What is Viscosity?
Viscosity is the measure of a liquid’s resistance to flow.
A lot of people will think of viscosity as how ‘thick’ a liquid is. Imagine honey versus water. Honey has an extremely high viscosity and thus has a high resistance to flow. Water on the other hand has a very low viscosity and low resistance to flow.
Why is Viscosity important for inkjet?
As inkjet chemists, viscosity is one of the most vital physical properties we need to control. Inkjet heads have an extremely strict tolerance for viscosity (and each printhead brand differs). If the viscosity of an ink is out of specification for a particular printhead, the ink will not flow properly into the print channels and when the ejection mechanism of the printhead is initiated, drops of ink will not be forced from the nozzles – resulting in poor print quality, banding and often printhead damage.
What can affect the viscosity of an ink?
All the raw materials that go into an ink will have an effect on the viscosity. A typical inkjet ink will have somewhere between 5-15 different raw materials blended together. All of these raw materials have a relevant function in the ink and all of them will effect the viscosity. Creating an inkjet ink is a balancing act between choosing the correct raw materials, blending together in the correct ratios and staying within the strict boundaries of the print head specifications.
Temperature also has a major influence on the viscosity of an ink. Think back to honey – as you heat up honey, it becomes less viscous (more runny). Temperature is therefore indirectly proportional to viscosity – i.e. as temperature increases, viscosity decreases (as honey gets warmer, it gets runnier). The same happens with inkjet inks, it is just harder to detect by eye. However, the printhead can notice! This is why printers are best running at as close to 20-25 degrees as possible to stabilise the whole system. A 10 degree change in ink temperature can have quite a drastic effect on the viscosity and thus print performance.
How do you measure viscosity?
There are few different tools for measuring viscosity. At Indie Ink, we use a Brookfield Viscometer with an ultra low adaptor. Inkjet inks tend to work in the extremely low viscosity range (3-10 cps range) so we need a special adaptor to measure at this level. All viscosity measurements are taken at a constant temperature (25 degrees C) controlled by a water bath. Without this constant temperature, external room temperature fluctuations will influence the measurements.
Can Viscosity change over time?
Yes! However, as ink chemists, this is exactly what we are trying to avoid. A change in viscosity over time can be a strong sign of instability in the ink. This is why when we develop a new ink, we test the viscosity profile over time (we accelerate the ageing process by keeping inks at higher temperatures which can ‘stress’ the inks). If the viscosity of the ink stays stable over this stressing period, it is a good sign towards stability of the formulation.