Yanjin Guan

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 (<10u2009MPa) 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.

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.

Formation mechanism and structural characteristics of unfoamed skin layer in microcellular injection-molded parts

The microcellular injection-molded part usually consists of a foamed core region and two unfoamed skin layers on the cross section. This paper investigated the formation process, formation mechanism and structural characteristics of the unfoamed skin layers in microcellular injection-molded part. It is found that the unfoamed skin layers are formed in two processes namely “during filling” process and “after filling” process. The shear flow and the fountain flow behaviors of the melt in the filling stage are the main controlling factors on the formation of the unfoamed skin layer in “during filling” process, and the cooling solidification of the melt in cooling stage is the fundamental reason for the formation of the unfoamed skin layer in “after filling” process. Further studies found that the unfoamed skin layer in microcellular injection-molded part has two distinct regions, the outer region is a thin frozen layer which contains deformed and broken cells, and the inner region is a relatively thick solid-like layer which has no visible cells in. The unfoamed skin layer has a minimum thickness in the gate location. The whole thickness of the unfoamed skin layer is decreased with the increase of injection speed and mold temperature, but is slightly affected by melt temperature.

Effect of Acrylonitrile-Butadiene-Styrene High-Rubber Powder and Strain Rate on the Morphology and Mechanical Properties of Acrylonitrile-Butadiene-Styrene/Poly (Methyl Methacrylate) Blends

Acrylonitrile-butadiene-styrene (ABS) high-rubber powder (HRP) toughened acrylonitrile-butadiene-styrene (ABS)/polymethyl methacrylate (PMMA) blends were prepared in a co-rotating twin screw extruder. The effect of ABS HRP on mechanical and morphology properties of ABS/PMMA blends was studied. It is shown that the toughness of ABS/PMMA blends was improved effectively with the incorporation of ABS HRP, while the elastic modulus and tensile strength decrease slightly. Tensile tests at different strain rates were carried out in order to investigate the influence of strain rate on mechanical properties of various blends. Finally, the strain rate sensitive behavior of different blends was explored by using the Eyring model.

Effect of rapid heating cycle injection mold temperature on crystal structures, morphology of polypropylene and surface quality of plastic parts

The present study mainly investigated the effect of mold cavity surface temperature in the filling process on the crystallization status of polypropylene (PP) in different thickness layers of injected plastic parts, and revealed the inner relationship between the surface quality of plastic parts, crystallinity and residual stress under rapid heating cycle molding (RHCM) process conditions. The morphology of PP was studied by using a transmission-polarized optical microscope (POM) and a scanning electron microscope (SEM). The crystal structures of PP on different thickness layers of plastic parts were examined by using a wide-angle X-ray diffraction (WAXD). The surface quality of plastic parts was characterized by using a glossiness tester. With the increase of mold cavity surface temperature, the thickness of frozen layer of plastic parts is drastically reduced, and the crystallinity of PP in the external surface first increases, then decreases, and finally tends to a constant, but the crystallization in the sub-surface layer first increases, then decreases, and finally increases again. When the temperature of mold cavity surface is close to the glass transition temperature of PP, the crystallinity in the external surface reaches the maximum and the crystallization status in the sub-surface and core layers is close to each other. Meanwhile, the surface glossiness of plastic parts also achieves its best. In addition, the high crystallinity and low residual stress in the external surface are both favorable conditions for obtaining a high surface glossiness of plastic parts.

Mechanical and Thermal Properties of ABS/PMMA/Potassium Titanate Whisker Composites

ABSTRACT Two kinds of potassium titanate whiskers were filled into ABS (Acrylonitrile-Butadiene-Styrene)/PMMA (Polymethyl Methacrylate) which is often used in rapid heat cycle molding process. The aspect ratio of potassium titanate whiskers 2 is larger than that of potassium titanate whiskers 1. Properties of ABS/PMMA/potassium titanate whiskers composites prepared in a corotating twin-screw extruder were analyzed. The results show that potassium titanate whiskers can improve thermal stability of composites while they cause toughness of composites reduced. Tensile strength of composites increases slightly with 5u2009wt% potassium titanate whiskers. Halpin–Tsai model can calculate the modulus of composites more accurately. Potassium titanate whiskers 2 is beneficial to the thermal stability of composites, while it makes the viscosity of composites increasing. GRAPHICAL ABSTRACT

Characteristic of rapid heating cycle moulding and warpage analysis of products

Abstract Rapid heating cycle moulding (RHCM) injection method can be used to solve many defects on the surface of the products produced by conventional injection method, such as weld mark, flow mark and so on. In this paper, the process of RHCM technology was presented. By comparing the new technology with the conventional injection method and the products produced by them, the advantages of RHCM technology were demonstrated. However, large warpage of the products was considered as the main defects of this technology. The reasons that caused the warpage of the products were analysed. Through finite element simulation and engineering experiment, the layout of the cold runners in the RHCM mould was optimised in order to decrease the warpage of the part. The results of the experiments and the finite element simulation were very close, proving that the simulation carried out in the paper was right.