Plastic fountain flow

FIGURE A.7 Plastic fountain flow inside cavity of injection molds. 

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. 

When the plastic is flowing into the mould, the so-called fountain effect makes the plastic at the leading edge of the stream turn outwards and create a rapidly-setting surface layer. This layer has a double role it is both a slip layer and an insulation layer, protecting the increasingly slow-moving core from premature solidification. 

Laminar flow is a phenomenon exhibited by plastics as they move through a cavity or runner system. As shown in Figure 3, the outermost laminates of material freeze as they contact the cool mold wall. The next laminates in the channel will then come into contact with the wall and freeze. This continues to happen, leading to a fountain flow of material originating from the center of the channel. 

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