This means that the melt temperature of the HDPE on the same screw design is going to be much higher than the PP. Processing conditions can be precisely optimized using capillary rheometers that measure polymer melt viscosity in relation to temperature and strain rate. Due to the multiplying effect of power-law coefficient and consistency index, an HDPE and a PP at identical shear rates and slightly different temperatures can have a difference in viscosity where the HDPE is three times as viscous as the PP. The melt viscosity at test temperatures is directly related to MFI, since a sample with higher viscosity will flow slower.

Melt-index and melt-flow measurements by capillary rheometer are at very low shear rates, where shear thinning is almost non-existent. The melt viscosity test showed that increasing the PC content increases the viscosity of the blends, and melt viscosities of blends are intermediate between the values of PC and SAN. Representative methods include melt mass-flow rate (MFR), melt volume-flow rate (MVR), a method using a measuring instrument such as a capillary rheometer, and bar flow evaluation using an actual injection molding machine. As the list of polymers in the table shows, the calculated viscosities from the chart at shear rates of 100 sec -1 do not fall in the same order as the power-law coefficients because of that consistency-index effect.Īs a result, two polymers of similar melt index or melt flow can have vastly different viscosities at the elevated shear rates during processing. Likewise, there are several methods of expressing the viscosity of a thermoplastic resin (fluidity) when it melts. The shear-thinning characteristics of various polymers are often categorized solely by the power-law coefficient, but the consistency index can have just as significant effect on the final viscosity and has to be considered.

Melt viscosity test