Last reviewed 26 November 2013
3D printing is a new method of manufacturing that is set to revolutionise industry and transform our relationship with consumer goods. Its impact on society could be as great as that of the internet. What’s more, proponents claim it is a zero-waste technology. But has the overall environmental impact been properly assessed? Caroline Hand investigates.
The concept of additive manufacture
3D printing, more accurately described as additive manufacturing (AM), begins with a computer-aided design or scan of a three-dimensional object, based on a large number of cross-sections. This data is used to programme a machine that builds up the object layer by layer. The first 3D printers built prototypes and models using plastics or inks, but as the technology has evolved, an expanding range of materials has been employed, including a variety of metals, ceramics, concrete, textile fibres, and even food.
Consumers can now dream of a future in which every home has its own 3D printer, churning out goods to the owner’s specification. Electronic gadgets, clothing, furniture, toys, even houses and cars — the scope is endless. It is already possible to print a futuristic metal car, shoes with an inbuilt mobile phone, and a jumper made to the wearer’s precise measurements. More seriously, individually printed hearing aids and dental appliances are improving the quality of life for their users.
From the environmental point of view, the big plus is that, with AM, 100% of the raw material goes into the product, with no wasteful offcuts. In theory, this makes it highly resource-efficient. The main downside is the high-energy consumption of the machines — in some cases, orders of magnitude higher than that associated with conventional mass manufacturing.
The greenest juicer
To date, most 3D printing has been done in plastic. There are two commonly used materials: acrylonitrile butadiene styrene (ABS) — a fairly dense and robust polymer; and polylactic acid (PLA) — a plant-based ink. Researchers from Michigan Technological University have carried out life cycle assessments on three simple plastic juicers: two made by 3D printing in ABS and PLA, and a third manufactured using conventional injection moulding. The study revealed that the overall energy demand was lower for the printed jugs, regardless of the material used. Looking specifically at carbon emissions, however, the 3D printed jug made from ABS was the poorer performer if the process was powered using fossil fuel generation, but had a lower impact than the moulded jug if solar energy was used. The PLA printed model produced the lowest emissions, which performed better whatever the energy source.
Printing in metal
Although plastic printing could bring some benefits in terms of waste reduction in manufacture, this could be offset by increased consumption as 3D printing grows in popularity — particularly if consumers dispose of various substandard items before finally turning out their dream design. Realistically, the cost of AM rules it out for low-value, mass-produced plastic goods, for which injection moulding remains a much cheaper (and not particularly wasteful) option. The picture changes, however, when the printed items are made from valuable metals, and this is the area where 3D printing appears to have the greatest environmental potential. Engineering giant General Electric (GE) has pioneered the use of AM for aircraft engine components and is already reaping the benefits. Other major engineering manufacturers, including Rolls Royce, Ford, Boeing and Siemens, are now following suit.
A variety of additive techniques are used to manufacture metal items. Most involve the use of lasers to fuse or melt a metallic powder into the desired cross-sectional shape (laser sintering). Metals used include titanium, niobium and platinum, plus alloys such as the copper chrome alloy used by GE (see below).
AM in the aerospace industry
Using conventional subtractive techniques, it takes 28 tonnes of material to manufacture an aero engine weighing seven tonnes. Other estimates from the engineering industry indicate that as much as 90% of the metal used is wasted. Although much of the waste consists of valuable metals, such as titanium, it is not generally feasible to reuse or recycle it; some is highly toxic, and recycling the swarf has as great an impact on the environment as using virgin material. Furthermore, some of the high-value metals used in the aerospace industry fall into the category of “strategic metals”: metals essential to the economy, which are not mined in the EU, and over which there is concern about future security of supply. (See Strategic metals: a priority for resource efficiency
An EU-sponsored study has calculated that, over the next 20 years of engine manufacture, 1.28 million tonnes worth of metal could be wasted, at a cost of 10,000 euros per tonne. This highlights the potential of AM to improve resource efficiency. What’s more, with AM it is possible to create complex structures that are lighter and stronger than those which can be made conventionally — for example, curved, scooped out surfaces or intricate interlocking structures. A lighter aircraft is more fuel efficient which, of course, will have a positive impact on carbon emissions during the use phase of the life of an aircraft. The same principles apply to cars, which can also be light-weighted.
Aerospace successes to date
GE uses laser sintering of a cobalt chromium alloy to manufacture fuel injectors for its new generation of jet engines. These highly complex nozzles were formerly made by welding 20 separate parts together but are now printed as a single piece. The printed nozzles are faster to make, lighter and more durable: their use reduces the total weight of a two-engine aircraft by nearly 40 pounds. The company also uses AM to manufacture titanium strips that are attached to turbine blades in order to improve the engine performance: previously, 50% of the titanium was wasted. While other companies like Ford use AM primarily for one-off prototypes, GE is rapidly scaling up the technology and, by 2020, plans to be printing tens of thousands of parts for its jet engines.
Here in the UK, Rolls Royce is working on the Merlin project, an EU-backed scheme to adapt AM technology for the manufacture of civil aircraft. This project uses a slightly different technique — laser metal deposition (LDM), in which a laser is directed onto a substrate to melt a pool of the desired shape. Metal powder or wire is then introduced to the pool. LDM creates a better surface finish than is possible with other additive techniques. The disadvantage of LDM is that it uses hundreds of times more electricity, per kg of metal processed, than conventional milling or casting. Cost is currently a limitation, but this was the case for CNC (computer numerical control) machines 20 years ago and they are now in widespread use.
Another major advantage of AM in engineering is that the technique can be used to manufacture replacement parts for engines or vehicles. The data for these can be sent via the internet to a 3D printing machine anywhere in the world, thus eliminating transport costs and the associated carbon emissions. Theoretically, this means that obsolescence is eliminated, and product life extended dramatically, as anyone can scan and replicate a part. This aspect of AM has already been exploited by an American airline whose ageing McDonnell Douglas planes were frequently grounded by leaking toilets. As the planes were no longer being built, spare parts became unavailable, but the company was able to manufacture its own plumbing in a durable, fire-resistant plastic using AM.
Rolls Royce is also using AM to repair components. The damaged part is scanned, compared to the ideal and a metal deposition process used to fill the gaps. Again, this prolongs the life of the aircraft and allows damaged parts to be reused rather than recycled or disposed of.
Weighing up the benefits
The machines needed to print metal components are expensive, energy intensive and slow — though, as the GE examples show, not invariably slower than conventional manufacture. Such technology is only accessible to large, cash-rich industries, such as aerospace or motor manufacturing. However, from the environmental point of view, the expansion of AM clearly embodies circular economy principles of eliminating waste, creating durable products that consume less energy in use, and enabling reuse and refurbishment rather than lower-value recycling or disposal of offcuts as hazardous waste.
In order to carry out a meaningful life cycle assessment, the energy consumed in manufacture needs to be offset against the energy saved during product use. Unfortunately, the large machines working 24/7 need a reliable, consistent energy supply and are less suited to being powered by (often intermittent) renewables: if greener energy could be used, either by developing energy storage technology or adapting the machines to a fluctuating supply, the environmental impact could be further reduced. As yet, not much is known about the recyclability of printed metal items, particularly those made from unusual alloys and composites. However, in the future, principles of ecodesign could be applied to make the technology even more sustainable.
3D printing raises many thorny issues for the society of the next few decades: the loss of engineering employment as skilled workers are replaced by huge, silent machines; intellectual property disputes as consumers start to replicate their favourite toys, phones and (more worryingly) guns at home; and safety concerns as new, untried products find their way onto the market. Although, in terms of resource efficiency, the benefits of AM for metals seem overwhelming.
A new partnership of 28 businesses and organisations in Europe has just launched a project called AMAZE (Additive Manufacture Aiming Towards Zero Waste and Efficient Production of High Tech Metal Products). At the same time, the European Space Agency is looking ahead to a time when spacecraft are built by AM on platforms in space. Here on earth, there are promising signs that this new technology could enable the development of commercial nuclear fusion. This is truly amazing — a revolution both in manufacturing and in our stewardship of scarce resources.