Engineers have several factors to consider when designing for a formed-in-place gasket:

● adhesion to substrate.
● compression set.
● variation in the height of the part.
● shear forces in the clamped assembly.
● pitch between clamp points on the mated interface.

Not considered here, but not to be overlooked are such factors as temperature range, chemical resistance, and bacterial resistance.

Adhesion of a gasket compound to a substrate is a complex issue. Certain materials adhere well to some plastics, but not others. The adhesion of some materials can be improved with surface treatment. Common surface treatments include flame, plasma and solvents. On the other hand, poor initial adhesion may actually be desirable if inspection and rework come into play. If a gasket does not pass quality control, it may need to be removed quickly and efficiently.

Compression set is the ratio between the height not recovered after compression to the compressed height as a percentage. The standard test compression is 25 percent. With this in mind, we get to the following equation:

Minimum Gasket Height = (Spacing Height) / (1 - Compression Set Percentage)

The lower the compression set, the lower the minimum gasket height needs to be. And the lower the amount of material that needs to be used to get a functional seal.

As a rule of thumb:

Miniumum Space Height = (Seal face one tolerance stack + Seal face two tolerance stack) / 0.25

Using a summation or “worst case” tolerance stack is a little overkill here and a “root sum of squares” (RSS) method is more appropriate. The small potential error of this method is far outweighed by the 25 percent compression we are choosing.

So, tolerance stack is the square root of the sum of the first tolerance squared, plus the second tolerance squared, plus the third tolerance squared, etc.

At this point an estimate should be made of the gasket width—a task made a bit easier when a material manufacturer is willing to publish typical aspect ratios for a material:

Minimum Gasket Width = (Minimum Gasket Height) / (Height-To-Width Ratio)

This minimum and the cross sectional area of a gasket resulting from this minimum must now be checked to be certain it does not exceed the enclosed channel area.

In the design stage, it’s helpful to have a calculator to test various configurations. Being able to quickly vary seal floors, aspect ratios, and channel heights helps to ensure details are not overlooked. Several online tools are located here.

Let’s look at an actual client example of a gasket in a channel that is intended to seal against a lid with a rib that rests down in the channel. In this case the channel is 0.1 inch wide by 0.1 inch high. The tenon is 0.059 inch high and 0.08 inch wide (leaving 0.01 inch on a side).

In this design, the gasket must be at least 0.041 inch tall to contact the tenon, and it cannot exceed (0.01-0.00472) 0.00528 square inch. Exceeding this will either cause the excess gasket compound to squeeze out of the groove or cause the parts to distend between the screw-down points.

The gasket compound has a 10 percent compression set rating. This makes the desired, dispensed height of the gasket 0.055 inch. The material has an average aspect ratio of 0.75, which yields a width of 0.073. The area of the gasket should be 0.0040 square inch, which is less than our 0.00528 cutoff.

Taking a look at an actual example we can see a dispensed gasket of width 0.076 inch and height of 0.064 inch. The scanned area of the gasket is 0.00412 square inch. Interestingly, the actual aspect ratio of this scanned sample is 0.84—a very good ratio. This sample is 0.009 inch higher than it needs to be according to our compression set based number. This works out to an excess of 35 percent and is costing the client approximately an extra 18.4 cents a part.

Interestingly, based on tolerances called for on for this customer’s part, he could have gone with a 0.032 inch gap which would have used a 0.043 inch tall gasket. This means he is paying 121 percent more than need be for this gasket! The customer could be paying 32 cents, and instead is spending 71 cents.