How to get all-metal parts with 3D FDM printing

Image 1: Model made of high carbon steel Filamet™ and sintered. Source: The Virtual Foundry

The Virtual Foundry is an American company, headquartered in Wisconsin, formed by great experts in the sector of molten metal, who have been constantly working since 2014 to improve and grow its range of filaments and accessories for 3D FDM metal printing. In the beginning they managed to manufacture filaments with high percentages of brass, copper or bronze but their structure after being sintered did not contain the properties of metals. Through a Kickstarter campaign, The Virtual Foundry got enough support to evolve its filaments to achieve the same properties as pure metal and expanded its range of materials (316L stainless steel, high carbon iron, 6061 aluminium and tungsten). All types of Filamet ™ consist of a base metal and a biodegradable and ecological polymer (PLA). This material is free of exposed metal particles and volatile solvents that can be released during printing. These materials are extremely simple to print, since their printing properties are similar to those of the PLA, which allows any user of a 3D FDM printer to create parts with these filaments, without the need to purchase expensive industrial 3D FDM metal printers . One of the main advantages of Filamet™ materials is that they achieve properties similar to those possible with DMLS technology but with certain limitations. Due to the need to sinter the printed pieces with this filament, where the PLA is removed, the pieces have porosity, loss of volume and non-isotropy. DMLS 3D printers manage to print completely solid parts (similar to casting), with great detail, layer heights of 0.02 mm and without the need for post-processing, the only disadvantage compared to Filamet™ 3D FDM printing is the cost of material, manufacturing and the printers themselves.

Image 2: Cones made of no sintered and sintered bronze Filamet™. Source: The Virtual Foundry

To get the printed part to be completely metallic, it must be sintered in an oven. Sintering is a process of manufacturing solid parts based on an object formed by compacted metallic powder, to which a heat treatment is applied at a temperature lower than that of fusion, but if high enough to bond the metal particles in a resistant way, resulting in a totally solid block. After printing with Filamet™ the pieces need to be sintered to remove the PLA that is part of the filament. Sintering can be done in an oven with an open environment or a vacuum or inert environment.

Sintered in open environment

For sintering in an open environment a coconut shell coal for sintering, a refractory vessel (crucible) and Al₂O₃ refractory powder is required. The process begins by sanding the rough edges of the piece for best results. 

First of all, the crucible should be filled with refractory powder, leaving a free space on the surface of the crucible. Then the workpiece must be immersed in the refractory powder, making sure to leave a gap of at least 15 mm between the surface of the workpiece and the walls and upper and lower parts of the crucible. The refractory powder must not be compacted.

At this point, depending on the material to be sintered, the free space on the surface of the crucible should be filled with sintered carbon or the crucible should be placed in the furnace.

Image 3: Sintering process. Source: The Virtual Foundry

Sintered in vacuum or inert environment

For sintering in a vacuum or inert environment, a crucible (cooking vessel) and refractory powder are needed. The piece is prepared for sintering by placing it inside the crucible and covering it with refractory dust, taking into account that there must be at least 10 mm of dust between the surfaces of the piece and the crucible. The crucible is then placed in the oven.

The Virtual Foundry provides the following table of recommended temperatures, either for sintering in an open environment or in a vacuum or inert environment:

Table 1: Materials and maximum sintering temperatures.

The user must take into account that these times and temperatures are a guide and that they can vary depending on many aspects, such as the model of oven they use, for example. Filamet range filaments not included in the table are considered experimental, so the manufacturer does not have data for sintering.

If you do not have a furnace that meets the requirements for sintering 3D printed parts with filaments Filamet™, you can contact us and we will inform you about the feasibility and conditions of sintering in our facilities.

After sintering (in an open environment or in a vacuum or inert environment) any filament of the Filamet™ range, all metal parts are obtained, with the real properties of the metal as electrical conductivity, post-processed by sanding and polishing or even bonding by welding; but with a certain porosity and a reduction in volume due to the loss of PLA. The user must also take into account that the mechanical properties of the final product are directly related to the time that the printed part is maintained at the sintering temperature. If the final product is powdery and brittle, the sintering time has not been sufficient. If the print shows a surface similar to wrinkled skin, it is over sintered.

Post-processing: Sanding and polishing

Once sintered the piece can be sanded and polished in the same way as a metal but following a series of indications. With water sandpaper it is possible to eliminate the printing lines and other small deformations because the loose particles during sanding adhere to the gaps due to the heat of friction. In case of using sandpaper or 3M radial disc it is recommended to start sanding with a grain of 120 (80 for 3M radial disc), being careful not to deform the most delicate areas, such as corners. Once the entire surface has been sanded, a sandpaper of the next grain should be used and so on until it increases 6 or 7 (4 times for the 3M radial disc). Before moving on to the final polishing it is recommended to use a 3000 grit sandpaper, with which a certain gloss is achieved. Finally and once the piece is cleaned with a flannel cloth, the piece can be polished. TVF recommends using a rotating tool with a polishing disc and polishing wax to make polishing faster and more efficient. Simply apply a little polishing wax on the polishing disc and polish with constant movements throughout the piece so as not to generate excess heat, which can deform the piece. In addition to sanding and polishing parts manufactured with Filamet™, they can be carved, melted, welded and smoothed with heat application.

Image 4: Polished copper bucket. Source: The Virtual Foundry

Seen how to get all-metal parts with 3D FDM printing we can conclude by saying that this printing technology, with the help of The Virtual Foundry metal filaments, has managed to reach the few sectors that it needed to conquer, especially some of the branch industrial.