Adhesive advice sought

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mwolczko

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Mar 14, 2008
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This German-made bathroom scale has been in use for 20 years. The glass plate on which one stands was attached to the raised metal disc in the middle of the base by a circle of adhesive, which failed this morning. It lasted an estimated 15k cycles of someone getting on and off (2 people, daily, 20yrs)!  What kind of adhesive would work to repair this?  When my foot first lands there’s 240lb being applied several inches from the disc.  Or is this something that can only be accomplished in factory conditions?
 

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Most likely they used a UV glue like the glue used to attach rearview mirrors to windshields or windscreens depending on what side of the pond you are on.  You can buy UV glue just run a google search for it.
 
Windshield adhesive is probably going to do a good job.  The “UV” represents the type of curing process.  It does not necessarily mean it is UV resistant.

You do want an adhesive that remains rubbery.  All that flexing will destroy a bond that is rigid.  I would not use epoxy for that reason.

My first inclination is to use E600 adhesive.  It is a styrene based adhesive that remains flexible.  It is like silicone adhesive on steroids. It is available in the big box stores like Home Depot, in hobby stores like Michaels and online from Amazon.com.

Amazon carries this product in squeeze tubes and caulking type tubes.  The others only carry the squeeze tubes.

This stuff cures in exposure to air. If you are just bonding to the perimeter, you will do fine.  If you are bonding to areas that will not have access to air, those areas will likely take a very long time to cure.

I like to use small pieces of mirror mounting tape to hold the piece in place, and use the E6000 everywhere else.

Also providing a good bond on non-porous surfaces is the adhesive that is used to glue rear view mirrors to the inside of the windshield.  But they come in very small squeeze tubes and it will cost a fortune to glue to a larger surface.

I recently bought a digital scale which reads to the tenth of a pound.  I don’t remember what I paid for it, but certainly under $40.00 and more likely under $30.00.

Unless you are emotionally attached to the scale, I’m not sure that making a repair is entirely sensible.
 
Update: I cleaned both surfaces, applied the E6000 and let it cure for 5 days.
It failed the first time I stepped on the scale.
Next I got some JB Weld ClearWeld epoxy (which is one the few adhesives that actually claims a specific strength), cleaned off the dried E6000, and applied that.
After another 5 days I just tried — and it failed immediately.
Next up is probably a cyanoacrylate, unless someone has a better suggestion.

 
Could be.  The ones for mirrors seem hellishly expensive to cover the area in question (eg $10 for 0.01oz).  I might end up there.

Aside: how come hardly any manufacturer advertises tensile strength for its adhesives? How do actual engineers who need certainty to design products deal with this?  Or is there a special secret place they go for this info?
 
Thanks.  Loctite has actual specs on its web site and this glue has an unimpressive tensile strength of only a few hundred psi.  By comparison, JB ClearWeld is advertised at 3900 psi, but under what conditions is not stated…
Also, I wonder how far 2g goes.
 
mwolczko said:
Could be.  The ones for mirrors seem hellishly expensive to cover the area in question (eg $10 for 0.01oz).  I might end up there.

Aside: how come hardly any manufacturer advertises tensile strength for its adhesives? How do actual engineers who need certainty to design products deal with this?  Or is there a special secret place they go for this info?
Click to expand...

I would be willing to bet that it is because they cannot control the condition of the surfaces you want to bond nor confirm after a failure that it was done to whatever specs they may provide. Basically it's a CYA situation, lawyers, etc...
 
One thing to consider is each failed attempt is likely to leave residual contaminants that will interfere with future bonding and your chance at success decreases. Also, each time you step on the scale and bond fails you have some risk of injury.

Personally Im all on on repairing and restoring things rather than tossing out as a life ethos, but at what point does purchasing adhesives that may or may not work stop making sense and simply replacing the scale become a preferred option?
 
How valuable is this scale?  I bought a very accurate digital scale for $30.00.  Perhaps you are better served by replacing it.

Or write to the manufacturer and see if they offer repair services.
 
Crazyraceguy said:
I would be willing to bet that it is because they cannot control the condition of the surfaces you want to bond nor confirm after a failure that it was done to whatever specs they may provide. Basically it's a CYA situation, lawyers, etc...
Click to expand...
I buy that. But I was hoping there’d be some kind of standard (ISO/DIN/BS/etc) which would yield a published result under carefully controlled conditions, and that could be used to engineer stuff.
 
Vtshopdog said:
at what point does purchasing adhesives that may or may not work stop making sense and simply replacing the scale become a preferred option?
Click to expand...
You’re absolutely right (as are the other posters suggesting replacement).
I’m doing this only because this particular item perfectly matches the decor of the room it’s usually in, and because I’m curious about how this could ever have worked and lasted for so long.  I’ll noodle for a bit, but I may end up just giving up.  I’m not sure I’ll ever persuade the missus to use it again, even if I think I’ve succeeded (and with good reason).

Having inspected the failed bond I attempted with epoxy, it seems to me the cure was not good — the cured glue feels soft and sticky.  One drawback of a twin-tube dispenser, when both sides contain clear liquid, is that you can’t really assess how equally both sides were dispensed.  I may try again, but controlling the mix a bit more carefully.
 
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