One can check the shear/viscosity curve to see where the melt viscosity will
drop off substantially and design that into the process.
Just raising the process temperature could lower the shear rate and be
MFI is such a coarse number. MW/MWD would probably give you more useful
Wrong! In practice, MFI is measured at different temperatures and the
higher the test temperature is, the lower MFI gets!
The original question was "how to increase MFI of HDPE during
extrusion", which implied to me the question "how to improve flow",
because "MFI during extrusion" is not a test.
If the question would have been "how to increase MFI of HDPE"
your replay would be valid, except, that it's not true that you can't
alter the materials MFI. Increase shear and you can decrease molecular
weight and thus increase MFI ...
First of all, that's the wrong way around - higher temperature would
lead to a higher MFI - not the other way around. So you're wrong there
to start with.
When I test MFI, I test it to a "standard" (as does everybody else). The
standard specifies *everything*, cylinder size, die size, load,
temperature etc. Under these circumstances, it is not possible to alter
the shear (or anything else), therefore it is not possible to alter the
So I'm not wrong, you are!
The only way to alter a materials MDF, is to get another material with a
Oops, that was my mistake, in the heat of the battle! Of course I meant "the
higher MFI gets",
according to my very first reply.
Well I guess you still didn't get my point. I assumed the original note
asked for ways
to improve flow IN AN EXTRUDER and not in the MFI instrument.
Rolf, You almost always provide very helpful answers to post,
including a number of useful links. This time you have come across
very harshly and with information that I don't think is accurate.
While MFI can be measured at very temperatures, to do so without
reporting explicitly the temperatures is very misleading. And simply
increasing the temperature in order to lower the MFI to an arbitrary
number is as helpful as adjusting the number by changing the units of
measurement. Measuring MFI in grains/min will always insure a real
high melt flow number, but is that what we really want?
I know that you know better, but I think things are getting muddled
here, in part due to the lack of clarity in the original post, as you
have already pointed out.
If my answer sounded a bit rude I apologize, but I think my contribution was
valid, since the original question referred to extrusion and not to the
measurement of MFI, in my opinion. Only the questioner could clarify what he
actually meant ...
Is absolutely correct. That's the problem with MFI, it can be and is
measured at any temperature using a number of different weights. That said,
there are commonly accepted conditions. Polyethylene is typically measured
at 190 C, polypropylene at 230 C. But it's quite common to find the melt
indexers of a compounder's QC lab set a one temperature and everything gets
measured at that temperature regardless of what it is. The typical weight
is 2.16 kg, but companies often use heavier weights (5, 10 and 20 kg) if the
MFI using 2.16 kg is less than 1.0 dg/min.
Fortunately as John indicates, you need to report these conditions so that
the user understands what the number being reported represents.
Unfortunately, not everyone reports their conditions, or more commonly, not
everyone takes the time to read the conditions reported.
All that said, I also agree with Rolf that the intent of the original poster
was how to lower his melt viscosity and increase the flow of his sheet line.
Again Rolf's recommendations are right in line, increase your barrel zone
temperatures and impart more shear to the HDPE.
Well, I'm sure this offering is well out of date, but having seen the post I
have to ask what Steven actually needed to know. If he really needed a lower
viscosity (as with a higher MFI), some temperature increase or more crunch
in the screw might help. If he is stuck with a resin that doesnt suit the
work, maybe he could blend it with a softer resin to get the result and use
it up. I successfully did this using a moulding grade resin (50 MFI) as an
additive some years ago, when the resin you wanted was hard to get. MFI is
a bastard measurement which is basicly only what MFI measuring devices
measure. It depends on standard not-very-realistic extrusion conditions in
the testing device and then measuring the time it takes to extrude a weighed
sample. It is some sort of indication of the likely characteristics of the
material, and thats about all.
All the best
Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.