# Experimental Comparison of Fastener Implementation Approaches in Fused Deposition Modeling
## 2 Design of Different Fastener Implementations  
Considering the inherent properties of most plastics, difficulties are encountered when trying to cut threads into them. In particular, the material yield strength of plastics used in the FDM method is lower than metals. Due to the limitations of FDM technology, small diameter tapped holes cannot be accurately manufactured with a 3D printer. Even with the latest reinforced plastics, these threads will not withstand excessive forces or wear down quickly [34].  
A more reliable solution for these situations is to add metal fasteners into the printed parts, such as nuts. Fasteners are implemented using various techniques, depending on the application and materials involved. Common techniques include heat-set insert and captive nut. In this study, four methods are examined, which are heat-set insert installation, side nut installation after printing, and nut installation into the part during printing. CAD designs of these inserts and square nut are presented in Figure 1. Each technique is chosen based on factors such as strength requirements, ease of assembly, permanence, and environmental conditions. In the first two methods discussed in this study, as it is presented in Figure 2a,b, the hole is designed to be adequately larger than the bolt size, and the heat-set insert presented in Figure 1a,b is heated and inserted into the hole after the manufacturing of the printed part. The heated metal insert melts a small layer of the hole surface and creates a bond. In the third method presented in Figure 2c, a suitable cavity is added to the design for the nut to be inserted in. After completing the printing process, a square nut presented in Figure 1c is inserted in its place through that added cavity. In the fourth method presented in Figure 2d, a nut is placed during the printing process. After printing the layer corresponding to the top surface of the nut, the printing process is paused. The nut is placed into the cavity designed just for the nut, either manually or automatically, and the printing process is continued with the remaining layers. In this approach, there is no need for a postprocess or extra cavity to place the nut.  
The cross-section of the samples was carried out in such a way that the impact of the walls was as small as possible, and the evaluation of the effect of the infill was easier. Samples were designed as 40 mm by 40 mm squares. For type II and type III samples, the distance between the top surface and the nuts is 8 mm.  
The tensile strength test samples in this study were also designed keeping less manufacturing time and material consumption in mind. A sheet metal was installed with three bolts, and the metal part was held with the pneumatic jaws of the test bench. The metal part was easy to exchange between test samples, so that only one metal part was enough. The bolt screwed into the nut in the sample was held with the upper part designed especially for this study.  
Samples are also designed for three-point flexural with each fastener implementation approach, and a blank design was added to the test for the reference. The test samples are designed according to ISO 178 standards. The width of the samples is 20 mm, and the thickness is 10 mm. The length of the samples is 80 mm, and the fasteners are placed in the middle of the part. Cross-sections of the flexural test samples are given in Figure 3.  
Figure 1: Dimensions of (**a,b**) heat-set inserts and (**c**) square nut.  
Figure 2: Solid models with cross-section of test samples: (**a**) type Ia, (**b**) type Ib, (**c**) type II, (**d**) type III.  
Flexural test sample design is also used for torque tests. Torque tests were executed by tightening the screw until the nut slips inside and spins free. A large washer is placed under an 8.8 standard M4 machine screw.  
Solid models were designed in SolidWorks, and CAM operations were executed on the AM software Prusa Slicer. Printing parameters were kept close to the most preferred values for parts, so a lighter design was appreciated.