When the print head of the 3D printer moves along the X and Y axes above the blank sneaker substrate, it deposits Voxel8's high-performance elastomers, which are manufactured in real time during the printing process. The digital image is then ink-jetted onto the elastomeric features, and finally a protective topcoat is added.

Fred von Gottberg, CEO of Voxel8, said: "Usually, when you make a shoe today, it requires hundreds of components and steps." "So we replaced hundreds of steps with a three-step process."

The technology of the Massachusetts-based company and its focus on sports shoes and textiles make one of the advantages of additive manufacturing (AM) a reality-reducing the number of steps or parts required to manufacture a product.

Voxel8 also achieves Industry 4.0's mass customization goals by depositing harder or more flexible elastomers where needed and inkjetting any images customers want.

Other companies that have overcome the challenges of using liquids and soft materials (such as polymers (including silicone and polyurethane)) for 3D printing are in the process of expanding existing market shares or creating new markets and mainstreaming additive manufacturing. Bring other benefits.

Among the benefits are that bioprinting can prevent prints from collapsing under their own weight, on-demand printing can replace spare rubber parts in storage, and silicone parts can be made without the expensive molds used in injection molding.

Although Voxel8 focuses on everyday products such as shoes and textiles, its technology uses technically challenging chemical processes.

A unique challenge is that the chemical reaction begins as soon as the raw material enters the mixing head. Von Gottberg said that the challenge lies in timing. "You have to make sure it doesn't react too fast," which will clog the print head, "but it reacts fast enough that when it hits your substrate, you can build a 3D structure when you set it up."

The company's material "ink" is isocyanate and polyol prepolymers, which react to form polyurethane. Different inks can be used to print silicone and epoxy resins. He said that Voxel8 has also developed inks made from 65% of bio-based raw materials or 25% of recycled components.

The ratio of the raw materials entering the four-input print head of the Voxel8 ActiveLab printer can be adjusted to change the hardness of the resulting thermoset plastic from the hardness of a soft and flexible rubber band to a hard plastic bottle. Uncured thermoset plastics can be extruded to the thickness and height of 3D structures, or atomized and sprayed for 2D applications. The material can also be extruded into a mold to obtain fine features and sharp edges.

In addition to sports shoes, the company's materials and technology can also be used to replace steel rims to increase the stiffness of bra support. They can encapsulate sensors, printed circuit boards and wires in electronic wearable devices to make them more ergonomic. They can be used in branded luxury cars.

Conceptually similar to Voxel8's technology, Chromatic 3D Materials' isocyanates and polyols react in the print head to obtain viscosity and reactivity suitable for printing. After deposition, the material forms chemical bonds between the layers, resulting in a very strong material with a very smooth surface.

Cora Leibig, CEO of Chromatic 3D Materials, said: “When you think of them as gels, you might think about how they flow in the right way,” because the material is discharged from the print head.

Unlike Voxel8, Leibig's company manufactures elastic spare parts for long-life industrial equipment such as trucks, tractors, buses and trains that have a service life exceeding rubber seals, gaskets and bushings.

Leibig's technology can change the hardness in a single part like Voxel8, but she doesn't care about this except for occasional living hinges or soft handles surrounding rigid parts.

When customers require different compression deformations, food-grade materials or different colors, the variability of the materials is really outstanding.

"It is often difficult to maintain spare parts inventories for equipment with a long service life, because rubber parts usually expire in about seven years," she said. "In addition, the inventory of these parts is very expensive because their demand is very low."

Usually, about 25% of the market only needs a few hundred parts per year. "So this is really the best place for 3D printing," Leibig said.

Although the demand of a single company may be low, the overall demand constitutes a huge market. She said that a company may spend 5 to 20 million U.S. dollars on elastic spare parts each year, and the global elastic seals and gaskets market is worth about 15 billion U.S. dollars.

"We must have found that it has a lot of demand," she added. "We will find out how far it can take us, but it will definitely allow us to start pursuing some new designs, such as things like multi-material parts and original equipment."

Leibig is a materials scientist and former Dow Chemical Research Director. When the inspiration for Chromatic 3D Materials burst, he was judging a high school debate on solving supply chain issues.

"I see such a huge demand for high-performance materials for 3D printing," she said. "I have also seen that the material palette of 3D printing lacks some chemicals that are heavily used in industrial applications."

Wacker Chemie also noticed that an industrial polymer, silica gel, is missing from the 3D printing palette. The company has accumulated 70 years of experience in silicone, but if 3D printing is possible, designs that cannot be achieved by injection molding can be performed, and expensive molds are not required.

To fill this gap, the company launched a research project in 2014 that resulted in its on-demand printing process capable of depositing silica gel drop by drop. In 2016, WACKER's Aceo division began to provide silicone 3D printing services.

In 2018, Aceo collaborated to develop a cold plasma device for wound healing, using its conductive 3D printed silicone for the first time. This year, the department launched a new combination of materials in a single print, which includes silicone elastomers and epoxy resin thermosetting materials.

The basic formulation of the material combines polymers with reinforcing fillers, crosslinking agents and addition curing catalysts.

According to the company, Aceo silicones are based on WACKER’s Elastosil liquid silicone rubber series and have similar mechanical properties. Aceo silicone has a hardness range of Shore A 20-60 and comes in different colors.

Like the products of Voxel8 and Chromatic 3D Materials, silicones of different hardnesses can be combined in the same print.

"This 100% silicone material is the biggest challenge we faced in the development process," said Vera Seitz, who was involved in silicone printing projects and now leads Aceo's business development. "Due to their high initial viscosity and long polymer chains, true silicone elastomers cannot be printed with traditional techniques."

Seitz went on to explain how the team overcome the challenge: “Due to the high viscosity and low surface energy of silicone, the formation of droplets requires a metering valve that can shear and eject the material at high frequency. Using this method, the nozzle of the valve and 3D printing There is no contact between the objects, so there is no string of material between the nozzle and the 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 discrepancies in subsequent layers.

The thickness of the layer will cause a "step effect" on the surface of the part, especially for complex geometries. But for a horizontal surface parallel to the build platform, the surface is smooth.

"Therefore, the part orientation in the build space plays an important role in the final quality of the part," Seitz said. "However, we noticed that layered surfaces are usually not a problem in the final application, because the material properties exceed the appearance of slightly differentiated layers."

The polymer provides support for the overhangs and inner cavity during the printing process, but is washed away by water during post-processing. Ultraviolet radiation crosslinks the various layers. The part is then cured at 200°C (392°F) for four hours, which is the standard for silicone elastomers.