Skin Thickness and β-Crystals Development in Injection-Molded iPP Along the Flow Direction

Abstract

In this study, iPP was injection molded at 180°C, 200°C, and 220°C. According to polarization optical microscopy (POM) results, for a given part, the skin thickness steadily decreases along the flow direction. However, at the same distance from the gate, the skin thickness of the parts molded at lower melt temperature is larger than that molded at higher melt temperature. It is found that flow time (here, the time taken for melt to pass the specific position along the flow direction) and melt temperature are two significant factors leading to this phenomenon, while the gate size is another one.

The DSC and WAXD results show that the relative fraction of β-form crystals, for a specific part, decreases along the flow direction, which is mainly determined by flow time. However, for the parts molded at different molding temperatures, the fraction of the β-form crystals is mainly determined by the molding temperature, though this influence is very complex.

Keywords:

• β-crystal
• flow direction
• injection molding
• shear rate
• skin thickness

Acknowledgements

The authors gratefully acknowledge the financial support of the National Nature Science Foundation (Grant No.50803060, 10590351, 10772164) and the Opening Project of The Key Laboratory of Polymer Processing Engineering, Ministry of Education of China.