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MFA and MFV
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.