Melt front velocity

MFV melt front velocity Mn number average molecular weight... 

Figure 4 Melt front velocity in the end gated mold...

Variable orientation as a result of variable melt front velocity with the part leads to differential shrinkage and the part gets warped. It is desirable to maintain a constant velocity at the melt front. Non-uniform velocity uses variable injection speed. Figure 6 illustrates the flow of polymer melt in melt front. 

During the injection phase, the plastic melt flows are speed-controlled and fill the cavity. This means, depending on the set injection speed and the set profile of the machine control, the cavity is filled under different conditions. These settings determine how the thickness of the boundary layer is formed in different areas of the molded part, that is whether the melt front is stagnant in certain areas or whether it continues to move continuously at a constant melt front velocity. 

The element side surfaces are formed by lines that connect the centroid of the triangular side and the midpoint of the edge. Kim s definition of the control volume fill factors are the same as described in the previous section. Once the velocity field within a partially filled mold has been solved for, the melt front is advanced by updating the nodal fill factors. To test their simulation, Turng and Kim compared it to mold filling experiments done with the optical lenses shown in Fig. 9.34. The outside diameter of each lens was 96.19 mm and the height of the lens at the center was 19.87 mm. The thickest part of the lens was 10.50 mm at the outer rim of the lens. The thickness of the lens at the center was 6 mm. The lens was molded of a PMMA and the weight of each lens was 69.8 g. 

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).]...

Using the preceding conservation equations and for an adiabatic system (i.e., no heat losses), subject to a prescribed inlet liquid velocity and liquid superheat (T)e0 - Tm flowing into a wettable solid matrix with porosity o, Plumb [140] determines the porosity distribution in the melting front. The approximate melt-front speed is determined from the overall energy balance and by neglecting the axial conduction and is... 

The numerical results show that the thickness of the melting front is proportional to the liquid velocity to a power of 0.4. At low velocities, the melt-front thickness can become nearly the same as the pore (or particle) size, and at very low velocities, diffusion dominates the axial heat transfer. 

Figtire 2 Foimtain flow. Formation of the frozen layer from the expanding melt front, showing also idealized shape of the velocity profile in the core behind the melt front. 

Pojman etal developed a system for studying Snell s Law of refraction in FP. Using trimethylolpropane triacrylate with 47% by mass kaolin clay (Polygloss 90), they created a formulation with the consistency of a putty, which could be molded into desired shapes. Viner et a used fillers that were inert but melted, so-called phase change materials, in an attempt to lower the front temperature without significantly reducing the front velocity. 

In the BETA facility two experiments were conducted on the failure of a cylindrical concrete wall which is eroded on the inner side by a heated melt while being cooled outside by stagnant water. No decrease of the radial erosion velocity is observed when the melt front approaches the outer concrete surface and wall failure occurs after the residual wall thickness is reduced to about one centimeter. The melt relocates into the water annulus. Melt/water contact in the second test led to an energetic steam explosion which destroyed the crucible and caused considerable damage of the facility. Applicability of the experimental results for LWR severe accidents is discussed. 

Finally, when two melt fronts coalesce, forming a weld line, the boundary conditions are that the pressure and normal velocity be continuous that is. 

A complete mold-filling computer simulations requires the calculations of the velocity and temperature profiles throughout the flow region, including the position and shape of the advancing melt front in the cavity. 

The computations are begun by assuming an initial value of the average injection velocity and a constant initial temperature [35]. This allows us to locate the position Xp of the melt front a time instant At later. At time At, Eq. (15.3.9) is solved to obtain the pressure variation so that we can find the temperature variation from Eqs. (15.3.15) and (15.3.18). Similarly, the velocity components are obtained from Eqs. (15.3.5), (15.3.6), and (15.3.18). Integrating the x component of the velocity over the mold cross section yields the volumetric flow rate Q into the mold. The quotient QjWH is the new front velocity ... 

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