Multi Jet Modeling

The ThermoJet and the Actua 2100, both made by 3D Systems

in Valencia, CA, fall into the growing area of the rapid prototyping
(RP) market known as concept modeling. Both systems apply the
Multi Jet Modeling (MJM) build style to produce wax prototypes
with an array of ink jets. The systems are one of the least expensive
in the line of RP technologies. They are also safe and clean enough
to operate directly in a design office environment. The ThermoJet is
actually a replacement to the Actua 2100, but there are still several
Actua units in operation throughout the world.

5.1 System Hardware

The ThermoJet/Actua 2100 comes as a single self-contained
unit (excluding the control computer), and is about the size of a
large photocopy machine. One of the more unique features of the
ThermoJet/Actua 2100 is the ability to network the system much
like a standard printer. This allows the download of build files into
a queue from various areas of the office. Parts are "printed" out in
the order they are received the same way that documents are printed
out on the standard printer. The ThermoJet/Actua 2100 also does
not require any post-processing units, as the support removal is done
easily by hand.
The key component of the MJM process is the material deliv-
ery system. The wax billet is loaded into a reservoir inside the cabi-
net of the machine. The material is kept molten there and is siphon-
fed to the multi-jet head. The Actua 2100 multi-jet head has four
44

Copyright © 2001 by Marcel Dekker, Inc.

 Multi Jet Modeling 45

rows of 24 jets each, a total of 96 jets for print-on-demand capabil-

ity, whereas the ThermoJet systems have over 300 jets spanning the
entire cross section of the part build area for a faster build capability.

5.2 Multi Jet Modeling Process Operation

5.2.7 System Software

The software for the Thermo Jet/Ac tu a 2100 system is very
user-friendly, where user input is kept to a minimum. It is available
on the PC platform, and accepts the standard STL RP file format.
Basically all of the slicing and operating parameters are default set-
tings that normally do not have to be changed. Parts can be nested
in the -x, -y plane with a unique auto-nesting capability, and are
then essentially "printed" into functional, accurate wax parts. Figure
5.1 shows the latest Thermojet system.

Figure 5.1 The ThermoJet MJM System (Courtesy of 3D Systems).

Copyright © 2001 by Marcel Dekker, Inc.

 46 Chapter 5

5.2.2 Build Technique

The MJM process builds parts by printing thin consecutive lay-
ers of the molten wax in the shape of the part cross sections. Like
most RP systems, the parts are built onto a movable z stage, which
lowers as the part is "printed." Currently, the Actua 2100 system
prints with a layer thickness of 0.0039 inches, or three passes of
0.0013 inches, whereas the ThermoJet system prints multiple passes
of thinner layers for higher resolution. The multi-jet head traverses
in the x axis direction (left to right), as the printer gantry that houses
the head increments the width of the affective print area in the y di-
rection. The multiple jets are turned on and off where needed at
precise intervals, which allows for an accurate final article in the xy
plane. The final z dimensions suffer a slight amount of inaccuracy
on the lower side due to support removal, yet the upward facing sur-
faces have excellent surface quality. Figure 5.2 shows the MJM
process.

Figure 5.2 The multi jet modeling process (MJM) (Courtesy of 3D

Systems).

Copyright © 2001 by Marcel Dekker, Inc.

 Multi Jet Modeling 47

5.2.3 Postprocessing
MJM process parts are fairly easy to post process. The sup-
ports are built with the part build material, yet are strategically
shaped for easy removal. The entire support structure consists of
very fine columns of the build material that reach from the build
platen up to any overhanging surfaces. These columns are thin
enough that most of them can be "rubbed" off the part with your
finger, and the rest can be sanded away. Again the upward-facing
surfaces already have excellent surface quality (detailed enough to
print a full-scale business card with raised print), so all of the post
processing needed is on the underlying surfaces where the supports
have been removed. For best results, the parts can be placed into a
freezer for an hour prior to support removal. This causes the fine
supports to become brittle and hence much easier to rub off. Care
must be taken not to fracture the part itself, however.
After the quick support removal, the parts are complete. They
require no post curing, infiltrating, or dipping. They are nontoxic
and can be handled immediately after processing.

5.3 Typical Uses of Multi Jet Modeling

The MJM system was designed specifically for concept mod-
eling in the office environment. The models can be built quickly
and effectively, and are durable enough to demonstrate designs in
presentations and meetings. The MJM process parts are also dimen-
sionally stable enough to use for limited fit-check analysis applica-
tions.
Another use of the MJM process parts developed later for a
more practical test hardware application. The wax makeup of the
build material makes it applicable as an investment casting pattern
material. The glossy surface finish and easy melt-out provide for
clean, crisp metal castings.
As materials develop for the MJM process systems, they may
begin to play a larger part in the more functional prototyping roles.
Until then they will continue to serve an important component in the
concept-modeling and casting realm. Figure 5.3 shows some sam-
ple MJM parts.

Copyright © 2001 by Marcel Dekker, Inc.

 48 Chapter 5

Figure 5.3 Sample parts from the MJM process (Courtesy of 3D

Systems).

5.4 Advantages and Disadvantages of Multi Jet Modeling

The advantages of the MJM systems include speed, afforda-
bility, reliability, ease-of-use, cleanliness, crisp resolution, and net-
working capability. All of these features stack up to make for a very
robust RP system.
The key disadvantages are not unlike those experienced by
other RP systems, which include low-strength material, rough sur-
faces on the support material side of parts, and the moderately high
cost of the build material.
Unfortunately, also, even though the post processing required
has been significantly reduced in the Thermojet system, sometimes
designers still find the effort required to clean up the parts is more
than what fits into their working schedules. Thus the systems may
tend to be used less than what would be ideal to maximize the de-
sign-to-manufacturing experience in a corporation.

5.5 Key Terms

Ink-jet technology. The same basic technology as used in desktop
publishing printers, ink-jet technology provides the core process for
concept modeling RP systems.

Copyright © 2001 by Marcel Dekker, Inc.

 Multi Jet Modeling 49

Networking. In this case, networking refers to electronic file trans-

fer networks, such as local area networks (LANs), over which STL
data can be transferred to the RP system. The MJM systems maxi-
mize the use of networking by allowing print queues, in which op-
erators from remote computer workstations can send multiple parts
to the systems to be manufacturing consecutively.

Concept modeling. Applying fast, inexpensive RP systems to the

concurrent engineering design loop by providing physical verifica-
tion models to designers, salesmen, etc.

For more information on MJM systems, contact 3D Systems at

805-295-5600.

Copyright © 2001 by Marcel Dekker, Inc.