A Comparison of Laboratory Melt Flow Rate Tester and ViscoIndicator Online Rheometer

The utilization of online rheometers in plastics processing and the recycling industry is growing. They are employed to supply a continuous data stream of rheological properties and real-time information with the extrusion process.

This is beneficial for reducing a waste stream, control of in-process product quality, and enhancing the process efficiency.

Dynisco® ViscoIndicator (VI) Online Rheometer

Dynisco® ViscoIndicator (VI) online rheometer is made to mirror the test conditions of a standard melt flow rate tester and monitor the melt volume flow rate (MVR) and melt mass flow rate (MFR) of the plastic materials whilst processing in an extrusion line.

How has dynisco’s viscoindicator been designed to closely match the standard testing condition (astm d1238/iso 1133) of a laboratory melt flow rate tester?
Dynisco's VI online rheometer is made up of a gear pump that isolates a side stream of molten polymer from the process and supplies a flow of the sample, which is precisely metered, through an interchangeable capillary die.

Dynisco supplies a range of capillary dies for the online rheometer in a variety of capillary diameters and lengths (e.g. diameter of 2 to 7.5 mm and L/D of 3.818).

The VI dies have been designed to have the same L/D ratio as the standard die of a melt flow rate tester, but their diameter sizes may be different from that of a standard die (2.095 mm). Correlation factors have been defined in our VI to take into account the die diameter variation as a result of this.

The amount driving pressure inside the rheometer is another key parameter. By manipulating the gear pump speed, the VI keeps a preset pressure (which corresponds to the pressure from a standard weight for a specific material in a MFR test). Usually the pressure drop in the channel of the rheometer is negligible when compared to the pressure drop through the capillary die. Yet, for short capillaries with a large diameter, this error can be huge and should be considered.

The pressure of the melt flowing in a capillary for a Newtonian isothermal fluid can be calculated as the equation below:

Where Q is the volumetric flow rate, n is the polymer melt viscosity, L is the capillary length, and R is the capillary radius.

The ratio of the channel pressure drop to the capillary pressure drop for an incompressible fluid can be shown as below:

The temperature is another important parameter. Extrusion processing temperature (which is the same as the set temperature in the online rheometer) is not necessarily the same as the standard temperature for a certain material in a MFR test. So, temperature correlation must be considered in the VI to define a shift factor and correlate the processing temperature to the standard temperature for a certain material.

Recycled high density polyethylene materials were extruded by utilizing a sampling extruder at a specific head pressure and screw speed. The extruder was connected to the rheological sensing unit of a VI by utilizing a connection heated tube and ½ mounting port which is compatible with the standard pressure port on extruders.

The rheometer ran in a pressure mode so as to keep a preset pressure of 59 psi. To capture the pressure, a vertex pressure transducer was mounted inside the rheometer.

Dynisco Online Laboratory

Laboratory Melt Flow Rate Tester vs. ViscoIndicator Online Rheometer

During the test about 10 grams of extrudate materials were collected from the extruder after 2.5 hours and 4 hours and their MFR value were measured in the laboratory using a melt flow rate tester (black circle points). The figure below shows the online reading of pressure (blue line) and MFR (red line) parameters from VI over a specific period.

Online Rheometer and Laboratory Melt Flow Rate Tester Results

A good agreement was found between the online and offline measurement which ensures that the ViscoIndicator is able to continuously control the product quality and open a window in to the process in any manufacturing or recycling industry.

This information has been sourced, reviewed and adapted from materials provided by Dynisco.

For more information on this source, please visit Dynisco.

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