At what bed temperature is it safe to remove PLA prints?

Question

I print with PLA with hotbed set to 60 °C. If I try to remove the print immediately after printing, the bottom part of the print might deform and keep the shape to which it was bent at the time of removal. If I don't remove the print immediately, and let the hotbed cool down, then the prints are perfectly flat when removed.

At what temperature is it safe to remove PLA prints? Would it be okay to modify my G-code so that the bed heating is turned off approximately 10 minutes before the end of the print? Would it make any sense to append a "cooling routine" to the end of my G-code that would move X and Y around for some time with the fan at full speed and heating turned off?

Answer 1

The reason we use heat in our platforms is to lower the temperature difference between filament being laid down and already hardened filament as some filaments shrink when cooling down (ABS is notoriously known for a larger shrinkage percentage). The temperature we use for the heated bed is close to the glass temperature of the plastic we print, at this temperature, the plastic is malleable. Because it is soft, it can better cope with the stresses caused by the relative cooler layers on top of the layers close to the bed. This prevents warping and prints getting loose from the build plate.

The reason for cooling down first after printing is to prevent the warm malleable first layers (as they have the bed temperature, especially with low height prints, or prints with a thin base) be deformed when removing from the build plate.

The temperature at which it is safe to remove the parts (so they won't get permanently deformed by the removal process) depends on the filament type. It has to be well under the glass transition temperature; so that it has hardened enough that it cannot be bend and maintain that shape. So, when printing different materials, the temperature is different. Furthermore, this is also depending on the geometric shape of the print, a larger volume print object with a relative high height to volume ratio cannot deform as easily as low height to volume ratio prints.

From personal experience, I remove large height to volume ratio prints directly after printing (printing on glass with 3DLAC), a quick tap will generally remove the print. With small height to volume ratio prints, like name tags, I wait for the bed too cool to either the print gets loose by itself (usually still temperature left in the bed) or the temperature has dropped to half the printing temperature difference (somewhere between 30 to 40 °C).

Because of the different materials and the print geometry, it is hard to device a strategy; some materials stick better (can also depend on the build surface). Cooling down the build plate before the print is finished is strongly discouraged, what if the print gets unstuck and the final layers are ruined on a long print? This waste of time and resources is not worth the few minutes to wait for the plate to cool down. If you cannot find the time to wait, you should look into alternatives; a flexible steel bed or multiple sheets of glass where you can quickly empty or replace the bed surface.

Answer 2

You haven't said what printer or bed you are using, or given any details of what kind of contact area you have with your print, so I'm only really able to give you a generic answer, but it will hold true for pretty much any consumer printer form an Ender E3 to a Bambu X1 and everything in between.

PLA is a very stable material that doesn't suffer badly from warping, which is one of the reasons why it's effectively the default for most printers these day.

If you have a removable flex plate you can remove your print almost immediately after the printer has completed the print. I printed a model with an 8" flat square base at 60 degrees a couple of hours ago and was impatient. I flexed the bed within a minute of the print stopping and had zero warping. This is my normal experience. I've only had warping if I've done something weird.

Just gently ease it off at a corner and you will be OK.

If I were less impatient it would probably have released automatically after 2-3 minutes.

I'm using the standard Bambu textured plate. For smooth PEI wait for the bed to cool for 5 minutes and it should release automatically as "sometimes" the cheaper smooth PEI beds can delaminate if you take the print off without them having cooled a little. Especially if they're cheap.

ABS is a different story.

Unless you're doing something uncommon, there is no need to modify your GCode, your slicer will manage the bed for you in a way that's suitable for about 90% of prints for about 90% of people. Unless you have a really specific use case or a printer with a custom slicer profile, your system will already be tuned in.