[printer]

1: A very thin layer of powder (plastic) is evenly spread. Then the print head prints a binder on the places where you want to create a slice of your 3D model. Then another layer of powder is layered on top of it and GOTO 1. When all slices are printed your 3D model will be covered in powder which is also the support material for overhanging slices.

It's very hard to make this up from the photos but I think you can see the container for the powder and model with a movable bottom.

-EDIT- This printer uses a binder, but other models melt the powder with a laser (Selective Laser Sintering).

[tocomment]

Thanks. A couple follow up questions:

So does the user sprinkle the powder after each slice, or does the machine somehow do it? How would the machine make an even layer of powder?

Why doesn't the powder stick to the print head, or get pushed around unevenly by the print head?

Why doesn't the powder fall out during printing?

[printer]

So does the user sprinkle the powder after each slice, or does the machine somehow do it? How would the machine make an even layer of powder?

I can't make it up from the pictures, but most printers just trow a pile of powder on top of the last layer and flatten this with an arm. But I think this video will show it much more clear: http://www.youtube.com/watch?v=wD9-QEo-qDk (take a look from 1:50).

Why doesn't the powder stick to the print head, or get pushed around unevenly by the print head?

Because the print head never touches the powder. Just like it doesn't touch the paper in your inkjet printer.

Why doesn't the powder fall out during printing?

Check the video ;)

[wetterhorn]

Well, you've covered pretty much all questions, but here is a reply from the builder :)

So does the user sprinkle the powder after each slice, or does the machine somehow do it? How would the machine make an even layer of powder?

The storage bin moves up a little and the build bin down, and then a counter-rotating roller (mounted on the XY-carriage) deposites the powder from one to the other bin.

Why doesn't the powder stick to the print head, or get pushed around unevenly by the print head?

Indeed, the printer head is mounted ±4mm above the powder bed and can never touch the powder. See the printer head moving across the powder bed in this video: http://www.youtube.com/watch?v=bzX5x1A1AMg

Why doesn't the powder fall out during printing?

Because the pistons form a seal and don't let the powder fall through.