4
Applications of Rapid
Prototyping
Rapid
prototyping is widely used in the automotive, aerospace,
medical, and consumer products industries. Although
the possible applications are virtually limitless, nearly
all fall into one of the following categories: prototyping,
rapid tooling, or rapid manufacturing.
4.1
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." 13 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. 14 Snecma, a French
turbomachinery producer, performed photoelastic stress
analysis on a SLA model of a fan wheel to determine
stresses in the blades. 15
4.2
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. 16 Rapid tooling can be divided
into two categories, indirect and direct.
4.2.1
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. 17 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. 18 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.
4.2.2
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. 20
Laser-Engineered
Net Shaping (LENS) is a process developed at Sandia
National Laboratories and Stanford University that can
create metal tools from CAD data. 21 Materials include
316 stainless steel, Inconel 625, H13 tool steel, tungsten,
and titanium carbide cermets. A laser beam melts the
top layer of the part in areas where material is to
be added. Powder metal is injected into the molten pool,
which then solidifies. Layer after layer is added until
the part is complete. Unlike traditional powder metal
processing, LENS produces fully dense parts, since the
metal is melted, not merely sintered. The resulting
parts have exceptional mechanical properties, but the
process currently works only for parts with simple,
uniform cross sections. The system has been commercialized
by MTS corporation (www.mts.com)
Direct AIM (ACES Injection Molding): A technique from
3D Systems in which stereolithography-produced cores
are used with traditional metal molds for injection
molding of high and low density polyethylene, polystyrene,
polypropylene and ABS plastic. 22 Very good accuracy
is achieved for fewer than 200 moldings. Long cycle
times (~ five minutes) are required to allow the molding
to cool enough that it will not stick to the SLA core.
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).
4.3
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 the 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. 23 NASA is
experimenting with using RP machines to produce spacesuit
gloves fitted to each astronaut’s hands. 24 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. 25 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.
Notes
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