Robin2:
I hate to be a wet blanket, and its a long time since my kids used Lego, so things may have changed.I would be very surprised if you can make effective Lego bricks with a 3D printer unless you can use the exact same plastic that Lego uses.
Also I suspect that the "grip" of Lego bricks depends on very fine control of tolerances.
Finally, I suspect 3D printed bricks would be very expensive as well as very slow to produce.
...R
Its been done and takes roughly 6 hours to print 20 bricks.
Abs is the preferred material as the more common PLA can be brittle when cold.
Use is not generally to make bricks but adapters for special purposes to mate with the bricks.