Xaar
Parts printed with Xaar's high viscosity printheads.
In a university research lab in West Lafayette, IN, a doctoral student is exploring the potential of a novel 3D printing technique to develop solid rocket propellant.
The motivation for trying to 3D print solid rocket propellant is to gain more control over its geometry – since this can affect how quickly propellant burns – and by the end of the research, Purdue University Assistant Professor Monique McClain will prove out the ability to 3D print a traditional mixture with all the volume loading content of traditional propellant and with no defects induced by the 3D printing process.
She does this with a 3D printing technology that utilises a vibrating nozzle which facilitates the flow of highly viscous materials. The key material is an ammonium perchlorate, which boasts particle sizes of around 200 microns, while the hydroxyl terminated polybutadiene binder used has a honey-like consistency. The viscosity requirement of the build material and binder is important for this application because, per McClain, there is a minimum number of particles required in rocket propellant for it to burn.
Purdue’s novel printing technology –called Vibration Assisted Printing – was developed by Emre Gunduz because there was no such printing technology to be bought off the shelf that could print parts in the viscosities Purdue needed. In 2018, that was not just true of ammonium perchlorate, but polymer materials too. There are markets out there yearning for things to change.
“What we’re doing with solid propellants, doesn’t just have to stay relevant to solid propellants,” McClain told TCT.
Earlier this year, BCN3D joined a select few others in the commercial 3D printing space looking to enable the printing of materials with higher viscosities with its Viscous Lithography Manufacturing (VLM) technology. A material’s viscosity is measured in centipoise (cP) or millipascal-second (mPas) – 1cP equating exactly to 1mPas – and refers to its resistance to flow because of its molecular makeup. Such flow resistance can make it challenging for some 3D printing technologies to process highly viscous materials.
Though BCN3D has so far specialised in extrusion-based methods, it has not followed in Purdue’s footsteps with a vibrating nozzle technique. Instead, the company landed on a lamination process in its bid to ‘enable manufacturing autonomy.’
“We wanted to bring value to this segment of the market by producing real functional parts, beyond prototyping, using resin,” BCN3D CTO Eric Pallarés Garcia explained.
To do that, BCN3D developed a patented photopolymerisation technology whereby resin materials are picked up by a roller, laminated to the underside of a transparent film, transferred to the printing area and when the build plate rises to meet the recently laminated resin, a UV light engine cures the resin in specific areas to build up parts. The build plate then retreats, peeling the recently cured layer from the film, with the unused material being recovered for the next print. BCN3D claims VLM can process resins that are ‘50x more viscous than standard market resins’ and says it can also tackle multi-material applications.
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Exhibit at the UK's definitive and most influential 3D printing and additive manufacturing event, TCT 3Sixty.
Per Garcia, VLM can process materials of higher viscosities because it is spreading resin like butter, rather than relying on the flow of liquid as in vat-based technologies. Other challenges in the 3D printing of highly viscous materials include the durability of the machinery required. Daniel Rothfuss, Global Head of Application Engineering for 3D Printing at Henkel – a BCN3D partner – noted how the high force load of more viscous materials can damage build trays, while there are also concerns with the creation of air bubbles on the resin. Heating resins can make them easier to process but that reduces the viscosity of the material – something the likes of BCN3D are trying to move away from.
It’s like giving an artist a full colour palette when they’ve had monochrome for their entire career.
In Xaar and ViscoTec there are two more companies who are channeling their expertise into the development of print heads that can facilitate the 3D printing of highly viscous materials. Four years ago, Xaar exhibited its High Laydown inkjet technology for photopolymer materials at TCT 3Sixty, with plenty of additional R&D ongoing since.
High Laydown allows Xaar to output ‘a lot of material very fast’ and increase the viscosities its print heads can handle from 100 centipoise to 130 centipoise, enabling parts with enhanced material properties.
“What high viscosity is enabling is true additive manufacturing,” Xaar Business Development Manager for EMEA + Israel Gareth Neal suggested. “There’s 3D printing, which makes parts in 3D, and then there’s additive manufacturing things which goes beyond making shape. We can put conductives into the ink, we can put dielectric properties into the ink, ceramics for structure or heat deflection temperature.”
ViscoTec, meanwhile, is developing printheads for filament 3D printing systems, with its vipro-HEAD range said to be able to process pastes with viscosities up to 1,000,000 millipascalsecond (mPas). These printheads are said to be able to gently flow material when building up parts and are compatible with off the shelf epoxy resins, acrylates, silicones, waxes and more.
BCN3D
Parts printed with BCN3D VLM technology.
Xaar has identified several niche applications within 3D printing for High Laydown, including windshield frits (those black dots that protect your eyes from the sun) and digital braille on packaging labels. ViscoTec, on the other hand, sees a lot of scope in automotive, medical and sealing applications, though Business Development Manager Simon Kasböck noted materials supply chain that makes sense for 3D printing volumes are lacking – particularly for silicones.
BCN3D’s solution on materials has been to align with Arkema and Henkel to work on the development and supply of highly viscous 3D printing materials for VLM. With this pair, BCN3D has set its sights on acrylic resins, epoxy resins, silicones and materials filled with inorganic fillers like metals or ceramics. With such an offering, BCN3D is looking at enabling elastomeric applications with good rebound energy return, while also hoping to facilitate parts with flame retardancy, low emissions and good insulation properties.
“If you bake a cake and only have flour, eggs and sugar, you can do a good cake, but if you have access to more ingredients, you would be able to do something more tasty,” Garcia said. “With high viscosity, you’re getting rid of limitations in terms of the ingredients you can use to make your formulation and to make your product.”
“It’s like giving an artist a full colour palette when they’ve had monochrome for their entire career,” Neal offered.
It would be fair to say that with this year’s launch of VLM, the print head developments for inkjet and extrusion technology by Xaar and ViscoTec, the material development at the likes of Henkel, and even the high viscosity capabilities of machines like the ETEC Xtreme 8K, a trend is emerging. McClain also suggested the vibrating nozzle technique developed at Purdue could, in theory, be used for high viscosity polymers.
And you don’t have to look too far to find the applications. Whether it’s in the implementation of braille on a label, the production of a medical device in a hospital, or the development of rocket propellant in an R&D lab, there is a demand for the durability of traditionally made parts, and the geometric freedom of 3D printed ones.
“The additive manufacturing industry is looking for more durable and functional resins and products to unlock new applications in automotive, railway, medical, everywhere where you have very demanding requirements,” Rothfuss said. “Also, long-term stability is one of the many requirements we are frequently asked [about]. And one possible solution to fulfil those needs and to develop new resins is to add functional additives into the formulation.
“What you’re enabling with a higher viscosity printhead, or a higher viscosity technology, is not only material capability, there’s [also] a resolution and speed which makes manufacturing viable,” Neal finished. “You’re giving materials engineers a wider gamut of materials to play with, you’re giving them a wider range of applications they can fix, so really what we’re giving is opportunity.”
