Melt Flow Rate (MFR)
Often a moulder has to change material grades. One of the first properties to be consulted on the new data sheet is usually the MFR. This is to establish if the new grade has the same melt viscosity performance.
Even if the new grade has an identical MFR, this is no guarantee it will perform the same in injection moulding. MFR values in data sheets are derived from a relatively simple test. This is easy and inexpensive to carry out and was originally intended as a method of crudely ranking materials. The test involves measuring the mass of melt extruded from a simple piston extruder, through a standard orifice and at standard temperatures. It is carried out at flow rates far removed from the flow rates experienced in the cavities and feed systems in injection moulding.
The problem is that thermoplastic melts are non-Newtonian fluids, which means that the resistance to flow (apparent melt viscosity) varies with flow rate (shear strain rate). Thermoplastic melts are shear-thinning fluids (pseudoplastic in old terminology). The melt viscosity can be several orders of magnitude lower at moulding flow rates than in the MFR test. Melt viscosity, and hence ease of mould filling, also decreases as the melt temperature increases.
Shear thinning is beneficial for the moulding process. However, it means that MFR is not a great predictor for what actually happens during mould filling.
To get a better comparison of the melt flow characteristics of one grade against another, most material suppliers can provide you with more relevant viscosity data, measured over a range of shear rates and temperatures. Much of this is accessible through databases such as CAMPUS.
This article was written by Dr. Charlie Geddes for Hardie Polymers.
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