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Table of ContentsIntroductionMFI Test
StandardsGuidelines for Choosing the Best MethodFactors that Affect Test AccuracyStandard
Reference Materials & Proficiency TestingEquipment
Issues that Affect Test ResultsProcedural Issues that
Affect Test ResultsProcedure A TestsProcedure B TestsConclusionsAbout Tinius Olsen
Flow Index (MFI), also known as the Melt Flow Rate (MFR), of a resin is a
materials property test used in the plastics industry. The test measures the
melt flow properties of resins (in g/10 min) at a particular shear stress
(related to applied load) and temperature. The test is done with an extrusion
plastometer, which is often referred to as a melt indexer. It is used to test
virgin, compounded, and post-process thermoplastics.
MFI Test Standards
The two main MFI test standards are ASTM D1238 - “Standard Test Method for
Melt Flow Rates of Thermoplastics by Extrusion Plastometer” and ISO1133 –
“Determination of the Melt Mass-Flow Rate (MFR) and Melt Volume-Flow Rate (MVR)
of Thermoplastics”. Both the test standards measure the same property but even
slight procedural and equipment differences may provide different results. Both
offer a manual method (Procedure A or Method A) and both offer an automatic
timed flow measurement (Procedure B or Method B). Theoretically, both methods,
if done properly will result in identical test results.
Guidelines for Choosing the Best Method
There is no clear-cut reason to prove which method is ideal for a particular
organization; however there are certain guidelines that can be used to make a
Procedure A is highly useful for organizations that test infrequently,
utilise a wide range of materials, use a range of additives in their materials,
or use regrind/recycled material.
Procedure B needs a “melt density” value and is ideal for organizations that
test the same material several times over and want to reduce the chance for operator
error. Some organizations may find that the alternative Melt Volume Flow Rate
(MVR) procedure, offered by both standards more useful.
Factors that Affect Test Accuracy
Irrespective of the method chosen, two organizations may test the same
material and obtain two different test results, wondering why the
The precision and accuracy of a test are affected by a range of factors that
include the following:
- The melt indexer needs to be in good working order
- The machine must have been calibrated by a certified metrologist, checking
temperature, physical dimensions, and distance and time measurement accuracy
- The machine cleanliness needs to be maintained
- The technicians must be properly trained and use the same testing technique
- Testing procedures and requirements need to be closely followed by periodic
testing of standard reference material (SRM) or control materials.
Hence it is important to examine the actual industry test standards. A
trouble shooting guide is offered by ASTM D 1238 in the appendix of the document
and certain salient features are highlighted below.
Standard Reference Materials & Proficiency Testing
A Standard Reference Material (SRM) is used by certain labs to crosscheck MFR
test results. There is a restricted supply of certain materials available from
national metrological standards bureaus such as the NIST in the United States
but they tend to be expensive, and only offer an indication of verification for
that particular material. It also raises the question whether the material is
being used to test the machine or the machine is being used to test the
Certain labs choose to participate in Proficiency Testing Programs (PTP).
Participation is voluntary and the programs charge a fee, but they provide a
standard to compare how one’s testing practices stand with other participants.
ASTM and Collaborative Testing Service are two vendors offering PTP services although,
there may be other programs available.
The range of factors contributing to questionable Melt Flow
Index test results is wide and deep. By being aware of each one, adhering
closely to test procedures, maintaining the equipment, and following overall
good testing practices, accuracy can be improved and ensure that small problems
do not become big.
Equipment Issues that Affect Test Results
In order to ensure good test results, it is important that the testing
machine and auxiliary equipment such as scales and micrometers are verified
using equipment traceable to national metrological standards.
Certain measures that need to be taken are listed below:
- The verification procedure will check the machine dimensions, temperature
control and distance measuring devices installed on the machine, for conformance
to the relevant test standard.
- The frequency of the verification is determined by an organization’s quality
program. Annual calibration is a common practice within the industry but regular
inspection of the critical components of the test instrument must be done,
especially for consumable items that include dies, piston rods, and piston feet.
These should be replaced when required.
- Before conducting any tests, a visual inspection of all components of the testing
machine shoud be carried out. The furnace of the instrument, which contains a heated metal cylinder
with a defined bore, is leveled using equipment typically supplied by the
- The equipment must be located in an area free from vibration and excessive
air currents. Dimensional checks are to be made when the machine is cold.
- The test machine components must be cleaned after every test. No residue
from prior tests must remain on the surfaces of the metal parts involved in the
test. These surfaces are cleaned with cotton patches and cloth and/or a brass
brush. Solvents are usually not needed or recommended.
- The barrel finish must be mirror-like and free of rust, scratches, and
imperfections. The barrel is cleaned by repeated swabbing with a cotton cleaning
patch using tools normally provided by the equipment manufacturer. Some material
is a little more difficult to clean and may require the use of a brass brush to
ensure a clean surface. After cleaning the barrel, a clean die must fall to the
bottom of the barrel and make an audible “click.”
- The die outer surface is cleaned with a cotton cloth and the die bore is
cleaned using a drill and/or a brush. The bore diameter, defined in D1238 as
2.0904 to 2.1006 mm (0.0823 to 0.0827”), must be periodically checked with a
go/no-go gage. The die must be visually inspected to make sure that the entrance
of the bore is not rounded or chipped. If the die fails the go/ no-go check or
is damaged, discard it and replace it with a new, conforming die.
- The piston rod is normally cleaned with a cotton cloth but use of a brass
brush may be needed for certain materials. The piston guide (if present) must
slide freely on the piston rod. It is essential to periodically verify if the
piston is straight and that the leading edge of the piston foot is sharp and
free of burrs or damage that would cause it to rub against the barrel wall. The
foot diameter when measured with a micrometer — must be 9.4676 to 9.4818 mm
(0.3727 to 0.3733”).
Procedural Issues that Affect Test Results
Precision and accuracy can also be affected by procedural factors. Test
conditions for test temperature and test load vary by material. Some materials
have several test conditions. It is essential that comparative tests are
performed using the same test conditions. Test conditions for most materials can
be found in Table 3 of ASTM D1238.
Certain procedural issues that need to be taken care of include the
- Moisture content can be a large variable for some materials that include
ABS, PMMA, PET, and Nylon. These resins must be dried in a suitable oven under
controlled conditions before testing; some materials need to be dried under an
- Variations in sample mass and sample charging technique can affect test
results of certain materials. Use of a tool to pack material into the bore
during the charging process is common, but it can result in variations in test
results for some materials if multiple machine operators are involved in the
testing process because the amount of force used to pack the material often
varies with operators.
- Similarly, the practice of purging excess material from the bore is normally
used to move the piston closer to the starting point of the test and can be a
source of variation in the test procedure.
- When the bottom of the piston foot is 46 +/- 2 mm from the top of die, a
proper test is commenced. The piston foot is required to reach that point within
420 +/- 30 seconds after the material charging has been completed. A preheat
time is needed to remove trapped air, ensure the material is sufficiently and
uniformly melted, and that the temperature of the material in the bore
stabilizes to within +/-0.2°C of the set point before the start of the test.
- A preferable alternative to packing the material purging is to use trial and
error to measure the optimum amount of material to be charged in the bore, based
on the expected flow rate, which will result in the test beginning at the proper
- It is common to weigh the material prior to charging or use a volume
reference when charging the bore.
- The test load needs to be unloaded from the piston or its travel needs to be
stopped during the preheat period in order to meet the preheat and start test
requirements. It may also be essential to plug the orifice to prevent the
material from running out completely before test commencement.
Procedure A Tests
Extrudate cutting technique can be a factor in Procedure A tests. A spatula
or a similar cutting tool can be used to cut the material as it extrudes from
the die at specific intervals. The extruded material must be cut at the exit of
the die. It is important to cut accurately to obtain desired results. Procedure
A can be used with materials that have flow rates up to 50 g/10 min. One must
note that there are chances of error because of manual cutting at increased melt
flow values. The first cut beginning at 46 +/- 2 mm is the reportable test
result. Making successive cuts and then averaging them is not acceptable.
Procedure B Tests
For Procedure B tests, permissible piston travel distances are 6.35 mm (1/4”)
to 25.4 mm (1”). The operator chooses the distance based on the expected flow
rate. The ¼” travel is used for low flow materials, while the 1” travel is used
for higher flow rates.
As with Procedure A, the first measurement of a Procedure B test is
considered to be the reportable test result. However, several modern testing
instruments are equipped with an encoder based measuring device that will allow
the user to divide up the specified travel distance into discrete “captures” and
then average the captures to obtain the reportable test result. This practice is
acceptable as long as the specified distances are used.
Procedure B requires the use of a melt density value, which is the density of
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 generic
melt density factors for virgin PP and PE. These factors will change with
additives and processing so it is more precise to measure the actual melt
density of the specific resin being tested. Procedure A and Procedure B test can
be combined to give both weight and volume data, which allows the calculation of
In case you are suspicious of the melt flow data you have obtained, there are
plenty of options for checking and updating equipment, examining procedures, and
comparing results with those obtained by others. There is a high likelihood that
answers to variances will be easily tracked down with some good old-fashioned
About Tinius Olsen
Olsen is the leading specialist manufacturer and supplier of static tension
and/or compression materials testing machines. Tinius Olsen
machines are designed for use in Research and Quality Control to measure
material’s strength and performance.
This information has been sourced, reviewed and adapted from
materials provided by Tinius Olsen.
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