Guilong Wang

Research on a New Variotherm Injection Molding Technology and its Application on the Molding of a Large LCD Panel

The polymer injection products produced by using the current injection molding method usually have many defects, such as short shot, jetting, sink mark, flow mark, weld mark, and floating fibers. These defects have to be eliminated by using post-processing processes such as spraying and coating, which will cause environment pollution and waste in time, materials, energy and labor. These problems can be solved effectively by using a new injection method, named as variotherm injection molding or rapid heat cycle molding (RHCM). In this paper, a new type of dynamic mold temperature control system using steam as heating medium and cooling water as coolant was developed for variotherm injection molding. The injection mold is heated to a temperature higher than the glass transition temperature of the resin, and keeps this temperature in the polymer melt filling stage. To evaluate the efficiency of steam heating and coolant cooling, the mold surface temperature response during the heating stage and the polymer melt temperature response during the cooling stage were investigated by numerical thermal analysis. During heating, the mold surface temperature can be raised up rapidly with an average heating speed of 5.4°C/s and finally reaches an equilibrium temperature after an effective heating time of 40 s. It takes about 34.5 s to cool down the shaped polymer melt to the ejection temperature for demolding. The effect of main parameters such as mold structure, material of mold insert on heating/cooling efficiency and surface temperature uniformity were also discussed based on simulation results. Finally, a variotherm injection production line for 46-inch LCD panel was constructed. The test production results demonstrate that the mold temperature control system developed in this study can dynamically and efficiently control mold surface temperature without increasing molding cycle time. With this new variotherm injection molding technology, the defects on LCD panel surface occurring in conventional injection molding process, such as short shot, jetting, sink mark, flow mark, weld mark, and floating fibers were eliminated effectively. The surface gloss of the panel was improved and the secondary operations, such as sanding and coating, are not needed anymore.

Research on optimization design of the heating/cooling channels for rapid heat cycle molding based on response surface methodology and constrained particle swarm optimization

Research highlights? We develop a method for optimum heating/cooling channels design of RHCM mold. ? A multi-objective optimization method is developed based on RSM and PSO. ? Energy equations in the heating and cooling processes of RHCM are deduced. ? Some guidelines to improve thermal response efficiency of RHCM mold are presented. The aim of this work is to optimize the layout of the heating/cooling channels for rapid heat cycle molding with hot medium heating and coolant cooling by using response surface methodology and optimization technique. By means of a Box-Behnken experiment design technique, an experiment matrix with three factors and three levels was designed. The design variables including the diameter of the heating/cooling channels, distances from the wall of heating/cooling channel to the cavity surface and between the adjacent heating/cooling channels were used to describe the layout and shape of the heating/cooling channels. The heating efficiency, standard deviation of the cavity surface temperature and the maximum von-mises stress were considered as the model variables. Thermal response and structural strength analyses of the mold based on FEM were conducted to acquire the objective variables for combination of process parameters. Some mathematical models of response surface were created by the mixed regression model and response surface method. The analysis of variance (ANOVA) method was used to check the accuracy of the developed mathematical models. With these mathematical models, the layout of the heating/cooling channels was then optimized to minimize the required heating time within reasonable temperature distribution and structural strength of the cavity by coupling the developed response surface (RS) models with the particle swarm optimization (PSO) method.

Influence of relative low gas counter pressure on melt foaming behavior and surface quality of molded parts in microcellular injection molding process

A complex medical instrument exterior shell was chosen as a studying object to investigate the influence of relative low (<10 MPa) gas counter pressure process on microcellular injection molding process. The gas counter pressure microcellular injection mould and related experiments were designed. The relative low gas counter pressure under which the melt can foam was mainly considered to improve the surface quality of molded parts without significantly prolonging production cycle. The effects of the gas counter pressure parameters on the surface quality, cell morphology, and cell density of microcellular parts were studied. A critical melt flow front pressure to effectively eliminate surface swirl marks of microcellular injection molded part was proposed. The mechanism of the influence of gas counter pressure process on foaming behavior of melt in filling process was analyzed. The reasonable gas counter pressure parameters to improve surface quality of products without significantly increasing molding cycle were obtained. By using the obtained reasonable gas counter pressure parameters, a sound microcellular injection molded product was injected finally.

Ultralow-Threshold and Lightweight Biodegradable Porous PLA/MWCNT with Segregated Conductive Networks for High-Performance Thermal Insulation and Electromagnetic Interference Shielding Applications

Lightweight, biodegradable, thermally insulating, and electrically conductive materials play a vital role in achieving the sustainable development of our society. The fabrication of such multifunctional materials is currently very challenging. Here, we report a general, facile, and eco-friendly way for the large-scale fabrication of ultralow-threshold and biodegradable porous polylactic acid (PLA)/multiwalled carbon nanotube (MWCNT) for high-performance thermal insulation and electromagnetic interference (EMI) shielding applications. Thanks to the unique structure of the microporous PLA matrix embedded by conductive 3D MWCNT networks, the lightweight porous PLA/MWCNT with a density of 0.045 g/cm3 possesses a percolation threshold of 0.00094 vol %, which, to our knowledge, is the minimum value reported so far. Furthermore, the material exhibits excellent thermal insulation performance with a thermal conductivity of 27.5 mW·m-1·K-1, which is much lower than the best value of common thermal insulation materials. Moreover, it also shows outstanding EMI shielding performance characterized by its high shielding effectiveness (SE) values and absorption-dominated shielding feature. More importantly, its specific EMI SE is as high as 1010 dB·cm3·g-1, which is superior to those of other shielding materials reported so far. Thus, this novel multifunctional material and its general fabrication methodology provide a promising way to meet the growing demand for high-performance multifunctional materials in sustainable development.

Fully Coupled Transient Heat Transfer and Melt Filling Simulations in Rapid Heat Cycle Molding with Steam Heating

Rapid heat cycle molding (RHCM) is a novel injection molding technology, in which injection mold is rapidly heated to a high temperature, usually higher than the glass transition temperature of the polymer material, before melt-injection and rapidly cooled down to solidify the shaped polymer melt in mold cavity for ejection. Since the elevated mold temperature can eliminate the unwanted premature melt freezing during filling stage, the melt flow resistance is greatly reduced and the filling ability of the polymer melt is also significantly improved. As a result, plastic parts with excellent surface appearance can be obtained. In this study, a three-dimensional numerical model coupled with heat transfer analysis and melt-filling processes for RHCM with steam heating was established. The thermal response analysis for the heating stage of RHCM process was performed by solving heat conduction equations. The heat transfer analysis results right after the mold cavity surface is heated up to the required temperature are taken as initial temperature conditions of the mold cavity for the following melt filling simulation. The pressure implicit splitting of operations solution algorithm was used to solve the pressure-velocity coupled Navier-Stokes equation for melt filling process. The moving interface between melt and air was captured by using the volume of fluid method. The energy equations for melt filling process were solved in a coupled manner for the cavity and mold domain at the matrix level. The proposed fully-coupled numerical model was applied in simulation of the molding processes, including a two-dimensional rectangular cavity with different heating times and a three-dimensional large scale LCD panel with a stem-heated stationary mold. The results show that the fully coupled numerical method provides reliable temperature and flow field predictions with the thermal response analysis and melt flow estimation.

Bubble morphological evolution and surface defect formation mechanism in the microcellular foam injection molding process

The filling stage of the Microcellular Foam Injection Molding (MFIM) process is a three phase flow process of polymer melt, super critical fluid (SCF) and air. It not only includes the nucleation and growth of spherical bubbles, but also the morphological evolution of the bubbles such as deformation, bursting, and vanishing. There are usually silver marks, spiral lines, pits and other defects on the product surface. In order to effectively control the surface quality, it is significant to reveal the morphological evolution law of bubbles and the formation mechanism of surface defects in the filling stage of MFIM. This paper established an incompressible, non-isothermal, and unsteady three-dimensional mathematical model of multiphase flow. A new setting method of the boundary conditions with the exhaust function on the mold cavity walls was proposed. The problem of temperature solution divergence on the interface between the two phases with a high viscosity ratio was solved through the coupling algorithm of energy equation and PIMPLE loop. The tracking accuracy of micron grade bubbles interface in macroscopic scale flow field was improved though the Adaptive Meshing Refining (AMR) technique. Based on the abovementioned mathematical model, the influence law of the temperature field and velocity field on the bubble morphological evolution in the thickness cross-section of the injection flow field was obtained. The deformation, bursting and vanishing process of bubbles with different initial sizes and locations in the shear and fountain flow field was predicted. Combined with a short shot experiment, the formation mechanism of pits, silver marks and collapses on the product surface manufactured by MFIM was also revealed.

Injection-molded microcellular PLA/graphite nanocomposites with dramatically enhanced mechanical and electrical properties for ultra-efficient EMI shielding applications

High-performance EMI shielding materials with renewable characteristics are needed to address the issue of electromagnetic radiation pollution. The use of traditional metal-based EMI shielding materials is limited by their high density, corrosiveness, and expensive processing costs. At the same time, using regular fossil-fuel-driven conductive polymer composite-based EMI shielding materials creates environmental pollution, exacerbates resource consumption, and offers poor electromagnetic shielding effectiveness. Moreover, most of the processing methods used for conductive polymer composite-based EMI shielding materials are focused on a batch-scale process, which cannot easily be scaled up. We studied a sustainable foam injection molding-based method to efficiently fabricate renewable microcellular PLA/graphite nanocomposite foams, with improved mechanical and electrical properties for ultra-efficient EMI shielding applications. A microcellular PLA/graphite nanocomposite foam, with a density of 0.7 g cm−3 and a thickness of 2.0 mm, exhibits an outstanding EMI shielding performance with a total electromagnetic interference shielding effectiveness (EMI SE) of up to 45 dB. More importantly, thanks to the reduced reflection, which resulted from the strong thin-film interference effect, this lightweight porous nanocomposite foam has an absorption-dominated EMI shielding feature with a radiation energy reflection of less than 15%. The nanographite reorientation, which resulted from foaming, led to the microcellular PLA/graphite nanocomposite foams electrical conductivity being dramatically increased by almost six orders of magnitude, in relation to the unfoamed sample. Furthermore, the microcellular PLA/graphite nanocomposite foam also exhibited outstanding mechanical properties. These were characterized by a strong specific strength and modulus, and super-ductile fracture behavior. Thus, this lightweight sustainable nanocomposite foam demonstrated great promise as an ultra-efficient EMI shielding material for future use in many applications such as aerospace and electronics.

Lightweight, thermally insulating, and low dielectric microcellular high-impact polystyrene (HIPS) foams fabricated by high-pressure foam injection molding with mold opening

In this study, we used a high-pressure foam injection molding process to fabricate microcellular high-impact polystyrene (HIPS) foams with a tailored cellular structure. The process is cost-effective, highly efficient and flexible, and can be easily scaled up to complex components. The cellular structure of HIPS foam can be tuned over a wide range by manipulating packing time, cooling time, mold temperature, and mold-opening distance. Microcellular HIPS foam with a weight reduction of up to 60% was prepared, which possesses a low thermal conductivity of 60 mW m−1 K−1 and an ultra-low dielectric constant of 1.25. Both the thermal conductivity and the dielectric constant can be tailored by regulating the expansion ratio of HIPS foam. Mathematical models based on the mixing rule were developed to clarify the dependence of thermal conductivity and dielectric constant on the cellular structure of the foam. The outstanding thermally and electrically insulating properties of HIPS foams come from a large amount of air in the microcellular structure. These lightweight, thermally insulating, and ultralow dielectric microcellular HIPS foams hold great promise as an ultra-efficient insulating material for future use in many applications such as microelectronics and microelectromechanical systems (MEMSs).