It may be cooling to fast. Try taking a card board box and putting it over the printer while printing.
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You could try to add a brim to the print, and the adhesion to the bed is probably not good enough, use a glue stick or hairspray (if it’s glass)
more infill will cause more warping (shrinking material) just keep it at 20%-25%.
I print PLA a lot, without a heated bed and on tape, warping is minimal.
maybe it’s just not so good quality PLA ?, maybe try another roll ?
Actually PLA is not tend to wrap in printing,Maybe you could print with bigger raft,So the bottom could stick to the hot bed.
You can also check your fan speed, I personally don’t use the extruder fan to cool my prints and I haven’t had any warping since. I have also however also changed my print bed material to glass and included a raft of about 5 lines to each print.
Which slicing software?
I’ve found that a glue stick + blue tape work really well to prevent warping. Whats the environment your printer is in? Are there are any weird airflows coming in or something similar?
Rule 1 : do not cool the first layer
Rule 2 : get a very clean bed. No fingers should touch it. use Isopropyl alcohol to clean it prior to applying any glue
Have you tried re-calibrating you z-height? Proper 1st layer “squish” can make all the difference in preventing warping, or the print itself from coming off the bed. I would imagine that may be the case if even with a raft the print still warps.
Another suggestion is to re-look at the design itself. Does the walls need to be solid like that or can you get away with designing relief holes into the walls to help reduce warping? Solid walls can cause stress on the print because they cool slower where as when designed with reliefs the design uses less filament and helps promotes even cooling which help prevents warping.
The best options to stop warping out of my experience 1) Calibrate your Z height and bed height 2)ensure, your first layer is set to 150% and not too high, you should see 75% height of first layer 3) Make sure, you don’t have heat bed ON. 4) Apply a thin layer of Glue Stick 5) Brim for atleast 15mm 6) First layer speed is 50% of actual speed 7) If your Printer is nearby FAN or Windows, keep them closed or switched off. 8) May be you can also keep your sides closed with carton paper or sheets 9) watch till you see the first layer is adhered properly 10) Switch ON the shroud FAN after 3 layers. 11)) let your first layer speed be atleast 5 °C more than other layers. I think, you will love your successful prints.
I had the same problem with my witbox 2 printer. Out of the box, warping at any print. For me , hairspray was the answer, but not just any hairspray. The strongest version of L’oreal. Now all prints on my - no heated - glass plate stick.
Pla should not need an enclosure to stop it from warping, in my experience Pla warps only when bed adhesion is poor and the corners were never “stuck” down to start with. If your bed is level and your first layer does not like overextruded (assuming your filament and flow rates are correctly calibrated) simply lower your z height slightly until it sticks or looks over extruded. If it still won’t stick you have another issue to solve, I print Pla on kapton tape heated to 50-60 degrees depending on print size and it sticks wonderfully if the z height is correct
Some great advice here from Botzlab
I’d recommend all these and add one more, keep it in reserve if you’ve tried everything and still can’t stop the warping - try adding a thin wall/shield around the model, at least a few layers tall, maybe 4mm tall or so, this will trap hot air around the part and help shield it from colder air. You might think this sounds weird - even in a well-heated house environment this can still make a difference.
Also coating your build surface with something like PVA glue can help, buildtak, 3D-EEZ etc.
Edit: I’d also recommend keeping the heatbed ON.
A shield around the model is a great tool to help, Simplify3D has a name called ooze shield for it. I totally recommend this add on for trouble free print. Cheers, PP Botzlab
18 days later
3 years later
Thank you, I can confirm as I had same experience. Most website recommend a build plate temperature of 70 degrees and till now I always used this temperature. But on my latest project (an large RC plane with thin walls) I had a lot of warping and I’ve spilled 1kg of PLA on it and a lot of time. I’ve lowered the build plate temperature to 30 degrees and now the print is finally ok. I think it’s because I currently print larger opjects so the nozzle needs to travel a longer distance.
Edit; unfortunatly the problem still persist. It starts warping and pulls even the brim from the build plate. Fan is off as well ;-(
Edit2; I think I’ve found it 
Recalibrated the Z-height using piece of paper (even the first layer looked perfect) but instead of using 0.1mm difference I’ve used 0.15 (tenth of a mm) to get closer to the build plate and now it seems like it’s sticking nice and not warping anymore. Still using a brim and temperature of the build plate a bit higher to 50 degrees. Temperature of the PLA is 190 degrees and I’m using a glass build plate and to stick hairspray.