05-09-2012, 05:01 PM
A Review of Additive Manufacturing
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ABSTRACT
Additive manufacturing processes take the information from a computer-aided
design (CAD) file that is later converted to a stereolithography (STL) file. In this
process the drawing made in the CAD software is approximated by triangles and
sliced containing the information of each layer that is going to be printed. There is a
discussion of the relevant additive manufacturing processes and their applications.
The aerospace industry employs them because of the possibility of manufacturing
lighter structures to reduce weight. Additive manufacturing is transforming the
practice of medicine, and making work easier for architects.
In 2004, the Society of Manufacturing Engineers did a classification of the
various technologies and there are at least four additional significant technologies in
2012. Studies are reviewed which were about the strength of products made in
additive manufacturing processes. However there is still a lot of work and research
to be accomplished before additive manufacturing technologies become standard in
the manufacturing industry because not every commonly-used manufacturing
material can be handled.
RAPID PROTOTYPING
The first form of creating layer by layer a three-dimensional object using computeraided
design (CAD) was rapid prototyping, developed in the 1980’s for creating
models and prototype parts. This technology was created to help the realization of
what engineers have in mind. Rapid prototyping is one of the earlier additive
manufacturing (AM) processes. It allows for the creation of printed parts, not just
models. One of the major advances that this process presented to product
development was the time and cost reduction, human interaction and consequently
the product development cycle [1], also the possibility to create almost any shape
that could be very difficult to machine. However at the present time it is not yet
adopted in the manufacturing sector, but scientists, medical doctors, students and
professors, market researchers and artists use it [2-4]. With rapid prototyping
scientists and students can rapidly build and analyze models for theoretical
comprehension and studies. Doctors can build a model of a damaged body to
analyze it and plan better the procedure, market researchers can see what people
think of a particular new product and rapid prototyping makes it easier for artists to
explore their creativity.
STEREOLITHOGRAPHY
Stereolithography(SL), developed by 3-D Systems, Inc., was the first and is most
widely used process of rapid prototyping, so in the past the two terms were used
synonymously. This is a liquid-based process that consists in the curing or
solidification of a photosensitive polymer when an ultraviolet laser makes contact
with the resin. The process starts with a model in a CAD software and then it is
translated to a STL file in which the pieces are ‘’cut in slices’’ containing the
information for each layer. The thickness of each layer as well as the resolution
depend on the equipment used. A platform is built to anchor the piece and
supporting the overhanging structures. Then the UV laser is applied to the resin
solidifying specific locations of each layer. When the layer is finished the platform is
lowered and finally when the process is done the excess is drained and can be
reused [2,5,11]. A newer version of this process has been developed with a higher
resolution and is called micro-stereolithography.
THE STL FILE
The STL file was created in 1987 by 3-D Systems Inc. when they first developed the
Stereolithography, and the STL file stands for this term. It is also called Standard
Tessellation Language. There are other types of files but the STL file is the standard
for every additive manufacturing process. The STL file creation process mainly
converts the continuous geometry in the CAD file into a header, small triangles or
coordinates triplet list of x, y and z coordinates and the normal vector to the
triangles. This process is inaccurate and the smaller the triangles the closer to
reality [2,14,18]. The interior and exterior surfaces are identified using the righthand
rule and vertices cannot share a point with a line. Additional edges are added
when the figure is sliced. The slicing process also introduces inaccuracy to the file
because here the algorithm replaces the continuous contour with discrete stair
steps [18]. To reduce this inaccuracy, the technique for a feature that has a small
radius in relation to the dimension of the part, is to create STL files separately and to
combine them later. The dimension in z direction should be designed to have a
multiple of the layer thickness value [16]. In figure 6 is shown the position of the
STL file creation in the data flow of a rapid prototyping process. In figure 7 is shown
the data flow in the STL file creation software.