increase MFI of HDPE

You could increase your temperature settings or generate more shear by making your screw design more severe ...

"steven lauwers" schrieb im Newsbeitrag news:BjNsc.130061$x snipped-for-privacy@phobos.telenet-ops.be...

In message , Rolf Wissmann muttered:

These would help 'flow', but would not increase the MFI. MFI is a property of the HDPE itself measured at a fixed set of conditions, you can't alter the materials MFI - only the melt viscosity.

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 counter productive.

MFI is such a coarse number. MW/MWD would probably give you more useful information.

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 ...

"andy" schrieb im Newsbeitrag news: snipped-for-privacy@NOHAMbigfoot.com...

In message , Rolf Wissmann muttered:

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 materials MFI.

So I'm not wrong, you are!

The only way to alter a materials MDF, is to get another material with a different MFI.

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.

John

Rolf

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.

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.

"andy" schrieb im Newsbeitrag news: snipped-for-privacy@NOHAMbigfoot.com...

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 ...

"John Spevacek" schrieb im Newsbeitrag news: snipped-for-privacy@posting.google.com...

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 Ian Macmillan