Visual observation and numerical studies of polymer foaming behavior of polypropylene/carbon dioxide system in a core‐back injection molding process

Thermoplastic foaming within a mold cavity was visualized as it was conducted in an 85‐ton core‐back injection‐molding machine. The core‐back molding process moved a section of the mold just after injecting a molten polymer into the cavity, quickly reducing the pressure to enhance the bubble nucleation. The foaming behavior during core‐back was observed directly through the glass windows of the mold. In the experiments, impact copolymer polypropylene was foamed with carbon dioxide. The effects of the gas concentration and the core‐back rate on bubble nucleation and growth were investigated. It was experimentally confirmed that the bubbles disappeared when the cavity was fully packed and that bubble nucleation occurred when the mold plate was moved and the cavity pressure dropped. Faster core‐back rates and higher gas concentrations increased the number of bubbles while decreasing their size. To analyze the experimental results, a bubble nucleation and growth model was employed that was based on batch foaming. The numerical results were a reasonable representation of the experiments, and this study demonstrated the applicability of the conventional free foaming model to the industrial core‐back molding process. Many aspects of the foaming in the core‐back molding aresimilar to the behaviors observed by batch foaming. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Novel injection molding foaming approaches using gas‐laden pellets with N2, CO2, and N2 + CO2 as the blowing agents

A novel method of producing injection molded parts with a foamed structure has been developed. It has been named supercritical fluid‐laden pellet injection molding foaming technology (SIFT). Compared with conventional microcellular foaming technologies, it lowers equipment costs without sacrificing the production rate, making it a good candidate for mass producing foamed injection molded parts. Both N₂ and CO₂ can be suitably used in this process as the physical blowing agent. However, due to their distinct physical properties, it is necessary to understand the influence of their differences over the process and the outcomes. Comparisons were made in this study between using CO₂ and N₂ as the blowing agents in terms of the part morphologies, as well as the shelf life and gas desorption process of the gas‐laden pellets. After gaining a good understanding of the SIFT process and the gas‐laden pellets, a novel foam injection molding approach combining the SIFT process with microcellular injection molding was proposed in this study. Both N₂ and CO₂ can be introduced into the same foaming process as the coblowing agents in a two‐step manner. Using an optimal content ratio for the blowing agents, as well as the proper sequence of introducing the gases, foamed parts with a much better morphology can be produced by taking advantage of the benefits of both blowing agents. In this study, the theoretical background is discussed and experimental results show that this combined approach leads to significant improvements in foam cell morphology for low density polyethylene, polypropylene, and high impact polystyrene using two different mold geometries. POLYM. ENG. SCI., 54:899–913, 2014. © 2013 Society of Plastics Engineers     

Experimental and numerical studies of injection molding with microfeatures

Injection molding with microstructures was investigated both experimentally and theoretically. A series of injection molding experiments with PP and PMMA was carried out in a long and a short rectangular mold containing microchannels with the thickness of either 50 or 100 μm and an aspect ratio of 5. The filling lengths in the microchannels were affected by injection speed, mold temperature, and channel location. A high injection speed or high mold temperature resulted in a longer filling length. The filling length in the microchannels decreased as the filling time in the main flow region increased. All filling lengths can be merged into a single curve vs. Fourier number based on the microchannel thickness. Comparison was also made between the experimental measurements and numerical simulation. The mold/melt heat transfer coefficient was found to be a critical factor in determining the filling lengths. The local heat transfer coefficient provided a much better agreement than a constant heat transfer coefficient. POLYM. ENG. SCI., 45:866–875, 2005. © 2005 Society of Plastics Engineers     

Experimental investigation and numerical simulation of injection molding with micro‐features

Injection molding of thin plates of micro sized features was studied in order to manufacture micro‐fluidic devices for bioMEMS applications. Various types of mold inserts—CNC‐machined steel, epoxy photoresist, and photolithography and electroplating produced nickel molds—were fabricated and tested in injection molding. The feature size covers a range of 5 microns to several hundred microns. Issues such as surface roughness and sidewall draft angle of the mold insert were considered. Two optically clear thermoplastics, PMMA and optical quality polycarbonate, were processed at different mold and melt temperatures, injection speeds, shot sizes, and holding pressures. It was found that the injection speed and mold temperature in injection molding greatly affect the replication accuracy of microstructures on the metal mold inserts. The UV‐LIGA produced nickel mold with positive draft angles enabled successful demolding. Numerical simulation based on the 2D software C‐MOLD was performed on two types of cavity fillings: the radial flow and the undirectional flow. The simulation and experimental data were compared, showing correct qualitative predictions but discrepancies in the flow front profile and filled depth.     

A strategy for the preparation of closed‐cell and crosslinked polypropylene foam by supercritical CO2 foaming

We report the preparation of a closed‐cell polypropylene (PP) foam material by supercritical carbon dioxide foaming with the assistance of γ‐ray radiation crosslinking. Styrene–ethylene–butadiene–styrene (SEBS) copolymer was added to PP to enhance radiation crosslinking and nucleation. Radiation effects on the foaming of the PP/SEBS blend with different ratios were investigated. A significant improvement in the foaming of the crosslinked PP/SEBS blend was achieved as compared to pristine PP. The cell density of the crosslinked PP/SEBS foam greatly increased at a dose of 10 kGy and a high closed‐cell ratio was obtained. The tensile strength of the crosslinked PP/SEBS foams (10 kGy) was improved from 14 to 20.7 MPa compared to pristine PP foam (0 kGy). In addition, the crosslinked PP/SEBS blend exhibited a wider foaming temperature window (10 °C) as compared to the non‐crosslinked ones (4 °C). © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45809.     

Identification of cell‐nucleation mechanism in foam injection molding with gas‐counter pressure via mold visualization

The mechanisms of cell nucleation and growth are investigated in foam injection molding (FIM) using gas‐counter pressure (GCP). An in‐situ mold visualization technique is employed. The application of GCP suppresses cell nucleation, and prevents the blowing agent from escaping during mold‐filling. The inherent structural heterogeneity in the regular FIM can be improved because of the uniform cavity pressure when employing GCP. The cavity pressure profiles show much faster pressure‐drop rates using GCP, because the single‐phase polymer/gas mixture has a lower compressibility than the two‐phase polymer/bubble mixture. Therefore, both the cell nucleation and growth rates are significantly increased through a higher pressure‐drop rate on the removal of the GCP. The effect of GCP magnitude on the cell morphology is explored. When the GCP is lower than the solubility pressure, bimodal foaming occurs. As the GCP increases above the solubility pressure, the cell density increases because of the higher pressure‐drop rate. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4035–4046, 2016     

Co‐injection molding of immiscible polymers: Skin‐core structure and adhesion studies

In this article, the microstructure and the adhesion developed in co‐injected specimens obtained with polypropylene (PP; core) and polystyrene (PS; skin) were studied as a function of process conditions and additives used. The study shows that the incorporation of low amounts of fillers such as Nanoclays and styrene‐ethylene‐butadiene‐styrene (SEBS) copolymer to the core material, working as compatibilizers, improves the adhesion at lower and higher polymer melt temperatures, respectively. The authors concluded as well that the use of such fillers, also improves the reproducibility of the process. The adhesion was assessed by shear tests using double lap shear specimens. A data acquisition system was attached to the mold to evaluate the pressure inside the cavity. Results of the in‐mold pressure profiles corresponded well when compared with MoldFlow predictions, and demonstrated that the adhesion of both materials is also related to their behavior and shrinkage inside the mold. POLYM. ENG. SCI., 2011. ©2011 Society of Plastics Engineers.     

Three‐dimensional simulation of multi‐material injection molding: Application to gas‐assisted and co‐injection molding

This paper presents an overview of the results obtained at the Industrial Materials Institute (IMI) on the numerical simulation of the gas‐assisted injection molding and co‐injection molding. For this work, the IMI's three‐dimensional (3D) finite element flow analysis code was used. Non‐Newtonian, non‐isothermal flow solutions are obtained by solving the momentum, mass and energy equations. Two additional transport equations are solved to track polymer/air and skin/core materials interfaces. Solutions are shown for different thin parts and then for thick three‐dimensional geometries. Different operating conditions are considered and the influence of various processing parameters is analyzed.     

Mechanical properties and morphological behavior of calcium carbonate‐filled polypropylene in dynamic injection molding

A custom‐made electromagnetic dynamic injection molding machine was adopted to study the mechanical properties and morphological behavior of calcium carbonate‐filled polypropylene (PP) in a dynamic injection molding process. The influence of vibration amplitude and frequency on the mechanical properties and morphological behavior of samples was investigated using tensile tests, notched Izod impact tests, differential scanning calorimetry, and scanning electronic microscopy. The tensile stress and the impact stress for all samples investigated were found to increase in a nonlinear manner with increasing vibration amplitude and frequency. The tensile stress reached a maximum value at about 8 Hz and 0.15 mm for neat PP and PP filled with 3, 20, and 30 wt% CaCO₃. For PP filled with 40 wt% CaCO₃, the tensile stress reached a maximum value at about 12 Hz and 0.2 mm. The impact stress reached a maximum value at about 12 Hz. From DSC experiments it was shown that the melting temperature slightly increased, but no new polymeric crystalline peak appeared under the vibration force field. The CaCO₃ particles were diffused easily and distributed evenly in the PP melt under the vibration force field, so it is very useful in improving the quality of injection products. Copyright © 2006 Society of Chemical Industry     

Theoretical and visual study of bubble dynamics in foam injection molding

This article presents an experimental observation and a theoretical prediction of bubble dynamics in foam injection molding process with a main focus on the cell collapse phenomenon under pressure. Using a visualizing setup, cell growth behavior under a nonisothermal condition was monitored. In conjunction with the growth behavior, dynamics of cell collapse under different pressures and the effect of growing time on collapse behavior and final cell size were studied. Theoretical simulation of bubble behavior included power law model, which predicted bubble dynamics during foaming process. The results show that collapse phenomenon strongly depends on both exerted holding pressure and growth time. The presented model can also give a reasonable prediction of growth and collapse of cells and could give insight to control of cell size in injection foaming process. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Pressure and temperature behavior of thermoplastic polymer melts during high‐pressure expansion injection molding

The development of expansion injection molding as new technology for producing thin plastic parts has raised questions about polymer melt behavior in the process of rapid high pressure compression and expansion. To investigate those phenomena, the new in‐line injection molding machine mounted measurement system has been developed. Measuring nozzle equipped with hydraulically driven closing bolts and contact fast response pressure and infrared temperature sensors enabled us to measure the compression and expansion of polymer melts at different starting conditions. Results presented for high impact polystyrene and polyamide show that polymer melt temperature rise is linearly dependent on compression pressure as well as it is dependent on compression speed. Comparable effect of temperature fall has been recorded during the polymer expansion, resulting in no noticeable loss of dissipation energy during this reversible process. Measurements have been compared with modified 2‐domain Tait equation of state. Curve fitting analysis to manufacturer's material data show that measurements fit well with the model, even at very high compression speeds. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Plastisol foaming process. Decomposition of the foaming agent,polymer behavior in the corresponding temperature range and resulting foam properties

The decomposition of azodicarbonamide, used as foaming agent in PVC—plasticizer (1/1) plastisols was studied by DSC. Nineteen different plasticizers, all belonging to the ester family, two being polymeric (polyadipates), were compared. The temperature of maximum decomposition rate (in anisothermal regime at 5 K min^?1 scanning rate), ranges between 434 and 452 K. The heat of decomposition ranges between 8.7 and 12.5 J g^?1. Some trends of variation of these parameters appear significant and are discussed in terms of solvent (matrix) and viscosity effects on the decomposition reactions. The shear modulus at 1 Hz frequency was determined at the temperature of maximum rate of foaming agent decomposition, and differs significantly from a sample to another. The foam density was determined at ambient temperature and the volume fraction of bubbles was used as criterion to judge the efficiency of the foaming process. The results reveal the existence of an optimal shear modulus of the order of 2 kPa that corresponds roughly to plasticizer molar masses of the order of 450 ± 50 g mol^?1. Heavier plasticizers, especially polymeric ones are too difficult to deform. Lighter plasticizers such as diethyl phthalate (DEP) deform too easily and presumably facilitate bubble collapse. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Morphological analysis of microcellular PP produced in a core‐back injection process using chemical blowing agents and gas counter pressure

A complete experimental analysis of the microcellular injection process using Chemical Blowing Agents (CBA) with Gas Counter Pressure (GCP) and core‐back expansion is presented. Three different types of polypropylene, neat and charged, were mixed with two different CBAs and injected into a plate mold with varying process parameters. First, an exhaustive cartographical mapping of the plate morphology is analyzed. In a second step, the relation between injection parameters and the resulting morphology is investigated. The results show that injection time affects the cellular structure. The formulation, especially the type of chemical foaming agent, controls the average bubble radius. Compared with classical injection process, the use of CBAs, combined to Gas Counter Pressure and core‐back process, allows obtaining parts with good surface aspect, more homogeneous cellular structures and smaller bubble radius. POLYM. ENG. SCI., 55:2465–2473, 2015. © 2015 Society of Plastics Engineers     

Warpage of a large‐sized orthogonal stiffened plate produced by injection molding and injection compression molding

Rectangular plates of the size of 1800 × 600 × 12 mm³ and 1200 × 600 × 12 mm³ were selected for injection molding and injection compression molding, respectively, in order to investigate warpage characteristics of the large‐sized polymer plates with orthogonal stiffener. To determine the mold system and to reduce warpage of the specimen, numerical analyses for injection molding and injection compression molding were performed by using a commercial simulation code. Experiments were performed to verify the suggested mold system and warpage of the specimen. Relatively large warpage of the injection molded product was observed and small warpage of the injection compression molded product was generated. Compression force of the injection compression molding was only 6% of the clamp force of the injection molding. Warpage of the product was reduced significantly by using the injection compression molding. The injection compression molding will be used to substitute expensive and disused wood forms with inexpensive and recyclable polymer plates for concrete casting. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Injection molding and injection compression molding of thin‐walled light‐guided plates with V‐grooved microfeatures

In this study, we investigated the feasibility of injection molding (IM) and injection compression molding (ICM) for fabricating 3.5‐in. light‐guided plates (LGPs). The LGP was 0.4 mm thick with v‐grooved microfeatures (10 μm wide and 5 μm deep). A mold was designed to fabricate LGPs by IM and ICM. Micromachining was used to make the mold insert. The Taguchi method and parametric analysis were applied to examine the effects of the process parameters on the molding quality. The following parameters were considered: barrel temperature, mold temperature, packing pressure, and packing time. Mold temperature in this investigation was in the conventional range. Increasing the barrel temperature and mold temperature generally improved the polymer melt fill in the cavities with microdimensions. The experimental results for the replication of microfeatures by IM and ICM are presented and compared. The height of the v‐grooved microfeatures replicated by ICM was more accurate than those replicated by IM. Additionally, the flatness of the fabricated LGPs showed that ICM was better than IM for thin‐walled molding. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

The synergy of supercritical CO2 and supercritical N2 in foaming of polystyrene for cell nucleation

This study examines the foaming behaviour of polystyrene (PS) blown with supercritical CO₂–N₂ blends. This is achieved by observing their foaming processes in situ using a visualization system within a high-temperature/high-pressure view-cell. Through analyzing the cell nucleation and growth processes, the foaming mechanisms of PS blown with supercritical CO₂–N₂ blends have been studied. It was observed that the 75% CO₂–25% N₂ blend yielded the highest cell densities over a wide processing temperature window, which indicates the high nucleating power of supercritical N₂ and the high foam expanding ability of supercritical CO₂ would produce synergistic effects with that ratio in batch foaming. Also, the presence of supercritical CO₂ increased the solubility of supercritical N₂ in PS, so the concentration of dissolved supercritical N₂ was higher than the prediction by the simple mixing rule. The additional supercritical N₂ further increased the cell nucleation performance. These results provide valuable directions to identify the optimal supercritical CO₂–N₂ composition for the foaming of PS to replace the hazardous blowing agents which are commonly used despite their high flammability or ozone depleting characteristics.     

Effect of die geometry on foaming behaviors of high‐melt‐strength polypropylene with CO2

This article reports on a systematic study that was conducted to investigate the effects of die geometry (i.e., pressure and pressure drop rate) on the cell nucleation and growth behaviors of noncrosslinked high‐melt‐strength (HMS) polypropylene (PP) foams blown with supercritical CO₂. The experimental results showed that the cellular morphologies of PP foams were sensitive to the die geometry. Furthermore, the initial expansion behavior of the foam extrudate at the die exit was recorded using a high‐speed CCD camera. This enabled us to achieve a more thorough understanding of the effect of die geometry on both the initial expansion behavior and the final cellular morphology of HMS PP foams. The effect of die temperature on cell morphology was also studied. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Microcellular injection molded polylactic acid/poly (ε‐caprolactone) blends with supercritical CO2: Correlation between rheological properties and their foaming behavior

A systematic study on the rheological properties helps to identify suitable processing and compositional windows for foaming polylactic Acid (PLA)/poly ε‐Caprolactone)(PCL) blends. In this article, the correlation between the rheological behavior, the blend morphology, as well as the resultant cellular structure of microcellular injection molded PLA/PCL blends was investigated. The addition of PCL had a significant effect on the storage modulus of PLA melts. With increasing the PCL content (less than 30%), the storage modulus increased due to the entanglement of polymer chains. The enhancement on their complex viscosities led to a better foaming behavior and pore microstructure. Porous structures with enhanced pore uniformity, decreased cell size, and higher cell density were observed in the PLA/PCL (70:30) specimens. POLYM. ENG. SCI., 56:939–946, 2016. © 2016 Society of Plastics Engineers