This guide is intended to give design engineers the basic facts needed to begin the process of designing effective rubber seals, gaskets and weather stripping. It is not intended to provide every piece of information available on the process or extrusion, the amount of information available is simply too great. However, it should serve as a starting point and give engineers the necessary information to better understand what they will need to consider during the design process.
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.)
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
- 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 2% to 10%. 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 60o load is applied as opposed to a 90o 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.