Which glass for heated bed

Question

I am building my CoreXY 3D printer with 350x350 mm heated bed at about 4 mm thickness. I just found out that borosilicate glass is unavailable at the local glazier. He offered tempered or ceramic glass instead. Which one should I go for? I read that both are a more expensive option to borosilicate glass, but I worry about the temperature stability, warping of the glass (in case of the tempered one), and parts (not) sticking to it. Any experience is appreciated.

PS. The ceramic glass of this size would cost me about 45 USD. The glazier did not tell me the price of the tempered option, but I reckon it will be around 30 USD, which is much more acceptable if it does the same job.

Answer 1

The primary advantage of borosilicate isn't that it's resistant to thermal shock -- as someone already noted, there's not much in the way of rapid heating or cooling in FDM printing (yes, the filament is hot, but its mass is low enough the glass it touches won't pick up much heat). Rather, it's the low expansion. Common soda-lime glass (window glass, mirror glass) expands several times as much as borosilicate, which means it will also shrink several times as much when it cools back down.

If you're holding constant temp throughout the print, and give adequate preheat time, that won't matter, but if (for instance) you heat the bed 10 °C hotter for the first layer, then cool it, plain glass will shrink enough to potentially cause the print to come loose, while borosilicate won't.

Ceramic glass is like borosilicate only more so (akin to Corningware) -- but again, not worth paying for unless you expect to change your bed temperature during the print.

For any kind of glass, my own preference is to use glue stick. I apply a "squiggle" (a precise metric volume), use 91% isopropyl to spread it evenly over the build area, and my prints stay stuck, but pop right off when the glass cools a little.

Answer 2

People choose borosilicate because of its thermal properties.

Between ceramic and tempered, I would suggest tempered glass, it's not good as borosilicate when comes to adapting to rapid temperatures but better than ceramic. The other option I would suggest will be "Aluminium", which is much better.

Answer 3

Mirror glass is commonly used. It's flat and cheap. There isn't really a sudden thermal shock to worry about, and if you're going to be printing at temperatures that need borosilicate glass, you might find the aluminium parts of the bed sagging. Remember to ask for the edges to be smoothed.

Thinner glass will transmit the heat better - but you might be able to get tempered glass as thin as 1/8" thick (~3.2 mm).

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I print with PLA and use 3 mm mirror glass secured with a bulldog clip near-ish to each corner, and Elnett "Normal Strength" hairspray for adhesion (applied holding the glass over the bath to avoid it getting everywhere) - it's easy to clean off under running water. Demounting the parts cleanly is a simple matter of popping it in the fridge for half an hour if they don't come off with a slight nudge.