Consequences of setting the bed temperature too high with PLA?

Question

On a printer with a heated bed, what are the consequences of setting the bed temperature too high went printing with PLA?

1. a small amount above the recommended setting, 5 Celsius for example
2. Considerably above it, 20 Celsius

I'm thinking of consequences for the print, not for the printer. Such as warping, adhesion, difficulty removing, and so on.

Answer 1

The biggest consequences of printing PLA with the bed too hot stem from difficulty cooling, especially for the layers close to the bed. PLA softens (or hardens, if you're looking at it from the other direction of cooling) around 50-65°C. When you have material in contact with a bed above that temperature, no amount of cooling fan will harden it. It will stay soft. If there's a big mass of material, just the viscosity may help it retain its shape well enough to meet your needs. If not, air pressure from the fan, friction with the nozzle, the pressure of the next layer beying extruded against it, etc. may displace the material from where it was supposed to be, giving you a print that's not just inaccurate but structurally unsound, or where support structures bond irremovably, or gaps that were designed into the model to be gaps fuse together, etc.

Even once you get a few mm away from such a hot bed, or even if the bed somewhat below the temperatures where PLA softens, you will still have greatly diminished cooling capacity. This is because the rate of heat transfer is proportional to the difference of temperatures. When PLA is at 70°C nearing hardening, hitting it with air that's 45°C will only cool it at half the rate as hitting it with air that's 20°C. Once the PLA is down to 60°C, the rates would differ by a factor of 3! This means you either need to give the PLA a lot more time to cool (print slower/longer minimum-layer-time) or use higher-flow fans that will pull in more cooler air from outside the print zone rather than just stirring around the hot air above the bed.

You can see some examples of how bed temperature's effects on cooling affect the print outcome in one of my older questions here: How do you solve PLA corner-curling short of printing really, REALLY slow?