Heated bed thickness
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@DaBit said in Heated bed thickness:
Although I must say that the kinematic mount using a couple of balls is quite charming too.
My attempt came about as I was experimenting with auto bed leveling and needed something that would not only allow for thermal expansion but also out-of-level conditions.
Frederick
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I'm going for cast aluminium tool plate. I can go for 10mm, 12mm, 12.7mm or 15mm
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10mm should do.
According to a quick&dirty simulation: when constrained at the 4 corners the plate will sag 0,048mm in the middle due to gravity, taking in account the reduced E-modulus due to the aluminium being heated to 180C.
In reality this will be slightly more since the corners are not fully constrained.
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check out these guys
https://clever3d.de/epages/7a4290fc-7c7f-46cc-9b99-eadef22228e2.sf/de_DE/?ObjectPath=/Shops/7a4290fc-7c7f-46cc-9b99-eadef22228e2/Products/c3d-DDP-PEI-naturcurrently their max size on offer is 450x450 but they say they can do custom sizes upon request, as each is individually made.
it is machined and pei covered. -
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@jay_s_uk For info, I use 10mm on my 400x400 bed and if I was doing it again, I'd probably settle for 8mm to improve the warm up time. It takes time for the heat to get transferred from the bottom surface where the heater is, through to the top surface where you want to print, so the thicker the plate, the longer that time. Having said that, if I was going for 500mm x 500mm as you are, I'd probably choose 10mm thick. IMO, there is nothing to gain by going thicker than that.
From your user name I'd guess that you are in the UK. If so, this is a good source https://www.aluminiumwarehouse.co.uk/aluminium-plate-cut-to-order. Go for the "Ecocast" which is quote "a fine milled and high precision continuous cast aluminium tooling plate."
It's also worth giving some thought as to where you fit the temperature sensor. With thick plate, the junction between the heater and the bottom of the plate is not the best place. I drilled a 3mm hole in the edge of the plate as deep as I could and as close to the top surface as possible without breaking through.
HTH
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One caveat with the sensor close to the top: during warmup there is 10's of degrees temperature differential between the silicone heater core temperature and the top of the build plate. If you really intend to go to 150-180C plate temperature you might end up in a situation where the top is not yet at the intended temperature, but the silicone heater is hot enough to break down the adhesive backing.
I have drilled the hole for an extra sensor, but I am still using the sensor in the mat itself for this reason after a friend of mine kept having issues with silicone heaters coming loose. I just allow a couple of minutes extra heat soak time. After all the heat must soak through the glass plate also. Usually turning on the bed heater before exporting STL/slicing/preparing gives plenty of time for the bed to stabilise.
The best solution would be to use two sensors, the one in the mat to put a cap on the maximum temperature there, and another close to the top of the build plate for control. Not sure how to do this with Duet though, maybe with some creative use of themostatic fan control.
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@DaBit I hadn't considered the effect of running the bed at 150 to 180 deg C on the silicone heater adhesive. I use semi rigid insulation under the heater, between the bed frame and the aluminium plate as a sort of sandwich. So if the adhesive gave way, the heater pad would remain intact, but I have no idea if that insulation would withstand temperatures.as high as 150 to 180 deg C.
But actually, at the start of warm up, the temperature differential between the pad/aluminium junction and the top of the thick plate can be a lot more than 10 degrees. Obviously it depends on the power of the heater and the thickness of the plate. When I first put my bed together, with a thermistor at that junction, I found that the temperature would rise very rapidly to the set point of (say) 60 deg C at which point the heater would cut out until the temperature dropped a degree or so, then cut back in. But of course, the top of the plate would remain relatively cool. This on off cycle would continue for a long while which meant that it took an awfully long time before the upper surface got anywhere close to the desired temperature. IIRC, it was about 15 minutes to get to 60 deg C but after moving the thermistor, that time dropped to less than 5 minutes.
So as I said, the positioning of the temperature sensor needs to be given careful consideration.
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e3d develop special beds to operate at that temperature.
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@deckingman: the adhesive handles 200-220C just fine, only not much more if you want it to last. And if you turn on your 2kW heater with the temperature sensor on top of the plate the adhesive will become hotter than that before the surface reaches 180C.
I agree that a sensor close to the top and away from the edge of the hotplate is best for sensing/controlling the bed temperature, and I agree that having the sensor at the heater location slows down reaching final temperature a lot. But I would still like to have a cap on heater core temperature, which is best sensed with the thermistor integrated by Keenovo. I have a hard time implementing these kind of things exactly as I want them with Duet hardware though. A lua interpreter, softPLC, conditional G-code that runs on a timer trigger, whatever would be extremely welcome.
Regarding insulation: I am using a ceramic mat that is used to rebuild two-stroke silencers between the hotplate/silicone mat and the support structure, available at your nearest moped dealer. Insulation properties are decent but not great. With the bed at 110C the support structure directly under the hotplate goes up to 50 degrees or so over a timespan of a few hours, I can live with that. At least that stuff won't decompose or burn when something goes wrong. Cork is another option that works well up to 200C and which can handle short-term temperatures up to 350C.
