Study on the Flow, Foaming Characteristics and Structural Strength of Polypropylene Structural Foam Injection Molding by Innovative Nitrogen and Molten Plastic Mixing Mechanism

Po-Wei Huang; Hsin-Shu Peng; Sheng-Jye Hwang; Chao-Tsai Huang

doi:10.3390/polym15092116

Study on the Flow, Foaming Characteristics and Structural Strength of Polypropylene Structural Foam Injection Molding by Innovative Nitrogen and Molten Plastic Mixing Mechanism

Polymers (Basel). 2023 Apr 28;15(9):2116. doi: 10.3390/polym15092116.

Authors

Po-Wei Huang¹, Hsin-Shu Peng², Sheng-Jye Hwang³, Chao-Tsai Huang⁴

Affiliations

¹ Program of Mechanical and Aeronautical Engineering, Feng Chia University of Engineering and Science, Taichung 407102, Taiwan.
² Department of Mechanical and Computer Aided Engineering, Feng Chia University of Engineering and Science, Taichung 407102, Taiwan.
³ Department of Mechanical Engineering, National Cheng Kung University of Engineering, Tainan 70101, Taiwan.
⁴ Department of Chemical and Materials Engineering, Tamkang University of Engineering, New Taipei City 251301, Taiwan.

Abstract

Plastic foam molding methods include thermoforming, extrusion and injection molding. Injection foam molding is a one-time molding method with high production efficiency and good product quality. It is suitable for foamed plastic products with complex shapes and strict size requirements. It is also the main method for producing structural bubbles. In this investigation, we developed a structural foam injection molding technology using the gas supply equipment connected to the unique plasticizing mechanism of the injection machine and studied its influence on the specimens' melt rheology quality and foam structures. In the experiment, the forming material was polypropylene (PP), and the gas for mixing/forming foaming characteristics was nitrogen (N₂). Additionally, in order to observe the rheological properties of N₂/melt mixing, a melt flow specimen mold cavity was designed and the change in the melt viscosity index was observed using a melt pressure sensing element installed at the nozzle position. With the nitrogen supply equipment connected to a unique plasticizing mechanism, the mixing of gas and molten plastic can be achieved at the screw plasticizing stage, where the foaming effect is realized during the melt-filling process due to the thermodynamic instability of the gas. It was also found that an increase in N₂ fill content increased melt fluidity, and the trend of melt pressure and melt viscosity index showed that the higher the gas content, the lower the trend. The foaming characteristic depends on the gas thermodynamic instability and the pressure release, so it can be seen from the melt fill path that, the greater the pressure near the gate, the lower the foaming amount and the internal structure (SEM) after molding; the farther from the gate, the greater the relative increase in the foaming growth/amount. This phenomenon will be more obvious when the N₂ fill content is increased.

Keywords: foaming process-sensing system; melt rheology; polypropylene; structural foam injection molding technology; viscosity index.

Grants and funding

This research received no external funding.