How the Metal FFF 3D Printing Process Works

These metal 3D printers are a highly specialized variation of FFF printers, optimized to print MIM (metal injection molding) feedstock. This type of printing creates “green” metal parts that must be debound and sintered before final use.

These “green” parts are always scaled up 15-20% from the final part dimensions to account for repeatable and predictable shrinkage during sintering.

• Printing: metal powder bound in plastic is printed a layer at a time into the shape of your part. Parts are scaled up to compensate for shrinkage during the sintering process, resulting in “green” parts.
• Debinding: After printing, green parts are placed into the debinding station, where an organic solvent is used to dissolve most of the plastic binding material. After washing, green parts are known as “brown” parts.
• Sintering: Washed “brown” parts are then placed in a furnace, where they are heated with a material-specific profile — first to burn away remaining binder, then to solidify metal powder into a finished part.
  

Metal FFF 3D printing technologies use a process that is nearly identical to conventional FFF printers. The only main difference is that metal FFF printers use a vacuum-sealed print sheet, rather than a conventional print bed.

For users, the printing process includes:

• Slicing parts on a software platform
• Placing a vacuum-sealed print sheet on the print bed
• Initiating the metal 3D printing process
• Removing the printed part after vacuum disengages
• Peeling the “green” part off from the print sheet

Dual Extrusion with MIM Feedstock: These types of printers extrude MIM filament in one nozzle, and ceramic interface filament in the other.

One extrusion nozzle is designed to print MIM feedstock, which is a metal powder bound in a two-part plastic binding material. This is the material used to form the part itself, as well as the supports and raft. For industrial 3D printer metal, Markforged currently offers six different commercial-grade offerings — A2 tool steel, D2 tool steel, Inconel 625, Copper, H13 tool steel, and 17-4 PH stainless steel.

The other nozzle prints ceramic release material. This provides the surface between parts and their supports and rafts. Without this release material, 3D printing metal parts that require supports would not be possible. The sintering process turns this material into powder, which enables the part to be easily separated from supports.

The debinding step removes the majority of the binding material. The result is a “brown” part ready for sintering.

First, the “green” part is first placed into a heated solvent bath in a wash station using a degreasing solution to dissolve the primary binding material. Markforged metal FFF machines use the Wash-1, a solvent based debinding solution. It primarily uses Opteon SF-79 as a solvent, a high-performance fluid designed to offer superior cleaning power, higher efficiency, and safety in an environmentally sustainable way.

Overall, the debinding process for metal FFF is fairly simple and only requires basic PPE. User steps for the debinding process include:

• Adding green parts to a washing basket
• Lowering it into the machine with the solvent and closing the lid
• Opening the lid after washing, and removing the parts from the washing basket
• Placing the parts in an air drying chamber in the wash station
• Removing the parts from the wash station station once the parts are dried
  

So, what happens to a part during washing? The debinding solvent bath dissolves the primary binding material in the green part. As it dissolves binding material, the solvent opens up microscopic fluid pathways into the part, which enables the solvent to flow deeper and dissolve more.

Sintering is the critical final step of this metal additive manufacturing process.

Sintering removes the “brown” parts from the debinding step, burning out the remaining binder, and then sintering the part at a near melting temperature. All of this is done in a precise, controlled atmosphere. The automated process is extremely complex and requires precise control of a high energy environment.

The sintering process involves several key stages. First, the part is heated to an intermediary temperature to burn out the remaining binding material — which exits the part through the micro-pathways created during the solvent debinding process. Second, the part is heated to the sintering temperature where it shrinks from the larger “green” or “brown” size to the final size of the 3D printed functional part. Solid geometries coalesce to 95%+ porosity while the closed cell infill remains. The ceramic interface material turns to dust, which enables users to easily separate the 3D printed metal part from the raft and supports.

Using a sintering furnace requires the following steps:

• Adding parts to the setter plate

• Placing the setter plate into the furnace
• Closing the furnace door
• Starting the sintering process
• Running the process to completion