Stabilization of Gas Counter Pressure in Supercritical Microcellular Injection Molding: A Case-Review

Chen SC; Wang YC; Feng CT; Lee SW

doi:10.37871/jbres2208

ISSN: 2766-2276

2025 October 23;6(10):1540-1543. doi: 10.37871/jbres2208.

Subject area(s)

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Case Study

Stabilization of Gas Counter Pressure in Supercritical Microcellular Injection Molding: A Case-Review

Chen SC^1,2*, Wang YC^1,2, Feng CT^1,2 and Lee SW^2,3

¹Department of Mechanical Engineering, Chung Yuan Christian University, Taoyuan, Taiwan
²R&D Center for Smart Manufacturing, Chung Yuan Christian University, Taoyuan, Taiwan
³Moldintel Co., Ltd. Taoyuan, Taiwan

*Corresponding authors: Chen SC, Department of Mechanical Engineering, Chung Yuan Christian University, Taoyuan, Taiwan E-mail:

Received: 15 September 2025 | Accepted: 22 October 2025 | Published: 23 October 2025

How to cite this article: Chen SC, Wang YC, Feng CT, Lee SW. Stabilization of Gas Counter Pressure in Supercritical Microcellular Injection Molding: A Case-Review. J Biomed Res Environ Sci. 2025 Oct 23; 6(10): 1540-1543. doi: 10.37871/jbres2208, Article ID: jbres2208

Keywords

Microcellular injection molding
Gas counter pressure
Cell uniformity
Green manufacturing

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Abstract

MuCell® Microcellular Injection Molding (MIM) is a key green technology that reduces energy consumption and material usage while facilitating recycling. Despite its benefits, surface defects and inconsistent foaming remain challenges. Gas Counter Pressure (GCP) has been introduced to improve foam morphology, but conventional methods often suffer from pressure instability. This study investigates the impact of a stabilization device in GCP-assisted molding. Results show that stabilized GCP increases cell density by approximately 41% compared to conventional GCP, and reduces the standard deviation of cell diameter from 2.54 µm to 1.77 µm. These improvements significantly enhance cell uniformity, underscoring the broader potential of MuCell® technology in plastic manufacturing.

Introduction

Microcellular injection molding (MuCell®) has emerged as a key technology in sustainable plastic production. By introducing supercritical fluids into polymer melts, MuCell® enables the formation of microcells, which reduce material usage and improve energy efficiency. Despite its advantages, the process often results in surface defects and non-uniform cell structures, limiting its application in high-precision industries. To address these issues, Gas Counter Pressure (GCP) has been employed to suppress premature foaming and improve cell morphology. The GCP process involves sequential steps of gas inlet, mold closing with gas compression, melt filling under counter pressure, gas release, and mold opening, as illustrated in figure 1. However, conventional GCP systems frequently exhibit pressure fluctuations, which compromise foam consistency. This study explores the integration of a pressure stabilization device into the GCP system to enhance process control and product quality [1].

Gas Counter Pressure in MuCell®

Gas Counter Pressure (GCP) was introduced to control bubble nucleation by applying a back pressure within the mold cavity [2-5]. Chen SC, et al. [2] first demonstrated that applying GCP above a critical threshold suppresses bubble nucleation during filling, enabling smooth, transparent foamed products without a skin layer. Later, Chen SC, et al. [3] proposed a Pressure-Temperature (P-T) path strategy for precise control of cell size and distribution, enhancing structural uniformity and mechanical strength. Agustion YH, et al. [4] further validated GCP in polycarbonate, showing that ~80 bar reduced average cell size from 40 µm to 20.9 µm and doubled cell density while maintaining 30% weight reduction. For soft polymers like TPU, Chen SC, et al. [5] combined GCP with dynamic mold temperature control, achieving 50% higher cell density and up to 60% weight reduction, producing lightweight midsoles with highly uniform morphology. While GCP is effective, unstable pressure profiles can still compromise cell uniformity. However, air compression during injection also influences counter pressure, yet this has been little studied.

Experiment: Stabilization of GCP

To overcome this limitation, a stabilization device has been proposed. By actively regulating cavity pressure during the filling stage, stable GCP enables more consistent control of bubble formation and growth. The device consists of a pressure sensor, timer module, solenoid valves, and a throttling valve, coordinated with a programmable controller. When the cavity pressure exceeds the preset value, the sensor triggers the solenoid valve to release gas until stability is restored, while at the end of injection the timer activates another valve to vent the cavity at controlled rates. This automated feedback loop ensures precise pressure regulation. Measurements on side-gated flat plaques at three positions (Figure 2)-Near gate (P1), Center (P2), and far gate (P3)-Show clear improvements (Figures 3,4). TPU was used as the thermoplastic material in this experiment. TPU is also an important material used in the midsole of spot shoes. With the stabilization device, average cell diameters decreased to 28.08 µm, 28.90 µm, and 31.47 µm at P1, P2, and P3, respectively, compared with 31.34 µm, 34.97 µm, and 36.22 µm without stabilization. Correspondingly, cell densities increased from 1.06×10¹², 7.35×10¹¹, and 6.94×10¹¹ cells/cm³ to 1.38×10¹², 1.20×10¹², and 9.48×10¹¹ cells/cm³, result summation show in table 1, under GCP settings in table 2. Moreover, the standard deviation of cell diameter was reduced from 2.54 µm to 1.77 µm, confirming enhanced uniformity and reproducibility of the foamed structures. Figure 5 illustrates the cavity counter pressure histories with and without stabilization, where the non-stabilized group exhibits a distinct pressure peak immediately after injection. The stabilization of counter pressure was achieved by using a pressure-regulation valve near gas inlet location.

Table 1: Result summation.
Position	Avg. Diameter (mm)-Stabilized	Avg. Diameter (mm)-Unstabilized
P1	28.08	31.34
P2	28.9	31.97
P3	31.47	36.22
Position	Cell Density (cells/cm3)-Stabilized	Cell Density (cells/cm3)-Unstabilized
P1	1.38E+12	1.03E+12
P2	1.20E+12	7.35E+11
P3	9.48E+11	6.94E+11

Table 2: Result summation.
Parameters	Value
Gas Counter Pressure	80
Release Rate (bars/s)	40
Holding Time (s)	3.5

Conclusion

This study demonstrates that incorporating a pressure stabilization device into the GCP system significantly enhances the performance of microcellular injection molding. The improvements in cell density and uniformity contribute to better surface quality and mechanical properties, making MuCell® technology more viable for high-precision and sustainable manufacturing applications. Particularly, traditional chemical foaming for shoe soles will be excluded in the shoe market. This research progress will accelerate the employment of physical foaming using supercritical foaming in the manufacturing of shoe soles. Future research may focus on refining stabilization mechanisms and exploring their effects across different polymer materials and industrial component.