I'm trying to print a calibration cube from PLA using a 70 °C heated build platform on a Prusa i3 Pro W. This results in:
Please explain why it prints like this.
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There are multiple issues that cause this result.
First, your nozzle is to far from the bed. This can be seen by the curly deposited filament on the build plate (I guess that is the brim or the skirt). Please properly level the bed and position the nozzle at a distance of a plain A4 paper as best as possible (should be doable as you have a glass sheet that are usually very flat as a result of the production process to make glass).
The second problem you face is layer shift. You see that the squares are printed further and further to the left, the print head does not return to original position. Layer shift is usually caused by improper belt tension or a loose grub screw of the belt pulley. This answer describes layer shifting in more detail. This question may be helpful too, the answer contains some references to layer shifting.
The layer shift can also be caused by the stepper motor "skipping" when it hits an obstruction. The obstruction can be a loop or ridge of filament sticking up higher than it should be.
As Oscar suggests, the first step is always to make the first-layer-thickness be right. This is often called "bed leveling", but really it is making sure that the bed defines the z=0 plane that the firmware is expecting. If it isn't planar, if it isn't constant Z (ie., it tilts in any direction), or it isn't at zero, problems follow.
It seems to have mostly shifted in X, so start with that motor and belt. Have you adjusted the tension correctly? Is the motor current set correctly?
Other things can also cause trouble, such as:
Are the slicer settings matched to the printer? If this is a commercial printer, are you using a profile from the manufacturer? If this is your design, or heavily modded, have you tried de-tuning the slicer parameters and/or the configuration parameters?
Any debugging proceeds from the first step, so adjust the Z=0 plane and see if you can print a uniform, single layer print covering the while bed. When that is good, you are most of the way to victory.