12 x 12 inch heated bed, how many watts?

Hello Everyone,

I'm trying to find out the optimal wattage for a 12 inch square heated bed, or about 30 x 30 cm.

To keep things simple(otherwise I will get confused, no one wants that!) my 20 x 20 cm Prusa PCB gets about 12 volts, and has 1.2 ohms resistance, and therefore needs 10 amps to put out 120 watts of heat.

In the real world, that means about 120°C and I can print ABS on 2mm glass.

So assuming the bed I want has 2.25 times the area, it will need about 270 watts. If we add a small buffer we could bump this up to 300 watts, a nice even number.

If I go to 24v, it would require about 12.5 amps, and have a resistance of 1.92 ohms.

Is this matching up with anyone elses numbers?

BTW, I'm hoping to buy one of this silicon rubber heat mats with the help of PR Iron Man on the forum here, so if anyone else wants to get a hold of one of these heaters chime in!

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Yvan

Singularity Machine

TAZ printer also has a 30x30cm printing surface. Some people are using this heater: [store.qu-bd.com]

If it were me, I'd go with a 120v silicone or kapton heater. All you need is an SSR, and depending on the amps you're pumping through it an SSR heatsink. Tops $20.

The advantages of stepping up to 120V: More efficient. Larger range of wattages available. Faster heating.

Disadvantages of 120V: More dangerous. More expensive.

I've got a 10" x 10" 1000W 120VAC silicone heater adhered attached to the underside of a very thin (0.20") 12"x12" aluminum heat spreader. 2.5mm mirror on top of that. Heats up to 100C in about 90 seconds.

Gone and done this to both my machines. Can't imagine ever going back to a low voltage heated bed.

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- akhlut

Just remember - Iterate, Iterate, Iterate!

[myhomelessmind.blogspot.com]

surface area in inches 12*12/8*8

144/64=2.25 times the surface area. to heat the same will require 2.25 times the energy.

edit- confirming the above post with the question does this seem about right.

Edited 1 time(s). Last edit at 08/15/2013 09:00PM by jamesdanielv.

jamesdanielv has the right idea. However, like I mentioned in our other thread I would bump up the power a bit more as I find that Prusa PCB heat beds have a hard time maintaining high temps during printing.

I was going to order a few custom heaters for Yvan and a few others as the Qu-Bd store has been out of stock for quite a while and I think the Qu-Bd heaters are a little underpowered anyway for printing Nylon.

I also agree with akhlut, 120V heaters are definitely the way to go but I have never bothered as it is more trouble to set up and was unnecessary for my 200mmx200mm bed sizes. I would consider this for larger bed sizes though.

Eric

My print bed is approximately 12x12 inches, and I use a silicone heating mat from McMaster ( [www.mcmaster.com] ). It's 360 W at 120 V. In hindsight I might have ordered the 720 W version for faster bed heating time. The 360 W one with 3 mm aluminum bed and glass top surface takes about 5 minutes to reach 110 C.

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misan, yes that heater would work, but it seems it is just barely enough to do the job? They had been out of stock for a while.

akhlut, good time comparison info. Fast heat-up time is good. Mine takes way more than 90 seconds!

James, I used the same numbers, so extrapolating from the Prusa at 120 W that give me 270 W minimum as a starting point. Bare minimum though.

New Perfection, Unfortunately McMaster doesn't ship to Canada! 5 minutes is not bad, it takes mine about three times as much.

Eric, how many volts are the heaters you can get? If the minimum is about 300 watts, and 360 watts takes about 5 minutes to heat up, anything higher than that would be great.

It seems nylon does not need a huge amount of heat, maybe only about 40°C, as long as the filament is nice and dry. So far it seems ABS needs the hottest heated bed.

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Yvan

Singularity Machine

Yvan, I can get the heaters made with any voltage and any power rating. Based on Cameron's experience with a 360W heater, it looks like 400W would be perfect.

Also, I imagine that Polycarbonate needs the highest bed temps seeing as it has a very high glass transition temp (about 150C).

Eric

whatever method you use, be sure to have a thermal cutout or thermal fuse. especially because the reason for the larger bed is likely for larger and longer printing times, probably 24hrs or greater.

That is a very good suggestion! I had been wondering about that myself.

What would be a good way of implementing some kind of thermal shutoff?

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Yvan

Singularity Machine

one built into the heated bed assembly. so if software glitch and sensory glitch the heater would loose power, and then prevent a fire. thermal cutouts for water heaters that can be reset are usable for example.