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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 test 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 laboratories use MVR directly without ever
determining MFR.
The piston’s downward travel time is determined from a counter initiated by
a digital encoder. The encoder senses
distance travelled by following the bottom of the test weight(s) which are at the top of the piston. With all Dynisco
Polymer Test Systems flags, Method A and B start in the same place. Flags may be any length desired and a test can
have any number as long as the total distance of flags is not longer than the distance from the start point and where
the piston would land on the top of the die (≈25.4mm). Recently, Method B has become the more common test
because it is simpler to run (hands free after material load) and more precise for routine analysis. In addition, the
encoder system makes it possible to get more from one run.
Calculations for the Method B test are as follows:
Where R= radius piston (cm), T (sec) is
time to traverse the distance L, L= length of flag (mm), ρ = apparent melt
density of polymer (g/cc).
You should determine the melt densities for your material using your 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, most likely, something is being
done incorrectly. Fillers, reinforcing agents etc. tend to increase the melt density of the material. Notice that appar-
ent 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
Temperature(ºC):
LDPE:
HDPE:
Polybutene-1:
Polypropylene:
120
.797
-
.806
.880
130
.791
-
.800
.872
140
.785
-
.794
.864
150
.780
.780
.787
.852
160
.777
.777
.780
.840
170
.770
.770
.774
.819
180
.765
.765
.767
.758
190
.760
.760
.760
.754
200
.755
.755
.754
.750
210
.748
.748
.746
.746
220
.744
.738
.740
.742
230
.738
.738
.733
.738
240
.733
.733
.726
.734
250
.737
.727
.719
.730
P/N:974179
Rev: 0918 ECO: 49667
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