These fast and rough 3d printers were critical to the early building activities in Moon Town. Powder regolith is sieved through screens into a hopper, which spreads thin flat layers of it over a giant crude printing bed. Then linear Fresnel lenses run along the bed, focusing sunlight to a very thin line across its width and melting the powder there into a glassy material. A set of motorized shutters block out portions of the lens, moving around as the lens travels the length of the bed. This allows simple flat shapes to be defined. Then the hopper spreads another layer of powder and the process is repeated. Successive layers allow simple objects to be built up to a depth of about 50 cm. 

The rails that the lens runs along can also accommodate other simple robotic equipment. In the SketchFab viewer below, a supplementary round Fresnel lens is shown. It can be used to heat spots in the piece being worked on for the purpose of bending, punching, or stamping it. When not doing that, this lens can be used to keep a pot of regolith molten, that can then be used to join pieces together. Perhaps a cutter tool could be added to the rails, that slices pieces when they have solidified just enough for that to work, thus aiding in making folds or cleaner edges.  Screens of insulation could be deployed to slow cooling.

This system is only really effective for about 10 of the 15 days of sunlight during each month, when the sun is high enough. Optionally, that could be extended by using a microwave emitter to melt regolith in the bed at other times. Note that the scale of the printer is, like many things in the town, as large as was practical when it was created. The effective printing area of the bed is 14 m wide and 59 m long. The Fresnel lenses focus the energy of 70 square meters of sun onto a line the width of a spaghetti noodle. 

If you click the arrow in the image below, you can then click on the numbers that appear and take a tour of the concept.