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Improving Flexographic Print Quality on Corrugated Cardboard

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Man checking colour accuracy on flexographic printing output

Flexographic printing has become a popular choice for printing on corrugated cardboard packaging due to its eco-friendly nature and cost-effectiveness. However, achieving photographic print quality can be a challenge. In this blog post, we will explore how to achieve photographic print quality with a 4 colour flexographic printing press on corrugated cardboard.

Flexographic printing was first patented in England in 1890, but it was not widely used until the 1950s when advancements in technology made it a more practical and efficient printing process.

Understanding the Basics of Flexographic Printing

Before diving into the specifics of achieving photographic print quality, it's important to understand the basics of flexographic printing. Flexographic printing is a relief printing process that utilizes flexible printing plates, also known as stereos, to transfer ink onto a substrate such as cardboard. The printing plates are made of rubber or photopolymer and have a raised image that transfers the ink onto the substrate.

Digitally controlled flexographic ink wells - ink tray, ink roller, anilox roller, doctor blade combined

Choosing the Right Ink

One of the most critical components of achieving photographic print quality is choosing the right ink. Water-based inks are a popular choice for flexographic printing as they are eco-friendly and produce vibrant colours. However, depending on the machine's limitations, they may not be suitable for achieving photographic print quality. Some machines require solvent-based inks as they provide better ink transfer and adhesion, resulting in sharper images, however in the context of postal packaging made from pourous corrugated cardboard, water based inks are the go-to option and offer the best balance on quality and sustainability.

There are two main types of inks used in flexographic printing: water-based and solvent-based inks.

Illustration / diagram of a basic flexographic printing press

Water-based inks are made of pigments or dyes suspended in water. They are eco-friendly and produce vibrant colours. They are also easy to clean up and have a low odour. Water-based inks are commonly used in flexographic printing for paper and cardboard substrates.

Solvent-based inks are made of pigments or dyes suspended in a solvent such as alcohol, acetone, or ethyl acetate. They provide better ink transfer and adhesion, resulting in sharper images. Solvent-based inks are commonly used for printing on non-porous substrates such as plastic films and metal foils.

UV-curable inks are a third type of ink that can be used in flexographic printing. They are cured by exposure to ultraviolet light, which causes the ink to harden and bond to the substrate. UV-curable inks are fast-drying and produce bright, vibrant colours. They are commonly used for printing on non-porous substrates such as plastic films and metal foils.

The type of ink used in flexographic printing depends entirely on the substrate being printed on and the desired properties of the final product. Water-based inks are a popular choice for eco-friendly printing on paper and cardboard substrates, while solvent-based inks are commonly used for printing on non-porous substrates. UV-curable inks are also a viable option for printing on non-porous substrates and provide fast curing times and bright colours.

Selecting the Right Printing Plate

Choosing the right printing plate is essential for achieving photographic print quality. Photopolymer plates are the most commonly used printing plates for flexographic printing. They are known for their durability, flexibility, and ability to produce fine details. When choosing a photopolymer plate, it's important to select a plate with a high resolution to ensure sharp and clear images.

UVR Photopolymer Printing Plate
Archive image of a traditional photopolymer printing plate (source)

Photopolymer plates are a type of printing plate that is commonly used in flexographic printing. These plates are made of a photosensitive material that hardens when exposed to ultraviolet light. The plate is first coated with a liquid photopolymer that is then exposed to a film negative of the desired image. The areas of the plate that are exposed to the light harden, while the unexposed areas remain soft. After exposure, the plate is washed with a solvent that removes the unexposed areas, leaving behind a raised image that is used to transfer ink onto the substrate. Photopolymer plates are known for their durability, flexibility, and ability to produce fine details, making them an excellent choice for achieving photographic print quality in flexographic printing.

There are other types of printing plates that can be used in flexographic printing, such as rubber plates and elastomeric sleeves. However, photopolymer plates are the most commonly used due to their durability, flexibility, and ability to produce fine details.

Adjusting the Press Settings

Another critical factor in achieving photographic print quality is adjusting the press settings. The pressure, ink viscosity, and printing speed must be adjusted to ensure optimal ink transfer and image clarity. The pressure must be set to the appropriate level to ensure the printing plate is in contact with the substrate, but not too much pressure so that it causes distortion.

The ink viscosity must be adjusted to provide consistent ink coverage, and the printing speed must be set to ensure the ink is transferred to the substrate at the right rate.

There are several ways to adjust a flexographic printing press to achieve optimal print quality and consistency:

  1. Pressure: The pressure between the printing plate and substrate must be set to the appropriate level to ensure that the plate is in contact with the substrate but not so much pressure that it causes distortion. Too little pressure can result in incomplete ink transfer and weak images, while too much pressure can cause distortion and smearing.
  2. Ink viscosity: Ink viscosity must be adjusted to provide consistent ink coverage. If the ink is too thick, it can result in uneven coverage and produce a rough texture. If the ink is too thin, it can result in a lack of ink transfer, leading to weak images.
  3. Printing speed: The printing speed must be set to ensure that the ink is transferred to the substrate at the right rate. If the speed is too slow, it can result in over-inking and produce a blurry image. If the speed is too fast, it can result in incomplete ink transfer and a weak image.
  4. Plate-to-substrate gap: The gap between the printing plate and substrate must be set to the appropriate level to ensure that the plate is in contact with the substrate. If the gap is too small, the plate can be damaged, and the substrate can be over-inked. If the gap is too large, the ink transfer can be weak, leading to a faint image.

By adjusting these press settings, businesses can ensure optimal ink transfer and image clarity, resulting in high-quality, eco-friendly packaging that meets their customers' demands for visually appealing products.

Conclusion

Achieving photographic print quality with a 4 colour flexographic printing press on corrugated cardboard is possible with the right ink, printing plate setup, and expert knowledge of press settings.

By following these guidelines, The Packaging Club can produce high-quality, eco-friendly packaging that meets customers' demands for sustainability, whilst also looking the part.

Talk to us today to learn more about our bespoke packaging services.

This article was written by...

Nathan Calvert

Nathan is Head of Digital at The Packaging Club. He has worked in the packaging sector for over 15 years across food, consumer electronics, FMCG ecommerce and more, both directly in-house or as a freelance consultant.

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