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What is Capillary Rheometer used for?

2017-03-16

Capillary RheometerCapillary rheometer also known as a viscometer is the most widely used apparatus/device for measuring shear viscosity and several other rheological properties. The device is used to determine the technique in which a suspension, liquid or slurry flows retorting any external applied force. Rheometer is specifically used for those fluids that can’t be characterized by a sole value of viscosity and hence need multiple parameters to be set and measured than in the case for a viscometer. Meaning, a rheometer measures the rheology of the fluid. Capillary rheometers guarantee conditions for testing that are genuinely an example of processing conditions, particularly for high speed and high pressure methods such as injection molding; consequently the rheometers are vital for optimizing the process. A few other rheological attributes which can be determined of predicted with the help of capillary rheometers data comprise of thermal stability, extensional viscosity, wall slip and extrudate swell. Additional measurements can be done for thermal conductivity, melt strength, density dependence on pressure and temperature (pvT). Research study done on capillary rheometer market, states that generally rheological properties are modulus, yield stress, viscosity, and relaxation times. All these attributes are dependent on the fluid material’s microstructure, and therefore depend on structural changes that are induced with strain or stress, and on time, i.e., ripening of structure.

The process of capillary rheometry has origination in polymer melt processing; however this process is also pertinent to several other material processes like high speed printing and coating applications. Rheometry allows the material to flow and deform its attributes to be portrayed under high pressure conditions and high shear rate and at escalated temperature, and all this is based on controlled extrusion of a test material.

Below are a few basics of capillary rheometry procedure:

  • The fluid sample which is to be tested is filled into a bore inside a barrel that is temperature controlled of the capillary rheometer
  • At the bottom of the barrel bore, a capillary die of set dimensions, i.e., length and diameter, is bestrode
  • With the help of a piston, the fluid sample is squeezed out via the capillary die, and the outcome pressure is determined at the entrance of the die
  • From the data collected from the capillary die dimensions, piston pressure and speed, shear viscosity is calculated
  • This shear rate can be altered to generate a flow curve
  • Simultaneous grit of extensional viscosity and shear is attained using a zero length die and a twin bore barrel