In order to understand the limits of the microstructure in the physical model fabricated by AM, a physical model with nine different holes was fabricated. Figure 3 shows the different holes in the physical model fabricated by AM. It was found that the fine feature with a diameter less than 1 mm cannot be printed directly. Therefore, a fine feature in the component or mold with a dimension less than 1 mm must be machined by computer numerical control machining. Figure 4 shows the surface quality of a test part before and after CNC milling. Two important phenomena were observed. One is the surface quality of the physical model can be improved significantly with amount of finish of 0.4 mm in the vertical direction. The other is no more amount of finish in the horizontal direction because the physical model has a dimensional error of about 0.2–0.3 mm in one side of the horizontal direction which can be mainly attributed to the drive motor positioning error. In order to maintain both dimensional accuracy and surface quality of the part after CNC milling, the height of the CAD model needs to add 0.4 mm as the amount of finish. Figure 5 shows the process flows for maintaining both dimensional accuracy and surface quality of the part using CNC machining. Figure 6 shows the result of a test part with microstructures. This result shows the part with excellent surface quality, good dimensional accuracy as well as a microstructure of 950 µm can be fabricated swiftly and economically thought AM, RT, and computer numerical control milling technologies.
The length, width, and height of a precision component are 118 mm, 60 mm and 34.5 mm, respectively. According to the experience of the test part, the length, width, and height are then changed to 118 mm, 60 mm, and 34.9 mm, respectively. The shrinkage of the silicone rubber and Al-filled epoxy resins are almost negligible because the shrinkage of the silicone rubber and Al-filled epoxy resins were approximately 0.5% and 0.8%, respectively. Figure 7 shows the variations in dimension of the precision component manufacturing process for length, width, and height. The average length, width, and height of a precision component after NC machining were 117.97 mm, 60.03 mm, and 34.53 mm, respectively. This result means that the dimensional accuracy of a precision component in the length, width, and height approximately 30 µm can be obtained. In is noteworthy that the quality of this precision component can meet the standards of the general industry because the dimensional accuracy of the precision component in the industry is about 50 µm. Figure 8 shows a precision component before and after CNC milling. As can be seen, this precision component has excellent surface quality and good dimensional accuracy.
The length, width, and microstructures of the injection molded product designed are 57 mm, 36 mm, and 0.95 mm, respectively. The average length, width, and microstructure of the injection molded product are 56.94 mm, 35.95 mm, and 0.94 mm, respectively. Figure 9 shows the variations in dimension of the precision mold manufacturing process for length, width, and microstructures. The shrinkage of the wax is about 1.5%. The dimensional accuracy of a molded part in the length, width, and microstructure approximately 60 µm, 50 µm, and 10 µm can be obtained. This result means a precision mold for injection molding can be fabricated in a short time and at low cost. Figure 10 shows a precision injection mold with microstructure. In order to evaluate the effectiveness of the fabricated precision mold, low-pressure wax injection molding was carried out. Figure 11 shows a wax pattern with microstructures fabricated by a precision injection mold through injection molding. This result shows the fabricated precision mold has excellent mechanical properties[26]for low-volume production of precision wax patterns to manufacture a variety of metallic components using investment casting technology [27–29]. Figure 12 shows a precision injection mold fabricated by only AM technology and hybrid manufacturing technology. This result clearly revealed that it is impossible to fabricate a precision injection mold with microstructures only by the use of AM. A distinct advantage of a precision injection mold fabricated by the proposed method is no residual thermal stress inside the mold compared to that fabricated by DMLS [30].
Generally, the production costs of a precision injection mold with microstructures fabricated by DMLS, EBM, SLM [31, 32], SLS [33], DB [34], DMD, or laser metal deposition (LMD) is expensive. However, the costs of materials used in this study, such as PLA filaments, silicone rubber, and Al-filled epoxy resins were inexpressive. The 3D physical models were fabricated with PLA filament using fused deposition modeling [35]. The intermediary silicone rubber mold was fabricated by liquid silicone rubber. The component and injection mold were fabricated by Al-filled epoxy resins. The surface quality of the component and injection mold was improved without losing the dimensional accuracy by CNC milling. The microstructures [36, 37] in the component or injection mold were also machined by CNC milling. Therefore, a precision mold with a microstructure can be fabricated swiftly and economically. According to the results discussed above, it can be concluded that a simple and cost-effective method to manufacture a precision mold with microstructures was demonstrated by integrating AM, RT, and CNC milling [38]. The findings of this work are very useful and provide the greatest application potential in both the precision machinery and investment casting industries since this technology can be employed to fabricate a precision mold for most general engineering purposes in the research and development stage. In this study, the material of the final precision injection mold is Al-filled epoxy resins. Ordinarily, the main disadvantage of the precision injection mold is that the mold service life was limited by the characteristics of injection mold materials. Therefore, some reinforcing additives, such as wollastonite, molybdenum disulfide [39–41], silica sand, glass sphere, zirconia [42–44], silicon nitride [45–47], or silica sand were recommended for adding to the injection mold. In this study, the injection mold was employed in wax injection molding for manufacturing precision wax patterns. Note that the proposed method can also be employed in plastic injection molds [48–50,], blow molding dies [51], metal injection molding molds [52], powder metallurgy molds [53], die casting dies [54], hot extrusion dies [55, 56], injection-compression molding molds [57], rotational molding dies, thermoforming molds, transfer molding dies, or hot stamping dies [58]. Finally, it is worth noting that two further studies are required for enhancing the surface quality of a precision mold machined and dimensional accuracy of a precision mold machined. One is accurate tool condition monitoring [59] is required during CNC milling. The other is that the surface quality monitoring [60] of a precision mold is also required during CNC milling. These issues are currently being investigated and the results will be presented in a later study.