Applications
of Rapid Prototyping
(categories
| industries)
Although the possible applications are virtually
limitless, nearly all fall into one of the following
categories: prototyping,
rapid tooling, indirect/direct
tooling or rapid
manufacturing.
Prototyping
As its name suggests, the primary use of rapid
prototyping is to quickly make prototypes for
communication and testing purposes. Prototypes
dramatically improve communication because most
people, including engineers, find three-dimensional
objects easier to understand than two-dimensional
drawings. Such improved understanding leads
to substantial cost and time savings. As Pratt
& Whitney executive Robert P. DeLisle noted:
"We’ve seen an estimate on a complex
product drop by $100,000 because people who
had to figure out the nature of the object from
50 blueprints could now see it." Effective
communication is especially important in this
era of concurrent engineering. By exchanging
prototypes early in the design stage, manufacturing
can start tooling up for production while the
art division starts planning the packaging,
all before the design is finalized.
Prototypes are also useful for testing a design,
to see if it performs as desired or needs improvement.
Engineers have always tested prototypes, but
RP expands their capabilities. First, it is
now easy to perform iterative testing: build
a prototype, test it, redesign, build and test,
etc. Such an approach would be far too time-consuming
using traditional prototyping techniques, but
it is easy using RP.
In addition to being fast, RP models can do
a few things metal prototypes cannot. For example,
Porsche used a transparent stereolithography
model of the 911 GTI transmission housing to
visually study oil flow. Snecma, a French turbomachinery
producer, performed photoelastic stress analysis
on a SLA model of a fan wheel to determine stresses
in the blades.
Rapid Tooling
A much-anticipated application of rapid prototyping
is rapid tooling, the automatic fabrication
of production quality machine tools. Tooling
is one of the slowest and most expensive steps
in the manufacturing process, because of the
extremely high quality required. Tools often
have complex geometries, yet must be dimensionally
accurate to within a hundredth of a millimeter.
In addition, tools must be hard, wear-resistant,
and have very low surface roughness (about 0.5
micrometers root mean square). To meet these
requirements, molds and dies are traditionally
made by CNC-machining, electro-discharge machining,
or by hand. All are expensive and time consuming,
so manufacturers would like to incorporate rapid
prototyping techniques to speed the process.
Peter Hilton, president of Technology Strategy
Consulting in Concord, MA, believes that "tooling
costs and development times can be reduced by
75 percent or more" by using rapid tooling
and related technologies. Rapid tooling can
be divided into two categories, indirect and
direct.
Indirect
Tooling
Most rapid tooling today is indirect: RP parts
are used as patterns for making molds and dies.
RP models can be indirectly used in a number
of manufacturing processes:
Vacuum Casting: In the simplest and
oldest rapid tooling technique, a RP positive
pattern is suspended in a vat of liquid silicone
or room temperature vulcanizing (RTV) rubber.
When the rubber hardens, it is cut into two
halves and the RP pattern is removed. The resulting
rubber mold can be used to cast up to 20 polyurethane
replicas of the original RP pattern. A more
useful variant, known as the Keltool powder
metal sintering process, uses the rubber molds
to produce metal tools. Developed by 3M and
now owned by 3D Systems, the Keltool process
involves filling the rubber molds with powdered
tool steel and epoxy binder. When the binder
cures, the "green" metal tool is removed
from the rubber mold and then sintered. At this
stage the metal is only 70% dense, so it is
infiltrated with copper to bring it close to
its theoretical maximum density. The tools have
fairly good accuracy, but their size is limited
to under 25 centimeters.
Sand Casting: A RP model is used as
the positive pattern around which the sand mold
is built. LOM models, which resemble the wooden
models traditionally used for this purpose,
are often used. If sealed and finished, a LOM
pattern can produce about 100 sand molds.
Investment Casting: Some RP prototypes
can be used as investment casting patterns.
The pattern must not expand when heated, or
it will crack the ceramic shell during autoclaving.
Both Stratasys and Cubital make investment casting
wax for their machines. Paper LOM prototypes
may also be used, as they are dimensionally
stable with temperature. The paper shells burn
out, leaving some ash to be removed.
To counter thermal expansion in stereolithography
parts, 3D Systems introduced QuickCast, a build
style featuring a solid outer skin and mostly
hollow inner structure. The part collapses inward
when heated. Likewise, DTM sells Trueform polymer,
a porous substance that expands little with
temperature rise, for use in its SLS machines.
Injection molding: CEMCOM Research Associates,
Inc. has developed the NCC Tooling System to
make metal/ceramic composite molds for the injection
molding of plastics. First, a stereolithography
machine is used to make a match-plate positive
pattern of the desired molding. To form the
mold, the SLA pattern is plated with nickel,
which is then reinforced with a stiff ceramic
material. The two mold halves are separated
to remove the pattern, leaving a matched die
set that can produce tens of thousands of injection
moldings.
Direct
Tooling
To directly make hard tooling from CAD data
is the Holy Grail of rapid tooling. Realization
of this objective is still several years away,
but some strong strides are being made:
RapidTool: A DTM process that selectively
sinters polymer-coated steel pellets together
to produce a metal mold. The mold is then placed
in a furnace where the polymer binder is burned
off and the part is infiltrated with copper
(as in the Keltool process). The resulting mold
can produce up to 50,000 injection moldings.
In 1996 Rubbermaid produced 30,000 plastic desk
organizers from a SLS-built mold. This was the
first widely sold consumer product to be produced
from direct rapid tooling. 19 Extrude Hone,
in Irwin PA, will soon sell a machine, based
on MIT’s 3D Printing process, that produces
bronze-infiltrated PM tools and products.
In another variation, cores are made from thin
SLA shells filled with epoxy and aluminum shot.
Aluminum’s high conductivity helps the
molding cool faster, thus shortening cycle time.
The outer surface can also be plated with metal
to improve wear resistance. Production runs
of 1000-5000 moldings are envisioned to make
the process economically viable.
LOMComposite: Helysis and the University
of Dayton are working to develop ceramic composite
materials for Laminated Object Manufacturing.
LOMComposite parts would be very strong and
durable, and could be used as tooling in a variety
of manufacturing processes.
Sand Molding: At least two RP techniques can
construct sand molds directly from CAD data.
DTM sells sand-like material that can be sintered
into molds. Soligen (www.3dprinting.com) uses
3DP to produce ceramic molds and cores for investment
casting, (Direct Shell Production Casting).
Rapid
Manufacturing
A natural extension of RP is rapid manufacturing
(RM), the automated production of salable products
directly from CAD data. Currently only a few
final products are produced by RP machines,
but txhe number will increase as metals and
other materials become more widely available.
RM will never completely replace other manufacturing
techniques, especially in large production runs
where mass-production is more economical.
For short production runs, however, RM is much
cheaper, since it does not require tooling.
RM is also ideal for producing custom parts
tailored to the user’s exact specifications.
A University of Delaware research project uses
a digitized 3-D model of a person’s head
to construct a custom-fitted helmet. NASA is
experimenting with using RP machines to produce
spacesuit gloves fitted to each astronaut’s
hands. From tailored golf club grips to custom
dinnerware, the possibilities are endless.
The other major use of RM is for products that
simply cannot be made by subtractive (machining,
grinding) or compressive (forging, etc.) processes.
This includes objects with complex features,
internal voids, and layered structures. Specific
Surface of Franklin, MA uses RP to manufacture
complicated ceramic filters that have eight
times the interior surface area of older types.
The filters remove particles from the gas emissions
of coal-fired power plants. Therics, Inc. of
NYC is using RP’s layered build style
to develop "pills that release measured
drug doses at specified times during the day"
and other medical products.
|
The term | Simple
