Visual observation and numerical studies of N₂ vs. CO₂ foaming behavior in core‐back foam injection molding

Abstract

We investigated, by visual observation and numerical calculations, the foaming behavior of polypropylene within a foam injection mold cavity with the environmentally benign physical blowing agents nitrogen (N2) and carbon dioxide (CO2). An 85‐ton core‐back injection‐molding machine with temperature and pressure monitoring systems as well as a high‐pressure view cell was used for the investigation. The experiments showed a prominent difference in bubble nucleation and growth between N2 and CO2 injection foaming. Even when the weight concentration of N2 dissolved in polymer was one‐third that of CO2, N2 injection foaming provided a bubble number density that was 30 times larger and a bubble size that was one‐third smaller compared to CO2 injection foaming. Classical bubble nucleation and growth models developed for batch foaming were employed to analyze these experimental results. The models reasonably explained the differences in injection foaming behavior between N2 and CO2. It was clearly demonstrated by both experiments and numerical calculations that N2 provides a higher number of bubbles with a smaller bubble size in foam injection molding compared to CO2 as a result of the lower solubility of N2 in the polymer and the larger degree of super‐saturation. POLYM. ENG. SCI., 2011. ©2011 Society of Plastics Engineers

Influence and prediction of processing parameters on the properties of microcellular injection molded thermoplastic polyurethane based on an orthogonal array test

“…Dissolved gas then starts emerging from the solution and generating bubbles inside of the molded part. 13 It has been reported that open porous polypropylene and polyether-urethane have been produced using microcellular injection molding. 14,15 The orthogonal array test is a systematic, statistical testing technique, which is particularly effective in finding errors associated with faulty logic within computer software systems.…”

Polystyrene Foam with High Cell Density and Small Cell Size by Compression‐Injection Molding and Core Back Foaming Technique: Evolution of Cells in Cavity

“…observed the shift from elongated cells into circular cells in the direction parallel to the flow in the FIM. In addition, FIM can sometimes be accompanied by a controlled mold‐opening step which is also termed as breathing or core‐back to increase the void fraction . For example, Takeshi et al .…”

A new insight into foaming mechanisms in injection molding via a novel visualization mold

“…The location of the viewing windows in the fixed plate, on the opposite side of the mold cavity, makes it possible to perform various experiments by simply changing the mold insert, some of which includes, but is not limited to, gasassisted FIM studies, shear/extension induced FIM, and the study of flow in various channels. Also, the mold can be utilized to investigate other FIM technologies such as gas-counter pressure (GCP) [12,13] and mold opening (core back) [9,10,14,15]. The designed mold can be utilized to further investigate the mechanisms of bubble nucleation/growth, interaction between bubbles and fillers [16,17], the effect of formed crystals on foaming [18], weld-line studies [19], defects [20], and model verifications [21].…”

“…Additionally, most of hypotheses or theoretical simulations were not experimentally verified [5,6]. In this context, the in-situ observation of foaming phenomena drew significant attention amongst researchers [7][8][9][10]. Villamizar and Han [7] designed a rectangular visualization mold with transparent quartz walls to investigate the bubbles' dynamics in conventional structural FIM.…”

“…Mahmoodi et al [8] used a visualization setup in which one side of the mold cavity was replaced with a multi-layer glass block in the moving part of the mold. They conducted fullshot FIM experiments using polystyrene (PS)/carbon dioxide (CO 2 ) system to visualize cell growth and [10] extended their study to compare the nucleation effectiveness of nitrogen (N 2 ) with that of CO 2 . Yamada et al [11] observed a multilayer core structure in FIM experiments of PS blown with N 2 , and justified this phenomenon by the local cavity pressure.…”