Gasket design question

[WolfHR]

I hope it would be OK for a newbie in this field to ask for help and advice here. I'm facing a problem that I have no experience with- but hopefully will not come across as trying to get others to solve my problem: I'm trying to figure out how to approach my problem correctly, have tried to come up with model of a sort...

I'm designing a way to attach a plastic part to a metal base (I'd say I would be safe to assume this metal part to be rigid compared to the rest of the structure), with a requirement that it is waterproof. The seal needs to be watertight solely by clamping forces (working loads will work in favour, providing extra clamping force).

I see a problem that at this stage I cannot assess rigidity of the plastic flange compared to that of rubber gasket, so would like to cover my bases by assuming that the deformation of the flange will not be negligible (on account of large spacing of clamps). Here's where I need help to see if I'm approaching the problem in right way. Firstly, I'm thinking to approach this as 2D problem at first- since width of the flange is relatively small compared to its length (say, less than 1:20). Secondly, if I assume the plastic flange to bend (and the gasket is loaded in compression only)- the continuous load applied to it would be proportional to its deflection, so I'm expecting the load to be in sine/cosine shape (so when integrated four times, the deflection will be of the same shape). This is illustrated in attached picture. Now, by fiddling with maximum allowable stress* of the parts, their thickness (or stiffness) and Young modulus I would be able to design a gasket that could be sealed with given clamp spacing Lx.

* this is part where my knowledge of rubber as a material is severely lacking and would be thankful for a further help. Firstly, is there some endurance limit to be observed? This would be frequently assembled/disassembled and I would like to avoid, within reason, permanent set of the gasket. Secondly, the clamping mechanism would be able to account for permanent set, but my idea was to use a cam to provide clamping force- are there any flaws to this method that I'm unaware of? (My reasoning is that rather than to use force as 'regulating element' of the seal, using the displacement might prove more reliable/consistent method.)

Thanks in advance.

 

[cczechowicz]

If all you need is a water tight seal for basic environmental conditions, I would suggest a closed cell foam product frequently a silicone or urethane.

Your deflection is a function of the reaction load of the gasket. By decreasing the reaction load, you will decrease the deflection.

If you need to use a solid rubber product, then you want to use something rather thick. Compression set resistance is usually a function of the amount the material is compressed. The same material will return to its original condition the less it is squeezed. If you have something .5" thick and you compress it 0.050", it will return closer to the .5" thickness when unloaded, as compared to a .1" thick piece of rubber. The .1" thick piece compressed to 0.05" will not return to its original condition when compressed 50% as compared with the thicker material compressed 10%.

Your design KEYS:
1) LOW COMPRESSION FORCE (LOW DUROMETER)
2) MAXIMUM THICKNESS
3) HIGH COMPRESSION SET RESISTANCE
4) LOW COMPRESSION %