Method "B" Run
General Description
Calculations: Method B
Method B is an assessment of a material's flow characteristics based on the
volumetric displacement rather than weight of extrudate with time as in Method
A. Unlike Method A, no cutting and weighing of the extrudate is required to
perform a Method B test. Results from Method B test can be expressed directly
as Melt Volumetric Flow Rate (MVR) in ml/10 min. To relate the results of a
Method B run back to Method A, the apparent melt density must be known.
The determination of the apparent density is illustrated in the previous section
using Method A/B. Without the apparent melt density MFR cannot be
calculated using Method B. Some companies use MVR directly without ever
determining MFR.
The piston’s downward travel time is determined from a counter initiated by an
optical sensor or digital encoder. The LED senses an opaque flag on a
transparent tape hung off the top of the piston rod. With all Dynisco Polymer
Test Systems flags, Method A and B
start
in the same place. Flags may be 1/8,
1/4, 1/2, or 1". Multiple flags are discussed in later. Recently, Method B has
become the more common test because it is simpler to run and more precise for
routine analysis. In addition, the encoder system makes it possible to get as
many as 15 results from one run.
Flow rate for method B is computed as follows:
MVR
MFR
R L
2
600
T
Where R= radius piston (cm), T is time to traverse the distance L (sec), L =
length of flag (mm),
= apparent
melt
density of polymer (g/cc)
You should determine the melt densities for your material using your own melt
indexer. Variations in technique and difference in material grades can cause
differences from user to user. The following table of melt densities can be used
as a general guide if you get values which differ by more than 10% from these
chances are you are doing something incorrectly. Fillers, reinforcing agents etc.
tend to increase the melt density of the material. Notice that apparent melt
density is a function of temperature. In general the solid state density is a very
poor estimate of the melt density and should not be used.
Some typical Melt Densities
Series 4000 Melt Indexer Operation Manual
Method "B" Run
32
