February 16,
1998
CAD/CAM/CAE
Rapid prototyping roundup
Competitive pressures are forcing
industry to reduce product development cycles and bring
out new products more and more frequently. During the
last ten years, rapid prototyping has become one of
the key technologies to help revolutionize the design
process.
Anna Kochan, UK
Rapid prototyping machines convert the CAD file which
defines a part into a solid three-dimensional object
that the designer can touch, hold, and show to colleagues.
The first stereolithographhy (SLA) machines, pioneered
by 3D Systems, (Valencia, CA), built up prototype parts
from successive layers of liquid resin.
Since the introduction of SLA in the late '80s, many
companies have entered the field. They are developing
ever-improving materials and faster processes. Prices
are also coming down. The following examples reflect
some of the most recent advances in rapid prototyping
and its application.
UV light, a photomask, and wax
RANAANA, ISRAEL--Solid Ground Curing (SGC), the technique
behind Cubital's Solider machine, involves shining ultraviolet
light through a photomask to harden successive layers
of resin. Here's how it works:
The system creates a photomask from layered CAD data
using electrostatic toner on an electrostatically charged
plate. The operator aligns the photomask with the build
platform on which a layer of liquid resin has been spread.
Applying a burst of UV light exposes and hardens those
areas of resin which the photomask leaves clear, while
the remaining liquid resin is vacuumed off. Cavities
are filled with molten wax; once the wax hardens, the
entire surface is precisely milled to give a defined
layer thickness and smooth surface finish.
While the first layer is being constructed, the system
cleans the electrostatically charged plate and prepares
a photomask for the next layer. Build time per layer?
About 70 seconds. Largest working envelope? 500 x 350
x 500 mm.
Ford Motor Company reports developing a new air intake
manifold for a 4.6-l V-8 engine with the aid of SGC.
The company claims a cost savings of 33% compared to
less accurate manual methods; time savings totaled four
months or more.
An emphasis on speed
BANBURY, UK--Based on technology acquired from IBM,
the new L51,000 Genisys machine from Stratasys focuses
on high speed. Build material is a plastic polymer,
initially supplied as a small solid wafer.
Delivered one-by-one into a liquefier, the wafers form
a reservoir of liquid polyethylene. An Archimedes screw
drives the liquid into an extrusion nozzle for deposition
onto the build table. Simultaneous nozzle motion in
the X-axis and platform movement in the Y-axis helps
optimize cycle times.
Using Genisys, prototypes measuring up to 203 x 203
x 203 mm can be produced to an accuracy within 0.0356
mm. The machine has sufficient capacity, once fully
loaded with polymer wafers, to build a model weighing
2.3 kg.
Michael Turner of Laser Lines, the company that markets
the Stratasys range in the UK, says Stratasys AutoGen
software automatically prepares a user's CAD file for
the Genisys machine, making the system quick and simple
to operate. Turner adds that Genisys complements the
company's Fused Deposition Modeling (FDM) technology,
which emphasizes accuracy and surface finish, rather
than speed.
FDM is similar to Genisys technology though it works
with a polymer filament. Parts measuring 254 x 254 x
254 mm, accurate to ±0.127 mm, are possible. A larger,
FDM 8000 has also been introduced for dimensions to
457 x 457 x 609 mm.
From CAD to sand
MUNICH, GERMANY--Cylinder heads are extremely difficult
to prototype due to their complex internal channels.
A rapid prototyping system that makes models directly
from sand changes the situation. Developed by EOS GmbH,
the EOSINT S has already earned praise from car companies
across Europe.
EOS sand sintering machines let engineers go direct
from CAD drawings to a sand core or mold without tooling.
As the EOSINT S scans each cross-section of the geometry,
its laser system builds the part in thin layers by locally
binding foundry sand. Once the part is complete and
has been through a post-process operation to cure the
sand, it can go straight to a foundry to be cast in
metal.
"While it is possible to make prototypes of single
cylinder heads using investment casting and existing
rapid prototyping technologies," says Chris Ryall,
operations manager at the Rapid Prototyping & Tooling
Centre, a joint facility between Rover and Warwick Manufacturing
Group, "multi-cylinder heads are a bigger challenge."
Ryall explains that complexity of the internal water
cores makes it difficult to create a consistent ceramic
coating of the prototype's internal geometry. In addition,
he points out that it is difficult to remove the ceramic
shell from inside the head once the prototype has been
burned out and an investment casting made.
Using the EOSINT S700, Ryall claims he can build a
"ready-for-casting" three-cylinder-head mold
in 52 hours--approximately 1/6th the time
and 1/7th the cost of conventional tool making
techniques.
Toner plus paper=prototype parts
HAZU-GUN, JAPAN--Essentially a 3-D photocopier, the
KIRA Solid Center rapid prototyping machine automatically
converts a 3-D computer model into a solid part using
sheets of ordinary paper. As with a photocopier, the
KIRA machine takes one sheet of paper at a time and
applies toner to a selected area of the paper. This
area corresponds to one particular cross-section or
layer of the model being built.
Printed sheets assemble one on top of the other. Pressed
together at a temperature which melts the toner resin
powder, they form the solid model. The toner, therefore,
acts as the adhesive.
Parts made from KIRA Solid Center have the consistency
of wood, and can be used as an investment die casting
pattern. Maximum model size: 400 x 280 x 300 mm.
A more productive ModelMaker
WILTON, NEW HAMPSHIRE--ModelMaker II, an upgraded and
larger version of the MM6-Pro, builds parts three to
five times faster than its predecessor. Developed by
Sanders Prototype Inc., ModelMaker II incorporates an
air conditioning system for quicker cooling cycles,
and faster servomotor speeds for the x-y plotter.
Like the MM6-Pro, MMII uses a patented ink-jet process
to build up models layer-after-layer. Two jets are involved.
One jet deposits droplets of a green thermoplastic material;
the other deposits droplets of a red wax material.
The actual model is constructed from thermoplastic
while all supports for overhanging parts and for cavities
inside the model are made from the wax. After deposition
of each layer, the top surface is milled to remove excess
height.
Immersion in a solvent bath dissolves the wax elements
once the model is complete. The resulting part requires
no finishing and can be directly investment cast. Maximum
model size is 15.24 x 30.48 x 22.86 mm--three times
the build volume of the previous generation machine.
One platform, many materials
HILDEN, GERMANY--The unique feature of DTM's selective
laser sintering (SLS) technology is that a single sinterstation
can process a variety of plastic, metal, and sand powders,
depending on the user's need at the time. Thus, by purchasing
just one platform, a company can make sand casting cores,
conceptual models, functional prototypes, patterns for
secondary processes, and metal mold inserts.
During operation, SLS builds up a part layer-on-layer,
using a heat-generating CO2 laser to selectively
"draw" each cross-section on a fine layer
of powder. The laser causes the particles to melt and
fuse, forming a solid mass. Once the part is complete,
any loose powder simply falls away.
A choice of 11 different powder materials is presently
available for use with the SLS process. The newest material
available is Somos 201, an elastomeric polymer allowing
flexible parts with rubber-like characteristics. Interest
in the Dupont-developed material comes from the automotive,
medical, sports equipment, and toy markets.
An additional benefit of Somos 201: resistance to elevated
temperatures and harsh chemicals such as gasoline or
automotive coolants.
Plastic sheet for LOM
BIRMINGHAM, UK--Laminated Object Manufacturing (LOM)
is usually associated with prototypes made from layers
of paper. Now, Helisys has introduced a plastic sheeting
for use with its existing range of LOM machines.
In principle, LOM machines use a laser to cut successive
layers of a part from a coated sheet material, and heat
to bond the layers together. With LXP 050 LOMPlastic
sheeting, LOM users can produce either paper or plastic
prototypes, or a mixture. Changeover from one material
to the other requires no mechanical adjustments, says
Simon Graham, specialist engineer at Umak, the company
responsible for UK sales.
The new material comprises a polyester film coated
with a proprietary polyethylene co-polymer adhesive.
Sheet thickness is 0.125 mm. Compared to models made
with paper, Graham claims parts made from the plastic
sheet have better moisture resistance, greater flexibility,
and a higher mechanical strength.
Better moisture resistance means that parts can be
used as patterns, masters, and molds for regular tooling
and liquid-based secondary processes. In one application,
LOMPlastic was used to make a water pump impeller measuring
300 mm in diameter and 100 mm in depth. The LOM model
was run at 1,200 rpm for a day, then stripped and examined.
As a result, it was possible to identify where cavitation
had occurred on the impeller and to carry out the necessary
design modifications.
Further new materials for the LOM machine are expected
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