Research on Structure Formation of Polymer during Injection Molding and the Apparatus for on-line Investigation

Gan-Ji Zhong*, Guo-Qi Ma, Zhao-Bo Sun, Jun Lei, Zhong-Ming Li

College of Polymer Science and Engineering, Sichuan University, Chengdu, 610065

ABSTRACT: Injection molding is the most commonly used molding method in the polymer industry for its unique combination of high automation, high efficiency, and the ability to mold products with complex shapes and high dimensional accuracy. Currently, there are about 32% of polymer materials are processed into products through injection molding in the world, the polymer used for injection molding over 20 million tons every year in China. The performance of polymer products is highly influenced by the hierarchical structure formed under the coupling of strong shear, pressure, and temperature field during injection molding. In this work, the formation and evolution of polymer aggregation structure and its relationship with properties are studied under the coupling of multiple strong fields during injection molding. Meanwhile, the on-line equipment for injection molding is developed to study the structure formation and evolution during the crystallization of polymer under the multiple strong external fields in the injection molding. The main results are shown as follows: (1) The high shear rate promotes the formation of α-form of nylon 6 (PA6), while the high cooling rate facilitates the formation of γ-form. The water absorption of PA6 is reduced by blending with poly(ethylene terephthalate) (PET), which processes low cost and excellent comprehensive properties. The micro-beam WAXD confirms the hydrogen-bonding interactions between PA6 and PET promote the crystallization of PET, which can also suppress the absorption and diffusion of water molecules. The dielectric and mechanical properties after water absorption of the blends are maintained due to the reduction of water absorption. (2) The high shear rate and cooling rate leading to the formation of imperfect crystalline of poly(butylene succinate) (PBS) with small size, while there forms banded spherulite whose size increase gradually with the decrease of shear rate and cooling rate. Additionally, to improve the processability and impact performance of PBS, the poly(butylene adipate-co-terephthalate) (PBAT) is introduced. The co-continuous morphology with a gradient orientation forms under the gradient distribution of shear rate and cooling rate during the injection molding by controlling the content of PBAT. The mechanical properties of PBS are enhanced significantly. (3) The on-line characterization for injection molding is developed, including an injection molding machine, a displacement platform, and a mold with on-line detection windows. The rapid evolution of the multi-scale and multi-layer structure under the strong external field can be studied by combining the equipment with X-ray diffraction technology that processing a millisecond time resolution. Therefore, the quantitative “parameters-structure-properties” relationship can be founded. It can also provide a new way to control the structure accurately and then to realize the high performance of injection molded parts.

Keywords: Polymer; Injection molding; Multi-layer structure; Properties; On-line research equipment