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Bed design/safety

Posted by AxeSlash 
Bed design/safety
February 18, 2020 07:22AM
So my current plan for my 400mm x 400mm bed, from top to bottom is:

Thin PEI
Spring Steel sheet (removable)
10mm Aluminium tooling plate (only because I can get 10mm with free carriage, making it cheaper than anywhere that does 6 or 8mm)
1400W 240V silicone heater
15mm Thermopanel (insulation)
2020 V slot frame

1. Does the silicone heater HAVE to be glued to the aluminium, or is just sandwiching it between the ali and the thermopanel OK? Do I risk damaging the heater by 'squashing' it?
2. I don't have access to a mill, so routing a slot into the ali isn't really an option for me; where's the best place to put the thermistor? Or do they generally come mounted in the heater already (heaters appear to have 4 wires coming off them)?
3. Thermal cut off fuse - where's the best place for this? DD seems to imply it needs to be stuck to the heater in case the heater detaches from the plate, but in my case I don't think it's physically possible for the heater to escape from it's sandwich, so presumably I *could* screw it to the plate in my case?


EDIT: there appear to be a number of products called "Thermopanel", one of which is Lexan; that's not what I'll be using. This is the stuff I'm looking at: [www.amazon.co.uk]

Edited 1 time(s). Last edit at 02/18/2020 07:25AM by AxeSlash.
open | download - Bed.png (836.8 KB)
Re: Bed design/safety
February 18, 2020 09:13AM
It sounds good the way you're doing it. The heater should have an embedded thermistor, and that's the best place for it. If your mechanical arrangement of the plate, heater, and insulator can't come apart, you don;t have to worry about the heater falling off the plate and you can mount the TCO on the plate. I would still glue the heater to the plate using high temperature silicone (not 468MP adhesive sheet) because just an air bubble between the heater and the plate will result in a burned heater. You don;t have to worry about squishing the heater- nothing will short.

With that thick bed plate there will be some time delay between the thermistor reading that the target temperature has been reached and the bed surface actually getting that hot. You'll probably want to preheat for a few minutes before you start prints.

Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Bed design/safety
February 18, 2020 09:22AM
Yeah I figured there would be a) a delay and b) maybe a slight difference in temperature. I intend to measure the delay and account for it in my starting G code; tbh I'll probably use a spare ABL sensor to measure the warp of the bed continually, and watch for when it evens out, then put that time value into the code.

Regarding air bubbles, should I be concerned about the opposite side of the heater (insulation side)? The thermopanel has a sort of ridged surface, not totally flat, so there will be some air in there...any idea why an air bubble causes burning? Struggling to figure the physics of that in my head.

Re: Bed design/safety
February 18, 2020 09:26AM
Also I've just discovered that the thermopanel has a max service temperature of 74C, so that might have to go. Any recommendations for a rigid, insulating material? I originally started with a cork design, but figured that would be too floppy to support the heater properly. Looked at DLPC as well but the insulation properties of it are nowhere near as good as cork or thermopanel.
Re: Bed design/safety
February 18, 2020 09:58AM
Air is a great thermal insulator. A down blanket is warm not because of the feathers but because of the air trapped by the feathers. The heater depends on good contact with the plate to keep from getting too hot- if the heater comes off the plate is will burn itself up in very short order. An air bubble between the heater and the plate will result in a hot spot on the heater because it isn't transferring the heat to the plate. Here's a heater that was toasted when the 468 MP adhesive started letting go of the plate:

When you mount the heater on the plate, use high temperature silicone and be careful to squeeze out air bubbles as you lay the heater down on the silicone.

Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Bed design/safety
February 18, 2020 10:48AM
Ah of course, that makes sense.

In terms of TCOs, I'm looking at one of these: [uk.rs-online.com]

Any disadvantages to those over a standard fuse-style TCO?
Re: Bed design/safety
February 18, 2020 04:35PM
That should be OK if it can handle the current your bed heater draws.

Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
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