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2
The Basic Process
Although
several Rapid
Prototyping techniques exist, all
employ the same basic five-step process. The steps are:
Create
a CAD model of the design
Convert the CAD model to STL
format
Slice the STL file into thin cross-sectional layers
Construct the model one layer atop another
Clean and finish the model
CAD Model Creation: First, the object to be built is
modeled using a Computer-Aided Design (CAD) software
package. Solid modelers, such as Pro/ENGINEER, tend
to represent 3-D objects more accurately than wire-frame
modelers such as AutoCAD, and will therefore yield better
results. The designer can use a pre-existing CAD file
or may wish to create one expressly for prototyping
purposes. This process is identical for all of the RP
build techniques.
Conversion
to STL Format: The various CAD packages use a number
of different algorithms to represent solid objects.
To establish consistency, the STL (stereolithography,
the first RP technique) format has been adopted as the
standard of the rapid prototyping industry. The second
step, therefore, is to convert the CAD file into STL
format. This format represents a three-dimensional surface
as an assembly of planar triangles, "like the facets
of a cut jewel." 6 The file contains the coordinates
of the vertices and the direction of the outward normal
of each triangle. Because STL files use planar elements,
they cannot represent curved surfaces exactly. Increasing
the number of triangles improves the approximation,
but at the cost of bigger file size. Large, complicated
files require more time to pre-process and build, so
the designer must balance accuracy with manageablility
to produce a useful STL file. Since the .stl format
is universal, this process is identical for all of the
RP build techniques.
Slice
the STL File: In the third step, a pre-processing program
prepares the STL file to be built. Several programs
are available, and most allow the user to adjust the
size, location and orientation of the model. Build orientation
is important for several reasons. First, properties
of rapid prototypes vary from one coordinate direction
to another. For example, prototypes are usually weaker
and less accurate in the z (vertical) direction than
in the x-y plane. In addition, part orientation partially
determines the amount of time required to build the
model. Placing the shortest dimension in the z direction
reduces the number of layers, thereby shortening build
time. The pre-processing software slices the STL model
into a number of layers from 0.01 mm to 0.7 mm thick,
depending on the build technique. The program may also
generate an auxiliary structure to support the model
during the build. Supports are useful for delicate features
such as overhangs, internal cavities, and thin-walled
sections. Each PR machine manufacturer supplies their
own proprietary pre-processing software.
Layer
by Layer Construction: The fourth step is the actual
construction of the part. Using one of several techniques
(described in the next section) RP machines build one
layer at a time from polymers, paper, or powdered metal.
Most machines are fairly autonomous, needing little
human intervention.
Clean
and Finish: The final step is post-processing. This
involves removing the prototype from the machine and
detaching any supports. Some photosensitive materials
need to be fully cured before use. Prototypes may also
require minor cleaning and surface treatment. Sanding,
sealing, and/or painting the model will improve its
appearance and durability.
Notes
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