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Printing |
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Printing |
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The final stage of map production is the printing of the map. Unless map quantities are small and a slight reduction in quality is acceptable the only economic way to produce bulk quantities of maps is using a traditional printing press.
Printing is by way of an offset litho press, generally with each colour on the map printed seperately. For the normal 5 colour O-map this will mean printing with 5 ink colours using 5 printing plates which are made from 5 colour seperated drawings.
Artwork
Before the printing stage can commence the printer needs to make the printing plates used by the printing press to print the map.
These plates are created from the artwork of the cartographer. These days the artwork can be supplied on computer disk direct
to the printer, the printer will generally sub contract the film making process to specialist firm specializing in film production
using an imagesetter.
Alternatively, the map maker could arrange for the films to be made himself. This has the advantage that the map maker can
check the films before the printer receives them.
Reproduction
When using pen and ink drawings or scribes the production of films was much more time consuming.
Pen and ink drawings are generally drawn at two times the final printing scale. The first stage for these enlarged drawings is to photographically reduce them using a special process camera, this will produce a negative film at the final printing scale. The drawings are normally held in a vacuum frame to ensure accurate scaling over the whole area. Normally the photograper uses refected light so it is important that all drawing is on the front of the film.
Once the artwork is at the printed scale a proof if required can be made. A proofing process such as Colorkey, Cromalin or Geva Proof is used, the process depends on whether the artwork is in positive or negative format. A single copy proof is produced on a plastic material, the mapper should note that the dye colours used will not match the printed colours of the final map. These processes can be expensive and you only get a single copy, it may be better to print a short run.
Any artwork that needs to be combined, for example, when the line work and screens are drawn seperately (always the case when scribed), is done by contact copying, again with the artwork held in a vacuum frame for each exposure.
Always check the the resultant films before printing for: the correct combination of artwork, the odd mistake that has slipped through previous checks and stray marks on the films. Retouching can be done to correctt faults including light patches that appear on the film. These are painted over with a suitable opaque paint.
The printing process
O-map printing uses the offset litho process. A printing plate is made for each colour from the colour separated originals as
already described. The principle behind the process is that the image areas of the plate are greasy while non-image areas are
damp. Printing ink is of a sticky greasy consistency and since water and grease do not mix ink is only taken up by the image areas
on the plate. Other areas are kept damp, so no ink is transferred to the rest of the plate.
The plate is clamped round the first cylinder (A). Ink is applied to the plate
via a bank of rollers which control the quantity and eveness. There is a similar
series of rollers for water which keeps the rest of the plate damp. As this
first cylinder rotates the ink image is transferred to a rubber blanket around
a second cylinder (B). The image is then transferred to the paper which is passed
between this second and a third cylinder (C). If the image were transferred
directly from the first cylinder to the paper, the printing plate would have
to be a mirror image (wrong- reading) of the printed work. The inclusion of
the "offset" blanket around the second cylinder (B) allows the image on the
printing plate to be the same as the printed work (right-reading).
The paper that will soon be a map is passed through the print machine once for each colour, the printing plate and colours are changed between each pass.
The print machine is constructed so that the paper sheets are aligned each time using the same two sides, one long and one short. This is necessary so that all the colours are positioned on the paper correctly. Normally the maps are printed in yellow, green, blue, brown and black. The sheet must therefore pass through the print machine 5 times. One of the major causes of dimensional change in the paper is it being stretched during its first pass through the machine. When large sheets are printed it is sometimes necessary to pass the paper through the machine once before printing to stretch the paper.
A multi colour printing press basically consists of several identical presses placed one after the other fitted with different printing plates and coloured inks. Sheets are moved between different printers via a system of gripped transfer drums and belts. As the paper passes through the different presses in a short space of time there are no dimensional changes to the paper and hence misalignment of colour.
Five colour printing
As previously mentioned the traditional way of print O-maps is using five spot colours (six including overprinting). Using spot colours (ie. colours that are made up before printing by mixing inks) will produce the sharpest lines as each line comprises of a single colour and any slight misalignment of colours will not affect the line quality and final colour. For a single colour press the paper will have to pass through the press five times, one for each colour. The order in which the colours will generally be either from lightest to darkest (ie. yellow, green, blue brown and black) as this makes the washing of the ink rollers between colours easier or printing black first as it is easier to check registration against black followed by yellow, green blue and brown.
Four / three colour printing
Four and three colour printing is the method used by printers for producing colour work requiring an infinite range of colours. It works on the principle that any colour can be created by combining cyan, magenta and yellow in the correct proportions on a white background. The difference between three and four colour printing is whether black is printed as a separate colour (black is created by combining cyan, magenta and yellow at saturated levels). This type of printing is highly suited for photographic reproduction but is less suited for line work as accurate registration is required to ensure colours are placed on top of one another.
Recent technological advances in three / four colour printing including the random placement of dots on a raterized screen and improved registration has meant that printing O-maps this way is now a possibility.
Other printing methods
There is a printing method that has been used by a printer in Sweden that can be good for some maps. All details that are to be black on the map are placed on both the blue and brown plates. As these will then be printed on top of each other, they appear as black. This saves one plate, and one passage through the printing process.
With this method, maps of size A2 or larger can suffer from a mis-register of black at the rear corners (as the map moves trough the machine). In normal printing this small movement is not noticable.
Which method should you use?
There are advantages and disadvantages to each method.
Until recently the only way to print quality O-maps was with five colour printing and this will still give the best results. The disadvantage of this is that not all printers are be prepared to mix small quantities of special inks and the increased cost of five colours (additional, films, plates and passes through the press).
Using three / four colour printing will never attain the quality of five colour printing and requires very accurate registration throughout. The advantage is the saving made by reducing the number of colours printed.
Printing plates
Some printers make plates from positives, others from negatives. The differences are not significant to the o-mapper. If the drawings are being produced at final scale and contact copied at the photographic stage, it is important to know whether the requirement is for a final positive or negative. In either case, it must be wrong reading so that the emulsion on the surface of the film is in direct contact with the printing plate during exposure. Exposure through the thickness of the film at any stage could cause the lines and dots to come out unevenly.
Paper
According to ISOM, maps should be printed on good quality paper with a surface weight of 100 - 120 g/m (preferably water 2 resistant). It should have a matte covering on the surface to provide a sharp image and prevents the inks spreading out causing swelling of the map details.
There are also a number of products which are more or less water resistant,such as Artosil and Syntosil, Syntetape, Tyveck and Polyart. Each of these materials have advantages and disadvantages. Most of them can be folded even with moisture without leaving colour or breaking .The main disadvantage is the price and in some countries availability. They are all two to four times the cost of paper.
Colours
The map colours are specified by the IOF using the Pantone Matching System (PMS) colour reference.
The following colours should be used:
Black Process black Brown 471 Blue 299 Green 361 Yellow 136 Violet Purple
Advertisements printed on the back of the map should be printed a light colour (low percentage tint) to avoid the image being visible through the paper which could affect map legibility.
Transparencies
At the same time as the maps are printed, some printers can produce one or more transparencies of the map by passing sheets of film through the press. These transpancies are ideal for use with overhead projectors, an ideal teaching / coaching resource. They are more stable than normal paper and therefore ideal as the underlay for course overprinting and revising the map at printed scale,
Trimming
Most maps are printed on paper sheets larger than the final size of the map and so the edges must be cut after printing. If you plan to overprint courses you should avoid cutting the two sides that where used to insert the map into the press. If the cuts are not in exactly the same place on each map it is difficult to get an exact register when overprinting.
Often you will print several maps on the same sheet. After printing, the maps are cut apart. A map to be used for overprinting should be placed so that the 2 sides used to insert the map remain uncut.
Bad register
This is the most common fault in printing. The importance of accurate register must be stressed to the printer at the beginning.
A misregister of 0.2 mm should be regarded as the maximum acceptable. Possible causes are:
Possible printing problems
Other mistakes may be caused by wrong exposure of plates, uneven colour or from marks left on the originals. To avoid this ensure all artwork passed to the printer is black and clean. Printers with experience of O-map printing may spare you most of these problems and guarantee a satisfactory result.
What to send to the printer
The printers delivery time is dependent on the size of the order. Make sure you contact the printers well before your order and agree on a date for the delivery of the original and the finished map.
When the map proofs are completely checked the original artwork or data disks are sent to the printshop. Printers who specialize in orientering maps should be used if possible, as they have experience in working with maps and know the requirements for colour, paper quality and registration.
If the printers work to a fixed price list, and a standard map is being printed, the cost can be easily worked out for each printer.
With the order, you should make sure your wishes are clearly stated. The following information should be given:
Preparing for course overprinting
Map underlay
The usual underlay used with course overprinting is a map from the final print run. The map should not be folded as this changes the map size and therefore fitting.
The printed map is even affected by air humidity and other factors, so size differences of up to 1 mm can occur between map copies of size A4 - A3.
To avoid these problems when making the EPS files for the colour seperations you can at the same time print an EPS file containing black, blue, and brown detail and have this printed out on film from the imagesetter. This film is then used as underlay for course overprinting.
Another alternative is to use a transparency printed at the same time as the map, this material holds its size better than paper.
Courses
In course overprinting, the originals are usually copied straight onto a positive
printing plate. This places high demands on the drawing quality, especially
the density of the lines.
The original is drawn with ink on drawing film, preferably on a somewhat thinner
film than with normal drawing, which makes it easier to use a backlight when
copying plates. Note that the density of the lines can be bad if you draw the
pen along too fast. This especially applies to drawing the lines between controls
with a rule. Use a pen with a width of 0.25 - 0.35 mm (wider pens with thicker
plastic). A pen of size 0.3 mm is fine in most cases.
For the control numbers, use transfer numerals such as Letraset. Mark each course
with the class symbols to reduce the chance of mixing the courses up.
Each course original should also have at least 2 register marks to fit it to
the map accurately. These should be placed on some thin line detail in black,
e.g. map registration crosses, the border, magentic north lines, or power line
crossings. You can use details in other colours, but all marks should be on
details of the same colour, as some movement between colours can occur, making
the mounting of the original more difficult. The marks should be drawn with
a rule using a thin pen, maximum 0.25 mm, and should be clear and accurate.
Out of Bounds and other information on the map that is common to all courses
should be drawn on one separate original. These can be printed on all maps as
in a single run or combined with the courses at the plate making stage.
Overprint detail can be drawn on computer with the courses printed to an imagesetter
but is much more expensive than traditional methods.
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