Some excerpts from a Rapid Prototyping article in Manufacturing
April 2009 Vol. 142 No. 4
Art to part with no machining
Robert B. Aronson, Senior Editor
"Rapid prototyping" is no longer a suitable catch-all term that defines
an entire industry. It is just one of many applications for a field
more properly called additive-fabrication (AF) technology. (Machining
operations are subtractive in that a product is created by cutting away
material. With AF, products are made by adding material.) In addition
to prototypes, this technology's applications include short-run
production, replacement parts, tooling, and casting patterns. Other
terms used for this type of work are rapid manufacturing (RM) and
additive manufacturing (AM).
Whatever the title, this industry is no longer dominated by service
bureaus. Lower costs and greater ease of use has made much AF equipment
both more attractive and affordable. Printers that make products in 3-D
are now available for $15,000, and there are hints that $10,000 units
are in the works.
In units that can make metal parts, a laser or electron beam focused on
a steel preform creates a molten-metal pool. Then a stream of powdered
metal or polymer is injected into the melted metal. The stream of
powdered material, under computer control, traces out the desired shape
and solidifies into a thin layer. The layers build up to form a part.
Currently metal parts are made using a process offered by EOS, a German
company with offices in Novi, MI. According to company Vice President
Jim Fendrick, "EOS systems work with two stainless steels,
cobalt-chromium, maraging steel, and titanium, and efforts are under
way to include the Inconel family of metals as well as aluminum. AF
technology has advanced to the point where it is a viable option to
more traditional manufacturing processes."
The unit now available from EOS has a work envelope of 10 x 10 x 12"
The EOS direct-metal process produces parts with properties said to be
superior to those made by conventional casting, chiefly because of
better grain structure.
Electron-beam melting (EBM) is the process used by the metal-part
making units from Arcam (Gothenburg, Sweden). This process is similar
to that employed by units based on a laser-heating system, but instead
Arcam units use an electron beam. The Arcam A2, the larger of the two
units the company builds, uses a 4000-W electron beam to heat metal
powder to 1100°C. The molten stream is used to form the part under
The Selective Laser Melting (SLM) process used by MTT Technologies
Group (Staffordshire, UK) produces fully dense metal parts direct from
a 3-D CAD program using a high-powered laser. Parts are built
layer-by-layer in thicknesses ranging from 30 to 100 µm. The process
uses a range of atomized powder metals including stainless steel 316L
and 17-4PH, H13 tool steel, aluminum alloys such as Al-Si-12Mg and
Al-Si-10Mg, titanium alloys including Ti-6Al-4V and Ti-6Al-7Nb,
commercially pure titanium, and cobalt.chrome (ASTM75).
"Build speed and surface finish are both material dependent. Tool
steels take longest to make a part and titanium and aluminum parts are
made faster. For fully dense parts, build speed reportedly ranges from
5 to 20 cm3/hr. Surface finish is in the range of 15?30µm for
horizontal surfaces and 13?25µm for vertical sides. Laser scanning
strategies can control surface finish," Weston concludes.
Here's What's Happening
One misconception is that additive fabrication (AF) materials are not
strong. This stems from the early days when stereolithography resins
were very brittle. There's still a great need and opportunity to
produce materials for a broader range of applications, but the
materials available today are meeting many needs.
Another misconception is that AF machine technology is not affordable.
In January 2009, Stratasys introduced the uPrint machine for $14,900. Z
Corp. offers a machine for $19,900. It is true that you can spend $1
million or more on equipment, but sub-$50,000 systems are available.
Two fields that are developing notably are dental copings for crowns
and bridges, and orthopedic implants. Companies are using metal-based
systems from Arcam, EOS, and MTT. The materials for these two
applications are mostly cobalt-chrome and titanium alloys.
AF is becoming an alternative to machining and even injection molding,
in some instances.
13 years ago