Final composite mandrel created from washout tooling.
The production of lightweight composite materials is high in demand for sectors like aerospace, automotive and defense where weight savings, strength and durability are crucial factors for winning a race or completing a mission. It is, however, a highly time-consuming and costly, multi-step process.
ExOne, a U.S. based additive manufacturing company which specialises in metal and sand binder jet printing, has developed an alternative that enables the manufacture of complex carbon- or glass-reinforced parts, doing away with multi-piece metal breakdown tooling or difficult to remove sacrificial tooling commonly used to build composite parts with trapped geometries. That means no need for solvents, deflating tools, or hacking away at parts with a chisel.
The technique, a washout tooling process, involves creating a robust yet dissolvable 3D printed support core of silica or ceramic sand. The part is coated with a water- soluble spray or Te on layer to leave an impenetrable surface before composite layup through lament winding or woven fabric depending on specific strength requirements. Once cured, the soluble support material is then easily washed away with tap water to leave a hollow composite part. Better still, that dissolved media can also be recovered and re-used for future print cycles.
“This is something that we have been kicking the tyres on for a while,” Rick Lucas, Chief Technology Officer at ExOne told TCT. “We really decided this year that this was a market we wanted to target and go after. We’ve got engineers here who have worked in the composites industry and [understand] that people have not been satis ed with the current options that are out there.”
Since 2013, when ExOne engineers first discovered they could bind silica or ceramic sand particles with a solvent that remains water soluble to 180°C, key areas of development have focused on the coating step and applying binder jetting experience to optimise the material. It's a complex challenge as customers require different resins to cater to various applications.
"The ceramic sand that we generally print with is porous," Lucas explained. “So, if you were just to lay a composite over that, and autoclave with pressure and heat, and cure that, the resin would soak into the tool and no longer be wash out. So, we had to put a coating on it to prevent that from happening. The coating methods and processes for that have been where we’ve spent a lot of our time in the last year or so, really fine tuning and making sure we’ve got good coating options for customers.”
Filament winding on 3D printed washout tooling.
This approach is now being used to create carbon and glass fibre reinforced composites, including aircraft ducting, pressure tanks, shrouds, struts and mandrels, particularly for designs that were previously unsuitable for manufacture through more traditional techniques. Take for example, Royal Engineered Composites, a company which creates air ducting for Sikorksky Aircraft Corporation’s heavy-lift helicopters. The company used ExOne’s washout tooling to create the mandrels for air ducts on two CH-35K helicopters which have already been successfully delivered to the Marines for demonstration.
“One of the things that we had to develop as a result of that programme was the ability to join long pieces,” Lucas said. “We generally print these in our smallest sand machines, the S-Print. It’s roughly 800 x 500 x 400 mm and so we developed a joining technique, where we can print these and on the ends we have designs [that can] only go together one way. That enables us to make really long shapes.”
For many customers, specific use cases can be highly sensitive due to the nature of their work but Lucas says it’s not uncommon for the company to receive orders from major aerospace and defense firms or racing teams. In one particular case, the process was used to supply a washout mandrel for an engine air intake manifold for the University of Texas at Austin’s racing team.
“What’s valuable to them is really our turnaround time,” Lucas said. “We can turn around a mandrel within a couple days and for that industry, time, it’s not just the performance there but the timing, and being able to turn it quickly is really, really important.”
Hollow composite fibre parts created with 3D printed sacrificial tooling.
ExOne has also sought to combat another common challenge found in the traditional autoclave process; thermal expansion and its effect on part geometries. With the washout tooling process, Lucas says the expansion can be controlled and distortion minimised by changing the powder. For example, silica sand’s high CTE may be suitable for some materials but in other cases where a lower CTE is needed, ceramic sand could be a more suitable option.
“What drives the thermal expansion is really the media, not the binder,” Lucas added. “That’s why those two materials are the two primary ones we refer to because one is close to glass composite in terms of thermal expansion, the other is close to carbon.”
Currently the technology is available on-demand through ExOne’s Adoption Centres primarily in the U.S. but the company says it is looking to expand these capabilities to Europe in the near future and potentially working with partners as demand increases. ExOne says it is also working on refining its binder even further with a next generation version that will deliver higher strength, finer resolutions and higher temperature capabilities. Building on that, it is also delving into an area many manufacturers are looking to increase their efforts in; automation.
Lucas added: “One of the big things that we’re looking to scale up is; we have some applications where the parts are not really large, they’re less than the size of a volleyball but they need hundreds of thousands of parts a month. So how can we automate the coating process? How do we take what’s primarily a manual process right now and convert that and automate that to where we have very high throughput?
“We have a great product as it stands now but we think we can improve it even further.”
