When a material selection comes down to flow rate, is using the Mass Flow Rate (MFR) or Melt Index (MI) really the best choice? In order to answer this, perhaps 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 of testing polymers, there was a need to determine the differences between polymers with regards to how they would flow when melted.
A method had to be chosen to basically keep all polymers on the same level playing field. This was accomplished by placing the material in an Extrusion Plastometer or Melt Indexer.
Extrusion Plastometer Furnace of the Plastometer
The ASTM D-1238 standard has the barrel of the melt indexer heated to a specific temperature. The user would obtain a sample of resin and place it in the barrel where a piston would then be inserted in the barrel. A specific load would be placed on the piston and the melted polymer would be extruded through a capillary die (with a specific orifice size). This 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 quickly allows one to compare them side-by-side therefore giving a respective idea of how each will flow relative to one another.
The limitation of this test method ,in fact, is that it reflects one point on the viscosity curve and is at a shear rate of nearly zero which is not indicative of the injection molding process (shown below on the Rheology curve).
When materials experience shear during the injection molding process, they might experience shear rates up to and possibly exceeding 100,000 1/sec. There are some materials which actually 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 (1950's), a much more accurate test method has been designed; a Dual Capillary Rheometer.
A dual capillary rheometer is able to 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 specific tested temperatures.
Notice how the Melt Index (MI) point does not provide any data relating to the injection molding process.
A Rheology curve like this will allow one to understand the exact behavior of the material and shear rate at the processing temperature used during molding.
When two materials are compared side by side the Rheology curve will be able to tell them if one material is going to flow more readily. Along with Dual Capillary Rhometers, Injection Molding machines have been instrumented to act as rheometers to capture an even broader spectrum of Rheology data.
In conclusion, the ASTM D1238 Melt Index Test Method is a great way to compare one material to another with regards to zero shear flow rates, but the reality is that when materials are injection molded, the shear rates are far from zero.
At higher shear rates, it's imperative to understand the flow behavior of a polymer and the only way to do this is to look at a Rheology curve for the specific material.
Bozilla Corporation utilizes these Rheology curves for each and every project in order to gain a thorough understanding of the polymer being used for the application.
Utilizing this data provides the greatest accuracy when analyzing projects within the Autodesk Moldflow software as well as when investigating polymers for project application and project troubleshooting.
Contact Bozilla Corporation for your FEA and injection molding troubleshooting needs and please visit our website at www.BozillaCorporation.com.
