The Inking System

The inking system has 4 basic functions

1. Move the ink from the ink fountain to the plate.

2. Break down the thick charge of ink into a thin uniform film around the rollers.

3. Work the ink into printing condition.

4. Remove image repeats on the form from previous printing cycles. The inking system on sheetfed presses usually has 10 rollers.

It includes the following parts

1. Ink fountain - a pan that contains the ink supply.

2. Ductor or ductor roller - a transfer roller that alternately contacts the ink fountain roller and the first roller of the inking system. Often an oscillator.

3. Oscillators or vibrators - gear or chain driven rollers that not only rotate but oscillate from side to side.

Distribute and smooth out the ink film and erase image patterns from the form roller.

4. Intermediate Rollers - friction- or gravity-driven rollers between the ductor and form roller that transfer and condition the ink. Often called the distributors if they contact two rollers or riders if they contact a single roller (such as an oscillator).

5. Form rollers - a series of 3 or 4 rollers (usually differing diameters) that contact the plate and transfer ink to it. The series of rollers is often called the roller train.

 

The roller train is composed of a series of hard and soft rollers. This long roller train is necessary because of the unique character of the paste ink. The ink is both thixotropic and pseudoplastic (shear thinning).

This property causes the initially large apparent viscosity to be greatly reduced under the shear provided by the rollers. The apparent viscosity decreases with time under a constant shear rate and also decreases with increasing shear rate. The hard rollers are usually steel covered with copper, ebonite or nylon. The soft rollers (ductor, intermediate and form) are made of synthetic rubber or other polymer. They may be PVC (polyvinl chloride), Buna-N (copolymer of butadiene and acrylonitrile) or polyurethane.

Ink fountain - Trough that holds the ink supply. The ink is formed by the fountain roller, a metal roller that turns intermittently or continuously, a fountain blade, which may be a spring steel plate, steel segments or plastic approaching the fountain roller at an angle and two fountain cheeks, which are vertical metal pieces that contact the fountain roller edges. As the fountain roller turns, the majority of the ink is held back by the blade, which is very close to the fountain roller. The distance between the blade and fountain roller is controlled by the fountain keys.

These can be used to control the amount of ink delivered to different areas of the plate. Ductor roller- The first roller in the train, feeds a metered amount of ink from the fountain to the inking system by alternately contacting the fountain roller and the first oscillator. A properly timed ductor roller contacts the oscillator when the form rollers are in the plate gap. This negates the effect of ductor shock, the vibration sent through the system when the ductor first contacts the oscillator.

Oscillators - Accepts ink from the ductor, passing it onto the remaining rollers in the train where the ink is worked down to a smooth film. An inking system has several oscillators (also called drums or vibrators) which are usually made of steel tubing covered with copper, ebonite, nylon or some other oil receptive material. They move laterally (side to side) at least once every revolution of the plate cylinder. This smoothes out the ink film and reduces banding.

Distributors - Resilient rollers that carry the ink from one oscillator to another. Driven by surface friction contact with oscillators.

Riders - Hard rollers that make contact with only a single roller and do not transfer ink. Help condition the ink by increasing the ink path and collect debris such as paper fiber and dried ink.

Form rollers - Several (2-4) resilient rollers that contact the plate. Usually have different diameters to reduce mechanical ghosting (ghost image appearing in the printed image due to uneven ink take-off from the form rollers). They lift off from the plate when the press is idling.