Will Acrylic overflow boxes work with silicone?

[reefsmurf]

I've been reading that silicone does not bond with acrylic.

I have a tank that's drilled on the bottom that I was going to have a custom acrylic internal overflow box built around.. kind of like a standard AGA internal overflow. But if silicone doesn't bind with acrylic, how is that going to work

Thanks for any advice you guys can provide!! Maybe I should just cap it off and do an external overflow

 

[JohnK]

Silicone won't bond with acrylic with enough strength to build a tank or other more stressful application, but it works just fine to attach an internal acrylic overflow, that's how AGA does it.

I wouldn't try to build the box itself with silicone, but installing it with silicone is fine. I've set up many tanks this way, the only tank I've ever had a leak in the overflow seal was a factory AGA reef ready tank.

Install;
before,

 

[JohnK]

after

 

[shawn]

As long as the box is built using acrylic solvent and not silicone, and only attached to the tank using silicone, it should be ok.

 

[reefsmurf]

Thanks guys.
If you ever need to redo the bulkhead and drain the overflow section, doesn't that edge of the overflow box+silicone become a tank edge? That seems like a lot of pressure against the silicone. For example, a 55 gallon has 345 lbs of force against a sidewall... is the tank water going to leak into the overflow when the overflow is empty, since the silicone bond is weak?

 

[shawn]

Yes, I agree - I'd suggest building the overflow box as a complete square tube, and not a "U" as shown in the pictures. It will be stronger and the silicon will only need to seal around the bottom where there is very little unsupported surface area on which the water will exert force.

 

[reefsmurf]

Thanks Shawn.. great advice.

 

[JohnK]

Done like the pic I posted does work fine though. The silicone actually does make a pretty strong connection, just not the incredible strength of a glass to glass joint.

I understand the caution, but FWIW, there are a whole lot of tanks with overflows siliconed in like that (like every "reefready" AGA/oceanic tank out there). I may be wrong, but I think that since the water pressure would be pushing the box against the glass wall there would be at least some physics helping it out?

Stick with what you feel safe with, but it does work.

 

[Jim Tansey]

As jimmy said the water pressure is pushing the box against the sidewall, silicone bond has more than enough strength to hold no need to make a four sided box. When I bond glass to acrylic I remove the gloss from the acrylic in the bond area with some fine emery paper.

Jim

 

[Dave McReeferson]

I third Jimmy's response. I have performed this very thing in my tank more than once with no leaks. 3 sided should be fine and dandy.

 

[shawn]

I third Jimmy's response. I have performed this very thing in my tank more than once with no leaks. 3 sided should be fine and dandy.

Your probably right. I design things for a living and I tend to over engineer to be safe. If it was a box I'd have no issue with stacking rock up against it, but with a "U" I'd worry that it might pull away from the tank side from the extra stress. What's the difference, about $2 more acrylic?

 

[JohnK]

I agree the cost difference would be negligable, and it can be done either way.

FWIW though, I'd feel safer with a corner or a U against a pane of glass since that way the overflow is supported against the rigid glass. With a box (I have seen professionally made tanks done that way also) the rock could lean against it in any direction, but it would only have the ring of silicone around the bottom to hold it secure. Again, however the rock were to lean against the overflow, the overflow is against the glass and more or less pushing against it. With a free standing box, the box has nothing to hold it secure except the silicone seal on the bottom.

Certainly both approaches could and would work, IMO it's a matter of preference.

I'm not an engineer, but I like the U or the corner and those are the most common styles used in the hobby, both professional and DIY - not to say dogma is always right, but it's working just fine on thousands of tanks.

 

[shawn]

I agree with what you said. But what I was thinking was that one side of the overflow box (the side that would have otherwise been the open end of the "U") would butt up against the side of the tank and would have silicone over their common surface. That would provide significant strength to prevent any movement, as opposed to just being sealed around the bottom. Anyway, like I said, I tend to over-design - probably due to my numerous failures!

 

[JohnK]

Ahh, I see what your saying. Sure, that would logically be stronger. I thought you meant a free standing type of thing.

The only tricky part would be fitting it against the silicone bead along the bottom edge of the glass. I'd think you could trim that clean but I don't know if that would affect the strength of the tank seam. Then again, you could also use a thick back wall on the overflow and sand it to fit - but that would take a while .

 

[randoma]

Instead of trimming the silicone, add an extra thickness of acrylic to the bottom of the box, that is shorter by the amount of silicone you have to make space for. Basically, make a little step on the bottom of the box.

 

[Dragon2115]

Will Acrylic overflow boxes work with silicone?

Yes, it will work.

 

[reefsmurf]

Thanks for all the advice.

Just out of curiosity... since silicone won't bind to silicone (right?).. how does it not leak at the point where you are siliconing over the original silicone that is joining the bottom and side of the glass tank?

 

[JohnK]

I'm not sure of the details on the strength of silicone on silicone, but I've done it several times and had no problems whatsoever. I suspect that it's something along the lines of the way silicone doesn't bond to glass, that is, not with the full strength that silicone can bond with other materials such as glass, but it definately does at least "stick" to it.