My printer has a (moving) 250mm x 290mm aluminium plate at the base -

My printer has a (moving) 250mm x 290mm aluminium plate at the base - this is not flat, it has screws in the middle of it.

Currently I have a 214mm x 214mm PCB heated bed, with a glass sheet of similar size on top, and a layer of cork board between the PCB and the aluminium. So:

Aluminium -> Cork -> PCB -> Glass

This is all sitting on top of each other, with foldback clips holding everything together. No levelling (I am using bltouch to deal with the lack of levelling right now). So the build area is maybe 190x190mm taking into account the clips encroachment.

I’d like to do two things - put in proper levelling, and increase the build area.

I am hoping for some advice as to a good approach. I can drill three points into the alum plate, and so add some levelling screws/springs.

But I am not sure on what to do next. If I want to use the existing PCB heater, then I’d need an aluminium spreader. So lets say I get a 250x250x3mm aluminium sheet. I can mount that on the three points, and put a 250x250mmm glass sheet on top of that, but I still need to somehow get the heater to affix to the underside of it.

And then I am wondering if two alum plates + glass + PCB is going to be too much for the NEMA17 + belt that moves the bed back and forth.

I am thinking that none of this is new, and so you might be able to give some tips?

I think maybe using thermal epoxy like permanent heatsinks are attached with to bond the PCB to an aluminum heat spreader might solve your problem, then level the aluminum and use clips to attach the glass so you can remove it to apply tape or glue, or another glass plate for faster turn around. I use Arctic Alumina thermal adhesive, any good thermal epoxy will do if it is for attaching heatsinks to stuff that gets hot.

@AlohaMilton Aah, right, so literally glue the PCB to the bottom of the aluminium plate? So my thinking with the stack seems sensible? I guess the alum plate should be the same size as the underlying one (290x250), so that I have some clearance on either side of the 250x250 glass sheet for the thumbscrews to protrude.

In this configuration also, where would the thermister go? Right now, I have it in the hole in the middle of the PCB. But in this scenario, if the thermister in the PCB, which has an alum plate on top of that, and then glass on top of that, does the thermister really represent the temp of the surface? Do people put the thermister somewhere else, or just wait a bit longer for the heat to migrate to the surface?

I used high temperature RTV from the auto parts store to bond my heatbed pcb to a cork tile and I’ve had no issues in 2+ years. When you bond the items together, be sure to press them between stacks of heavy books or similar so that the bonded pair is held flat - this way, they actually work together to be more rigid.

@Alan_Thomason Cool. So what is the order in your stack? Cork, then PCB, then glass? What is under the cork?

Nothing is under the cork. The way my printer (Rigidbot) is designed, there is a smallish aluminum “carrier” plate where the linear bearings are mounted and it is the bottom of the stack, then springs with leveling screws that pass through, then the pcb, the cork is bonded to the bottom of the pcb, and the glass sits on top and is held by binder clips. I think I have a picture posted long ago, I’ll look for the link and post it soon.

ideally the thermistor would be in the center touching the aluminum heat distributor so if the pcb has a hole leave it open so you can attach a thermistor and replace it if needed, mask that hole area on the aluminum before bonding them together. I have not bonded a PCB to an aluminum plate for a heatbed before only for cooling electronics. theory is the same though and its considered a good solution in electronic hardware. It will be permanent though so if the aluminum warps you wont be able to replace it and loose both, but it will bive much better efficiency of thermal transfer than a few contact points from just a few screws. But maybe wait for some more advice before doing that though, or at least some confirmation. printers are finicky and I wouldn’t want to steer you wrong with bad advice if there is some unknown issue… anyone used thermal epoxy to permanently bond a pcb and aluminum heatbed composite? figure its basicly what an aluminum heatbed is, isn’t it?

@Paul_Murphy ​​ here is the link to a post with pictures of my bed setup. Pictures 2 and 3 show the best detail. Again, my setup is unique to the design of the Rigidbot’s heatbed.

@Alan_Thomason Ah, I see. So the cork isn’t part of the supporting structure, it is to insulate the underside?

@Paul_Murphy exactly, the cork is for insulation; however, when bonded to the pcb, the result was more flat and much stiffer than the individual components.

@AlohaMilton Yeah, no problem, I get the permanency thing. Mostly I need to find a source of flat aluminium in Australia. I don’t think building suppliers would generally worry too much about how flat the aluminium is.

@Paul_Murphy several others who have the Rigidbot have used a stack like mine with a thin aluminum heat spreader between the pcb and the glass. I don’t know if they bonded it or not. If the spreader is relatively thin, it may conform the bottom of the glass and the top of the pcb without any “help”.