By Debbie Sniderman
Additive manufacturing (AM) technologies speed prototyping by allowing design engineers to make prototypes on demand, rather than using large amounts of energy and raw materials for a small number of items. That, in itself, is environmentally friendly.
Parts can be produced using many materials: plastics, waxes, paraffins, glass, metals, ceramics, composites, sand, even paper. Though many of these materials are recyclable, parts produced by some AM techniques can’t be recycled because their composition is changed with additives, stabilizers, finishes and binders. As AM becomes mainstream, some are beginning to look toward recyclable and biodegradable plastic material options.
On the Horizon
According to Dr. Ian Gibson, founder and advisor of Global Alliance of Rapid Prototyping Associations (GARPA) and associate professor in the Department of Mechanical Engineering at National University of Singapore, sustainability in the AM industry is not yet an immediate concern for two reasons:
- “The processes are not so cheap as to make parts in a throw-away fashion,” he says. “People are still concerned about cost when they build, and therefore, need to justify the actual fabrication.”
- The volume of machines and parts has not yet intensified, Gibson points out: “One city probably creates more polymer waste than all the AM machines of the world.”
But, he notes, sustainability is likely to become a concern in the future, when the industry experiences a rapid increase as a result of low-cost machines aimed at personal use.
“None of the large printer manufacturers have yet to introduce machines to the world. When they do, they will have automated assembly plants capable of generating a veritable flood of machines. This is the tip of a potentially huge iceberg,” says Gibson.
Currently in Use
Polycarbonate (PC), recycled in some cities with resin code 7, and acrylonitrile butadiene styrene (ABS) recycled with resin code 9 or ABS, are the most commonly known recyclable plastics used in AM.
ABS and PC are among some of equipment producer Stratasys’ fused deposition modeling (FDM) recyclable material choices, which, according to Bill Macy in a Direct Part Manufacturing Workshop at the March 2011 Midwest SAMPE Conference, “are 100% recyclable.” Others he mentioned currently in use include Ultem 9085 and FDM-PPSF (polyphenylsulfone thermoplastic).
Over the last year, technologies for recycling items made from the biopolymer polylactic acid (PLA), formed from glucose and currently available for 3D printing, have improved. Near-infrared recycling separation processes have matured, and are now able to distinguish among PLA, polyethylene terephthalate (PET) or HDPE for effective sorting in mixed waste, so recycling PLA is becoming easier and more widespread. PLA will also degrade in commercial composting environments, according to Cargill’s Natureworks, maker of Ingeo PLA biopolymers.
Polyamide (PA) 11 (Nylon 11), a bio-based engineering thermoplastic polymer produced from renewable castor oil sources, is available in fine powder form and is used in the selective laser sintering (SLS) market. It can be recycled in some cities under plastic resin code 7, Other or O. Material producers such as Arkema, through its French partner Agiplast, collect, sort and reuse production scraps, pellets and end-of-life articles made from Rilsan technical polymers, including PA11.
Biodegradable Biomedical Materials
The biomedical industry is developing structures from composite materials, combining biopolymers as binders with bioceramics, which are more rigid (and brittle) to achieve their desired material properties.
“There are numerous biodegradable materials available for AM technologies that were developed specifically for medical applications,” Gibson says, “but there is no reason why they cannot be developed for mainstream manufacturing. Many of the materials have sufficient strength to withstand heavy and regular use, and it is possible to ‘tune’ the degradation rate to suit a particular application, including its recycling.”
Some biodegradable materials currently used with AM technologies include:
- Mirel bioplastic, made through fermentation of sugar.
- Poly-L-lactic acid (PLLA), a biodegradable semi-crystalline
polymer derived from lactic acid used with AM for bone tissue and other biomedical applications.
- Hydroxyapatite (HA or HAp), a biodegradable bioceramic available in powder form for SLS, a complex phosphate of calcium (Ca (PO4)3OH) that occurs as a mineral.
- PGA poly(glycolic acid), a newer material PLGA — bioresorbable poly(lactic-co-glycolic acid), and PLGLA.
Gibson says the AM industry needs to look at sustainability in many ways, and material use is just one of them.
“Currently, very few AM applications include plans for material degradation,” Gibson says. “Most products are designed according to the desired function, and very few of these functions focus on the lifetime of the product. This issue should be given more attention, since we are aware that most AM materials do not have considerable longevity when compared with many commonly used industrial polymers.”
Usually, other manufacturing methods are considered when high durability is concerned, Gibson adds, and AM is used to fabricate the less-durable, but customizable features of a product.
“One concern would be to examine the use of composites in AM,” he points out. “Many processes use a non-polymer filler material to enhance the overall material properties. Another thought might be to specifically develop recyclable materials and processes for short-term part applications, like prototyping.
“Finally, we really need to raise the awareness of this potential problem to the increasing number of users through a comprehensive training and education program. If the potential risks are realized, we can prepare this new technology in a way that won’t result in the next plastic-bag nightmare.”
Fully Recyclable Digital Printing
Some AM processes, such as FDM and fused filament fabrication (FFF), that lay down streams of layered plastic such as ABS, produce parts that could be recycled by “starting over” and melting down the whole object as if it were raw material. But most parts created by AM processes are still not recycled for re-use.
Cornell University researcher Jonathan Hiller explains: “The fundamentals of some AM technologies cause materials to undergo irreversible processes, such as polymerization, infiltration with a binder, or UV curing, making further re-use of the material impossible. At the current low volumes, there are prohibitive costs, a lack of recycling infrastructure, and virtually no incentives for consumers and manufacturers to recycle–financial or otherwise.”
In 2009, Hiller explored an AM approach that enabled the same physical material to be used multiple times. Under a Defense Advanced Research Projects Agency Programmable Matter grant and National Science Foundation research fellowship, he demonstrated a method in which printed items could be completely recycled using “digital material elements.”
His fabricator assembled repeated elements. In the future, they could be made small enough and with sufficiently high accuracy to form products that appear natural to the human eye. Units called “voxels” can be pre-made in a range of sizes, and lock together or be used with binders that do not change their properties, but hold them together during assembly and use.
A structure was “printed” using steel and Delrin spheres and binder. At the product’s end of life, the binder was dissolved, disassembling the repeated elements, which were sorted and placed back into the fabricator bins. A different structure was then printed from the reclaimed material.
Hiller says he believes multi-material AM is the way of the future, so when other materials are mixed together in an object, they could not be melted down and reformed, but disassembled voxels could be sorted by material. The goal of his research was “to draw the attention of printing manufacturers and consumers to encourage a focus on total lifecycle analysis, and think about how to fully recycle,” he says. Over time, as more users consider the lifecycle of the materials they use, and demand grows, the cost-effectiveness of re-usable materials should help make them feasible.
Debbie Sniderman is an engineer, writer and consultant in manufacturing and R&D. Contact her at VIVLLC.com.