3D printing army parts on demand with a new filament

Materials World magazine
,
3 Jun 2020

A new multi-polymer filament could mean that military components are printable when and where they are needed. Idha Valeur reports. 

The US Army has created a polymeric material that allows common desktop 3D printers to fabricate strong and tough military parts at a moment’s notice for soldiers on deployment. This eliminates the need for costly and, potentially dangerous, resupply operations.  

The filament consists of two materials, describes Dr Eric Wetzel, Researcher at the Combat Capabilities Development Command’s (CCDC) Army Research Laboratory, in Maryland, USA. It is made from ‘an acrylonitrile butadiene styrene (ABS) body with a polycarbonate (PC) core. These two polymers are commonly used in printing applications because they behave nicely in desktop 3D printers and have good intrinsic mechanical characteristics’. 

The dual-material filament has a more intricate thermal response in comparison to those made from one material, explains Wetzel. He notes that it ‘has two separate material flow temperatures. It is printed at a temperature higher than these two but, after printing, can be annealed to high strength and toughness at a temperature intermediate between the constituent flow temperatures. 
‘During the annealing process, one of the component polymers softens and flows locally to increase interlayer bonding, while the other component remains rigid to prevent geometric creep due to gravity, or distortion due to residual stresses. The result is a mechanically robust, geometrically accurate part’. 

Using this material with common desktop 3D printers would eliminate the current reliance on a process called fused filament fabrication (FFF), which works by depositing thermoplastics layer by layer to achieve a 3D structure. Wetzel outlines how this can cause weakness in between layers and is therefore better suited for prototype models and not the final parts. ‘Our approach captures the cost and accesses advantages of FFF, while dramatically improving mechanical properties so that FFF objects can be used as load-bearing parts,’ he says. 

To create the dual-material filament, the researchers used a thermal draw process, eliminating the need for specialised dies that are costly and have long waiting times. ‘Thermal drawing is a dieless filament production process. It begins with a large cylinder of material, called the preform, that is progressively heated and placed under tension, necking down the preform to produce a continuous filament,’ says Wetzel. ‘Because of the viscous flow conditions, the filament adopts the same geometry and material arrangement as the preform. Thermal drawing is used, for example, in the manufacture of glass optical fibres.’ 

Wetzel adds, ‘One of our innovations was using 3D printing to build multi-material, complex thermoplastic preforms. The combination of 3D printing and thermal drawing allows us to very rapidly go from a design concept on the computer to physical filament, really in a matter of hours. This quick turnaround time made it possible to work through many more design ideas, and discover the best design in a manner that would not have been practical using conventional die-based extrusion processes…[The filament] can be implemented in most existing desktop FFF printers without requiring specialised or proprietary hardware.’ 

Currently, the team is working to scale up the filament design to create numerous kilometres of it. ‘We will be sharing this filament with the Department of Defense partners first, primarily for expeditionary manufacturing. If this exercise leads to increased demand, we will then work with industry partners to commercialise the filament for both military and consumer use,’ Wetzel concludes.