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Material to hold the heated bed?

Posted by Trakyan 
Material to hold the heated bed?
December 05, 2017 01:11AM
I made a thread a while ago about putting MDF directly the print bed and in contact with the heater, I'm not sure whether reviving that thread makes sense.

Anyways, I've been looking for alternatives to MDF since it's starting to look like more and more of a poor design choice. The bed heater will be directly embeded in whatever material I choose, so it will basically act like a bezel for the heater. I basically have three requirements for this mystery material and need some help picking it out.

One, it needs to be a thermal insulator, otherwise it will just act like a giant heat sink. Two, it must stand up to high temperatures, since it will be in direct contact with the heated bed. Three, it needs to be fairly sturdy and rigid as it will double as part of the printer's frame (so no glass wool, cork, etc.). The printer is a GUS simpson style design so the bolt that go through the bed are also the pivots for the arms, so floating the bed on springs to avoid the direct contact issue is a no no. Another option is to add rigid spacers (possibly some hex nuts?) to float the heated bed over the rest of the frame, and I'm wondering what people's opinions if this will be rigid enough. Yes, I'm aware few people have experience with a GUS simpson design but at a glance, what are your opinions?

Anyways, I've looked at acetal/delrin, but what little information I could find on its glass transition temperature says it transitions around 80 degrees C, which would limit the temperatures the heated bed can reach. Anyone know of any other materials I could take a look at?

EDIT: The original simpson had the heated embedded in some plywood like I planned to do with MDF, but I think he used it to print in PLA so I don't think it was brought up to ABS temperatures.

Edited 1 time(s). Last edit at 12/05/2017 01:29AM by Trakyan.
Re: Material to hold the heated bed?
December 05, 2017 02:05AM
PTFE sheat?

Putting heated bed in something is an odd design...
Re: Material to hold the heated bed?
December 05, 2017 02:31AM
The best insulator you can pick is air. But only non-moving air is insulating. Also a good idea is to reflect the IR rays.
So I made an insulator of aluminum foil ( reflecting IR ), 5mm styrofoam (air captured in bubbles) and then MDF.

I wouldn't build a frame with this composite, except you can think of a sturdy method to connect the bed to the MDF plate and fill the gap with the above mentioned materials.
Re: Material to hold the heated bed?
December 05, 2017 05:41AM
Im not really looking for an insulator, I'm looking for something I can embedd the heater in, the insulation factor is just something preferable. Air is not an option since it cant hold the heater or bed.

I considered ptfe but from what i know its difficult to machine and the forms i need arent really off the shelf.
Re: Material to hold the heated bed?
December 05, 2017 08:23AM
One of my printers uses M3 threaded pillars to hold the bed off the frame. The pillars conduct heat, but their cross sectional area is small, so not much heat is lost. The frame is also metal, so a little heat passing to it does no harm. Stainless steel threaded pillars will conduct the heat less well than brass pillars, but cost more.

You can buy PTFE pillars from [www.gandbprojects.co.uk] but I expect they are expensive.

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Material to hold the heated bed?
December 05, 2017 09:01AM
Hi-Temp Silicone baking sheet maybe?

Like dc42, I use M3 threaded pillars, but inserted through 3mm ID automotive Hi temp silicone hoses.
Re: Material to hold the heated bed?
December 05, 2017 04:02PM
The problem with something like teflon, besides cost, is that plastics expand a lot when heated. The side that's in contact with the heater will expand, and the side that isn't won't, and the whole thing will warp. PCB bed heaters are a great example of this problem.

How about a stone or ceramic floor tile from Home Depot? You can even pick out a pretty pattern...

Edited 1 time(s). Last edit at 12/05/2017 04:05PM by the_digital_dentist.

Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Material to hold the heated bed?
December 05, 2017 05:48PM
There are some specialty plastics with a low thermal expansion. The problem with tiles is similar to teflon, i cant machine the shapes, pockets and holes into it like i need to. I'll probably just use standoffs and hope they dont conduct too much heat or go back to mdf.
Re: Material to hold the heated bed?
December 05, 2017 07:20PM
If you can't machine grooves, etc., build them up by stacking pieces. You could even use glass if you like the look.

Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Material to hold the heated bed?
December 05, 2017 09:08PM
Id still need to cut the shapes to make it by stackinv, it just turns grooves into holes. Glass is pretty hard to cut holes on accurately or cut accurately.
Re: Material to hold the heated bed?
December 05, 2017 10:02PM
I don't understand why the robot arms have to be attached to the bed plate. Why can't the bed be mounted on leveling screws or standoffs a few mm above the base to which the arms are mounted?

Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Material to hold the heated bed?
December 06, 2017 01:35AM
They don't have to, but it reduces part count and helps avoid misalignment from several mating parts.
Re: Material to hold the heated bed?
December 06, 2017 07:42AM
You can still bolt the arms to a single plate, just not the plate that's heated. It would probably be a bad idea to bolt the plastic bases of the arms to a heated plate.

Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Material to hold the heated bed?
December 06, 2017 08:37AM
My original plan was to have a pocket machined in the frame for the heat bed to pop into, so the arms wouldnt be in direct contact with the bed. That and the plastic shoulders may end up on the other side of the frame to the heated bed like in the original simpson.
Re: Material to hold the heated bed?
December 18, 2017 11:19AM
I'm not sure why MDF would be a bad idea, aside from the fact that it's heavy.

With an auto-ignition (flamepoint) temperature in excess of 200 deg C, it's not in danger of catching fire under any kind of normal operation. Its thermal expansion coeficient is also very low, so that's not likely to cause a big problem. Warping can certainly be controlled via structural reinforcement. I'd be more concerned about environmental humidity myself, which is easily controlled with a coat of sealant, which can also help in increasing the auto-ignition temperature to over 300 deg C. By comparison, the fibreglass printed circuit boards, made with polyester resin and glass fibre has a softening point of about 290 deg C, and therefore we should never want to have a PCB heated bed come to higher than that point.

Perhaps making your heated bed from MDF and sealing it with a thin coat of polyester resin (Fibreglass resin) would be the best combination. The MDF will then easily withstand any temperature that wouldn't cause problems with the actual heat bed itself. In that configuration it will be waterproof, will have no problems with warping that the actual heated bet itself won't have, and won't burn.

MBot3D Printer
MakerBot clone Kit from Amazon
Added heated bed.

Leadscrew self-built printer (in progress)
Duet Wifi, Precision Piezo parts
Re: Material to hold the heated bed?
December 19, 2017 01:11AM
For the moment I'm planning to have the bed on stilts above a machined delrin frame. putting it on stilts should hopefully make sure the acetal below doesn't hit its 80-90 degree glass transition temperature. This has also helped me get rid of a few idlers and create a (possibly unnecessary) auto calibration sequence for steps per mm and endstop correction. The downside I'm mulling over is that with the bed no longer being bolted with the same bolts that act as the pivots for the arm, it's more open to misalignment which would throw off the aforementioned calibration routine and require an extra 3 bolts to float the bed. I'm yet to build and test the printer though so I don't know how much of an effect this misalignment could have.

At this point I'm considering bolting through the bed as per my original plan and just using a thicker aluminium plate and foregoing the MDF. Of course if I find an easy to machine, heat resistant material I'll grab that and go back to plan A. MDF just doesn't seem suitable anymore, yes I can treat and coat it but that seems like more effort than it's worth and makes the printer harder and more complicated to make, which is the antithesis of what I'm going for.

Edit: Now that I think about it, acetal's glass transition temperature may prove to be a non issue and I can just use that. I only really found one source for this and it didn't specifically state it was a glass transition temperature, just that it 'softened' but I don't know by how much. Also, the bed/frame assembly will be completely stationary so weight is a non issue.

Upon some extra searching prompted by this post, I've found this
which puts it's continuous use temperature at ~100 celcius which is what I need it to hold up to, and its "Vicat softening temperature" at ~138 celcius, so acetal might be fine for what I need.
Dupont's specs for delrin put its glass transition temperature at -60 celcius (not a typo). It seems to firm up closer to room temp and then gradually soften starting at ~50 C
Another couple of sites put its max operating temperature at just over 80 C

I realize that delrin is a specific type of acetal, but I figure the two should have similar enough thermal properties.

Edited 1 time(s). Last edit at 12/19/2017 01:39AM by Trakyan.
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