Fountain Flow Effect

Previous numerical studies have been performed by several authors (Kamal et al. 1986, 1988 Mavridis et al. 1986, 1988 Coyle et al. 1987 Zheng et al. 1990 Jin 1993 Sato and Richardson 1995 Bogaerds et al. 2004 Baltussen et al. 2010 Mitsoulis 2010). These investigations have shown that the fountain flow has significant effects on the melt-front temperature, and also on the molecular or fiber orientation distributions in the skin region near the cavity waU. 

The levels Ci and C2 are related by the following condition, based on mass conservation. 

The same method can also be used for the simulation of fountain effect on orientation (Crochet et al. 1994). 

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Hence, uz -C ux, uy and uz can be ignored. We must point out that this velocity plays a significant role in heat transfer and orientation in the flow front region, because the free flow front is dominated by what is usually referred to as a fountain flow effect. 

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. 

No thermal boundary conditions are required along the edge boundaries in the X -X2 plane due to the neglect of the thermal conduction in the x -X2 plane. The inlet temperature is assumed uniform and taken as the injection melt temperature. The flow-front temperature boundary condition requires a special treatment to mimic the fountain flow effect, which will be discussed later in Chap. 8. 

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. 

The true 3D numerical approach developed in the past few years [1-2] is applied to the simulation. Hele-Shaw approach method is not suitable for the simulation because it lacks transverse flow information and the fountain flow effect around the flow advancement. Under the circumstances, the thermal... 

Correct modeling of the flow near the front of a stream requires a rigorous solution of the hydrodynamic problem with rather complicated boundary conditions at the free surface. In computer modeling of the flow, the method of markers or cells can be used 124 however this method leads to considerable complication the model and a great expenditure of computer time. The model corresponds to the experimental data with acceptable accuracy if the front of the streamis assumed to be flat and the velocity distribution corresponds to fountain flow.125,126 The fountain effect greatly influences the distribution of residence times in a channel and consequently the properties of the reactive medium entering the mold. 

Detailed kinematic investigations of flow near the front of a stream were undertaken.284 A diagram of the experimental device is shown in Fig. 4.49. In the experimental procedure, a liquid was placed in a chamber with transparent walls above an aluminum piston, which was driven downwards by connection to a suitable drive. This resulted in the appearance of streams inside the liquid,and three different flow zones could be distinguished. The so-called "fountain effect discussed in Section 2.11 appeared near the free surface, while a reverse fountain flow was observed below the moving surface. It is interesting to note the movement of two liquids with different densities, when one liquid is used as a piston to push the other (analyzed experimentally and theoretically).285 If the boundary between the two liquids is stationary and the walls of the chamber move at constant velocity, then the pattern of flow is as shown in Fig. 4.50, where flow trajectories corresponding to front and reverse fountain effects are clearly shown. Two other flow patterns -developed flow inside the main part of the chamber and circulation near the surface of the aluminum piston - were also observed. 

The term fountain effect ox fountain flow was coined and discussed by Rose (18), and it is essentially the reverse of the flow observed near a plunger emptying a fluid out of a channel of the same cross section. The two-dimensional flow in the... 

Fig. 13.18 Predicted velocity field showing fountain flow around the melt front region for non-Newtonian fiber suspension flow at about half the outer radius of the disk. The reference frame is moving with the average velocity of melt front, and the length of arrow is proportional to the magnitude of the velocity. The center corresponds to z/b = 0 and wall is z/b = 1, where z is the direction along the thickness and b is half-gap thickness. [Reprinted by permission from D. H. Chung and T. H. Kwon, Numerical Studies of Fiber Suspensions in an Axisymmetric Radial Diverging Flow The Effects of Modeling and Numerical Assumptions, J. Non-Newt. Fluid Mech., 107, 67-96 (2002).]...

Flows in injection molding are dominated by viscometric flows, but extensional flows are also encountered, for example, in the mid-surface of a center-gated cavity, or in a cavity having a sudden change in thickness, or in the fountain flow . Calculations of flow through runners and gates usually need to consider the effect of extensional viscosity (Brincat et al. 1998 Gupta 2000). 

Mavridis H, Hrymak AN, Vlachopoulos J (1988) The effect of fountain flow on molecular orientation in injection molding. J Rheol 32 639-663 McLeish TCB, Larson RG (1998) Molecular constitutive equations for a class of branched polymers the Pom-Pom polymer. J Rheol 42 81-110... 

Tadmor [136] used this description to interpret the orientation profile in amorphous polymers (see Fig. 15.33) the orientation in the skin is due to the fountain flow and the secondary maximum to the shear flow behind the front, with, in both cases, some relaxation of the orientation during cooling. This can be generalized to semicrystalline polymers, where the same type of orientation profile is found for the crystalline phase (e.g. [137]), even if it is sometimes difficult to find an orientation maximum at the surface, perhaps for experimental reasons. Additional effects may be introduced by the packing stage. The packing flow may induce a new secondary maximum in the orientation profile, but lower than the first one [137]. 

Weldline is a serious problem in injection moldings which causes visual defects and reduction of mechanical properties. The main factors leading to the reduction are considered to be poor intermolecular entanglement at the weldline interface, molecular orientation indnced by the fountain flow, and the stress concentration effect of surface V-notch and so on [1-6]. However, in conparison to other factors, there are less papers regarding molecular orientation in weldline region. Only a few methods for detecting molecular orientation have so far been reported, for exanple, inlfared dichroism [7] and observation of birefringence [8]. However,... 

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