Ink dispensing the basics

Ink dispensing: the basics

For printers working with ‘liquid’ inks – flexo, gravure and screen – ink dispensing is a fast, efficient and reliable way of creating a brand’s unique ‘spot colours’.

Many brand owners depend on unique colours to build a recognizable package and strong brand identity. Packaging suppliers can buy these colours ready-mixed from the ink supplier, or can produce these themselves by mixing different colours.

When using flexo, gravure and screen printing processes, spot colours are a reliable way of reproducing unique colours smoothly. Spot colours, or Pantone colours, are produced by blending various shades to get the desired colour and are relatively easy to correct. A separate printing unit is needed to print each spot colour.

Ink blending

What is a colour formula?

To make a spot colour, you need a ‘formula’ (or ‘recipe’): the right ingredients in exact proportions, measured in weight, to get the accuracy demanded by brand owners. Those ingredients may include:

  • base colours with pigments – to get the correct hue, lightness and intensity
  • varnishes, hardeners and binders – to provide technical characteristics
  • water or solvents

Example of a spot colour formula:

Let’s suppose a soft drinks brand’s colour is a special orange that has to be printed on a carton box. Its formula is: 45% varnish, 25% red, 25% yellow and 5% white. We assume that 20kg is needed for a job on a wide-web press. The amounts of each colour ink ingredient needed for the formula would look like this:

Ingredient% RequirementAmount needed for a 20kg job

Printers can get their formulas in three ways:

  • create the formulas yourself – but this requires much trial and error and manual labour
  • measure colours with a spectrophotometer and create the formula by means of special software, such as from X-Rite or Datacolor
  • get them from your ink supplier (this is the easiest and most common way)

Be consistent with your base ingredients!

Note that every ink supplier has special formulas for their series of inks. So, to get accurate spot colours with the same formula, you must use the same supplier’s inks for your base ingredients. That’s why you should also look a long-term partnership with an ink supplier that assures you of consistent colours.

How to mix colours

There are two ways for a printer to make colours in house:

  • Manually mixing colours
    Some experienced colour specialists at printing houses may mix (and correct) colours by eyesight, but most use a weighing scale. This is slow, results in lots of ink waste, is labour-intensive, and can cause lost production time. The quality is often incorrect at the first attempt, requiring corrections. To improve this process, a scale can be used in combination with a control computer with ink management software. This system scales up the selected formula to a desired batch size, calculates the individual weight of each formula ingredient and then leads the operator through a step-by-step controlled weighing process.
  • A gravimetric ink dispenser – automating the mixing of colours. As we will see, this is faster, repeatedly accurate and saves waste.

What’s an ink dispensing system?

The automatic dispensing system doses the exact amount of special ink colour, mixed from a number of connected inks. It features a control computer with a user interface and a dispensing station. Under the head with the dispensing valves is a weighing scale, on which you place the empty buckets: this is a ‘gravimetric’ system that records all dosed amounts in kilogrammes.

Ink dispensing

The pneumatic valves in the dispensing head are connected with hoses and pumps to containers, each of which is filled with a ‘base component’ – a colour or varnish that can be used as an ingredient. The container’s size depends on what you’re printing on and run lengths: it could be cartridges or 20 L buckets for labels, or 200 L drums or even 1,000 L containers for higher-volume flexible packaging and paperboard.

And how does an ink dispenser work?

Imagine that a printer receives an order for printing cardboard boxes, for that same soft drink brand in our example above. The operator looks for the right orange colour in the list of formulas in the user interface and enters the required weight.

Once the dispensing job is entered, the computer then starts dispensing each ingredient to make that special orange colour. Each container with the selected ingredients is activated one-by-one: the valves of their connecting hoses are opened for a specified time, so that a precise quantity of each base ingredient (the varnish, red and yellow in our example) is pumped to the dispensing head and into the bucket. Activating the valves with split-second accuracy means precise ink quantities can be dispensed, to an accuracy of 1g (or tighter on request) – and in turn, precise colour targets can be achieved.

Gravimetric versus volumetric dispensing

In an unforeseen case of overdosing one of the ingredients, systems equipped with a weighing scale automatically recognize the overdose and calculate how much of the other ingredients can be additionally dispensed, to reach the required colour target.

This automatic feedback loop to ensure colour precision is one of the main reasons that gravimetric dispensing systems are used in industrial printing, rather than dispensers that work by volume that you can find at paint shops and DIY stores.

Volumetric tinting machines do not include a weighing scale and do not have this feedback and correction capability. This equipment therefore does not meet the colour consistency demands of the brand owners.

After each ingredient is dispensed and blended, the bucket of ink is ready for the press. The operation for making a single spot colour should take just a few minutes. Simply repeat this cycle for your job’s other spot colours.

Returning ink

In flexo and gravure, a quantity of ink is left in the chamber or connecting hoses at the end of the printing run. On the wide-web press in our example, this can be about 7kg per printing station.

In a next blog we will explain how these return inks can be reused in repeat jobs, or even better, recycled in new job with other colours!

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