These are actions that are taken during the press run.
In the Sheetfed Offset Press, paper is fed through the press individually one sheet at a time. Press speeds can reach up to 10,000 impressions per hour. An impression is equal to one full press sheet of 38 inches by 22 inches.
Inspection of the press sheets
During the pressrun the operator will periodically remove a sheet
for inspection. The operator will be looking for hickeys, a particle on
the blanket giving a white spot on the sheet, as well as misalignments
of the color dots. For this inspection the operator will use a few tools
to aid in his inspection. About once every 50 sheets two are pulled and
cheked for the following:
- Side-guide marks - these are vertical and horizontal lines at the edge of a sheet that allow the operator to determine if the side guide is operating properly.
- Register fit - Improper register or "misregister" happens when the conditions of the paper change. This could happen with a change of humidity causing the paper to become tight or wavy edge paper. A poor paper trim could also cause a misregister.
- Halftones - The plugging of halftones results when the dispensed ink is excessive or there is not enough alcohol substitute.
- Drying of the plate
- Excessive Ink - If there is to much ink on the paper it will smear when bushed lightly by a finger.
- Printing defects - items such as ghosting, improper register, hickies, etc.
- Color consistency when compared to the OK sheet
- Dampening solution
- Paper feeder operation
- Paper delivery
- Quality Control
Color and registration control is a process that is aided by the use
of the control panel. The alignment of the printing plates as they apply
their respective color portion of the image that is being printed is done
by the press operators at the control panel. The images from the plates
must align perfectly otherwise a blurred or “out of focus”
image will appear on the paper. Color control is a process that involves
the way in which the ink appears to blend together, and is tied closely
to the plate registration. The amount of ink that is released into the
units depends on how much ink is needed to achieve a desired look. The
ink is adjusted via the control panel that is part of the overall control
console.
Pic 1.16
This is a digital representation of a halftone. The duck is represented
by a series of dots. The dots do not overlap, or the colors blend. Your
eye is fooled into believeing you see a solid color. Take 5 steps back
and then look at this picture of the duck.
Pic 1.17
The red circle above is what is known as a "Hickey" in printing.
Hickies occur when a piece of debris (made of dust, ink, or paper) sticks
to the plate. The ink sicks to the piece of debris, causing the center
of the hickie to print with the desired color.