Rapid Manufacturing
The product development landscape has evolved tremendously over the last 20 years.
Product designs have become much more complex in both their shape and
functionality, while the need of reducing time-to-market has increased. The evolution
of CAD and computer technologies have brought forth a new way to produce
prototypes know as Rapid Prototyping (RP). Rapid Tooling (RT) while developing
from the same requirements as RP is generally is concerned with the production of
tooling inserts. Both RP and RT help compress time-to-market of products and are
therefore competitiveness enhancing technologies. Rapid Manufacturing (RM) is a
term that embraces both RP and RT.
Where it once took many weeks and lots of money to produce a prototype or tool
from a design, with RM it is now commonplace to produce the same component in a
matter of hours for a fraction of the cost. This evolution of RM has afforded
companies the ability to verify and change designs more often while spending less
time and money doing so. In general, RM is only cost effective for low volume
component production. Such components meet high tolerance, temperature and
strength requirements. Today, RM parts can be used as:
 Functioning components
 Moulds for plastic injection moulding
 Patterns for other tooling and manufacturing process
 Focus group models for sales analysis
 Marketing models to create literature and ad campaigns
There are number RP technologies on the market, most of which work under the same
fundamental principals. CAD data for the designed part, in a specific file format
(usually STL nowadays), is processed and oriented in an optimal build position. The
data is then sent to the RM machine where it is numerically sliced into thin layers.
The RP machine then fabricates each two-dimensional cross section and bonds it to
the previous layer. A complete prototype is thereby built by stacking layer upon layer
until the prototype is completed.
The automotive and aerospace industries are using RP to check the fit of specific parts
as well as product design. Architects can use it in designing scaled models of
proposed buildings and other structures. In the medical industry, Baxter Healthcare
Corporation is using rapid prototyping to aid in the design of their medical products.
Also, it is used in the design of telecommunications, computer, and consumer
products. Rapid prototyped parts can also be tested for air and fluid flow.
There are two main types of RM, subtractive and additive. Subtractive techniques
involve the removal of material from a block to produce the prototype. The CAD data
is converted into CNC data in order to control the CNC equipment for automated
cutting of the block. Generally softer materials are machined to produce the prototype
so that high cutting speeds can be used (for example polystyrene or plastic). Similarly,
a prototype aluminium injection mould tool may be produced for evaluation, with this
technique, whereas conventional prototyping methods would produce a steel die.
Most modern RP techniques are however additive. The main RP techniques are
discussed below.
Rapid Prototyping Techniques
Stereolithography (SLA)
Patented in 1986, stereolithography started the rapid prototyping revolution. The first
commercial system was introduced in 1988 by 3D Systems of Valencia, CA. The
technique builds three-dimensional models from liquid photo-curable polymers that
solidify when exposed to ultraviolet light.
As shown in the figure below, a platform is lowered into the resin (via an elevator
system), such that the surface of the platform is a layer-thickness (~ 0.1 mm) below
the surface of the resin. A low-power highly focused UV laser then traces the
boundaries and fills in a two-dimensional cross section of the model, solidifying the
resin wherever it touches. Once a layer is complete, the platform descends a layer
thickness, resin flows over the first layer, and the next layer is built. This process
continues until the model is complete. Once the model is complete, the platform rises
out of the vat and the excess resin is drained. The model is then removed from the
platform, washed of excess resin, and then placed in a UV oven for a final curing. The
model is then finished by smoothing the "stair-steps".
Materials
The materials used by SLA equipment are photo-curable epoxy-based resins that offer
strong, durable, and accurate models. It is ideal for form, fit, and function testing as
well as for visual aids and patterns for tooling. In many cases, SLA is capable of
reproducing snap fits. In general SLA materials have a low heat tolerance with typical
heat deflection temperatures around 46 ºC.
Figure 1
Stereolithography machine.
Selective Laser Sintering (SLS)
DTM Corporation introduced selective laser sintering to the commercial world in
1992. SLS uses a laser to sinter powder based materials together, layer-by-layer, to
form a solid model. The system consists of a laser, part chamber, and control system.
The part chamber consists of a build platform, powder cartridge, and levelling roller.
A thin layer of build material is spread across the platform where the laser traces a
two-dimensional cross section of the part sintering the material together. The platform
then descends a layer thickness and the levelling roller pushes material from the
powder cartridge across the build platform, where the next cross section is sintered to
the previous. This continues until the part is completed. Once the model is complete,
it is removed from the part chamber and finished by removing any loose material and
smoothing the visible surfaces
Materials
Duraform is a hard plastic and an example of a material used with this process. It
processes high temperature resistance with a heat deflection temperature of around
140ºC @ 455kPa and has a tensile strength of about 44 MPa.
Figure 2
Selective laser sintering machine.
Fused Deposition Modeling (FDM)
Stratasys Inc. introduced fused deposition modeling in 1990. FDM is a solid-based
rapid prototyping method that extrudes materials, layer-by-layer, to build a model.
The system consists of a build platform, extrusion nozzle, and control system. The
build material is melted and then extruded through a specially designed head onto a
platform to create a two-dimensional cross section of the model. The cross section
quickly solidifies, and the platform descends where the next layer is extruded upon
the previous. This continues until the model is complete, where it is then removed
from the build chamber and cleaned for shipping. FDM is a good choice for
conceptual models, as its dimensional tolerance is adequate (~ ±0.25 mm typical).
Parts with finely detailed features are not recommended with this process as layer
thickness used is generally around 0.25 mm. Layer thickness has an effect on the
model dimensional exactness.
Materials
ABS is a hard plastic and an example of a material that is used with this process. It is
ideal for form, fit and function testing as well as for other visual aids. In its finished
form it is offered in several colours. The melting temperature of ABS is around
177ºC.
Figure 3
Fused deposition modeling machine.
Laminated Object Manufacturing (LOM)
In this process, the laser cuts the slices from a sheet of paper which is then attached to
previously cut layers. Unlike other processes, only the outline of the shape need be
cut, but areas which are to be internal free space in the final model are finely crosshatched by the laser, producing a collection of small cubes in the final model. These
provide support for material which may come on top, and can normally be removed at
the end of the process. Occasionally, the unwanted cubes could become entirely
enclosed within the model, and thus the process has to be interrupted so that the
extraneous material can be removed before enclosure occurs. After the first layer is
cut, the platform lowers out of the way and fresh material is advanced. The platform
rises to slightly below the previous height, the roller bonds the second layer to the
first, and the laser cuts the second layer. This process is repeated as needed to build
the part, which will have a wood-like texture. Because the models are made of paper,
they must be sealed and finished with paint or varnish to prevent moisture damage. In
recent years Helisys, CA (the original developer) has developed several new sheet
materials, including plastic, water-repellent paper, and ceramic and metal powder
tapes. The powder tapes produce a "green" part that must be sintered for maximum
strength.
Figure 4
Laminated object manufacturing machine.
Solid Ground Curing (SGC)
SGC was developed by Cubital Ltd., which is jointly owned by Harwix GmbH, Clal
Electronics Industries, Scitex Corporation, and private investors. Cubital is
headquartered in Israel with subsidiaries in the United States and Germany. SGC (also
known a the solider process) is illustrated in Figure 5. The production machine uses
the CAD data to cure an entire layer of photopolymer in a solid environment. An
ultraviolet light completely cures the material through a photomask. No post-curing is
required. The machine currently marketed by Cubital can build parts to a maximum
size of about 500 x 350 x 500 mm, which is around average for relatively large RP
producing machines, and has building speed of about 55 layers per hour. This
machine costs $470,000 [Cubital, 2000].
In this process, firstly photosensitive resin is sprayed on the build platform. Next, the
machine develops a photomask (like a stencil) of the layer to be built. This photomask
is printed on a glass plate above the build platform using an electrostatic process
similar to that found in photocopiers. The mask is then exposed to UV light, which
only passes through the transparent portions of the mask to selectively harden the
shape of the current layer. After the layer is cured, the machine vacuums up the
excess liquid resin and sprays wax in its place to support the model during the build.
The top surface is milled flat, and then the process repeats to build the next layer. The
entire building process concludes with the part embedded in a block of wax. The
water-soluble wax is melted in a microwave oven, and the part is cleaned in warm
water.
Figure 5
Solid ground curing machine.
Ink-Jet Printing
Unlike the above techniques, Ink-Jet Printing refers to an entire class of machines that
employ ink-jet technology. The first was 3D Printing (3DP), developed at MIT and
licensed to Soligen Corporation, Extrude Hone, and others. As shown in Figure 6a,
parts are built upon a platform situated in a bin full of powder material. An ink-jet
printing head selectively "prints" binder to fuse the powder together in the desired
areas. Unbound powder remains to support the part. The platform is lowered, more
powder added and levelled, and the process repeated. When finished, the green part is
sintered and then removed from the unbound powder. Soligen uses 3DP to produce
ceramic moulds and cores for investment casting, while Extrude Hone hopes to make
powder metal tools and products.
Sanders Prototype of Wilton, NH uses a different ink-jet technique in its Model
Maker line of concept modellers. The machines use two ink-jets (see Figure 6b). One
dispenses low-melt thermoplastic to make the model, while the other prints wax to
form supports. After each layer, a cutting tool mills the top surface to uniform height.
This yields extremely good accuracy, allowing the machines to be used in the
jewellery industry.
3D Systems has also developed an ink-jet based system. The Multi-Jet Modeling
technique (Figure 6c) uses an array of 96 separate print heads to rapidly produce
thermoplastic models. If the part is narrow enough, the print head can deposit an
entire layer in one pass. Otherwise, the head makes several passes.
Figure 6
Ink-jet printing machines.
Rapid Tooling Techniques
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 millimetre. 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, moulds 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 up the process.
Instead of several weeks to make a prototype mould, RT allows for the prototype to
be made within a few days or hours. RT technology is also much less expensive than
conventional mould prototype production, which can run into the €100,000’s. There
are two RT methodologies, indirect and direct. Indirect methods use a RP model, as
produced for example by one of the above RP techniques.
Indirect RT methods
The most widely employed RT techniques use RP masters to make silicone room
temperature-vulcanising (RTV) moulds for producing plastic parts, and as sacrificial
parts for investment casting of metal parts. These processes which are suitable for
batches of 1-20 are known as ‘soft tooling’ techniques.
RTV moulds
RTV tools (also known as silicone rubber moulds) are an easy, relatively inexpensive,
and fast way to fabricate pre-production tools. The main steps in this process are:
1. Start with a vented and gated RP pattern.
2. Place the pattern, supported by plasticine, flat against the bottom surface of a
mould box.
3. Pour silicon rubber to form the first half of the mould.
4. Invert the first half of the mould, remove the plasticine, and attach the top
mould box.
5. Pour silicon rubber to produce the second half of the mould.
RTV tools can be used for moulding parts in wax, polyurethane, and a few epoxy
materials. This process is best suited to parts where form, fit, or functional testing can
be done with a material that mimics the characteristics of the production material.
Investment casting
The use of RP sacrificial models for investment casting was one of the first
applications of RP. The RP model for this purpose can be made by most of the RP
techniques. Investment casting is considered to be the quickest and most robust
method available to accurately produce complex metal prototype parts with good
surface finish. The integrity of investment cast parts means that cast parts are ideal for
the most demanding applications. The technique of using a disposable pattern (the
investment) as the basis for casting a metal object dates back to the 15th century. In
the modern era, World War II provided an urgent need for accurate, complex metal
parts with good surface finish that traditional machining could not address. The initial
discovery that a lost art provided a practical metalworking technique caused great
enthusiasm. Investment casting was found practical for many wartime needs, and
during the postwar and modern periods it expanded into many applications where
complex metal parts were needed. Growth of the investment casting industry has
continued. The process has an excellent reputation for reliability, value, accuracy,
versatility, integrity and good surface finish. The marriage of investment casting and
rapid prototyping techniques as a means to rapidly produce the investment pattern has
resulted in even greater economies and timesavings in the manufacture of small
quantities of complex metal parts. The investment casting process follows these steps:
A gating system is added to the pattern(s) to form a ‘tree’.
The tree is dipped into a ceramic or plaster slurry.
The ceramic (plaster) coating is allowed to dry and forms a shell.
Step 2 and 3 are repeated several times to build up a thicker shell coating that
is strong enough to hold the molten metal during the casting process.
5. The pattern is melted or burned out of the shell.
6. Metal is poured into the shell to form the parts.
1.
2.
3.
4.
Specific example:
Thermojet Wax Pattern fabricator is a RP machine that produces foundry-friendly
wax patterns directly from the 3D model. The Thermojet Wax material is very similar
to production investment casting wax material, and has proven over time to be a
consistent high-quality performer for the casting of both ferrous and non-ferrous
metals. The wax has a melting point of 85-95 C, and can be deposited to achieve an
accuracy of 0.5% and a layer resolution of 0.04 mm.
- Tolerances of ± 0.05 mm/cm on most parts and features
- Aluminum (A356), stainless steel (304), and zinc (ZA8) (other alloys are available)
- Part Sizes up to 500 x 500 x 500 mm
- Typical IC surface finish is 125 RMS
Direct RT methods
Direct RT methods can be divided into two main groups, ‘firm’ tool producing
methods and ‘hard’ tool producing methods. The first group produces tooling that is
less expensive and with shorter lead times. These tools are capable of short prototype
runs of approximately 50-100 parts using the same material and manufacturing route
as for the final products.
The second group includes RT methods that allow for pre-production and production
tools to be built. This latter group is based on the fabrication of sintered metal (steel,
iron and copper) powder inserts infiltrated with copper or bronze. These tools can be
used to produce similar numbers of parts as conventionally produced moulds (250100,000 parts). Commercially available examples of RT processes that produce these
moulds include KeltoolTM from 3D Systems, DTM RapidToolTM process, EOSINT
Metal from EOS, and Three-Dimensional Printing of parts from Soligen.
Aerospace Industry Example
The Boeing Commercial Airplane Company has used rapid prototyping technology to
produce parts for design support, proof of concept, and as a visual aid. Their rapid
prototyping machines have been used to confirm the design of door seal support
structures and to build parts and silicon rubber moulds for wind tunnel projects.
Pratt & Whitney has been actively involved with rapid prototyping technology since
1988. This company finds a significant benefit in that this new technology aids in the
identification of design errors prior to manufacturing involvement. Also, rapid
prototyping facilitated concurrent engineering practices by improving
communications among design engineers and between engineering and manufacturing
departments. A physical part is more consistently interpreted than a sketch. Rapid
prototyping aids in manufacturing producibility studies. The plastic replica of the part
can help the manufacturing shop prove out tooling and fixturing while the actual parts
are being produced.
Automotive Industry Example
Chrysler Corporation has been actively involved with rapid prototyping technology
since 1990. Here are some instances of how this new technology has helped Chrysler.
In the area of design verification, two engineers on an engine upgrade program were
given the task of designing the mating of a distributor cap and body. The parts were
built on an SLA, and when they were put together, they did not fit. The distributor cap
engineer made a design change and built another part in 24 hours. The new part fit. If
conventional methods had been used, the problem would not have been seen until the
prototypes came back weeks later.
In the area of air flow testing, Chrysler used SLA to find the optimum size for an
ambient air duct. Three models were made in one day as opposed to four weeks using
conventional prototyping. The models were tested, and the best choice was selected.
Chrysler has also used rapid prototyping to quickly create precise master patterns for
secondary tooling applications such as room-temperature vulcanising (RTV)
moulding, sand casting, resin transfer moulding, vacuum forming, and squeeze
moulding. Parts produced include centre consoles, interior trim panels, and instrument
panel components.

Other Examples
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.

Specific Surface (Franklin, MA) uses RP to manufacture complicated ceramic
filters (which cannot be made by conventional means) that have eight times
the interior surface area of older types. The filters remove particles from the
gas emissions of coal-fired power plants.