§ 4.7.1 Vapor and Droplet Deposition in Vacuum
Vacuum vapor deposition in space utilizes an electron beam to vaporize a metal sheet, billot or ingot target, and the vapor sputters off onto a mold, or is manipulated by a magnetic field to paint a mold. Parts of any shape can be made with great ease, speed and purity, using electron and particle beams. This will work much better in the vacuum and zero gravity of space.
Aluminum sheet and steel sheet have been commercially produced on Earth by vapor deposition despite the costs of creating a special environment to minimize oxygen embrittlement. (Of course, in the vacuum of space, there's no problem with oxygen embrittlement.) Electron beam guns have also been used to coat millions of square feet of architectural glass each year. "Extensive work has been done on developing high rate physical vapor deposition of metals and alloys and evaluating the mechanical properties of metals so deposited. ... [R]esearchers [have] determined that the mechanical properties of vapor deposited metals and alloys can be comparable to those of the same metals made by casting, rolling and annealing." (General Dynamics report) Thus, many standard Earth processes for deforming material by application of large mechanical forces can be replaced by vapor deposition in space.
Droplet deposition is a relatively new technique on Earth used to make unique parts, as opposed to mass produced parts, without using a mold, by adding small increments of material to an existing structure, slowly building it up into any shape desired. Machines are already available which do this by programming, using plastics, and are used to make prototypes for product development. Indeed, they have opened up a whole new industry called "rapid prototyping". It has been pointed out that there are advantages to doing this in zero gravity and without air. As of 1992, commercial droplet deposition was using only plastic, but a number of companies were working to develop rapid prototyping machines able to make metal and ceramic prototype products. This process is related to "shape welding" or "shape melting" whereby layers of weld material are fed and built up. This method has been used to make vessels up to many tons in mass.
The energy for all these processes to melt or vaporize the metal can be direct solar oven heating or electrical heating (induction, resistance, or electron beam).
Droplet deposition is slow, whereas vapor and spray deposition is fast. The process used depends upon whether the product is mass produced (justifying a mold) or unique. Droplet deposition lends itself to computerized design so that producing a new product just means reprogramming the computer to move the droplet depositor in a different sequence. Droplet deposition requires less power and lends itself to simple, flexible robots sent out to perform tasks without being in a great hurry.
One such company, Incre, Inc., has produced products by incremental droplet deposition using aluminum and other metals, presented a paper on the technique at the 1993 SSI/AIAA Princeton conference, and mentioned plans to do work with nickel and steel alloys such as is found in asteroidal metal.