Bed heater recommendation
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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.
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So I ordered 350W heater for 30cm2 bed. Lets see how it works.
After some more research on how these things are built I realized that one can not conclude the max temp just based on the power density but that there will be some variation depending on the resistive wire that is used.
As an experiment I think I'll also get few PTC heaters bolt them to the alu plate and see how that works. If that works I might have a silicone heater for sale
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I know this may be a mute subject but think twice before using aluminum. The expansion rate of aluminum is quite high. Instead think of Borosilicate glass instead. I am currently using 305mm X 610mm X 10mm for my print bed.
I am using 2 Kapton heaters 300 X 300 on mains through a SCR controlled by the Duetwifi. I do have a safety cutoff wired in series to the mains. I have tested this to 140C without issues heatup time is less than the heatup time of the E3CV6. The bed stays flat does not warp
