Mold filling fountain flow

The third region of flow near the front is of special interest. The important feature of this region is the fountain effect, which must be considered in modelling all types of mold filling. It is important not only for estimating the hydrodynamic flow pattern, but also because the deformation of the macromolecules near the front influences their orientation and the properties of the end product. 

Rose examined the flow pattern in a capillary tube where one immiscible liquid displaces another one. In the front end of the displacing liquid the flow pattern is one he termed fountain flow, and in the other reverse fountain flow. In polymer processing the significance of the former was demonstrated in the advancing melt front in mold filling (see Chapter 13). 

Fig. 13.23 Two-color, PP injection, high-rate mold-filling short shots documenting fountain flow instability. [Reprinted by permission from A. C. B. Bogaerds, G. W. M. Peters, and F. P. T. Baaijens, Tiger Stripes Instabilities in Injection Molding, in Polymer Processing Instabilities, S. G. Hatzikiriakos and K. B. Migler, Eds., Marcel Dekker, New York, 2005.]...

The cycle starts with the plastification of the core component in the injection unit. Then the extruder moves to the bottom position, the injection unit moves forward to the extruder nozzle to link the nozzles of the extruder and the injection unit. The extruder starts plastification of the skin component and extrudes the melted skin component into the screw antechamber of the injection unit. Thus the skin and core components are located one after the other in the screw antechamber. After the extruder moved back to the top position, the injection unit moves forward to the mold followed by a conventional filling phase. Due to the fountain flow effect the first injected material forms the skin layer followed by the second component forming the core. Compared to the standard sandwich process the injection phase of the monosandwich process is less complicated as it is identical to the conventional injection molding process. 

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. 

Injection molded plaques or bars of PLC have a skin—core structure [33]. The molecular chains in the skin regions are largely aligned in the mold fill direction while the chain orientation in the core is more or less random. The high molecular alignment in the skin layer is induced by the elongational stress in the fountain flow and is immediately frozen upon contact with the mold surface. 

Coyle DJ, Blake JW, Macosko CW (1987) The kinematics of fountain flow in mold filling. AIChE J 33 1168-1177... 

Figure 4.52 Schematic representation of flow patterns during the injection filling of an endgated rectangular mold whose width is much greater than its thickness, (a) advancement of the flow front with fountain flow and...

Kamal, M.R., Chu, E., Lafleur, P.G., and Ryan, M.E. (1986) Computer simulation of injection mold filling for viscoelastic melts with fountain flow. Polym. Eng. Sci., 26, 190-196. 

Kamal, M.R., Goyal, S.K., and Chu, E. (1988) Simulation of injection mold filling of viscoelastic polymer with fountain flow. AIChE J., 34, 94—106. 

To account for this distribution of shrinkage (orientation) Tadmor (1974) proposed the fountain flow mechanism, which occurs in the advancing front. In Figure 10.5 the flow patterns in normal mold filling are shown schematically. As the melt leaves the gate, the front is found to occupy various positions in the mold at different times. The velocity profiles in the fully developed flow behind the front are shown in Figure 10.5b. The flow well behind the front is primarily... 

The fountain flow associated with the advancing front is extremely important to the properties of materials generated by means of injection molding. In the case of homogeneous polymer systems we have already seen how the molecular orientation is affected. For fiber-filled systems, the flow at the front can lead to highly oriented fibers at the surface of... 

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