Offset Printing: How it works and process

Offset printing, also known as offset lithography, is a widely used commercial printing method that produces high-quality printed materials such as newspapers, magazines, brochures, books, and more.

It is called “offset” because the ink is not directly applied to the paper but transferred (offset) from a metal plate to a rubber blanket and then onto the paper.

Sometimes, especially when visiting our facilities, customers ask us how offset printing works.

The offset printing system is quite unknown even to people who work in relation to printing companies, so we will explain it briefly

The offset printing process

Offset printing is a printing method in which a printing plate is used to transfer texts and images to a sheet of paper.

The plate is made of a light-sensitive material that is exposed to the content to be printed. The ink is transferred from the plate to a series of rollers and then to the sheet of paper.

Here’s how the offset printing process works:

1. Image Creation: The first step involves creating the printing plates. A computer-to-plate (CTP) system is used to transfer the digital image or design onto metal plates. Each plate represents one color used in the printing process—typically cyan, magenta, yellow, and black (CMYK).
2. Ink and Water Separation: Offset uses the principle that oil-based ink and water do not mix. The metal plates are designed in a way that keeps the ink-receptive areas separate from the non-image areas, which repel ink and attract water.
3. Ink Transfer: The plate cylinders are coated with ink, and water is applied to the rest of the non-image areas. The ink adheres to the image areas while the water covers the non-image areas. This ink-water balance ensures that only the image areas will be transferred to the next stage.
4. Offset Blanket: The inked image from the plate is transferred to a rubber blanket cylinder. The rubber blanket acts as an intermediary, receiving the inked image from the plate and allowing for a smoother transfer.
5. Paper Transfer: The paper is fed into the press and passed between the blanket cylinder and the impression cylinder. The blanket cylinder applies the inked image onto the paper with considerable precision and clarity. The impression cylinder applies pressure to ensure proper contact between the paper and the blanket cylinder.
6. Drying: After the ink is transferred to the paper, it needs to dry. In offset printing, the ink is typically oil-based, and it dries through evaporation or absorption into the paper. Additional drying methods, such as heat or air, may be used to expedite the process.

Offset offers several advantages over other printing methods:

1. High Image Quality: Offset printing produces sharp, detailed, and consistent images, making it ideal for projects that require precise color reproduction and fine details.
2. Cost-Effective for Large Runs: Offset printing becomes cost-effective when producing a large volume of printed materials. The setup costs are higher initially, but the per-unit cost decreases as the quantity increases.
3. Versatility: It can print on a variety of paper types, sizes, and thicknesses. It is suitable for printing on textured papers, envelopes, cardboard, and more.
4. Pantone Color Matching: Offset can accurately reproduce Pantone colors, ensuring consistency across different print jobs.

Next, the information to be printed is recorded on aluminum plates: one plate for each color of four-color printing: cyan, magenta, yellow, and black. The machines responsible for this recording process are called CTP (Computer-to-plate). From the CTP, the plates are placed in the printing machine.

In this video, we show you what a CTP looks like in operation:

To understand how offset printing works, it’s necessary to know that the printing plate is covered with a polymer layer that has the property of attracting (fixing) ink and repelling water throughout its surface. The bare surface of the plate, on the other hand, repels ink and attracts water. And you know… water and oil (ink) never mix.

The function of the CTP is precisely to remove from the plate, which is initially completely covered with polymer, the areas of the plate that are NOT going to be printed. This is achieved by “burning” the surface of the plate with a laser.

Finally, the areas that will accept ink retain the polymer, while those that are not going to be printed do not.

Inside the offset printing machine

Once the plates have been created, this is the sequence of steps that will allow us to print from paper, ink, and water:

The plates are placed on the plate cylinder holder (also called the illustration cylinder).

When the machine is started…

A set of rollers (dampening rollers) transmit a water-based solution from a bucket to the plate, wetting the surface. As explained above, only the areas with exposed metal will accept water.

Another set of rollers (inking rollers) transfer ink from another tray (ink fountain) to the plate. The ink will be fixed only in the areas where polymer is present. The areas with exposed metal will repel the ink, since a thin film of water is already on them.

However, and this is another point that distinguishes offset printing from other printing systems, the inked plate will not directly contact the paper: instead, a second cylinder called the offset cylinder is interposed, covered by the blanket, an elastic rubber-based surface.

The ink passes from the plate cylinder to the blanket.

And then…

The ink passes from the blanket to the paper. The paper circulates between the offset cylinder and a third cylinder whose purpose is to exert the right pressure that allows both printing and paper circulation.

The intermediate step from the blanket to the paper is necessary since if the paper were in direct contact with the plate, the image would deteriorate quickly because the paper is a very abrasive material (and the image on the plate, very delicate).

It should be noted that this process is repeated for each printing unit. To achieve a color image (four-color process), the paper will therefore pass through at least 4 printing units. In this other post, we explain in more detail how color printing is achieved.

And that’s how offset printing works!

In the following video from Sappi, the entire offset printing process is explained in a very enjoyable way. We recommend that you take a few minutes to watch it:

Types of Offset Printing

There are several types of offset printing, each designed for specific applications and requirements. Here are some common types:

1. Sheet-Fed Offset: In sheet-fed offset printing, individual sheets of paper are fed into the printing press. The sheets are typically pre-cut to the desired size and fed through the press one at a time. Sheet-fed offset is versatile and suitable for a wide range of projects, including brochures, postcards, stationery, and packaging.
2. Web Offset Printing: Web offset printing uses large rolls of paper instead of individual sheets. The paper is fed through the press in a continuous roll, allowing for high-speed production. Web offset printing is commonly used for high-volume projects like newspapers, magazines, catalogs, and newspaper inserts.
3. Coldset Offset: Coldset offset printing, also known as non-heatset offset, uses inks that dry through absorption into the paper without the need for additional drying methods. It is commonly used for printing newspapers, directories, and other publications that require fast production and cost-effectiveness. Coldset offset lithography is often done on uncoated papers.
4. Heatset Offset Printing: Heatset offset printing utilizes inks that dry through a combination of evaporation and heat. The paper passes through a dryer unit after the ink is applied, which speeds up the drying process. Heatset offset is suitable for glossy or coated papers, making it ideal for magazines, catalogs, and high-end brochures that require vibrant colors and sharp image reproduction.
5. UV Offset Printing: UV offset printing involves the use of ultraviolet (UV) inks that are cured instantly using UV light. The UV curing process results in faster drying times, vibrant colors, and enhanced durability. UV offset is commonly used for materials that require immediate finishing or further processing, such as packaging, labels, and high-quality marketing collateral.

Offset Lithography vs Digital Printing

Offset lithography and digital printing are two different printing methods, each with its own advantages and applications. Here’s a comparison between the two:

Offset Lithography:

1. Quality: Offset lithography offers high-quality and consistent results with sharp details and vibrant colors. It is particularly well-suited for projects that require precise color reproduction and fine details.
2. Cost-effectiveness for large runs: Offset becomes cost-effective when producing a large volume of printed materials. The setup costs are higher initially, but the per-unit cost decreases as the quantity increases.
3. Pantone Color Matching: Offset can accurately reproduce Pantone colors, ensuring consistency across different print jobs.
4. Versatility: It can print on various paper types, sizes, and thicknesses, including textured papers, envelopes, and cardboard.

Digital Printing:

1. Quick Turnaround: Digital printing offers fast turnaround times as it eliminates the need for time-consuming plate setup. Files can be directly sent to the printer for immediate printing.
2. Cost-effectiveness for short runs: Digital printing is cost-effective for small to medium print runs. There are no setup costs or minimum quantity requirements, making it suitable for on-demand or personalized printing.
3. Variable Data Printing: Digital printing allows for easy customization and variable data printing, where each printed piece can have unique text, images, or other elements.
4. Proofing and Prototyping: Digital printing is ideal for producing proofs and prototypes quickly and cost-effectively. It allows for easy modifications and adjustments before finalizing a design.

Choosing between offset lithography and digital printing depends on factors such as project requirements, quantity, budget, and turnaround time.

Offset is typically preferred for large-scale commercial printing jobs that demand high quality and cost-effectiveness for large quantities.

Pros and cons

Here are the pros and cons of offset printing:

Pros of Offset Printing

1. High-Quality Results: Offset printing delivers high-quality printed materials with sharp details, vibrant colors, and excellent color consistency. It is capable of reproducing intricate designs and fine lines accurately.
2. Cost-Effective for Large Runs: Offset becomes cost-effective when producing a large volume of printed materials. The setup costs are higher initially, but the per-unit cost decreases as the quantity increases, making it economical for large print runs.
3. Pantone Color Matching: Offset printing can accurately reproduce Pantone colors, ensuring consistency across different print jobs. This is particularly important for brands that require precise color matching for their logos and branding elements.
4. Versatility: Offset print is versatile and can be used to print on various paper types, sizes, and thicknesses. It can handle different textures, coatings, and specialty papers, providing flexibility for different project requirements.

Cons of Offset

1. Longer Setup Time: Offset lithography requires the creation of printing plates and initial setup, which takes more time compared to digital printing. This can result in longer turnaround times, especially for small or time-sensitive print runs.
2. Limited Customization: Offset is not as easily customizable as digital printing. It is more suitable for static designs rather than variable data printing or personalized content.
3. Minimum Quantity Requirements: Offset lithography may have minimum quantity requirements due to the setup and plate creation process. It may not be cost-effective or practical for very small print runs.
4. Potential Waste: Offset print can generate more waste during the setup and color calibration process, particularly when adjusting colors or making changes mid-print run. This can impact cost-effectiveness and environmental sustainability.

It’s important to consider these pros and cons when deciding whether to choose offset for a particular printing project.

Factors such as budget, quantity, quality requirements, turnaround time, and customization needs will help determine whether offset is the best choice or if another printing method like digital printing may be more suitable.

We are your offset printer

At Cevagraf, we have extensive experience as an offset printer. Do not hesitate to contact us if you need professional services to print your products.