Fountain flow

The parameters of mould filling, packing and cooling have been shown to be more important to final part quality than the plastication stage. All of these processes will be described in greater detail in Chapter 8. 

Fountain Flow material pushed progressively against cavity wall as later hotter material penetrates into centre of melt stream.  

Contact on tool surface rapidly cools melt to form skin layer.  

This chapter has presented a general introduction to polymers and plastics and some of their important properties that relate to injection moulding. There are more detailed chapters on thermoplastics (Chapter 6) and thermosets (Chapter 7) later in this book. 

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Volume central spout volume fountain Flow rates... 

Seek for that Stone which has no fleshly nature, but out of which a volatile fire is extracted, whence also this stone is made, being composed of white and red. It is a stone, and no stone therein Nature alone operates. A fountain flows from it. The fixed part submerges its father, absorbing it, body and life, until the soul is returned to it. And the volatile mother like to him, is produced in her own kingdom and he by his virtue and power receives greater strength. The volatile mother when prepared surpasses the sun in summer. Thus the father by means of Vulcan was... 

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 volume of liquid in these experiments must be large enough so that circulation flow near the piston does not influence the flow pattern in other parts of the liquid. These experiments were performed with different liquids, both Newtonian and non-Newtonian (shear-thinning). The introduction of tracers allowed the authors to obtain pictures showing the movement and deformation of the tracers when they appeared near the front of the stream. In the beginning, a Poiseuille-like velocity profile is present (Fig. 4.51 a). Then the material reaches the region of fountain flow,... 

Figure 4.S1. Consecutive stages of tracer deformation in the region of fountain flow.

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. 

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). 

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. 13.14 Numerical simulation of the velocity field behind an advancing liquid front, moving at constant speed inside a two-dimensional channel. Calculations were carried out with a standard, general purpose FEM program. [Reprinted by permission from H. Mavridis, A. N. Hrymak and J. Vlachopoulos, A Finite Element Simulation of the Fountain Flow, Polym. Eng. Set, 26, 449 (1986).]...

Clearly, according to this model the rate of elongation increases with injection rate, with decreasing gap and increasing n. Because the shape of the front is not flat but, as shown in Fig. 13.16, bends backward and becomes tangent to the walls aty = H/2, the fluid elements that were oriented by the fountain flow in the y direction are deposited on the cold wall with an x-direction orientation. 

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).]...

Fig. 13.22 Schematic representation of stable (left) and unstable (two right) fountain flows as causes of surface defects. [Reprinted by permission from A. C. B. Bogaerds, G. W. M. Peters, and F. P. T. Baaijens, Tiger Stripes Instabihties in Injection Molding, in Polymer Processing Instabilities, S. G. Hatzikiriakos and K. B. Migler, Eds., Marcel Dekker, New York, 2005.]...

Multicomponent Systems The flow kinematics of the multicomponent system is of considerable interest in molding. Vos et al. (59) used multilayer polymer tracers to study experimentally and to simulate the fountain and reverse fountain flows occurring in the driven and driving piston regions of the simple capillary experimental device shown in... 

Fig. 13.24 Schematic representation of the fountain and reverse fountain flows at the driving and driven pistons.

Peters et al. (46) utilized their fourth-order approximation of the fountain flow velocity field, Eqs. 13.1-9 and 13.1-10, and the particle tracking numerical technique they incorporated, to calculate the temperature and conversion fields in that region. They assumed that the very flow front material particles experience an adiabatic thermal history, which is reasonable. 

B. Friedrichs and S. I. Gujeri, A Novel Hybrid Numerical Technique to Model 3-D Fountain Flow in Injection Molding Processes, J. Non-Newt. Fluid Meek, 49, 141-173 (1993). 

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. 

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