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I was having an issue with flat sheets raising up during initial layers. I had my bed at 80 °C and kept raising it thinking it was an issue with cold. After reducing the temperature to 50 °C the sheets sit flat now but sometimes sections come loose. I ended up using masking tape and a glue stick and everything finally seems to print flat and keep adhesion. I am using 210 °C hotend but I might try lowering it to 205 °C.

How do you check if your printing temperature is too hot/cold or if your bed temperature is too hot/cold? None of the articles I read ever said that a bed too hot would cause PLA to rise but that's what happened.

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Too Hot

If you're printing too hot (with any filament, not just PLA) you're going to see stringing and blobs/oozing because the material is getting runny and exiting the nozzle in an uncontrolled manner. Because it's uncontrolled, you will also likely see artifacts showing up in your prints.

You might also see your filament burning. Instead of coming out of the nozzle as whatever color it should be, it will look brown or discolored because it was overcooked.

If your bed is too hot, you might start to see "elephant's feet" where the lowest layer(s) are being heated to the point of becoming soft and the weight of layers on top of them are pushing down, causing that layer to "pooch out."

You might also have problems removing the print from the print bed's surface because the plastic has seeped into the details of the print surface and hardened, essentially welding the part to the surface.

Too Cold

If you're printing filament that is too cold, you're going to run into an issue where the material being pushed into the hot end is not getting melted sufficiently. This means that pressure will build up in the hot end that can't be released through extruding material through the nozzle. If you've ever tried to pipe frosting using a bag/tip and the frosting was too thick, you'll know what I'm talking about.

When material is being fed into the hot end but not being allowed to flow out of it, something has to give. That thing is your extruder. It has a wheel with little teeth on it to grip the filament and feed it into the hot end (or bowden tube which leads to the hot end). It can exert a certain amount of force on that filament. When the hot end's back pressure builds to the point that it becomes greater than the extruder's force, it will start skipping.

Imagine trying to push a large, heavy object. Your feet will begin to slip as your the force of your exertion overcomes the friction between your feet and the ground. That's what will happen to your extruder. It will make clicking/clunking noises as the extruder unsuccessfully tries to push the filament through and the friction between the teeth of the gear and the filament is overcome. This is called grinding.

If your bed is too cold, you simply just end up with problems getting the print to adhere to the surface.


Warping

In your particular case, you're describing warping. Remember from physics 101 that cold things contract and warm things expand. Your print bed is warm, and so, too, are the first layers that are near it because the bed's heat is transferring up into them and keeping them warm.

Obviously, your active (topmost) layer will also be a bit warmer as it as just come out of the hot end. However, in general as you move higher up away from the printed bed, the printed layers get colder. Because they are getting colder, they are undergoing thermal contraction.

This creates a thermal gradient where layers go from greater thermal expansion to greater thermal contraction. The combination means that the bottom of the print will start to curl up (away from the expansion and towards the contraction).

This is not an issue with your printing temperature. It's a problem with your ambient temperature.

The easiest way to fix this issue is to put your printer in an enclosure. This isolates the air immediately around the printer from the rest of the air in the room. Because your heater's bed and nozzle are throwing out a lot of heat, they will heat up the print chamber quite a bit (mine typically runs over 30 degrees celsius, even in the dead of winter).

Because the ambient temperature in the print chamber is so much warmer than the outside air, that temperature gradient is much, much smaller. As a result, warping will stop becoming a problem.

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