Mold cavity frozen-layer

Fig. 13.9 Cross-sectional view of a center-fed, disk-shaped mold cavity. Indicated schematically are the frozen-skin layer that can form during filling, as well as the nipple -shaped velocity profile.

Polymer orientation varies through the thickness of the injection-molded part owing to the fountain flow of the melt in the mold cavity. The flow at the center of the cross-section is deformed through extension and the highly stretched flow front rolls up to the cold mold surface, where orientation is frozen in a thin surface layer. The rest of the melt required to fill the cavity flows under this stationary frozen layer in more or less a plug fashion, with minimum orientation. Surface orientation in an injection-molded part can be significantly different from that in the core of the part. 

Most injection-molded parts are thin waUed, i.e., they have a small thickness compared to other typical dimensions. Therefore, one can reduce the three-dimensional flow to a simpler two-dimensional problem, using the lubrication approximation (Richardson 1972). We consider a polymer flow through a thin cavity with a slowly varying gap-wise dimension and arbitrary in-plane dimensions. Assume that x, X2 are the planar coordinates, x is the gap-wise direction coordinate. The flow occurs between two walls at JC3 = h/2. Adjacent to each wall there is a frozen layer of the solidified polymer so that the polymer melt flows between two solid-liquid interfaces at xj, = s (xi,X2) and JC3 = s x, x2) (see Fig. 3.1). 

The plastic flows inside the cavities to form the part. The flow of a plastic materials inside the mold is characterized by fountain flow as described in Figure A.7. The hot plastic flows through a gate and then into runners in the mold and finally into the cavity of the injection mold. The hot plastic flows at the center of the gap until it reaches the edge of flow and then flows to the walls of the cavity and cools. This results in a thin frozen layer of plastic on the mold walls and forms a skin layer of plastic. The hot plastic in the middle of the gap is called the core of plastic. The thickness of the core can be adjusted with processing conditions of pressure, injection speed, mold temperature, and melt temperature. 

Another important result to review from the cooling phase is the frozen layer fraction. The frozen layer fraction reaches 100% for both cavities therefore it is guaranteed that both parts are solidified at the end of the cycle. This is the same result when molding the parts with HIPS (Fig. 5). 

The flow-induced stress plays an important role especially in thin-wall molding, for the incomplete relaxation of polymer molecules that freeze too fast and the less shrinkage that generates less the thermal-induced stress. Frozen layers on the surface of mold cavities act as poor thermal conductors. The molecular orientation in the hot core is allowed to relax. Yet the whole parts are cooled and frozen in a very short period and thus could not relax completely in thin-wall molding. Therefore, the flow-induced stress is a significant amount for thin-wall parts, while the thermal-induced stress dominates in thick-wall parts. 

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