I want to build my own heated bed.Need help with the math for the heater.

Hi everybody,

So as the title says, I want to try my hand at building a heated bed out of a tooling plate 6 mm thick and a custom made heater, from self adhesive copper tape (3-4 mm wide) between two sheets of kapton foil. Basically, something looking like these commercial versions :


The heatbed will be around 350 x 350 mm in size.

I was planning on using a high power 12 volts supply, up to 60-70 Amps if needed, but now comes the tricky part: calculating the length and width of the copper traces to get the correct heat dissipation and power needed. The copper foil has a thickness of around 0.65 - 0.7 mm.

I played with this tool to get some values, but I'm not sure they are correct for my case:
29 Amps with a temp rise of 100°C, Cu thickness of 2.6 mil, conductor length 450 cm, I need to use a 4mm wide external trace, with a voltage drop of 11.76 Volt and 341 Watts of power loss.
I mean... Isn't that a little bit too under powered ? Or because of the heat transfer to the copper plate, I should double the amperage without damaging the copper trace ?

Can someone with a little bit more knowledge help me out on this ?

Thanks !

edit: found another tool and tried other values, looks a little bit better, with 200°C temp rise, but with 6 mm wide trace: [mustcalculate.com]

Edited 1 time(s). Last edit at 07/06/2016 07:47AM by unromeo21.

Hi!

Very interesting project.

I'm not sure about using self adhesive tape. You may have problem with adhesive and smoke at 100°C.

Regarding the heatbed, is there a reason why you want to get such high power from 12V? Why not building something from AC instead of DC?

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DYZE DESIGN
Hotends, Extruders, Liquid Cooling and Accessories.

Aim for a power density of about 0.4W or 0.45W per square cm. The usual choice of heater for aluminium tooling plate is a silicone heater. These can be ordered in custom size and power ratings from China at relatively little cost.

---

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

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dc42
Aim for a power density of about 0.4W or 0.45W per square cm. The usual choice of heater for aluminium tooling plate is a silicone heater. These can be ordered in custom size and power ratings from China at relatively little cost.

Yeah, I am aware of those silicone heaters, I'm just aiming at sourcing locally or doing it myself, you know, learning by doing. I'm just not that good with the math. So based on what you are saying, the values that I get from the last link I posted should work just fine.. It gives me a total of 514W power loss. I'm just not sure if this is the right way of calculating it.

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Dyze_Design
I'm not sure about using self adhesive tape. You may have problem with adhesive and smoke at 100°C.

Regarding the heatbed, is there a reason why you want to get such high power from 12V? Why not building something from AC instead of DC?

That adhesive is for high temperatures. Kapton tape is also self adhesive and it doesn't smoke when heated to over 200°, does it ? And at the bed temperatures of 80- 110 °C max, this should be no issue.
12V is because I am trying to stay away from high voltage inside the build chamber. and a used server power supply that can deliver the rated amperage is really cheap - 15-20 € or so.

Quote
Dyze_Design
Regarding the heatbed, is there a reason why you want to get such high power from 12V? Why not building something from AC instead of DC?

I would agree with Dyze; AC is useful and with a well made ground wire and gfci plug you "Should" be fine.

35cm x 35cm = 1225cm²
1225cm² x .5W per cm² = 613W

613W / 12V = 51A (DC)

613W / 120V = 5.1A (AC)

I am going to change my plan, decided to make a 220V heater after all. Switching a 12V at 55Amps is not an easy task. I do have to proper MOSFETs but for such powers, it's better to go with AC and SSR. Just have to be very careful regarding the insulation. I found flat Nichrome wires of 0.8mm wide and 0.08mm thick. They are used for electronic cigars (ebay). 17 Ohm per meter. This would be perfect to sandwich between sheets of Kapton tape. I am aiming for 700W of power on a 350x350mm surface. Cost should be really low, less than 20€ for the whole heater.

I will keep you updated on the building details and success/failure, if some of you are interested in this. I know this has been done before, but not with flat wire.

Good luck! make sure to use a gfci plug, a ground wire, and a fuse / circuit breaker.