The Back Extrusion Rig - an Alternative to Rotation Viscosimeters

With the back extrusion rig Zwick offers an alternative solution to rotation viscosimeters. The principle of the back extrusion rig is based on the displacement of the test material through an annular gap between piston and back extrusion cell. The cell is filled with the test substance and the piston is lowered into the container, pressing the material up through the annular gap. When the piston is raised, the material flows back through the annular gap into the cell. 

The force is measured in the down and up cycle at a defined position. The dynamic viscosity is evaluated from the measured force difference and the flow rate of the liquid in the annular gap. 

The Zwick system can repeat this cycle as often as desired with freely adjustable shear rates. The test can be optimally adapted to the test material, and the viscosity can be measured across the entire shear range with one single test. 

Advantages of the Back Extrusion Rig

• The viscosity can be determined directly in containers from the running production. This rules out prestressing of the material caused by transfilling.
• Flow influences which occur in the production plants can be reproduced by altering the annular gap and the moving speed.
• This method enables tests on masses with chunky pieces as fruit preparations and food systems: Fruit pieces or other bigger ingredients aren´t pushed out and thus are measured together with the yogurt or the fluid.
• Individual selection of the number of test cycles and the possibility of increasing the shear rate for each of these cycles enables recording of the viscosities over the entire shear rate in just one test.
• The test results on Newtonian fluids correlate with the results from measurements with rotation viscometers.
• For non-Newtonian fluids reproducible, comparable test results are obtained, usually with more sensitive characteristics than those gained from rotation viscometers.

This special test program includes a lot of extras especially for viscosity testing. Examples: automatic preconditioning, determination of cycle numbers and speed, selection of results etc.. 

The test software is clearly arranged and enables comprehensive evaluation and presentation of data as well as graphical evaluation and reporting.