# Experimental Comparison of Fastener Implementation Approaches in Fused Deposition Modeling
## 1 Introduction
Although being able to design and produce threaded holes on the part for screw connections is one of the useful features of AM methods, it may not be applicable in cases where thread sizes are small in the FDM technique [23; 27]. In applications with a thread diameter below M6, the thread tolerances increase, and the strength of the threads decreases significantly. In cases where it is not possible to use large mounting elements or where frequent assembly and disassembly are required, the use of metal fasteners is inevitable. The methods that can be used for FDM technology are generally mechanical or heat inserting of the metal nut. In the literature, there are special designs for the assembly of parts produced by AM as well as studies on the insertion of standard fasteners [28; 29; 30; 31]. When AM is used in situations that require the manufacturing of special parts, such as unmanned aerial vehicles, electric vehicles, and prosthetic designs, situations frequently arise where the manufactured parts need to be connected to each other. In these cases, whether the manufactured part should be resistant to tensile-compressive force or bending changes the type and design of the connection. In this study, three different connection techniques were produced by additive manufacturing, and the response of these methods to bending and tensile forces was examined. In this study, the four most commonly used fastener implementation methods in FDM were manufactured in three different infill ratios, and experiments were performed. For the first two methods, brass inserts used in injection molding are heated and mounted in parts after the printing process is completed. In these methods, the cavity for the insert is designed to be adequately wide. The installation is carried out by melting the surfaces of the cavity where the heated insert is placed. In the third method, a cavity is designed for both fastener and installation. After the printing is completed, the material is installed into the cavity of the part. In the fourth method, a cavity is designed only for the fastener. When the printing process reached the layer lined with the upper surface of the fastener, the fastener was placed manually or automatically, and the printing process was completed with the remaining layers. In the last approach, there is no need to perform postprocessing after printing. In the last two methods, square nuts were used instead of a hexagonal nut to reduce damage to the cavity surface during the tightening of a shell. Square nuts can hold more torque compared to hexagonal nuts [32; 33]. The CAD designs of these four methods were manufactured with a 3D printer and compared experimentally. Firstly, test samples were designed for tensile strength and three-point flexural and torque testing. The experimental setup was designed to assess four different sample designs across varying infill ratios. According to the experimental setup, four designs for tensile strength tests, five designs for flexural tests with a blank one without a fastener, and four designs for tightening torque tests were performed. For each of these designs, samples were manufactured with 10%, 20%, and 40% infill ratios. For each test, 12 repetitions were run, and a total of 144 tensile, 180 flexural, and 144 torque tests were performed. In Section 2, the key points of the four approaches and properties of the solid models are explained, along with the parameters for FDM printing and details of the manufacturing process. The test results are presented in Section 3, and the conclusions are given in Section 4.