## 1 Introduction  
Researchers have focused more on biodegradable and sustainable materials research due to the ever-growing global energy crisis and ecological risks. Poly (lactic acid) (PLA), derived from renewable resources (e.g., corn, wheat, or potato) [1], is a biodegradable, recyclable, and compostable thermoplastic with good biocompatibility and processibility. Whereas the slow degradation rate, high cost, hydrophobicity, and lack of reactive side-chain groups have limited its use in the biomedical field [2]. Natural fibers are being used currently as reinforcement for PLA, such as seed (e.g., cotton and milk weed), fruit shell (e.g., walnut, almond and peanut), bas (e.g., flax, lamp and jute), leaf (e.g., sisal and banana), and grass/cane/reed fibers (e.g., bamboo) [3]. Natural fibers possess a lot of advantages, including a high degree of flexibility, low cost, light weight, non-toxic, and high specific modulus. Additionally, they are biodegradable and recyclable in contrast to traditional synthetic fibers [4]. As a kind of natural fiber, Walnut Shell (WS) is a very common by-product of walnut food. A health claim that was authorized by Food and Drug Administration (FDA) indicated that walnuts could reduce the risk of heart disease [5]. It takes up 67% of the total weight of a walnut. According to the data from the United States Department of Agriculture, China's walnut production was about 850,000 metric tons in the marketing year of 2018/2019 [6]. However, although a small amount of WS was used as adsorbent [7; 8], most of them were scraped as waste. Like other natural fibers, WS consists of cellulose, hemicelluloses, lignin, and a small part of wax, oil, and other impurities on its surface [9]. These impurities prevent WSP from reacting with the hydrophobic polymer matrix, which results in poor interfacial compatibility, which further leads to the debonding and formation of voids [10; 11]. Normally, this issue could be partially solved through physical and chemical modifications [12]. Many chemical treatment methods have been reported in prior literatures, such as alkali [4], silane coupling [13], maleation [14], and so on. Beyond that, some agents have also been used to improve the interfacial compatibility, such as montmorillonite [15], metakaolin [16], and talc [17]. Several researchers have conducted valuable work on the polymer/WSP composites. [PERSON] [PERSON] et al. [18] introduced 40-60% WS into Polypropylene (PP) and found that the bending and tensile modulus of composites were significantly improved with the increase of filler content. [PERSON] et al. [19] fabricated a WSP/urea-formaldehyde particle board with a tensile strength of 3.8 MPa. [PERSON] et al. [20] found that a 40% addition of WSP greatly enhanced the tensile and flexural strength of WS/Polyester composites. In addition to these papers, the mechanical properties of Epoxy/WS composites were reported in some papers [21; 22]. [PERSON] et al. [23] evaluated the effect of WS on the tensile property of the Starch/WSP composite. [PERSON] [24] investigated the effect of WS and KH550 on the properties of PLA/WS composite. However, these WS/polymer composites were all prepared by traditional methods, including injection molding and compression molding [25; 26], which would limit its application in some parts with a complex internal and external structure. Fused Deposition Modeling (FDM) has advantages over traditional technologies of reducing product development times and free fabricating complex-shaped parts [27]. FDM is currently one of the most popular Additive Manufacture (AM) techniques, due to its low cost and easy operation, along with its wide application in bio (polymer) and recent biocomposites [28], including PLA/wood composites [25; 29], PLA/kraft pine lignin [26], PLA/jute and flax strands [30], PLA/basalt composites [31], etc. Even though, FDM of polymer/natural-fibers composites was rarely reported in the literature.