## 3 Results and Discussion  
For 3D printing technology, the most widely used printing method is the extrusion method [1]. Detailed mechanisms for 3D printing technology were reported in the literature [1]. In a 3D printing system based on the extrusion method, also named fused deposition modeling, a gear system drives an amorphous or low crystallite polymer filament into the heated nozzle assembly for extrusion. Several possible materials such as PLA, poly-vinyl alcohol, or ethyl-vinyl acetate have been employed as the base polymers in the applications of 3D printing technology [1]. In order to inject the polymer filaments into heated nozzle smoothly and to avoid a high temperature set in the heated nozzle from damaging the gear system, a metal tube with length of around several centimeters is used to connect the heated nozzle and the gear system. Melted polymer filaments dropped from the heated nozzle form as the stalagmites. Using a control unit of the 3D printer, a 3D object can be built using layer-by-layer deposition. An important parameter for 3D printing is the temperature set in the heated nozzle of a 3D printer. Materials used in a commercial 3D printer are the low-crystalline polymers (e.g., PLA), of which the glass transition temperature (T\({}_{g}\)), crystallization temperature (T\({}_{c}\)), and melting temperature (T\({}_{m}\)) are lower than those for PEEK. Therefore, the temperature of the heated nozzle has to be set at a higher temperature than the original design for commerical 3D printers (around 220 \({}^{\circ}\)C). In our previous study [5], the crystallization and melting temperatures of around 292 \({}^{\circ}\)C and 340 \({}^{\circ}\)C for PEEK were reported. It indicates that the temperature of heated nozzle has to be set higher than 340 \({}^{\circ}\)C. However, at around 600 \({}^{\circ}\)C, PEEK will decompose accord to the results proposed by [PERSON] et al. (2015) [21]. Therefore, two temperatures of the heated nozzle of around 340 \({}^{\circ}\)C and 360 \({}^{\circ}\)C were set, respectively, in order to make the PEEK filaments melt in the heated nozzle and drop in the form of stalagmites for the building of a 3D object. The temperature of holder was also set at 45 \({}^{\circ}\)C, which was the same as in our previous study [5]. Figure 1 shows the 3D printing results of PEEK samples with a different temperature set in the heated nozzle. From the results shown in Figure 1, poor qualities of 3D printing PEEK samples were observed. Some dark parts in the PEEK sample were observed with the temperature of the nozzle kept at 360 \({}^{\circ}\)C, which indicated that the carbonization of the PEEK sample occurred when the temperature of the  
Figure 1: Printing results of polyetheretherketone (PEEK) samples with temperatures of (**I**) 340 \({}^{\circ}\)C and (**II**) 360 \({}^{\circ}\)C in the heated nozzle at the initial printing stage. nozzle was kept at 360 \({}^{\circ}\)C. For the temperature of the nozzle kept at 340 \({}^{\circ}\)C, although no carbonization of the PEEK sample was observed, the failure percentage of around 20% for the PEEK objects was still observed. It indicated that the heated nozzle or the metal tube must be remodified. After we carefully examined the structure of the heated nozzle and the metal tube connected between the heated nozzle and the gear system, the major problem for a high failure percentage of 3D printing objects was the diameter of the heated nozzle. The original diameter for the heated nozzle loaded in the commercial 3D printer is 0.4 mm. However, a higher viscosity of PEEK compared with that for PLA made the PEEK material block in the heated nozzle and resulted in the high failure percentage of a 3D object. The loading of the metal tube has to be remodified in order to inject the polymer filaments into the heated nozzle smoothly. Figure 2 shows comparisons of the metal tube and heated nozzle for the original and new designs in our own 3D printer.