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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:
MVRρ = MFR = πR2Lρ 600
T
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
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
.
P/N: M0726(3)
| Rev: 012020 | ECO: 51182
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