Having problem gluing laser-cut acrylic parts

[Frederic Gagnon]

Hi everyone!
I've been laser-cutting acrylic parts for a year now, mostly for sign letters. Up to now, I used double-sided tape for putting together my work.

Recently, I was asked to make acrylic displays to protect some collector items, basically boxes with an open bottom. I laser-cut the parts in 1/8'' acrylic and used methylene chloride (a solvent) to glue them. To my surprise, all the smooth, laser-cut edges would immediately crack in contact with the solvent.

After research, I learned that laser-cut acrylic edges are brittle and under a big mechanical stress, and will crack with solvents or alcohol... unlike routed or sawed edges. Wow, this is a big drawback no one told me about!

So now, I am looking for a bubble-free, clear glue that will do a nice job without cracking the laser-cutparts... I was thinking about trying Weld-On #16 glue. Anyone as any advice?

Thanks!
Fred

[Joe Pelonio]

Yes, any solvent will cause cracks almost before your eyes. The same happens to flame-polished acrylic. I have glued laser cut acrylic edges together but only black. With clear, you can still glue flat pieces on top of each other, or clear onto black etc.

The Weld-On #16 is merely a thicker version of the same solvent based cement, and might still cause the cracking.

The only thing I have used that works is clear silicone adhesive, but it's not perfectly clear and certainly not as strong a bond. You have to "cut" off the excess that squishes out with a razor blade or xacto knfe.

[Bill Cunningham]

I Haven't done much quality acryic bonding, other than for underwater camera housings years ago.. (long before lasers).. all the acrylic bonding I do now, is simply to hold jigs together, and I use acetone. The finish is not important, as long as it sticks!
Is it possble to cut the parts a 'hair' oversized, then sand the edges on a tabletop belt sander etc.. 'Then' glue them? wouldyou still get cracking? how far does the heat affected zone go? Have you ever tried this Joe?

[Pete Simmons]

How about Weldbond?

I think ACE hardware carries it.

I have not tried plexi with it but I use it on everything else.

[Joe Pelonio]

I Haven't done much quality acryic bonding, other than for underwater camera housings years ago.. (long before lasers).. all the acrylic bonding I do now, is simply to hold jigs together, and I use acetone. The finish is not important, as long as it sticks!
Is it possble to cut the parts a 'hair' oversized, then sand the edges on a tabletop belt sander etc.. 'Then' glue them? wouldyou still get cracking? how far does the heat affected zone go? Have you ever tried this Joe?

Actually this will work, but I have no idea how far the "zone" is. I have used a 1/4" roundover bit on the router after cutting and that will take the solvents without the cracking. I tested it by rubbing it with denatured alcohol which normally cracks it within less than a second. That bit takes off quite a lot though. Maybe something for Frederic to try and get back to us.

[Tom Majewski]

I've had this problem even without gluing. After the parts sit and are handled for a few weeks they show cracking along the sides. I wonder (for smaller parts) if tempering them in boiling water might help to relieve the edge stress.
Next time I'll try.

[art baylor]

Frederic:

If memory serves it is a mixture of methylene cloride with diacetone alcohol,
60/40. The alcohol slows the evaporation. Use an applicator for capillary action, or draw in. It sucks moisture out of the air, don't let it sit around a long time after it is mixed or it won't work. Not stress crack proof, but much better.

Art

[art baylor]

Disclaimer:
Haven't tried the methylene chloride/ diactone alcohol mix specificly on laser cut parts. However it use to work well on flame polished edges without stress cracking. Cast acrylic, we never used extruded.

Art

[art baylor]

Tom:

There is a formula for stress relief by annealing. Check Google or Rohm & Haas. Something like 180 degrees in an oven ( on a flat surface) for an hour for 1/4 inch. Don't trust this, look up 'annealing acrylic'.

Art

[Rodne Gold]

Fistly use tensol for bonding , it will ameliorate the stress cracking problems secondly use cast acrylic as its way more resistant to this than extruded and thirdly , use a lower PPI when cutting parts to be bonded so as to put less heat into the edge. You can also scrape the sharp edges with a razor blade or deburring tool.
The formula for annealing is 180 degree for cast , 1 hour for every mm and 170 for extruded.
PS routering the acrylic can also put the same stresses into it , unless you keep it cool and use the right tool.

[Frederic Gagnon]

Many thanks for all the tips.

Right now, the acrylic sheet I have is extruded, so there is one cause there for sure.

Pete mentionned Weldbond glue. On their website they have a detailed list of compatible materials, but they don't mention acrylic, so I have doubts about it. And they say that one of the two surfaces at least has to be porous.

I will do a test with the oven annealing suggestion.

As for routing or sanding, since I don't have a well equipped shop besides my laser, I'm afraid it would be unproductive.

I will also check into the alcohol+methylene chloride mix.

Will keep you posted!

[Keith Outten]

I use Weld On number 3 or 4 and have never seen a crack on cast or extruded acrylic. Maybe I have been lucky.

.

[Tom Majewski]

Art, thanks for the tip. Thought I post this here for everyone.

http://www.polyfab.biz/annealing-acrylic.htm

[Richard Rumancik]

If you want to anneal you will probably have to make or buy a dedicated oven. Your kitchen oven is not accurate enough (plus your spouse may not approve.) You need good circulation and probably baffles between the element and parts. Plus a better temperature controller and perhaps a timer to ramp up/dwell/ramp down. You could re-work an old range if you were doing a lot of parts.

I tried a part using a kitchen range some time ago and had poor results. The surface was affected plus the part shrunk. Do some preliminary testing and research before you get too far into annealing as a solution. It creates some new problems. Supporting acrylic at high temperature so it doesn't end up with marks on the surfaces and so it doesn't warp is quite a challenge.

[Bob Tate]

I have not had very many cases of the acrylic cracking, except on a very important job. I was cutting Flight Simulator panels, and the windows that go into them. They crazed. I annealed them in my normat kitchen oven, by another table source but very similar in times and temperature. It worked fine except the surface I had them on, a cookie sheet, was not totally flat. I went to the glass shop and bought a 12 X 12 square of plate glass, the stuff like in store windows, it was like 3/16 thick. Annealed again on the glass. Worked perfectly. I think the only consideration is not getting it too hot. I did, and it did melt on one occasion.
It works for me!

[Phyllis Rhodes]

I bought the Weld-On 4 from laserbits. I've used it several times with no problems whatsoever.

They charge $7.69 for the glue and $3.79 for the syringe-type applicator.

~phyllis

[Frederic Gagnon]

I read the annealing info page and I agree it's not as simple as it looks : it needs time (2hours + 1hr cooling), precision which my kitchen oven lacks I'm sure, and jigs to support the parts flat (a tempered glass seems a good idea)... But I was still hoping for something more straightforward!

I tried reducing the PPI of the laser to a minimum and It still cracked. But I am still using the extruded acrylic I have.

I bought a glue called ''Amazing Goop'' which is supposed to glue everything. Will try that now.

[Frederic Gagnon]

Ok, so I tried gluing together a display case using "the Goop", a glue I found at Home Depot that is recommended for plastics and even large size aquariums. Well, it looks like a kind of silicone glue, which stays elastic even after cured. It's very strong and it's transparent.

The problem is that it's almost impossible to spread an even and thin layer of it on the acrylic edges. When I join the parts together , the glue overflows and it looks nasty. I think a water-thin glue is really needed to make a good-looking bond.

Weld-on #4 is my next try!

[Mike Null]

I glue acrylic plaques and bases regularly and have never experienced a cracking or crazing problem.

The correct solvent cement to use is Weld On 3 or 4.

Cast acrylic works better than extruded.

One surface should be rough as if it were sanded with 400 grit paper. It doesn't matter which one. (If you have a coin edge from the laser sand it.)

The entire contact area must covered with solvent cement. Spot welds won't work.

Hold one surface slightly off vertical on the other surface and let the applicator feed the cement into the joint. Position the pieces and hold for a minute or less until they are joined. If you have a perfect joint you will see no cement or bubbles.

Low humidity is important (40%).

[Mike Null]

While I can't attest to the fact that annealing will help I do take issue with those who say your kitchen oven is not accurate enough.

Having retired from the appliance business as product manager for cooking products I have some knowledge in this area.

Even annealing ovens do not hold a technically constant temperature but work much like a kitchen product in that there is a temperature amplitude that is critical to the performance.

For example if you set your oven to 300 degrees F the thermostat will cycle it at 285 and 315. (depends on the thermostat spec.) But the important thing is that it's this fluctuation that causes the air movement within the oven and therefore more evenly distributed temperatures throughout the oven.

[Tom Majewski]

I've had long term success using 30 to 90 minute epoxy. The Loctite and Tower Hobby brands remain clear and dont yellow. The edges have to be sanded with 220-300 grit paper. To sand a flat area, I mask off a 1/8" or 1/4" strip, (whatever thickness you're using) then sand between the masking tape. Wipe the area with alcohol, leave the tape there and apply a narrow bead of epoxy, remove the tape about 5 minutes later.

This was all before I had my laser when I had to cut the plexi on a table saw and file and buff the edes.

[Vaughan Raymond]

I have experience in aerospace manufacturing and looked this up in some specs that I have from a major aircraft manufacturer. After any processing like machining, sawing or thermoforming, they require annealing.

Quote:

"Except for laminated acrylic parts, anneal all acrylic plastic parts at a temperature of
160 ± 15°F for 24 to 48 hours, followed by air cooling to room temperature."

We used to use a flat sheet of plywood covered with felt. At that temperature, there is no mark-off.

[Tom Bull]

There is a water clear silicone available from cr laurence (a glass supply place) that is OUTRAGEOUSLY expensive, but very useful.

[Larry Bratton]

I use Weldon #16 all the time and have not experienced any of this. I also use #3 and have had no issues with it.