moisture and UV or sunlight. If a gasket needs to prevent moisture passing through it's best to use a closed cell material. Foam is a result of gas bubbles forming in liquid polyurethane. Depending on the conditions and additives used, those bubbles may be completely closed, or open to one another. An open cell foam is generally softer and recovers faster from compression, but will allow moisture through.
When a gasket will be used outdoors it's likely to be exposed to UV light. Some foams break down under these harsh conditions, so material should be selected with this in mind.
In pipe and flange applications it's possible a gasket could be exposed to abrasive or corrosive materials. In such situations expert advice should be sought on the choice of gasket material.
A second consideration is the presence of a pressure differential across a gasket. While uncommon in enclosure sealing, pipes used for moving liquids or gases often have a significant differential from outside to in. This may place considerable lateral loads on the gasket and material should be selected accordingly.
Foam gasketing materials available from Merryweather
Merryweather offers several types of foam suitable for gaskets. Low permeability foam works well as a low-pressure gasket for sealing against dust, vapor and light water exposure. Highly cost-effective, it can be PSA-laminated and is a popular choice for seals in automotive, appliance and electronic equipment applications.
When water ingress is a concern it may be best to select a closed cell material. Merryweather offers a range of sponge rubbers and PVC's that perform well in weather-stripping roles as well as providing good insulation and sealing.
Silicone foam is often preferred for high temperature gasketing, Available in hardnesses ranging from very soft to very firm, silicone also has excellent compression set resistance.
Another option for demanding gasket applications is PORON®. Developed by Rogers Corporation, PORON® has an open structure with high pore density and provides repeatable elastic performance. It resists cleaning solvents, is flame-retardant material, and has a low outgassing nature.
Make the Right Foam Choice
A foam fabrication will perform well as a gasket, providing the material is selected to suit the application. Clamping force and compressibility, plus the need for compression set resistance and exposure to high temperatures and other environmental factors are all important considerations. Modern foam materials exist to handle a wide range of gasketing applications and the specialists at Merryweather are always happy to help customers select the most appropriate. Feel free to schedule a call with us today to learn more about what we can offer you and your team!