Mold-cavity melt fountain flow

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. 

Fig. 9 Fountain flow behaviors of polymer and diffusion of CO2 and additives into polymer melt front in a mold cavity. (From Ref. l)...

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. 

For thermosets, the velocity profile of the flow front has been established to be relatively flat and plug-shaped. A thin melt region exists between the charge core and mold surfaces, which ensures that core fluid layers move together in the direction of flow. In the case of thermoplastics, the flow front assumes a more parabolic shape, characteristic of laminar flow. However, due to the fact that solidification initiates in the material in contact with the cooler mold, resin at the core migrates toward the cavity surfaces creating a secondary flow pattern that is known as the fountain flow effect. 

---