Deep Dive: Bound Metal Deposition (BMD)
What is BMD?
Bound Metal Deposition™ (BMD) is an extrusion-based metal additive manufacturing (AM) process where metal components are constructed by extrusion of a powder-filled thermoplastic media. Bound metal rods—metal powder held together by wax and polymer binder—are heated and extruded onto the build plate, shaping a part layer-by-layer. Once printed, the binder is removed via the debind process, and then sintered—causing the metal particles to densify.
Prevalent metal AM technologies involve melting powder or wire feedstock using lasers or electron beams. While viable, these systems have substantial facilities requirements to accommodate power and safety requirements. Additionally, localized melting and rapid solidification create complex stress fields within parts, requiring rigid support structures to aid heat dissipation and resist shrinkage. As a result, support removal often requires machining.
The Studio System leverages BMD to deliver an office-friendly metal 3D printing solution. There are no loose powders or lasers associated with fabrication. In terms of support removal, parts are printed with their supports which are separated by ceramic interface media (or the Ceramic Release Layer™) that does not bond to the metal. This material disintegrates during sintering, making it easy to remove supports by hand.
The printer has two extruders - one dedicated to printing bound metal rods and the other to the ceramic interface media rods. The rods are fed from the media cartridges into the extruders, heated to soften the binder, and then dispensed through the nozzle. Precise tool paths and extrusion rates are calculated to ensure reliable extrusion, start/stops, and feature accuracy.
The raft is printed first, and then the part with its supports. The interface layer printed between the part and its supports is engineered to ensure controlled shrinkage throughout the part. While advanced support structures are designed to fully support the part geometry throughout the print, debind and sinter processes.
In the second step of the process, the part in the debinder where a significant portion (30 to 70%) of primary binder is removed by chemical dissolution while the remaining binder helps the part to retain its shape. An open-pore structure is created throughout the part in preparation for sintering.
In the furnace, part is heated to temperatures near melting. Remaining binder is released and metal particles fuse together, causing the part to densify up to 96 to 99.8%. Depending on the material, the part shrinks about 17 to 22% during densification. Understanding and controlling shrinkage due to sintering is critical to achieve dimensional accuracy. Optimized through dilatometry, the sintering cycle is tuned to each build and material to ensure repeatable shrinkage and densification.
To enable Separable Supports. The interface layer printed between the part and its supports doesn't bond to the metal and prevents the part from sintering with its supports. The ceramic media disintegrates in the furnace, making it easy to remove parts from their supports.
The role of infill
As an extrusion-based process, BMD enables the fabrication of parts with fully-enclosed, fine voids. With the exception of extremely small geometries, all parts are printed with closed-cell infill—a fully-enclosed, internal lattice structure printed within the part. Closed-cell infill is not possible with powder-bed AM methods, such as SLM, which are restricted to open-cell lattices in order to remove unbound powder from the void spaces. Both print and debind time are directly affected by infill. The time it takes to debind a part is directly related to cross-sectional thickness which is reduced by printing with infill. Infill also reduces the weight of a part while maintaining the design-intent of the part surfaces.
BMD can be applied to virtually any sinter-able powder that can be compounded in a thermoplastic media. This includes industrially-relevant metallic alloys such as stainless steels, tool steels, and other metals that are difficult to process via other AM techniques such as refractory metals, cemented carbides, and ceramics.
Parts & capabilities
Extrusion-based additive manufacturing can build structures and geometries previously unachievable via bulk manufacturing processes—including MIM, press-and-sinter powder metallurgy, and reusable mold casting techniques. BMD results in near-net-shape parts with the strength and accuracy needed for functional prototyping, jigs & fixtures, tooling applications, and in some cases, low-volume production.
Cast vs printed
The Studio System printer has a build volume of 30 x 20 x 20 cm and can accommodate a maximum part size of 25.5 x 17 x 17 cm (post-shrink).
A wide range of materials
For example, copper is difficult to process via powder bed fusion due to its high thermal conductivity and laser absorption characteristics. Copper media can be bound, printed, and sintered with BMD.
The non-sintering interface layer enables printing of encapsulated assemblies, such as a the hinge, shown here. Traditionally, this is made by forming, assembly and joining of multiple parts.