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I have printed a MPCNC machine. It has a print area of about 30" x 30" and up to 11" tall. (yes, those numbers are correct).

I found a perfect piece of glass at a garage sale for $5.00 to use as my print bed.

My problem now is how to heat the glass? I was wondering if there is some sort of tape that would perhaps mimic what is on the rear window of a car, but I couldn't find it anywhere.

Any ideas or links to something that can get me some progress on my search would be greatly appreciated.

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Your best option may be to seek out a silicone rubber heating mat, using those terms for your web search. A quick search on my part shows many resources, some of which are known to the 3d printing manufacturing world, while others are equally suited for that purpose.

Don't bond the heater to the glass. You'll need to replace it when it breaks. Consider to use borosilicate glass for better heat tolerance and smaller chance of breakage. A quick search for such a large size pane comes up empty, invalidating that suggestion.

I've read of some people using water bed heaters for large area coverage, but they may heat the area unevenly.

It could be to your advantage to use multiple heater panels with temperature controls for each one. This would provide more uniform heating although more complex temperature management.

I would post links, but there are so many from which to choose.

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  • $\begingroup$ Silicone heating pads such as those for reptile tanks (i.e. fishtank but no water, just a snake or lizard :-) ) might be a good choice, tho' they probably won't get as hot as you want. $\endgroup$ – Carl Witthoft Aug 29 '16 at 15:08
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It will be very difficult to heat such a large bed, simply because of the enormous power required. Thomas Sanladerer recommends at least 0.6 W/cm² but notes 0.4 W/cm² also works (but takes "forever" to reach the target temperature).

For a 30"x30" bed, 0.6 W/cm² would come out to 3.5 kW. At 110 V that would require 32 A and at 220 V, 16 A. These are extremely large currents, perhaps more than you can draw from a single circuit: both in the EU and US sockets tend to be fused at around 15-20 A (the standard EU Schuko plug itself is only rated for 16 A).

You will be forced to go with a lower amperage, for 0.5 W/cm² you "only" need 27 A@110 V or 14 A@220 V. 0.4 W/cm² is 21 A and 11 A respectively

As such, if you are in the US, then it will be impossible to heat such a bed from a standard wall outlet. In the EU it might just about be possible, but make sure that the wiring in your house is in good state and capable to carry the current required (and note that running electrical equipment at its maximum rating for an extended period of time is never a good idea).

If you are in the US, you should definitely look into getting 3-phase power installed. If you are in the EU, you might also consider this as an option. It is able to deliver more power, but you'd need a special type of heater.

There are some suppliers that make custom silicone heater mats. If you can get one in this size that would be a good option, though it would be considerably more expensive than your \$5 piece of glass.

In any case you should not attempt a DIY solution because this is extremely dangerous. The currents you are dealing with should not be underestimated. People have already set their printers on fire due to using high currents at 12/24 V; you do not want to make a similar mistake using even higher currents at 110 V.

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  • $\begingroup$ There is an intermediate step of using a 240V outlet, such as a dryer outlet. Often these are 30A or 50A, providing 7200W or 12000W respectively. $\endgroup$ – Technophile Nov 5 '19 at 16:11
  • $\begingroup$ If using 240V, electrical safety precautions become more important, because (1) what would have been a 120V 'tingle' becomes 2X the current and cardiac arrest, (2) the increased power can make short circuits burn hotter and faster. Probably want a GFCI outlet or breaker. $\endgroup$ – Technophile Nov 5 '19 at 16:36
  • $\begingroup$ A water bed is a nice idea and, together with heavy insulation of the printing chamber, may work fine. Using the CNC he can mill a duct that ensures quite uniform heating, especially with a alu plate and a heat conductive rubber between water and glass. $\endgroup$ – FarO Nov 6 '19 at 14:10
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I am heating my 1 meter by 1 meter plate (yes those numbers are correct) (in progress long term project) of tempered glass with a silicone heater bed I bought as overstock on ebay. Silicone Heating Mat

Silicon bed

From Reprap Wiki

Silicone Heater Pad in sizes silicone heater pad Pros: Fast heating Reliable Most use mains voltage - DOESN'T require any amps from the 12 volt supply Very low height Easy to install (adhesive backed) Can be relatively inexpensive 12 V types can be run directly from the controller MOSFET without a relay Integrated thermistor Long life span Cons: Can be expensive - silicon mat (50€) plus solid state relay (16€). Mains voltage, but can be purchased for 12 V. Need GROUND line to aluminum bed for safety. Relatively high temperatures possible (safety problem if thermistor dies/falls off).

I also talked to a few alibaba companies and they will happily make you a custom order. Just be wary working with them. You will also need to start another question about the particular parts you need. Actually I think I have a second one I bought from Aliexpress about that size. So no custom orders needed. Ran about 100 USD. I will note I used a MIC6 aluminum plate, custom cut at a local shop. That will have a more even heat profile than glass.

I will note that I have to run mine from 220 VAC not 110 VAC power lines.. But the actual electrical costs are pretty low!

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As @Tom pointed out, heating that much area is a pretty big deal. I would just add:

  • You can't draw anywhere near that much power from the normal heatbed output on a PC board like RAMPs. You could, however, use the normal output to control a big relay (semiconductor or not); that also lets you keep the high-power wiring away from the rest.

  • You'll want multiple temperature sensors and heat zones, for at least 2 reasons: First, variation may be high over such an area; and second, you can save a lot of power by only powering up the portions you're going to use.

However, just like with home heating, having multiple zones makes things much more complicated. The software for managing heatbed temperature isn't likely to support multiple heaters (especially since most cards don't have enough outputs to make use of it).

I've done a bit with this since my printer is about 4x as long as normal (but not 4x as wide too, like yours). So far I just use one standard PC-board heatbed, and if I have to print something longer than that I do without the heatbed entirely (mine is easy to take in and out). Present plan is to get a few more of the same PC-board heaters, and have an entirely separate box with temperature dials to control each one.

But that still means the printer software won't be in control. So I couldn't (for example), have a different temp for the first few layers, or have the print stop if the heatbed gets out of range.

To put the printer back in charge, I'd have to do something like calculate the average (or maximum?) temperature of the zones, and create a signal that looks like what a thermistor or thermocouple would generate, to send to the printer software. The software would then turn the heat on or off as usual, but that output would go to the control box, which would decide what specifically to do about it. For example, even when the printer says to turn off, the heat control might keep the cooler zones on until they catch up.

In other words, it gets pretty messy... :(

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Maybe you can stick a nichrome wire under the glass using a heat resistant tape. You'll have to make the appropriate calcs (or just trial/error) to achieve the desired temperature at a consistent timing.

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