Direct Tooling Technology Using DMLS

28 Apr 08

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. Direct Metal Laser Sintering provides a wide range of part properties, from controlled porosity for venting or filtering to fully dense structures which can have a higher strength than castings and forgings.

How Direct Metal Laser Sintering (DMLS) Works

The technology works like this: Offline, a technician imports 3D-CAD data as an stl file into the process software to position and orient parts. A layer thickness is chosen to strike a balance between accuracy/resolution and manufacturing speed (thinner layers have a higher accuracy, but takes longer to produce). After the operator selects a material and layer thickness, the software assigns correct building parameters and “slices” the 3D data into layers. This data is then sent to a Direct Metal Laser Sintering machine. Next, the operator fixes a steel plate inside the machine on which parts will be built.

A dispenser in the machine delivers raw powder to the plate and a coater arm with a blade spreads the powder on top of the plate in accurate layers according to the layer thickness chosen. The 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 is the laser sintering part of the process.

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 bottled nirogen gas is not required. In cases where nitrogen is not suitable, such as for titanium, bottled argon is used.

To find Australian suppliers who have the laser sintering process available, visit our Selective Laser Sintering directory.

Direct Tooling Using DMLS

Manufacturing of direct tools is one important DMLS application. Early on users of direct metal laser sintering often built simple rapid tooling — so-called because of its short lead times, to eliminate the cavity machining required. Today's improved technology also allows building what's called advanced tooling with features such as built-in cooling channels and inserts. The technological changes now include harder, tougher materials so rapid and advanced tools are now durable enough to injection mold millions of plastic parts and die cast thousands of metal parts.

Reduced Reliance on Conventional Tooling

Conventional tooling is usually costly and can entail 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 sinker EDM machining the part features. Tooling for complex parts can require sliders, removable inserts, and other machined components that make production even more complex and costly. Direct metal laser sintering can eliminate much of this costly work.

A good example of where DMLS can reduce tooling costs would be a plastic assembly containing 10 separate parts and having a production run of around 5000 units. Each component in the assembly will require its own tool made. Some of the components are complex and will require inserts and sliders. A tooling and manufacture project such as this would likely have a lead time of up to 16 weeks to complete and would be costly considering the low volume of production. In contrast, direct tooling using DMSL could complete the tooling in less than 6 weeks with minimal machining required and cost a good deal less.

Another good example is the manufacture of a moulding tool for a complex rubber part. Due to the nature of the material, rubber parts can be removed from the moulding tool despite the existence of undercuts and intricate features. But tools such as this require generation of complex tool paths or the breaking up of the moulding tool into smaller inserts to enable machining. The direct tool process can manufacture the complex rubber moulding tool in one setup in a fraction of the time required.

Higher Quality Parts From Conformal Cooling

Typically in a moulding tool, cooling channels are drilled starting at an external point. Providing adequate cooling channels is a difficult exercise as components such as ejector pins, inserts and sliders have to be avoided. Cooling channels are a combination of straight drillings which do not follow the contours of a rounded cavity. The result is often a less than ideal cooling solution.

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. The built-in cooling channels provide lower and more-uniform temperatures in the mold, and more-rapid cooling, or heating.

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. Direct tool inserts with conformal cooling in a standard machined production tool can reduce cycle times drammatically with no loss of quality.

Metal powders

A variety of metals are available for direct metal 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 Rc. Parts are easily post-hardened (6 hr at 490C, 914F) to 53 to 55 Rc 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, a bronze-nickel based alloy is often used. 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.

Click here to locate Australian suppliers who have the laser sintering process available.