Understanding humidity and the inkjet printing process

Deflected nozzle filmed on an industrial drop watcher.
A deflected nozzle printing

Colin Marsh, managing director of the Resolute Print Group, takes an in depth look at humidity and how it affects the water-based and non water-based inkjet process used in both DTG and DTS.

We normally associated humidity problems with very hot or very cold countries where the humidity fluctuation can be high. In a recent study of installations throughout Europe it would seem humidity can still cause some issues if its not kept within a reasonable level. This is not just associated with the DTG process but inkjet in general, one of the symptoms of poor humidity can also be excessive static electricity, this can cause very strange results leaving the un experienced thinking there equipment is faulty when it is actually the environment or process that is at wrong.

Water-based ink

The vehicle solution (in this case modified water) carries the pigment or dye in a water-based ink. The viscosity of the ink (we will use 7.5 as an example) is set in line with the type of print head used. If the viscosity of the ink changes to below 6.9 or above 8.1 the print head cannot function correctly, it’s a little like trying to ice a cake with either milk or hard cheese. It just doesn’t work and this can cause the ink to deflect or even worse clog. The humidity effects the evaporation rate of the vehicle solution, modified water, changing the viscosity resulting in pigment being left in the exit holes of each jet. This causes the ink being jetted to misalign commonly known as deflection. Removing this ink and correcting the humidity should remove the problem, but only if the viscosity of the ink is still correct. If the vehicle solution has evaporated from the ink storage it will not jet correctly no matter what the humidity.

Storing your ink at the correct temperature and making sure evaporation of the water back into the environment does not occur is an easy but important point to manage.

Non water-based ink

Moving on to non water-based ink the symptoms differ but are still just as critical. When UV or similar inks are manufactured the vehicle solution will be different. Although evaporation of the vehicle solution is no longer such an issue poor humidity will now start to generate high levels of static electricity. This will cause the ink to be deflected when jetted from the head and will also give the symptom of deflection. This time caused by the static electricity in the substrate which deflects the ink away from its intended target making prints blurred and often clouds of ink appear in non printed areas. This is not a machine fault, but a symptom very often found in clean rooms especially where air conditioning is used to regulate the temperature leaving levels of humidity below the recommended. Many substrates printed with UV inks are acrylic or some form of plastic, these are prone to accumulating excessive levels of static when cleaned before printing commences. Using an anti static or IPA based wipe approved by your supplier to prepare the substrates for print can help reduce high levels of static.

A UV printer fitted with simple static inhibitor
A UV printer fitted with simple static inhibitor

What should my humidity be

This will be specific to the ink and process being used and does vary from machine to machine. In general, you should aim for higher than 35% humidity in some cases 40% humidity for water based and UV inks with a room temperature of around 21⁰C. This is normally fine for DTG printing and in most print rooms should not be to hard to achieve.

In larger print rooms and clean rooms very often other digital equipment can sap the humidity and generate static making it much harder to achieve a consistent level. In these cases, multiple machines of the same make can give inconsistent and different results by simply being closer to or situated in an area that is in the path of an air conditioning vent or close to a digital press. Fitting static inhibitors may be necessary in some extreme cases to achieve consistent printing where the environment is just no good enough.

A simple and inexpensive hygrometer will allow you to monitor your environment and help you avoid the pitfalls of printing in low humidity.

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