Choosing the right ink for your coding application

Dr. Richard Marsden – Chief Chemist, Linx Printing Technologies Ltd

When you buy your groceries, the chances are that you read the ingredients and the price, and glance at the sell-by date.

But that sell-by date or batch code is a vital part of the packaging. Not only does it tell the consumer by when to use the product, but it also includes traceability information to help protect you, the consumer.  And that information is a legal requirement.

So it is important that the code is clear and legible and remains so during the lifetime of the product, so in the event of a recall for instance all the affected products can be correctly identified and returned.  Manufacturers need to be able to trace their products back to the raw materials, to help identify the possible source of a product problem, and the batch code forms an essential part of this process.

CIJ, or Continuous Ink Jet, is widely used as a method of applying batch codes onto products because it is quick, non-contact and is supported by a vast range of ink types.  New inks are continually being developed in response to changing packaging, and ink innovation is becoming faster as consumer and market drivers speed up this packaging development.

Inks are therefore as varied as the number of applications: from edible inks for coding onto eggs and pharmaceuticals, to robust inks for extruded drainpipes that withstand UV exposure.

All inks will have slightly different performance characteristics, often by virtue of their solvent base, and these need to be considered when determining the correct ink to use.

Examples include:

• General-purpose inks offering excellent adhesion and contrast on a wide range of porous and non-porous substrates, such as paper, metal, card, plastic and glass.
• Pigmented inks offering a higher level of opacity are especially useful if printing onto dark surfaces. These inks do not bleed on PVC or other flexible plastics and have a higher level of resistance to heat and light, making them ideal for use on items intended for the open air. They also offer good chemical resistance and are therefore suitable for use in situations where solvents such as perfumes or aerosol products are used.
• Specialist inks designed to operate under particularly challenging conditions or to perform in a specific way such as UV-readable inks, colour-change inks, removable inks and food grade inks.

How to choose an ink for coding

When choosing an ink for coding purposes, there are five key areas to consider:

1. What does your application require?

• Drying time
  • What is the speed of your production line? Certain inks dry faster than others, so if you run a high speed production line you may need a fast-drying ink. Particularly if the coded products come into contact with each other, production line equipment or operators soon after coding
• Adhesion requirements
  • Will the ink stick to the material you are printing onto? Is there a risk of smudging or smearing? Non-porous materials such as some plastics can be harder to print onto; whereas highly porous materials could cause the printed code to ‘bleed’
• Special requirements
  • Such as withstanding sterilization or pasteurization processes? Ensure that any print samples produced by your CIJ supplier are then put through your processes to test how well they stand up to them
• Post-printing exposure
  • Once printed, codes may be exposed to a range of chemicals and environments during their life cycle, including UV light. Ensure that your supplier has tested their inks against these

2. Is the ink designed specifically for your printer?

Beware using third party inks not produced specifically for your CIJ printer.  CIJ manufacturers will test their inks in their own printers, and using alternative

inks may produce inferior results eg poor adhesion or clarity of code.  Ultimately they could cause poor printer reliability, resulting in costly downtime and complications for your production line.

Comprehensive testing of inks is carried out by CIJ manufacturers in their printers to ensure:

• The ink’s characteristics, such as viscosity, are specifically matched to the printer. Incorrect viscosity will cause print and printer problems
• The ink performs consistently in a wide range of operating environments including humidity and temperature. If it does not then the print quality will suffer, the printhead may suffer build-up of ink and require more frequent cleaning, which will result in extra cost and downtime for the user

Inks often contain mixtures of solvents which evaporate as the printer is used. It is important to add the correct type of solvent to ensure the inks ingredients remain stable and to maintain ink adhesion.

3. What quality assurance does the ink have?

Quality assurance should play an integral role in your decision-making process. Given the responsibility of manufacturers to print quality, durable codes onto products, they will want reassurance that the inks they choose have been rigorously tested at the development stage and are manufactured according to strict protocols and specifications.

Check that your supplier holds the relevant certifications:  quality inks will have their properties tested against certified and verifiable ISO 9001 quality control procedures.

These procedures include:

• Close control of critical ink parameters
• Micro-filtration of ink and solvent to ensure impurities are removed
• Full traceability of all new materials used in every batch of ink.

The shelf life of the ink may also be a factor in the decision. All containers of ink and solvent should display a use-by date, after which they must not be used. If an ink or solvent is stored longer than the recommended time, adverse chemical changes can take place. Quality ink manufacturers will be able to provide prospective customers with details of tests they have carried out to confirm that when the ink is used within its shelf life, users can be confident of ink performance.

4. What health and safety requirements must you conform to?

Original equipment manufacturers are aware of their responsibilities to customer safety. Quality inks will have accurate product data, labels and safety datasheets that are up to date with the latest international regulations. Manufacturers will also be able to give help and advice on many issues relating to the safe use of inks and solvents.

Other issues to consider include:

• Does the chosen ink comply with your health and safety policies? Will you need to provide ventilation or protective clothing on your production line?
  • Choose a printer which is designed to minimize operator contact with ink and solvent, for example by using a clean process for adding ink and solvent to the printer.
• What solvent base should you consider? If coding will take place in a ‘closed’ or ‘clean room’ environment, such as that found in the pharmaceutical or food sectors, then a low odour solvent or one designed specifically for these applications should be considered.
• Inks must be packaged and labelled according to the latest EU legislation, with appropriate safety guidelines. This also identifies those that carry hazardous pictograms such as ‘Acute toxicity’, ‘Serious health hazard’ or ‘Hazardous to the environment’. A good manufacturer will be able to supply regulatory information such as heavy metal content, and statements regarding hazardous materials such as phthalate plasticisers, ozone depletors, endocrine disruptors, etc
• Environmental responsibility is another factor. ISO 14001 (which is held by the most reputable ink jet companies) demonstrates sustainable policies for reducing the environmental impact of inks and solvents, and commits the company to continuous environmental improvement.

5. Printer Type

Finally, the model of printer used will impact on the choice of ink. As we have already covered, inks are developed for, and tested in, specific printers. For example, heavily pigmented inks will require dedicated printers which are designed to keep pigmented particles dispersed.  So choosing your coding solution has to be made with the printer in mind, as well as the ink.

Coding and marking is a necessity in today’s society, but there is no straightforward choice when it comes to choosing an ink; the range of inks available is extensive, and making the wrong ink choice can have serious consequences for a business.

Reputable manufacturers are there to help provide you with the right ink (and printer) for your application.

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Dr. Richard Marsden has a degree and Ph.D. in Colour Chemistry from the University of Leeds, followed by postdoctoral fellowships in Synthetic Organic Chemistry at McMaster University in Canada and Hull University, UK.

He first worked on CIJ inks for binary systems with Elmjet, before joining Linx Printing Technologies in 1990.  He is a chartered chemist and member of the Royal Society of Chemistry.