# A Material-Recycling Unit for the Fused Deposition Modelling of Three-Dimensional Printing Systems
## 1 Introduction  
Fused deposition modelling (FDM) is a melt extrusion process that exhibits the ability to fabricate intricate geometries through the deposition of molten materials in the form of a filament [1; 2]. It has gained substantial recognition as the pre-eminent commercially viable technique for three-dimensional (3D) printing [1; 3]. The 3D-printed structure is created using a layer-by-layer approach, where the print head assembly is moved over a platform to extrude the molten filament tip in precalculated positions [3]. Materials such as Acrylonitrile Butadiene Styrene (ABS), Polylactide (PLA), Thermoplastic Polyurethane (TPU), etc., are commonly used in FDM manufacturing [1; 4]. Products of FDM are increasingly present in many biomedical [5; 6; 7; 8; 9], tooling [10; 11; 12; 13], electronic [14], automobile, and aerospace applications [15; 16; 17], etc., playing increasing roles in the industry [14]. However, at the same time, the continued build-up of plastic waste in landfills has become a significant concern for environmental sustainability [18]. Ideally, FDM technology should exclusively employ materials that are essential for the final product. However, this is not feasible due to the need for support structures in various geometries and the frequent need to reprint defective objects, especially during prototyping [16]. To tackle the issue of plastic waste in FDM, several studies have explored the generation of support materials, focusing on modifying the material behaviour or optimising the support structure without recycling [19; 20; 21]. Still, this option is highly restricted by the geometry of the 3D-printed object. Recycling plastic waste, especially post-consumer, is a different method [22]. On a global scale, plastic recovery and recycling rates tend to be lower compared to other commonly utilised materials, such as paper, glass, and metals, even in countries with advanced waste management systems and extensive recycling expertise [23; 24; 25]. This relatively low recycling rate can be attributed, in part, to the diverse range of plastics, additives, and composites used in various applications [23]. In the realm of plastic recycling methods, while there have been advancements in chemical recycling,mechanical recycling remains the predominant technology used for recovering plastic waste, which involves the employment of mechanical processes to transform the plastic waste into new plastic products [26; 27; 28; 29], which is a highly effective method of plastic recycling based on time, economic cost, and environmental impact [30]. Typically, the recycling of plastic mechanically involves melt blending, sorting, shredding, and reprocessing [23; 30]. The existing plastic-recycling machines are expensive and occupy a large amount of space. Their design target is to recycle wide-ranging plastic waste, not limited to FDM waste, into 3D printer filaments or make filaments [31; 32]. A comprehensive machine or machine attachment unit explicitly designed for the recycling of waste from 3D printers is not widely available on the market and is typically associated with high costs. The Filabot company (Barre, VT, USA) offers an all-in-one configuration for 3D-recycling setups, which includes a reclaimer, extruder, airpath, spooler, and pelletiser. This setup is available at a price of approximately GBP 1500 and requires a large setup volume [33]. The ProtoCycler+ Filament Maker and Recycler introduced by ReDecTec (Toronto, ON, Canada) is an all-in-one extruder machine with a built-in manual material shredder, mainly focusing on research purposes and is estimated to be an expensive unit [34]. Similar products were commercialised by Precious Plastic but require a large amount of setup space and are more suitable for the manufacturing end of recycling process [35]. In short, the commercially available FDM-waste-recycling machines are generally either expensive or do not have all-in-one configurations.