Materials

Large statue made by stereolithography.	Flexible hoses made by selective laser sintering (SLS).	Detailed connector housing fabricated by fused deposition modeling (FDM).	Fan fabricated by high resolution PolyJet.
(Courtesy, Materialise, Belgium)	(Courtesy, DSM Somos)	(Courtesy, Stratasys Corp.)	(Courtesy, Objet Geometries, Israel)

Selective laser sintering (SLS) is the technology which produces plastic parts which most closely emulate those from other processes. However, there is always some porosity of at least a few percent because the parts are sintered from powder layers, and powders always have spaces between the particles. In a similar fashion, fused deposition modeling (FDM) while nominally using materials such as ABS and several other thermoplastics, produces parts with a grain structure because they are vectorially extruded a layer at a time out of a nozzle. In general, the physical properties of parts produced by a rapid prototyping system, such as tensile strength and elongation, will be somewhat poorer than those produced by other methods. Indeed, if it comes to pass some day that two RP technologies use the same, chemically identical material - which today none do - there will be differences in the physical properties of parts realized by each method because the operation of the machinery will be different.

The parts will also be anisotropic. That is, they may have different physical properties depending on which direction measurements are made, and differences can also arise if the exact same part is made in a different way. This can happen if the building orientation of the part in the machine is changed, and also from the sequence in which the part's elements are fabricated.

Stereolithography and other photopolymer-based methods offer plastic parts, too. However, liquid photopolymers are not exactly thermoplastics. Thus, the best we can hope for is that the resulting materials closely emulate or replace certain engineering plastics whose properties we admire or require. The chemistry of photopolymers is very rich, however, and these materials are improving at a rapid pace. Photopolymers that imitate polypropylene, ABS, polyethylene and a number of other plastics are available, as well as specialty materials for optical, medical and other applications.

Inkjet systems also output plastic parts. MultiJet Modeling (MJM) offers a soft, thermoplastic material which is essentially a hot melt adhesive. Other inkjets offer polyester or wax-like materials. None of the materials from inkjets are very usable as is, except perhaps in lost wax casting processes, but at least it's not hard to use that technology and others to change them into a more desirable material.

Plastics from an RP system may not replicate the characteristics of an injection molded plastic part very well. That's because, as mentioned previously, while the chemical composition might be exactly the same the physical processing is completely different. Many plastics have long, linear molecules that provide an anisotropic "grain structure" to a part because as they flow in the heated liquid state through the mold under pressure, they orient themselves with respect to one another. The process of cooling within the mold also has an effect. This is not what happens during an RP process and as a consequence the physical properties can be quite different. If the need is to test how a mold might perform and what final physical properties might be attained in a part, a better choice may be to make a simplified mold using RP and run that on an injection molding machine as close as possible to final parameters.

It should be noted that while the choice of plastic-like materials is the greatest available in the rapid prototyping field, it still is a very limited one - really just a handful compared to the literally thousands of materials and grades available for other processes. Also, the materials available are not well-characterized for many important properties, and most are also very expensive. It's thus a very good thing that numerous secondary processes exist for changing the output from a rapid prototyping machine into some other material. Many of these processes are listed in the tables for tooling applications.