What is
Rapid Prototyping?
(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.
|
The term | Simple
