Valorizing scraps from the leather industry through additive manufacturing: Direct Ink Writing and nonplanar slicing for personalized products in the watch sector

The global leather market is expected to grow in the next years, with an estimated value of more than $400 billion in Europe by 2030 However, the leather industry generates different greenhouse gas emissions, volatile organic compounds, scraps and by products, e.g., tanned and untanned leather residues. Circular economy principles represent a potential way to use leather by products as secondary raw materials for new applications. This work aims to use leather residues as fillers in 3D printable epoxy based inks for customizable products in the fashion industry. A flexible thermal curable epoxy resin was selected as the matrix for ink formulations containing 10 or 15% wt. of grinded leather filler with a granulometry >0.5 mm and 5% wt. of silica. The formulations were processed through a custom desktop size Direct Ink Writing (DIW) 3D printer with a pneumatic extruder and a plastic conic nozzle (1.6 mm diameter). The tensile properties of the formulations and the neat resin were tested on cast and 3D printed specimens following the ASTM D3039/D3039M 17 standard. Conventional and nonplanar slicing techniques were used to fabricate rectangular samples for surface roughness measurements assessing the staircase effect from 3D printing. A demo product was then fabricated to evaluate the printability of the inks. The inks were successfully 3D printed, obtaining planar tensile specimens and rectangular planar and nonplanar samples. The tensile tests highlighted the reinforcing effect of the leather particles with minimal differences between cast and 3D printed samples confirming good printability. Using custom Grasshopper slicing definitions reduced the staircase effect from DIW 3D printing. A demo product for the watch sector, i.e., a customizable watch strap, was designed and fabricated as a proof of concept for future applications with custom nonplanar patterns and ink formulations. The shape retention before the thermal curing was obtained by combining the shredded leather and silica weight content in the inks, achieving good extrudate processability. Combining these ink formulations with nonplanar slicing for the fashion sector can pave the way for new circular economy practices, reintroducing leather scraps in the same loop and reducing the need for virgin raw materials.