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I have a 15x15 cm heating resistor from my current printer (printing area: 12x12 cm).

I would like to switch to a glass bed and to rework my printer to increase the printing area to 20 cm (22x22 cm glass plate).

Would it be possible to use the old heating resistor placed only in the centre? this way I would have a smaller heated bed for ABS and a bigger one for PLA.

Would the glass crack due to non uniform heating? This is because glass has a conductivity of less than 5 W/mK, therefore the hot area will stay hot and basically never really spread the heat to the surrounding area. So the frame will be cold and the center hot, causing stresses.

Related: https://engineering.stackexchange.com/questions/31842/how-much-an-unevenly-heated-glass-plate-bows

FarO
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    This could be a problem with ambient temperature below to 5°C and the heater raises the temperature too fast. for example the heating of cooking oven. When the temperature raises smoothly the closer areas get warm also, so the thermal shock won´t occur. So if you are so worried you can get a tempered glass. – Fernando Baltazar Apr 20 '18 at 18:12
  • it will work without breaking. you can pre-heat with a towel over the bed to increase uniformity. – dandavis Apr 28 '18 at 10:31
  • How will you secure the heated pad? On my printer, the corners that hold the leveling screws also hold the corners of the pad (unless it had a peel & stick face on it, I didn't see that part was assembled). – CrossRoads Nov 06 '19 at 15:26
  • @CrossRoads I could prepare a 3 mm thick plywood plate as big as the bed, and use it to squeeze the heated PCB against the glass – FarO Nov 07 '19 at 12:36
  • I would think plywood would dry out and become a fire hazard over time with 60C/140F temperature applied to it for hours at a time. – CrossRoads Nov 07 '19 at 12:49
  • @CrossRoads Plywood is fine up to 90°C, after which mechanical properties are affected. It takes more to get to the fire hazard range, but thanks for letting me know, because I never thought about it. Anyway that's a detail... I could use an aluminium plate, the question doesn't change. The advantage of plywood is that it acts as insulator to reduce losses to the bottom. – FarO Nov 07 '19 at 13:50
  • So I guess plywood, then some thermal insulator, then heat pad, then AL plate. – CrossRoads Nov 07 '19 at 14:00
  • I thought of a thermally conductive rubber instead of alu, to compensate for deformation of the heating PCB and to ensure optimal contact with the glass – FarO Nov 07 '19 at 14:05

3 Answers3

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If you're using borosilicate glass (aka pyrex) then it won't crack. You can get squares of ~20x20 cheaply off aliexpress.

If you're using window glass, picture frame class etc then you might have issues with cracking, it will depend on lots of factors like the wattage of your heater and the temperature of the room.

Your biggest problem will be that the bed heats very non-uniformly, which is unlike to give good print adhesion. I'd suggest a sheet of aluminium underneath the glass if possible, it will allow much more even heating.

Funcan
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I purchased some very, very cheap thin glass from walmart (or home depot, cant remember for sure). Probably the thinnest kind available. Went through dozens of prints heating it up to 85C only at the center and never experienced issues with cracking. Personally I believe you’ll be fine. I can look for specifics if you need them.

Jose Maytorena
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The glass will be taking up the heat to slowly cover its full area. That means two things:

  1. it will not crack as there is no thermal shock
  2. it will put more pressure on the heater as there will be bigger heat absorption, so in an edge case, you could end-up with not getting a required temperature on the glass surface and/or the heating process will be very slow.

In that case, I personally will try that out and then if I have an issue with getting proper temperature, then I will buy a new heater.

A comment regarding heat expansion:

my glass is about 20mm longer than the bed and it is warm during the printing process. The 'breaking' stress is connected with temperature shock (an immediate temp change), not a relatively slow heating process. However, borosilicate glass also known as Pyrex (TM) expands very little. Much less than most metals. This is why it doesn't break when exposed to sudden hot and cold changes. So in short words - check tea lighter under your tea-pot and see how te heat is comming out from the centre

source

profesor79
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    Glass conducts heat so badly that the hot center part will not spread significantly the heat to the unheated area around it. It may cause internal stresses. – FarO Apr 20 '18 at 10:06
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    my glass is about 20mm longer than the bed and it is warm during the printing process. The 'breaking' stress is connected with temperature shock (an immediate temp change), not a relatively slow heating process. However, borosilicate glass also known as Pyrex (TM) expands very little. Much less than most metals. This is why it doesn't break when exposed to sudden hot and cold changes. So in short words - check tea lighter under your tea-pot and see how te heat is comming out from the centre – profesor79 Apr 20 '18 at 10:15
  • This is only true for sheets of small and intermediate dimensions. Very large beds (400mm upwards) would have considerable thermal gradients over the area. – Trish Nov 06 '19 at 14:22
  • @profesor79 for info: https://engineering.stackexchange.com/questions/31842/how-much-an-unevenly-heated-glass-plate-bows – FarO Nov 07 '19 at 14:28