The scientists involved in the project believe this advance could improve operational readiness, reduce dependence on outside supply chains and enhance safety.
The team presented their findings this week at the 256th National Meeting & Exposition of the American Chemical Society (ACS).
"Ideally, soldiers wouldn't have to wait for the next supply truck to receive vital equipment," said Nicole Zander, PhD a research chemist at the US Army Research Laboratory (ARL). "Instead, they could basically go into the cafeteria, gather discarded water bottles, milk jugs, cardboard boxes and other recyclable items, then use those materials as feedstocks for 3D printers to make tools, parts and other gadgets."
A number of front-line units are equipped with 3D printers that can produce spare parts and other equipment, but they rely on conventional feedstocks, such as commercially available plastic filaments, that must be requisitioned and be delivered, a process that can take days, weeks or even months.
Zander, US Marine Corps Captain Anthony Molnar and colleagues at ARL explored the possibility of using recycled polyethylene terephthalate (PET) plastic as a starting material for 3D printers.
PET plastics are common waste materials found around military bases, and the researchers determined that PET filaments, produced by recycling, were just as strong and flexible as commercially available filaments for 3D printers. In tests, the team used recycled PET filaments to print a vehicle radio bracket, a long-lead-time military part. This process required about 10 water bottles and took about two hours to complete.
Initially, the researchers determined that other types of plastic, such as polypropylene (PP), used in yogurt or cottage cheese containers, or polystyrene (PS), used for plastic utensils, were not practical for use in 3D printing.
The team is then said to have strengthened PP by mixing it with cardboard, wood fibres and other cellulose waste materials to create new composite filaments. Furthermore, brittle PS was blended with ductile PP to generate a strong and flexible filament.
The researchers used solid-state shear pulverisation to generate composite PP/cellulose filaments. In this process, shredded plastic and paper, cardboard or wood flour was pulverised in a twin-screw extruder to generate a fine powder that was then melt-processed into 3D printing filaments.
After testing using dynamic mechanical analysis, the scientists concluded that the new composites had improved mechanical properties, and they could be used to make strong 3D-printed materials.
A mobile recycling trailer is being built by Zander's team that will allow front-line soldiers to fabricate 3D-printing filaments from plastic waste. She is also exploring ways to print materials from plastic pellets instead of filaments, which could help soldiers quickly produce larger 3D-printed parts and machinery.
"We still have a lot to learn about how to best process these materials and what kinds of additives will improve their properties," Zander said. "We're just scratching the surface of what we can ultimately do with these discarded plastics."