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umFi t

hw g b mFi ruBy Harry Yohn,Tinius Olsen Testing Machine Co., Inc.

t fw dx (mFi), kw

fw (mFr), d

fw pp (ud /10 )

material at a specic shear stress (related to applied load) and

temperature. An extrusion plastometer, more commonly reerred

to as a melt indexer, is used to determine the MFI o virgin,

compounded, and post-processed thermoplastics.

The MFI test is used widely among the various segments o

the plastics industry. Resin suppliers, or example, look or MFI

variations as a QC check to detect and troubleshoot variations

in their polymerization and/or compounding processes. As part

o their materials inspection procedures, some processors use the

test to determine the amounts o recycled material that can be

used in their product while still maintaining end-product speci-

cations. The diculty is that any two labs can easily come upwith dierent results, making it dicult to determine whether a

material meets a desired MFI spec. Here are some reasons why

such discrepancies occur and what can be done about them.

KNOW YOUR STANDARDS

There are two major MFI test standards:

ASTM D1238: Standard Test Method for Melt Flow Rates

of Thermoplastics by Extrusion Plastometer,

ISO1133: Determination of the Melt Mass-Flow Rate

(MFR) and Melt Volume-Flow Rate (MVR) of Thermoplastics.

Both measure the same property, but slight procedural and

equipment dierences can yield dierent results. Both oer amanual means (Procedure A or Method A) and automatic timed-

fow measurement (Procedure B or Method B). In theory, both

methods, i properly perormed, will yield identical test results.

There is really no consensus as to which method is best;

however there are some guidelines that can be used to make a

choice. Procedure A is typically most useul or companies that

test inrequently, use a wide range o materials, use a variety o

additives in their materials, or use regrind/recycled material.

Procedure B requires a melt density value and is best or com-

panies that test the same material repeatedly and want to mini-

mize the chance of operator error. Others may nd that the

alternative Melt Volume Flow Rate (MVR) procedure, oered

by both standards, is more useul.No matter what the procedure selected, it is common or two

organizations to test the same material and come up with two

dierent test results, setting o a search to determine the reason

or the dierence. A variety o actors aect the precision and

accuracy o a test. Is the melt indexer in good working order?

Has the machine been calibrated by a certied metrologist

checking temperature, physical dimensions, and distance- and

time-measurement accuracy? Is the machine clean? Are the

operators properly trained and using the same testing technique?

The chances o obtaining precise and accurate test results

can be greatly increased by closely ollowing proper testing

procedures and requirements, by machine verication, and byperiodic testing o control materials or a standard reerence

material (SRM). Even so, bad or variable MFR results attrib-

utable to the equipment and/or procedures can still be encoun-

tered. Examining the actual industry test standards offers direc-

tion here. ASTM D1238 offers a Troubleshooting Guide in the

Appendix o the document and the ollowing thoughts are based

upon it. (Consult ISO 1133 for appropriate dimensions and

procedures when testing to that standard.)

Some labs do use a Standard Reference Material (SRM) to

crosscheck MFI test results. There is a limited supply o se-

lected materials available rom national metrological standards

bureaus (including NIST in the U.S.) but they tend to be expen-

Variations in sample mass can affect MFI/MFR results, so always use

a scale or balance to weigh samples.

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34 sePtember 2011 Plastics technology

sive, and can only help veriy testing or that particular mate-

rial. Some labs maintain their own supply of reference mate-

rial, which they use to maintain in-house quality programs.

However, using a reerence material raises the question, Are

you using material to test the machine or the machine to test

the material?

Other labs choose to participate in Prociency Testing Pro-

grams (PTP). Participation is voluntary and the programs charge

a ee, but they provide a benchmark to compare how your testing

practices stand up against other participants. ASTM (astm.org)

runs one of these programs. Collaborative Testing Services (col-

laborativetesting.com) is another vendor oering PTP services.

Practices like these are useul in that they can point out when

there is something wrong. Unfortunately, they dont tell you

specically what is wrong. The range o actors that can contrib-

ute to questionable MFI test results is wide and deep. By being

aware o each one, adhering closely to test procedures, maintain-ing your equipment, and ollowing overall good testing prac-

tices, you can improve accuracy and keep small problems rom

growing into larger ones.

VERIFY YOU R E QUIPMENT

To ensure good test results, it is important that the testing ma-

chine (and auxiliary equipment such as scales and micrometers)

be veried using equipment traceable to national metrological

standards. The verication procedure will check the dimensions

o the machine, as well as the temperature-control and distance-

measuring devices installed on the machine, or conormance to

the relevant test standard. The requency o verication is deter-mined by an organizations quality program. Annual calibration

is common practice within the industry but more requent in-

spection o the critical components o the test instrument is

needed, particularly or consumable items such as dies, piston

rods, and piston eet. These should be replaced as necessary.

Prior to running any tests, visually inspect a ll components

ASTM species 420 30 sec of preheat time for most materials.

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actor in Procedure A tests. Proper cutting technique is an ac-

quired skill and requires practice to get it right. A spatula or

similar tool is used to cut material as it extrudes rom the die at

precise intervals. The extruded material should be cut directly at

the exit o the die. Automatic cutters can be useul but may not

work with all materials. Procedure A can be used with materials

that have ow rates up to 50 g/10 min. However, it should be

noted that the chances or error due to manual cutting increase

with higher melt fow values. It is also

important to note that the rst cut (start-

ing at 46 2 mm) is considered to be the

reportable test result. The practice o

making successive cuts and averaging

them together should be avoided.

For Procedure B tests, permissible

piston travel distances are 6.35 to 25.4

mm (0.25 to 1 in.). The operator selectsthe distance based on the expected fow

rate. The 0.25-in. travel is used for low-

ow materials, while the 1-in. travel is

used or higher fow rates.

As with Procedure A, the rst mea-

surement o a Procedure B test is consid-

ered to be the reportable test result. How-

ever, many modern testing instruments

are equipped with an encoder-based

measuring device that will allow the user

to divide up the specied travel distance

into discrete captures and then averagethe captures to get the reportable test

result. This practice is acceptable as long

as the specied distances are used.

Procedure B requires the use o a

melt-density value, which is the density

o the material when it is in its molten

state. The melt density is a multiplier

that converts volume back into a mass

value. ASTM D1238, Table 4 lists ge-

neric melt-density actors or virgin PP

and PE. These factors will change with

additives and processing, so it is moreaccurate to measure the actual melt

density o the specic resin being tested.

You can combine a Procedure A and

Procedure B test to give both weight and

volume data, which allows calculation o

melt density.

I you are suspicious o the melt-fow

data you are obtaining, you have plenty o

options or checking and updating equip-

ment, examining procedures, and com-

paring results with those obtained by

others. There is a good likelihood that

answers to variances will be easily tracked down with some

good old-ashioned detective work.

about the author

Harry Yohn pdu pp p t u o h, P., 1996 d 25 xp d p p. h astmsu D20.30 t Pp P, su- D20.10 ip Pp, d v k up w committee. Contact him at (215) 675-7100 hyohn@tiniusolsen.com.