Elsevier

Chemical Engineering Science

Volume 155, 22 November 2016, Pages 27-37
Chemical Engineering Science

Effect of foam processing parameters on bubble nucleation and growth dynamics in high-pressure foam injection molding

https://doi.org/10.1016/j.ces.2016.07.040Get rights and content

Highlights

  • A visualization mold is used to study high-pressure foam injection molding process.

  • The injection speed and gate resistance had no significant effect on cell density.

  • The injected-gas content and the melt flow index affect bubble nucleation.

  • The cell growth rate is inversely proportional to cell density in confined volume.

Abstract

We used an innovative visualization mold to investigate the effect of foam processing parameters on bubble nucleation and growth. This was also done to uncover the mechanisms responsible for cellular structural development in the high-pressure foam injection molding process. The effects of the injection speed, the injection gate geometry, the blowing agent content, the melt flow rate and the use of talc as a heterogeneous nucleating agent on the formation and dynamics of cell bubbles were all explored. In the high-pressure foam injection molding process with a proper packing pressure, the overall cell density did not change with the injection speed nor with the injection gate resistance. However, the cell density increased significantly with the blowing agent's concentration and with a nucleating agent. We also observed the growth mechanism of the bubbles in a confined mold cavity, and concluded that the bubble growth rate decreased as the cell density increased. In addition, the satelliting phenomenon, i.e. bubble nucleation around the previously nucleated cells, was observed. This was due to the induced stress fluctuations in the surrounding melt, which could eventually affect the final cellular structure.

Introduction

The popularity of plastic foams in manufacturing engineering is dramatically increasing. They offer enhanced mechanical properties (Matuana et al., 1998, Rachtanapun et al., 2004, Seeler and Kumar, 1993, Shimbo et al., 2007), improved heat and sound insulation (Ghaffari Mosanenzadeh et al., 2015, Gong et al., 2015, Jahani et al., 2014), enhanced electrical conductivity (Ameli et al., 2014b), and charge storage capability (Ameli et al., 2015b). In particular, the functionality of foams is improved by reducing the cell size to the sub-micron levels (Liu et al., 2015, Sundarram and Li, 2013). Due to the Knudsen effect, for instance, the thermal conductivity of nano-cellular foams can decrease dramatically to manufacture super-insulators (Lu et al., 1995, Notario et al., 2015, Schmidt et al., 2007).

Foam injection molding (FIM) is unique among foam processes because it can produce light-weight parts with high geometrical accuracy and a high stiffness-to-weight ratio in fast production cycles (Xu et al., 2008). Before molding, it is essential to prepare a homogeneous, one-phase melt/gas mixture in order to achieve a uniform cell structure in FIM products (Xu et al., 2008). The gate's resistance should produce a sufficiently high back pressure to avoid premature bubble nucleation in the injection nozzle or in the sprue. Once the gas-charged polymer melt is injected into the confined mold cavity, bubble nucleation occurs according to the governing nucleation mechanism, depending on the injection-molding method. In low-pressure FIM, where a short-shot, typically ranging 65–95% of the full-shot, is used to partially fill the mold cavity, the required pressure drop to induce bubble nucleation is obtained over the gate. The nucleated bubbles then grow and expand, even during injection, until the entire cavity is filled. In high-pressure FIM, on the other hand, a full-shot is used to fill the entire cavity. In an ideal high-pressure FIM, all the cells nucleated during filling dissolve back into the melt, and the pressure drop for bubble nucleation can only be achieved through melt shrinkage during the solidification process (Shaayegan et al., 2016b). It has been verified that the high-pressure FIM can produce uniform cell structures (Shaayegan et al., 2016b). But despite all its advantages, FIM suffers from an inherent heterogeneity in its cellular structure, which deteriorates the physical and mechanical properties of its foamed parts. Therefore, a wide-ranging understanding of the governing mechanism(s) of bubble nucleation/growth in the FIM process, and the relationship between its processing parameters and the final foam structure, is essential to achieving a uniform and fine-cell structure with the desired properties.

Extensive research has been undertaken on structural and morphological development in the FIM process. Some studies have investigated the relationship of the processing conditions, the cellular structures, and the mechanical properties of pure and composite resins in a low-pressure FIM process (Kharbas et al., 2003, Turng and Kharbas, 2003, Yoon et al., 2009, Yuan and Turng, 2005, Yuan et al., 2005, Yuan et al., 2004). Ameli et al. investigated the effects of the cellular structure formation on the functional properties of the injected-molded foam through fiber orientation (Ameli et al., 2013a, Ameli et al., 2013b, Ameli et al., 2015b, Ameli et al., 2014c). Huang and Wang (2008) studied the effects of the injection speed, the nozzle temperature, and the shot size on the microcellular structure of polystyrene (PS) and the formed skin layer. Barzegari and Rodrigue (2009) investigated the effect of processing conditions on the final cellular morphology of structural foams and concluded that the injection pressure and the blowing agent's concentration were the most influential processing parameters. Pilla et al. (2009) studied the microstructural development and evaluated the mechanical properties of microcellular injected molded PLA by adding chain extender. Ameli et al., 2014a, Ameli et al., 2015a explored the effects of talc and clay on the cellular structure of PLA foams, and showed that the foaming behavior of PLA was significantly enhanced by the presence of additives. The application of gas-counter pressure on both the cellular structure and the mechanical properties of FIM parts was examined by Chen and co-workers (Chen et al., 2013, Chen et al., 2012). Lee et al. (2008) analyzed how the effects of processing parameters such as the blowing agent concentration, the injection speed, and the shot size in relation to the cavity pressure profile affected the cellular structure in low-pressure FIM. Wang et al. (2015) argued that the morphology of the cellular structure in high-pressure FIM is a function of the shot size.

Besides the experimental approaches, theoretical and numerical attempts have also been made to model the development of bubbles and to predict the effect of such processing parameters as the gas concentration, the melt temperature, and the mold temperature on FIM cellular morphology (Amon and Denson, 1986, Arefmanesh et al., 1990, Han et al., 2003, Ramesh et al., 1991). Han and Yoo (1981) used the DeWitt equation (Bird and Hassager, 1987) to model the isothermal growth of a single bubble in a large rectangular cavity of a viscoelastic fluid. Upadhyay (1985) applied the Leonov constitutive equation (Leonov, 1976) to model the non-isothermal growth of bubbles in FIM. Osorio and Turng (2004) adopted a numerical approach to solve the energy, mass diffusion, and continuity equations in order to predict cell growth in FIM in a non-isothermal condition. Mahmoodi et al. (2010a) used the power-law model to simulate the bubble dynamics.

Each of these earlier studies contributed to a better understanding of the cellular structure development in FIM, and significant insight was gained with respect to the effect of processing conditions on the final bubbles’ nucleation and dynamics. However, the bubble nucleation mechanism and the growth phenomena in FIM are still not well understood. This can be attributed to the fact that most of the analyses were done on the finally obtained foam samples without clear understanding of the interim development of the cellular morphology, especially in the case of high-pressure FIM.

In this context, a few researchers had used in-situ visualization techniques to study the FIM process. Villamizar and Han (1978) studied the bubble growth phenomena at different processing parameters by means of visualization. However, their study focused mainly on low-pressure experiments in conventional FIM, and used chemical blowing agents with a limited gas content. Yamada et al. (2012) visualized the structure development in FIM of PS/nitrogen system, and observed a multi-layer central region. Mahmoodi et al. (2010b) carried out high-pressure FIM experiments using PS as the matrix and carbon dioxide (CO2) as the physical blowing agent to visualize cell growth and cell collapse during non-isothermal mold filling. Ishikawa and Ohshima (2011) investigated the foaming behavior of polypropylene blown with CO2 during mold opening using visualization techniques. Ishikawa et al. (2012) extended their study to compare the nucleation effectiveness of nitrogen with that of CO2. Recently, Shaayegan et al., 2016a, Shaayegan et al., 2016b demonstrated the bubble-nucleation mechanism in high-pressure FIM is different from that in low-pressure FIM with a newly designed visualization mold equipped with numerous pressure transducers.

To uncover the FIM's complex dynamics and bubble-nucleation processes, and to interpret each processing parameter's effects on the final cellular structure (and thereby on the resultant foam properties), a systematic and wide-ranging research is required. Such a study would include the investigation of phenomena in real time. Further, the theoretical explanations of the FIM phenomena, proposed mechanisms, and numerical simulations need to be verified by reliable point-to-point experimental data. Thus, we used an on-line visualization method to explore the effects of the injection speed, the injection gate resistance, the blowing agent content, the melt flow rate, and the nucleating agent on bubble formation and growth in the high-pressure FIM process.

Section snippets

Materials and procedures

Two grades of PS, PS675 and MC3650 with MFR 7.5 g/10 min and MFR 13.0 g/10 min (200 °C/ 5 Kg), respectively, were provided from Americas Styrenics. Carbon dioxide with 99.8% purity from Linde Gas Canada was used as the physical blowing agent. To conduct FIM experiments, a 50-ton Arburg ALLROUNDER 270C injection molding machine (Lossburg, Germany, 30-mm diameter screw) was used. The injection of CO2 in a super critical phase was done using MuCell® technology (Trexel, Inc., Woburn, Massachusetts). The

Effect of the injection speed and the gate resistance on bubble nucleation and growth in high-pressure FIM

Fig. 2 shows the visualization snapshots performed at Location A (see Fig. 1b) for high-pressure FIM experiments of PS675-3 wt% CO2 at different injection speeds. A melt packing pressure of 8 MPa in the machine specification (which would be the hydraulic pressure) was used for 1s to ensure that all primary nucleated bubbles, that is, bubbles nucleated during the feeding stage at the gate, were dissolved back into the melt. The cell density information, with respect to the unfoamed volume (Xu et

Conclusion

The effects of the injection speed, the resistance of the gate, the content of the blowing agent, the melt flow rate, and eventually the addition of talc as a heterogeneous nucleating agent on the bubble nucleation and growth behaviors were explored in the high-pressure foam injection molding process. The injection speed and the resistance of the gate did not influence the overall cell density. The dissolved gas content, however, increased the cell density greatly. A lower melt flow rate PS

Acknowledgment

This research was financially supported by the Natural Science and Engineering Research Council of Canada (NSERC 154279-2010). The authors are thankful to the Consortium of Cellular and Micro-Cellular Plastics (CCMCP) for their financial support. The polystyrene used in this research was provided by Americas Styrenics, TX, United States.

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