Busted! Again!

The mirror print bed broke when I tried to heat the bed up to ABS temps. It looks like a 2nd temp sensor is just a touch high and created a stress point. PLA temps didn't cause any problems with it.

One thing I noticed is when I used just the shard of mirror, the temperature quickly climbed that last 10 degrees. The temperature sensor was no longer under the mirror. The mirror must be radiating heat faster than the plain PCB board? Anybody have any science on this?

I put 2 pieces of tape down to hold some heat around the part as the printer is out in the open in my cold basement. I got a lot less lifting printing straight to glass with a bit of hairspray than I expected.

In album 2013-08-01

Broken bits of my print surface laying on the edge of the aquarium next to my printer. It’s a cheap IKEA mirror tile, so I will cut another and edge it this time. I also will adjust the sensor that the glass broke over.

Printing on a shard of my broken bed. The blue tape is to create a warm pocket of air to reduce ABS curl.

This part lifted surprisingly little for it’s shape & printing in an open basement. Less than a mm lift that worked back less than a cm. First print with the ABS after switching from PLA, so I am sure the first few layers where contaminated with PLA and I didn’t have the temps tuned right for clean printing.

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  1. If I edge the glass on a belt sander and remove the stress points under the glass I can temp cycle it more than once. I had a piece of window glass that I edged last me a hole roll of ABS. I broke it when I got rough with it peeling a PLA part off of blue tape on the glass.

    I've got 1 more tile left that I am going to cut and edge. We shall see how long it will last.

    I love this thin mirror for PLA. I may need to get something else for ABS. Time (and broken glass) will tell.

  2. I have a piece of plain 3mm glass sitting on top of two heated beds and have never had any problems with cracking. I can even crank one up all the way and leave the other one totally cold – the only issue that arises then is that the glass heats up a bit unevenly.

  3. The band gap on the glass is probably higher than the effective band gap on a PCB. The mirror will also cause directionality in emission of thermal radiation. Notice radiator panels in vacuum environments like the ISS tend to be shiny. The smooth metallic surface emits IR far better than just a painted or white surface. I have a book i can scan some information from regarding space systems design. Black things emit heat almost as well as they absorb it, but there's something really weird about spectrally reflective items, since they don't absorb heat very well (or at least they absorb it with a bit of anisotropy)

  4. Oh, also, have you thought about putting down some copper foil underneath? We solved a problem with a hot plate/stir bar dodad screwing up at my old lab by tossing a chunk of copper on it. Turned out we only needed something less than 1mm to even out the wonkyness.

  5. +Rion Motley I have not thought about that. A lot of people like using aluminum instead of glass. I've not tried it yet. I wonder if a thin aluminum (lighter than copper, yes?) plate between the heater and the glass could help? Maybe thermal paste the PCB heater to the aluminum plate?

  6. That's exactly what I'm doing. There is a 0.5mm sheet of aluminum between my glass and the PCB, which should help spread the heat a bit better. I like to think that the bed's corners are a bit more evenly warmed up than without the aluminum in there, but I haven't done any scientific tests.
    I'd advise against thermal paste, though, since it's only meant to fill microscopic gaps and not the huge distance between our flexing beds and the smooth glass. Thermal paste is actually a fairly bad thermal conductor, and it also makes a huge mess if you try to apply it to larger surfaces evenly.

  7. I've had great results with Garolite (FR4-G10) and ABS.  It can take the heat and has some texture that i think helps with adhesion.  It is not flat as a nice piece of glass but still very flat.

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