What is the Melt Flow Rate (MFR)?

The most commonly used test is usually referred to as MFR or MFI. Both terms refer to the same test and stand for melt flow rate and melt flow index, respectively. This test is popular, particularly for polyolefins, since the test is easy to do and understand and the instrumentation is inexpensive. A specified force, produced by an accurate weight, extrudes a heated plastic material through a circular die of a fixed size, at a specified temperature (Figure 12.). The amount of PE extruded in 10 minutes is called the MFR and results are reported as, for example, MFR (190, 2.16) = 2.3. This means that the temper- ature was 190˚C and a force, producing load of 2.16 kg, was used. Under these conditions, 2.3 g of the plastic material was extruded in 10 minutes.



MFR is general a low shear rate test where the shear rate may be as low as
1 sec-1. Weights larger than 2.16 kg may be used as well as different temper- atures. These will depend upon the material and the grade of material being tested (Table 12.). With UPVC a weight of 20 kg may be employed, while the suggested temperature (in ASTM D 3364) is 175˚C (347˚F). (See Flow Rate Testing).

Method A

Method A, also known as Procedure A, is a manual cut-off operation used for materials having flow rates that fall within 0.15 to 50 g/10 minutes. The piston position during the timed measurement, (that is, the position of the piston tip) is required to be between 51 and 20 mm (2.0 and 0.8 in) above the die. The test specimen can be in any form that can be introduced into the heated bar- rel. The conditions of temperature and load can be selected by consulting the standard. For example, for a PE with an expected MFR of 2, the temperature would be 190˚C and the load would be 2.16 kg. After packing a precisely measured amount of PE (in the range of 3 and 5 g) into the barrel, the sample is allowed to heat for 4 minutes with the piston in contact with the material.

60   Then the weight (2.16 kg) is placed on top of the piston. After a further 3 min- utes (usually between 2 and 4 minutes) the lower reference mark on the pis- ton must be level with the top of the barrel. The material is then allowed to freely extrude for a specified time (such as 3 minutes), after which the extru- date is carefully cut off and saved. This should be repeated three times if pos- sible, before the upper reference mark reaches the top of the barrel. The three samples are then individually weighed, to the nearest milligram (0.001g), and the average mass in grams is found. The flow rate is reported as the rate of extrusion in grams in 10 minutes (obtained in this case, by multiplying the average mass of polymer extruded (Mp) by 3.33).
Thus
MFR = MFI = Mp (600/t)   (Eqn. 16)
where:   t = the cut-off interval expressed in seconds (s) Mp = the mass of polymer extruded

Method B

Method B is also known as Procedure B and is an automatically timed flow rate measurement used for materials having flow rates that fall within 0.50 to 1,200 g/10 minutes. To ensure reproducibility, the timing device must be accurate to within ±0.1 seconds and the position of the piston foot at the end of the test must be 25.4 mm (1 in) above the die. During the measurement, the length of timed piston movement must be measured to within 0.025 mm (0.001 in) over a prescribed distance. This measurement may be achieved by using an opaque flag, hung from the piston, which interferes with the passage of light to an electronic eye. It is more common today, however, to use a mechanical encoder connected to the piston. If the system is computerized, then all the operator has to do is to select the test conditions of temperature and load by consulting the standard or the memory and load the material. The test specimen can be in any form that can be introduced into the barrel. The electronics will automatically control the temperatures and time the measure- ments (and even change the weight during the test run if required). The apparent melt density of the polymer (see Method A to B Conversion) at the test temperature must be entered to allow the calculation of the weight of extrudate from the piston displacement. Once the test has been performed then the electronics can calculate flow rate, flow rate ratios, viscosity, shear rate and shear stress. The electronics also may contain the statistical equa- tions for SPC/SQC analyses of the data for quality control purposes.

Method A to B Conversion

Conversion from Method A to Method B may be obtained on machines where an appropriate electronic optical eye, mechanical encoder or computerized control system is used. The operator runs the test as if it is a Method A test, while the machine conducts a Method B test. Upon completion of the test the apparent density is obtained using the data from both tests and setting the equations equal and solving for melt density.
By Method A, the melt flow rate (MFR) is MFR = Mp (600/t)
Where Mp is the average mass extruded (M) within the cut-off interval t (expressed in seconds).

By Method B,
MFR = R2 L DM (600/T)   (Eqn. 17)
Where R is the piston radius in cm, L is the effective length of the flag in cm, A is the apparent melt density in g/cm3 and T is the time taken for



the test measurement (in seconds). If the two MFR's are the same then, the equations may be solved for A (Table 5.). The advantage of this system is that by using an apparent density obtained from any given machine, Method B will always equal Method A and it takes into account frictional and other variations for that specific instrument.

Increasing Shear Rate and Stress

Melt Flow Rate (MFR) testing is a low shear rate test (the shear rate may be as low as 1 s-1). Larger weights and/or different temperatures may be used depending on the material and grade of material (Table 12.) to produce higher shear rates or to make the test practical. With un-plasticized polyvinyl chloride (UPVC) a weight of 20 kg/44.1 lb is often employed at a temperature (ASTM D 3364) of 175˚C/347˚F. When the test is run with such a high load, or with the even higher load of 21.6 kg/47.62 lb, then it may be referred to as "the high load melt index" or the HLMI.

Flow Rate Ratio

Since polymer viscosity varies as a function of shear rate, the single point determination by a melt indexer does not fully characterize material flow. To obtain more useful information, flow rate testing may be done with more than one load. For example, in blow molding the sag behavior of the parison is best measured by the low shear flow, while the flow through the die lips is best measured by the higher shear flow. The Flow Rate Ratio is obtained by divid- ing the flow rate obtained when a large load is used by the flow rate obtained under a smaller load. The load ratio, which is generally 10:1, may, on some instruments, be measured automatically during a single test run.

Melt Volume Index

An abbreviation used for this term is MVI. MVI is defined as the volume of thermoplastic material extruded through a die during a specified time and is expressed in cm3/10 min (MFI is in g/10 min). It is measured by performing a Method B test and doing the calculation without using the melt density.
Specifications for this test can be found in DIN 53735. A rating of MVI (250/5) means that the melt volume index, in cm3/10 min, was measured at 250˚C and under a load of 5 kg.

Source :
John Goff and Tony Whelan
Edited by: Don DeLaney