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1mx1m hot plate?

Posted by rowow 
1mx1m hot plate?
October 05, 2015 08:25PM
Hello, Currently I am making a 1000m+ cube 3d printer. I already have all the mechanics planned out (currently all of the material will ship in in the next few weeks which costed under 400$) but the only issue I cant wrap my head around is the heat bed. I want to print is abs and later homemade 3d printer filament/PETE/PET which warps as much as ABS,
*A head bed which can go 120C-150C
*Be very large 1m x 1m
*Be relatively cheap, under $100
Currently the only thing I can think of is nichrome wire, but does give alot of heat? I can get some 36awg wire 100ft for 5$, but is 36awg thick enough?
Thank you, this is my final struggle in my DIY 3d printing, once I get this dealt with I will start recording and building the printer.

*EDIT* I also forgot to add that the heat plate will be stationary, so you can go for the heaviest solution possible, I dont care, just as long as it's cheap and works.

Edited 1 time(s). Last edit at 10/06/2015 05:22AM by rowow.
Re: 1mx1m hot plate?
October 06, 2015 03:41AM
What´s the specific resistance per meter/feet of this nichrome wire?
Such a large bed would best be heated with 3 phase mains. With option to heat only one phase/area of the bed for smaller parts.
A cast aluminum plate split in several sections as independent heat spreader, covered with PEI to get a homogeny surface.
100$ for the bed is a very low limbo.....

Edited 1 time(s). Last edit at 10/06/2015 03:44AM by o_lampe.
Re: 1mx1m hot plate?
October 06, 2015 04:19AM
IMO there is no way you can build a large printer successfully for a low price. To heat a 1m x 1m plate, you will need a mains voltage heater with around 3kW of power (which is close to the limit of a mains plug and socket here in the UK). There is no way you should be messing around with nichrome wire at AC mains voltages.

My suggestion is that you buy a cast aluminium tool plate, attach one or more mains-voltage silicone heaters to the underside, and put cork insulation below that. If you can't get a single tool plate of that size, use 4 sections instead as Olaf suggests. These people [alirubber.en.alibaba.com] will make a silicone heater to your size, voltage and power specifications at a good price. Use a heater or heater(s) that are a few cm smaller than the plate so that you can support the plate at the edges. For a 1m x 1m plate, I guess you will need support at the centre too.

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].
Re: 1mx1m hot plate?
October 06, 2015 05:10AM
26/ohms/ft is the resistance, and yes I was thinking I need first a sheet of aluminum plate as a heat spreader, which I can do but anyone has any better ideas and and tips I don't know.
Re: 1mx1m hot plate?
October 06, 2015 05:21AM
Like I said I already have all the material for under 400$. Only thing I need is to make a design for the heat bed, and if I really wanted to I could go ahead and use nichrome wire and a glass sheet which will cost 50$ and produce enough heat to print PLA. But like I said, I dont want to.

Also 400$ is a little to expensive because that this includes a
dual extruder, LCD screen,
50a power supply,
8mm acme lead screw rod designed for shaft movement,
aluminum antibacklash nuts,
j head extruder with cooling,
aluminum heatsink and fan for each and every stepper motor (total of 7)
aluminum extended heatsink with fan for stepper motor controller

All of which I can go without and save 120-150$
Re: 1mx1m hot plate?
October 06, 2015 05:23AM
I also forgot to add that the heat plate will be stationary, so any heavy solutions will work.
Re: 1mx1m hot plate?
October 07, 2015 05:35PM
Re: 1mx1m hot plate?
October 07, 2015 06:09PM
You can print ABS on a room temperature piece of polyisocyanurate (PIR) foam. It is fire resistant, and prints stick very well. No power supply, no heater, no SSR, and no bed leveling system are needed. The extruder nozzle is buried about 1mm or so into the foam for the first layer, and you print on a raft. The foam is readily available and cheap- about $0.50 per sqft in the US for a 1" thick piece that can be printed on multiple times.

Stratasys has been printing on sacrificial foam beds for years.

Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: 1mx1m hot plate?
October 07, 2015 06:13PM
As I stated in the post, I also want to print plastics such as PET/etc. Something that require 120c+, and even still I have invested a good amount of money, and I don't quite want to make the final component so cheap. Plus I am planning to make a thermal cover in order to prevent cracking.
Re: 1mx1m hot plate?
October 07, 2015 07:11PM
The 120C bed temperature used for PET is there to get the print to stick to whatever the bed surface is. ABS is normally printed on a 105C bed. It doesn't matter. I have only tested ABS but I would bet that just about any plastic you can squirt out of an extruder will stick to PIR foam because it forms a mechanical attachment where the molten plastic flows into tiny voids in the foam. Of course, if you're going to print ABS you want an enclosed and heated build chamber. That doesn't mean you have to heat the bed- the stratasys machines are set up exactly that way- unheated foam bed, heated chamber (70C).

Here's some video of test prints I did on PIR about two years ago:




You can test it easily with any plastic- just get a piece of PIR foam, tape it to your printer's bed, and use the Z offset in slicer to start printing about 1mm into the surface of the foam.

Here is a part I printed directly on the foam without a raft:

and here's what the bottom side looks like after breaking it free of the foam:

As you can see, some of the foam has remained embedded in the part, clear evidence of a mechanical bond, which is why I suggest printing on a raft.

Edited 1 time(s). Last edit at 10/07/2015 07:13PM by the_digital_dentist.

Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: 1mx1m hot plate?
October 07, 2015 07:19PM
I agree its a cheap and easy solution, but too cheap, and I later want to add insulation to prevent cracking, so aka having a case.
Re: 1mx1m hot plate?
October 08, 2015 02:41AM
Don´t know if there´s a big difference between PET and PETG, but I print PETG on a cold bed masked with painter tape.
Re: 1mx1m hot plate?
October 08, 2015 10:50PM
What is your plan to maintain an accurate distance between the big bed and the nozzle in 0.1mm for the first layer? I am designing a printer too but not that big. I will do my best to mechanically get everything squared and flat. But still plan to use grid z levelling to make sure the print can start successfully. It is very hard to get the distance between nozzle and bed in 0.1mm for the whole 1mx1m area.

Edited 1 time(s). Last edit at 10/08/2015 10:51PM by bonmotwang.

Printer I bought: 2015 Sunhokey Prusa i3
Printer I am designing: Another big CoreXY machine
Re: 1mx1m hot plate?
October 09, 2015 05:27AM
Re: 1mx1m hot plate?
October 09, 2015 05:44AM
Auto bed Levelling is one of 3d printings biggest misnomers.

You cannot correct for a warped or none flat bed with this.

All it does its average out the points of measurement and define a plain based on that

Any low points, will still be low.
Any high points will still be high.

Worse than that if you happen to measure at a high or a low point, without enough other data points, the entire plain will be wrong.

If your flat bed is on an angle, you can correct for that. But you should really just level it for real, or you just end up wearing out your Z threaded rods. (Presuming its a Cartesian)

Edited 3 time(s). Last edit at 10/09/2015 05:57AM by Dust.
Re: 1mx1m hot plate?
October 09, 2015 06:14AM
Thanks Dust! I thought it is doing much more complicated calculation to "follow" the bed surface.
Now I understand. I always wondered how it is been done.

Well a little disappointed. But then the next question is for the thread owner, how is it possible to get the first layer reliably stick to the bed? Because the 1mx1m bed needs to be flat and the XY plane of the nozzle needs to be flat. Understood the tolerance of the final print doesn't need to be in 0.2mm, but the two big "flat" planes I mentioned do need to be palellel and in that tolerance for the first layer to start.

PS. I have done the flat work surface for desk top CNC, so I understand 300x300 is absolutely doable, but 1000x1000 requires a lot of $$$ or I am missing something here.

Edited 1 time(s). Last edit at 10/09/2015 06:16AM by bonmotwang.

Printer I bought: 2015 Sunhokey Prusa i3
Printer I am designing: Another big CoreXY machine
Re: 1mx1m hot plate?
October 09, 2015 09:21AM
I agree its a cheap and easy solution, but too cheap, and I later want to add insulation to prevent cracking, so aka having a case.

Using a foam bed would not prevent adding an enclosure to the printer. In fact, you could use the same PIR foam material to insulate the walls of the enclosure. See: [www.thingiverse.com]

As has been pointed out, keeping a 1m square surface flat, level, and hot is going to be difficult/expensive. OTOH, with such a big bed you're most likely planning on printing in 1mm or even thicker layers to keep print times reasonable, so maybe maintaining flatness isn't such a huge problem. A metal plate that size is definitely going to need a heavy duty support structure and leveling system. Heating a bed that size to print temperatures will require relatively costly heater(s). That's where a sacrificial foam bed can save a lot of money. It won't need the heavy support structure, leveling system, or the heaters. Just a relatively light weight surface to support the foam. A 1 m square piece of the foam is about $9 US, a mere drop in the bucket compared to the cost of filament that will go into a really huge print. If you're printing smaller objects you can use a piece of foam that is cut to fit the base of the print before printing and keep the foam cost quite low for most prints (how often are you really going to print a 1m square base model?).

If you want to heat the enclosure, you can pick up a small room heater for about $10 (http://www.walmart.com/ip/Pelonis-Fan-Forced-Heater-with-Thermostat/21804031) that will easily heat a 1m^3 volume to the 45-50C that ABS prints require to prevent delamination. Such heaters are a LOT cheaper than the sort of thing that sticks to a metal plate. You can use the unused bed temperature sensor input in the printer's controller board to regulate the enclosure temperature by switching the enclosure heater on and off with an SSR just as you would a bed heater.

Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: 1mx1m hot plate?
October 09, 2015 12:27PM
I think you missed the memo.
Cheap, effective, and doesn't look like sh*t. And I also think you missed the fact that the bed will be stationary, so if I really need to I will go with nichrome wire under a sheet of aluminum, under a sheet of glass I will have no problem with weight even if its 100lbs. I already found a glass sheet for 50$, and I already have some scrap aluminum, so I could just do that.
And not only that but I can get some strips of silicone pad heaters as Jim has done
For under 100$ which will fit my budget of $150

And I have no clue why flatness is an issue with large printers, for materials such as aluminum I understand but glass?... only ones that I heard complain about flatness are the ones in this post, and I guarantee you a foam sheet wont be flatter then glass.

I expected some help from this community, and all I got was inexperience and misinformation. If anyone that has a better solution then the two I listed, or has some tips/tricks, then PLEASE post, because I am losing hope in this community.
Re: 1mx1m hot plate?
October 09, 2015 01:30PM
Hello rowow

The flatness is something cost money to achieve.
What is the thickness of your glass? Then find a deflection calculator to see what type of the support you will need. Glass is made by float technologies so it is flat but also flex.
If you need support, then the support needs to be flat, just one thing leads to another.
From the calculator you can judge by yourself if it is good enough.
If you want to use aluminum extrusion, again, use the 8020 defection calculator and judge from there.
Also the flatness and straightness of your rail system is very hard to be controlled in 0.2mm accuracy for a 1m x 1m range.

Remember, what you are doing is 9 times or 16 times bigger than most of the printers here.

Don't expect people here tell you if it will work or not as a conclusion. People here are helping you to solve your problem and giving you warnings and direct you to the right directions.

You didn't understand Mark's foam solution. Foam doesn't need to be flat. The nozzle is into the foam by about 1mm or 2. So the first layer will follow the nozzle moving plane and stay there. Somewhere the nozzle will be in by 2mm, somewhere the nozzle will be in by 0.2mm, as long as they are contacting the foam, the plastic will stay. Google will give you better explanation.

We are all encouraging you to proceed, and would like to see a decent solution for 1m x 1m printing, hopefully low cost too.

Printer I bought: 2015 Sunhokey Prusa i3
Printer I am designing: Another big CoreXY machine
Re: 1mx1m hot plate?
October 09, 2015 02:06PM
It makes no difference if the bed is stationary or moving. Maintaining flatness is an issue in a large bed because all materials are flexible and all will sag under their own weight. If, for example, you use a 1/8" sheet of aluminum that is 1m square, you can count on the center to sag unless you provide a support system for it. Even glass will sag. You haven't said anything about the print layer thickness you're planning to use, but a 1m square piece of almost anything that isn't very thick will sag a few hundred microns under its own weight. You also have not said anything about the frame design/structure/materials, but they too will sag under their own weight and the weight of other parts of the printer bolted to them. What you thought will be straight or flat will not be unless you've specifically designed things to minimize the effect. All the sagging is less important if you're going to print in 1mm layers. The sagging and leveling is a real problem if you're planning to print in 100 um layers. I hope you aren't trying to use 8mm end-supported guide rails for this project!

Forgetting about the bed sagging for a moment, if your bed weighs 100 lbs you're going to need a very strong support system for it and the bed's weight will most likely distort the printer's frame unless you make that extra rigid, too. All that extra rigidity costs $.

A printer's bed surface normally has to be very flat, smooth, and level so that it can be aligned with the true XY plane which is defined by the printer's guide rails. The whole purpose of aligning the bed surface with the true XY plane is to ensure that the first layer of the print will stick to the bed. The reason PIR foam doesn't require a heavy support system is because it is low in mass. The reason it doesn't have to be particularly flat is because you're not printing on its surface. The reason it doesn't have to be particularly level is because the extruder nozzle will follow the true XY plane in the foam and create a perfectly aligned first print layer. The reason it doesn't have to be heated is because the molten plastic forms a mechanical bond with the foam by filling in tiny voids throughout the material.

The reason most people have glass on their print beds is because it is relatively cheap and provides a flatter surface than the thin sheet of aluminum (on a poorly designed "leveling" system) usually provided by printer kit makers. Glass is actually a poor surface to use because it is a thermal insulator and will have large temperature variations over its surface. The thicker the glass sheet is the more variation in surface temperature you can expect to see (as well as longer warm-up times due to the increased mass). The variation in surface temperature is a problem because high temperatures are necessary to ensure that the entire first layer of the print sticks to the bed. Cast aluminum tooling plate is a much better choice than glass because it is flat and thermally conductive which minimizes variations in the surface temperature. A 1m square piece of cast tooling plate in any thickness is pretty expensive. It, like everything else, will sag under its own weight.

You haven't asked any questions after your original post, just demonstrated through your comments that you don't understand what others have been posting. Everyone who has posted here has tried to be helpful, and no one has provided any misinformation. If you've already decided what you're going to do and why you're going to do it, why ask for input? If you receive the input you asked for, why be offended? Multiple people tried to help you and your response was not to thank people for their input but to state how disappointed you are in their responses. THAT is the real disappointment here.

In your original post you emphasized the need for cheapness. I can't imagine a cheaper, more easily tested way to make a 1 m^2 print bed that prints are guaranteed to stick to than to use an unheated piece of PIR foam, but you apparently know a better way. I wish you great success and look forward to seeing pictures of your completed machine and its prints.

Edited 6 time(s). Last edit at 10/09/2015 04:06PM by the_digital_dentist.

Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: 1mx1m hot plate?
October 09, 2015 04:21PM
The thickness is 1/4 if im correct, im at my laptop currently and later today I will send the thickness.
And my general question here is a cheap heating element, not a solution to flatness. This is why I mentioned that the bed will be stationary, which is a VERY important factor because that I don't need as a complex support as I would need if I had a moving bed.

So I have no issues with having a flat bed, glass is flat enough for a 3d printer with a very thick first layer.
The only issue I have is a heating solution. I was thinking nichrome but have no experience which is what the post is for, tips or unless someone has a better idea?

Am I missing something?
Re: 1mx1m hot plate?
October 09, 2015 08:00PM
Assuming you have some means by which to attach the nichrome wire to an aluminum plate and you have some means by which to keep the aluminum/glass flat enough to print on, let's run some numbers:

Your wire's resistance is 26 Ohms per foot and you have 100 feet of it. That's a total of 2600 Ohms. My printer has a 450 watt heater for a 1 sq ft plate, so let's say you have about 9 sq ft of bed area. That means you'll need somewhere around 9 x 450 = 4050 Watts.

Assuming you have an AC power source than can deliver that much power without blowing a circuit breaker, let's calculate the voltage and current required to dissipate 4 kW in your 2600 Ohm resistor:

Power = E^2 / R
4050W = E^2 / 2600 Ohms
E^2 = 4050 x 2600 =10,530,000
E= 3245 Volts

I = E/R
I= 3245/2600 = 1.25A

So you need a transformer than will deliver 3245 Volts @ 1.25A in order to dissipate 4 kW in your bed heater using the wire you selected because it was cheap and it was heater wire, and well, you know...
Here are some 5 kVA transformers (5 kVA transformers are more common and threfore cheaper) that have sufficient power rating to deliver the 4 kW that your bed needs to get up to print temperature: [www.ebay.com] however, none of them can deliver 3245V.

Normal 36 ga wire is limited to 0.035A, but since this is a heater, a little more current is probably OK, but not 1.25A

Now, lets say you decide OK, I'm going to get a big transformer to run the heater because well, a 4 kW heater in the printer is definitely better than using a cheap, light weight, readily available, unheated piece of foam to achieve the same result, and it will look so much better! Those transfomers seem to have some common voltage ratings- 240V and 480V show up a lot. Let's say we go with 480V. In order to deliver 4 kW using 480V:

4050W = 480V ^2 / R
R=480 V ^2 / 4050 W = 230,400 / 4050 = 56.9 Ohms. This is the bed heater resistance needed to dissipate 4 kW at 480V.

I=480 V / 56.9 Ohms = 8.436 A This is the current in the bed heater.

I hope you have a VERY safe way to connect this to your printer!

[www.heatersplus.com] says that 16 gauge nichrome wire will get up to 316C at 8.3A in free air, so it's probably about right when coupled to the aluminum heat spreader in your printer (and still assuming you have an electrical circuit in your house where you can draw 4 kW without blowing a circuit breaker).

16 gauge nichrome 60 wire has 0.2595 Ohms per foot resistance at room temp. You will therefore need about 56.9/0.2595 = 219 feet of it to achieve the required resistance. At $80 per 100 ft you're looking at about $175 for the nichrome wire for your heater. Add at least $100 for the transformer to power it and $50 for an SSR to switch power into it and you're up to $325 without the cost of the glass and aluminum and its support structure. Didn't you say you were going to build the whole machine for $400? I don't think it's going to be enough...

You haven't done even these basic calculations, but on the bright side, what you lack in knowledge you make up for in self-esteem.

I suggest that before you spend $400 on a bunch of parts to make a big printer, you study a LOT more and ask a LOT more questions, and try being polite about it.

You're welcome.

Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: 1mx1m hot plate?
October 09, 2015 08:49PM
Actually you miscalculated alot.
1: I wont be using the whole 100ft roll, somewhere around 70 will do
*2~ inch spaced apart
*1m = 40 inches
2/40 = 20
20 x 40 = 800
800/12 = 66.6

2: Are you serious? I guess you googled up your equations because anyone with ANY basics in electrical engineering knows a VERY simple solution to your issue. Instead of having 1 strait run it would be MUCH more efficient to split it into sections to run off 12v/etc to solve your transformer problem...
(But I wont have too for the next point)

3: Are you REALLY SERIOUS? Now I am guessing you have elementary education for the fact that your calculations are for the maximum power the nichrome can take, aka highest temperature, aka thousands of degrees... I am not melting aluminum here, I only need 150c maximum which is a joke for nichrome wire.
And instead of doing the calculations myself (which there is a quite few of) I'd rather use this website to do the work for me.
154 C
71 inches
32 gage
36 volts .6 amps 21 watts

And NOT 3245 volts at 1.25 amps like you said.

I can keep on going on how your previous comments are as much of a disappointment to the community at this one but why bother. And when I was saying "I was losing hope to this community" I meant SPECIFICALLY at you and NO ONE else. I just didnt realize all of the comments I was laughing at was from one person. Please, I would REALLY not like your help, its a waste of my time more then anything.
Re: 1mx1m hot plate?
October 09, 2015 08:50PM
And I would like to state that thank you bonmotwang, from everyone here at this post you have helped me the most. But I would still like to keep the post open for any more tips.
Re: 1mx1m hot plate?
October 10, 2015 12:10AM
Final attempt, I promise...

Here is how you should approach this problem: "I need to heat a massive piece of metal and glass, 1m^2, to 150C in a reasonable amount of time. How much power will it take to do so?"
I used my bed heater as an example- 450W applied to a 1 sq ft x 1/4" aluminum plate heats it to 105C ABS print temperature in about 5 minutes. If you want to heat a plate that is about 9-10x larger to the same temperature in about the same amount of time, it is reasonable to say (for a first order approximation) that you will need 9-10X the power, therefore about 4 kW.

It will take 4 kW whether it comes from a 12V power supply or a 10000V transformer. Where and how you get the power doesn't matter- it takes the same amount of power input to heat the plate so the power source has to be able to deliver that amount of power, in this case, 4 kW.

There are two pieces to this problem- where do you get 4 kW and how do you make a resistor that converts that 4 kW into heat?

Making the resistor: you want to use nichrome wire. That's fine, the stuff is used to make heaters all the time. In my previous post I showed that your 36 gauge wire would not be adequate- it will melt like a fuse if you try to shove 4 kW into it (which would require a 3200V power supply). I then showed you that 16 gauge nichrome wire would be up to the task (based on the wire specs) if you used 219 feet of it with a 480V transformer.

Let's talk about that power source for a moment. You said it would be better to divide up the resistance into a bunch of parallel resistances so you could use a 12V power supply. While you could do that, you still need 4 kW into the wire/plate to get the necessary heat. A 12V power supply that can deliver 4 kW would have to supply over 300 amps. I don't know where you would find such a power supply but it would be very expensive. A 480V transformer would be a lot cheaper.

Whatever power supply or transformer you use to deliver 4 kW to the heater will need 4 kW+ coming in from the power line. You won't get 4 kW from any normal household electrical circuit. They are typically fused at about 2-3 kW.

So the answer to your original question about a cheap heater is this: there is no cheap way to electrically heat such a large, massive bed plate to 150C in any reasonable amount of time. The faster you want it to heat up or the higher temperature you want to achieve, the more power input you need. It's that simple.

In my original post I skipped all the calculations and suggested the foam bed because I knew from experience that it would be impossible to make a dirt cheap heater for your plate. The foam bed is still your best bet for making this thing work for a low cost.

If you do manage to get a 4 kW heater working the next problem you'll have to deal with is the print chamber overheating- that much heat in a 1 m^3 enclosure will cause its temperature to soar out of control very quickly.

If you doubt the legitimacy of my calculations you can easily verify them- I used nothing more than Ohm's law and provided a link to a site that explains things in simple language.

I know that personal attacks are one of the standard coping mechanisms for folks who are "willfully ignorant". If you want to attack my qualifications in this matter you should know that I was an electrical engineer for 22 years before I went back to school and became a dentist. I have a modicum of experience in the subject of 3D printer design and construction- see the link in my sig below.

Edited 3 time(s). Last edit at 10/10/2015 09:39AM by the_digital_dentist.

Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: 1mx1m hot plate?
October 10, 2015 03:43AM
Hmm, my heated bed (200x200mm ) has 100W and takes forever to heat up to 100°C...
It must have missed reading this "Jacobs-online.biz" page.... eye rolling smiley

Maybe you can heat up the wire to 154C with 36V/21W, but not the aluminum sheet or the glass.

I´m out
Re: 1mx1m hot plate?
October 10, 2015 10:11AM
I dont need 154c, I need 100c, which is why I added the extra 50c to compensate for the thermal lose in the aluminum/glass. And its much more easier to show an item in practice rather then having to explain it.
The same exact post I sent earlier in which another guy was making a 1mx1mx1m 3d printer and only used silicone heat pads, (8 total) for 480w, UNDERNEATH GLASS AND ALUMINUM
So like I said, I wont need to pump these 1500c like you want to and your calculations show, the most that I need would be 300c for faster preheats. But otherwise this set up would be using less then half a kilowatt for warm up, and 50watts for keeping its temperature.
Re: 1mx1m hot plate?
October 10, 2015 10:13AM
Olaf, your heat bed is most likely a pcb heat bed in which its maximum temperature is 150c or so. Copper also has a much lower resistance then nichrome and therefore heats up much slower.
Re: 1mx1m hot plate?
October 10, 2015 10:58AM
I give up. Good luck.

Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: 1mx1m hot plate?
October 10, 2015 11:19AM
I agree with what digital dentist has said, except that I am not convinced that you need as much as 4KW. There are two considerations:

1. Power needed to bring the thermal mass of the bed to 100C in a reasonable amount of time. This depends on how thick the bed is. The specific heat capacity of aluminium is 0.91 kJ/kgK and the density is 2700kG/m^3. From this you can work out the kW minutes needed to raise the bed temperature. For example, if the bed is 5mm thick then the mass is 2700 * 1 * 1 * 0.005 = 13.5kg and to raise its temperature by 80C you need 13.5 * 0.91 * 80 kWsec = 983kWsec or about 16Kwmin. So a 2kW heater would raise the bed to 100C in 8min if it were perfectly insulated (ignoring the thermal mass of the heater).

[EDIT: corrected the above calculation]

2. Power needed to counter heat loss. This does not scale up linearly with bed size, because the ratio of length of the periphery to area decreases with increasing size. So convection does not scale linearly, especially if you can shield the sides of the bed from draughts (which is easy of the bed is fixed).

I have a 350W heater and a 330mm diameter by 5mm thick bed. The heating power is more than adequate, and if left on full power it will reach nearly 200C. I reckon I could manage with 250W (or even 200W at a push). Even if we scale that up linearly with bed area, that's 2.5 to 3kW, not 4kW. Here in the UK, you can draw 3kW from a single mains outlet - meaning a 1m square heated bed powered from a normal mains outlet is feasible.

Of course, if you use a thicker bed plate then it will take longer to heat up with the same power - but the ultimate temperature reachable would not change.

However, I still think it is a bad idea to mess around with nichrome wire and mains voltage, when you can get silicone heating pads.

Edited 2 time(s). Last edit at 10/11/2015 10:47AM by dc42.

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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