Offset Printing · The Complete Guide

Offset printing is the dominant printing process for commercial and packaging work worldwide, and in India, it accounts for the vast majority of brochures, catalogues, cartons, labels, and books produced each year. Understanding why offset works the way it does, rather than simply accepting that it does, is the foundation for every quality and production decision in print.

The process is built on a simple chemical principle: oil and water do not mix. An offset printing plate has two zones, image areas that accept oil-based ink and repel water, and non-image areas that accept water and repel ink. When the plate is dampened with fountain solution and then inked, ink sticks only to the image areas. The plate transfers its ink image to a rubber blanket cylinder, and the blanket in turn transfers it to the substrate. The ink goes from plate to blanket to paper, it is "offset" by the blanket, which is why the process is called offset printing.

Why offset instead of printing plate directly onto paper?

The rubber blanket serves two critical purposes. First, it conforms to minor surface irregularities in the substrate, the compressible rubber makes contact with the peaks and valleys of a coated paper surface more uniformly than a rigid metal plate could. This produces more consistent ink transfer and sharper dots. Second, it protects the delicate photopolymer plate surface from direct contact with abrasive substrates, the blanket wears instead of the plate, which is far more economical to replace.

Understanding what happens at each of the two nip points (plate-to-blanket and blanket-to-substrate) is essential for diagnosing quality problems.

Plate-to-blanket nip

The plate cylinder and blanket cylinder rotate in opposite directions and are set with a small amount of squeeze (typically 0.10–0.15mm of compression between the two cylinders). At this nip, the ink film on the plate image area splits, approximately half transfers to the blanket and half remains on the plate. The efficiency of this transfer depends on ink tack, ink film thickness, blanket surface condition, and the balance between the ink and fountain solution. Too much fountain solution emulsified into the ink reduces ink tack and degrades transfer.

Blanket-to-substrate nip

The blanket cylinder and impression cylinder squeeze the substrate between them. The impression pressure is set to achieve full ink transfer from blanket to substrate across the entire sheet. At this nip, most of the ink on the blanket transfers to the substrate, typically 75–90% under good conditions. The rubber blanket conforms to the substrate surface, ensuring contact even with slight caliper variation across the sheet.

Ink film thickness in offset

The ink film in offset printing is extremely thin, approximately 1–3 microns (micrometres) on the printed sheet. For reference, a human hair is approximately 70 microns in diameter. This thin film is what allows offset to produce fine halftone dots and sharp detail. It also means that ink density is very sensitive to small changes in ink film thickness, a 0.5 micron change in film thickness produces a visible density difference on the printed sheet.

Offset printing comes in two fundamental configurations, sheetfed (printing on individual cut sheets) and web (printing on a continuous roll). Each has a different economic and quality profile, and each dominates different market segments.

For the purposes of this guide and the work produced at Paper Print Services, sheetfed offset is the relevant configuration. All subsequent sections refer to sheetfed offset unless stated otherwise.

A sheetfed offset press is described by the number of printing units it carries, its sheet format (maximum sheet size), and whether it can print both sides of the sheet in one pass (perfecting).

Printing unit count

• 1-colour press, one printing unit. Prints a single ink per pass. Used for simple stationery, forms, spot colour work.
• 2-colour press, two units. Can print two colours in one pass. Common for 2-colour commercial stationery and simple packaging.
• 4-colour press (4C), four units. The standard for full-colour CMYK work. One unit per colour: C, M, Y, K. The most common press configuration in Indian commercial printing.
• 5-colour press (4C + 1), five units. Four CMYK units plus one additional unit for a spot colour, UV coating, or aqueous varnish in-line. Very common for premium packaging and commercial work requiring an in-line finishing unit.
• 6-colour and above, for specialist work: 4C + 2 spot colours, or 4C + coating + sealing, etc. Found in premium packaging press rooms.

Sheet format

Press format is described by the maximum sheet size it can handle. Common formats in India:

• SRA3 / B2 format, approximately 450×640mm. Common for smaller commercial press rooms, suitable for A4 work with bleed in 8-up layouts.
• B1 format, approximately 720×1020mm. The most common format for general commercial and packaging printing in India. Handles A1 posters, A4 booklets in 16-up, and standard packaging blanks.
• Large format (beyond B1), 750×1050mm and above. Used for large-format posters and wide packaging blanks. Less common but present in larger Indian press rooms.

Perfecting

A perfecting press can print both sides of the sheet in one pass by turning the sheet between printing units. This requires precise mechanical alignment and is more complex to set up than single-side printing. Most standard Indian commercial presses are not perfecting, jobs requiring printing on both sides (booklet inners, double-sided packaging inners) are printed in two passes: first the front, then the back after the first side has dried.

Makeready is the complete setup process that takes a press from idle to producing saleable printed sheets. It is the most labour- and time-intensive part of any offset print job and the primary source of paper waste. Understanding makeready helps clients understand why short-run printing costs more per copy than long-run, the makeready cost is amortised over the total run quantity.

The makeready sequence

Makeready waste

The sheets used during makeready, from the first test sheet through to the OK sheet, are waste. On a standard 4-colour job on a modern well-maintained press, makeready waste is typically 100–250 sheets for commercial work and 150–400 sheets for packaging (which has tighter colour tolerances). On older presses or with difficult jobs, makeready waste can reach 500–800 sheets. This waste is a fixed cost per job, it is why minimum print quantities exist, and why the cost per copy drops significantly with longer runs.

In a 4-colour press, the four inks are printed in sequence, one unit after the next, with each ink printing wet-on-wet over the previous one. The order in which the inks are printed affects ink trapping (how well each subsequent ink adheres to the previous one), final colour appearance, and dot gain. The colour sequence is not arbitrary.

Standard CMYK sequence

The most widely used sequence in Indian sheetfed offset printing is: Cyan → Magenta → Yellow → Black (first to last unit). This sequence is derived from ink tack considerations, each subsequent ink must have lower tack than the one before it to achieve good wet trapping. In a 5-colour press with a spot colour, the spot colour is typically the 5th unit.

Why this sequence specifically

• Cyan first, cyan has the highest tack and provides the base for overprinting magenta and yellow. Cyan is also the most transparent of the four inks and lays down well as the first layer.
• Magenta second, slightly lower tack than cyan, traps well over cyan. The C+M overprint produces the blue tones.
• Yellow third, lowest tack of the three primaries. As the most transparent ink, yellow can be printed over darker inks without significantly affecting them. The Y+M overprint produces red, C+Y produces green.
• Black last, black is the densest, least transparent ink. Printing it last ensures maximum black density and sharpness, particularly for text and fine detail. Black printed over wet CMY produces cleaner shadow detail than black printed under the colour inks.

Wet trapping

Wet trapping is the ability of a subsequent ink to adhere to an ink that has not yet dried (wet-on-wet printing). In offset, every colour after the first prints onto a wet ink surface. If trapping is poor, the subsequent ink sits on top of the previous ink film without bonding, producing flat, weak secondary colours and incorrect hue in overprint areas. Trapping is measured from the printed sheet using a densitometer: the trap percentage should be above 85% for good colour reproduction.

Once the OK sheet is signed off and production begins, the press operator's job shifts from setup to monitoring and control. An offset press in production is a dynamic system, dozens of variables can drift during a run, and the operator must identify and correct deviations before they produce unacceptable sheets.

What to check, how often

The OK sheet · its role during the run

The approved OK sheet must remain at the press throughout the run, protected from light and handling. The press operator compares production sheets to the OK sheet throughout the run, visually and with measurement. If a production sheet deviates from the OK sheet beyond the agreed tolerance, the press is stopped, corrected, and a new comparison sheet is pulled. Sheets produced during the deviation period may be quarantined and inspected before being included in the final count.