Best type of plastic for underwater (saltwater) use....

[gophaster]

I am going to have some gaskets made to press fit some glass lenses into a 316/316L housing for a permamnent underwater closed circuit camera that I am working on. I know there are many types of plastic out there.... which plastic would be best suited for prolonged exposure (several months/years at a time) underwater in saltwater?

Thanks for any leads and/or ideas.

[jehardiman]

All plastics will swell over time in water (especially if they cycle in depth) and you will need to do the calculation if the lenses are not up to the squeeze. We have used acetal (Delrin http://modernplastics.com/pdf/Ensinger_Delrin.pdf ) and GUR UHMW ( http://tools.ticona.com/tools/docume...hMan_GU-7.pdf).

Stupid Note: I have no business intrest in these companies/products and would recommend detailed analysis for suitability before using. OVWP YMMV.

[PAR]

Can you provide more description about these "gaskets"? Will they be a real gasket or will the lens get bonded as you have thus far found satisfactory (epoxy)? From and engineering stand point there is a huge difference. Do you have working drawings?

Generally, this isn't a particularly different problem for an engineer. The question lies with what type of seal are you looking to create (assuming clean glass, with or without coatings), with a water proof one being a given. Will it be and interference fit, mechanical and/or adhesive attachment, other design requirements, such as the need to remove it for maintenance or repairs, will it "float" in a rabbit or groove or be locked down with an external ring, etc. There are several materials that will work, depending on how the interface is engineered. On the other hand, do you just want to squirt some goo around the perimeter and call it a day?

[gophaster]

Par, once again I appreciate your input on this. After looking at all the "glue on" methods and speaking with many manufacturers of adhesives/epoxies I have had a hard time finding a manufacturer that would endorse one of their products for my exact application. It seems they all have an issue either with the stainless or with the glass (or both).

I have disassembled some dive watches in the past and the crystals are held in place by what seems to be a PTFE ring and they are simply pressed into place (interference fit with no adhesive, etc). In fact I have a watch rated to 300M that the crystal is pressed on this way...if you remove the back from the watch and take out the movement you can simply "pop" the watch crystal+ring right out with your thumbs. Then to reinstall you just press it back in with your hand from the front!!! Doesn't get much simpler than that.

At this point and since I have access to a full CNC shop rather than trying to "rig" something up by gluing it I think it might be better to put a little more thought into it and try something that might be more permanent and even allow for removal if needed.

I can have the glass made and I can have the PTFE ring made to specs...I am just not certain what the specs (sizes) should be to achieve this interference fit. Keeping in mind that these enclosures will see at most 10-15 feet underwater.

I am thinking about trying a 40mm diameter by 3mm thick flat mineral glass disk and having a 40mm ID x 3mm high x .5mm thick PTFE ring made. Then making an opening at 40.5mm and pressing the glass + PTFE ring into place. Does this sound like it would work? Am I totally on the wrong track?

Thanks again for any further input.

[jehardiman]

gophaster;

This is really simple, you know, don't make it any harder than you need to.

Pressure squeeze on the face of the lens is what seals the lens. Assuming a brittle external lens assembly to the PV housing, all you have to do is to provide a backing (i.e. your gasket) for the lens that will support it without point loading it or side squeezing it at depth. The lens just lays on the gasket bedded in a compatable waterproof grease. A retaining ring then screwed around the outside to provide squeeze at or near the surface. If you really want to, you can put in a conic taper between the lens and the backing ring.

The viewports of the deepest diving submersibles are put in this way, and I've yet to hear of one failing. Get a copy of Manned Submersibles by Busby (on page 260) or Submersible Vehicle Systems design by SNAME (page 165).