OFFSET PRINTING

Offset printing

How offset lithography actually works, why ink and water balance decides almost every colour argument, and what to check when a litho press will not hold.

How offset works

Offset lithography prints from a flat plate. There is no relief and no engraving. The image area and the non-image area sit on the same plane, and they are separated chemically: the image area accepts ink and repels water, the non-image area accepts water and repels ink.

The plate never touches the substrate. It transfers the inked image to a rubber blanket, and the blanket transfers it to the sheet or web. That indirect step is what the word offset refers to, and it is why litho can print cleanly onto textured stock that a direct process would struggle with.

The consequence of the chemistry is that a litho press is always running two fluids at once. Almost every instability people blame on ink, on plates or on the operator is a dampening problem underneath.

Ink and water balance

Too little water and the non-image area starts accepting ink, which appears as scumming or toning in the background. Too much water and the ink emulsifies, density falls, drying slows and the sheet can pick up dimensional problems from the moisture.

The working window between those two states is narrower than most people expect, and it moves during the run as temperature, fountain solution concentration and substrate all change. The practical approach is to control what you can measure: fountain solution conductivity and pH, dampening roller settings, press and solution temperature, and ink density at fixed intervals rather than by eye.

One useful diagnostic: if density is drifting and the operator keeps increasing ink to compensate, the actual problem is almost always on the water side.

Where offset fits in packaging

Folding cartons are the main packaging application. Sheetfed offset gives high image quality on coated board at commercially sensible run lengths.
Labels, both sheetfed and narrow web, particularly where fine type and detailed images matter.
Litho-laminated corrugated, where a litho printed sheet is laminated to fluting to get print quality that post-print flexo on brown liner cannot reach.
Metal decorating for cans and closures, which is offset applied to a very different substrate.

Where offset is weak is film. Conventional offset inks need an absorbent substrate to set. Printing offset on non-absorbent material means UV or LED UV curing, which is a different ink system, a different cost and a different set of adhesion questions.

Keeping colour stable

Offset responds well to calibration because the process is fundamentally repeatable once the mechanics are right. The order of work matters.

Fix mechanical and chemical stability first: rollers, blankets, packing, dampening, fountain solution. Calibrating an unstable press produces a correction that will not survive the week.
Fingerprint the press as it currently runs, on the board you actually use.
Apply plate curves so the tonal response and grey balance land on target. This is the G7 calibration step, described in detail on the G7 page.
Verify on a live job and set tolerances the press can genuinely hold on that stock.
Measure routinely and keep the records, so drift is visible before a customer finds it.

Total area coverage is worth checking at prepress rather than at press. Too much ink in the shadows on coated board causes set-off and slow drying that no amount of press adjustment fixes. The TAC calculator gives you the figure, and explains why the acceptable limit depends on your process and substrate rather than on a universal number.

Common offset problems

Symptom	Usual causes	First checks
Scumming or toning	Insufficient dampening, plate or roller condition, fountain solution chemistry	Solution conductivity and pH, dampening roller pressure, plate age
Density drifting during the run	Ink and water balance, temperature rise, roller settings	Solution temperature, roller stripe, ink duct condition
Ghosting	Ink starvation across the form, roller pattern, layout	Form layout and ink demand across the cylinder, roller condition
Set-off in the delivery	Excessive total ink, slow drying, stack pressure	TAC value, spray powder, stack height, drying time
Misregistration	Sheet feed, paper dimensional change, cylinder packing	Feeder timing, board moisture and storage, packing measurements
Mottled solids	Substrate absorbency variation, blanket condition, ink transfer	Blanket surface, ink tack, substrate uniformity

More defects, across all processes, are covered on the troubleshooting page.

Frequently asked questions

What is offset printing?

Offset lithography prints from a flat plate where the image area accepts ink and repels water while the non-image area does the opposite. The plate transfers the image to a rubber blanket, and the blanket transfers it to the substrate, which is why it is called offset.

Why is ink and water balance so important in offset?

Because the separation between image and non-image area is chemical rather than physical. Too little water lets ink appear in the background; too much water emulsifies the ink and drops density. Most offset colour instability traces back to the dampening side rather than the ink side.

Can offset print on plastic film?

Conventional offset inks need an absorbent substrate to set, so film normally requires UV or LED UV curing offset. That works, but it is a different ink system with its own cost and adhesion considerations. For most film packaging, flexo, gravure or digital are the usual choices.

What is litho lamination in corrugated?

A sheet is printed by offset and then laminated onto corrugated fluting, instead of printing directly onto the brown liner. It gives much higher print quality than post-print flexo, at a higher cost and with an extra converting step.