E & H Graphic Service
E & H Graphic Service

Checking Form Roller Pressure Problems

The symptoms of excessive or uneven form roll pressure are a series of horizontal streaks, sometimes blamed in the ink. Often, additives such as varnish or reducer are added to compensate. This can bring on a complex range of new problems, especially if running an integrated system which requires tackier ink than conventional units. Gear wear is also a suspect, remember however, these same effects can be caused by an overpacked blanket, undersized center oscillator or vibrator roll, over acidity and worn parts.

Form roller bounce is caused by the roller slamming up against the lead edge of the master cylinder at each revolution. The average press operates at speeds up to 10,000 impressions an hour. The slamming action then occurs up to 80,000 times a day!

A small amount of bounce is acceptable in the offset process but excessive pressure will cause problems. You cannot achieve quality press work with too much form roller pressure.

Imagine for a moment that you are at the wheel of a car and you momentarily lose control. A similar situation occurs when a form roller hits against the edge of the master cylinder. You momentarily lose control of the ink and the moisture flow. The roller then skids as it hits hard on to the smooth surface edge of the plate. The skid caused breakdown of the image structure; ink and water emulsification, short plate life, streaks and damage to the roller surface.

With excessive pressures, when the ink form rollers drop into the cylinder gap at the relatively high speed of a press, they momentarily separate from the internal oscillator. This separation completely disrupts the milling action of the ink train. As you are aware the objective of the train of the rollers must remain in contact throughout the revolution cycle.

When the rollers separate as they bounce into the cylinder gap, two thing happen; first, a light streak across the width of the form roller. This voided area causes a visible variation in the ink film thickness around the circumference of the form roller. Second, the ink which was not transferred now presents a build up or film and causes a zebra effect often incorrectly interpreted as gear streaks.

The moisture form roller reacts in a similar manner to the ink form roller in this situation with an excess, or starvation of fountain solution.

When these two factors are combined the results are horizontal streaks that for years have been blamed on gears. If the condition has been in existence for some time, you can bet that the rollers should be replaced. Install new rollers; eliminate all end play and set the pressures carefully and evenly to 1/8" or as recommended by the manufacturer, and use the Lith-O-Roll pressure gauge to measure the strip accurately. When using a compressible blanket, more pressure from plate to blanket and impression will be required than if using a conventional blanket. The end result will be fine line and half tone copy, good solids without streaks and true colors.

Free Lith-O-Roll pressure gauges are available from E & H Graphic Service.

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Interpreting Ink Form Roller Stripes

 

Drop your ink form roller on the plate and observe one of these conditions:

  1. This is a nice even stripe which indicates your ink form roller is properly set and not swollen on the ends or in the center.
  2. Here is proof your roller is swollen on the ends and should be replaced. The low condition in the center will cause excessive ink to accumulate here.
  3. Here is the opposite of "B" above. Your roller is high or swollen in the center and low on both ends causing ink to accumulate on both ends.
  4. This is an indication of improper roller setting. Adjust your roller so that both ends are the same. If this produces an even stripe like "A" above, indications are that your roller is not swollen on ends or center.

Your ink roller stripe should be approximately 1/8" on most offset duplicators for best results.