Offset Printing (also called Offset Lithography) is a printing technique that uses aluminium plates, rubber blankets, and rollers to transfer an image on paper, paperboard, plastic sheets, synthetic paper, cardboard and other print media.
Although the process sounds simple enough, it is actually a very intricate combination of different components working together.
This article explains how offset printing works, explores different offset printing techniques, and examines both its benefits and drawbacks.
Offset Printing, also known as offset lithography, prints by transferring ink from a metal plate to a rubber blanket. The paper then passes between the blanket cylinder and the impression cylinder, where the image is printed.
This indirect transfer is why it’s called “offset.”
The offset printing process relies on the principle that oil and water don’t mix. The printing plate is treated to have both oil-attracting (oleophilic) and water-attracting (hydrophilic) areas.
Ink adheres to the oleophilic areas, while water covers the hydrophilic areas, repelling the oil-based ink.
Let’s look at the units or parts of a typical offset-printing machine.
The offset printing process starts at the feeding unit, which guides sheets of paper into the printing unit. This system controls paper feeding for the entire process.
Feeding units typically operate at speeds between 10,000 and 20,000 sheets per hour. Heidelberger Druckmaschinen AG’s Speedmaster XL 106 is the world’s fastest offset printer which can print 21,000 70 x 100 format printers per hour.
Now let’s look at the components of this incredible printing machine.
There are two basic configurations of feeder sections.
The feeding unit must be carefully calibrated by the press operators to the specific paper stock being used. Factors such as paper GSM, texture, and even humidity also come into play when calibrating an offset printer.
The printing unit is the core of the offset press, where ink is transferred to the paper. It consists of several critical components, each playing a vital role in the printing process:
The plate cylinder assembly is the heart of the offset printing process. Modern plate cylinders are designed to work with precision-made printing plates.
Today’s printing plates are typically crafted from aluminium or polyester, with aluminium being the more common choice for its durability and stability.
These plates are incredibly thin, usually ranging from 0.15 to 0.30 mm in thickness. Thinner plates have finer reproduction details.
To prepare the plate surface for printing, manufacturers use a process called anodizing and graining. This treatment creates a microscopically rough surface that improves the delicate balance between ink and water adhesion – crucial for the offset printing process.
The plate’s surface is divided into image and non-image areas. During printing, the inking system applies ink to the image areas, while a dampening system wets the non-image areas with a water-based solution.
This clevers use of the oil-and-water-don’t-mix principle is what makes offset printing so effective. Modern plates have resolutions up to a 2,540 dots per inch (dpi), resulting in incredibly sharp and detailed prints.
The longevity of these plates is impressive too – a well-made metal plate can handle between 1 million to 1.5 million impressions before it needs replacing.
This lifespan of the plate makes offset printing cost-effective for large runs. Once the plate is inked, the image is transferred to a rubber blanket on the blanket cylinder.
The blanket cylinder is the intermediary between the plate and the paper. It’s wrapped with a special rubber blanket that’s both tough and flexible.
The rubber blanket picks up ink from the plate and transfers it cleanly to the paper. They’re typically made from a multi-layer rubber compound.
The top layer, which comes into contact with the ink and paper, is designed to be resilient yet soft enough to conform to the paper’s surface. This layer usually has a hardness between 75 and 85 on the Shore A scale – firm enough to hold detail, but with enough give to ensure even ink transfer.
The overall thickness of a typical blanket ranges from about 1.95 to 2.10 mm. This might not sound like much, but in precision printing, every micron counts.
Within this thin layer, manufacturers build in a compressibility of 7-15% which allows the blanket to adapt to slight irregularities in the paper surface, ensuring consistent ink transfer across the entire sheet.
The surface of the blanket is another critical factor. It needs to be smooth enough to pick up and release ink cleanly, but with just enough texture to hold the ink in place during transfer.
Most modern blankets have a surface roughness measured at 0.5 to 1.0 micrometres RMS (Root Mean Square). This ultra-fine texture is barely perceptible to the touch but makes a world of difference in print quality.
A well-maintained blanket can last for millions of impressions before needing replacement.
The impression cylinder applies a controlled pressure, ranging from 0.69 to 1.38 MPa (100 to 200 psi), pressing the paper against the blanket cylinder to transfer the inked image onto the sheet. Controlled pressure helps achieve a clear and precise print.
Impression cylinder has a surface hardness of 58-62 HRC (Rockwell C scale) and a diameter tolerance of ±0.0127 mm (±0.0005 inches), the impression cylinder ensures accurate, consistent image transfer while maintaining durability and precision throughout the printing process.
The purpose of the inking system is to transfer the correct amount of ink from the ink reservoir to the plate cylinder. The inking system is a precision-engineered setup with 15 to 25 rollers per colour unit.
The form rollers, which touch the plate, are softer – about 25 to 35 on the Shore A scale. The distribution rollers are a bit harder, around 40 to 50. This system keeps the ink at just the right consistency – between 40 and 100 Poise at room temperature.
The ink layer it applies is mind-bogglingly thin, just 0.5 to 1.5 micrometres. For perspective, that’s about a hundredth the thickness of a human hair. Temperature matters too. The system keeps the ink within one degree Celsius of the target temperature.
The rollers run at different speeds, typically with a 2:1 or 3:1 speed ratio between the fastest and slowest. This speed difference distributes the ink evenly.
The inks come from a central reservoir through pipes. The ink management software constantly tweaks the ink flow and other parameters to make sure every part of the image gets just the right amount.
The damping system applies a water-based solution to keep ink away from the non-image areas of the printing plate. This solution is a mixture of alcohol, acid, and synthetic gums.
The alcohol helps the solution spread evenly, the acid keeps the pH in check, and the gums protect the plate.
This solution is applied cloth covered rollers. These rollers distribute the solution across the plate, making sure every non-image area gets a thin, even coat.
Some presses use a conventional system where the solution is applied intermittently, while others use a continuous system that keeps a steady flow going
Getting the chemistry of the dampening solution right is important. The solution needs to be slightly acidic, with a pH between 4.8 and 5.5. Conductivity should be between 800 and 1500 microsiemens per centimetre.
Older systems used about 8-12% alcohol, but many printers are moving away from that now.
Water hardness matters too for preventing mineral buildup on plates. 8-12 degrees of German hardness is ideal. If the solution is softer it will lead to emulsification, foaming, and uneven distribution over non-image areas.
The temperature is also kept within half a degree celsius of the ideal temperature for consistent ink viscosity and drying time.
Modern systems are getting better at automatically controlling these variables. Some even recycle and filter the solution to reduce waste.
The delivery unit stacks the printed sheets and prepares them for post-press handling. Let’s have a look at its components and their specifications.
The gripper bars are made of hardened steel with rubber-coated gripping surfaces.
They grab printed sheets from the impression cylinder and transport them to the delivery section. Each gripper provides a grip force of 5-10 N, with opening and closing times of less than 20 milliseconds. The system has a typical precision of ±0.1 mm positioning accuracy.
Rear and side joggers in the delivery unit vibrate at speeds of 1,000 to 3,000 times per minute to align freshly printed sheets. Their movement ranges from 0.5 to 2 millimetres, which is sufficient to ensure accurate alignment of the paper stack.
Some modern joggers incorporate air jets to help separate sheets, and others have sensors to detect and correct misalignments during operation.
Good “jogging” is a prerequisite for subsequent steps like cutting, folding, or binding, ensuring that sheets are ready for the next stage of production.
Stoppers in a printing press align sheets as they come through during production. They are made from hardened steel and are applied with low-friction coatings. They catch the front and back edges of each sheet, and align them.
These stoppers can be adjusted by up to 2 millimetres, for smooth stacking and avoiding jams. Some presses now use servo-controlled stoppers that can adapt automatically based on changes in paper weight. Stoppers also take a lot of impact as sheets hit them constantly, so manufacturers are always exploring new materials and coatings to extend their lifespan.
Suction slowdowns and blowdowns manage the speed and placement of freshly printed sheets as they exit the press.
Suction slowdowns use negative pressure between 0.2 to 0.5 bar to hold and slow down the sheets, with airflow rates of 50 to 100 cubic metres per hour. Blowdowns, applying pressure from 0.5 to 1 bar, guide the sheets into place, preventing shifts or misalignment.
Slowdowns and blowdowns are controlled by servo motors that adjust in sync with the press speed. Some advanced presses adjust automatically based on paper type.
Skeleton wheels, typically made from lightweight aluminium with diameters ranging from 100 to 150 millimetres, guide freshly printed sheets through the delivery system in offset printing. These wheels are coated with non-marking material to prevent smudging and damage during high-speed operation.
Some presses use ceramic coatings for durability and smoother sheet transport.
Skeleton wheels are synchronised with the press to make sure that each sheet is guided properly to the pile. The design of these wheels helps prevent wrinkles and maintains the quality of the printed material.
Some presses include perforations for airflow cushioning the sheets during transport.
In the final stage of offset printing, anti-set-off powder is applied to prevent freshly printed sheets from sticking together.
The powder is typically made from natural starches like corn, wheat, or rice. Depending on the paper type, particle size ranges from 15 μm for lighter sheets (150 g/m²) to 70 μm for heavy boards (700 g/m²). Application rates vary between 0.5 to 1.5 g/m².
The system uses 4-8 pneumatically controlled spray nozzles per unit and has a powder container capacity of 5-10 kg. Special features include hydrophobic properties for use with water-based varnishes and electrostatic properties for improved sheet separation.
Delivery systems can either be manual (Clute Delivery) or automated (Chain Delivery), with speeds ranging from 3,000 sheets per hour for smaller runs up to 20,000 sheets per hour for high-speed presses.
Chain delivery systems can adjust their speed to match the press output. They use ion bars, which generate 5-10 kV, to eliminate static electricity from the sheets. To cool the sheets, these systems rely on infrared sensors and air knives.
Many modern presses include automated powder application systems that precisely control how much powder is applied and where it is distributed. Newer systems are also moving toward using environmentally friendlier powders and improving quality control by integrating in-line spectrophotometers and cameras to monitor the output in real time.
Sheet-fed offset printing uses individual sheets of paper fed into the offset press. Sheet-fed technique gives options with paper type and thickness – a requirement for sharp details and fine print quality.
For example, models like Heidelberg’s Speedmaster CX series can print on substrates with a maximum thickness of up to 1.0 mm, making it ideal for more printing on bags, art paper, metal laminated paper, and plastic sheets.
Sheet fed systems can print at 15,000 to 18,000 sheets per hour.
Sheet sizes for sheet-fed offset printing range from small presses like 12″ x 18″ (commonly used for business cards, flyers, and envelopes) to larger formats like 25″ x 38″ or even up to 55″ x 78″, offering versatility for different types of print projects and paper types.
Web offset printing is the traditional method that uses large rolls of paper that are continuously fed into the press.
It is widely used in high-volume commercial printing, especially for newspapers, magazines, brochures, and catalogues.
For example, in newspaper printing, web offset printing can print at 60,000 impressions per hour, making it one of the fastest and most cost-efficient options for high-volume print jobs.
Typical paper sizes are:
Once a web press starts running it does not stop till the end of the batch. If the paper breaks, it is manually re-threaded through the press, which is time-consuming and expensive.
Wet offset printing works by maintaining a crucial balance between water and ink to ensure high-quality prints. A dampening system applies a water-based solution to the non-image areas of the printing plate, preventing ink from adhering to those sections.
The balance—typically 1 part water to 3 parts ink—is vital; too much water can cause emulsion, leading to blurred prints, while insufficient water results in scumming, where ink transfers to non-image areas.
As the name suggests, waterless offset printing works without a water-based dampening system. This technique uses silicone-coated plates which repel ink from non-image areas.
By removing the need for maintaining the complex ink-water balance, waterless offset printing minimises issues like emulsification and scumming, which are common in traditional wet offset printing.
To ensure exceptional print quality and optimal performance, waterless offset presses must maintain precise temperature control between 25°C to 30°C (77°F to 86°F), as the lack of water increases the likelihood of heat buildup, which can affect ink viscosity and print quality.
A prominent example of waterless offset technology comes from Toray Industries, a leader in the field. Their waterless plates, designed for use with specialised inks, can handle print runs of up to 600,000 impressions, making them ideal for high-margin applications such as security printing (e.g., banknotes) and metal decorating.
In summary, offset printing is a solid choice for handling large-scale, high-quality print jobs.
It’s adaptable, works with a wide range of materials, and becomes cost-effective as the print volume increases.
While offset printing has higher fixed costs, it becomes more economical over large print runs.
For reliable equipment that supports your printing and production needs, visit Triton Store.
We supply thermal printers, thermal papers, check weighing machines, scanners, and a variety of other industrial solutions to help keep your operations on track.
Auckland
Christchurch
Phone 09 579 2057
Live Chat – Widget below
Auckland
Christchurch
Phone 09 579 2057
Live Chat – Widget below
