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Composition Rollers
The qualities of a good Composition roller
* They must be perfectly cylindrical in shape and the same diameter overall.
* They should be, as far as possible, unaffected by atmospheric conditons.
* They should be capable of receiving a sufficient supply of ink from the table or __.inking cylinder and be able to impart an even film of ink to the whole forme.
* They must have good elasticity.
* The stock must be perfectly straight and in good repair.
The method of manufacturing Composition rollers
1. The stocks are coated with white lead and then bound with tape or coarse twine.
2. The stock, mould, base or stand and star are warmed. The inside of the mould is __'lightly oiled and then the parts are assembled.
3. The composition, made from gelatine and glycerine is then poured over the stock __,so as to fill the mould from the centre.
4. When the roller has cooled, it is removed from the mould and the ends are trimmed.
Defects in casting
* Pin holes caused by air bubbles.
* Oil streaks caused by too much oil inside the mould.
* Chill streaks caused by using a cold mould.
Setting the rollers
Good printing depends largely on good rollers and thin correct setting. The precautions to be taken when setting are:
1. Rollers must be set accurately.
2. When adjusting to the ink table (cylinder machines) ensure that rollers are not _i_touching the riders.
3. When adjusting to the riders (cylinder machines) make sure that rollers are not in _i_contact with the forme or ink table.
Methods of setting
(a) Run up ink on machine, remove the riders and wash up the ink table (which must be type high). Lower the forme and distributor rollers in turn to the ink table and adjust each to leave an even strip across the table of 3/16'' to 1/4'' wide (less on small machines, platens etc and when presses have precision synthetic rubber or poly rollers installed). The riders are then washed and replaced, the rollers are set to leave the same ink strip on them. Double check the setting with the rollers and riders making contact.
(b) Replace the forme rollers in the machine one at a time and adjust each to lightly grip the roller gauge. Adjust one end, making sure that the opposite end is not disturbed. Place the riders in and adjust the rollers to the riders using strips of thin paper. Having set the rollers, always make sure that they are returned to the same position in the machine from whence they came or if ever they are removed.
Incorrect setting
Damage to rollers will occur if they are not set correctly. Cuts, cracks or melted rollers are often caused by incorrect settings. The printed sheet may show:
* roller wipes or slurring.
* ghosting.
* incorrect or irregular inking.
* set-off and inking of non-printing areas.
Advantages:
1. Low in cost (standard composition is re-usable).
2. Can roll formes of various heights.
3. Easy to wash.
4. Good tack.
5. Good distributor of ink to forme.
Disadvantages:
1. Susceptible to variation in condition to humidity.
2. Melt at high speeds.
3. Easily cut or damaged.
4. Tendency to swell on ends.
Synthetic Rubber Rollers
Advantages:
1. They are tough, therefore they have a long life.
2. Can be run at high speed without melting.
3. They do not require seasoning.
4. Suitable for all climates.
5. Do not require frequent setting.
6. They may be re-ground if damaged.
Disadvantages:
1. They are costly.
2. Affected by certain penetrating oils resulting in the filling in of fine halftone __screens.
3. Are sometimes difficult to clean for light colours.
4. They do not hold dust but allow it to be transferred to the forme.
Polyurethane Rollers
(Cast in mould. Polymer and Elastomer are the two main ingredients)
Advantages:
1. They do not need seasoning.
2. They are suitable for all climates.
3. Easily cleaned.
4. Tackiness is life-long.
5. Durable.
6. They will retain fluff and dust.
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