A rapid-manufacturing technology called direct metal laser sintering (DMLS) builds solid metal parts directly from powdered metals, complimenting or sometimes even replacing traditional machining. DMLS provides a wide range of part properties, from controlled porosity for venting or filtering to fully dense structures with a higher strength than castings and forgings.

The technology works like this: Offline, a technician imports scan or 3D-CAD data into the process software to position and orient parts. After the operator selects a material, the software assigns correct building parameters and “slices” the 3D data into layers. This data is then sent to a DMLS machine such as our EOSINT M. Next, the operator fixes a steel plate inside the machine on which parts will be built.

A dispenser in the machine applies raw powder and a coater arm with a blade spreads a 20 or 40-μm (8 or 16 micro-inch) layer on top of the plate. Machine software controls the laser beam's variable focus and positioning as the spot travels across the cross-section. Wherever the beam strikes, it melts the powder into a solid, and melts the solid onto the metal below as well.

This process continues, layer-by-layer, until the build completes. The steel plate acts as a heat sink, so the melted metal solidifies rapidly. For most materials, the build chamber is filled with nitrogen gas to protect parts from oxidation. A nitrogen generator in the machine uses compressed air and separates-out nitrogen, so there is no need to buy bottled gas. In cases where nitrogen is not suitable, such as for titanium, bottled argon does the trick.

What we call DirectTool is one important DMLS application. For example, early users often built rapid tooling — so-called because of its short lead times. In the past, relatively soft materials were available, so most of this tooling went to fabricating prototypes for plastic processing. Today's improved technology also allows building what's called advanced tooling with features such as built-in cooling channels. Rapid and advanced tools are now durable enough to injection mold millions of plastic parts and die cast thousands of metal parts.

Goodbye conventional tooling

Conventional tooling is usually costly and entails long lead times. Even a relatively simple two-part (open-shut) injection mold typically requires EDM and CNC milling to fabricate the cavity (injection side) and core (ejector side). Milling might involve removing large quantities of material from a metal block, as well as separate steps for roughing and finishing. Deep slots and sharp internal corners that cannot be milled require first cutting electrodes for EDM, positioning them, and then EDMing the part features. Tooling for complex parts can require sliders, removable inserts, and other machined components that make production even more complex and costly. DMLS can eliminate much of this costly work.

A good example comes from a company that recently produced 5,000 joystick device assemblies, each of which included 14 injection molded parts in PA6.6 GF. A combination of mold inserts built with DMLS and minimal machining completed the assemblies within six weeks. In contrast, conventional tooling would have had a 16-week delivery time and also cost twice as much.

In another case, DMLS produced tooling for intricate parts that would otherwise have involved generating complex tool paths, machining, and post-machining. The process easily built a mold (for rubber or elastomers) with undercuts, features that typically require complex machining.

The built-in cooling channels provide lower and more-uniform temperatures in the mold, and more-rapid cooling, or heating. DMLS allows positioning and shaping channels or other elements in a freeform way. When freeform channels follow molding surfaces, it is known as conformal cooling.

Conformal cooling can improve cooling in a cost-efficient way. For example, in blow molding PE bottles, the cycle time is typically limited by the cooling time for the necks of the bottles, which have the thickest cross-section. DirectTool inserts with conformal cooling in a standard machined production tool reduced cycle time from 15 seconds to just eight seconds, with no loss of quality.

Conventional machining, in contrast, adds such channels by drilling, which restricts cooling channels to combinations of straight lines. To make matters worse, the channels must avoid intersecting ejector pins, forming surfaces, and the like.

Metal powders

A variety of metals are available for EOS laser-sintering machines and new powders are continually being developed. A high-grade 18 Maraging 300 type steel (1.2709, X3NiCoMoTi18-9-5) is best-suited for production tooling. The material is melted in the machine to produce fully dense parts with a hardness of 36 to 39 R_c. Parts are easily post-hardened (6 hr at 490C, 914F) to 53 to 55 R_c and produce an ultimate tensile strength of more than 1,900 MPa (280 ksi). Components built with the material can be machined, eroded, and polished much like conventional tool steels.

When lower strength and hardness are sufficient, the material of choice is often a proprietary bronze-nickel based alloy called DirectMetal 20. It is quick and easy to build and finish, making it well-suited for prototype and low-volume production tooling. The high build speed comes from setting processing parameters, such as laser scan speed, to produce parts with a higher-density surface and a partially porous internal structure. Also available are stainless steel materials for molding corrosive plastics, as well as cobalt chromium and titanium alloys.