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Neil Hopkinson working with SAF printer

Behind the scenes: Shaping our production-focused vision with the making of SAF.

Lynn Cohen Kolevsohn
Lynn Kolevsohn July 12, 2023
July 12, 2023

Curiosity, vision and tenacity. Three characteristics inventors have in spades, not least Prof Neil Hopkinson, Vice President of Additive Manufacturing Technology at Stratasys. But to take an idea from prototype to in-demand technology with a far reaching impact on the world also requires a willingness to take risks and to put yourself “out there” to find the resources and collaboration you need.

A gifted sportsman and musician with a lifelong interest in “making things”, Neil put aside dreams of being a full-time bass guitar player, and buckled down to complete a degree in Manufacturing Engineering and Operations Management at the University of Nottingham. It was here, in the early 1990s he learned the fundamentals of manufacturing. 

Later, while he slogged away at his doctorate in Additive Manufacturing for injection molding tooling, he developed a deep appreciation for the almost limitless design potential of AM. 

While acknowledging the low cost-point of injection molding, he also recognized its limitations; not least restrictions on the design of parts the technology can produce. This conundrum, he says, led to his life’s raison d’etre: to achieve the best of both worlds: the design freedom of AM and the low costs of injection molding. 

In the early 2000s, as an academic at Loughborough University in UK, he started publishing papers on the economics of AM and became recognized as a global champion for AM at production volumes. As he gained traction and support, he also faced doubters. Sceptics believed AM could not be used beyond prototyping or printing out a few parts, it would never be feasible as a mass manufacturing option.  

Searching for a scalable 3D printing process 

Not dissuaded by this scepticism, Neil sought to develop a 3D printing process that was scalable to volume production. In 2003 he broke new ground, filing his first patent for a process employing print heads to print infrared absorbing fluid onto a powder bed and irradiating the powder bed to fuse parts at a rate much higher than SLS – which at that time was the leading AM technology for production of end-use parts. He coined the term “High Speed Sintering” (HSS) for this idea. What he needed next was a proof of concept.  

On a work bench he prepared two mounds of nylon powder.  One mound was neat white nylon and the other mound had nylon mixed with carbon black. He applied heat via an infrared lamp. The powder that contained carbon melted, while the other mound remained in powder form. Says Neil: “We received looks of utter bemusement as we high-fived each other for making a gunky piece of black plastic.”

To further develop his ideas, Neil started the arduous process of pitching for funding, but many proposals got knocked back. One stroke of luck was that, at Loughborough University in the early 2000s, Neil had access to a block of funding for the UK Government’s Engineering and Physical Research Council (EPSRC). This allowed him to avoid the usual lengthy external peer review process and quickly secure a modest, but critical, £50,000 to conduct initial proof of concept for HSS. With funding he started testing in earnest. 

Testing High Speed Sintering on a re-purposed SLSTM printer.

Birth of high-speed sintering.

Following on from the gunky piece of black plastic on the work bench, the next step was to use a print head to deposit some infrared absorbing material onto nylon powder and irradiate it. Neil wanted to put a printhead into a SLS machine to see if using a print head and a lamp, in place of lasers and mirrors, could produce parts but much quicker and cheaper.

The problem was the printhead would need to operate in a very hot environment with a bed temperature surface at around 329 Fahrenheit (200 degrees Celsius). Most printhead manufacturers said their product wouldn’t work at that temperature, but Xaar, a leading UK-based printhead manufacturer, said they were prepared to give it a go. So, using an old SLS machine, they disconnected the laser, added a printhead and lamp and after a couple of false starts started making decent parts.

Some early parts printed using HSS showed much promise.

The very first print using HSS was a small S-shaped tube that was designed to represent ducting that was being created by laser sintering in aerospace, an industry that produces ducts and tubes in shapes that can’t be produced in any other way. Without a means to convert a CAD model into printable data, Neil’s team used simple 2D bitmap images and printed several layers of one shape followed by the next and back to the original shape. Essentially, they printed, flashed, and deposited powder layer after layer. 

Neil jokes that the first print almost worked too well. “The bitmap we created was quite rough having been created on very basic software. The roughness from original bitmap was there on the part, but happily this showed that we had excellent feature resolution.” Within the first week of testing, he was producing extremely detailed parts.

First ever individualised 3D soccer boot printed using AM.

HSS ventures into footwear.

While filing his initial HSS patents at Loughborough University, in 2003, Neil had also worked on AM printed sports footwear, resulting in the world’s first individualized 3D-printed football boot, which received much interest from global sports shoe brands. With his initial success with HSS, he secured a further £1 million from EPSRC to develop the technology specifically for making running shoe soles in collaboration with footwear brands. 

Since this pioneering work, multiple footwear brands have released 3D-printed footwear and many elite athletes have competed in the Olympic games wearing 3D-printed footwear.

3D printing of clothing and accessories is now commonplace with technology capable of printing directly onto fabric creating high fashion clothing and accessories.

Commercial development of HSS.

In academia Neil was told that the last mile is always the hardest and that making something work in a lab for an experiment is one thing; developing a commercially viable product that is reliable, repeatable, and valuable and from which your customer can make a good profit is a much harder thing to do. This proved to be true!


The first ever HSS printer build from the ground up - dubbed “Little Blue”.

Commercial development of HSS proved very difficult indeed.  For various non-technical reasons HSS development stalled for five years. Neil defended his patents in the Hague and faced restrictions on licensing, but in 2011, he was finally free to build the world’s first HSS machine from the ground up. This milestone marked the start of the biggest change in 3D printing in 20 years, and, through Loughborough University, Neil licensed the patents to several 3D printing companies.

With multiple licenses in place, Neil wanted to join one of the companies that was developing the technology commercially. “I joined Xaar, a company with which I’ve had great affinity, as they not only supplied the first printheads, but its management also understood the revolution that had taken place in 2D printing, making digital printing at volume cheap and agile, so it was easier for me to sell the concept of achieving this scale up with 3D printing,” Neil explains. At Xaar he teamed up with a group of engineers from Copenhagen who had also developed a powder bed 3D printer and thus had valuable expertise in controlling temperatures and working with powder. Together they developed the first prototype, a 3D printer development process that started in early 2017.   

Also in early 2017, Neil was at Stratasys headquarters in Rehovot, Israel, on unrelated business and, completely by chance, heard that Scott Crump, the founder of Stratasys, was also there at the time. Neil had initially met Scott in Detroit ten years prior and had occasionally kept Scott up to date with HSS. The chance meeting in Rehovot led to an impromptu dinner where the two agreed to meet for a demonstration. “We started a discussion about partnering with Stratasys to sell our printers and agreed to demonstrate our first prototype – which we were able to do in late 2017,” says Neil.

The team at Xaar wanted to partner with Stratasys because they knew that while Xaar could build the business up organically and sell printers, growth would be exponential if they partnered with the best and most trusted brand in 3D printing and its global sales channel. 

The H350 using SAF technology – a high-performing 3D printer resulting from decades of research.

Stratasys and Xaar: a perfect partnership.

The result of the 2017 demonstration to Scott Crump was a joint venture between Xaar and Stratasys, formed in 2018 and called Xaar 3D.  The technology developed at Xaar 3D built on the original HSS approach of printing an absorber and fusing material but did so with a unique process architecture, which was called Selective Absorptive Fusion (SAF) technology. In 2021, Stratasys acquired Xaar 3D in full. The first 3D printer using SAF™ technology was the H350,™ which was launched in early 2022.

Neil had achieved his vision. SAF technology allows for maximum freedom of design, and thousands of parts to be printed economically and consistently. And it now is an anchor technology for Stratasys’ mission to transform polymer manufacturing through 3D printing. 

When looking at his journey, Neil recounts many highlights. He is particularly proud of how the technology he invented and licensed became a real product during the nightmarish first year of the Covid-19 pandemic. Many frontline workers were desperately short of PPEs and medical equipment, and additive manufacturing technologies quickly mobilized to help fill gaps in the supply chain. This  response woke many business leaders to the possibilities of 3D printing – accelerating product development and production without enduring long lead times to develop tooling for traditional methods such as injection molding.  

Should there be another global shortage of medical equipment for whatever reason, the 3D community is now organized with a repository of vulnerability-assessed models for medical equipment and a network of 3D printing and health care experts able to mobilize quickly for stopgap solutions.

More recently the H350 has been used to print tourniquet parts to treat casualties from the war in Ukraine.  

The H350 continues to find applications in a range of industries and Neil and his team now focus breaking new boundaries with the addition of new materials with attributes that will allow for new applications.


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