The 3D simulation models were firstly imported from CAD software to the simulation software through a data exchanges STEP format. The 3D solid mesh involves four different kinds of meshes, including prism, tetra, pyramid, and hexahedron. The number of nodes for tetra, pyramid, and hexahedron are 4, 5, and 6, respectively. In this study, the simulation models are composed of meshed with pyramid, tetrahedron, and hexahedron. Figure 10 shows the mesh sections of the injection mold, conformal cooling channels, and injection molded parts. To ensure accuracy of simulation results, the boundary layer mesh (BLM) was used in this study since it is suitable for simulation models with complex geometries. The higher the number of meshes denotes the longer the computing time of the simulation. In particular, the cooling time of the injection molded part reaches the steady state when the mesh element counts of exceeding 400,000. Thus, the mesh element count of 400,000 seems to be the optimal number of meshes based on both the correctness of the cooling time and the computing time of the simulation. The simulation model include injection molded part, CCC, mold base, and runner. The number of elements and nodes are 53,018, 68,008, 261,604, and 8,000, respectively. The total elements and nodes are approximately 39, 0630. The average edge length is about 0.4 mm.
In the injection molding simulation, the melt front time (MFT) result showed the position of melt front with respect to time during the filling stage. Optimization of MFT provides the balanced flow contribution of each gate. The wax injection molding process includes three stages: filling, cooling, and ejection stage. Figure 11 shows the simulation results of the filling of the molded part at the end of filling (EOF). The filling time of the molded part is approximately 2 s. In the injection molding simulation, the average part temperature results showed the distribution of temperature on the front face of part at the end of cooling (EOC). Figure 12 shows the numerical simulation results of the part temperature difference at the EOC for the injection mold with and without CCC. The results revealed that the part temperature at the same location of the molded parts fabricated by the injection mold with CCC can be lower than 2 ℃ compared to that for the injection mold without CCC. In the injection molding simulation, the magnitudes of deformations in three directions in each position inside the wax patterns can be estimated. Figure 13 shows the numerical simulation results of the x-displacement, y-displacement, z-displacement, and total-displacement of the molded part for the injection mold with and without CCC. The x-displacement, y-displacement, z-displacement, and total-displacement of the molded part for the injection mold without CCC are -0.049-0.049 mm,-0.023-0.035 mm, -0.049-0.049 mm, and 0.017-0.06 mm, respectively. The x-displacement, y-displacement, z-displacement, and total-displacement of the molded part for the injection mold with CCC are -0.033-0.033 mm, -0.014-0.023 mm, -0.033-0.033 mm, and 0.01-0.04 mm, respectively. As can be seen, x-displacement, y-displacement, z-displacement, and total-displacement of the molded part for the injection mold with CCC are lower than those of the injection mold without CCC.
The cooling time can be estimated from end of packing (EOP) to the instant that wax pattern temperature has been cooled down to the ejection temperature. Figure 14 shows the numerical simulation results of the cooling time of the molded part for the injection mold with and without CCC at the EOP. The theoretical cooling times of the molded part for the injection mold with and without CCC are 13 s and 18 s, respectively. It should be noted that about 27.7% improvement in the cooling time of the molded part can be obtained when the designed CCC was embedded in the injection mold.
In this study, a series of HT experiments were conducted on the MSS samples. In general, HT experiments involve three categories: solution treatment (ST), direct aging treatment (DAT), and solution & aging treatment (SAT). According to the literature reviews, the temperatures of ST include 780 ℃, 840 ℃, 900 ℃, 960 ℃ or 1020 ℃ and the duration is 0.25, 0.5, 1, 2 or 4 hours. The temperature of DAT is 400 ℃, 440 ℃, 480 ℃, 520 ℃ or 560 ℃ and the duration is 1, 3, 6, 9 or 12 h. The general SAT HT procedures are at 900 ℃ followed by 400 ℃, 900 ℃ followed by 440 ℃, 900 ℃ followed by 480 ℃, 900 ℃followed by 520 ℃ and 900 ℃followed by 560 ℃ for 6 h . The temperature of ST is 820 ℃ for 1 h and the temperature of AT is 460 ℃ for 5 h . The temperature of the AT is 490 ℃ . The temperature of AT is 840℃and the temperature of AT is 480℃.The temperature of the AT is 510 ℃for 1 h . Figure 15 shows the surface hardness of the test specimens processed by three different HT methods. The surface hardness of the test specimens can be enhanced due to nanometer-sized Ni3 and Fe2Mo intermetallic particles precipitated during the AT. According to the surface hardness of the test specimens, three phenomena were found：(a) the surface hardness of the test specimens after SAT is the highest, followed by the DAT, (b) the surface hardness of the test specimens is the highest after ST at 760 °C for 1 h, and (c) the optimum HT procedure is ST at 850 °C for 1 h, followed by AT at 480 °C for 6 h. The highest surface hardness of the test specimens can be obtained by SAT with the optimum HT procedure. It was seen that the average surface hardness of the test specimens is about HV 545.9 which meets the requirement of the injection mold.
The process parameters for fabricating high densification injection mold with CCC include hatching space of 60 µm, layer thickness of 30 µm, laser power of 50 W, and scanning speed of 200 mm/s. The general process parameters for fabricating injection mold with CCC include hatching space of 100 µm, layer thickness of 50 µm, laser power of 40 W, and scanning speed of 240 mm/s. It takes 149 hours to manufacture the injection mold by using the high densification process parameters. However, it only takes 49 hours to manufacture the injection mold by the using general parameters. This means that the injection mold manufacture time about 67% can be saved using the general process parameters. To evaluate the performance of the optimum HT process procedures, a preliminary experiment was conducted. Figure 16 shows the coolant leakage test results before and after HT of the test injection mold with CC. The results revealed that the coolant leakage for test injection mold after HT during the test was not found. However, the test injection mold before HT has coolant leakage of about 38 g, 76.5 g, and 115 g during the test of 1, 2, and 3 h. This means that the proposed method enables quick fabrication of an injection mold with CC by optimum HT process procedures resulting in no coolant leakage in the injection molding process. In this study, the internal surface of the CCC was found are remarkably not smooth. Therefore, improving the internal surface of the CCC is also an important research issue. The potential polishing methods , including abrasive blasting , abrasive flow machining , electrochemical polishing , chemical polishing , laser polishing , or ultrasonic cavitation abrasive finishing  could be employed to improve the internal surface of the CCC .
To verify the effectiveness of the optimum HT process procedures, two sets of injection molds shown in the Figure 17 were fabricated using general process parameters. After optimum HT process procedures, post-process finishing operations of the mold injection inserts was performed for obtaining the desired dimensions of the injection mold using a computer numerical control (CNC) milling machine . In addition, positioning pin holes were maching using a CNC drilling machine. The theoretical relative density was increased from 70.02 to 85.03%. In this study, the MSS powder was used to fabricate injection mold. Some alternative powders, such as 17-4 PH stainless steel, Al-Si alloy, Ni-Ti alloy , 304 stainless steel , 316-L stainless steel, CoCrMo , IN 718 alloy, Ti6Al4V , W-Ni-Cu , TC4 , Ni , Al-Fe-V-Si , A357 , Cu-15Ni-8S , or Inconel 625  could also be used to make functional components for industrial applications. Thus, molds, dies, automotive, aircraft, aerospace, or gear with high mechanical properties can be manufactured by MAM technology with above powders.
To investigate the cooling time of the wax patterns after injection molding, a series of experiments were performed using low-pressure wax injection molding. The wax injection process parameters involve injection time of 2s and injection pressure of 0.06 MPa. Figure 18 shows the part temperature as a function of the cooling time of the wax pattern after injection molding. The coolant temperature and the coolant flow rate are 25 °C and 3 L/min, respectively. Especially, the molded part will cause short shot since the temperature of the injection mold was influenced by the leakage coolant. The cooling times of the wax patterns fabricated by the injection mold with and without coolant leakage are 23 s and 38 s, respectively. The cooling stage is a sophisticated heat transfer process in the injection molding process. Generally, there are four distinct stages, i.e. filling, packing, cooling, and demolding in the injection molding process. To study heat transfer process during the cooling stage, the cycle-averaged temperature distribution represented by the steady-state Laplace heat conduction equation was widely employed to simplify the analysis of the cooling process. Figure 19 shows the schematic illustration of the heat fluxes during the cooling stage after low-pressure wax injection molding. Generally, the heat conduction is usually governed by the partial differential equation. The heat transfer rate must be in equilibrium when the heat balance was established. The heat transfer rate from the molding materials to the mold materials, heat transfer rate from the mold materials to the coolant, and heat transfer rate from the mold materials to the ambient air are symbolized by,, and , respectively. Therefore, the heat balance can be expressed by the equation of .The heat from the molten wax material in the mold cavity is taken away by both coolant and exterior surfaces of the mold. The heat balance equation can be simplified by neglecting the heat lost to the surrounded environment since is less than 5% of the . In addition, the mold materials boundary is assumed to be adiabatic. Therefore, the heat of the molten wax material in the mold cavity is taken away by the coolant moving through the conformal cooling channels after the injection molding. Based on the solidification of the wax patterns, the required cooling time (tc) of the wax patterns can also be calculated by the following equation [36-38]:
where s is the thickness of the wax patterns, Tm is the melt temperature of the molding material, Te is the average demolding temperature of the wax patterns, is the thermal diffusivity, and Tw is the mold cavity surface temperature.
Figure 20 shows the part temperature as a function of the cooling time of the wax pattern for five different coolant temperatures. Two phenomena were found. On is that the cooling times of the wax patterns fabricated by the injection mold without coolant leakage are about 21 s, 33 s, 45 s, and 112 s when the coolant temperatures are 21 °C, 23 °C, 25 °C, 27 °C and 29 °C, respectively. The other one is that the cooling time of the wax pattern was obvious longer when the coolant temperature is 29 °C. According to the results described above, determination of the coolant temperature is an important factor affecting the injection molding yield and molding cycle time based on the cooling shrinkage rate and cooling time of the wax pattern.
The coolant flow rate is an important issue on the cooling efficiency for injection mold with conformal cooling channels. In general, the turbulent flow (Reynolds number > 4,000) provides three to five times as much heat transfer as laminar flow (Reynolds number < 2,100). The coolant flow performs the turbulence when the Reynolds number exceeds the 4,000. In this study, four different coolant flow rates were used in this study, i.e. 2.5 L/min, 3 L/min, 3.5 L/min, and 4L/min. The Reynolds number for four different coolant flow rates is about 4927, 5913, 6897, and 7883, respectively. To understand the effects of coolant flow rates on the cooling time of the wax pattern, a series tests was carried out. Figure 21 shows the part temperature as a function of the cooling time for four different coolant flow rates. In particular, the cooling time of the wax pattern is approximately 38 s. This means that the cooling time of the wax pattern was found not affected by the different coolant flow rates while the coolant reaches the turbulent flow. However, the layout of CCC was not optimized. Therefore, optimization of CCC using the design of experiment method is also an important research issue. In particular, the a discrepancy in the cooling times of the wax patterns between the experimental and numerical simulation results was attributed to the inconsistency in initial and boundary conditions. Thus, reducing the discrepancy in the cooling times of the wax patterns between the experimental and numerical simulation results is also an important research issue. The CCC embedded in the injection mold is series circuits. Mixing series circuits  to keep turbulent flow and parallel circuits  to improve temperature homogeneity is also an important research issue.
In general, a high-temperature HT was widely employed to join small particles for reducing the pore size to obtain sufficient mechanical or thermal properties since precipitation in solids can produce many different sizes of particles during optimum HT process procedures. Figure 22 shows the microstructural developments after optimal HT. It was shown that the irregular pores or void defects were reduced gradually through the recrystallization HT, resulting in significant reduction in the porosity of the injection mold. In addition, the textural anisotropy and internal residual stresses of the direct metal printed injection mold built with DMLS can also be improved significantly through the recrystallization HT. This result reveals that the mechanical properties and microstructure were improved after optimum HT process procedures. Based on the results described above, the remarkable findings of this study can be used for the fabrication of molds or dies efficiently and economically for trial production in the mold or die industries. The wax pattern can be fabricated by wax injection molding via MSS injection mold processed by optimum HT procedures, which can be employed for investment casting (IC).
According to the foregoing results, the findings of this study are very practical and provide the greatest application potential in the IC industry. The main contributions in this study is to propose a low-cost and highly efficient method of reducing coolant leakage during wax injection molding 3D printed conformally cooled injection molds. However, some distinct mold defects, including melt, ball formation, swelling, cracking, residual stress, or delamination were not addressed. In addition, some alternative MAM technologies, such as directed energy deposition, electron beam melting, diffusion bonding, selective laser sintering , or selective laser melting can also be used to make injection molds. The molds or died fabricated by the MAM technologies could also be employed for micro-injection molding, thermoforming, forging, hot embossing, blow molding, metal injection molding, die casting, hot extrusion, injection-compression molding, rotational molding, thermoforming, transfer molding, or hot stamping. The microstructure of the injection mold manufactured by DMLS can be manipulated by laser power , hatch space , scanning speed, scanning strategy , or powder layer thickness. Normally, slower scanning speed, or higher laser power will contribute to grain size growth. These issues are currently being investigated and the results will be presented in a later study.