Heated bed glass like an automobile rear window defroster?

A quick search on "defroster" hasn't turned up any hits here or on the wiki, so I figured that this was worth asking.

Has anyone ever proposed or tried using conductive paint or etched traces placed directly onto the back side of the bed glass as an electric heater? It works for an automobile rear window defroster, albeit at lower temperatures. Could this technology generate enough heat for a good stick with ABS? Just curious, since I am not thrilled with the heat transfer from my existing PCB bed heater through glass, and was wondering if taking the PCB-glass gap out of the picture would improve the heat flow.....

... won't work for ABS with normal glass and heating traces because of thermal gradients and mechanical stress.

You'll need special glass like Ceran with embedded tungsten wires ...

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While it's good to think outside the box, I don't think this would be practical. It would need to be custom made and if you break your glass your heater is gone as well. Also I almost always remove my glass to make it easier to remove prints (I have them stick very well, takes some work to get them loose). A defroster temp also does not get very hot, just barely hot enough to remove frost.

I think heaters we use (PC board or Silicone mat type) are already pretty efficient and work well with almost no failures. No need to reinvent the wheel in this case.

This is exactly the kind of product we need. 3mm safety glass, with the traces much closer to each other. We don't need to see through it. Great thinking.

I wonder if one could glue nichrome wire on to the back of a piece of borosilicate glass?

Would high temperature epoxy work?

Would the expansion and contraction of the wire break it free?

Would steel or stainless steel wire work better?

Seems like an aluminum heat spreader might be a good idea in between the glass and the heat source to even things out anyway. But then we need an electrical insulator that is a good conductor of heat.

When I get some spare time I'll have to break out the thermal camera at work and do some testing.

Quote
garak
I wonder if one could glue nichrome wire on to the back of a piece of borosilicate glass?

Would high temperature epoxy work?

Would the expansion and contraction of the wire break it free?

I did some digging around yesterday after my original post, and found that Airtripper made a glass heated bed by taping nichrome traces to the back of the glass using nichrome tape:

http://airtripper.com/698/resistance-wire-heated-build-platform-diy-tutorial/

A more interesting tangent that I also found was discovering that airplane cockpit windows and freezer display doors in supermarkets have heated glass that doesn't use wire traces, but rather a solid transparent conductive coating. They stick parallel bus bars on opposite ends of the glass, and the entire conductive surface becomes the heating element! They can even make hotter or colder spots by varying the thickness of the coating, and it supposedly is good for well above the temperatures that we need. This glass is also commercially available as heated glass shelves for food display cases, so I don't think that it is too much of a stretch to find some either new or in a scrap freezer display door to cut up and see if it can be made to work in heated bed size.

The only pitfall for me is that my personal need is for a round glass bed for a Delta printer, which doesn't lend itself to parallel bus bars. I'm assuming that a center electrode and outer ring on circular glass won't produce even heating without layer thickness engineering beyond home/garage skills and tools.

In response to tmorris9's comment above, I don't think that this question is re-inventing the wheel. The China-made MK2 PCB heated bed that I bought is not precisely flat when mated to glass, and my IR pyrometer shows a 15 C variation in glass surface temperature across the surface at 80 C. (It is a 30 C variation at 110 C!) There is plenty of room for improvement on the popular PCB heater and bulldog clip setup IMHO, and removing the gap between the PCB heater and glass seems to me like a good first move.....

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airtripper
Uneven heating may result in the glass cracking or breaking and could cause issues with 3d print’s bond
to the heated build platform during printing because of wide temperature differences and changes.

[airtripper.com]

An idea to improve upon AirTrippers heated glass bed, diffuse the heat, (i.e. eliminate all air gaps):
Clean the glass of oil, and dirt, don't get finger oil get on the glass.
Draw the wire over a rounded corner to straighten, and remove kinks, pull the wire straight as you do this to reduce curling.
Use as small as possible pieces of Kapton tape to hold the NiCr wire in place, crease the tape to reduce air gaps.
Apply a layer of high temp RTV automotive Silicone, or equivalent, equal to the height of the NiCr wire.
Add a layer of Exhaust wrap (fiber glass).
Using a flat surface covered with wax paper (mold release) press it (weight it down) against the
Exhaust wrap (fiber glass) to evenly distribute the silicone.
Remove the wax paper after it has cured.
Done.

I am using a silicon heater pad glued with acrylic heat resistant tape to and aluminium plate. Thermistor built into the heater pad.
The aluminium spreads the heat evenly.
I then have a glass plate clipped (just two clips, not four) to the aluminium plate.
No distortion, even heating, can remove the glass plate to take off part printed and change over to a new glass to continue printing while the first glass cools down.

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[regpye.com.au]
"Experience is the mother of all knowledge." --Leonardo da Vinci

Get a sheet of copper foil and glue it too your glass. Use a silkscreen or toner transfer method to deposit resist, etch the copper using ferric acid, solder leads, then cover the copper with a layer or the same glue you used to attach it for protection. I'm thinking either high temp epoxy or silicone would work.