Materials for Static Seals
Introduction
A re-usable elastic seal—the sort used in vacuun and pressure systems—consists of a cylinder of material compressed between two flat surfaces. The seal must form the largest possible contact width, while keeping the contact stress sufficiently low that it does not damage the flat surfaces. The seal itself must also remain elastic so that it can be re-used many times. What materials make good seals? Elastomers—everyone knows that. But perhaps we can learn more, and identify some promising candidates.
Figure 1 Schematic of an elastic seal
Design requirements
| FUNCTION | Elastic Seal |
| OBJECTIVE | Maximise conformability |
| CONSTRAINTS |
(a) Limit on contact pressure (b) Low Cost |
Table 1
The Model
A cylinder of diameter 2R and Modulus E, pressed on to a rigid flat surface by a force f per unit length, forms an elastic contact area of width
(1)
This is the quantity to be maximised: the objective function. The contact stress, both in the seal and in the surface, is adequately approximated by
(2)
Now the constraint: the seal must remain elastic, that is, the stress it experiences must be less than the yield stress or failure strength of the material from which it is made. Combining the last two equations with this condition and eliminating the 'free' variable f gives
(3)
The contact width can therefore be maximised by maximising the index
(4)
This index measures conformability. The same index measures the ability of a seal to bed-down on a surface with a wavy or irregular profile.
It is also required that the contact stress be kept low to avoid damage to the flat surfaces. For a given contact width b, the local stress is found by eliminating f from equations 1 and 2:
(5)
The contact stress can therefore be minimised by maximising the index
(6)
The Selection
An initial survey, using the Generic filter, is shown in Figures 2a and 2b. The search region is in the top left corner of the chart and it can be seen that elastomers, polymers and foams make the best static seals.
Figure 2a Tensile Strength plotted against Young's Modulus in the Generic filter using M1
Figure 2b Tensile Strength plotted against Young's Modulus in the Generic filter using M2
In figures 3a and 3b, the selection criteria are swapped to the polymers filter. We can see that if the surfaces to be sealed are hard and the contact pressure is not critical, then solid elastomers make the best seals (upper part of Results Table 2). But if the surfaces are delicate, requiring low contact pressures, then soft elastomers and foams are the better choice.
Figure 3a Tensile Strength plotted against Young's Modulus in the Polymers filter using M1
Figure 3b Tensile Strength plotted against Young's Modulus in the Polymers filter, using M2
The analysis highlights the functions that seals must perform: large contact area, limited pressure, environmental stability. Elastomers maximise the contact area. PTFE and Silicone rubbers best resist heat and organic solvents. Foams (and cork, from Figure 1) minimise the contact pressure. The final choice depends on the conditions under which the seal will be used. Further 'protective' stages in CMS would be necessary to ensure that these conditions are met.
Finally, we said that cost is a consideration. A ranking by cost is facilitated by Figure 4. Results Table 3 shows the reduced list of suitable materials.
Figure 4 A Cost Limiting Stage in the Polymers filter
Results
| Material | COMMENT |
|---|---|
| Butyl Rubbers | The natural choice, but poor resistance to heat and some solvents |
| Polyurethanes | Widely used for seals |
| Silicone Rubbers | Higher temperature capability than carbon-chain elastomers, chemically inert |
| PTFE | Expensive, but chemically stable and with high temperature capability |
| Polyethylenes | Cheap |
| Polypropylenes | Cheap |
| Nylons | Near Upper limit on contact pressure |
| Cork | Low contact stress, chemically stable |
| Polymer Foams | Very low contact pressure; delicate seals |
Table 2 The results of the selection
| Material |
|---|
| Butyl Rubbers |
| Polyurethanes |
| Polyethylenes |
| Polypropylenes |
| Nylons |
| Cork |