In injection molding, the viscosity describes the flow resistance of the plastic in the cavity.
Higher slow-moving of the material means higher viscosity.
Flow curves show that the viscosity of a plastic varies by different shear rates and temperatures.
These curves need to be measured with the help of a viscometer or rheometer.

The viscosity is described by the formulaic character 𝜏 (Tau).
Its unit is Pascal seconds.

Examples of Viscosities:
Water: 0,001 Pas
Oil: 0,03-0,1 Pas
Glycerin: 1,5 Pas
Honey: 10 Pas
Easy flowing plastic melt: 1000-5000 Pas
Heavy flowing plastic melt: 10000 – 100000 Pas

Viscosity in Plastic Industry
Viscosity in Plastic Industry by STAK

Image 1: Plastic: The temperature curves describe the viscosity over shear rate.
Image 2: Plastic: The higher the viscosity the better the dimensional stability.
Left: A material with higher viscosity; right: lower viscosity.

Related topics:

pvT – Diagram

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This glossary of plastic industry is provided by PLEXPERT Canada Inc.