On a quiet, crosstown street in Chelsea, New York City, 10 machines resembling microwave ovens line the walls of the factory-cum-store of Normal, an earphone manufacturer. The machines – which are three-dimensional printers – allow Normal to custom build earphones that fit a buyer’s ears precisely in a couple of days, based solely on photographs.

The 3D printing machines at GE Aviation’s low-rise additive technologies laboratory near Cincinnati are similar to those at Normal but are undertaking a far more sensitive task. They are forming, from thin layers of metallic dust, light components for jet engines that will ensure these are far more fuel efficient than previous products made by General Electric, the conglomerate that has a large aerospace division.

These contrasting scenes highlight how 3D printing– widely expected to revolutionise some of the world’s most complex types of manufacturing – is starting to become a mainstream technology used across many industries rather than a niche technique for prototype products and highly personalised consumer goods. But as it spreads into safety critical sectors such as aerospace and healthcare, 3D printing is drawing heavy scrutiny from regulators.

Proponents of the technology say manufacturers’ problem proving to regulators that printed products are safe is holding back widespread adoption of 3D printing. There is no suggestion the technology is inherently less safe than conventional manufacturing, but US regulators are struggling to decide how to test products.

The market for 3D printed products and services is predicted to reach $21bn worldwide by 2020, compared with just $3.1bn last year, according to Wohlers Associates, the consultancy. But the need to satisfy regulators about safety is an “obstacle” to the technology’s advance in certain sectors, according to David Reis, chief executive of Stratasys, manufacturer of more than half the world’s industrial 3D printers, and supplier of printed products to Boeing.

“Every part has to go through testing,” says Mr Reis. “Every technology, every material has to go through certification.”

Regulators’ concerns stem from how 3D printing is a fundamentally different way of making things compared to conventional manufacturing.

Also called additive manufacturing, 3D printing involves building products up in thin layers from their constituent particles of plastic or metal – in contrast to conventional manufacturing, where materials are moulded, pressed or lathed.

The issues before regulators include how printed products will behave over time, the consistency of their quality, and the materials used. For example, only 2,000 of the tens of thousands of plastics used in orthodox manufacturing work in 3D printers, so materials have to be changed when switching to the technique.

Ed Lowney, head of manufacturing for Custom Control Concepts, which builds tailored interiors for high-end luxury jets, describes 3D printing as a “transformational technology” but says it is hard to win approval from the US Federal Aviation Administration. “The FAA doesn’t mess about,” he adds.

Dave Scott, manager for product development at Acist Medical Systems, which makes catheters for coronary artery procedures by 3D printing, says the constraints in his industry come from the US Food and Drug Administration.

“It [3D printing] is a process that is difficult to evaluate, because every time you make something it’s different,” he adds. “It doesn’t have the same feeling that you have when you mould something.”

Pete Basiliere, an analyst at Gartner, the consultants, says regulators are simply unfamiliar with the issues involved. “The regulators want to make sure [the new product] meets or exceeds the performance characteristics that are being used already,” he adds.

In spite of the regulators’ concerns, manufacturers are keen to stick with 3D printing. While the cost per item of printed products is higher than for conventionally-manufactured items, set-up expenses are far lower, meaning the technique is particularly attractive for short production runs.

Stratasys estimates one aerospace customer cut 70 per cent of its construction time and reduced costs by 80 per cent when it switched to making cable housings for aircraft by 3D printing, rather than orthodox manufacturing. Many other companies report similarly striking results.

The emphasis consequently is on finding ways to satisfy regulators about the safety of printed products.

Custom Control Concepts says it has managed the challenges by setting up a testing laboratory at its site approved by the Federal Aviation Administration. It has switched to using Ultem, one of the few 3D printable plastics that the regulator approves as sufficiently fire-resistant for aircraft.

“Once you have this material that’s approved, you’ve cut down most of the barriers between you and getting the item on an aeroplane,” says Mr Lowney.

GE Aviation, meanwhile, uses “in-process monitoring” during 3D printing to make sure parts meet the required standards. It discards any that fall outside acceptable parameters, and puts every third part through a scanner and X-rays it.

“That’s a huge part of the 3D printing technology,” a company spokesman says of this technique. “If you don’t have those, you cannot be sure of the parts you’ve made.”

Mr Reis insists that such techniques will in time allow manufacturers to satisfy regulators about safety. “We’re going to have more tools to do the testing faster and it will cause an acceleration of the penetration,” he says.

Meanwhile, the regulatory challenges are pushing some manufacturers to concentrate on less safety critical areas. For example, Mr Scott predicts it will be a while before companies make the most sensitive healthcare devices – such as heart valves – by 3D printing.

The FAA says it is making efforts to understand the implications of 3D printing in the aerospace industry.

The Food and Drug Administration – one of the regulators most intimately involved in the safety issues surrounding the technology – welcomes 3D printing’s potential to create new products custom-shaped to a patient’s body. The regulator, which last month held a workshop on the technology’s use in medical devices, says 3D printing could greatly expand the range of products that are matched to patients’ anatomy.

But a spokesman adds: “Because the manufacturing process is very different, devices produced using this technology may require additional or different testing to show safety.”