Bonding Aluminum to itself

By rlitman (***)

Date 06-09-2011 19:17

That's a tough one to answer. What's the surface area and shape where the parts are in contact? That can make a huge difference in whether or not this can work.

By weldwade (***)

Date 06-09-2011 21:31

He just sent me an email with some dimensions. The top is 14" square 1/2" t-6 and the legs will be 1"x3" t-6 rect stock. The stand will sit 2' off the floor. These are left over "drop" or "oops" parts they have on hand that have already been anodized.

By Joseph P. Kane (****)

Date 06-09-2011 19:44

I have seen some large, heavy signs and logos held to a frame with 3-M double sided HMB Tape. (High Molecular Bond) However, I do mot know the compactness and configuration of your application. This tape would not ruin the original part.

By rlitman (***)

Date 06-10-2011 14:49

You mean this?
http://solutions.3m.com/wps/portal/3M/en_US/3M-Industrial/Adhesives/Product/Bonding-Tapes/VHB-Tape/

That's a close relative of 3m 4011, which can be purchased at a local hardware store as "Scotch Permanent Outdoor Mounting Tape" if someone wants to play with a small sample.
Pretty darned strong stuff. It has some good uses, but I can't see this miracle tape, or even epoxy holding on the legs as described. Especially if the part has already been anodized. Maybe epoxy could work if you could insert a dowel pin of some sorts.

By Joseph P. Kane (****)

Date 06-10-2011 18:59

Rlitman

Many of the outdoor sign structures that you see in the big cities are put together with the VHB tape Ten years later, these signs are still serviceable. In 1991, I saw a large heavy clock attached to a building and cantilevered over the sidewalk with this stuff. It is still there today. Now that clock base had a lot of square inches of surface area, to stick the tape to.

By rlitman (***)

Date 06-10-2011 20:59

Yeah, it is pretty amazing stuff. I know many glass walls supported by it, and I've heard it's used on some aircraft exteriors. The 3M document "Design Guide for Architectural Metal Panels" includes a picture of the Gehry Concert Hall. I guess it's used on that building's skin (not much else I can think of that would work there).
In those applications, it's "give" allows it to hold together flexing structures, without overstressing the corners.
http://multimedia.3m.com/mws/mediawebserver?mwsId=SSSSSu7zK1fslxtUn8_e48txev7qe17zHvTSevTSeSSSSSS--&fn=ArchitecturalPanelbro.pdf
It shows a lot of aluminum being held this way.

You won't be able to glue or tape 2 foot long 1"x3" legs to a 14" square piece of aluminum safely, BUT if those legs could be bolted (or welded) to a different aluminum plate (maybe 12" square so it hides under the part being displayed), you could probably tape the two flat surfaces together successfully and hide the bolts that way. You just need sufficient surface to bond the plates, and a way to compress the tape with the recommended 15 psi of compression to set the initial bond.
The spec sheet I'm looking at, shows a bond strength (which takes 72 hours to be fully reached) of between 55 and 160 PSI (depending on the exact tape) in tension, on aluminum. About 1/3 less in dynamic shear, and between 12 and 25 lbs per lineal inch of tape (depending on which one), to pull up at 90 degrees off of stainless.
See:
http://www.markingsystems.com/uploads/media/3M_4926_foam.pdf

The catch, is that these materials are prone to "creep" in static shear. From that same document:

Use the right amount of tape: Because 3M™ VHB™ Tapes are viscoelastic by nature their strength and stiffness is a
function of the rate at which they are stressed. They behave stronger with relatively faster rate of stress load (dynamic
stresses) and will tend to show creep behavior with stress load acting over a long period of time (static stresses). As a
general rule, for static loads, approximately four square inches of tape should be used for each pound of weight to be
supported in order to prevent excessive creep.

That may be your undoing if you don't plan this well. So, do you know the angle of the mounting? Multiply the sine of the angle from horizontal by the weight of the piece, to calculate the shear force, and that will let you know how much tape you'll need to make this work.

By Lawrence (*****)

Date 06-09-2011 19:54

There are some pretty sexy rivits and fasteners out there.

They look professional and no worries about adhesives.

By weldwade (***)

Date 06-09-2011 21:35

Thats what I told him :lol:

but he is hell bent on not having a mechanical fastener??? I looked up a 3m product on google that may work? I sent 3m an email asking for product info.

By Tommyjoking (****)

Date 06-09-2011 22:48

I know lotus built an aluminum framed car a few years ago and the entire thing was bonded together to prevent tempering issues....there are some incredibly strong epoxies out there....I think you are on the right track in contacting 3m, might try contacting these folks as well http://www.loctite.com/.

By DaveBoyer (*****)

Date 06-10-2011 01:52 Edited 06-10-2011 02:07

Epoxy will have a tensil strength of about 2 KSI, but You need a good bond.West System suggests sanding wet unthickened epoxy into the surfaces with coarse [36 grit?] sand paper. This allows the epoxy to wet the material before it has a chance to oxidise. The down side of epoxy adhesives is they are not real flexable, so if the stand flexes in handling, the bond can be point loaded to beyond failure. Use a 24 hour setting epoxy for a job like this, the 5 minute stuff isn't up to the job. I have used a lot of West System products http://www.westsystem.com/ss/product-selection-chart/ I would use 105 resin 206 hardener and 404 filler on a job like this, but JB weld or PC-7 would do OK as well, but it would be hard to sand PC-7 into the surfaces.

Another alternative is pollyuethane adhesive. 3M 5200 comes to mind. It is flexable and provides a tenacious bond to properly prepped surfaces. It has a long cure time in the standard version, but there is a fast cure [still would take a while in this aplication] version.

You might be just as well off with a structural adhesive like Liquid Nails

If You put cleats on the frame to prevent the part from sliding off, there would be less load on any fastening method.

300f-400f from a torch on the back of the frame will make any of these adhesives removable, if the time comes when it has to come off.

Edit: Here is a newer West System product that might work well: http://www.westsystem.com/ss/g-flex-demo/ I would still sand it in and then thicken with #404 filler.

By weldwade (***)

Date 06-10-2011 02:04

Thanks Tommy and Dave! Very good info I really appreciate it. I will check those links and get some more info on the way.

By rlitman (***)

Date 06-10-2011 14:38

I've seen warnings about using epoxy on aluminum. The epoxy can allow oxygen to get through, and if the aluminum oxidizes underneath, it will let go.
The wet sanding WITH epoxy is a really good start, but the best thing I can recommend, is to not use a filler, and keep your epoxy and joint as thin as possible. A thinner joint will expose less epoxy to the air (the aluminum itself will keep the interior airtight).

I've used a bit of West System with fiberglass. It is more flexible than the polyester more commonly used in fiberglass, and if it's flexible enough for that, it's plenty flexible to match with a heavy chunk of aluminum.

Along with polyurethane, I would suggest looking at polysulfide.

By DaveBoyer (*****)

Date 06-13-2011 02:26

I didn't word that right, the epoxy has little elasticity. In thin layers it has some flexibility.

I find unthickened West System to be pretty runny to use as an adhesive if the joint isn't pretty tight.

You can use up to about 80% high density filler without a reduction in strength. The #406 Colidial Silica works well too.

In West's literature, there is a section on hardware bonding, probably worth reading if You are using epoxy as an adhesive.