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Li S.,Shandong University | Zhao G.,Shandong University | Dong G.,Shandong University | Wang J.,Qingdao Hisense Mould Co.
Journal of Cellular Plastics | Year: 2016

As a novel injection molding process, microcellular injection molding process has the characteristics of saving material, decreasing warpage and surface sink mark, improving dimensional accuracy, etc. But for the plastic part with thick reinforcing ribs, if selection of process parameters are not reasonable, foaming quality of melt will be affected and obvious sink mark defects will appear on the surface of plastic part. This paper selected a medical appliance shell with many reinforcing ribs as research object. Simulation experiments of microcellular injection molding process were performed by using orthogonal experiment method. The influence of different process parameters, such as mold cavity surface temperature, melt temperature, injection rate, cooling time, weight reduction ratio and supercritical fluid level, on the surface sink mark of microcellular injection molding part was studied by using signal-to-noise ratio analysis and analysis of variance. The results showed that mold cavity surface temperature was the most important influence factor on surface sink mark depth of microcellular injection molding part, followed by weight reduction ratio, cooling time, supercritical fluid level, injection rate and melt temperature. Meanwhile, the optimal combination of process parameters was obtained for minimizing sink mark depth of microcellular injection molding part. The average surface sink mark depth of microcellular injection molding part molded by using the optimized process parameters was only 2.62 μm, compared to 4.87 μm of average surface sink mark depth of microcellular injection molding part molded by using the process parameters before optimization, the average sink mark depth of microcellular injection molding part was reduced by 46.2%. Finally, the forming mechanism of sink mark of microcellular injection molding part at locations of reinforcing ribs was discussed, and the influence mechanism of different process parameters on surface sink mark defects of microcellular injection molding part was also analyzed. © 2015 The Author(s).


Li S.,Shandong University | Zhao G.,Shandong University | Wang G.,Shandong University | Guan Y.,Shandong University | And 2 more authors.
Journal of Cellular Plastics | Year: 2014

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. © The Author(s) 2014 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav.


Wang X.,Shandong University | Wang X.,Qingdao Hisense Mould Co. | Zhao G.,Shandong University | Wang G.,Shandong University
Materials and Design | Year: 2013

Rapid heat cycle molding (RHCM) is a recently developed innovative injection molding technology to enhance the surface quality of the plastic parts without extending the molding cycle. Most of the common defects that occur in the plastic parts produced by conventional injection molding (CIM), such as flow mark, silver mark, jetting mark, weld mark, exposed fibers, short shot, etc., can be well solved by RHCM. However, RHCM is not a nostrum for all the defects in injection molding. Sink mark and warpage are two major defects occurring in RHCM. The purpose of this study is to investigate and further solve the sink mark and warpage of the molded parts in RHCM. To solve the problem of sink mark, a new " bench form" structure for the screw stud on the product coupling with a lifter structure for the injection mold was proposed. The external gas assisted packing was also proposed to reduce the sink mark in RHCM. To solve the problem of warpage, design of experiments via Taguchi methods were performed to systematically investigate the effect of processing parameters including melt temperature, injection time, packing pressure, packing time and also cooling time on the warpage. Injection molding simulations based on Moldflow were conducted to acquire the warpages of the plastic parts produced under different processing conditions. A signal to noise analysis was conducted to analyze the effect of the factors, and the optimal processing parameters were also found out. ANOVA was also conducted to quantitatively analyze the percentage contributions of the processing parameters on the warpage. The verification results show that part warpage can be reduced effectively based on the optimal design results. © 2012 Elsevier Ltd.


Wang G.,Shandong University | Zhao G.,Shandong University | Wang X.,Shandong University | Wang X.,Qingdao Hisense Mould Co.
International Journal of Advanced Manufacturing Technology | Year: 2013

The influences of the cavity surface temperature just before filling on part surface appearance and texture in rapid heat cycle moulding are investigated. It is observed that the cavity surface temperature just before filling has a very significant influence on part surface appearance. As the cavity surface temperature increases, aesthetic quality of the moulded part can be greatly improved by reducing surface roughness, increasing surface gloss and reducing weld mark. There is a critical cavity surface temperature just before filling for each plastic material. As the cavity surface temperature reaches the critical value, the part surface appearance will reach the optimal level with lowest roughness, highest gloss and without any weld mark. The critical cavity surface temperature on surface gloss and roughness is close to the Vicat softening point of the plastic material. The critical cavity surface temperature on weld mark is 10-20 C higher than that on surface roughness and gloss. The mechanisms for generating the rough surface of the part moulded with a low cavity surface temperature and improving part surface appearance by increasing cavity surface temperature are disclosed. © 2013 Springer-Verlag London.


Dong G.,Shandong University | Zhao G.,Shandong University | Guan Y.,Shandong University | Li S.,Shandong University | And 2 more authors.
Journal of Cellular Plastics | Year: 2016

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. © SAGE Publications.


Wang G.,Shandong University | Zhao G.,Shandong University | Wang X.,Shandong University | Wang X.,Qingdao Hisense Mould Co.
International Journal of Heat and Mass Transfer | Year: 2014

A new rapid mold heating and cooling method is developed in this study. For rapid mold heating, cartridge heaters are assembled in the holes of the mold. Between the heaters and the corresponding mounting holes, there are annular gaps which are full of water. During mold heating, the heat generated by the heaters passes through the water gaps firstly and then transfers into mold base to raise cavity surface temperature. For rapid mold cooling, pressured cooling water is passed though the annular gaps. Firstly, a cell model was established to evaluate the effectiveness of the new rapid mold heating and cooling method. Thermal response analysis based on numerical simulations was conducted to investigate the influences of the gap size, the power density of the heaters and also the layout of heaters on thermal responses of the cavity surface. Further, the injection mold of a large LCD TV frame was designed and manufactured based on the developed rapid mold heating and cooling method. Both numerical simulations and experiments were performed to evaluate the thermal response efficiency of the cavity surface. The results show that the cavity surface temperature can be changed in a large temperature range within relatively short time. The simulation results are consistent well with the experimental results, which verifies the effectiveness of the established analysis method. Finally, production testing was conducted to produce the LCD TV panel. The results show that the weld marks on the outer surface can be eliminated completely and the outer surface gloss of the part can reach higher than 90 GU with a molding cycle of about 60 s. The consumption of energy and water can be greatly reduced by comparing with other conventional rapid mold heating and cooling methods. © 2014 Elsevier Ltd. All rights reserved.


Wang W.,Shandong University | Zhao G.,Shandong University | Wu X.,Shandong University | Li X.,Harbin University of Science and Technology | Wang C.,Qingdao Hisense Mould Co.
Polymer Composites | Year: 2015

In this study, standard test specimens with flame-retarded short glass fiber-reinforced PC/ABS materials were fabricated under rapid thermal cycle injection molding condition by selecting a potassium perfluorobutane sulfonate flame retardant specially used for PC, FR2025, and two kinds of aryl phosphorus halogen-free flame retardants, UN707 and PX-220. The flame-retardancy effect of the above different flame retardants on the studied systems was compared through combustion tests of the specimens. Meanwhile, the thermal and mechanical properties of flame-retarded composites were studied by using the thermogravimetry analysis, dynamic mechanical thermal analysis (DMTA), and universal testing machine. The results show that the "candlewick effect" of fibers exacerbates the fire behavior of composites. With the increase of the aryl phosphorus halogen-free flame retardants, the flame-retardancy effect of composites is obviously improved, and the maximum thermal degradation rate of composites is significantly decreased. The UL94 combustion rating is improved, and the time of residual flame is substantially reduced with the increase of PC content under the same content of flame retardant. The DMTA results show that the flame retardants have a reinforcement action on PC/ABS matrix. However, the macroscopic mechanical properties are slightly decreased in the glass fiber-reinforced composites because of the destructive effect of the flame retardants on the interface compatibility between matrix and fibers; the scanning electron microscopic micrographs of tensile fracture fully prove this action mechanism of flame retardants. In addition, the addition of toughener and antidripping additive significantly affects the flame retardancy and mechanical properties of composites. © 2014 Society of Plastics Engineers.


Wang G.-L.,Shandong University | Zhao G.-Q.,Shandong University | Wang J.-C.,Qingdao Hisense Mould Co. | Zhang L.,Shandong University
Polymer Engineering and Science | Year: 2015

This study investigates the formation mechanisms and control of external and inner bubble morphology in MIM. First, the related theories about foaming and filling flow are analyzed. Second, the assumptions for the formation of inner bubble morphology, external bubble morphology, and the compact skin layer in MIM process are proposed based on theoretical analysis. Finally, experiments of MIM process are conducted to verify the theoretical assumptions. In addition, gas counter pressure (GCP) and rapid mold heating and cooling (RMHC) technology are used for control of bubble morphology. It is found that foaming process in MIM can be divided into foaming during filling and foaming during cooling. Foaming during filling produce oriented and deformed bubbles while foaming during cooling produce spherical or polygonal bubbles. As the bubbles formed by foaming during filling can reach melt flow front, they will be pushed to the cavity surface where they are stretched further and frozen to generate the silver or swirl marks. The compact skin layer is formed due to the redissolution of the gases within bubbles into polymer melt and also restraint of foaming by high cavity pressure. GCP and RMHC are two effective methods for controlling external and inner bubble morphology. © 2014 Society of Plastics Engineers.


Wang G.-L.,Shandong University | Zhao G.-Q.,Shandong University | Wang X.-X.,Shandong University | Wang X.-X.,Qingdao Hisense Mould Co.
Materials and Design | Year: 2014

Rapid heat cycle molding (RHCM) is a recently developed innovative injection molding technology. Rapid heating and cooling of the injection mold is the most crucial technique in RHCM because it not only has a significant effect on part quality but also has direct influence on productivity and cost-efficiency. Accordingly, Heating and cooling system design plays a very important role in RHCM mold design. This study focuses on the heating/cooling system design for a three-dimensional complex-shaped automotive interior part. Heat transfer simulation based on finite element analysis (FEA) was conducted to evaluate the thermal response of the injection mold and thereby improve heating/cooling channels design. Baffles were introduced for heating/cooling channels to improve heating/cooling efficiency and uniformity of the mold. A series of thermal response experiments based on full factorial experimental design were conducted to verify the effectiveness of the improved heating/cooling channels design with baffles. A mathematical model was developed by regression analysis to predict the thermal response of the injection mold. The effects of the cavity surface temperature on weld mark and surface gloss of the part were investigated by experiments. The results show that the developed baffle-based heating/cooling channels can greatly improve thermal response efficiency and uniformity of the mold. The developed mathematical model supplies an efficient approach for precise predication of mold thermal response. As the cavity surface temperature raises to a high enough level, automotive interior parts with high gloss and non-weld mark surface can be obtained. © 2014 Elsevier Ltd.


Dong G.,Shandong University | Zhao G.,Shandong University | Guan Y.,Shandong University | Wang G.,Shandong University | And 2 more authors.
Journal of Applied Polymer Science | Year: 2014

In this article, we studied the cell forming process of microcellular injection-molded parts. Using a modified injection molding machine equipped with a Mucell® SCF delivery system, microcellular-foamed acrylonitrile- butadiene-styrene parts with different shot sizes were molded. The cell structure on the fractured surfaces along the direction both vertical and parallel to melt flow in the molded parts was examined. The results showed that a regular spherical cells region and a distorted ellipsoidal cells region exist in the molded parts simultaneously. The length of the distorted cells region along the melt flow direction in the molded parts remained basically unchanged for different shot sizes and it is about 195 mm away from the flow front in this study's conditions. The cell formation mechanism was analyzed, two cell forming processes in microcellular injection molding, the "foam during filling" process and the "foam after filling" process, were proposed. It was also found that the melt pressure in the filling stage is the dominant factor affecting the cell forming process, and there is a critical melt pressure value in the filling stage, 20.9 MPa, as the dividing line of the two cell forming processes in this study. © 2014 Wiley Periodicals, Inc.

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