ASTM 1238 and the ISO 1133 test methods are "technically equivalent" but there are some minor differences. Both ASTM 1238 and ISO 1133 offer two variations of the standard melt-flow test: Method A and Method B. Method A describes a basic manual melt indexer. It involves making "cuts" of the extrudate at timed intervals as it exits the extrusion die. The "cuts" are weighed on an analytical balance to determine the average mass and this value is extrapolated to the mass that would be extruded in 10 minutes—the MFR. Weights range from 1.2 to 21.6 kg to permit measuring materials ranging from very low to very high viscosity (more viscous melts require more weight to force them through the die). The load and temperature to be used are determined by the material and the ASTM test method.

With most Method A instruments, only temperature is automatically controlled. All the measurement is done by the operator. All new melt indexers today are provided with automatic timers, and some have automatic extrudate cutters. Although the latter work well with some plastics, some resin melts are sticky or difficult to manipulate to obtain a clean cut. In most cases, automatic cutters cannot match the human hand.

Method B testing is simpler in that there is no extrudate cutting or weighing because it is volumetric. The volume of the resin extruded is obtained from the geometry of the barrel and distance of piston travel over a measured time period. This determines the mass volume rate (MVR) in cc/10 min. From this value and the resin's melt density, the MFR can be calculated automatically. With Method B testing, the operator simply loads the barrel with resin, starts the test, and the instrument calculates the results. Semi-automatic units have a sensor that detects where the piston is at any moment.

This is the first advantage over basic Method A instruments. These units measure 20, 30, or 40 measurement points in one test run and statistically calculate which results are within the norm and which are outside the norm due to an air bubble or other impurities. The second advantage is that there is less user interference. Results, then, are generally more accurate and more reproducible.

On the other hand, Method B testing requires an accurate value for the resin melt density at the test temperature. Such values are available in published literature—for example, typical values for "generic" PE and PP are cited in the ASTM 1238 standard. However, most sources consider it more accurate to measure the actual melt density of the specific resin being tested. This is done by combining elements of Method A and Method B in a single test run so as to obtain data for both weight and volume—hence, density. Today's melt indexers for Method B can be used to perform both tests. If desired, an automatic extrudate cutter could be added to the Method B instrument.

There is no general consensus as to which procedure is best. Some recommend Method A for most processors claiming taht it tests the 'bulk property' of the material and does not presume that you know or trust a stated melt-density value for the actual resin.

Others view Method A as best for compounders who use a broad range of materials and additive packages and see Method B as best suited to labs that make frequent measurements of the same type of material with a known melt density.

Processors working with very low-MFR resins (under 0.5 g/10 min) are best off with Method B—provided the instrument has a digital encoder to measure piston movement. With polymers of such low flow rate, there is virtually no material extruded, and the piston hardly moves at all. To rely on a manual cut or a visual determination of how far the piston has moved leaves too much room for error. A digital encoder takes care of the problem. The same holds true for very high-flow-rate materials (over 75 g/10 min).

There is also a multi-weight melt flow test, which unlike standard Method A and Method B, provides more than one measurement on a single charge under two or three different load conditions (using different weights). The multi-weight test is referenced under ASTM D1238 as a variation of the melt-flow test, but instrument suppliers are working to write a separate standard for this test.

Several suppliers offer multi-weight capability. In an instrument that can calculate shear rate and viscosity in addition to MFR, multi-weight measurement provides the equivalent of a "poor man's capillary rheometer," suppliers say.

Most suppliers agree that multi-weight instruments are mainly suitable for use by resin producers or compounders who want to provide their customers with more information on how a material will behave.