Capillary Rheology: Measuring Success


The competitive nature of the plastics industry together with the invention of new and innovative materials and techniques can make success difficult. One mistake can result in an economic loss from which a company will have difficulty recovering. It makes it essential that companies utilize on their technology, instruments and skills to ensure the material they employ performs to expectations under the selected processing method. To help achieve this, many industries apply capillary rheology. While melt flow testing is another option, using a capillary rheometer can prove to provide more information on the behavior and various properties of a material such as a polymer and various plastics.

Capillary Rheometers

A capillary rheometer is an instrument companies employ to measure the changes in a material’s viscosity as they are relative to the shear rates. If the rheometer is controlled-stress and high-shear, its parts will consist of:

  • A heated barrel – single or double bore are the two basic options. Double bores come into play if the technician wishes to conduct two tests under diverse conditions simultaneously. If a twin bore is combined with a “zero length die” this will allow the technician to determine both shear and extensional viscosity concurrently
  • A piston
  • A calibrated die – it is changed when the company requires the die to determine the rheological properties of the material under different conditions

In this manner, the rheometer can measure not only the load, but also the piston speed and the die geometry. In addition, technicians can calculate the shear viscosity by knowing these three critical factors:

  • Die dimensions
  • Piston speed
  • Pressure

Technicians employ the capillary rheometer in various types of material processes. These include extrusion and injection molding where the rheometers track the flow of the plastic or polymer through the defined space to achieve a measure of true or absolute viscosity, something not achieved by torque rheometers.

Why Employ Capillary Rheology?

Several reasons lie behind the use of a capillary rheometer. By adopting this method, a technician can supply the company with valuable information including:

  • Determination of the optimal working strictures for various processing methods including blow molding, extrusion and injection molding
  • Examination of various processing concerns swifter and with less disruption
  • Discovering which specific materials are the most suitable for long flow lengths or complex components
  • Replication of the manufacturing strictures for various reasons including design, product or numerical simulations and troubleshooting

While other reasons exist, including reducing the instances of lost time, wasted material and economic efficiency, capillary rheology is first and foremost about measuring true or absolute viscosity.