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2. What are the limitations?AccuracyStairsteppingSince rapid prototyping builds object in layers, there is inevitably a "stairstepping" effect produced because the layers have a finite thickness. Those methods that produce the thinnest layers have less stairstepping than others, but it's almost always visible. Inkjet-based technology from Solidscape Inc. produces the thinnest layers. These machines have the advantage that each layer is milled flat after it's deposited and as little as 0.0005 inches of material can be left as a layer thickness. Of course, that makes the building process very slow. Variable layer thickness can sometimes be used to speed things up, however. Stereolithography can also produce thin layers, although not quite at that level and this feature is mainly used to make small parts in the several millimeters or less range. A close runner up in the best stairstepping category are Objet’s jetted photopolymer-based machines. These units are capable of 16 micron resolution in the Z-axis, equal to about 0.0006 inches. Stereolithography can also produce thin layers, although not quite at that level and this feature is mainly used to make small parts in the several millimeters or less size range.
Everything else produces more pronounced stairstepping. Obviously methods based on one or another form of laminated object manufacturing (LOM) will be limited in what can be accomplished because of the raw material thickness. Methods based on powders, for example selective laser sintering (SLS) or three dimensional printing (3DP), have the finite size of the powder as a lower limit. Layer thickness will almost always be greater than the minimum particle size. One cannot indefinitely grind a powder smaller since at some point it starts to acquire a static self-charge that makes it difficult to spread evenly. In some experimental methods of LOM, variable layer thickness is used and cutting means may be employed that shapes edges so that less stairstepping is visible. No commercial systems are available at the moment that do this, however, and this technique is used mainly for very large parts which may be made from plastic foam or ceramic materials.
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