The central impression press, sometimes called a drum, common impression or CI press, supports all of its color stations around a single steel impression cylinder mounted in the main press frame, Figure 4-7. The impression cylinder supports the web, which is thereby "locked" to the cylinder as it passes all color stations. This configuration helps prevent register shift from color to color.
Since the greatest advantage of the central impression cylinder press is its ability to hold excellent register, it has become the mainstay of the converting industry. Also, with graphic designs becoming more complicated and the demand for process printing remaining steady, the positive register ability of the CI press makes it suitable for all types of substrates. The most common press in use today is still the six-color central impression press, although this is being superceded by the eight-color CI press. Even ten-color CI presses are being now built.
Impression cylinders of various diameters have been used. At first, four-color presses were the most common, and they generally used 30 or 36-diameter impression cylinders. To get better speed and allow room for interstation drying, impression cylinders up to 60" were used for four-color presses. The first six-color central impression cylinder presses used 83" diameter cylinders. The latest eight-color central impression presses have cylinders up to 94" in diameter. As drying techniques have improved and the distance required for drying between colors has decreased, smaller impression cylinders have come back into use. The most common eight-color single impression cylinder press today has an 89" (2.26 meter) diameter cylinder.
Thanks to advances in between-color drying, the adage that "larger cylinder presses usually offer higher speeds" no longer applies. In general, however, it is still possible to get longer printing repeat lengths on the larger impression cylinder presses than those of smaller design.
The central impression press has found limited use when it comes to printing both sides of a web during one pass through the press, most commonly in tubular film printing.
Central Impression Drum Drum construction in a central impression press can be of double-wall steel or cast iron. In either case the drum will be temperature-controlled by a heating/cooling device. As markets have advanced in flexography, and the product expected from a CI press has improved to high degree of excellence, press manufacturers have been compelled to hold more demanding tolerances in regard to the CI drum. Very commonly employed today is the use of digitally controlled heating/cooling elements, which hold drum temperature within a range of ±1°F. This close tolerance is a necessary element in the printing excellence being demanded of press manufacturers and converters today. If the press experiences variation in drum temperature, which causes CI drum size variation, the operator will be continually attempting to compensate, causing missed impressions or overimpression.
The CI cylinder, independent of its method of construction, must be manufactured to meet high Total Indicated Runout (TIR) characteristics. It is very common today that specifications on a new press dictate that the drum not exceed 0.0003" TIR. Ideally, the drum will be held to a lower actual number if possible. Realizing that the TIR of the drum will only be as good as the supporting journal and bearings, manufactures of presses are demanding the use of printer-quality roller bearings with a TIR of around 0.0002". The use of custom hand-fit bronze bushings, which were very prevalent in the past, is losing favor with manufacturers today. The bronze bushings must be constantly lubricated, usually with a lube pump. The advantage of printer-quality roller bearings manufactured to acceptable tolerances is that they can be lubricated in a similar manner to gearbox (enclosed oil bath or grease pack).