MFI Test Machine

[jonturka]

Hello,

I am asked if it is necessary to make MFI test for incoming materials. I said it is nice to have, but I have never used this machine before and I do not know exactly how to use information obtained from test. I would like to ask for your short feedbacks.

We process ABS, MABS, PP and we work with international reliable suppliers. My process is not precise as much as medical devices.
My questions;
1- How to use the information from MFI test ? Rejecting the lot before pumping to silo or adjust the parameters accordingly and warn the company?
2- Is it common that they send out of spec material companies like BASF ?
3- Can I use MFI test machine for other purposes than accepting lot ?

[Suhas]

Hi Jonturka,
Yes, it is nice to have. If you are using only virgin material then the certs supplied with the resin should be good. For your questions:
1. Not sure if you can reject the lots without any basis. Also remember that this is a very 'operator dependent' and 'filler(glass etc) dependent' test and is not consistent. You may reject perfectly good material. Adjusting some molding parameters is always required.
2. Not sure, I do not believe it is common but can happen. I have written the book on Scientific Molding but I do sometimes forget to collect some data or make a mistake. To err is human
3. Yes, you can use to study material degradation. Test the molded parts and the base resin. There will always be an increase in MFI - dropping viscosity - tells you degradation and filler breakage. Talk to the manufacturer and ask them the % acceptable drop. They all differ and can go up to even 25% acceptable.
Regards,
Suhas

[brentb]

The batch/lot cert should show MFI. Your purchase contract and resin spec sheet should show nominal or "advertized MFI Generally a molder will accept plus or minus 2 on MFI, sdepending as you say application.
Just the fact that resin companies know you test will sometimes lead to tightened MFI on shipments, I mean like when you buy rail cars and tank trucks of resin.

As learned in Suhas book, a good robust process will tolerate variations in MFI, but not humungous variations.

Keep on Molding!

brent

[dhoffman]

Here are my thoughts on the questions:

1- Where I worked before we used MFI test to inspect incoming raw material (this was back in the mid 90's anyway). We compared the value to the value and range on the spec sheet. If it was suspect, we ran it again and then had others run the test also to verify consistency of the results. We used the data to accept/reject a given lot of material, and also to let the processor know if he may see differences in filling/pressure/quality based on the variation we saw.
2- I don't think it is common that they send out of spec material, but then again if people are not checking it how do we know? Do we just chalk it up to process variation or whatever? I did reject a lot of material from a certain high level supplier. They didn't believe our MFI results so we sent them material for them to test. They confirmed our data and agreed to replace the lot of material with a new lot that met the spec. Came to find out back then that material suppliers may not test every lot of material, even though you may still find a value on the data sheet.
3- It is a good test to get an idea of molecular weight. As such and as Suhas already mentioned, it can give indications of material degradation. He mentioned 25% variation, but I believe Mike Sepe says 40% may still be acceptable. May I also suggest you search "ptonline melt flow index mike sepe". You will find a series of 7 or 8 articles he has written on this test recently.

EDITED: forgot to mention that MFI is NOT a good indication of how the material will mold. It can (and will be at some point) be very misleading if you are using it to select one material over another for viscosity reasons. You need to understand the test, what it is intended for, and don't over use it.

Good luck.
-Dave

[Suhas]

The last point by Dave is a good point to keep in mind. (Thanks Dave). MFI is not a good indication of how the material will mold, especially for a particular part design. Beaumont Inc has done a lot of excellent work on that.
Also, yes Mike Sepe is THE guy!
Suhas