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MFA and MFV

Posted by admin on July 21st, 2012
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Definition

Here we present two simple yet important design and process parameters: melt-front velocity (MFV) and melt-front area (MFA). As its name suggests, melt-front velocity is the melt-front advancement speed. Melt-front area is defined as the cross-sectional area of the advancing melt front: either the length of the melt front multiplied by the thickness of the part (see the diagram below), the cross-sectional area of the runner, or a sum of both, if the melt is flowing in both places. At any time, the product of local MFV and MFA along all moving fronts is equal to the volumetric flow rate.

FIGURE 1. Melt-front velocity and melt-front area. Note that a constant volumetric flow rate does not necessarily guarantee a constant velocity at the advancing melt front, due to the variable cavity geometry and filling pattern. With a variable MFV, the material element (shown in square) will stretch differently, resulting in differential molecular and fiber orientations. Varying MFV For any mold that has a complex cavity geometry, a constant ram speed (or, equivalently, a constant volumetric flow rate) does not necessarily guarantee a constant velocity at the advancing melt front. Whenever the cross-sectional area of the cavity varies, part of the cavity may fill faster than other areas. The figure above shows an example where the MFV increases around the insert, even though the volumetric flow rate is constant. This creates high stress and orientation along the two sides of the insert and potentially results in differential shrinkage and part warpage.

Equation

The relationship of volumetric flow rate, MFA, and an averaged MFV can be expressed as:

How flow dynamics affect orientation

During the filling stage of the injection molding process, the polymer molecules and reinforcing fibers (for fiber-filled polymers) will orient in a direction influenced by the shear flow. Since the melt is usually injected into a cold mold, most of the orientation in the vicinity of the part surface is almost instantaneously frozen-in, as illustrated below. The state of molecular and fiber orientation depends on the flow dynamics of the melt front and the evolution of fiber orientation. At the melt front, a combination of shear and extensional flows continuously force the fluid elements from the center core to the mold wall, a phenomenon commonly referred to as the “Fountain flow.” Fountain flow behavior greatly influences the molecular/fiber orientations, especially in areas close to the part surfaces.

FIGURE 2. Fiber orientations on the part surface and in the center mid-plane of the part Why constant MFV is important The dynamics of the melt front are perhaps the least well understood aspect of mold filling, and are beyond the scope of this design guide. However, it is well recognized that the higher the velocity at the melt front, the higher the surface stress and the degree of molecular and fiber orientation. Variable orientation within the part, as a result of variable velocity at the melt front during filling, leads to differential shrinkage and, thus, part warpage. Therefore, it is desirable to maintain a constant velocity at the melt front to generate uniform molecular and fiber orientation throughout the part.

Flow balance

MFV and MFA are important design parameters, especially for balancing the flow during cavity filling. For example, MFA can be used as an index to quantitatively compare the degree of flow balance. More specifically, when the flow is unbalanced, portions of the melt front reach the end of the cavity while other portions are still moving. The melt-front area changes abruptly whenever such an unbalanced situation occurs. On the other hand, balanced flow generally has a minimum variation of melt-front area in the cavity. For a given complex part geometry, you can determine the optimized gate location by minimizing the variation of MFA in the cavity. As an example, the diagram below shows the MFAs that correspond to a balanced and an unbalanced filling pattern.

FIGURE 3. a) Variation of MFA with balanced and unbalanced flows, and (b) the corresponding filling patterns.

Author: ZHILIAN MOULD
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