Bed heater recommendation
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@akstrfn said in Bed heater recommendation:
300 degrees on hotend is not quite the same as 300 degrees on the bed imho. Voron parts are printed in abs so it would fall appart entirely at that bed temperature and silicone would fail but with smaller max temperature i.e. around 180 degrees plus aluminium or metal holders for the bed would basically make the whole thing much more stable.
I thought you were either mistaken or joking so I took a gander at the Voron and see that you were neither. I just assumed that the bed would be "all metal". IMO, plastic bed mounts are a seriously bad idea for so many reasons.
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@mrehorstdmd said in Bed heater recommendation:
Insulating the underside of the heater will increase the cool-down time, and it's generally a bad idea to pry prints off the bed until it has cooled, so you'll have a wait a little longer to take your prints out of the machine.
..............Unless you use a removable build surface, in which case you can slide one out, slide another in a be printing again with the minimum of down time between prints (but we've had this discussion before so I guess we must agree to differ)
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@akstrfn Your diagrams of temperature vs heater power are very nice but assume perfect adhesion to the metal plate, that is hard to guarantee. This is what happens when there's an air bubble between the heater and the plate:
It got hot enough to char the silicone in that area.
This heater was stuck to the plate using the 468MP adhesive that came on the heater from Keenovo. The air bubble developed after some use, probably because I did not follow Keenovo's instructions to seal the edges of the heater with silicone. I suspect the heat and exposure to oxygen causes chemical changes in the adhesive that cause it to fail over time.
I've had PEI sheets glued down with 468MP come off the bed, too. I think my next bed plate will be about 4 or 5 mm larger than the PEI so I can seal the edges with silicone. I have switched to using high temperature silicone to glue the heater to the bed. Keenovo sells the heaters with and without adhesive.
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@deckingman I'm gonna replace the abs parts that hold the bed to aluminium ones after I get the printer up and running.
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@mrehorstdmd I saw you posted this on few places and saved it as a reference. It is an important point to take care of. I am testing this one at the moment https://www.amazon.de/-/en/gp/product/B00GFACVWO/ref=ppx_yo_dt_b_asin_title_o01_s00 and the smell is very strong I hope it goes away. I believe you recommend TRV one?
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@mrehorstdmd Thats a nice calculator and it estimates 11 minutes for my setup with 300W heater. Nice
Edit: do you know how the calculations are done? -
@akstrfn Your words on thermal cutouts are well taken when it applies to a mechanism. There are better and worse.
I'm really referring to the ones that are "just physics". They contain a selected alloy and/or salts that have a known melting point. They are in a whole different league of reliability, or really trust of being a "safety net", than any mechanical, human designed, cutout.
Heater safety is always a very far ranging discussion, and there is fact and truth (and at least grains of truth) in many many threads of that conversation. And LOTS of opinion.
At the same time, I tend to push all that aside and focus on these "physics based" devices. To me, their presence makes the far ranging discussions moot.
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@akstrfn Replacing the bed mounts with metal might be tricky- With plastic mounts the leveling screws (assuming there are some) will fit tightly in the mounts and won't allow the bed to wobble. As soon as you go to metal, if the screws have to be adjustable, the bed may wobble on the screws. Tapped holes always allow clearance in the threads - that's why a screw will wobble loosely inside a nut. If there are leveling screws, consider using a higher temperature plastic like PTFE- you drill undersized holes and let the screws roll their threads into the plastic. It will be solid, but still allow easy adjustment of the screws. Stainless/steel screws are not very good thermal conductors so the temperature of the PTFE will be well below the bed temperature, and far below any temperature that will cause the PTFE to soften or break down.
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Point taken
The fact that there are so many opinions is what makes it hard to find the right answer... I spent a while looking for the right sources for this and generally duet forum and reprap forum so far have the highest quality imho.
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@akstrfn said in Bed heater recommendation:
@mrehorstdmd I saw you posted this on few places and saved it as a reference. It is an important point to take care of. I am testing this one at the moment https://www.amazon.de/-/en/gp/product/B00GFACVWO/ref=ppx_yo_dt_b_asin_title_o01_s00 and the smell is very strong I hope it goes away. I believe you recommend TRV one?
That stuff should be fine. The acetic acid smell goes away as the stuff cures.
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@mrehorstdmd the bed screws are not plastic but the holders for the platform are. The printer is open source but I dont know if its available on the github. Here is a link from their discord https://a360.co/2zVYtGY. As you can tell if the bed heats to dangerous temps the whole platform would drop. Thats why they spec thermal fuse to 120-125C I believe.
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Coming a bit late to this conversation, but recently purchased (but haven't yet received) a silicone heater pad, so thought I'd share my experiences and what I learned in the process...
300x300x8 is a massive slab of aluminum to heat with only 300W. Keep in mind that while the heater is trying to transfer heat energy into that thick metal over a surface area of 900 sq cm , that thick slab of aluminum is also trying to transfer the same heat energy into the surrounding air via the other side of that 900 sq cm surface area. I don't know how easy it would be to reach and maintain 100C.
When I was shopping for my 310mm diameter (round) bed (~750 sq cm), I considered 400W to be the bare minimum. I ended up with a 280mm diameter 500W heater. (The heater is smaller than the bed to allow room for mounting screws, etc.)
The thermistor (or PT100) built into the keenevo silicone heaters is embedded in the heater itself. The heater itself will reach any set temperature (such as 100C) much faster than the aluminum bed will and the duet, seeing a temp of 100C on the bed, turns off the heater. As the heat energy transfers from the silicone heater to the aluminum bed, the heater cools down and the duet turns it back on. Until the heat energy fully equalizes between the heater and aluminum, the duet will show an inaccurate temperature and will turning the heater on/off somewhat blindly. This extends the heating time considerably.
If possible, install a thermistor (or RTD) in/on the aluminum bed itself. For myself, I ordered a little aluminum block that should (I hope) accept a e3d 3mm thermistor cartridge and screw under the edge of my build plate. (There should be room enough to attach this because my bed is larger than my heater.) Here's the block: https://www.seemecnc.com/products/copy-of-thermistor-cartridge-ntc-100k-3mm-dia
Keep in mind that the calculator that @mrehorstdmd posted likely assumes that the heater will run at full power until the aluminum bed is up to temperature. If the thermistor (or RTD) is installed in the heater (and not the bed itself), the times to heat will be much longer (for reasons described above.)
Finally, I also purchased a keenevo bed with the built in "thermostat." If your is the same as mine, keep in mind that it may be self-resetting. (Keenovo isn't completely clear on how it works other than to say that it is mechanical bi-metal and snap action.) That could mean an endless cycle of the heater turning itself off when it reaches ~150C, and then back on again when it cools down a bit. A secondary thermal control would probably be a good idea.
"I'm not saying that you are wrong - I'm just trying to fit it into my real world simulated experience."
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Funny how I went through all these points in my considerations as well, I guess I did my homework
@garyd9 said in Bed heater recommendation:
Coming a bit late to this conversation, but recently purchased (but haven't yet received) a silicone heater pad, so thought I'd share my experiences and what I learned in the process...
300x300x8 is a massive slab of aluminum to heat with only 300W. Keep in mind that while the heater is trying to transfer heat energy into that thick metal over a surface area of 900 sq cm , that thick slab of aluminum is also trying to transfer the same heat energy into the surrounding air via the other side of that 900 sq cm surface area. I don't know how easy it would be to reach and maintain 100C.
I got idea for 300W heater from dc i.e. check his answer with his delta here https://reprap.org/forum/read.php?1,807307,807400#msg-807400
When I was shopping for my 310mm diameter (round) bed (~750 sq cm), I considered 400W to be the bare minimum. I ended up with a 280mm diameter 500W heater. (The heater is smaller than the bed to allow room for mounting screws, etc.)
The thermistor (or PT100) built into the keenevo silicone heaters is embedded in the heater itself. The heater itself will reach any set temperature (such as 100C) much faster than the aluminum bed will and the duet, seeing a temp of 100C on the bed, turns off the heater. As the heat energy transfers from the silicone heater to the aluminum bed, the heater cools down and the duet turns it back on. Until the heat energy fully equalizes between the heater and aluminum, the duet will show an inaccurate temperature and will turning the heater on/off somewhat blindly. This extends the heating time considerably.
Since aluminium is good heat conductor I've found user report that the bed itself does not lags just a few minutes (300x300x8mm size). If you check one of my posts I mention that one user tested with 40% heater that equates to 280W for him and got heatup time in 5 minutes and said few minutes more for alu bed itself to get to temp.
If possible, install a thermistor (or RTD) in/on the aluminum bed itself. For myself, I ordered a little aluminum block that should (I hope) accept a e3d 3mm thermistor cartridge and screw under the edge of my build plate. (There should be room enough to attach this because my bed is larger than my heater.) Here's the block: https://www.seemecnc.com/products/copy-of-thermistor-cartridge-ntc-100k-3mm-dia
This is the plan. I saw this from others that have posted in this thread.
Keep in mind that the calculator that @mrehorstdmd posted likely assumes that the heater will run at full power until the aluminum bed is up to temperature. If the thermistor (or RTD) is installed in the heater (and not the bed itself), the times to heat will be much longer (for reasons described above.)
Finally, I also purchased a keenevo bed with the built in "thermostat." If your is the same as mine, keep in mind that it may be self-resetting. (Keenovo isn't completely clear on how it works other than to say that it is mechanical bi-metal and snap action.) That could mean an endless cycle of the heater turning itself off when it reaches ~150C, and then back on again when it cools down a bit. A secondary thermal control would probably be a good idea.
I think if powers on and off just like a thermostat works, I believe that it is not precise but thats not very important. I am not sure why many people find it problematic that it turns on/off? RRF would react in that case and turn it off entirely if SSR is not dead. Considering that it survives thermal fuse blowing :paranoid:
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@garyd9 It's true- the heater will reach the set temperature before the bed plate will, but moving the thermistor to the bed plate means you'll be running the heater at a much higher temperature. I haven't tried it so I don't know if it will get hot enough to damage the heater.
I use the heater's built in thermistor and it doesn't seem to add a lot of time to the bed heat up. I usually just do an "upload and print" which starts printing as soon as the bed reaches the set temperature and don't have any problems with prints letting go.
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@akstrfn said in Bed heater recommendation:
@deckingman do you mind doing a test with a PWM limit on your bed to around 0.3W/cm2 and let me know how did it go?
I didn't do exactly as you asked but tested my bed warm up time "as is" (100% PWM). The bed spec is as follows:
400mm X 400mm X 10mm thick aluminium tooling plate. 6mm float glass on top. Thermistor situated in a hole about 40mm deep in the edge of the aluminium, close to the top surface. 800 watt silicone heater stuck to the underside of the bed (so about 0.5 Watts/cm^2). Twelve mm (2 off 6mm thickness) of semi-rigid insulation under the bed. Holes bored in the insulation to accept hollow pillars. Countersunk bolts through the aluminium plate, through the pillars, into the 2020 extrusion frame (so the only thermal transfer between heated bed and its' frame is via the 4 hollow support pillars - thermal expansion of the bed frame is hence negligibly small).
Ambient start temperature 20.0 degrees C.
Time to reach 50 deg C (30 deg increase) = 6 minutes 10 seconds
Time to reach 60 deg C (40 deg increase) = 7 minutes 55 seconds
Temperature after 10 minutes = 71 deg C (51 degree increase).
Time to reach 100 deg C (80 degree increase) = 16 minutes 55 seconds (17 minutes near enough). -
@mrehorstdmd if your heater is properly speced there is more time for temperature to spread properly:)
I think that moving the thermistor could damage the heater if its overpower as per the graphs I found.
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@deckingman clearly 0.3 would not work for you since your termal capacity is high... Why did you decide on such a thick alu piece + 6mm glass on top?
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@akstrfn said in Bed heater recommendation:
@deckingman clearly 0.3 would not work for you since your termal capacity is high... Why did you decide on such a thick alu piece + 6mm glass on top?
TBH, 10mm was hardly any difference in price to 8mm so I thought "why not" (but I could have used 8mm aluminium). I like to print on glass with 3DLac - I've tried just about every other print surface but never yet found anything that will match it. I can print the first layer without using a slower speed (so 90mm/sec upwards), I never use any "squish", I can print edge to edge, once cool, parts just fall off (or at worst need a light tap). I like to be able to slide one sheet out and another in so that I can print multiple parts with minimum down time in between. Anything less than 6mm float glass is a bit too fragile.
Note that the bed warm up time as measured is with the thermistor in the side of the aluminium, so the glass on top is largely irrelevant to the measurements I took. The difference would be a function of heat transfer from the top of the aluminium to glass vs heat transfer to air. So in theory, without the glass, the warm up time for the aluminium plate might be slightly longer due to the slight insulating effect of the glass.
Edit. Note also that the top surface of the glass is about 1.9 degrees cooler than the top surface of the aluminium when the aluminium is at 50 deg C and the ambient air is at around 22 deg C.
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Just as a data point: I have a 24" diameter (610 mm) glass bed on a delta that is 6mm thick. I have a 600 mm dia Kenovo heater directly adhered to the bottom of the glass. No aluminum anywhere.
Works great.
(and, you can see if it the heater is coming off, which has been a non-issue for three years now).
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@akstrfn said in Bed heater recommendation:
From these diagrams https://www.horngmbh.com/en/heating-systems/#tech3 I got an idea that maybe 0.35W/cm2 could do the job perfectly (unfortunately this company doesnt seem to sell to private customers):
That fits with my experience. I specified the heater for my delta at 0.4W/cm^2 and it tops out at just below 200C at full power.
