Silicone Washers

[Zed]

I want to make some silicone washers, to achieve it I will need some sort of mould release, some thing that silicone will not stick too. Any words of wisdom? Anyone tried this?

[PAR]

I've made polyurethane washers and gaskets. A dry mold release agent or my personal favorite, automotive paste wax should work. Apply several coats and if wax give it a few days to truly dry.

[Zed]

Thanks Par, polyurethane may well be a better choice than the silicone, I will try both.

[Landlubber]

Zed, you can make removeable washers by simply putting silicon onto standard washers and allowing it to harden before application, I have used these removeable washers in many situations and they work well.

[Zed]

Hi Landlubber,

They are going to be sitting on a curved surface and against aluminium, they also will be quite large, say 10cm wide/round, in fact two 'washers' may become one long unit so I guess a gasket is a better description of what I'm thinking about. I see what you are saying, and I can think of a couple of other instances that it would be useful but in this case I want to end up with a flexible gasket type of arrangement. Even if I can simply set a sheet of Sika at a constant thickness and then cut the shape I want. Buying sheet material might be better but I have not seen anything that is a soft as I think is needed.

[PAR]

I recently made a gasket for the cover to a centerboard case. I made a mold of the perimeter and interior edges out of a plywood base and some solid stock. I used a 1/16" thick piece of cardboard as a shim around the perimeter, then laid a thick bead of polyurethane between the shims. I mashed this down flush with a putty knife and let it cure. 48 hours later I peeled the perfectly formed gasket off the waxed from, trimmed the edges and used a hole punch to knock out the fastener holes. The gasket waited a few more days until it was really needed and could cure more. It takes a good amount of time the get these type of gasket goos to fully cure. Increasing the humidity (a lot) will accelerate the cure.

[Zed]

YES! That is exactly what I had in mind, that or moulding the shape to some degree. Currently the two parts are bedded in silicone, which either leaks if and is not so bad to pull a part if you use low grade silicone OR doesn't leak and is a real pain to part if you use the better product. The boat only gets pulled down every few years so it no big deal but it would be save hours of messing around if I could make a successful gasket.

So I take it that the auto paste did that job fine?

[PAR]

Silicone has no business being on a boat, except when attaching glass or acrilic windows to frames. Other then this, it's worthless.

I use plane old Turtle Wax and apply several coats, letting them dry as long as possible. I've also used dry release agents, like the stuff used in molds. This works well too. Often you can use the part as a portion of the mold, just cover it with release, packaging tape, polyethylene, etc.

The keys to a good gaskit are; let it dry, let it dry, let the damn thing dry. It's easy to wan to install the newly made gasket, but it may be dry to the touch, but will ooze goo when you apply pressure, defeating the abilities of a gasket. Let it fully cure into a nice rubbery part, then install it. Make it uniformly thick. I use shims to do this, which the putty knife rides on as I spread out the bead of goo. Use damp towels over the work (not touching the goo) to speed up the cure of polyurethanes and polysufides.

[Zed]

Sometimes I think it has its uses. In the absences of a suitable gasket silicone is probably the only other thing you would use for this application because it needs to be pulled down every so often. The one I use seals well but it is possible to pull it off when needed, possible but painful. My fear would be that if I used polyurethane I would never get it a part.

Having said that I think that this gasket idea may prove to be a much better way to go... suck it and see, I guess.

I think I will get two sheets of MDF (18mm?) a fair bit bigger than the job, seal it with polyurethane varnish then apply the mould release. If I run some bolts around the edge and place 4 or 8 spacers around the job to the right thickness, say 3-2 mm. Then I can overfill the area required and clamp it down with the bolts, hopefully making a smooth surface both sides. (practice required?)

Would you go for high bond strength polyurethane? or one of the cheaper sealant grades?

Thanks again Par.

[PAR]

When the polyurethane is fully cured, it's not going to stick to things unless there's a substantially amount of heat involved. This is the idea behind a gasket as opposed to a sealant. Polyurethane is much more likely to tolerate removal and replacement from time to time, if it's installed as a cured piece. Another option is to buy sheet rubber, in the thickness you need and cut a gasket. You could always wax the gasket surfaces if you're afraid the gasket will stick to the parts.

3M makes a slow cure hot glue that gives you time to work. Don't use any release agent but pucky it and bolt it down.

[Zed]

Originally Posted by PAR

When the polyurethane is fully cured, it's not going to stick to things unless there's a substantially amount of heat involved.

I was talking about using it as a sealant as opposed to silicone... not as a gasket material. If I used say Sika where I have used silicone my fear is that the boat may never come a part without damage! Making gaskets is a whole new approach, I wouldn't expect to have any issues with the material sticking if it has been cured first. At this time I have to used a 4' gal pipe, a block, a lot of grunting, leverage and a blade to break the seal... its fun! :-) God forbid I used a really sticky PU! A nice non sticky, cured, PU gasket sounds like heaven... so long as it seals well!

Thanks
Z

[ancient kayaker]

What about a closed-cell foamed material? Something in a suitable thickness and size should be available. I've seen electronic equipment arrived packed in a white material with a distinct shine and a kind of waxy feel, came in various thicknesses from about 2 mm up, not sure what it was called but it was at least as tough as silicone would be.