How Lens Silicone Sealing Rings Improve IP68 Waterproof Ratings in Underwater Lens Enclosures
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- publisher
- Suey
- Issue Time
- Apr 22,2026
Summary
IP68 is a statement about a specific depth and duration, and lens silicone sealing rings earn it through gland geometry rather than hardness alone. This guide covers compression and fill ratios, groove design, salt-water and UV ageing, and the five failure modes that break underwater lens seals.
Sealing design guide
IP68 is not a material property. It is a pass statement for an agreed depth and duration, and a lens silicone sealing ring meets it when three numbers are right at the same time: 20-30% compression after ageing, 75-90% groove fill, and a parting line that never crosses the sealing face. Hardness alone tells you almost nothing about whether the ring will still seal in year three.
- Specify the IP68 condition explicitly: depth, duration and number of cycles, because pressure scales linearly with depth.
- Groove volume, not cord diameter, decides whether the ring can extrude or be over-compressed.
- Salt water and UV attack the housing and the glass bond as much as the silicone itself.
What IP68 actually demands from lens silicone sealing rings
The second digit of IP68 is assigned by agreement between buyer and supplier: continuous immersion under conditions that must be stated. In optical housings the usual reference is 1.3 m for 30 minutes, but a housing rated for a swimming pool and one rated for a 30 m ROV are not the same product, and the seal design differs.
Pressure rises by roughly one bar for every 10 m of water depth, so a 30 m rating loads the seal with about three bar of differential pressure before any thermal expansion is added. Write the condition down — depth, duration, temperature, number of cycles — and test the assembled housing, because a ring that seals on a test fixture may leak once the glass, the bezel and the cable entry are stacked on top of it.
Gland design: compression ratio, fill ratio and groove geometry
Three numbers govern whether a ring seals or extrudes. Compression ratio is the reduction in cord section when the housing closes, normally targeted at 20-30% for static optical seals. Below 20% the ring cannot follow surface imperfections; above 30% the material is over-stressed and takes a permanent set.
Fill ratio is the cross-sectional area of the ring divided by the groove area, targeted at 75-90%. A groove that is too small cannot accept thermal expansion and the ring extrudes past the gap; a groove that is too large lets the ring move and nibble during assembly. Groove width is usually 1.1-1.2 times the cord diameter, with radiused corners and 1-2° draft so the ring is not shaved on entry.
- Compression 20-30% — measured on the closed assembly, not from nominal dimensions.
- Fill ratio 75-90% — computed from the actual groove drawing, before the tool is cut.
- Hardness 40-60 Shore A — low enough to conform, high enough not to extrude at depth.
- Parting line off the sealing face — a flash ridge on the contact surface is a leak path by definition.
Material and environment: salt water, UV and thermal cycling
Silicone holds its elasticity from deep cold to well above the temperature of a sunlit housing, which is why it suits outdoor optics. Its weakness is compression set and gas permeability: a ring that takes a set under a tightened bezel will not recover when the housing is opened for service, and a high-volatility compound will fog the lens from the inside.
Specify a post-cured, low-volatile grade for anything sealing an optical path, and set a compression-set limit with an ageing condition attached — for example 22 hours at 175 °C, a common reference for silicone. In salt water, the seal itself is rarely the first failure: the stainless fasteners, the anodised bezel and the glass-to-housing adhesive usually go first, so the seal specification has to be read together with the hardware it sits against.
The four numbers to put on the seal drawing
If these are not on the drawing, the moulder is guessing and the test house is guessing with them.
| Parameter | Design target | What fails if it is missed |
|---|---|---|
| Compression ratio | 20-30% on the closed assembly | Under-compression leaks at low pressure; over-compression takes a permanent set |
| Fill ratio | 75-90% of groove area | Extrusion at depth, or ring movement and nibbling during assembly |
| Compression set | Under 25% after 22 h at 175 °C | Seal does not recover after service opening, second assembly leaks |
| Volatile content | Post-cured low-volatile grade | Condensation on the inner lens, permanent fogging |
Five failure modes of underwater lens seals
Extrusion at depth
Pressure pushes the ring into the clearance gap. Reduce clearance, raise hardness or add a back-up ring.
Nibbling during assembly
A sharp groove edge shaves the ring as it is fitted. Radius the entry and use a fitting tool, then inspect every unit.
Compression set
The ring flattens and stops recovering. Move to a lower-set compound and reduce over-compression.
Chemical swell
Detergents and chlorine change the section. Confirm the compound against the cleaning agents actually used on site.
Bond-line leak, not a seal leak
Water enters at the glass-to-housing adhesive. Test the assembly, and treat the adhesive as part of the sealing system.
Questions from optical housing programmes
Is a harder ring better for deep water?
Up to a point. Higher hardness resists extrusion, but it also needs more compression to conform to surface finish, and it seals less well against a machined groove. The usual compromise for optical housings is 50-60 Shore A with reduced clearance rather than a harder compound alone.
Can the same ring be reused after the housing is opened?
In practice, no. A compressed ring has taken a set; re-torquing it produces uneven loading and a leak path. Specify a service kit with a new ring, and design the groove so the old ring can be removed without scoring the seat.
How is fogging separated from a seal failure?
Fogging without water droplets points to volatile carry-over from the compound or the adhesive, not ingress. A low-volatile post-cured grade plus a controlled adhesive cure removes it; a leak test will pass on a fogging unit.
Send the groove drawing before the seal is quoted
Share the groove cross-section, the IP68 condition and the housing materials. FromRubber will compute compression and fill ratio, propose a compound and hardness, and flag the geometry that will not seal before a tool is cut — custom silicone seals moulded in Dongguan.
Request a seal design reviewRelated reading: preventing lens fogging and outgassing and custom silicone seals.