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Oliver_Klosoff 02-10-2010 12:29 AM

45 joint in a glass tank can it be done?
1 Attachment(s)
Hey guys and gals.

Down the road (along way down the road) I want to build a poly tank with a bay window like front. I am not worried about the poly I have a lot of experience with welding poly tanks for fire trucks but I have never tried to join it with glass. The tank drawings are below. I am mostly worried about the 45 angle joints in the glass up front. I know there are angled joints in octagonal tanks but I have never seen one over 30-40g and this tank will be 200g +. The glass will be " thick and came from a old 120g tank. Any toughts I would be happy to hear them.

Thank you Wayne

Oliver_Klosoff 02-10-2010 12:31 AM

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Sorry missed a picture.

jsoong 02-10-2010 11:38 AM

You are trying to join plastic (acrylic / poly?) to glass?

Those joints will be my main worry. People who tried to silicone acrylics to glass ended up with split seams. I would suspect that it would be similarly unsafe for poly to glass, esp. with a big tank. The big tanks with plywood or other non-glass backs normally have a complete frame with the base material, and the glass is a window that is gasketed/epoxied to the frame. Having a frameless design with 2 different material like yours would make me worry.

I am not that worry about the 45 degree glass to glass joint, as long as you can cut/grind a proper bevel on the edge. I would not try to join 2 square edges at 45 degrees ... that would leave a large gap for the silicone to fill & I wouldn't trust that to hold. you see them butting square edges at an angle in small octagonal tanks, but I have not see any big tanks with that construction.

BigJim 02-10-2010 02:12 PM

+1 on the angles not being an issue if you get the glass properly beveled. I've seen 100g hexagon aquariums and a 500g+ hexagon terrarium held together with silicone.

I would think about making a frame for the tank that would separate the poly from the glass with wood or another suitable material. I'd be hesitant to try to silicone glass to poly since silicone doesn't stick to acrylic well. If silicone sticks to poly, I might give it a shot, but for piece of mind, I'd go a different route.

Oliver_Klosoff 02-10-2010 02:51 PM

2 Attachment(s)
Hey Jsoong

thank you for the reply. I would be using poly plastic. I have used it in sumps in the past with no problems but I have never joined it with glass. There are some pictures attached to this reply of how I wanted to make the joints. I was thinking if I routered a channel in the poly to receive the glass that would add the strength I need. Also I know I am going to have to have a top frame also shown in the drawings this can be welded into place. What do you think Jsoong? Every one else have at it what do you think?


Oliver_Klosoff 02-10-2010 03:33 PM

Sorry I missed you BigJim. I am doing some experiments on how well the silicon will stick to the poly. Hope it gose well. I will let you know.


jsoong 02-11-2010 01:58 AM

I've never made anything like you are planning, (ie joining glass to plastic) so all comments will be conjectures!

From reading about acrylics & silicone, the problem is that silicone simply don't adhere well to it. So rabbeting it to increase the bonding surface isn't a real solution, esp. shallow dados like yours. I think you need to find out the bonding strength of silicone (or epoxy, or whatever you are finally using) to the poly & make an educated decision. It is good that you are experimenting first. <I know for sure that silicone will not stick at all to PVC ... it all depends on how slippery your poly is (ie what is the exact molecular composition) .> You might want to research some industrial materials articles to gain some insight.

If I were to do the joint, I think I would feel a lot safer making a frame rather than just a dado. <That is assuming that you find an acceptable bonding agent.> You might want to weld on a good size frame at the joint and make a deep dado (like 1-2") to accept the glass. If you have a strong frame all around the glass, the frame will take all the structural stress so you don't need to worry about the glass-poly bond. With a deep dado, the glass is trapped in the frame, and the water pressure will push the glass against the frame, helping to seat & seal it. You are looking then only at a water tight joint, rather than a structural joint ... a much easier task. That at least is a tried & true design in plywood aquarium. <I never want to be the poineer when it comes to a couple hundred gallons of water inside the house ... it is always nice to find someone who had done it successfully before.>

Oliver_Klosoff 02-11-2010 01:11 PM

Thank you again Jsoong. When you talk of a frame you mean a brace from the top of the tank to the bottom at the 45 joints as well or do you think the top frame I have pland to go all the way around could be beefed up? I would like to keep the front of the tank as clean as I can. But I agree 200+g of water on the floor would suck. I am going to look into acrylics for the front wall instead of the glass. I didn't realy want to go with acrylics but acrylics likely have a better chance of making a bond with the poly if I was to use an epoxy.


jsoong 02-12-2010 06:42 AM

No, I think the 45 degree joints will hold. once you joint the 3 glass pieces, you can think of that as a window, and you need to wrap a frame all around that.

This is the GARF plan for plywood & glass:
It calls for a frame of 3" x 1.5" (a 2x3/4" sandwich) all around the flat glass window.

I think you need something beefy like that around your "bow-window" front. You would prob need to brace the top & bottom frame members so it will keep the 1/2 hex shape under load. Since there is no precedence in your construction, it would be safer to go overboard in the bracing & frame thickness ....

Acrylic would work better if you can weld it to the poly. <No idea if it is possible.> Epoxy isn't that strong, so being conservative, I would still make a frame around an acrylic if a structural weld is not possible.

Oliver_Klosoff 02-12-2010 08:01 PM

Thank you again jsoong I think I am on the same page as you will let you know how things go.


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