Citation: Kai-qiang Guo, Han-xiong Huang. Cellular Structure and Warpage of Microcellular Injection Molded Electrical Boxes Fabricated at Higher Injection Speeds[J]. Acta Polymerica Sinica, ;2019, 50(8): 850-856. doi: 10.11777/j.issn1000-3304.2019.19030 shu

Cellular Structure and Warpage of Microcellular Injection Molded Electrical Boxes Fabricated at Higher Injection Speeds

Kai-qiang Guo ,
Han-xiong Huang ^,

Department of Industrial Equipment and Control Engineering, Guangdong Provincial Key Laboratory of Technique and Equipment for Macromolecular Advanced Manufacturing, South China University of Technology, Guangzhou 510640

Corresponding author: Han-xiong Huang, mmhuang@scut.edu.cn
Received Date: 4 February 2019
Revised Date: 2 March 2019
Available Online: 12 April 2019

A series of microcellular polypropylene (PP) electrical boxes were fabricated by microcellular injection molding with supercritical nitrogen (Sc-N₂) as the physical foaming agent. The effects of higher injection speeds (250 and 290 mm/s) on the cellular structure, unfoamed skin layer thickness, and warpage of the microcellular boxes were quantitatively investigated. According to the simulated gapwise distributions of shear rate, temperature, viscosity, and pressure of PP melt/Sc-N₂ solution at the last moment of the filling stage in the molding of the PP boxes, the formation mechanisms of cellular structure and skin layer within the walls of the boxes were analyzed in detail. It was demonstrated that microcellular cells with smaller diameters (less than 90 μm) were developed at the core layer of the boxes. This is mainly because higher injection speeds led to higher pressure drop rate and shear rate, which thereby promoted bubble nucleation. It should be noted that tiny cell area with diameters less than 20 μm and more regular shape appeared near the skin layer of the boxes. This is due to the synthetic effects of higher shear stress combined with lower melt temperature. Along the filling direction, an exponential relationship was found between density and mean diameter of the cells within box sidewall, and the skin layer thickness increased in a nearly linear manner. The latter is mainly attributed to an almost linear decrease in the corresponding surface temperature. Warpages at the open end of the boxes were decreased through increasing the injection speed from 250 mm/s to 290 mm/s. The results demonstrate that more uniform and compact cellular structure as well as thinner unfoamed skin layer is beneficial to lowering the warpage of microcellular injection molded parts. This is worthy of further investigation in the future work.
- Microcellular injection molding,
- Supercritical fluid foaming,
- Injection speed,
- Polypropylene,
- Cellular structure,
- Warpage
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