An in-depth guide to lithographic printing

Lithographic printing, also known as offset printing or offset lithography, is a widely used printing technique that produces high-quality, consistent, and cost-effective printed materials.

This guide will provide a comprehensive overview of lithographic printing, covering its principles, processes, advantages, and applications.

Whether you're a printing professional or simply interested in learning more about this technology, this guide will be valuable.

Contents

• An introduction to litho printing
• Pros & cons
• The lithographic print process
• Types of litho printing
• Materials used in offset litho printing
• Typical applications
• Environmental considerations
• Future trends
• Conclusion

1. An introduction to litho printing

Definition and history

Lithographic (aka offset printing method) is a print technique that transfers ink from a printing plate to a rubber blanket and then onto the printing surface.

It is based on the principle that oil and water do not mix.

Lithography was invented in 1796 by Alois Senefelder and has become one of the most widely used commercial printing methods.

Principles of lithography

Offset (printing): The term "offset" refers to the fact that the printing ink isn't applied directly to the printing surface but is instead 'offset' (or transferred) onto an intermediate surface, such as a rubber blanket, before progressing to the final substrate.

Oil and water repulsion: Lithography relies on the principle that water and ink repel one another. The litho printing plate gets chemically treated to attract oil based ink and repel water, creating the image on the plate.

2. Pros & cons

The advantages of lithography are as follows:

• A high-quality output: Lithographic printing offers excellent colour accuracy and image detail.
• Cost-effective for large runs: It becomes more cost-efficient as the print run quantity increases because the cost of the plates gets spread over more units.
• Versatility: It's used on many substrates, including paper, cardboard, plastic, and metal.
• Consistency: Lithographic printing provides consistent results across long print runs because it works with pre-mixed inks, and the plates incur very little wear.

The disadvantages of lithographic printing:

• A long setup time: The initial setup can be time-consuming, making it less suitable for small print runs. Each plate can cost hundreds of pounds; you need one per colour separation. Compare this to digital printing machines, and you can see why digital printing is fast becoming the de-facto printing method.
• High cost for short runs: Litho is less cost-effective than digital printing because of the setup costs. The ink is generally cheaper than digital ink.
• Not suitable for variable data: It's less suited for personalised or variable data printing because the plates would have to be changed for each variable, which is cost & time-prohibitive. 

3. The lithographic print process

Lithographic printing involves several stages:

Step 1 - Prepress

The pre-press stage takes and creates a piece of artwork and makes it ready to be printed. It involves:

• Design and layout: Create the digital artwork using design software.
• Image processing: Process and optimise the images for printing, ensuring the file's resolution is not too small or large.
• Plate making: Transfer the digital design onto printing plates.
• Plate alignment: Accurately align the plates for colour registration.

Step 2 - Printing

When the printing process starts, this is the process:

• Ink application: Firstly, the ink gets applied to the printing plates.
• Ink transfer: The ink gets transferred from the plates to the rubber blanket.
• Offset: The image is then 'offset' from the rubber blanket onto the final substrate.
• Printing multiple separations: Multiple plates and inks are used for full-colour printing. Up to 16,000 colours can be created by layering CMYK process colours on one another.

Step 3 - Post-press

• Drying and curing: The printed materials are dried or cured using IR or UV light to prevent smudging.
• Cutting and finishing: The printed sheets are cut, folded, and finished as needed.
• Quality control: Inspect the final product for defects and colour accuracy.

4. Types of litho printing

There are 2 main types of lithography, with arguably a 3rd called Digital Offset. This method, however, is more of a digital print technology based on lithographic printing.

The 2 main types are sheet-fed and web offset printing. These two really refer to the media that are put through the print machine, not the actual print technology itself.

We cover each briefly below:

Sheet-fed offset printing

This is what a sheet-fed lithographic printer looks like:

Sheet-fed machines take single sheets of paper into the press.

Sheet-fed machines are better suited for short to medium-print runs. They are regularly used for printing high-quality brochures, posters, and packaging.

Sheets are ideal when printing on paper; however, they can be problematic when printing onto self-adhesive substrates such as vinyl for labels. The problem is that self-adhesive materials are more susceptible to curling, which causes issues when printing.

Web fed machines

This is what a web-fed lithographic printer looks like:

Large, continuous rolls of media up to 2000m get fed into one end of the machine. Roll-fed machines print faster than sheet machines and can reach speeds of up to 120 linear meters every minute!

This makes web-fed machines suitable for high-volume printing like newspapers and magazines.

Digital offset

This is more of a digital technology inspired by lithography, combining digital and offset lithography technologies.

Instead of etching the image onto an aluminium plate, a laser positively charges a thin sheet of foil that attracts ink molecules. 

It's an incredible technology that combines the best digital (low setup costs) with the quality and speed of traditional offset printing processes.

5. Materials used in offset litho printing

Printing plates

Printing plates are the main topic when discussing lithography.

A metal plate is typically aluminium coated with a photosensitive emulsion to hold the image.

The plate quality has the most significant effect on print quality.

Inks

Offset lithography uses oil based ink to ensure a consistent viscosity and colour consistency.

Remember, the lithography process depends on water & oil repelling one another, which is why water-based inks aren't used.

Substrates

The printing surface can be a wide range of materials, including paper, cardboard, plastic, or metal.

Different substrates impact print quality and finishing options. The most popular choices tend to be paper or card.

6. Typical applications

The top uses of the lithographic printing process media are:

Packaging

• Carton packaging for consumer goods.
• Labels, folding cartons, and product inserts.

Publishing

• Books, magazines, and newspapers.
• Brochures, catalogues, and promotional materials.

Promotional materials

• Flyers, posters, and direct mail pieces.
• Business cards and stationery.

Labels & tags

• Self-adhesive labels for products.
• Hang tags for clothing and accessories.

7. Environmental Considerations

Lithography is an old process and could be more environmentally friendly.

However, it has recently made some advances, including using vegetable oil-based inks. 

Vegetable-based inks are less harmful than older petroleum-based oils. Crude oil is used to make petroleum inks, and we're all aware of the impact this has on the planet.

Inks used for lithography consist of 3 main elements. A colourant (the pigment or dye), a binder (such as resin), and a carrier. It's the carrier which differentiates vegetable oil-based inks from petroleum alternatives.

Vegetable-based inks generally have their carriers made from soy, palm, corn, rice or linseed oil.

8. Future trends

The main 2 trends we expect to see in the lithographic world are to increase the inks' environmental sustainability and to increase automation around plate making, set up and colour control.

We've already discussed vegetable oil-based inks, how they differ from petrol-based oil and their benefits. We expect to see this continue at an even faster pace.

With robotics, AI and other technologies improving rapidly, we expect to see more automation, robotics, and data analytics integration to improve production efficiencies.

These will help to reduce setup times and give more consistent & accurate colour that all helps speed up and reduce waste.

9. Conclusion

In conclusion, the world of litho printing remains constantly evolving & adapting to meet shifting market demands and leverage the latest technological advancements.

Whether you're a seasoned printing professional or a business owner looking to understand the printing process, I hope this comprehensive guide has offered valuable insights into lithographic printing and its diverse applications.

As you navigate this dynamic industry, staying informed about the latest trends and embracing best practices will empower you to harness the capabilities of this versatile printing technology to your advantage.