Glass Beds and Even Heating

I am having adhesion problems (nobody else does, right?) and what I seem to see, is that when my prints are over a certain size the print skirts and rafts do not stick. At about 2x2" things have always worked fine, but now I am printing some 6 inch "hoops" and the rafts (and subsequent layers) dont stick. the same occurs with a 4x8" device. Measuring bed temperatures it is obvious that the bed is cooler near the edges by about 10C. (I am running PLA at 210C on a Creality ender 5). I have done some of the usual things-double checked leveling, slower speeds on the skirt layers etc but none work. SO I am looking a bit at the bed-I would think that it should heat evenly and I would assume a glass bed would to this. However some of the guides on the web are rather "vague" about this and the need for a glass bed, or even its adhesion capability seems questionable. They do of course give even surfaces.Anyone got experience with glass beds? CheersFritz

Glass actually conducts heat pretty badly. It's not uncommon for a glass bed to have an aluminium heat spreader between the heater and the glass. Also means that the heater isn't stuck directly to the (relatively fragile) glass. <BR><BR>

The other thing to check is whether the glass is bowing/dishing as heat increases.

I agree about the conduction for glass-I think it heats very slowly - but I was imagining that due to its greater thickness it would spread the heat out more evenly. And of course it is flatter. But in the end (agreeing with you!) I am not sure why people are using it then. Creality is low-end and I would be interested in how well other printers do here-especially how they handle the beds--I note that the crealtiy has a heading pad of about 75x75cm under the aluminum bed-probably should really extend across the whole bedThanks for the replyFritz

if this is the heatbed you've got, then you've actually got a decent sized PCB heater on an aluminium plate under the glass. In that case, most of the heat loss is more likely due to the geometry (edges and corners area always easy to cool, hard to heat). The gold coloured tape in the middle just holds the thermistor in place. https://www.creality3dofficial.com/p...-hot-bed-board

Most printbeds have a temperature variation. You could try bumping the temp to compensate for edges to see if it sticks better. If it still doesn't, then heat up the bed and check it's still actually flat.

Thanks- I don't see any warp in the bed, at temperature-but it is a touchy measurement of course. I guess that is the incentive to use glass. Do you know if this is this pretty standard with other printers?

In my experience, the heat variance across a heatbed can be a bit of a nuisance with regard to achieving a suitable sticking temperature. However, usually you can get around it by bumping the temp a bit. What I've seen as a more common problem is height variation across a heatbed as it warms up. As commented, edges and corners are usually cooler, which means that those areas also expand less. This means you get greater expansion in the middle of the bed, and less at the edges, and in response, a bed will often dish a bit as temps increase.

When you do the larger prints, do you see any difference in "squish" in the filament as it is deposited on the bed?

well-frankly-I can't see much of anything on larger prints because they don't stick long enough to get a reasonable test. I definitely saw raising in one that "stuck" for a while. but I can certainly see where warp should be an issue as you mention.
I am a bit surprised that a much thicker block is not used for the bed (I think the creality is about 3/16". So I am considering replacing the bed with something a lot thicker and perhaps a broader heater to distribute the heat faster-but I will probably try a simple glass bed first simply because warpage should be less.
My only goal is to get "reasonable" runs with a cheap printer like this one-once I have it done for checking fit etc I would typically send the CADs to a printer.
Thanks
Fritz

Post some photos of the bottom of your prints. It could also be plastic shrinkage. Most plastics shrink a bit as they cool, and can then pull themselves off the heatbed if not securely stuck.

There's a few brands/types of filament that are particularly problematic. It may be worth trying a few different brands to see if you can find one that doesn't warp much.

Thanks Martin- all good points. I am presently printing a group of "small" parts for my optics bench but I will send some photos as soon as I get some large prints that prints. (The bottoms of the small ones look flat etc). Your comments on filament interest me-I have heard about this but as new user I don't have much experience. I presently had failures with 2 different brands of filaments-one from Creality and the other from "Tecbears". Just a note: I printed a "large" 6 hoop last night using a raft and it worked--but the initial stages of the Raft had trouble sticking. Fortunately not enough of a problem to destroy the print. Rafting is a curiosity to me--if the "regular" print does not stick, it seems as though the Raft would not stick either-and that is what I see. I guess the idea behind the raft is it has enough dense runs to insure that at least some sticking occurs. I think I am stretching things to their limits on this build-the 6" ring goes right to the edges of my creality (8.1" or 183mm). Also the hoop geometry does not lend itself well to adhesion. My other build-an 8"x4" flat device, should have adhered better but it did not. I will redo it with a raft in due timeThanksFritz

OK--back at it--I ran a partial print of my 4x8" "dovetailed rail" tonight. It lifted off the be so severely that I stopped the 5 hour print. This has to be within about 0.1mm flatness because two dovetail chasers ride up and down the lands and the must be perfectly flat.I have attached a few photos-showing the print from the top (z) axis plus a side view to demonstrate the warp. Also an STL of the half image.I have played a lot with bed-leveling, speeds and temperatures now and I don't think there is a lot more to try here. A steel ruler check shows the bed to be perfectly flat, both heated and cooled. And of course I go through a lot of time leveling it.So I think this could relate to the plastic (PLA) I am using. Or perhaps a perfectly flat item that covers the whole bed is just not a good fit for this printer creality ender 5) . I went ahead and drafted the print into two 4"x4" pieces and was surprised to see that even at this size there was about 0.8mm warpage at one end-more than is acceptable for a set of dovetails running on lands. Not sure when to throw in the towel here-when I have all the material easily available I can machine this construct in metal in about 30 minutes. Fritz

Any chance you can post larger pics. Those are tiny.

Sorry-i was worried about space limitations (the board says 19KB) . Here are originals.
Fritz