A rheometer is a specialized type of instrument used to measure the flow of a liquid in response to specific types of applied force. This liquid may be a slurry or a suspension as well as a pure liquid.
It is important to realize that the flow of the material is not the same as the viscosity or thickness of the material. Viscosity measures how a liquid is deformed or changed when shear stress is applied. This is stress applied in a parallel fashion to the surface of the liquid. While viscosity is a single measure, the flow measured by a rheometer will have a wider range of parameters and measurement options.
There are many different models of rheometers on the market, which typically fall into one of three basic types. These include torque, dynamic rotation and capillary rheometer models.
Polymers and Plastics
One of the most common applications for the capillary rheometer is in the testing of polymers, plastics and rubber. With these testing devices, the material to be tested is melted to liquid form and then forced through a fine tube. This tube is carefully controlled for exact specifications and consistent cross-sectional dimensions throughout the length.
With this precise diameter of the tube and the known pressure, either the pressure drop or the flow-rate can be fixed. This allows the rheometer to measure the other based on the settings selected.
Additionally, with the fixed parameter, the measurement and the known dimensions of the capillary, the shear rate can be calculated from the flow rate and the shear stress from the pressure drop.
The resulting measurements can be used to create a curve rather than a single point measurement. This makes the data from a capillary rheometer more applicable than a melt indexer or a viscosity reading. With multiple points of data the curve is very accurate, providing a complete set of information when choosing a material for injection molding under specific use conditions.
Considerations
When selecting a rheometer, it will be essential to consider the size and footprint of the system and how it will fit into a current lab. There are smaller, more compact models available that have to option to add features and functions to increase the versatility and testing ability of this essential piece of equipment.
Consider the reputation of the manufacturer, the years in the industry and the customer service and support offered by the company. While the rheometer itself is a critical factor, these additional considerations will help you to find the right match for your specific testing needs.
At GOETTFERT we have provided industry leading testing equipment, including capillary rheometer models, since 1962. To learn more about our products, visit us online at www.goettfert.com.