A Look At Capillary Rheology

There are many applications for the technology behind capillary rheology. This was originally developed to test polymers in melt processing, but it can be used to test virtually any type of similar material.

The process involves extruding a small amount of test material under controlled conditions to test the material flow under specific levels of force or pressure. It is used to test for deformation properties based on high level of force, increased or elevated shear rates and at high temperatures.

A Simple Explanation

For those new to capillary theology, it is actually a relatively simple technology with an advanced level of theory. The entire process uses a capillary or a hollow tube of a known diameter and length.

The sample product is loaded into a barrel or tube which is correctly known as the bore. This is controlled at a specific temperature. The temperature can be adjusted to the specific requirements of the test and for the specific material sample being tested. Next, the capillary, which is carefully selected for its diameter and length, is attached securely to the bottom of the bore.

Pressure is applied from above through piston that presses down on the sample material that has been loaded into the bore. Remember, this is held at a specific known temperature. Using an extrusion process the heated sample is pressed through the capillary by the piston. At the entrance of the capillary there is a die, an opening, where the testing equipment measures the pressure.

From the measurement of the pressure at the capillary die entrance, the dimensions of the capillary and the piston speed it is possible to calculate the shear viscosity of the test material. In fact, the equipment will automatically complete this calculation.

Advanced Options

It is possible with capillary rheology to vary the shear rate of the test to produce what is known as a flow curve. This shows how the test material viscosity changes when the shear rate is increased and decreased. This is typically a more effective representation for determining if a particular material is suitable for a specific type of application as compared to a single point test.

It is also possible to complete capillary rheology using a barrel with two bores, also known as a twin bore and a zero length die. This will allow for the determination of shear and extensional viscosity of the test material at the same time. Other types of measurements including melt strength, die swell and melt fracture can also be completed using this same testing process.

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