## 1 Introduction
Researchers have shown that multimodal powder distributions can increase flowability and final sintered density [17]. Therefore, unique particle size distributions are needed for BJT, and these special distributions increase the cost of the powder [16; 18]. In addition, relative to MEX, BJT equipment is more costly [19]. Thus, for low-volume applications of medium-sized specimens, MEX could be a more economical alternative. The most common type of MEX additive manufacturing is a relatively simple process that uses filaments as starting materials. The material with a liquid-like consistency is pushed through a die and then selectively deposited, layer-by-layer, to shape a three-dimensional object [15]. MEX allows the production of complex-shaped parts not only from thermoplastics [20], fiber-filled composites [21], and low melting point (\(\sim\)450 \({}^{\circ}\)C) metallic alloys [22], but also from high-melting-point (\(>\)1000 \({}^{\circ}\)C) metallic alloys [23; 24; 25; 26; 27; 28; 29; 30], such as copper [31], ceramics [15; 32; 33; 34; 35; 36; 37; 38; 39], and hard metals and cermets [40; 41; 42]. Highly-filled thermoplastic filaments are used as the feedstock material to obtain metallic, ceramic and cermet parts. As is the case with binder jetting, the shaped part is then subjected to a process of binder removal [43; 44] and sintering to densify the parts [15]. Recently, new machines that can work with pellets of metal injection molding (MIM) feedstocks have been developed, and research is being carried out to maximize the performance of the printed specimens with steels [45], hard metals [40], and copper [46]. The advantages of using filament feedstocks are low-cost shaping equipment [47], availability of powdersused in traditional powder metallurgical processes, and same feedstock materials are usable in metal injection molding (MIM) or metal extrusion molding (MEM) [48]. It is also easy to prepare multi-material parts when co-sintering is possible [15; 49; 50]. The main disadvantage of using filaments as feedstock is that binder systems must be adjusted to have enough mechanical strength and flexibility so the filament can be reliably processed in standard commercial MEX machines. Nevertheless, new binders are available, and filaments with copper and other sinterable powders have been reported in the literature [15; 29; 31; 32; 39; 49; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64]. Therefore, due to the increasing popularity of filament-based MEX, this technique was selected for shaping copper specimens in this study. Many studies available in the literature concentrate on producing parts as dense as possible since the density is directly related to the mechanical performance and the conductivity of the specimens. For example, the thermal and electrical conductivities of specimens shaped by MEX and sintering have been discussed previously by [PERSON] and [PERSON] [31]. They observed that the tensile properties of sintered copper parts were consistent with other sintering approaches once the porosity of the parts is considered [31]. This investigation focuses on producing copper specimens that are not 100% dense; that is, one of the main advantages of additive manufacturing, i.e., the fabrication of lightweight specimens with different infill patterns. The production and characterization of copper parts with different infill grades and patterns by MEX, solvent debinding and sintering is here presented. A unique feedstock material with 55 vol.% of copper particles was prepared using a proprietary binder. A particular emphasis is placed on studying the debinding rates with solvents and flexural mechanical properties depending on printing patterns, infill degrees, and covers to prepare hollow specimens. The current study aims to close the gap of the lack of material properties for materials and components produced by MEX debinding and sintering that are not necessarily 100% dense. It aims to provide additional guidelines for the fabrication of lightweight specimens.