Enclosure math- why they're so important and how to make one.
December 22, 2014 09:14PM
Hi guys,

I've been think a lot about print quality and exactly what sets repraps back from the pros (I'm not talking about all of these little startups, most of you have them beat) but companies like Stratasys. I think there is a very good reason that commercial printers are all enclosed and actively heated.


I think one of the main RepRap setbacks is trying to account for an absurd amount of thermal contraction. Most are limited to ABS and PLA, while other plastics like Nylon and Delrin (acetal) just shrink too damn much for repraps. Currently ABS and PLA are the only plastics that are really sold everywhere, but if everyone in the community could suddenly print acetal easily I'm sure it would be everywhere in weeks. I think we're limiting our community's potential by not giving heated chambers more attention. I've seen people with them, but as I realized the other day most enclosures usually aren't enough. Hear me out:



Let's assume the total surface area of an enclosure is about a square meter. That's a five-faced cube with ~450mm sides, and we're going to assume the table or whatever it's on isn't giving us any heat loss. I should probably account for a few more things, and my system is a little too ideal but this is just to illustrate a concept.

So now let's use 6mm polycarbonate. It seems to be a pretty simple, clear solution to a bed. at 1 m^2 and .19 W/mK for polycarbonate, and a temp difference of 60C (Room temp of about 20, goal is about 80) That's 1900 watts. This exact number doesn't mean a whole lot because of the crude assumptions, but consider that a 12V heated bed pushing 10 amps is only giving your system 120 watts. Not even one-tenth.

In reality you are helping to stall the air around your print which helps, but with a moving print head there are still lots of air currents going on, and at that point all your chamber is doing is preventing against a draft or two and raising the ambient temperature by maybe 10C.

Let's try another example- 1" insulation foam. It comes a lot thicker, but 1" is still very lightweight and easy to work with. I know it's hard to get in some places but here in MN it's cheap as hell and you can get it anywhere (it's pretty necessary) Their site lists the thermal conductivity at only .029 W/mK, and rounding to 25mm thick puts us at like 70 watts. That's it! Move up to the 2" stuff and you're at like 35 watts which is crazy low. Your heated bed would provide all the power you needed to keep the enclosure nice and warm.

But this foam is ugly, thick, hard to find for some, not clear, not healthy, etc. etc.

I had another idea I would like to do the math for: What about two very thin sheets of plastic with an air pocket in between them? The air in the middle is so stagnant that it can mostly be modeled as a solid as far as conductivity goes. This is how windows are made- I have a 6' square bay window, and if it was solid glass there's no way I could heat my house when it gets down to -20 every year.

So let's try this: 5 mil polycarbonate sheets with 1" of air between them: at .024W/mK that's only 60 watts. I'm not even going to add in the polycarbonate. Obviously it's not going to function exactly like that, but it's definitely in the right range.




In conclusion:

Your heated bed puts about 110W into your enclosure. Here's very roughly how much heat is lost to a 20C room through a 1m2 area enclosure:

6mm polycarbonate: 2000 W
1" Insulation Foam: 70 W
Thin plastic, 1" Air Gap: 60 W

To achieve the same insulation as the air or foam, you would need to make a solid polycarbonate enclosure 20 CM THICK. To do the same out of cork (common bed insulator) it would still need to be 60 mm thick!

I'm about to do a series of test prints with both kinds of enclosures. I figure the foam one can be done for under $10 and the plastic one even cheaper. I'm guessing in reality I might have to do 3 plastic layers or a few inches of spacing for the air pocket idea. We'll see.


Does anyone have any other ideas or comments? Let me know what you thinkThanks.
Re: Enclosure math- why they're so important and how to make one.
December 22, 2014 10:37PM
A 1" gap between thin sheets of plastic doesn't really work because the internal air currents between the sheets vastly increases the effective thermal conductivity. In twin pane windows, you usually only see about a 1/4" gap max. That's why some places actually use triple pane windows to create two small gaps.

Probably the best solution for printers would be to use twin wall or even triple wall polycarbonate sheets used for green houses. Maybe something like this.
Re: Enclosure math- why they're so important and how to make one.
December 23, 2014 02:20AM
Thanks for the input, that's what I was thinking. A key aspect of that enclosure is that it doesn't touch the printer so the walls don't act like a diaphragm from the vibration and such, but you're right a full inch will allow lots of convection. I was thinking of using like seran wrap or something stupid cheap and somehow printing corner brackets thst would allow me like 6 thin layers....although as soon as you lose visibility you're better off with foam. Hmmm.
Re: Enclosure math- why they're so important and how to make one.
December 23, 2014 02:57AM
For my next enclosure i want to use this

It is a polycarbonate material that is mostly hollow. Here in germany you can get 6-8mm thin plates of this stuff pretty cheap, it is sold to cover greenhouses.
Thermal insulation is pretty good and it is easy to cut.

Edited 1 time(s). Last edit at 12/23/2014 02:57AM by Srek.


[www.bonkers.de]
[merlin-hotend.de]
[www.hackerspace-ffm.de]
Re: Enclosure math- why they're so important and how to make one.
December 23, 2014 12:27PM
I was under the impression that stratsys had a patent on chambers, at least for cartisian printers. So while individuals could build their own, they couldn't be made commercially available via kits or complete printers and thus the market would never be saturated enough to justify other plastics.
Re: Enclosure math- why they're so important and how to make one.
December 23, 2014 02:08PM
I'm not a patent lawyer but it I would think getting a patent on a box would be tricky, if not impossible.

Sounds like 1" rigid foam would be cheapest and easiest. Maybe have one side as a double or triple pane window to keep tabs on how things are progressing.
Re: Enclosure math- why they're so important and how to make one.
December 23, 2014 03:38PM
I just made an enclosure from this material (well 5mm) because I had a sheet laying around from another project. Not only does it keep the heat in but you can still see what is going on though a little blurry looking. Have not got to fully test it yet but I plan to use a 40watt reptile heater inside as well (used it with an older printer and it worked well).
Re: Enclosure math- why they're so important and how to make one.
December 23, 2014 03:45PM
Here is mine
Attachments:
open | download - Enclosure.jpg (124.9 KB)
Re: Enclosure math- why they're so important and how to make one.
December 23, 2014 04:55PM
It looks good, but I'm still quite skeptical about maintaining a decent temperature with just the bed in those kind of enclosures, however an external heating element might get it a lot warmer.

If you're looking to maintain a decent temp though, I don't think that greenhouse stuff (at 5mm anyway) will work. If I were you I would find out the equivalent heat coefficient of that material, because even if it's like .05 W/mK or something like that you'll be wasting your money on a 40W heater...40W is really small. Remember, my math gave the IDEAL scenario so although it probably helps a lot, to hold 80C you'd need a hell of a low more. I think you can get regular sized heated bulbs in the 200-300 rage and if I were you I'd go for that with only a 5mm wall.

As far as the patent goes, you're right ryan_m, Stratasys has like 5 patents on filament spools so I don't know exactly where the enforcement line is. Check out all of their patents- it's absurd. [www.google.com]

What I'm proposing though is that we come up with something so easy that you don't really have to buy anything special for. No one would sell anything besides maybe the brackets because it would be absurd to- seran wrap, garbage bags, foam, etc. The reason I think this is possible is that I'd bet 99% of reprap owners who can fit a build chamber don't care if it's 5mm thick or 50 mm thick because space isn't usually that crucial.

Is getting insulation foam easy in Europe? How about Australia/NZ? I know there are some strange restrictions on it in California so I guess I just assumed it was scarce.
Re: Enclosure math- why they're so important and how to make one.
December 23, 2014 05:05PM
The heater I use is ceramic and it's made for reptiles, screws into a standard light socket and can be purchased in many different wattages. I choose the 40w for my old printer as a guess and I did not want to overheat anything as the electronics were also inside the chamber. I got the inside up to an average of 60c with the heated bed and this 40w heater. So yes if 80c is the target point I would need a higher wattage and my new chamber is quite a bit bigger as the printer is larger. I did buy some heatsinks to go onto my stepper motors though I am not sure if they would be needed at 80c.
Re: Enclosure math- why they're so important and how to make one.
December 23, 2014 06:05PM
A couple of things to consider:

Lexan panels are fairly pricey so you may want to reserve those for the door. I used double panels which I bent.

You may want to consider foam panels like these: R-matte Plus 3 has R3.2
Only about $13.00 from Home Depot here in the US for a 1/2" x 4' x 8' sheet, and can take reasonable heat. That's what I used.

They also have 3/4" available and possibly thicker (probably not needed).

I'm using a 200w bed and 100W in incandescent lights. The interior is about 60C and I have little warping. Testing is ongoing tho as my printer is new.


My printer: Raptosaur - Large Format Delta - [www.paulwanamaker.wordpress.com]
Can you answer questions about Calibration, Printing issues, Mechanics? Write it up and improve the Wiki!
Re: Enclosure math- why they're so important and how to make one.
December 23, 2014 06:32PM
tmorris oh I see, yeah good call with the electronics inside. I kept all my wires long (and messy) so I can put mine outside but I guess I should assume most people have their ramps trimmed pretty close to their printer somewhere.

Paul, yeah that's exactly what I had in mind. Here in Minnesota they regularly stock up to 2" (maybe not that R value but still) Have you gotten the chance to get an IR profile of your heat loss or anything like that? How'd you do the joints? I'd be interested to know where/when the most heat loss is coming from.
Re: Enclosure math- why they're so important and how to make one.
December 23, 2014 06:46PM
The enclosure for my printer is 3/4" MDO plywood, that in itself is joined tightly. I just cut the foam at a bevel to fit tightly together and tacked it to the interior with a tiny amount of urethane glue - minimal gaps if any. I used some aluminum tape to cover the foam edges around the inside of the door - Nashua 324A. That's good stuff, also from H.D.

Most of the heat is lost from the belt holes on the top. That's the only place I can feel heat at all, even touching the lexan. I will be printing some covers to block most of those openings on top.

My brother just got an IR camera, so we will be looking at it with that after Christmas.


My printer: Raptosaur - Large Format Delta - [www.paulwanamaker.wordpress.com]
Can you answer questions about Calibration, Printing issues, Mechanics? Write it up and improve the Wiki!
Re: Enclosure math- why they're so important and how to make one.
December 23, 2014 07:17PM
About the idea of double paneled casing, that is similar to a window-
Windows protect from heat loss because there is a vacum in between the panels (or it should be if they are made correctly)
..so no air current possible there.
Re: Enclosure math- why they're so important and how to make one.
December 23, 2014 09:32PM
Actually most of the time they fill them with Argon or Krypton. The vacuum idea is very new and not reliable yet.
Re: Enclosure math- why they're so important and how to make one.
December 24, 2014 05:33AM
I stand corrected, it's actually dried air and/or the gasses you mentioned. A total vacuum would ofc break the glass

Anyway, I am getting my enclosure done from lexan and we'll se how it goes. Especially when I install a hexagon hotend and try some high temp prints

Edited 1 time(s). Last edit at 12/24/2014 06:51AM by t0bb.
Re: Enclosure math- why they're so important and how to make one.
February 15, 2015 05:29AM
hello everyone
i don't have 3d printer yet, but i will buy one soon, and i'm already looking for an enclosure, and i found your thread. So, to answer one of your questions : yes we can easily find isolation foam here in France if this is what you meant
also i was wondering if using some reflective sheet (like this) could help maintening the warm inside (or maybe just alu sheet like the one we use for cooking)
also, i think it would be "safer" to include some charcoal filter somewhere in the enclosure to avoid bad smell when opening
and finally, what about some dessicant (like silicagel) inside to avoid 'vapor' due to temperature ?
Re: Enclosure math- why they're so important and how to make one.
February 15, 2015 05:47PM
Here's what I came up with: [www.thingiverse.com]
If you use 1/2" sheet, just scale the parts by 50% and print. If you don't need to take the enclosure apart and reassemble it just use tape to hold the pieces of foam or whatever you use for the walls and top together.

My printer has a 450 watt heater and keeps the enclosure very warm without any additional heat source. I keep the door propped open a little to keep it from getting too hot inside the box. You don't have to go nuts with insulation to keep things warm. If your bed heater is insufficient to keep the box warm enough it is very cheap to put in a few incandescent light bulbs that will heat and light the enclosure at the same time.
Re: Enclosure math- why they're so important and how to make one.
April 06, 2015 10:18PM
Here is mine:

The heated bed heats it up. I am running it now with PC filament at 60C inside using a bed set to 120C.




[www.matter-replicator.com]
Re: Enclosure math- why they're so important and how to make one.
April 07, 2015 06:14AM
I like the idea and even calming down the airflow would be positive, however my main concern would be hotend cooling. Which would require a either a big hose getting outside air onto the hotend and model cooling fans or a pressurized fan that sits outside and blows in air though a smaller tube.

I have been thinking about pressurized fan already independent of the enclosure. It would make aiming the model cooling airflow much easier if you had 2-4 little tubes blowing on you model instead of mounting a fan (or two) and funnels that cover only half of the model anyway,
Re: Enclosure math- why they're so important and how to make one.
April 07, 2015 07:40AM
With a 60C chamber, my hot end has run for 50 hours already and has been cool. But if I wanted a hotter chamber, then water cooling is the way to go.


[www.matter-replicator.com]
Re: Enclosure math- why they're so important and how to make one.
April 07, 2015 07:56AM
Rigth, and the model? how is PLA model cooling behaving?
Re: Enclosure math- why they're so important and how to make one.
April 07, 2015 08:57AM
We have a Stratasys machine at the makerspace that has a couple flexible hoses attached to the hot-end for bringing cool air in from outside the box to keep the hot-end cool. I don't know what sort of blower they have attached to it to force the air through the hose.

Quote
sungod3k
I like the idea and even calming down the airflow would be positive, however my main concern would be hotend cooling. Which would require a either a big hose getting outside air onto the hotend and model cooling fans or a pressurized fan that sits outside and blows in air though a smaller tube.

I have been thinking about pressurized fan already independent of the enclosure. It would make aiming the model cooling airflow much easier if you had 2-4 little tubes blowing on you model instead of mounting a fan (or two) and funnels that cover only half of the model anyway,
Re: Enclosure math- why they're so important and how to make one.
April 07, 2015 09:41AM
Ah interesting. Do you have an estimation of how much air it uses? an aquarium air pump or something like this should suffice [www.alibaba.com]
Re: Enclosure math- why they're so important and how to make one.
April 07, 2015 10:51AM
I think you'd need more air than an aquarium pump can provide. A blower like the things used in CPAP machines would probably do the job since they are designed to blow air through a hose.
Re: Enclosure math- why they're so important and how to make one.
April 07, 2015 02:11PM
Quote
SamS
Let's assume the total surface area of an enclosure is about a square meter. That's a five-faced cube with ~450mm sides, and we're going to assume the table or whatever it's on isn't giving us any heat loss. I should probably account for a few more things, and my system is a little too ideal but this is just to illustrate a concept.

So now let's use 6mm polycarbonate. It seems to be a pretty simple, clear solution to a bed. at 1 m^2 and .19 W/mK for polycarbonate, and a temp difference of 60C (Room temp of about 20, goal is about 80) That's 1900 watts. This exact number doesn't mean a whole lot because of the crude assumptions, but consider that a 12V heated bed pushing 10 amps is only giving your system 120 watts. Not even one-tenth.
I know it's an older post, but your numbers are incorrect for what you're trying to compute as you are confusing thermal conductivity with heat transfer coefficient.

Thermal conductivity is how well heat flows within a material. For our heater blocks, we want heat to flow very well so that it's even and no hot spots right next to the heater but not on the far side of the melt chamber. For this, we choose aluminum or brass because they have a "k value" in the hundreds of W/(m K). But because the thermal conductivity is so good, this is why all metal hot ends heat breaks aren't made of aluminum...it would wick all that heat away up into the cooling fins defeating the purpose of a heat break. So instead they are made of stainless steel which has a k value of around 16 W/(m K). Polycarbonate has a k value of .19 W/(m K). It would make a excellent material for a heat break as it resists the transfer of heat...if only it could withstand the heat (it can't). In terms of your enclosure, 1900 Watts is the rate of heat transfer WITHIN THE POLYCARBONATE if you had the inside surface at 80 degrees Celsius and the outside at 20 degrees Celsius. Slowly over time though the outside surface would raise in temperature as more heat from the inside was generated. That is then where the heat transfer coefficient comes into play...

The heat transfer coefficient is basically how well the material gives off heat, usually in the form of convection between solids and a fluid or gas. This multiwall polycarbonate sheet like was shown above has a coefficient of 3.7W/(m² K). I found a few solid 6mm sheets that had a coefficient of 3.5W/(m² K). A box with a surface area of 1 meter made of solid polycarbonate 6mm thick would lose 3.5 W/(m² K) * 1m² * 60 Kelvin = 210 Watts, NOT 1900 Watts.

Edited 4 time(s). Last edit at 04/07/2015 05:25PM by cdru.
Re: Enclosure math- why they're so important and how to make one.
April 07, 2015 03:38PM
Quote
cdru
Quote
SamS
Let's assume the total surface area of an enclosure is about a square meter. That's a five-faced cube with ~450mm sides, and we're going to assume the table or whatever it's on isn't giving us any heat loss. I should probably account for a few more things, and my system is a little too ideal but this is just to illustrate a concept.

So now let's use 6mm polycarbonate. It seems to be a pretty simple, clear solution to a bed. at 1 m^2 and .19 W/mK for polycarbonate, and a temp difference of 60C (Room temp of about 20, goal is about 80) That's 1900 watts. This exact number doesn't mean a whole lot because of the crude assumptions, but consider that a 12V heated bed pushing 10 amps is only giving your system 120 watts. Not even one-tenth.
I know it's an older post, but your numbers are incorrect for what you're trying to compute as you are confusing thermal conductivity with heat transfer coefficient.

Thermal conductivity is how well heat flows within a material. For our heater blocks, we want heat to flow very well so that it's even and no hot spots right next to the heater but not on the far side of the melt chamber. For this, we choose aluminum or brass because they have a "k value" in the hundreds of W/(m K). But because the thermal conductivity is so good, this is why all metal hot ends heat breaks aren't made of aluminum...it would wick all that heat away up into the cooling fins defeating the purpose of a heat break. So instead they are made of stainless steel which has a k value of around 16 W/(m K). Polycarbonate has a k value of .19 W/(m K). It would make a excellent material for a heat break as it resists the transfer of heat...if only it could withstand the heat (it can't). In terms of your enclosure, 1900 watts is the rate of heat transfer WITHIN THE POLYCARBONATE if you had the inside surface at 80 degrees and the outside at 20. Slowly over time though the outside surface would raise in temperature as more heat from the inside was generated. That is then where the heat transfer coefficient comes into play...

The heat transfer coefficient is basically how well the material gives off heat, usually in the form of convection between solids and a fluid or gas. This multiwall polycarbonate sheet like was shown above has a coefficient of 3.7W/m²K. I found a few solid 6mm sheets that had a coefficient of 3.5W/m²K. A box with a surface area of 1 meter made of solid polycarbonate 6mm thick would lose 3.5 W/m²K * 1m * 60 degrees = 210 watts, NOT 1900 watts.


I think that there is a Celsius, Fahrenheit, and Kelvin conversion issue in your equation.
Re: Enclosure math- why they're so important and how to make one.
April 07, 2015 05:27PM
Quote
Djkingsley
I think that there is a Celsius, Fahrenheit, and Kelvin conversion issue in your equation.
Feel free to point them out. Or if I need to provide sources to back up my math I can do that as well. All my units should be metric (Watts, meters, Kelvin) and the temperature component is actually a delta temperature change so degrees Celsius or Kelvin doesn't matter...they are the same just offset 273.15 from each other. I originally had just degrees and implied Celsius so I did update my post so that I removed any ambiguity about what "degrees" I was talking about. I also double checked my units to make sure they used the proper abbreviations.

If you still don't believe me, punch in the numbers into google.
Re: Enclosure math- why they're so important and how to make one.
April 07, 2015 06:26PM
60 degrees Kelvin is -213.15 degrees Celsius. I believe you meant to use 333.15 degrees Kelvin which would give you 3.5 W/m^2K * 1m * 333.15 K = 1166 W

Edited 1 time(s). Last edit at 04/07/2015 06:28PM by Djkingsley.
Re: Enclosure math- why they're so important and how to make one.
April 07, 2015 09:39PM
Quote
SamS

Is getting insulation foam easy in Europe? How about Australia/NZ? I know there are some strange restrictions on it in California so I guess I just assumed it was scarce.

yes you can get it from ebay that withstands very high temps

you can also get it recycled or from standard sources. I had some that came in a box padding that seemed to be an unusual dark foam, i tested it, it shrivels slightly at 240 and withstands 100 degrees just fine.

it's the same foam as velux use in it's side pads to stop rain splases going from window edge to under tiles. made to be under hot red tiles that can get to 100 degrees.

[www.metrotile.co.uk] it's a common foam of some kind i got it in a box from something.

For an enclosure, i am tempted to find a cardboard box of the right size and fit a thermometer to the side of teh print surface and a plexiglass to a window at the front of the cardboard box, insulative and you can lift it on and off so easily from the table.
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