One of the key factors in controlling the sealability of your device is using the correct grommet hole size. A grommet is a child part used in assemblies to seal the device. It is often used to smoothen the rough edges of a surface to eliminate friction caused by the otherwise ragged edges when another part is passed through it.
Grommets can be rubber molded, die-casted, sheet metal formed or plastic molded. A grommet hole size can be a tricky thing due to a phenomenon called shrinkage. Sheet metal-formed grommets are precise but lack the strength or sealability characteristics because of metal on metal assembly.
When choosing the correct grommet hole size for a rubber molded grommet, the hole size is measured from the center of the part that requires sealing. We have started the practice of cryogenic rubber deflashing which provides accurate grommet hole size. However, shrinkage remains a major issue for which the tolerances need to be adjusted while dimensioning. Another way to attain the correct grommet hole size would be to keep the grommet in the upper tolerance zone at the highest point.
In the case of die casting, the shrinkage that occurs is negligible. However, in the case of seat locks or seat latches where a die-casted or a sheet metal grommet is used, the shelf life due to friction is reduced tremendously. If the die-casted grommet has bubbles or warpage, then it might lead to breakage.
Grommets are available in various sizes and the grommet hole size is critical when designing your device. One has to be diligent when choosing the correct material, correct dimensioning, and tolerance of grommet hole size and the environment this child part interacts with. If a rubber grommet interacts with fuel fumes in a fuel tank, then its shelf life is affected.
Grommet hole size is a cognitive factor in successful design and development. They come in different shapes and sizes, one must be wary about the shape one decides to use as well, taking into account the friction caused in the case of metal grommets or even plastic grommets.