Silicone Heated Bed
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@mike definitely, that's a great idea too, with the inherent dangers running an AC bed something as simple as that could save your bacon if your SSR fails and your heater ends up on thermal runaway.
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@mike Wouldn't a bimetallic switch just cycle on and off if the SSR fails? A thermal cutout would guarantee a shut off. Or are the Keenovo bimetallic switches the manual reset kind? An automatic one would be unsuitable, and the one-shot kind would render the heater useless if it's buried within the silicone?
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@klcjr89 A resetting bimetallic may reset, but not a problem if the bed is designed to take upto the switch point. In addition the abnormal behaviour of the heater should be detected by the duet and power supply to the heaters killed when the PSU_ON signal drops.
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@dino also how many 12 / 24v P.S.U have you seen which is capable of 500 watts or more, that`s unless to go for some sort of ATX psu.
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@peter247 I don't quite understand what you mean. When using an AC heated bed you can get away with using a much lower wattage PSU than if you are using a large DC heatbed.
Since the power from the wall is AC it doesn't need to be converted to DC by the PSU. Therefore the PSU only needs to power fans, control board, steppers, hotend heater and anything else.
My AC heatbed is connected to the IEC where I get mains power for the whole printer. I'm considering switching my PSU to a lower wattage passively cooled one since the mean well SP-320-24 has a loud fan and has a ton of overhead.
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@klcjr89 It'll end up cycling - usually these switches flip back on at when the temperature drops by 10-15 degrees. I was previously using a thermal fuse caked onto the heater surface with some RTV silicone, but if this one trips, you have to rip it all apart and solder a new one in it's place.
So I much prefer the bimetallic switch. It will still prevent my arguably massively overpowered heater (500 W for a 200x300 mm bed) from reaching dangerous temperatures if the worst (SSR fails closed) happens. And besides, even in this case the Duet will still post an error message, so I'll eventually see it and shut everything down.
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i recently installed a 200x200 110v, 500w Keenovo on a glass bed and it will go from 25c-160c in less than 2 minutes. I'm pretty sure it will easily hit 200c but have yet to find the need to try it. Pretty cool when you can create a puddle of molten plastic with your bed. At the same time I also added a 12v 50w cartridge heater to the hotend and it takes less than 3 minutes to get from 25c to 400c. Wish I would have done this years ago.
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What controls have you got for the environment of the machine? Enclosure or is your machine made out of a low thermal expansion material? With a linear thermal expansion rate of 23.1 * 10^-6 for Aluminium I work that as a 0.23mm expansion per metre of extrusion for a 10C temp change.
There's something to be said for a slow warm up as it gives the rest of the machine a chance to warm up and settle before the bed probe. The expansion itself isn't so much of a problem - you can tune / change procedures to account for it. What is a problem is the bits closest to the heat sources get hot first, then conduct heat away taking a while to reach equilibrium, geometrically changing the chassis in a non-linear manner over a longer period of time than it takes to heat the bed.
I've been meaning to ask if there is feature request for some sort of average duty report on the heaters or a warning of under powered heaters for the chosen set point. Having whinged about the negatives of big heaters I have to conceded my bed heater is lacking.
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@doctrucker said in Silicone Heated Bed:
I've been meaning to ask if there is feature request for some sort of average duty report on the heaters or a warning of under powered heaters for the chosen set point. Having whinged about the negatives of big heaters I have to conceded my bed heater is lacking.
See the M573 command.
Duet WiFi hardware designer and firmware engineer
Please do not ask me for Duet support via PM or email, use the forum
http://www.escher3d.com, https://miscsolutions.wordpress.com -
@dc42 Thanks.
In the python coding world this sort of thing was referred to as Guido's time machine! I've still many tricks to learn with the V0.6 boards. Watching the RTOS updates for the Wifi/Ethernet with interest.
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@3dpmicro In the end I went for a 300 watt heater which for my 70c max looks to get the job done.
But will have to try a external Thermistor because it gets upto temperature quickly , BUT 55c is 48c and it get 55c I need to set it to 70c , using a IR gun on the print side of a 6mm bed on tape. -
@peter247. Awsome. I switched back from aluminum to glass bed with thin adhesive backed fg/foil insulation on the back of the pad. Small cutout in insulation to expose thermistor so its not insulated. Surface lags about 10 deg behind thermistor on heat up then catches up within a few degrees after a couple minutes.
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I've had problems, never getting the bed level , which was killing the nozzles very quickly .
Today after various mods , rebuilds , and resigns to make it stronger I've found the problem I hope !!! , with the aid of a new straight edge ? , my aluminum 6mm bed is bowed at the corners , YES , ( how do you bend a 6mm plate ) the centre looks level but not the edges , so I`ve edited my bed level macros which now levels 50mm from each corner.
All I need now is a 410x410mm glass bed to level it off ?. -
@peter247 the 6mm Mic 6 tooling plate I was using is in the annealed state and very easy to bend. Identifying exactly where the bent areas are, marking those areas then clamping to a table and carefully working out those areas might be possible. Are you using bed leveling?
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@3dpmicro said in Silicone Heated Bed:
Are you using bed leveling?
Depends on what you mean bed leveling ? , I use dual Z and auto level them , But my bed leveling is via a macro to the bed corners and some paper.
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@peter247 I basically do the same thing you do. Seems mesh bed leveling with a probe is all the rage these days. Great first layer even with warped bed. I'm probably going with 8 standoff adjusters on the next machine. Adjust it flat
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@3dpmicro said in Silicone Heated Bed:
I'm probably going with 8 standoff adjusters
Wow , that is a lot of adjusters ?
I was thinking of getting a bed leveling probe , but can`t find how to mount it anywhere near the nozzle ( titan aero ) .
Now I've come to the opinion the best way is thin aluminium plate 3 or 4mm and flat glass / shims , what is the point of a heavy plate if it bowed ? -
@peter247 said in Silicone Heated Bed:
.Now I've come to the opinion the best way is thin aluminium plate 3 or 4mm and flat glass / shims , what is the point of a heavy plate if it bowed ?
@peter247 sounds like a great idea. I didnt like the aluminum bed because of the weight and because 40% of it is outside the printable area. Not an efficient approach in my opinion on a machine that moves the bed in x or y. A lot of wasted energy. I here many claim the poor thermal conductivity of glass as being the reason for aluminum but the glass is thin (relative) and has a high rate of heat flow even though it is a poor conductor.
If you ended up with a flat piece of aluminum you may be able to plop it down on a layer of PVA glue and call it a day bypassing the shims
