and uniform heating of the mold surface and the infrared-heating-assisted micro-injection molding process for fabricating a microneedle array can improve the mold-filling capability of the polymer melt and optimize the replication quality of the parts, such as filling height, uniformity, and shrinkage (31).
1.6.3.5 Carbon-Bonded Graphene Coating
A carbide-bonded graphene coating on silicon insert was proposed to realize rapid thermal cycling (RTC) in injection molding. A continuous and dense carbon-bonded graphene coating was prepared on the surface of the silicon cavity through chemical vapor deposition (32, 33).
Serving as a thin film resistance heater, the graphene coating was able to heat the mold cavity rapidly to above the glass transition temperature of the polymer, even if the applied power source was of relatively low voltage. When the voltage was 240 V, the coating was heated up to 145.6°C in a short period of 10 s. So, the average and transient heating rates were able to be as high as 11.6 ◦C s–1 and 16.1 ◦Cs–1, respectively.
This injection molding technique of RTC could be successfully implemented to produce plate samples with uniform sizes at a thickness of 600 μm (32).
Also, the influence of RTC on the weld line, internal stress, and replication fidelity was investigated. Compared with CIM, the here described method was able to mold products with smaller weld mark, less internal stress, and better replication fidelity. The tensile strength and elongation at yield of the products were also enhanced by 37.77% and increased by 265.11%, respectively, with much less energy consumption (32).
The graphene heater with a 48 Ω surface resistance made by a 60 min coating time proved to be the best choice for rapid heat cycle molding with regard to the heating efficiency (33).
The graphene heater was used in the rapid heat cycle molding of long-glass-fiber-reinforced PP composites to reduce the width and depth of the weld lines, decrease the floating fiber phenomenon, and improve the surface quality (33).
1.6.3.6 Induction Heating
Induction heating is a method that allows obtaining a rapid thermal cycle, so the overall molding cycle time is not increased (34).
Finite Element Model. To analyze the heating and cooling phase of an induction heated injection molding tool accurately, the temperature-dependent magnetic properties, namely the nonlinear B-H curves, an induction heating simulation has been performed (35).
A finite element model has been developed, including the nonlinear temperature-dependent magnetic data described by a three parameter modified Fröhlich equation fitted to the magnetic saturation curve, and solved with an iterative procedure.
The numerical calculations were compared with experiments conducted with two types of induction coils built into the injection molding tool. The model shows a very good agreement with the experimental temperature measurements (35).
It could also be shown that the nonlinearity can be used without the temperature dependency in some cases, and a method was proposed for estimating an effective linear permeability to use with simulation codes that are unable to utilize a nonlinear solver (35).
Multi-turn Induction Heating Coil. An integrated multi-turn induction heating coil has been developed and was assembled into an injection molding tool. This tool contained a glass window, so the effect of induction heating can directly be captured by a high speed camera. In addition, thermocouples and pressure sensors are also installed, and together with the high speed videos, the induction heating and filling of the cavity is compared and validated with simulations.
Two polymer materials, i.e., ABS and high viscosity PC, were utilized during the injection molding experiments. A nonlinear electromagnetic model was used to establish an effective linear magnetic permeability. The three-dimensional transient thermal field of the mold cavity was then calculated and compared with the experiments. This thermal field was transferred to an injection molding flow solver to compare the simulations and experimental results from the high speed video, both with and without the effect of induction heating.
A rapid thermal cycle was proved to be feasible in a mold with an integrated induction coil. Furthermore, it was shown that the process can be modeled with good accuracy, both in terms of the thermal field and in terms of the flow pattern (34).
High-Frequency Induction Heating. The recent trends of miniaturization and multifunctionality in electrical parts have driven the development of molded interconnect devices (MIDs) that contain conductive tracks on a nonconductive base (36).
A polymer/metal hybrid molding technology was developed to fabricate MIDs in a single manufacturing process, without an additional assembly procedure. For this purpose, injection molding was performed to fabricate a thermoplastic carrier that contained negative circuit channels, and die casting was used to fill the circuit channels with metal alloy of low melting point. To increase the flow length of the molten metal through the narrow circuit channel, high-frequency induction heating was used prior to the die casting stage.
The effect of heating conditions on the mold temperature was investigated numerically, and the relevant induction heating conditions were determined accordingly. Induction heating was then applied to the die casting process to increase the flow length enough to be used as a circuit path for fabrication of MIDs (36).
1.7 Microcellular Injection Molding
Injection molding is a well-established replication process for the cost-effective manufacture of polymer-based components (37). The process has different applications in the medical, automotive and aerospace fields among others. To expand the use of polymers to meet growing consumer demand for increased functionality, advanced injection molding processes have been developed that modify the polymer to create microcellular structures.
Using the creation of microcellular materials, additional functionality can be gained through polymer component weight and processing energy reduction. Microcellular injection molding shows a high potential for creating green manufacturing platforms (37).
The process conditions as well as nano/micro-fillers such as nanoclay and core-shell rubber have a strong influence on cell density and cell size, hence, the final material properties of the molded parts (38). The addition of nano/micro-fillers at optimum loading levels can generally facilitate the formation of microcellular plastics with higher cell density and smaller cell size, leading to superior mechanical properties.
The integration of a solid plastic surface with a microcellular plastic core via the co-injection molding technique has been investigated to achieve Class A surfaces and improved material performance. An improved mathematical model has been developed to simulate the cell growth behavior in the microcellular injection molding process (38).
The significant developments that have been achieved in different aspects of microcellular injection molding have been reviewed (37). The aspects covered include core-back, gas counter pressure, variable thermal tool molding and other advanced technologies. Also, the resulting characteristics of creating microcellular injection molding components through both plasticizing and nucleating agents were presented. In addition, the review highlights the potential areas for research exploitation, in particular, acoustic and thermal applications, nanocellular injection molding of parts and developments relating to more accurate simulations
