Elsevier

Polymer

Volume 48, Issue 19, 10 September 2007, Pages 5486-5492
Polymer

Polymer Communication
Hierarchical crystalline structure of HDPE molded by gas-assisted injection molding

https://doi.org/10.1016/j.polymer.2007.05.047Get rights and content

Abstract

To understand the crystalline morphology of the parts molded by gas-assisted injection molding (GAIM), in this work, the hierarchical structures and the crystalline morphology of gas-assisted injection molded high-density polyethylene (HDPE) were investigated. According to the comparison between the results of the GAIM part and those of the conventional injection molded counterpart, it is found that gas penetration can remarkably enhance the shear rate during GAIM process and oriented lamellar structure, shish–kebab structure and common spherulites arise in the skin, subskin and gas channel region, respectively, owing to the different shear rate in these regions. Meanwhile, cooling rate also plays an important role in the formation of the oriented crystalline structure.

Introduction

Gas-assisted injection molding (GAIM), an innovative molding process [1], [2], has been developed on the basis of conventional injection molding (CIM). Its main feature is that the mold cavity is partially filled with polymer melt, and then compressed gas penetrates the molten polymer and drives it further into the mold end until the cavity is completely occupied. A schematic representation of this technology is shown in Fig. 1. GAIM has drawn intensive attention for its excellent advantages [3], [4], [5], [6], such as significantly reducing material cost, clamping tonnage, cycle time and prolonging the life-span of injection machine. In addition, some defects encountered in CIM, such as residual stress, warpage, sink marks can be substantially eliminated, especially for the large parts whose quality and rigidity are the main concerns.

It is well known that polymer melt is subjected to a complex thermo-mechanical field characterized by rapid cooling rate and severe stress field during CIM process, and these fields vary along the flow path and through the thickness of the parts. As a result, a CIM part usually shows an intrinsic heterogeneous microstructure, featuring a gradual and hierarchical variation of the morphology that evolves throughout the part. Many studies [7], [8], [9] showed that neat crystalline polymers molded by CIM are always characterized with multilayered structure in accordance with the crystalline morphology and size, that is, a highly oriented nonspherulitic skin, a row or shear-nucleated spherulitic subskin, and a typically spherulitic core.

It should be noted that, during CIM process, the melt is merely confined by the mold wall, i.e. a rigid and cold medium. However, during the gas penetration of the GAIM process, the melt is confined not only by the mold wall but also by the compressed gas. Whether the different confining media with widely different physical properties would lead to distinct different flow behaviors, and then the flow behaviors give rise to different microstructures? Recently, we reported the unusual phase morphology of PC/PE blend molded by GAIM, in which the stronger shear field brought by gas penetration is responsible for such a phenomenon [10], which has substantiated our hypothesis. Now that the unusual phase morphology can be formed during GAIM process, could fascinating crystalline morphology come into being? Unfortunately, little work has been done in this field.

In this communication, our interest is in the crystalline morphology of neat HDPE molded by GAIM. For comparison, the crystalline morphology of that molded by CIM was also investigated. From a practical point of view, attempting to explore the crystallization characteristics during the GAIM process which has been widely used in practical production is of great significance.

Section snippets

Material

The resin used in this study is HDPE (5000S). It was a commercial product of DaQing Petroleum Chemical Co., China, supplied in pellets with number-average molecular weight of 5.28 × 105 g/mol. Its melt flow rate is 0.9 g/10 min (21.6 N, 190 °C).

Sample preparation

Using the same mold depicted in literature [10], two kinds of columnar parts of the same shape were molded, respectively, by GAIM and CIM. In order to provide a routeway for gas penetration, the main body of the parts is designed in a columned shape. The main

Results and discussion

Fig. 2a and b shows the 2D-WAXS patterns in different zones of G16 and CIM part. For G16 (Fig. 2a), one observes obvious reflections of (110) plane of the skin and subskin in equatorial direction, which indicates that there is obvious orientation along the flow direction (i.e. vertical direction). In the gas channel zone, one observes two circles rather than arcs, indicating a random orientation. The orientation for G19 layer was also characterized via 2D-WAXS which appeared to follow the same

Conclusions

Apart from the common spherulites, parallel lamella stacks and fascinating shish–kebab structure are formed in the GAIM part. The fierce shear rate brought about by the gas penetration and the fast cooling rate are two principal factors in such a structure formation. Since the shish–kebab structure may greatly improve mechanical properties, naturally, it will be our future work to study the further mechanism of such preferred crystalline morphology formation and the relationship between such

Acknowledgements

The authors gratefully acknowledge the financial support of this work by the National Nature Science Found of China (Grant nos. 10590351, 50673067 and 10372095) and the Major State Basic Research Projects (Grant no. 2005CB623808).

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