Understanding dot gain and its common causes in flexo printing

17 Jun 2021
Dot gains

When setting up print operations to deliver packaging and labels that wow, there are many varied and intricate variables that can affect the quality of the result.

 

After all, a design can look great on screen, but translating a great CAD render faithfully and consistently onto a physical medium or substrate is another matter.

 

Getting it right has significant bottom line implications – even the slightest deviations, defects and imperfections can cause high volumes of packaging to go to waste before ever reaching the store shelves. Of course, this brings additional cost in terms of materials, time and energy use, bumping up costs and increasing CO2 output at a time when energy conservation is paramount.

 

One of the more complex defects in flexographic printing is dot gain. Flexographic print relies on countless tiny dots of colour across the substrate to give sharply defined detail, vibrancy and effective colour gradient. These dots or halftones, which are imperceptible from standard visual ranges, combine to create an image or a printed colour in Cyan, Magenta, Yellow & Black (CMYK) colours.

 

However, dot gain is where these dots increase in size when applied to the packaging material. The result is that colour and lines are not authentically recreated, with colours often appearing much darker and details becoming fuzzier – like looking through a blurry lens.

 

Using today’s modern flexographic techniques, dot gain cannot be avoided altogether, but it can be managed, minimised or compensated for. A dot gain curve is one approach to this, which aligns press settings with the effect dot gain, in order to mimic the target press tone response.

 

From halftones, midtones and beyond, a dedicated prepress partner like Creation is a valuable support for printers and designers in preparing for dot gain and anticipating its effects.

 

No two print presses or operations are exactly alike, which means there’s no ‘one-size-fits-all’ approach to addressing dot gain. One setup may print with higher dot gain than another, despite seeming to be very similar. Variables that can influence dot gain include the type of press, the substrate being printed onto, ink type and colour, roller pressure and even how regularly the printing press is maintained. Dot gain is measured with a densitometer and colour bar, usually using 1%, 2%, 5% and 50% tones as key reference values.

 

All too often, when speed and volume become a priority, printers forego full dot gain checks to speed up the production process, but this false economy can be a major source of errors and waste – and unexpected press downtime.

 

So, what are some of the common causes of dot gain?

 

Ink viscosity is too low

When creating the dots needed to add colour, detail and sharpness to packaging graphics, ink viscosity plays a big role. Too thick and the ink takes on an adhesive effect – too thin and it’s more likely to spread across the substrate beyond its intended print areas.

 

High pressure on print cylinder

Excessive pressure is one of the most common causes of dot gain – and one of the easiest to avoid. Flexo is a pressure-sensitive process, and too much or too little pressure can have a big impact on the quality and consistency of print. If in doubt about what’s causing dot gain, trial and error with the print cylinder pressure settings is a good first port of call.

 

Incorrect thickness of flexo plates

Due to the forces involved in the flexographic print process, the plates or sleeves may change shape. Swelling can be a side effect from the platemaking process, where solvents used have not fully evaporated during the drying process. Precision is everything when it comes to delivering quality and speed, and a swollen or enlarged plate behaves very differently on-press. This effect can increase pressure and as a result, dot gain.

 

One way to tackle this challenge is to switch to water-washable flexo plates, such as our own Asahi AWP™ plates, which remove solvents from the equation altogether.

 

Worn press machinery

Like with any modern technology, the flexo press used to create labels and packaging can be subject to the rigours of use. For example, worn or unstable cylinders can affect how the plate contacts the substrate, or a misaligned gear could create unwanted vibrations that make it tough to get a clean transfer. Regular maintenance and inspection should help negate this or flag up potential issues.

 

Avoid dots entering the anilox

Pay close attention to the dots on the plate of printing sleeve – they should not be smaller than the cell on the anilox roll. Otherwise, the dot can enter this cell, picking up more ink than intended, creating larger and more pronounced dots than desired. Naturally, this increased ink pickup impacts its effect on the substrate, increasing dot gain.

 

Incorrect mounting tapes

Engineered to be micron-precise, plate mounting tapes are an essential piece of the flexo print puzzle and need to be selected with care for each print job. Even by just thousandths of an inch, harder or thicker tapes can push the image carrier onto the substrate with enough increased pressure to make a noticeable difference and create unwanted dot gain. For the same reason, operators must also take care to ensure there’s no trapped air under the tape that could impact plate pressure.

 

Dot gain is a common occurrence in flexo printing and is often unavoidable. What matters most is how well printers are able to compensate and minimise its impact. However, with the speeds printers are now required to run at to compete, an outsourced specialist often makes more sense than using in-house resources to anticipate dot gain.

 

A powerhouse reprographic partner like Creation Reprographics can help printers deliver quality, accuracy and speed on every job, no matter the substrate or project complexity.

 

Looking to shift gear and turn your flexographic prepress into a key business advantage? Contact the Creation team today to find out how simple it can be – https://www.creation-repro.com/say-hello/

Questions?
Contact us on +44 (0) 1327 312444  or info@creation-repro.com
and we’ll be happy to help