Melt Flow Index

Hi,

This is Himu from Bangladesh.

I want to learn about MFR/MFI of plastic resin. How MFR is related with other properties like hardness, fragility, shrinkage, strength etc?

Could anyone reply, please?

Himu

Melt flow index, spiral flow and the rest of the flow indicators have one problem. They are lab certified standard tests. While nice indicators you run into the complexities when you make a part.

In your instance you're now faced with MFI and shear rates (specifically through small restrictions) are a variable you'll have difficulty having a lab test duplicate.

Here's another stinker - Pressure is resistance to force. Force in this case is the injection pressure. But apparent viscosity is governed by speed. Try this. Try filling the mold at the fastest possible fill rate (do an apparent viscosity test)for both materials. The speeds will be different for each. Now look at the fill pressures at the optimal fill rate.

MFI doesn't take into account apparent viscosity.

Stinker #3. What about cooling? Styrene and PC need to lose a different amount of Kcals before they solidify. The cooler it gets the viscosity changes. You only need one cool spot in the runner/cavity to change the pressures.

Personally #3 is probably bogus. I believe your confusion comes from apparent viscosity. At the same fill speeds the pressure readings will be different with identical MFI.

Send money or attend my seminars or just keep posting. This is fun. This gets tied up in the complexities of ultra long chain polymer behavior. The snooty East Coast Colleges perpetually flood the technical conferences with topics along with equation and graph laden explanations for this. However their conclusions generally don't translate into good (hands on applicable) common sense. Just some PhD feeding the publish-or-perish beast.

Comfort the Confused, continue to Confuse the Comfortable.

Hope this helps.

Bill

I haven't tested the PC's melt index here yet to see if what we are buying is actually 38 melt. I would do a rheology curve myself here with both materials but when I run the PC I am seriously pressure limited which will provide false data. Maybe I'll try to run both materials in a more capable press to see what I get for rheology.

Joe you've asked a question that could take a thesis to explain.

Some of the obvious though is the polymer chain makeup of the two - PC being more like a benzene ring than a chain and more responsive to heat than shear.

If the two were tested under exactly the same test method/conditions you'd see apple to apple results but that's the problem many molders have in making a comparison of melt index test results- results are often not presented under the same test conditions.

In this case 100 degrees C is a big difference and if the PS were taken that high for the test it would run out like water or be an index of like 100 possibly.

I don't know if this helps or not but if you look in your IDES reference book you'll see that there are like 43 + conditions that the melt index results are reported. You probably already knew all this so that's why I say I don't know if what I mentioned here has helped or not.

Hello Joe,

Just as you indicate, the Melt Flow Index is pretty much meaningless for typical injection molding processes. Yes, you can make a stiffer flowing resin fill a mold easier than an easier flowing resin relatively speaking in terms of MFI...even within the same plastic family.

I have not looked at the data but here is one possible explanation...

Take a look at the supplier's shear rate vs. viscosity graphs for each. You may find the slopes of the graphs for the two resins are very different. The CS may have a lower viscosity than the Makrolon at the SAME initial low shear rates (MFI test), but when you reach typical injection shear rates and compare the viscosity of the two at the SAME higher shear rate the Makrolon's viscosity may actually be much lower than the CS's viscosity. You can even easily calculate the actual shear rate to make sure you are looking in the right area of the graph = (4*Flow Rate)/(pi*r^3)