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(2. What are the limitations?)

Materials

Plastics
Plastics account for the preponderance of materials used in additive fabrication systems. While in most cases, the plastics are familiar ones such as nylon or ABS with the usual chemical compositions, there are substantial differences in what comes out of an additive fabrication system compared to the results from machining or injection molding the same materials. That's partly because the material has to be in a special form to be used by a the system in the first place, and secondly because additive technologies operate on it in a different way.

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Large statue made by stereolithography. Flexible hoses made by laser sintering (LS). Detailed connector housing fabricated by FDM®. Model made by high resolution PolyJet™.
(Courtesy, Materialise) (Courtesy, DSM Somos) (Courtesy, Stratasys) (Courtesy, 3D Creation Lab)

Laser sintering (LS) produces plastic parts which most closely emulate those from conventional 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, thermoplastic extrusion methods such as Fused Deposition Modeling™ (FDM®) while nominally using materials such as ABS and several other thermoplastics, produce parts with a grain structure because they're extruded in thin, linear tracks, a layer at a time out of a nozzle.

In general, the physical properties of parts produced by an additive system, such as tensile strength and elongation, will be somewhat poorer than those produced by conventional methods. One rule of thumb sometimes used is that they're degraded by about 30% in mechanical properties. Indeed, if it comes to pass some day that two additive 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 upon in 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 build orientation of the part in the machine is changed, and also from the sequence in which the part's geometric elements are fabricated.


Stereolithography and other photopolymer-based methods offer plastic parts, too. However, photopolymers are not 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. Photopolymers that imitate polypropylene, ABS, polyethylene and other plastics are available, as well as specialty materials for optical, medical and other applications.

Inkjet systems not based on photopolymers also make plastic parts. Some offer polyester or wax-like materials, although none of the materials from these inkjets are very usable as is, except perhaps in lost wax casting processes. At least it's not hard to use that technology and others to change them into a more desirable material.

Plastics from an additive 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, freezing the oriented molecules into place. This is not what happens during an additive 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 an injected molded part, a better choice may be to make a simplified mold using an additive method and to 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 additive fabrication field, it still is a very limited one - really just a handful compared to the literally thousands of materials and grades available for conventional processes. Also, the materials available are not well-characterized for many important properties, and many are also very expensive. It's a good thing that numerous secondary processes exist for changing the output from an additive fabrication machine into some other material. A comprehensive list of these can be found in the section describing functional parts and tooling.

 


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REV 3 - - - 10/25/12