Finished gaskets follow a formula. Seal design begins with compound, hardness, and profile selection. Custom seals and gaskets that account for these factors can fill the gap, withstand the environment, and meet business and technical requirements.
Let’s take a closer look at what you need to know.
Compound selection is about asking and answering three main questions:
- Which compounds can withstand the environment?
- What are their advantages and disadvantages?
- What are their material costs?
To determine which materials can withstand the environment, consider media, temperature, application, and pressure. The table below contains some specific questions.
|Media||Is there exposure to freshwater, saltwater, fuels, oils, or chemicals?
Is this exposure incidental or long-term?
|Temperature||What are the minimum and maximum service temperatures?
Are heat aging or thermal cycling potential risk factors?
|Application||What is the flange type, flange material, and bolt-hole pattern?
Will the seal or gasket be installed or applied with PSA tape instead?
|Pressure||Does the sealing application involve high or low pressures?
How could the pressure level affect seal compression?
When you compare compounds, consider their advantages and disadvantages along with material costs. For example, both EPDM and TPEs can resist outdoor environments. EPDM rubber costs less, but TPEs are available in tighter tolerances. TPEs also come in custom colors and are recyclable.
You may also need to compare commercial compounds to specialty compounds such as UL approved materials. Specialty compounds have extraordinary material properties but are more expensive. They also have higher minimum order quantities (MOQs).
Compounds come in a range of durometers, a measure of hardness. Depending on your sealing application, you may need a lower-durometer rubber that’s soft like a pencil eraser or a higher-durometer that’s hard like a hockey puck.
Remember the following about harder and softer rubber:
- Harder rubber provides greater impact resistance but is less elastic.
- Softer rubber provides greater cushioning but is more prone to compression set.
Compression set is the permanent deformation of a material when an applied force is removed. A gasket that’s over-compressed won’t provide proper sealing, but a gasket that’s too hard can also cause seal failure. Consider the example of a door that won’t shut because the seal is too hard to compress.
During durometer selection, you may find yourself comparing sponge and solid materials. Solid rubber is usually harder, but that’s not always the case. The distinguishing feature of sponge rubber is a cellular structure that either permits (open cell) or prevents (closed cell) the passage of air, water, and gases.
Rubber profiles for seals and gaskets come in a variety of shapes. Four of the most common types are D, P, E, and lip seals. The bulb or lip fills the gap, but profile selection is also about the amount of compression and the installation or attachment method.
- D-seals have a half-moon bulb that provides sealing under pressure and a flat, straight section that attaches with an adhesive such as PSA tape.
- P-seals have a rounded bulb and a flat stem. The bulb provides sealing under compression. The stem or tail is used for attaching the gasket with mechanical fasteners.
- E-seals have a half-moon bulb that supports compression, a tail that lays atop a substrate, and a gap or slot where an installer inserts metal bar stock.
- Lip seals have an edge or extended lip instead of a round bulb. Unlike other types of profiles, they don’t need full compression to provide effective sealing.
More Seal Design Tips
Seal design starts with compound, hardness, and profile selection. Gasket designers also need to consider extrusion and fabrication tolerances – the subject of our next blog entry. To support your seal designs, Elasto Proxy is offering a Gasket Design 101 Lunch and Learn session. Click here to sign-up.