Annealing Plexiglas

[EricU]

I've been doing some projects in plexiglass (poly methyl methacrylate,
PMMA) cut on the laser. The finished parts are perfect and beautiful
until you try to bond them. Solvents used to join plexiglass cause
crazing which is unsightly and significantly weakens the joints
produced. It seems laser cutting introduces high internal stresses along
the cut edge of the plexiglass which causes problems during bonding.
These stresses can be reduced by annealing the parts after cutting.
Annealing involves heating the parts in an oven from room temperature up
to some temperature just short of the point where the parts start to
deform due to softening, around 90°C. Ramp up and ramp down times and
'soak' time are critical and vary depending on part thickness, plastic
type, etc. Rapid annealing is do-able for thin sheets but it is obvious
I need to control the temperature in a small oven rather precisely.

I'd like to modify a toaster oven for the purpose of annealing plastic
parts. Maximum control temperature only needs to be 200°F. I've got a
clean new dedicated toaster oven just for this purpose. It has a fan for
convection and four tube elements inside, two above and two below the
work tray, which dissipate 1200 watts total, overkill in this
application. The controls are all mechanical which make the oven easy to
modify. I am wondering if there is a reasonably priced solution already
available. I know it's mostly a function of time vs. money when it comes
to projects like this. I'd like to control the tube elements in pairs or
individually with a solid state relay or some kind of modulation. The
toaster oven has a soft start feature so it does not bang on 1200 watts
all at the same time. Unsure of the mechanism. I have not had the covers
off yet. A micro controller and temperature probes seem to be the only
other big prerequisites.

I'm handy with electronics and some programming but don't wish to
reinvent the wheel. Maybe someone at the space has already tackled this
problem. Any input on toaster oven modification would be appreciated.
Thanks, EricU

[Steve Baker]

I wonder if a water bath would be a good way to do it.  The extra thermal

inertia might make it easier to control...depending on what the ramp

up and down rates are.

[EricU]

Good idea. Never considered it but a water bath might solve some issues. No need to go above the boiling point of water. It certainly would be slow and even heating. And it would give new meaning to the term soak time.

It seems everyone is modifying toaster ovens to reflow solder while making surface mount PCBs. The temperatures involved are higher and cycle time is shorter but same idea. I annealed some cut plexiglass with an unmodified toaster oven using its built in mechanical thermostat. Parts deform very easily when the temperature gets too close to the melting point of the plastic. I think I might try sandwiching the parts between two pieces of heavy gauge sheet metal. It would press parts to help keep them flat and evenly distribute the heat. There would be more symmetry in the oven and I like symmetry. Two lamps above and two lamps below the two metal plates with the plexiglass parts in between. But I still need a controller.

They also recommend an anneal step after bonding. It's always something. All I wanted to do was cut some plexi on the laser and make a simple little box. That was more than a month ago. First I needed an elaborate jig and clamping system to hold the parts while gluing. OK, done. Now I need to anneal. Alright. But only because I know it's going to be an awesome little box. Plexiglass is beautiful stuff when done right.

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[Steve Meyer]

Eric

You might also want to check with Interstate Plastics here in town.  They sell all the engineering plastics commercially (including Ultem and PEEK) and they keep all the bonders on hand.  They can get technical info on annealing from their suppliers.  I had some issues with UHMW and bonding polycarbonate for my wood shop and they were very helpful and reasonably priced.

Steve Meyer

Solid Tech Inc

720 217-8894

[Paul Stiverson]

I have done this exact project (to make a reflow soldering toaster) and it came out great! I think you're describing the exact same toaster that we used; it was a big silver thing from GE. Ours was great because the PCB holding the relays was separate from the PCB holding the other electronics. We were able to hack the cable between and connect it to an MCP23017, this was connected to a raspberry pi. We used a type K (I think) thermocouple to give temperature feedback, and I homebrewed a pcb to amplify and clean up its signal. We used an A/D chip (MCP3004?) to allow the raspberry pi to read it. I also have working code that will follow an arbitrary temperature profile (well, it tries really hard and gets close enough).

You could do without the MCP23017 if you had a ULN2803 (or some darlington array), in fact a Gertboard has almost everything you need.

I could bring my toaster up to the space if you want to see how we rigged it, or I could see about hosting it if it would find some use! And I would be happy to help with your project.

-paul

[Joseph Bridgewater]

Paul if you were to host your hack reflow toaster at the space, I'm
confident it would get use. Not necessarily immediately (maybe so),
but certainly sooner or later, and whenever it did, whomever is using
it would be VERY glad that it was there and already tested out for
that purpose.

[Paul Stiverson]

I haven't tested it for annealing per se, but I'm sure it would do fine at that. All I need is an ideal temperature vs time profile that I can program in.

As it happens I might need to anneal some acrylic for my own project so I may be able to test it at home.

I'm interested in hosting it, but its origin story is a tapestry and I'll have to talk to the donor of the toaster oven itself as well as the donor of the code that makes it run before I can offer it for hosting.

-paul

[Paul Stiverson]

Just found this document (http://www.plexiglas.com/export/sites/plexiglas/.content/medias/downloads/plexiglas-expert-pdf/Fabrication-Guidlines-Plexiglas-Annealing.pdf) pertaining to the preferred heating profile for plexiglass. The 5mm material will take approximately 2.75 hours anneal completely!

We only tested the oven briefly and we never held it at temperature for any extended period. I'm unsure that it will give a stable temperature for that length of time, but we will see.

-paul