The list of items currently known to be printable can be found at List of 3D Printable Objects.
First, you must scan an item to recreate. Scanning the item usually costs 25% of the recreation cost in pixels, e.g. if you want to scan a decoration that costs 300 pixels, the scan itself will cost 75. Also scanning an item destroys it. Once an item is scanned, it will remain in the 3D Printer's blueprint database permanently. You can now create as many of that item as you desire, as long as you have the required pixels to do so.
Note that currently material items such as Coal, Iron, Titanium, etc. cannot be scanned at this time. It is unknown as to whether this will change in the future.
- NPC spawners can be scanned, but won't be seen in the 3D printer, thus making the player waste the spawner and pixels.