Key Processing Parameters for Microcellular Molded Polystyrene Material

Journal of Cellular Plastics

Guan

et al. 2015

The microcellular injection-molded part usually consists of a foamed core region and two unfoamed skin layers on the cross section. This paper investigated the formation process, formation mechanism and structural characteristics of the unfoamed skin layers in microcellular injection-molded part. It is found that the unfoamed skin layers are formed in two processes namely “during filling” process and “after filling” process. The shear flow and the fountain flow behaviors of the melt in the filling stage are the main controlling factors on the formation of the unfoamed skin layer in “during filling” process, and the cooling solidification of the melt in cooling stage is the fundamental reason for the formation of the unfoamed skin layer in “after filling” process. Further studies found that the unfoamed skin layer in microcellular injection-molded part has two distinct regions, the outer region is a thin frozen layer which contains deformed and broken cells, and the inner region is a relatively thick solid-like layer which has no visible cells in. The unfoamed skin layer has a minimum thickness in the gate location. The whole thickness of the unfoamed skin layer is decreased with the increase of injection speed and mold temperature, but is slightly affected by melt temperature.

Section: Introductionmentioning

confidence: 99%

Formation mechanism and structural characteristics of unfoamed skin layer in microcellular injection-molded parts

Dong

Journal of Cellular Plastics

Guan

et al. 2015

“…Behravesh et al believed that a high pressure drop rate of the polymer melt is required in MIM for a fine bubble morphology with a small bubble size and a high bubble density. They also concluded that shot size is a dominant factor affecting bubble morphology by MIM of a small part having a weight of about 10 g. By using a mold, which is able to produce 10 cylindrical samples with an outer diameter of 12.5 mm, Leicher et al investigated the effect of processing parameters in MIM on bubble morphology. They reported that a higher injection speed, a lower melt temperature or a smaller weight reduction contribute to smaller bubble size and homogeneous bubble morphology.…”

Section: Introductionmentioning

confidence: 99%

Research on formation mechanisms and control of external and inner bubble morphology in microcellular injection molding

Wang

Polymer Engineering & Sci

Wang

et al. 2014

This study investigates the formation mechanisms and control of external and inner bubble morphology in MIM. First, the related theories about foaming and filling flow are analyzed. Second, the assumptions for the formation of inner bubble morphology, external bubble morphology, and the compact skin layer in MIM process are proposed based on theoretical analysis. Finally, experiments of MIM process are conducted to verify the theoretical assumptions. In addition, gas counter pressure (GCP) and rapid mold heating and cooling (RMHC) technology are used for control of bubble morphology. It is found that foaming process in MIM can be divided into foaming during filling and foaming during cooling. Foaming during filling produce oriented and deformed bubbles while foaming during cooling produce spherical or polygonal bubbles. As the bubbles formed by foaming during filling can reach melt flow front, they will be pushed to the cavity surface where they are stretched further and frozen to generate the silver or swirl marks. The compact skin layer is formed due to the redissolution of the gases within bubbles into polymer melt and also restraint of foaming by high cavity pressure. GCP and RMHC are two effective methods for controlling external and inner bubble morphology. POLYM. ENG. SCI., 55:807–835, 2015. © 2014 Society of Plastics Engineers

“…All of the above studies showed that it is not an easy work to obtain an ideal cell structure in microcellular injection molding, and many efforts have been made in the cell structure control technology. Leicher et al investigated the influence of key processing parameters on the morphology of the microcellular injection‐molded porous structures. It is concluded that the increase of the injection speed and the decrease of the polymer melt temperature will decrease the pore or cell sizes, whereas the increase in the degree of weight reduction will increase the pore or cell sizes.…”

Section: Introductionmentioning

confidence: 99%

The cell forming process of microcellular injection‐molded parts

Dong

J of Applied Polymer Sci

Guan

et al. 2014

In this article, we studied the cell forming process of microcellular injection-molded parts. Using a modified injection molding machine equipped with a Mucell V R SCF delivery system, microcellular-foamed acrylonitrile-butadiene-styrene parts with different shot sizes were molded. The cell structure on the fractured surfaces along the direction both vertical and parallel to melt flow in the molded parts was examined. The results showed that a regular spherical cells region and a distorted ellipsoidal cells region exist in the molded parts simultaneously. The length of the distorted cells region along the melt flow direction in the molded parts remained basically unchanged for different shot sizes and it is about 195 mm away from the flow front in this study's conditions. The cell formation mechanism was analyzed, two cell forming processes in microcellular injection molding, the "foam during filling" process and the "foam after filling" process, were proposed. It was also found that the melt pressure in the filling stage is the dominant factor affecting the cell forming process, and there is a critical melt pressure value in the filling stage, 20.9 MPa, as the dividing line of the two cell forming processes in this study.