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Support: Unlike granular binding systems, fused plastic extruders cannot deposit plastic in mid-air and have it remain there. For overhangs and wide spans, support material must be added to the design and later removed after printing.
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Raft: Depending on the type of material and build plate used, it may be useful to add a raft or flat printed layer that forms a base on the build plate for your model. As in the case of support, the raft is later removed in finishing the object.
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Fill: Because additive systems like the RepRap are unaffected by complexity, it is possible to define any solid object as a solid outer shell and an interior space that can be completely solid (100% fill), empty (0% fill) or some midway point in which a regular pattern of thin walls provide support with voids interspersed. The amount of volume that includes plastic is defined as its "Fill" and allows you to produce the same object printout using far less plastic than solid equivalents.
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Slicing: The virtual model is calculated as a series of layers, with each slice then translated into codes that will direct the printer to move a particular distance while extruding or not and then again, until that layer is done and the z-axis can be moved to the next layer with the process begun again. This code is called "g-code" and many slicers can add support and raft elements automatically, along with an automatic "fill" pattern to reduce the amount of plastic needed for each print.
