By Joyce M. Conant, ARL Public AffairsMarch 19, 2018
ABERDEEN PROVING GROUND, Md. -- A collaboration between the U.S. Army Research Laboratory and the U.S. Marine Corps has resulted in the discovery of using waste plastics -- such as water bottles, milk jugs and yogurt containers -- for 3-D printing parts that Soldiers may need on the battlefield.
Reclaimed materials may be the next materials used in additive manufacturing, or 3-D printing, to improve the self-reliance of service members on forward operating bases by cutting costs and decreasing the demand for the frequent resupplying of parts by the supply chain.
"The potential applications for additive manufacturing technologies are extensive -- everything from pre-production models and temporary parts to end-use aircraft parts and medical implants," said ARL researcher Dr. Nicole Zander.
Additive manufacturing offers many advantages over traditional manufacturing, she said, including increased part complexity and reduced time and cost for one-off items, such as a bracket for a radio, one of the long-lead parts evaluated in the work.
The research by Zander and co-collaborator Capt. Anthony Molnar from the U.S. Marine Corps, generated fused filament fabrication, or FFF filament, from 100-percent recycled polyethylene terephthalate, or PET, from bottles and plastics without any chemical modifications or additives. Work is also underway to generate filament from other recycled plastics and reinforced filaments.
Zander said that while PET is widely used in many applications, it is not widely used as a feedstock for FFF due to its high melting temperature, water absorption and issues with crystallinity, which can make printing difficult.
Steve Post, business development manager for Thermo Fisher, the maker of the equipment the Army used to produce its filament, said this is a strong statement on sustainability.
"The Army really thought out of the box on this application, turning a troublesome waste product into a valuable resource," he said.
Zander said recycled PET was shown to be a viable new feedstock, with mechanical properties of printed parts comparable to parts made from commercial filament. In addition to small parts for evaluation, several larger long lead item military parts were also printed with the filament.
"In terms of mechanical properties, most polymers used in FFF have bulk strengths between 30 and 100 MPa," Zander explained. Recycled PET has an average strength of 70 MPa, and thus may be a suitable 3-D printing feedstock."
Mechanical testing, including uniaxial tensile and three point bending experiments, were conducted in the laboratory. In these tests, the tensile strength of 3-D printed recycled PET was compared against commercial filaments and found to have similar strength. In addition, a custom test fixture was made to test a 3-D printed radio bracket (a long-lead military item). Brackets made from recycled PET failed at a similar load to brackets printed with commercial ABS filament. The recycled PET filament may have the capability to replace commercial filament in printing a diverse range of plastic parts.
In addition to mechanical testing, the recycled plastics underwent chemical analysis, thermal stability and a host of other tests.
"Recycled polymers have a variety of different additives, fillers and dyes and may have experienced different processing conditions -- even for the same polymer type," Zander said.
To get a better understanding of different recycled plastic feedstocks and the best properties to expect from such materials, chemical, thermal and mechanical analyses were performed.
Molnar, project officer with the mobility and counter mobility team in Quantico, Virginia, said PET plastics such as water bottles and packaging are one of the most prolific wastes found on the battlefield. Both U.S. and coalition forces produce large volumes of this waste, and being able to repurpose this on location by forward deployed forces will reduce the logistic burden of transporting parts to forward operating bases and the additional costs of disposing of the recyclable material.
"Nikki's groundbreaking research will provide U.S. forces with the ability to 3-D print replacement parts on demand," Molnar said. "This will not only increase readiness of equipment but also provide troops with the ability to manufacture mission specific gadgets in the field.
"As our enemies have shown us, they can often out pace our ability to react to their new tactics and equipment," he continued. "This new technology will enable the warfighter to more rapidly develop tools necessary to defeat an ever changing enemy technology.
"With Nikki's continued research in incorporating additives, stronger filaments will continue to increase the capability to print stronger replacement parts. This will further enable Soldiers to push the boundaries of expeditionary manufacturing into not only larger parts, but also other areas such as printing building materials, unmanned platforms, and force protection devices."
Researchers said the driving force for this work is to enhance warfighter capability and readiness by enabling repairs while deployed and to reduce dependence of the logistical supply chain.
"While each unit carries large stockpiles of spare parts for emergencies, this is costly and increases the risk to warfighters during the convoy of those assets. It is also difficult to predict the failure and lifecycle of these parts," Zander said.
Zander and Molnar are in the process of building a mobile recycling facility to enable Soldiers to be able to repurpose plastics into feedstocks for 3-D printing.
"The MRF will be a plastic processing laboratory housed in a 20-foot ISO container, with all equipment and tools needed to fabricate 3-D printing filament from plastic waste," Zander said.
Researchers determined that recycled plastics have shown to be suitable material for 3-D printing, provided the material is properly cleaned and dried. The tensile strength of printed parts from recycled PET was equivalent to printed parts made from commercial off the shelf PET pellets and commercial filaments. But the research will not stop here.
Zander said blending with other plastics, or the addition of fillers such as reinforcing or toughening agents, may further improve the mechanical properties of the recycled plastics, or rPET, filament and expand the realm of applications in how it may be used.
"Ultimately, we'd like to produce the best possible feedstock we can from recycled plastics and waste materials," Zander said. "Future work will involve testing select 3-D printed long-lead parts against original parts to determine if they can be a suitable long-term or at least a temporary replacement."
The U.S. Army Research Laboratory is part of the U.S. Army Research, Development and Engineering Command, which has the mission to provide innovative research, development and engineering to produce capabilities that provide decisive overmatch to the Army against the complexities of the current and future operating environments in support of the joint warfighter and the nation. RDECOM is a major subordinate command of the U.S. Army Materiel Command.