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AM for liquid and soft materials goes mainstream

Ilene Wolff
By Ilene Wolff Contributing Editor, SME Media

As the printhead of a 3D printer moves along the X and Y axes above a blank athletic shoe substrate, it deposits Voxel8’s high-performance elastomers that are made in real time during printing. Then a digital image is inkjetted onto the elastomer features and, finally, a protective topcoat is added.

“Typically, when you manufacture a shoe today, it requires hundreds of components and steps,” said Voxel8 CEO Fred von Gottberg. “So, we’ve replaced hundreds of steps with a three-step process.”

The Massachusetts-based company’s technology and its focus on athletic shoes and textiles makes real a touted benefit of additive manufacturing (AM)—that of reducing the number of steps or parts it takes to make a product.

Vera Seitz, right, is one of the engineers who developed a process to 3D print silicone at Aceo, a division of Wacker Chemical. (Provided by Wacker Chemical )

Voxel8 also realizes the mass-customization goal of Industry 4.0 by depositing stiffer or more flexible elastomers where needed and inkjetting any image a customer desires.

Other companies that have mastered the challenges of 3D printing with liquid and soft materials like polymers, including silicone and polyurethane, are bringing other benefits into focus as they grow their share of existing markets or create new ones and take AM mainstream.

Among the benefits are bioprinting that keeps a print from collapsing under its own weight, on-demand printing that replaces warehousing spare rubber parts and making silicone parts without the costly molds used in injection molding.

Although Voxel8 focuses on everyday objects like shoes and textiles, its technology uses technically challenging chemical processes.

Voxel8 CEO Fred von Gottberg. The company’s 3D printing technology reduces the number of steps it takes to make athletic shoes and makes mass customization possible. (Provided by Voxel8 )

A unique challenge is the fact that the raw material begins a chemical reaction as soon as it goes into the mixing head. The challenge is in the timing, von Gottberg said. “You have to make sure it doesn’t react too quickly,” which will plug the printhead, “but reacts quickly enough to, when it hits your substrate, set up so you can build 3D structures.”

The company’s material “inks” are isocyanate and polyol prepolymers that react to make polyurethane. Different inks can be used to print silicones and epoxies. Voxel8 has also developed inks made from 65 percent bio-based raw material or with 25 percent recycled content, he said.

The ratios of the raw materials fed into Voxel8’s ActiveLab printer’s four-input printheads can be adjusted to change the durometer of the resulting thermoset from that of a soft, flexible rubberband to a hard plastic bottle. The uncured thermoset can be extruded for thickness and height of 3D structures or aerosolized and sprayed for 2D applications. The material can also be extruded into a mold to achieve fine features and sharp edges.

In addition to athletic shoes, the company’s materials and technology can be used instead of underwires to add stiffness for support in bras. They can encapsulate sensors, printed circuit boards and wires in electronic wearables to make them more ergonomic. And they can be used for branding luxury vehicles.

Market demand hits sweet spot

Similar in concept to Voxel8’s technology, Chromatic 3D Materials’ isocyanate and polyols react in the printhead to have the right viscosity and reactivity for printing. After deposition, the material forms chemical bonds between layers that result in very strong materials with a very smooth finish.

“When you think of them as gels, you are probably thinking of them the right way in terms of how much they flow” as the material exits the printhead, said Chromatic 3D Materials CEO Cora Leibig.

Unlike Voxel8, Leibig’s company makes elastomeric spare parts for long-lived industrial equipment like trucks, tractors, buses and trains whose life exceeds that of their rubber seals, gaskets and bushings.

Leibig’s technology can change the durometer in a single part like Voxel8’s can, but she doesn’t focus on that aspect except for an occasional living hinge or a soft handle around a rigid part.

The variability of the material really shines when a customer requests a different compression set, a food-grade material or different colors.

“Oftentimes it’s very difficult to maintain spare parts inventories for those long-lived pieces of equipment because rubber parts usually expire at about seven years,” she said. “Also, these parts are very expensive to keep in stock because they’re very low demand.”

Typically, about 25 percent of the market needs only a few hundred parts per year. “So that’s really within a 3D printing sweet spot,” Leibig said.

While the demand may be low for an individual company, the combined demand comprises a large market. A company may spend $5 million to $20 million yearly on elastomeric spare parts, and the global elastomeric seals and gaskets market is worth about $15 billion, she said.

“We’re definitely finding a lot of demand for it,” she added. “We’ll find out how far it can take us, but it can certainly take us to the point where we can start going after some of those new designs like multi-material parts and things like that for original equipment.”

Leibig, a materials scientist who previously was research director at Dow Chemical, was judging a high school debate about fixing supply chain problems when inspiration hit for Chromatic 3D Materials.

“I saw that 3D printing has this huge demand for higher-performing materials,” she said. “I also saw that some of the chemistries that are used to a large degree in industrial applications were just missing from the materials palette in 3D printing.”

Another material for the palette

Wacker Chemical also noticed an industrial polymer that was missing from the 3D printing palette—silicone. The company already had amassed 70 years worth of experience with silicone but to be able to 3D-print it would allow for designs not possible with injection molding and would eliminate the need for expensive molds.

To remedy the void, the company started a research project in 2014 that resulted in its drop-on-demand printing process that deposits silicone drop by drop. In 2016, Wacker’s Aceo division started offering silicone 3D printing as a service.

Aceo in 2018 collaborated on a cold plasma device for wound healing, introducing the first use of its electrically conductive, 3D-printed silicone. And this year, the division introduced a new material combination with silicone elastomers and epoxy thermoset in a single print.

The basic formulation of the material combines polymer with a reinforcing filler, crosslinker and addition-cure catalyst.

The Aceo silicones are based on Wacker’s Elastosil line of liquid silicone rubbers and have similar mechanical properties, according to the company. The Aceo silicones range in hardness from shore A 20-60 and come in different colors.

Like Voxel8 and Chromatic 3D Materials’ product, silicones of differing hardness can be combined in the same print.

“This 100 percent silicone material has been our largest challenge during the development,” said Vera Seitz—who worked on the silicone printing project and now leads Aceo’s business development. “Due to their high starting viscosity and long polymer chains, real silicone elastomers cannot be printed with conventional technologies.”

Seitz went on to explain how the team overcame the challenge: “Due to the high viscosity and low surface energy of silicones, the formation of droplets requires a dosing valve capable to shear and eject the material with high frequency. With this method, there is no contact between the nozzle of the valve and the 3D-printed object and consequently no string of material between the nozzle and object.”

Aceo’s 3D printer measures the thickness of each printed layer, compares it with the target value in the CAD model and corrects any difference in the subsequent layer.

The layers’ thickness can result in a “staircase effect” on part surfaces, particularly for complex geometries. But for horizontal surfaces that are parallel to the build platform, surfaces are smooth.

“So, the part orientation in the build space plays an important role in the final quality of the part,” Seitz said. “However, we notice that the layered surface is often not an issue in the end application where the material properties outweigh the slightly layered appearance.”

A polymer provides support to overhangs and inner cavities during printing but is washed away with water during post-processing. Ultraviolet radiation crosslinks individual layers. Parts are subsequently cured at 200°C (392°F) for four hours, which is standard for silicone elastomers.

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