3D Printers in Risk of Rain 2 can be a bit of a risk. They will draw a random item from your loadout and print a copy of a very specific item instead. Each 3D Printer has one item you can get from it, but you can sacrifice as many items as you like to stack them.
The Scrap machine is a new addition to the game that works hand in hand with the 3D Printer. If you find a scrap machine, you can interact with and then pick an item to turn into scrap. You can do this with as many items as you like, but there is a short animation between each use.
Now, if you can find a 3D Printer, it will take the Scrap and build an item before it starts to random take items. If you have 15 Scrap in your loadout, it will take all 15 before it starts to randomly chew through the items you like.
Every time you find a scrap machine, it is good to go through your loadout and get rid of items that just do not really add to your build. When you find the right 3D Printer, you can turn all that scrap into something actually useful.
It’s a risk either way, as you could be giving up worthwhile but not top tier items in the hope of a perfect 3D Printer that will never appear in your game. That is just how Risk of Rain 2 works, it is all about taking calculated risks and putting your faith in the gods that what you need will show up.