<p>The glass will be taking up the heat to slowly cover its full area.
That means two things:</p>

<ol>
<li>it will not crack as there is no thermal shock</li>
<li>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.</li>
</ol>

<p>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.</p>

<hr>

<p>A comment regarding heat expansion:</p>

<p>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</p>

<p><a href="http://glassproperties.com/expansion/ExpansionMeasurement.htm" rel="nofollow noreferrer">source</a></p>

CC BY-SA 3.0
2018-04-20T10:06:06.660Z
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
CC BY-SA 3.0
2018-04-20T10:15:11.423Z
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
CC BY-SA 4.0
2019-11-06T14:22:04.477Z
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
CC BY-SA 4.0
2019-11-07T14:28:14.410Z
@profesor79 for info: https://engineering.stackexchange.com/questions/31842/how-much-an-unevenly-heated-glass-plate-bows
FarO
CC BY-SA 3.0
2018-04-20T09:29:50.793Z
2018-04-20T10:17:53.060Z
2018-04-20T10:17:53.060Z
profesor79
<p>If you're using borosilicate glass (aka pyrex) then it won't crack. You can get squares of ~20x20 cheaply off aliexpress.</p>

<p>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.</p>

<p>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. </p>

CC BY-SA 3.0
2018-04-20T21:10:06.977Z
2018-04-20T21:10:06.977Z
Funcan
<p>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.</p>

CC BY-SA 3.0
2018-04-21T08:31:11.573Z
2018-04-21T08:31:11.573Z
Jose Maytorena
<p>I have a 15x15 cm heating resistor from my current printer (printing area: 12x12 cm).</p>

<p>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).</p>

<p>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.</p>

<p>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.</p>

<p>Related: <a href="https://engineering.stackexchange.com/questions/31842/how-much-an-unevenly-heated-glass-plate-bows">https://engineering.stackexchange.com/questions/31842/how-much-an-unevenly-heated-glass-plate-bows</a></p>

CC BY-SA 3.0
2018-04-20T18:12:30.910Z
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
CC BY-SA 3.0
2018-04-28T10:31:27.023Z
it will work without breaking. you can pre-heat with a towel over the bed to increase uniformity.
dandavis
CC BY-SA 4.0
2019-11-06T15:26:40.413Z
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
CC BY-SA 4.0
2019-11-07T12:36:31.220Z
@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
CC BY-SA 4.0
2019-11-07T12:49:03.870Z
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
CC BY-SA 4.0
2019-11-07T13:50:55.450Z
@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
CC BY-SA 4.0
2019-11-07T14:00:55.737Z
So I guess plywood, then some thermal insulator, then heat pad, then AL plate.
CrossRoads
CC BY-SA 4.0
2019-11-07T14:05:55.260Z
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
<heated-bed><glass-bed>
Can I have a glass bed with only the center heated?
FarO
# Can I have a glass bed with only the center heated?

(Asked by: username_0 on 2018-04-20)

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>

**Tags:** heated-bed, glass-bed
> **Comments:**
> **username_1**: 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.  
> **username_2**: it will work without breaking. you can pre-heat with a towel over the bed to increase uniformity.  
> **username_3**: 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).  
> **username_0**: @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  
> **username_3**: 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.  
> **username_0**: @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.  
> **username_3**: So I guess plywood, then some thermal insulator, then heat pad, then AL plate.  
> **username_0**: 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  

---

## 3 answers

### Answer by username_4 on 2018-04-20. Score: 0 | Accepted answer

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](http://glassproperties.com/expansion/ExpansionMeasurement.htm)

> **Comments:**
> **username_0**: 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.  
> **username_4**: 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  
> **username_5**: This is only true for sheets of small and intermediate dimensions. Very large beds (400mm upwards) would have considerable thermal gradients over the area.  
> **username_0**: @profesor79 for info: https://engineering.stackexchange.com/questions/31842/how-much-an-unevenly-heated-glass-plate-bows  

---

### Answer by username_6 on 2018-04-20. Score: 5

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.


---

### Answer by username_7 on 2018-04-21. Score: 3

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.


---

