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What is viscosity?
What is viscosity?
Viscosity indicates how thin or thick a liquid material is. For silicones, polyurethanes and resins, viscosity determines how well a product flows, de-aerates and reproduces detail. On this page we explain what viscosity is, in which units we express it and how you use the value in practice.
Viscosity in brief
Viscosity is the resistance of a liquid to flowing. A low viscosity means thin, like water. A high viscosity means thick, like honey or syrup. The higher the viscosity, the slower a material flows and the harder it is for air bubbles to escape.
In which unit is viscosity expressed?
Viscosity is usually expressed in millipascal-second (mPa s). This unit equals the older unit centipoise (cP): 1 mPa s is exactly 1 cP. In product data sheets you will therefore often see values in mPa s or cP interchangeably, but they refer to the same quantity.
For comparison, a number of reference values at room temperature.
| Liquid | Viscosity (approximate) |
|---|---|
| Water | 1 mPa s |
| Thin silicone oil | 50 to 100 mPa s |
| Cooking oil | 60 to 80 mPa s |
| Thin casting resin or thin silicone | around 1,000 mPa s |
| Honey | around 10,000 mPa s |
| Thick, pasty silicone | 30,000 mPa s and higher |
Why is viscosity important when moulding and casting?
In practice, viscosity determines how a material behaves.
- A low viscosity flows easily into fine details and de-aerates faster. Ideal for moulding models with a lot of detail.
- A high viscosity stays in place better and is suitable for applying brush layers or for applications where you do not want the material to run off.
- Processing time and temperature affect viscosity. Many materials become thinner when heated and thicker when cooled.
A thinner viscosity de-aerates more easily but also runs off faster. A thicker viscosity stays in place better but more often requires vacuum de-aerating to remove air bubbles. Choose the viscosity that suits your model and method.
How is viscosity measured?
There are various methods to measure viscosity. Two commonly used ones are the rotational viscometer and the flow cup.
Rotational viscometer
A rotational viscometer measures the resistance that a rotating spindle experiences in the liquid. This gives a direct value in mPa s. This method is accurate and widely used for product specifications.
Flow cup (Ford cup)
With a flow cup, such as the Ford cup 4, you measure the time in seconds that a fixed amount of liquid needs to flow out through an opening of fixed diameter. The longer the flow time, the higher the viscosity. This method is simple and practical for checks on the work floor.
Viscosity values always apply at a certain temperature, usually 23 or 25 degrees Celsius. Therefore only compare values if they were measured at the same temperature, and bear in mind that the actual viscosity in your workspace may differ.