購買設(shè)計請充值后下載，，資源目錄下的文件所見即所得，都可以點(diǎn)開預(yù)覽，，資料完整，充值下載可得到資源目錄里的所有文件。。。【注】：dwg后綴為CAD圖紙，doc,docx為WORD文檔，原稿無水印，可編輯。。。具體請見文件預(yù)覽，有不明白之處，可咨詢QQ:12401814
Chapter 5
Introduction to Rapid Prototyping
Eef Moeskops and Frits Feenstra, TNO Science and Industry, Netherlands
Abstract
The term rapid prototyping (RP) refers to a class of technologies that are used to produce
physical objects layer-by-layer directly from computer-aided design (CAD) data. These
techniques allow designers to produce tangible prototypes of their designs quickly, rather
than just two-dimensional pictures. Besides visual aids for communicating ideas with co-
workers or customers, these prototypes can be used to test various aspects of their de-
sign, such as wind tunnel tests and dimensional checks. In addition to the production of
prototypes, rapid prototyping techniques can also be used to produce molds or mold
inserts (rapid tooling) and even fully functional end-use parts (rapid manufacturing).
Because these are nonprototyping applications, rapid prototyping is often referred to as
Figure 5.1. Prototype of a talking barcode scanner
100
5 Introduction to Rapid Prototyping
solid free-form fabrication or layered manufacturing. For small series and complex parts,
these techniques are often the best manufacturing processes available. They are not a so-
lution to every part fabrication problem. After all, CNC technology and injection molding
are economical, widely understood, available, and offer wide material selection.
In rapid prototyping, the term “rapid” is relative; it aims at the automated step from
CAD data to machine, rather than at the speed of the techniques. Depending on the di-
mensions of the object, production times can be as long as a few days, especially with
complex parts or when long cooling times are required. This may seem slow, but it is still
much faster than the time required by traditional production techniques, such as ma-
chining. This relatively fast production allows analyzing parts in a very early stage of
designing, which decreases the resulting design cost. The costs can also be reduced be-
cause rapid prototyping processes are fully automated. and therefore, need the skill of
individual craftsmen for no more than finishing the part.
General advantages:
· Freedom of design: The production of complex parts is reduced to the accumulation
of layers.
· Well automated: No supervision is needed during the build process.
· Relative easy to employ: Only little preparation and postprocessing are required.
· Avoiding the high cost of prototype tooling, allowing (more) design iterations.
· Physical models are easy to check for errors.
General disadvantages:
· Accuracy generally >0.1 mm.
· Material properties: products can be very fragile, and some need postprocessing be-
fore they can be handled (as with 3-DP).
· Staircasing effect. Because an inclined surface is constructed using several layers,
staircasing will occur.
5.1 The Basic Process
Rapid prototyping techniques share the following process steps (see Figure 5.2):
1. Creating a CAD model either by designing a new or scanning an existing
object.
2. Converting the CAD data to STL format. Because the various CAD packages
apply a number of different algorithms to represent solid objects, the STL
format (Standard Triangulation Language) has been adopted as the stan-
dard of the rapid prototyping industry to establish consistency. This STL file
is a concrete visualization of the product geometry, built up from triangles.
Using triangles to describe a surface, curved surfaces can only be ap-
proached. Increasing the number of triangles (i.e., increasing the resolution)
yields a better approach. However, it also enlarges the STL file. So, one has
to find the optimum balance between file size and part accuracy.
5.1 The Basic Process
101
Figure 5.2. The basic RP process (FDM). A color reproduction of this figure can be seen in the Color Section
(pages 219–230).
3. Slicing the STL file into thin cross-sectional layers. After the STL file has
been sized and oriented, it is sliced in layers with a predefined thickness.
4. Generation of a support structure. This additional step is not required for all
techniques. Because the model is built up in layers, there may be areas that
could float away or have overhanging features, which could distort the re-
sulting model. A base and support structures have to be added, which can be
easily removed after the building step.
102
5 Introduction to Rapid Prototyping
5. Producing the model layer-by-layer. The generated slices are reconstructed
in the machine by building one layer at a time. This can be fully automatic.
6. Postprocessing. This step enhances cleaning and finishing the model and (if
a base or support structure was built) removing the support structure. Some
materials need to be postcured or infiltrated to achieve optimal properties.
5.2 Current Techniques and Materials
A wide range of techniques and materials can be used for rapid prototyping. There
are more than ten commercial rapid prototyping processes and more than five
concept modeling processes; all have unique properties. Due to worldwide re-
search, this range is growing quickly. Commercial techniques are available to pro-
duce objects from numerous plastics, ceramics, metals, and wood-like paper.
Among these techniques are
·
·
·
·
·
·
·
Stereolithography
Selective laser sintering
Fused deposition modeling
Three-dimensional printing
Laminated object manufacturing
Multijet modeling
Laser-engineered net shaping
5.2.1 Stereolithography
Stereolithography (SLA-stereolithography apparatus), launched by 3D Systems
Inc. in 1987, is the first and most commercially used rapid prototyping method.
A platform is placed in a bath of photosensitive UV-curable resin at a level that
leaves a small layer of resin between the top of the platform and the surface of
the bath. A laser (often He-Cd or argon ion to produce UV radiation of about
320–370 nm wavelength) then strikes the desired areas, thereby curing the resin
selectively.
As the layer is completed, the platform descends allowing liquid resin to flow
over the previously cured area. A wiper blade clears the excess fluid from the top
of the surface. This sweep is essential to achieve consistent layer thickness and
prevent air entrapment. As the new layer is cured, it sticks to the preceding layer.
This process continues until the object is completed. On completion, the object
raises above the fluid, so that resin can drain out. The object is carefully removed
and washed in a solvent to remove uncured resin. The cleaned object has to be
placed in a UV oven to ensure that all resin is cured. During the process, features
that lean over have to be supported. This support structure can easily be gener-
ated by software and consists of a series of slender sacrificial columns or lattices.
5.2 Current Techniques and Materials
103
Figure 5.3. Schematic of the SLA process
Figure 5.4. A detailed DMD mirror reproduction using microSLA
A lattice structure is also created as a base to prevent the model from sticking to
the building platform. Thus, additional hand-finishing will be needed to remove
these supporting structures and to sand any small stubs from the surface.
A large variety of photosensitive polymers is commercially available, includ-
ing clear, water resistant, and flexible resins that simulate the properties of, for
example PA, ABS, PP, and rubber-like materials. Process times, tolerances, and
surface finish depend on layer thickness, which is controlled by the amount the
platform is lowered into the resin. Generally, layer thicknesses vary from 0.05–
0.5 mm. Thinner layers can be applied with digital light processing using a tech-
nique called perfactory, which is based on the standard SLA process. Instead of
describing a cross section with a laser, a normal beamer covers the entire cross
section at once. Due to the high resolution of the beamer (pixel size: 39 ìm) and
the accurate positioning system of the platform (layer thickness: 25 ìm), the
parts produced can contain highly detailed features.
Characteristics:
·
·
·
·
Long-term curing can lead to overcuring which leads to warpage.
Parts can be quite brittle.
Support structures are required.
Uncured material can be toxic.
104
5 Introduction to Rapid Prototyping
5.2.2 Selective Laser Sintering
Selective laser sintering (SLS) is a process that was patented in 1989 by Carl Deck-
ard, University of Texas. A layer of powder (particle size approximately 50 ìm) is
spread over a platform and heated to a temperature just below the melting tem-
perature. A carbon dioxide laser needs to raise the temperature only slightly and
selectively to melt the powder particles. As the layer is finished, the platform
moves down by the thickness of one layer (approximately 0.10–0.15 mm), and
new powder is spread. When the laser exposes the new layer, it melts and bonds to
the previous layer. The process repeats until the part is complete.
On completion, the built volume has to cool down to room temperature after
which the processed objects can be removed from the powder bed by brushing
away excess powder. Sandblasting the objects removes all unsintered particles.
Surrounding powder particles act as supporting material for the objects, so no
Figure 5.5. Schematic of the SLS process
Figure 5.6. Accurate positioning elements with internal hinges produced by SLS
5.2 Current Techniques and Materials
105
additional structures are needed. Furthermore, more objects can be built at the
same time because they can be meshed above/in each other. Excess powder can
be reused. However, it needs to be mixed with virgin powder to guarantee good
part quality. Commonly used materials for SLS are nylon (polyamide-12), glass-
filled nylon, and polystyrene. The method has also been extended to direct fab-
rication of metal and ceramic objects and tooling inserts.
Characteristics:
·
·
·
·
·
Key advantage of making functional parts in essentially final materials.
Good mechanical properties, though depends on building orientation.
Powdery surface
Many variables to control
No support required
5.2.3 Fused Deposition Modeling
Fused deposition modeling (FDM), developed by Stratasys, is the second most
widely used rapid prototyping process. A filament thread of plastic is unwound
from a coil and fed into an extrusion head, where it is heated and extruded
through a small nozzle. Because the extrusion head is mounted on a mechanical
stage, the required geometry can be described, one layer at a time. The molten
plastic solidifies immediately after being deposited and bonds to the layer below.
Support material is laid down similarly through another extrusion head. The
platform on which the object is built steps down by the thickness of a single layer.
The entire system is contained within a heated oven chamber which is held at
a moderate temperature above the glass transition temperature of the polymer.
This provides much better control of the process because stresses can relax.
As in the SLA process, overhanging features need to be supported. This support
material needs to be removed in secondary operations. Commercially available
Figure 5.7. Schematic of the FDM process
106
5 Introduction to Rapid Prototyping
Figure 5.8. Scanned archery handle and an FDM reproduction
water-soluble support materials facilitate this final step. ABS, polycarbonate, and
poly(phenyl)sulfone are commonly used materials in the FDM process.
Characteristics:
·
·
·
·
Office-friendly and quiet.
FDM is fairly fast for small parts.
Good mechanical properties, so suitable for producing functional parts.
Wide range of materials.
5.2.4 Three-dimensional Printing
In some textbooks, the term “three-dimensional printing” (3-DP) is used for all
rapid prototyping processes. The process developed at MIT is referred to here.
In this process, a layer of powder is spread over a platform. The particles are
bonded together selectively by a liquid adhesive (binder solution). This liquid is
deposited in a two-dimensional pattern by a multichannel jetting head. As the
current layer is completed, the platform moves down by the thickness of a layer,
so that a new layer can be spread. This process is repeated until the entire object
is formed within the powder bed. On completion, the object is elevated and the
5.2 Current Techniques and Materials
107
Figure 5.9. Schematic of the 3-DP process
Figure 5.10. 3-D printed landscape
extra powder is brushed away, leaving a fragile “green” object. It is necessary to
infiltrate the part with another material to improve mechanical characteristics.
No support structures are required because the surrounding powder particles
support overhanging features. By adding color to the binder solution, objects
can be produced in every desired color. Starch, plaster, medicines (for produc-
ing controlled-dosage pharmaceuticals), ceramics, and metals are commonly
used materials (powders) for 3-DP.
Characteristics:
· Limitations on resolution and surface finish.
· Fragile objects need to be infiltrated.
108
5 Introduction to Rapid Prototyping
5.2.5 Laminated Object Manufacturing
In laminated object manufacturing (LOM), a sheet of paper (unwound from
a feed roll) with a polyethylene coating on the reverse side is placed on a plat-
form. This coating is melted by a heated roller, making the paper adhere to the
platform. Then, a carbon dioxide laser cuts out the cross section of the object
and a border. The laser also creates hatch marks, or cubes that surround the
pattern within the border. These cubes behave as a support structure for the
model. When the laser has finished the layer, a new paper sheet is applied.
Upon completion, the model is captured within a block of paper. When all of
the surrounding cubes have been removed, the unfinished part is sanded down.
The humidity and temperature dependency of the paper material can be reduced
by coating the model. The finish and accuracy are not as good as with some
other methods; however, objects have the look and feel of wood and can be
worked and finished like wood.
Figure 5.11. Schematic of the LOM process
Figure 5.12. Trumpet prototype using LOM
5.2 Current Techniques and Materials
109
5.2.6 Multijet Modeling
Multijet modeling (MJM) uses multiple print heads to deposit droplets of mate-
rial in successive, thin layers. Two major MJM techniques can be distinguished
(see http://www.3dsystems.com/ for more information): ThermoJet?. A 96-
element print head deposits droplets of wax. Because of its relatively fast pro-
duction, this technique is marketed to the engineering or design office for quick
form studies (concept modeling). However, wax models can also be used as
master patterns for investment casting, as will be explained later.
InVision?. A print head jets two separate materials, an acrylic UV-curable
photopolymer-based model material and a wax-like material to produce sup-
port structures for the model. Due to the relative good quality of the models,
Figure 5.13. Schematic of the ThermoJetTM process
Figure 5.14. Wax models produced by M