CELL NUCLEATION IN HIGH-PRESSURE FOAM INJECTION MOLDING

Transcription

1 CELL NUCLEATION IN HIGH-PRESSURE FOAM INJECTION MOLDING Raymond K.M. Chu, Lun Howe Mark, and Chul B. Park Department of Mechanical and Industrial Engineering University of Toronto, Toronto, ON Canada Abstract Manufacturing polymeric foams with high cell densities with injection molding is of great interest to industry, primarily because of the flexibility and costeffectiveness of the technology. Foams manufactured from high-pressure foam injection molding processes, in general, possess relatively uniform cellular morphology. When used in conjunction with the mold-opening technique, high-pressure foam injection molding would also enable the manufacture of foams with higher void fractions. This work undertook an experimental approach to study the difference in cell nucleation and growth behavior in high-pressure foam injection molding with and without implementing the mold-opening technique. Introduction Foam injection molding (FIM) possesses the advantages of reduced material usage [1], greater dimensional stability [2], and faster production cycle [1]. The introduction of foam structure could also improve the mechanical and thermal insulation properties of the material [3-6]. Hence, FIM has been widely used in a broad spectrum of industries such as automotive and electronics for manufacture of lightweight foam parts and products of complex geometries. FIM can be classified into low-pressure (LP) and high-pressure (HP). For LP FIM, a short-shot polymer/gas mixture will be injected to partially fill the mold cavity. The cell density and morphology are mainly governed by the pressure drop rate (dp/dt) achieved as the melt is injected through the mold gate into the mold cavity. Depending on the degree of mold filling in LP FIM, different void fractions can then attainable. For LP FIM, cell nucleation and growth take place as the melt enters and flows across the mold cavity. Owing to that, the cell density and structure of the foam may suffer from notable deterioration along the melt flow direction. On the other hand, for HP FIM, the polymer/gas mixture will be injected full-shot to fill up the entire cavity. It is believed that the degree at which the material contracts or shrinks as it is cooled inside the mold cavity determines the foam s structure. Since the cell nucleation and growth activities occur after mold filling, foams manufactured from HP FIM tend to possess more uniform cellular morphology. Nonetheless, the void fractions obtainable are relatively limited [7-12]. In light of this, research and development of moldopening foam injection molding has received increasing attention [13-15]. Mold-opening (MO) FIM can be considered as a variant of HP FIM, where HP FIM is performed with full-shot injection followed by precise MO to an intermediate distance. With MO, foams with uniform morphology and void fractions of 50% and above could then be attainable [13-15]. In this work, we undertook an experimental approach to examine cell nucleation and growth in HP FIM and HP FIM with MO. Theoretical Background As described, the process of HP FIM decouples cell nucleation and growth activities from mold filling. Hence, cell coalescence and collapse activities present in LP FIM as a result of the shearing and smearing of adjacent cells will be prevented. Thus, foams manufactured by HP FIM tend to possess uniform cell density and void fraction across its melt flow direction. In HP FIM, the process begins with filling and packing of the polymer/gas mixture into a closed mold cavity under high pressures. In [16-17], it has been visualized that by packing the mold under sufficient pressure, majority of the gate-nucleated cells would collapse and dissolve back to the polymer melt. As the melt is cooled inside the mold cavity, volume contraction and blowing agent solubility change from material cooling would lead to the formation of the cellular structure. For the cases when HP FIM is used in conjunction with MO, the MO step will initiate a rapid depressurization of the cavity pressure, and hence, foaming occurs. In MO FIM, varying the holding times would also permit the polymer/gas mixture to cool and crystallize prior to MO, and this would determine the resultant cellular structures [13-14]. Figure 1 shows a schematic of the HP FIM process with and without MO. SPE ANTEC Anaheim 2017 / 2411

2 (b) Figure 1. Schematic of the HP FIM process: without and (b) with MO Materials Table 1. Foam injection molding parameters Parameter Value Injection temperature ( o C) 170 Mold temperature ( o C) 50 Injection flow rate (cm 3 /s) 110 CO 2 content (wt%) 3 Mold opening distance (mm) 0, 3 Mold opening speed (mm/s) 20 Initial mold cavity pressure (MPa) 20 Holding time (s) 0-20 Packing/holding pressure applied (MPa) 37.5 Commercial grade polylactide (PLA), Ingeo 2003D supplied by NatureWorks LLC was used in this study. The density and melt flow rate (MFR) of this PLA was 1.24 g/cm 3 and 6 g/10min (at 210 o C and 2.16 kg), respectively. The PTFE, Metablen A3000 by Mitsubishi Rayon Company Ltd., was provided as masterbatch of 10 wt%. Prior to the experiments, PLA and PLA/PTFE masterbatch pellets were oven-dried at 65 o C for 6 hour. The masterbatch was later dry-blended with neat PLA pellets to a PTFE content of 3 wt%. The blowing agent used in the foam injection molding experiments was carbon dioxide (CO 2, purity of 99.8%) supplied by Linde Gas Canada. Foam Injection Molding Experimental Pre-determined amounts of neat PLA and PLA/PTFE masterbatch were dry-blended and added to the hopper of a 50-ton injection molding machine, Arburg Allrounder 270C, equipped with the Trexel MuCell technology. A fixed amount of blowing agent (3 wt% CO 2 ) was then metered into the barrel and dissolved with the polymer melt. The PLA/CO 2 mixture, at 170 o C, was injected in full shot to completely fill and pack the mold cavity. The injection flow rate used was 110 cm 3 /s. After full-shot injection, the injected material was allowed to pack and hold inside the mold, regulated at 50 o C, for different times. For the cases of HP FIM, the packing pressure was removed after the pre-determined holding time elapsed. For the cases of HP FIM with MO, after the holding period, the mold was precisely opened to an intermediate position of 3 mm to induce a rapid depressurization. A mold opening distance of 3 mm would correspond to an overall void fraction of 50%. The foam was allowed to cool further for stabilization. Details about the processing parameters used during the foam injection molding experiments were summarized in Table 1. A schematic of the mold cavity geometry can be found on Figure 2. Figure 2. Geometry of the mold cavity Sample Characterization Foamed PLA specimens were cryo-fractured along the machine direction in liquid nitrogen and examined under a scanning electron microscope (SEM, JEOL6060). The cell density of the foams with respect to the unfoamed volume was evaluated via: Cell Density æ = ç è 3 # 2 of cells ö area ø where v.f. is the void fraction. æ 1 ö ç è1- v. f. ø Results and Discussion The cavity pressure profiles at the sampling location were recorded and shown in Figure 3. It can be observed that the pressure inside the cavity was able to kept at approximately 20 MPa by using 37.5 MPa packing pressure. For the cases with HP FIM only (Figure 3a), it could be seen that the pressure drop rates obtained from material cooling and shrinkage were relatively low: from about 2.5 MPa/s for the 0 s holding time case to about 0.5 MPa/s for holding times >= 8 s. Meanwhile, for the cases with HP FIM and MO, the pressure drop rates, obtained (1) SPE ANTEC Anaheim 2017 / 2412

3 from the slope of the pressure profiles around the solubility pressure, were all approximately 140MPa/s for the different holding times examined. It could be seen that the pressure drop rates in HP FIM with MO were almost 2 orders of magnitude higher than that from HP FIM. (b) Figure 3. Pressure profiles achieved with different holding times for: HP FIM and (b) HP FIM with MO Figure 4 shows the cellular morphologies of foams fabricated using the two HP FIM processes over different holding times. As shown in the figure, it could be seen that without applying the packing pressure (i.e., holding time = 0 s), large gate-nucleated cells (labeled with red arrows) were observed; for the case with MO, cell growing and coarsening activities associated with these large cells had led to the formation of macroscopic voids. (b) Figure 4. Cellular morphology of PLA foams prepared at different holding times: HP FIM and (b) HP FIM with MO The cell densities of the foams were evaluated and were summarized on Figure 5. It is to be noted that large coarsen cells or macroscopic voids were excluded from cell density calculation. Cell densities were notably higher for foams prepared with MO. It is well accepted that a higher pressure drop rate will induce higher thermodynamic instability for cell to nucleate [18]. According to Figure 3b, the pressure drop rates, estimated from the slope of the pressure profile around the solubility pressure, were similar for all of the cases during HP FIM with MO. It could then be assumed that the cell nucleation SPE ANTEC Anaheim 2017 / 2413

4 rates the polymer/gas experienced would be similar. As a result, the cell densities of the foams obtained from our MO experiments were all in the same range of 10 6 cells/cm 3. Similar to our earlier results, at short holding time (e.g., <= 8 s), the temperature of the melt was still high and a large temperature gradient was apparent. Hence, coalesced and coarsened cells were observed particular at the core of the foam where slow melt cooling resulted in a weak melt strength. As the holding time was lengthened, the time for the polymer/gas mixture to cool and crystallize was also extended and hence, the cellular morphologies were more uniform with uniform temperature and higher melt strength. investigated via an experimental approach. With MO, the rapid depressurization of mold cavity helped induce a high thermodynamic instability for cell nucleation. With longer holding times, the melt was able to cool further and foams with uniform morphologies were achieved. Meanwhile, for HP FIM without implementing the MO, cell densities were notably lower. It was also found that packing with longer holding times could reduce volume shrinkage of the melt and further lower the overall cell densities. Acknowledgement The authors would like to acknowledge the members of the Consortium of Cellular and Micro-Cellular Plastics (CCMCP), NatureWorks LLC, and Linde Gas Canada for their support towards this work. References Figure 5. Cell densities of foams prepared at different holding times For the cases with HP FIM only, the cell densities were, in general, lower as the pressure drop rates were significantly lower. Cell nucleation and growth were also governed by the rate at which the material shrinks as it cooled. Without applying packing pressure (i.e., holding time = 0 s), the presence of large gate-nucleated cells greatly reduced the amount of gas for nucleating additional cells. Hence, very low cell densities could be achieved. With packing pressure, it could be seen that, unlike the cases with MO, the cell densities were higher with shorter holding times. This opposite trend could be attribute to the fact that the application of longer packing would reduce volume shrinkage of the melt and thereby, lowering the overall cell densities. In addition, cooling the melt under pressure would prevent cells from nucleating with a low thermodynamic instability. Therefore, as the melt cooled, gas could be frozen and could not be utilized for cell nucleation or growth. Conclusions In this work, the difference in cell nucleation and growth behavior in HP FIM with and without MO was 1. Lee, J.W.S., Wang, J., Yoon, J.D., Park, C.B, Industrial & Engineering Chemistry Research, : p Kramschuster, A., et al., Polymer Engineering Science, : p Yuan, M., et al, Polymer Engineering Science, : p Wong, S. Lee, J.W.S., Naguib, H.E., Park, C.B., Macromolecular Materials and Engineering, : p Kramschuster, A. Gong, S., Turng, L.S., Li, T. Li, T., Journal of Biobased Materials and Bioenergy, (1): p Pilla, S., et al, Materials Science and Engineering: C, : p Lee, J.W.S., Park, C.B, Kim, S.G., Cellular Plastics, : p Lee, J.W.S., Park, C.B, Macromolecular Materials and Engineering, : p Park, C.B., Xu, X., US Patent Application 11/219,309, 2005; Canada Patent Application CA2,517,995, Xu, X., Park, C.B., Lee, J.W.S., Zhu, X. Applied Polymer Science, : p Ameli, A., Jung, P.U., Park, C.B., Carbon, : p Ameli, A., Jung, P.U., Park, C.B., Composite Science and Technology, : p Chu, R.K.M., Mark, L.H., and Park, C.B., SPE - Annual Technical Conference, 2015: Paper # Chu, R.K.M., Mark, L.H., and Park, C.B., SPE - Annual Technical Conference, 2016: Paper # Wang, L., Ishihara, S., Ando, M., Minato, A., Hikima, Y., Ohshima. M., Industrial & Engineering Chemistry Research, (46): p SPE ANTEC Anaheim 2017 / 2414

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