Bonding soles via digital printing of adhesives

Bonding soles via digital printing of adhesives

Atom Lab, the research and innovation business unit of shoe machinery manufacturer Atom, will present a fully-automated process for shoe production for the first time at the Simac Tanning Tech trade fair in Milan, Italy, from 20 – 22 February 2018. At its stand in hall 14, visitors can select 3D-printed soles and uppers, and have customised shoes made for them.

Covestro´s digital print experts Dr. Jörg Tillack (right) and Lukas Breuers examine a shoe sole with an adhesive printed on it (Source: Covestro)

According to Covestro, the most technically challenging step in this process is bonding the outsole to the shoe upper. The company has developed a solution for this, which is based on polyurethane raw materials for adhesives, which are applied to the outsole or upper by means of digital printing.

For the application, Atom Lab used a variation of the melt layer method (Fused Filament Fabrication, FFF, or Fused Deposition Modeling, FDM). This method uses strands of melt-processable adhesives – so-called filaments – which are melted. The molten adhesive is applied according to a digital print layout. The sole and upper are then joined under pressure. This creates a solid, permanent adhesive bond.

This application using adhesive filaments is quick and efficient, said the company. Furthermore, it is said to be precise and reproducible, as well as well suited for absorbent substrates. Due to the use of solid filaments, for example, the subsequent drying process may be dispensed. The adhesive features a high initial strength. Since application takes place from the melt, the adhesive is already activated. If the open time is sufficiently long, the fabricator can dispense with prior heat activation. Thus, two process steps can be saved in total.

Covestro said it benefited from its experience in additive manufacturing with the technology and material solution developed on short notice. The company is involved in conventional printing methods and is currently developing a range of materials. Among these are filaments and raw materials for the FFF method, which so far has been used primarily in rapid prototyping.

Polyurethane adhesives have proven to be successful in shoe manufacturing. They are easy to process and result in flexible yet strong and durable adhesive joints with high impact resistance. Covestro offers a comprehensive range of raw materials for one- and two-component adhesives that are often developed in close collaboration with formulators.