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San
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The
Printing Industry's Premier Supplier of |
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San
Diego Printing |
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Location: |
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San Diego Printing Parts and
Supply |
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From the Service Department of SDPP |
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Volume
2 |
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Offset Blankets & Good Impressions Much attention is focused on the ink – water balance, plates and ink rollers as key ingredients for obtaining good printing impressions – but, there may be a lack of understanding or attention paid to the contribution played by the blankets, the pressure settings and the maintenance of the blanket. All are needed to print quality copy on the small offset press. Under normal conditions, blankets used on the small offset press fall under two categories: -Conventional: The harder and less thick blanket -Compressible: The more versatile blanket, which has become the favorite of the small press user. In addition, there are blankets with characteristics and price of economy blankets, and higher quality blankets – a fact that becomes apparent when printing half tones, solids, and the useful longevity of the blanket. The degree of quality is in the manufacturing and finishing. The manufacture begins with layers of fabric material, separated by a pliable or elastic material. This is called the carcass. If the blanket is manufactured as a compressible, the material used to add the compressibility is added beneath the final layer of fabric – which adds to the thickness and durability. A composition rubber is added – known as the face to give the blanket a finished surface – needed to both receive the image from the plate and release ink onto the paper. This is an important factor in determining the quality of the blanket. Blankets can be damaged the instant they are installed on the press. They are intended to be firmly stretched around the cylinder – however, over tightening can strain and pull the fabric causing an uneven printing surface and loss of resiliency. A blanket that has not been properly tensioned will shift the image each time the lead of the plate and blanket cylinders come into contact, causing a double image. Mounting a New Blanket With a minimum of pressure between the cylinders – mount the blanket and run it in by allowing the press to run on impression 50-60 revolutions, and retighten the excess created by the stretching. Repeat the operation a few times taking up just the stretch from the blanket. Both plate and impression cylinder must be set to the blanket with proper printing pressure to achieve good impressions. Operating under constant over pressure can cause wear or damage to the gears and related parts on the small offset press. Larger – and even more sophisticated small offset presses have slightly over-sized steel rings (bearers) on the ends of each cylinder, which cause the cylinders to be undercut in relation to the bearers. The bearers make a light contact when the machine is on impression. The operator must use a micrometer or more modern measuring methods now used to correctly compensate for the undercut cylinders using gauged packing sheets to apply correct printing pressures between cylinders. The small offset machine cylinder adjustments are made with the use of eccentric shafts with the adjustment for turning the shaft located as an operator control. Turning the elliptical contour of the eccentric shaft raises or lowers the cylinder. The small offset press is not intended to use packing. The pressures are normally set using ink stripes between cylinders ranging from 1/8 – 3/16 – with cylinders on impression, and the plate mounted. However, the mechanics of certain presses make this setting difficult, if not an impossible means of setting the impression cylinder, and instead operations manuals direct the pressure be set by running the press – then applying pressure until a dark image just appears. If applied correctly, this should approximate a minimum setting. Press Pressures & Maintaining Blankets To understand why a minimum pressure creates good impressions, you need to realize that when the cylinders meet on impression, the pressure at the nip creates a slight raise in the blanket, which travels the circumference of the cylinder. The pressure between the cylinders must be enough to compensate for uneven surfaces – which would include manufacturing tolerances and slight depressions. Excess pressure would cause flattened type, dot-gain and tracks in solids. Many press operators feel printing is squeezing ink between cylinders. However, you are using the movement of the rolling cylinders, combined with just the needed pressure to transfer ink from the plate to the blanket, then releasing the ink from the blanket onto the paper. Most common problems small press operators experience are brought on by themselves. Unparallel cylinders; excessive use of cleaning solvent will penetrate and swell the blanket edges causing an increase in pressure needed to obtain uniform copy; over pressured cylinders; using fast drying or harsh chemicals causing swelling, uneven printing surfaces and hardening of the blanket; chemicals – including fountain solution can be absorbed into a poorly cleaned blanket, causing a glaze which affects ink transfer to and from the blanket. Through daily use, blankets will stretch and wear causing frequent adjustments. When installing a new blanket all adjustments should be backed off, and reset to the new blanket. Attention given the blanket can eliminate problems and improve printing impression. Damaging and discarding blankets do add to the cost of printing. SDPP stocks a complete line of good quality blankets – and blanket treatments.
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