3D printed polyurethane footwear makes technological strides

All of the big sportswear brands have been investigating the possibility of printing high-performance footwear. And they have been supported by the manufacturers of materials and printing equipment.

Adidas set up a strategic partnership with Redwood City, Calif.-based 3D printing specialist Carbon to create its Futurecraft 4D high-performance footwear. The German sportswear brand says it used 17 years of running data to design the new midsole. This was made using a digital footwear component creation process that removed the need for traditional prototyping or molding.

It uses Carbon's Digital Light Synthesis (DLS) process. This employs digital light projection, oxygen-permeable optics and programmable liquid resins to create high-performance, durable polymeric products.

The running data in Adidas's library was used to shape functional zones into a midsole design. Using the data allowed Adidas to design midsoles which combined movement, cushioning, stability and comfort.

In contrast to standard 3D printing manufacturing methods, the DLS process is claimed to overcome issues such as low production speed and scale, poor surface quality and restrictions in color and material.

The midsoles are grown from liquid, and Adidas says the lattice network that is formed has 20,000 individual struts that can be adapted to optimise energy return.

Nike has started printing the uppers, too, and Eluid Kipchoke ran to victory in the 2018 Virgin London Marathon in a pair of shoes with uppers 3D printed out of TPU. The company claims this is the first time the uppers of performance footwear have been created in this way.

The uppers are made using solid deposit modeling. In this method, filament is unwound from a coil, melted, and then laid down in layers. The company claims printing allows prototypes to be created 16 times faster than other production processes.

Nike also is using the technique to incorporate athlete-specific data into the textile geometry. Nike uses data captured from the athlete and this is analyzed to confirm the material's ideal composition. That analysis is used to design the textile.

The company claims that 3D printed fabrics are more dynamic than traditionally woven fabrics. This is because the warp and weft fibers are connected.

"An advantage of Flyprint textiles comes in the fused nature of the material," the company said. "In a knit or woven textile there is frictional resistance between the interlaced warp and weft yarns. In a printed textile there is greater potential for precision-tuned containment."

It added that the 3D printed textile is "lighter and more breathable" than earlier fabrics.

The Zoom Vaporfly Elite Flyprint shoes Kipchoge wore were based on his feedback on the shoes he wore in the 2017 Berlin marathon. His 2018 London marathon shoes were .4 ounces lighter than the first iteration.

Reebok's Liquid Factory process was introduced in 2016. It uses software and robotics to draw shoes in three dimensions. A liquid urethane material created for Reebok by BASF is used to draw the shoes' components precisely in three-dimensional layers.

The Liquid Speed shoe is created using this polyurethane system, which is drawn onto the entire shoe as the outsole rather than using a mold to create the outsole separately. As soon as it is drawn on, the material starts to cure and solidify. The outsole has "wings" that wrap around the sides, giving a more custom fit and better support for the whole foot.

Improve performance

New Balance signed a deal in 2017 with its Boston neighbor 3D printing specialist Formlabs to implement 3D printing on a larger scale. The two companies are developing footwear-specific materials and printers that will be able to create products that will improve athlete's performance. Its stereolithography printer Form 2 powers the printing process.

This is not New Balance's first foray into 3D printing: back in 2013, Jack Bolas became the first track athlete to compete using customized 3D printed spike plates. To design these, individual biomechanical data was gathered via race simulation using a force plate, in-shoe sensors and a motion capture system.

This initial 3D printing project led on to further work on the custom printing of the softer components required for midsoles. The Zante Generate, introduced in 2016, features what New Balance claimed to be the first full-length 3D printed midsole. It used 3D Systems' DuraForm TPU elastomer laser sintering powder in a selective laser sintering manufacturing process that built the midsole up layer by layer. The result was a midsole with an intricate honeycomb structure that the company says has an "optimal balance" of flexibility, strength, weight and durability.

Once 3D printed soles and uppers have been created, they still have to be bonded together. According to Atom Lab, the research unit at shoe machinery manufacturer Atom, the most technically challenging step in this process is bonding the outsole to the shoe upper. The company has worked with Covestro to develop a process where the polyurethane raw materials for adhesives are applied to the outsole or the upper via a digital printing process.

Atom used a variation of the fused filament fabrication melt layer method to apply the adhesive. Strands of melt-processable adhesives are first melted, and then the molten adhesive are applied according to a digital print layout. Pressure is then applied to join the sole to the upper, creating a solid and permanent adhesive bond between the two.

According to Atom, this application using adhesive filaments is both quick and efficient. Precise and reproducible, it is suitable for absorbant substrates. Solid filaments are used, so an additional drying process is not necessary.

Additionally, the adhesive is applied from the melt and is preactivated. If the open time is long enough, prior heat activation is not necessary. This saves two process steps.

The technology is still in its early stages, with developments focused on high-end, limited-edition products. But these projects indicate that larger scale implementation of additive manufacturing in the sportswear market is likely, and polyurethane materials have the potential to make it possible.