Is Using MFR the Best method for Material Selection?
When a material selection comes down to flow rate, is using the (Mass Flow Rate) MFR or Melt Index (MI) the best choice? To answer this, we need to understand why the Melt Index test initially came about
The Origin of the Melt Index Test Method (ASTM D-1238)
ASTM D 1238: Test Method for Flow Rates of Thermoplastics by Extrusion Plastometer
Before there were standards to test polymers, there was a need to determine the differences in how polymers would flow when melted A method was created to keep all polymers on the same level playing field This method places the material in an Extrusion Plastometer or Melt Indexer
The standard has the barrel of the melt indexer heated to a specific temperature The user would obtain a resin sample and place it in the barrel where a piston would be inserted A specific load would be placed on the piston, and the melted polymer would be extruded through a capillary die (with a particular orifice size) The extrusion would take place for 10 minutes, and the amount of polymer would be weighed in grams yielding an output in g/10 minutes
Having MFR data for all materials allows one to compare them side-by-side, giving a respective idea of how each will flow with the other
The limitation of this test method is that it is, in fact, one point on the viscosity curve and is at a shear rate of nearly zero, which is not indicative of the injection molding process
When materials experience shear during injection molding, shear rates may be experienced up to and possibly exceeding 100,000 1/sec Some materials become more viscous at higher shear rates, but these are uncommon
So how do we compare materials at these higher shear rates?
Since the inception of the melt indexer (1950s), a much more accurate test method was designed using a Dual Capillary Rheometer
A dual capillary rheometer can produce a series of viscosity data points over a range of shear rates, such as the image below
A Rheology curve provides exact viscosity data based on specific shear rates at specifically tested temperatures Notice how the Melt Index MFR point does not provide any data relating to the injection molding process A curve like this will allow one to understand the exact behavior of the material and shear rate