Rotary presses are the workhorses of modern print. They print on paper that passes between a supporting cylinder and a cylinder holding the printing plates. This design stands in stark contrast to the flatbed press, which uses a flat printing surface. You will find these machines in high-speed, web-fed operations. They take paper from a roll, exactly like newspaper printing does.
The speed is staggering. Paper can pass through some presses at nearly 20 miles (30 km) per hour. What limits that speed? Mostly the tensile strength of the paper. Large presses can print up to 60,000 copies of 128 standard-size pages in an hour. That is not just printing. Many of these large machines cut, fold, and even bind in a cover. All in one continuous automatic process.
How the cylinder mechanism works
In its simplest form, a rotary press consists of two cylinders turning in opposite directions. One is the plate cylinder, which has curved printing plates attached to its surface. The other is the impression cylinder. It presses the paper against the inked plates as the paper travels between them.
A simple two-colour rotary press uses two plate cylinders in succession. Each bears a different typeform. Each has its own inking system. The same side of the same sheet receives two successive impressions of two different colours. It happens as the paper passes through.
Printing on both sides of a sheet, or in three, four, or even five colours, requires different combinations and successions of plate and impression cylinders. The geometry changes, but the principle stays the same. Ink transfers from plate to paper under pressure.
Scaling up for mass production
Extremely high rates of production happen in very large, highly automated roll-fed rotary presses. These machines have cylinders with a circumference large enough to accommodate two or more plates. With each revolution, the cylinder prints two or more copies of the same page.
Similar arrangements allow a single cylinder to print eight copies of the same page in one revolution. This multiplies output without multiplying machinery. It is efficient engineering.
Why color registration is hard to get right
Quality control is extremely important in colour printing. Why? Because the various colour impressions on a page must be in perfect register with each other. If they are not, the image looks blurry or misaligned.
Rotary presses handle this with photoelectric cells. These cells selectively pick up guide marks printed in each colour as they go by. They react to any irregularities in the distance between these guide marks.
Any error is automatically corrected. The system modifies either the speed of one group of cylinders or the pressure of the rollers that control the tension of the paper between one cylinder group and the next. It is an active feedback loop, not a passive one.
The machine adjusts in real time. It keeps the ink marks aligned. The result is sharp, consistent color, even at 20 miles per hour.
