Gasket need a bulb that’s large enough to fill the gap but that will compress to form a reliable, long-lasting seal. For example, a door seal needs to fill the gap between the edges of the door and a frame. When the door is closed, the seal compresses to prevent the entry of wind, water, dust, or dirt. If the seal works properly, the rubber will “rebound” when the door is open and then compress again when the door is closed.
Determining the bulb-size you need is a two-step process that’s represented by the following calculation:
Bulb Size = Average Jam Dimension + Compression Percentage
First, calculate the average jam dimension. An easy way to do this is to place some modeling clay in each corner and then close the door, hatch, or enclosure to compress the clay. Next, insert calipers into the clay and record the measurement for each corner. Now divide this number by four to get the average jam dimension or gap size.
Next, apply a compression percentage to this amount. As a rule of thumb, apply between 25% and 50%. Do not apply more than 50% because over-compressing the bulb won’t create a better seal. In fact, over-compression can reduce seal life and cause compression set, a problem we’ll examine in the Durometer section of this Gasket Design Guide.
Rubber profiles for gaskets come in a variety of shapes. Four of the most common types are D, P, E, and lip seals.
Rubber compounds come in a range of durometers, a measure of hardness that’s expressed on the Shore A scale. Depending on your gasketing 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:
Compression set is the permanent deformation of a material when an applied force is removed. A rubber 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.
Compound selection is about choosing the right type of rubber for your sealing application. If you think that all you need is “black rubber,” it’s time to think again! Rubber doesn’t just come in black. You can choose other colors as well. To chose the right material, you’ll need to ask and answer these questions:
To determine which rubber materials can withstand your environment, consider media, temperature, application, and pressure (MTAP). The table below in this Gasket Design Guide contains some specific questions to ask about your application.
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 rubber 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).
Rubber gaskets can be attached to plastic, metal, or glass surfaces. Mechanical fasteners like screws and bolts are strong and reliable, but installation is time-consuming and requires the use of hand tools. Moreover, metal fasteners add greater indirect material costs to projects. Plastic push pins provide an alternative to metal screws and bolts, but these plastic fasteners are used mainly in automotive-style applications.
Gasket attachment with liquid adhesives can also extend installation times. For starters, liquid adhesives are messy. Many products contain volatile organic compounds (VOCs) and must be used in well-ventilated areas. Liquid adhesives can also introduce concerns about employee health and safety. When gasket attachment is complete, project timelines are extended by cleanups.
Taping can keep rubber gaskets in place for temporary or permanent fastening. Pressure-sensitive adhesive (PSA) tapes hold profiles in place on rough or porous surfaces. Heat-activated taping system (HATS) products adhere well to the paints and plastics used with vehicles. Both types of tapes come with peel-away release liners for ease-of-installation.
Gasket design starts with compound, hardness, and profile selection. Yet engineers also need to consider part tolerances both for rubber extrusions and for fabricated gaskets. To learn more about gasket design and fabrication, contact Elasto Proxy.