Printing Systems based on Non-Impact

Printing Technologies

A system architecture that it is possible to set up print media production systems based on non-impact printing technologies. Such systems enable the entire printed product to be produced in-line. With non-impact printing technologies it is of fundamental importance that the print image is already dry after the print (as described for electrophotography with subsequent fixing or ink jet processes using special drying techniques or using hot-melt ink). In this way, finishing (collating, stitching and folding) can be done in-line, thereby making the intermediate storage of printed sheets superfluous.

A printing system that delivers a completely finished brochure copy by copy. The printing process is digitally controlled and a data file contains all the text, image, and graphics information to be printed page by page. Additions or modifications of this file are still possible shortly before printing starts, for example via the connected flatbed scanner. This production system could be summed up as a “data in-product out” system.

If printing systems based on non-impact technologies

are to be used, a full description of the printed product

is required in digital form.

LECTURE 4

Assembly

Page Make-up

The combination of text and images to form a page

occurs during the page “make-up”. In the production

of conventional printing plates the complete page is

put together manually by repro experts from text and

images that exist mainly as cut films.

The films are placed on a carrier sheet, adjusted for

position, and fastened in place. The quality of the print

depends directly on the precision and care of the makeup.

Register errors caused at this stage are unlikely to

be able to be compensated for later in the print. Film

stripping is carried out on a light table (see also fig.

1.2-24). Its work surface consists of a glass plate that is

illuminated from below with diffuse white light. Two

steel rulers at exact right angles, which may be moved

in parallel, are used to ensure accurate stripping.

In preparation for the stripping a millimeter sheet is

first placed on the light table with the carrier sheet on

top and fastened with adhesive tape. The carrier sheet

is a plastic film that is dimensionally stable, insensitive

to moisture, crystal clear, clean, and free of scratches

and should be as resistant as possible to build-up of

electrostatic charge. Polyester film with a thickness of

0.15 mm has proved useful for this purpose. Medium

and large-format assembly work (e.g., sheet assembly)

makes use of 0.3 mm film.

The several films are always mounted with the coating

facing uppermost so that during subsequent copying

exposure occurs coating to coating to avoid undercutting.

This means that for an offset plate wrong-reading

film copies must be produced. The films for text

and image are placed in the correct position according

to the layout and attached to the carrier sheet with liquid

adhesive or adhesive tape. In addition the films

must be cut to the required size without burring, using

for example a scalpel and a glass base or a film cutter.

The films on the carrier sheet must not overlap since

intimate contact between the film and the printing

plate will be affected by the thicker overlapping layers

of film during subsequent copying leading to undercutting during exposure of the plate and an imperfect copy (undercutting) (fig. 3.1-38).

Offset printing employs mainly positive printing

plates (see sec. 2.1.1.2). These require a wrong-reading

positive film assembly, during which difficulties may

occur due to the cut edges of the film, particles of dirt,

or dust as well as scratches and the edges of the adhesive tape, which act as image areas during platemaking by increased light absorption.

Adhesive tapes for fastening the film securely to the

carrier sheet are single-sided adhesive strips of 0.05

mm thickness. Crystal clear or light blue transparent

adhesive tapes are used for positive assembly. Despite

their thinness, they still disrupt the direct contact between the film and the printing plate during platemaking, which may lead to undercutting. Adhesive tapes must therefore only be placed on an image-free margin on the film that is sufficiently wide; there should be a minimum of 5 mm between the edge of the image and the tape. The adhesive strips must also be cut burr-free.

If there is insufficient space available the film must be

attached to the carrier sheet with a liquid adhesive or a

spray.

The assembly of multicolored images must be carried

out with particular care as imprecision translates directly into register errors. In particular, errors of parallax caused by registering the color separations when viewing from an angle must be avoided. It is possible to improve precision during assembly by using a magnifying glass (pocket lens) or a microscope, which enlarges the image element under scrutiny and automatically provides a vertical viewing angle.

Two technologies have become established in the assembly of color separations:

• Guide assembly. A guide assembly is first produced

with the separation films of the color having the

clearest visible details. It acts as a base for all of the

other separations that are each mounted on other

carrier sheets. The contrast between the guide assembly

and the separation being worked on is improved if a matte film is placed in between to make the guide assembly appear gray.

• Blue key. Better registration is achieved by this

process. A positive or negative blue key is produced

from the guide assembly on precoated light-sensitive

plastic film. The light-sensitive coating is dyed

blue or bluish red; the dye is transparent to the

copy light. The separation is mounted on this film.

Cut edges and minor imperfections during positive

platemaking are exposed away by placing a dispersion

sheet/foil in between (fig. 3.1-39).

Negative assembly (i. e., ink-carrying/image areas appear light in the film) is frequently used in newspaper production. Dark brown or black adhesive tape is used to mount the negative films on the carrier sheet. The whole assembly area is covered with a backing sheet that is transparent on the light table but opaque to the UV-containing copy light. The backing sheet is cut away in all the image areas, which means there are no defects caused by cut edges or dust as may occur during positive platemaking. An imperative for negative assembly is that the image areas are perfectly transpar- ent (D<0.05) and non-image areas have good opacity (D>2.5).

Since manual registration of four-color images is dif-

ficult in negative assembly and is therefore fraught with greater errors, it is used predominantly for single-color work. It is advantageous to copy the images during a second stage in exposure onto a plate that has already been exposed with the text and graphics. The image areas are blackened out in the text film copy and the plate at first remains unexposed in these areas during copying. Alternatively, the non-image areas of the image film copy are black in order not to destroy

the already exposed areas of text by a double exposure.

For quality control a blueprint proof is produced

from each finished assembly,which is used to check the

position, completeness in all color separations, and the

text and image. These checks should be carried out very carefully as changes to the assemblies during the makeup stage may be made without a great deal of extra time, or financial or personal expenditure. 3.1.5.2 Sheet Assembly

If the ready made-up films (text and images are put together

in the correct position) are available for all pages

(full-page film), such as in the form of manually created

assemblies or as electronically made-up full-page

films output by the imagesetter, they must be mounted

as a full-sheet size film, corresponding to the sheet

to be printed. The positioning of the full-page films in

sequence and orientation as well as the precise location

of the individual pages on the sheet must be established

by taking into account the following list of points (in

addition other control and editing elements must be

added to the assembly):

• printing process (halftone film copy for autotype

printing processes, continuous-tone copy for variable

depth gravure printing; right-reading film for

direct printing processes, wrong-reading for indirect;

positive film for positive-working plates, negative

film for negative plates),

• type of printing press (gripper edge for sheet-fed

machines; pin edge, roller gaps, register marks,

transverse register balance of stretching due to humidity for web-fed machines),

• print format of the press used,

• method of turning the sheet for printing on both

sides (work and tumble with sheet-fed presses

equipped with a perfecting unit; work and turn or

work and tumble with sheet-fed presses without

perfecting unit; simultaneous printing on both

sides with presses equipped with blanket-to-blanket

units),

• imposition diagram of the folding machine or

apparatus,

• method of binding the whole printed product (allowance in the inside margin for cutting off the

folds in adhesive binding),

• method of finishing (overfold for gathering or inserting drum processing in the dispatch room),

• direction of paper travel (long grain, short grain

paper for sheet-fed presses, see fig. 1.5-3, portrait or

landscape formats for web presses),

• position of the folded sheet in the end product

(arrangement of page numbers, e.g., one to thirty

two on the first sheet as well as on the pages for the

other sheets),

• number of colors per side of sheet,

• auxiliary marks (registration marks, trimming

marks, folding marks, allocating marks, register

marks, signature line, color identification, ID

number),

• control patches (print control strips crosswise to

the print direction over the total width of the sheet,

copy control wedges on a non-printing area of the

plate).

Other specifications must be observed according to the

job.

This makes sheet assembly stand out as an information- gathering point of central importance for achieving trouble-free results. Although the handling of each individual stage is almost identical to page assembly, the content is much more comprehensive.

The first step in producing a sheet assembly [3.1-7,

3.1-12] is to draw a layout sheet containing the correct

measurements (fig. 3.1-40). It is the same format as the

sheet with an additional border and is made of dimensionally

stable transparent material/paper. If we take a

layout sheet for a sheet-fed offset press as an example,

after positioning on the light table parallel to the axes

and fastening with adhesive tape, the sheet format,

gripper edge, the beginning of the print, and the center

line are marked in. The data required for this are in

the documentation referring to the press or may be obtained from the job itself.

According to the measurements given in the imposition

diagram (which should be checked in each case

with the aid of a hand-made folded sample) the exact

outlines of all of the pages to be accommodated on the

top surface of the sheet must be accurately drawn, in

reverse (wrong-reading). These include the distances

between the pages taking into account allowances for

folding and trimming. The page number must be

entered in the bottom outside corner of each page.

This clearly indicates the position and orientation

(upright or upside-down) of each page. After marking

in the position of the control elements and auxiliary

marks, the layout sheet is complete. Cut and fold marks

should be designed so that they cannot be confused

during the finishing process and so that they cannot be

seen on a final print that has been badly trimmed.

Color control bars should be placed in the border so

that they can be trimmed away from the print. The

space for the sheet signature, the ink description and

the ID number must be marked.

For the reverse side of the sheet the layout sheet should

be marked according to the same considerations while

taking account of the type of inversion to be used. Hence two layout sheets must be produced for each sheet.

For the sheet assembly the layout sheet is accurately

positioned and fastened on the light table – if necessary

with a millimeter grid placed underneath. A carrier

sheet/assembling film is placed and positioned above.

The completely assembled films for the individual pages are registered on the layout sheet and stuck down. As with page assembly, sheet assembly of process color sets is carried out either using guide assembly technology or with a blue key.

Дата добавления: 2015-11-16