Making Sense of Plastic Melt Flow Rate, Melt Volume Rate & Melt Viscosity

By Michael Sepe, Materials Analyst

If we can’t speak in tongues, we should at least speak in the same language when referring to units of measure.

One of the lesser known of Murphy’s Laws is that test results will always be in the least useful units, such as furlongs per fortnight. People whose job it is to review property data sheets can probably relate to this statement. The move to international standards has replaced foot-pounds with joules (Newton-meters), inches with centimeters, and pounds per square inch (psi) with megapascals (MPa).

The drive to so-called SI (Système International) units is unstoppable. The only three countries left in the entire world that still adhere to the so-called inch-pound system, sometimes erroneously referred to as British units, are the United States and those other industrial superpowers, Liberia and Brunei. (In the interest of full disclosure, the speed limit signs in parts of England are in miles per hour, not kilometers per hour.)

A survey of different data shows that the metric system is not as uniform as one would think. However, it would be reasonable to suppose that a property that has always been expressed in SI units would be free from any confusion. The melt-flow-rate (MFR) test has always been expressed in g/10 min, an SI expression of mass divided by a time unit, which thankfully is the one type of measurement that has always been universal. But even this fundamental measurement is not free from the ferment of the global marketplace. The changes being brought to this test have caused a lot of confusion with our clients lately, so this article is designed to take the mystery out of the data.

Volume vs. Mass
The MFR test has been defined for many years by ASTM D 1238 and ISO 1133. While there are variations in the way the test can be performed and the specific test conditions are material dependent, the values have always been reported in g/10 min. In the past few years, however, some material suppliers have started to provide the results as a volumetric flow rate rather than a mass flow rate. Generally, the units are cc/10 min.

Now, there is a certain logic in expressing flow rates in volumetric terms because in the real world of processing, the shear rate, which is the primary determinant of a material’s viscosity, is determined by the volumetric flow rate, not the gravimetric flow rate.

In addition, the volumetric flow rate measurements eliminate contributions from the density of the material. This can be an important consideration in evaluating different colors of a certain grade of material. Pigment ingredients like titanium dioxide (TiO2) are approximately twice as dense as most polymers. Materials with a dark natural color require very high loadings of TiO2 to achieve certain color matches, while achieving a black color typically employs the much less dense carbon black added at much lower levels. Melt-volume rates (MVRs) of the two colors based on polymers of the same average molecular weight will be the same, but the MFRs will not be, driven by a difference in density.

But to interpret MVRs in relation to MFRs, it is necessary to know the density of the material—not the solid-state density that can be found readily on the data sheet, but the melt density. Plastic melt density is virtually impossible to find in the literature, but it can be measured with an MFR tester.

The piston that is used to force material through the orifice of the MFR tester is scored with two lines that are 1 inch (2.54 cm) apart. With a known diameter and a known height, the volume of the resulting cylinder is easily calculated. The mass of material extruded when the piston travels 1 inch divided by this volume is your melt density. This number may be anywhere from 80% to a little more than 90% of the solid-state density, depending on the polymer.

It’s Not the Same
There is another way of using the MFR instrument to express the manner in which a material flows. This method was developed by General Electric in an attempt to obtain an actual measurement of viscosity. It is still employed on many of the company’s PBT polyesters and blends.

Viscosity is defined as resistance to flow, and it is important to understand that this property is inversely related to the flow-rate values. Typically, viscosity measurements are made in a capillary rheometer, where the shear rate can be controlled and varied. MFR tests cannot control the shear rate, but there is always a particular shear rate associated with an MFR test. With some empirical work, it is possible to come up with a relationship between melt viscosity and MFR.

The GE technique calculates the melt viscosity of a material by multiplying the time required to extrude a fixed volume of material by an instrument constant. That fixed volume of material involves that same 1-inch travel used to calculate the melt density. The units most often used to express viscosity are Pascal-seconds (Pa-sec), or poise. (One Pa-sec equals 10 poise.)

If you know the melt viscosity and the instrument constant, you can calculate the time required to extrude a fixed volume of material. Knowing the melt density allows you to calculate the mass of material extruded. And knowing the time required to extrude the mass of the material allows you to calculate both the MVR and the MFR. Simple, right?

Consider this example: One supplier of an unfilled PBT polyester provides a certification that gives the melt viscosity as 9576 poise. Another supplier sends in an “equivalent” material with a certification that states the MFR as 22 g/10 min. How do we tell if we are really dealing with equivalent materials?

In order to determine if these materials are the same, the melt viscosity of 9576 is divided by the instrument constant of 228. This gives the time required to extrude a fixed volume of 1.81 cc.

9576 poise/228 = 42 seconds

The melt density of unfilled PBT polyester is 1.18 g/cc, so the mass of the material in the standard volume is:

1.81 cc x 1.18 g/cc = 2.14g

Calculating an MFR in g/10 min now becomes a matter of solving a simple proportion. If a mass of 2.14g is extruded in 42 seconds, then how many grams will be extruded in 10 minutes?

2.14g/42 seconds = Xg/600 sec
X = 30.6 g/10 min

The MFR of the material is 30.6 g/10 min and the two materials are not equivalent. This, of course, assumes that both of your material suppliers are using the same test conditions. PBT resins are usually tested at 250°C, but some manufacturers used a constant load of 5 kg while others use 2.16 kg. Obviously, this will have a significant effect on the numbers. Read the fine print. If you think the need for this type of gymnastics is an unrealistic scenario, you have not had the pleasure of dealing with the different methods found in various corners of the global economy. Language barriers have nothing on the problems associated with units.

Should Everyone be Accommodated?
As if this were not bad enough, there is a new trend starting to emerge in data sheets. This can only be characterized as political correctness for the metrically challenged. Material suppliers have now started to publish two sets of data sheets, one completely in SI units and one that is completely in inch-pound units. The result is that now we have data sheets that express those few properties that were familiar to us in SI units in terms that now look awkward.

MFR tests are given in oz/10 min. Imagine obtaining a data sheet for a 5-MFR polyethylene and finding out that the MFR is .18 oz/10 min. Is this really meaningful? (28.35g = 1 oz). Or just as you are getting used to the fact that your familiar PBT polyester that once had an MFR of 33 g/10 min actually has an MVR of 28 cc/10 min, you are presented with a data sheet that gives the MVR as 1.7 in3/10 min (1 in3 = 16.4 cc).

How do we get out of this mess? Hopefully, in the long term the efforts to harmonize testing methods will actually bring about a uniform approach to reporting. In the meantime, there is no substitute for a thorough understanding of how these numbers are generated. And keep your conversion tables handy inside your Chinese-English phrase book. It’s going to be a bumpy ride.

May , 2006 - Reprinted with permission from Injection Molding Magazine. Copyright © Canon Communications LLC.