Stereolithography-based Tooling
Several efforts have been made over the years to directly fabricate molds using epoxy-based stereolithography materials. Direct AIMTM from 3D Systems has probably received the most attention, but has not been widely adopted because of its limitations. A special stereolithography build style is used. (AIM stands for ACES Injection Mold and ACES is an acronym for Accurate Clear Epoxy Solid.) The method has been said to be useful to manufacture short run or prototype quantities up to approximately 50 pieces of small, less-complex thermoplastic parts. Depending on the part geometry, it may be necessary to back the mold with epoxy to provide strength. The molds typically require secondary finishing operations before use to remove stair-stepping and improve finish.
Molds made using stereolithography are used in injection molding machinery, but here again the parts fabricated won't be identical to those created in a high volume mold. Cycle time must be considerably longer due to the poorer thermal conductivity of the material compared to metal, and lower pressures must be used to accommodate its lower strength. The slower cooling cycle required may actually provide advantages for specific part geometries and materials. Glass-filled plastics aren't recommended and its very important to study the limitations of the process as it applies to the specific application.
The delicate nature of a mold fabricated using stereolithography can lead to failure on the very first shot according to some users, so appreciable caution is necessary. The introduction of higher strength and temperature-resistant composite photopolymers may breathe new life into this method, however.
Laser Sintering from EOS GmbH and 3D Systems
Soft Tooling Using Metals
The EOS GmbH Direct Metal Laser SinteringTM (DMLS) process can use a bronze alloy which offers a step up in soft tooling over plastic-based stereolithography methods. Harder, more thermally-conductive materials mean that it's possible to run an injection molding machine closer to final mold conditions. There will still be differences between parts made with bronze alloy and tool steel molds, but not as great as that with epoxy tools. The resultant mold core and cavity inserts are porous, and it may be necessary to infiltrate them with epoxy or low melting temperature metal prior to use. As many as several thousand relatively simple parts have been produced from such DMLS molds. In addition to limited life, soft metal tools generally can't reproduce fine detail and must be finished before use.
3D Systems formerly offered a similar process using a copper polyamide material.
Hard Tooling and Metal Parts
3D Systems' selective laser sintering process for metals uses polymer-coated steel powders. The resultant green part is burned out, sintered and infiltrated with bronze in secondary furnace operations to produce a fully-dense mold with about 70% steel content. These burn-out and infiltration procedures typically take about a day. The SLS metal part fabrication process has been greatly improved over the years to improve accuracy and resolution, and reduce stairstepping. The latter are critical improvements for hard tools because it doesn't make much sense to be able to quickly produce a hard steel tool if lengthy hand finishing and final machining eat up that advantage compared to CNC.
EOS's competing DMLS process for bronze alloys and steel powders doesn't require secondary sintering and burnout cycles in a furnace because the parts produced are already at 95% density. This is a result of a difference in the basic philosophy of how each company has designed its machinery. EOS has chosen to build separately optimized systems for plastics and metals, while 3D has chosen to build more flexible systems which can utilize both classes of materials. The EOS metal part producing machines use more powerful lasers which result in increased density after sintering. Indeed, in some scanning areas the metal is completely fused.
EOS has also paid a great deal of attention to limiting the amount of secondary finishing required and they claim that customers often use their molds for production after simply a quick shot peening. Both steel and alloy metal powders are available that produce 20 micron (0.0008 inches) layers.
These steel-based processes offer the greatest benefit for small, complex geometry parts that would be difficult to machine. Conformal cooling channels can be incorporated into the molds which should last for hundreds of thousands of shots of almost any plastic. Some thought has to be given to conformal cooling channel geometry, however, to make certain that unconsolidated build powder can be removed after fabrication.
EOS introduced a cobalt-chrome alloy in August, 2006, which was initially developed for dental applications. Better operation of parts at elevated temperatures, and with improved strength and corrosion resistance are said to result. The company indicates it is also working on titanium, and 3D Systems has supported much research in aluminum materials at Queensland University (Australia).
MCP-HEK Tooling GmbH
MCP-HEK (Germany) is marketing the RealizerTM selective laser melting (SLM) system. The technology was originally developed by the Fraunhofer Institutes and initial commercialization was done by Fockele & Schwartze, both also of Germany. The process is similar to selective laser sintering, but in contrast fully melts metal or ceramic powders to directly form fully-dense parts. According to MCP, no post processing steps such as burnout or infiltration are required as with the porous parts produced by SLS.
MCP says almost any metal or ceramic powder can be used, and a very high polish is achievable. The process is capable of forming conformal cooling channels for injection molds and is also being applied to short production runs for direct manufacturing. Layer thickness is about 20 microns (0.0008 inches) resulting in sharp edge definition. Several materials have been run in the system including tool steels, stainless steels, pure titanium and cobalt alloys.
In 2003, EOS GmbH announced a cross-licensing and joint venture agreement with Trumpf GmbH to develop a similar technology, called Direct Selective Laser Melting. The technology is now available from inno-shape GmbH. In 2008, 3D Systems began selling MCP's selective laser melting systems in the US.
Arcam AB
Arcam AB (Sweden) provides CAD to Metal® technology based on the Electron Beam Melting (EBM) process originally developed at Chalmers University. It's a powder-based method also having a lot in common with selective laser sintering (SLS), but replaces the laser with a scanned 4KW electron beam which produces fully-dense parts. Materials available include H13 tool steel, Arcam Low Alloy Steel, titanium alloy (Ti-6Al-4V) and pure titanium. Arcam Low Alloy Steel is an easier to machine material for prototyping applications. Systems sell for about $475,000.
Parts are fabricated in a vacuum and at about 1000 deg C to limit internal stresses and enhance material properties. The cooling process is also controlled to produce well-defined hardening. As with other processes, the parts require some final machining after fabrication, although the company claims the finishes produced are better than those available from laser powder forming and other competitive processes. Arcam also says that processing in a vacuum provides a clean environment that improves metal characteristics.
The EBM process may ultimately be applicable to a wider range of materials than competitive processes and also has the potential to offer much better energy efficiency.
In January, 2008 the company announced plans to directly market its products in the US after its agreement with Stratasys came to an end.
ProMetalTM from ExOne Company
ProMetal TM technology is an application of MIT's Three Dimensional PrintingTM process to the fabrication of injection molds. Steel powder layers are bonded by photopolymer selectively applied by a wide area inkjet head. The photopolymer is cured layerwise by a UV lamp mounted on the head assembly. The green part thus formed is then sintered to form a porous steel matrix which is subsequently infiltrated with bronze. Considerable surface finishing is said to be required. Molds can be used to make hundreds of thousands of parts out of nearly any plastic.
ExOne Company was formed by spinning out several disparate technologies from Extrude Hone in 2005 when that company was purchased by Kennametal.
Optomec
Laser Engineered Net ShapingTM (LENS®) technology developed by Sandia National Labs has been commercialized by Optomec. The company's trade name for this technology is Directed Metal Deposition SystemTM (DMDS) which is very similar to POM's name for its technology. See below. Since this method of additive fabrication is not self-supporting, removing fully-dense steel support structures can be difficult in some applications. Injection molds don't have this limitation however, and are consequently one of the major applications the company has pursued.
The method offers fully-dense, tool steel molds made from multiple materials. Using several materials permits optimizing costs and thermal characteristics at any location in the mold. Among the materials the system supports are H13 tool steel, 304 and 316 stainless steels, nickel-based alloys such as Inconels, and cobalt-chrome and Ti-6Al-4V alloys. The method also permits conformal cooling and molds have shown a 20% improvement in cycle time, in line with other processes. DMDS is also said to produce better metallurgical properties than available from intrinsic materials. On the down side, the molds or parts must undergo considerable secondary finishing operations before they can be used.
These are not prototype or bridge tools. Instead, this is a way to more quickly make a true high volume steel mold with characteristics that can't be achieved with CNC technology alone. The technology can also be used to repair or modify existing parts or molds made using conventional technology.
All such laser powder forming technologies offer the additional important advantage of being frugal with expensive materials. Making as few chips as possible is an important consideration when using materials such as titanium.
POM-Group
The Direct Metal DepositionTM (DMD) technology that POM-Group is pursuing is very similar to LENS®. The differences are mainly in the details of machine control and implementation. The process offers similar benefits in terms of multiple materials, conformal cooling and lead time, and also has the same limitations. The process is fairly slow, depositing just 0.5 to 1 cu in/hr, making it more appropriate to build details on a pre-form of the part rather than build it from scratch. POM has concentrated much of its marketing effort on the auto industry.
The process can be used with a wide variety of materials including: nickel-based alloys such as Inconel and Hastalloy, cobalt-, copper- and tungsten-based alloys, cermets, and also stainless-, tool- and precipitation-hardened steels. It is also possible to use more exotic metals such as titanium and to make gradient materials. A different material can be fed from each of the system's four feed cartridges.
Similarly to EOS GmbH, the company also has an agreement with Trumpf Inc. which supplies 5- and 6-axis CNC machines incorporating POM's DMD technology. Trumpf sells the machinery in Europe under the TrumaForm trade name. The machines have two processing chambers allowing one to be used for cooling of finished parts while the other continues with fabrication. Similar combinations of laser powder forming and subtractive technologies are also being investigated in academic and research institute settings.
