These and other factors, such as a change in pigment source, mean that formulations can change over the years. One thing is certain: ink makers take their responsibilities seriously and will do everything possible to provide consistent formulations. The UK boasts a rich vein of specialist ink manufacturers, including Small Products, Davison Chemographics and Apollo Colours while the multi-national players such as Sun Chemicals and Fujifilm Sericol still sit astride the screen printing market.

Screen printing is an ink friendly process and there are a selection of ink chemistries available so most people use one or two different types of ink without really knowing what is available.

Choose wisely

The greatest division is between radiation cured and solvent based inks. Ultra violet curing is the most common kind, with microwave and electron beam curing in specialist applications. UV cure inks are fast drying and because of the lack of solvents will stay open in the screen mesh for some time. They will only dry when exposed to high intensity ultra violet light.

Of the other ink types, water based ink and reactive curing two part inks are the most likely to be used. Reactive curing are generally the most resistant ink systems available. Often solvent based, they consist of an ink base and a catalyst. The two are mixed together at a specific ratio and drying is a combination of solvent evaporation and the polymerisation between the base and the catalyst. Left to dry at room temperatures, they can take several days to cure. Heat reduces the time and improves the product resistance.

The cured ink film boasts a high chemical resistance to solvents, acids and alkalis and can withstand high temperatures. It is particularly suited to metals, glass and ceramics.

A useful tip with two component ink systems is to ensure that the temperature of the curing ink film does not drop below five degrees Centigrade as this can stop the curing chemical reaction, which cannot be restarted. Once fully cured a drop in temperature will not be damaging.

Alongside these systems are baking inks, oxidation drying, sublimation, textile inks, adhesives and specialist mediums adapted for printing.

The main determining factor for choosing ink is the facilities available for drying and or curing. Assuming the correct means of drying/curing is at hand, the next question is whether the ink will stick to the desired substrate and whether the printed results will withstand the destined environment. Also consider whether it is suitable for use in your printing machine.

To solve this problem, ink suppliers produce many different ink types. Ink suppliers often provide charts that show which of their ink systems are suitable for a particular application. Technical data sheets will give information but for a new substrate, it is worth sending a sample for ink trials.

Sourcing solvent inks

Solvent based inks are very versatile, but can present some production issues. From the printers perspective, maintaining stability of the mixed ink on the screen is challenging, but the effects and characteristics that can be achieved are exciting. The inks work by suspending pigments in resins dissolved in a combination of solvents, which makes it possible to create inks for every printable substrate as well as mirror gloss inks, inks that react to light and heat, inks with electrical properties and photoluminescent inks.

Any application that requires light fast colour that sticks well to a substrate and withstands abrasion and chemical attack is likely to be met with a solvent based ink.

Once the inks are mixed, the solvents will start to evaporate unless they are kept in a closed container. As the solvents evaporate, the ink thickens and flow through the mesh alters, resulting in colour variation. It is possible to slow evaporation by using retarder, but this results in longer drying time. It may be necessary to introduce more drying racks, or a longer conveyor dryer to run more slowly at a higher temperature or with higher airflow. Take care not to overheat prints as the substrate may shrink or distort.

Opting for UV inks

UV curing inks also come with their own limitations. Despite the fact that additives make up only three per cent of the basic constituents of UV curing inks, take great care when using them, as their incorrect use can destroy the properties of the ink.

Adhesion promoters, for example, improve the adhesion of the ink yet they can also affect the usable life of the ink. Gloss/matte thinners can adjust the colour and reduce cure time, but they may also reduce ink adhesion. Cured UV ink is normally a hard film which, if bent will crack. Flexibility additives provide additional elasticity, but may also reduce adhesion.

Water based UV ink was produced by ink manufactures specifically to overcome the ink build problem that can occur with conventional UV. This is an issue when printing fine four colour halftones where skipping can occur. The result is that much of the third and forth colours do not get printed onto the substrate.

Water based UV ink addresses this issue with its 45 per cent water content. When the ink is printed the water starts to evaporate and this can be accelerated by applying infra red energy or hot air. The heat drives off the water leaving a dried and cured thin film. The build is still slightly more than solvent based systems, but low enough to reduce the skipping problem.

Another solution is to use a low build stencil that gives an emulsion over mesh (EOM) of three microns. When used correctly, fine line rulings can be printed with conventional UV curing inks without the skipping problem.

During ink selection, remember that all elements of the screen printing process are linked together. The successful printer understands how they interact to can make a profitable technology choice.

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