Material Selection
The first seal or gasket design consideration is to determine whether an application calls for sponge or dense
material. Along with this initial determination, closing force requirements need to be considered.
The primary difference between sponge and dense from the design standpoint, by example:
- If the application requires a very low closing force, such as a door seal on a consumer-type product,
then a sponge cross-section is most likely best suited, or;
- If the application requires a great deal of interference between the two surfaces, such as the bolting
together of two components in an industrial setting. Then a dense section is the preferred choice.
In either instance, material selection depends upon the physical characteristics and attributes needed in the
application.
A second consideration in the material selection process is the sealing environment. Will there be heat or cold
present? Are there solvents or other chemicals present? Is it static or dynamic sealing application? Will the
material be required to stand up against UV and ozone exposure? Once these questions are answered, it becomes easier
to match the right material to the application.
A third consideration is exactly how the seal or gasket will be attached. Is a particular part to be attached by
compression fit into a channel? Will an adhesive be applied to the seal? Or, will there be some sort of mechanical
attachment, i.e.; staples, nails, or clips? (For an explanation of pressure sensitive adhesive attachment systems,
refer to this section in the guide on Pressure Sensitive Adhesives.)
Design Criteria
Cross-Sectional Design – Several things need to be known about the application before you can properly determine what
good cross-sectional design will be.
- Knowing the attachment system is important – how the seal will be attached to the substrate or to the
application.
- What is the gap or the opening the seal is going to fit into and be expected to seal? This includes both the
minimum and maximum amounts of gap that exists throughout the application, taking into account tolerances of the
enclosure and sealing material. In most cases, the general effective compression range is 25% to 50%. It is
important to remember that the end use always needs to be considered. For example, there is a very significant
difference between sealing a box to keep out dust, and sealing that same box to keep out moisture when it is
submerged in ten feet of water.
- What is the amount of closing force that will be tolerated in the application? A seal requiring 50 pounds of
force to seal is not going to work in an application where only 20 pounds of force will be tolerated. How much
force is required on the seal, how much surface area, the type of material that is used, and the size of the cross
section all need to be considered.
- The direction in which the load is applied to the seal is also a critical factor. Very different results will
occur if a 60lb load is applied as opposed to a 90lb load.
- There are also ways to help reduce or control the total amount of load required to get a good seal: (1) The
shape of the cross-section can be changed, depending on how the load is applied; and (2) holes or hollow
sections can be introduced into the cross-section to help reduce the load required to compress the seal. For
example, a dense “D” section will have a higher mechanical load than one that has been “hollowed" out.
Compression Application Design
When designing compression fit applications, it is necessary to have the same 15% to 25% compression in the channel
for sponge sections to hold the seal in place. By the same token, for dense sections, the 2% to 10% compression
listed above is necessary to hold the seal in the channel. As before, this is somewhat dependent upon the
application, such as the size of the channel itself and the direction from which the load is applied.
Adhesive Attachment Application Design
The attachment surface would be wider that 1/8” to ensure proper attachment. For dynamic situations, the minimum
strip of adhesive to hold the seal in place until the mating substrates are joined. In addition, the adhesive
placement should be on flat surfaces on both the seal and the substrate. Transfer adhesives generally perform better
than supported adhesives (adhesives that use a carrier) when it comes to bending around radii. On the other hand,
supported adhesives help prevent stretching of the seal during application better than transfer adhesives. The
thickness of the PSA itself should be taken into consideration when determining the overall height of the profile.
Mechanical Seal Attachment Application Design
When designing seals that will be attached mechanically, accommodation must be made for the insertion of the
attachment device (clip, nail, staple) to the seal. The seal design has to be such that the mechanical attachment
device does not interfere with the compression of the seal.