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Bed heating and avoiding PLA warping

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Especially for prints with high density or solid infill, but also for anything with several wall lines...

I've had issues quite often with prints peeling up at the corners when printing in PLA, and yet the two things which have fixed them on different occasions run completely counter to each other. That is to say sometimes a hotter bed helps, other times a colder one helps, and I've no way to know which in advance for planning future print jobs.

I'm using PLA, I don't have an enclosed build chamber, I have a textured bed with a flexible texture sheet which is magnetically held to the rigid heating element plate.

I always print with rafts, Cura does them with a first layer f thicker over-extruded lines, then a criss-cross of normal amounts of extrusion, and then starts printing the actual part and any support struts on a layer atop that.

I usually work at 205 degrees (C) nozzle, 65 bed* for the first layer, and then start the bed cooling to a lower temperature as soon as the first layer is down. For some parts though, particularly wider parts taking up a good fraction of the build plate (this issue never applies for small parts in the central few square cm only), I've had them warp up from the bed at the edges, and as they are mechanical parts for robots this isn't acceptable, a bit of warping and the whole part is useless. I usually had the bed cool to 60* degrees and stay there for the full print, this lead to warping of big parts. So I tried cooling to 45*, and it worked.

But then for another part, which I knew was a wide job, I pre-emptively set it to be 45* degrees throughout the print, except 65* on the first layer, but this warped up, so badly infact it fell off the bed. And when I say warping, I mean the raft layer coming up rom the print bed, with the part still firmly attached to the raft, not the raft staying on the print bed and the model peeling away from the raft. I tried widening the excess "brim" of the raft, but it didn;t help at all, even when the edge of the rat was several cm beyond the edge of the model the whole thing still warped up at the edges, with this warping travelling towards the centre until the whole raft detached from the bed. This print job however then did work when I ran it with the bed at 60* for everything but the first layer.

What on earth is going on here?

What is the trick to avoid warping with PLA and wide high density prints?


*this is the temperature of the heated element plate, I'm sure the actual surface where the printing happens is rather cooler though due to some insulating effect of the flexible magnetic textured layer, 65 as the printer regards it is infact cool enough on the top of the surface to be almost but not quite on the pain threshold when one holds a hand against it, 65 on the heated element layer itself is rather painful if one catches it when peeling away the magnetic layer to remove a part.

Curious problem, though the first thought would be to fasten a thermometer to the bed surface to check what the temperature really is and if its maintained when the print is running.
Have not had and need to do this on our Ender3 but will check it out later today.

Normally do not have to use a raft or brim for smaller PLA parts, though wonder what is the typical base size of what you are printing ?

Is this a new problem to just this reel or PLA or have you always had it ?

When you say you have no enclosure are you printing inside the your house, so would assume it is a resonably stable 20c -ish with no really cold drafts.

Cleaning the bed with non-alcohol: I've had situations where alcohol was useless but cleaning the bed with water and ordinary dish detergent suddenly made things stick properly.

Increase first layer contact area: Brims and brim-like shapes are useful for small terminations of large items.  I also run my first layer very thick (0.35mm thick on a 0.4mm diam nozzle and "Extrusion width" set to 200%), this makes it much easier to get right.

Glue-stick + isoprop spreading: I've only tried this once recently but it seems to work.  Apply a small bit of ordinary glue stick to the bed, spray with isopropanol (apparently this is important) and then spread around on the bed with a piece of paper towel.

Masking tape: I'll cautiously note this one.  It can work great.  it can also work like shite.  It depends on the brand of the tape, some brands stick better to the prints than the bed (especially when heated) which is very frustrating.  I did this for a long time, only occasionally replacing bits of tape when they wore out or were torn off.  Required regular cleaning with soap and water. 

Feeling the bed surface temp with your hand: keep in mind your ability to judge 65degC depends on the thermal conductivity of the item you touch (they have to heat your fingers to reach your thermoreceptors, you can only sense the temperature of your fingers not what's outside them).  Some materials are safe to touch at 90degC, but metals can burn you down near 40degC.  A contact thermometer and a bit of sticky tape is probably the best option.

Just checked the bed temperature with a metal tipped digital thermometer and when set at 55c it reads 51c when covered and held down with some cloth, but it does take some minutes to get to that temperature.    TA -  18c

Wonder if you are starting the print off very soon after powering up,  though the control panel shows its temp sensor reaches 55c in a couple of minutes think it needs at least 10mins+ for the whole bed /surface to get fully heated.

There might be an effect there, the top surface of the bed taking rather longer to come up to temperature than the part of the bed where the inbuilt temperature sensor is. I haven't any handheld thermometers to hand, but I might have some thermometer ICs buried in a box somewhere, I'll see if I can find them and then run a test with one of those on the top surface and see how it compares to temperatures the printer measures from lower down in the bed structure.


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