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Plate Wear - How It Affects Quality

Recently our offset press has been having trouble with plate wear after about 300,000 impressions.  Not that the plate wear is significant enough to cause serious quality concerns, but since we have recently been trying to meet ISO standards, it has caught our attention.  After doing research as well, I know we are not the only ones experiencing this in offset printing.

Achieving ISO Standards

As most who have implemented it know, ISO standards are very strict when it comes to dot gain and loss.  We have been trying to achieve a plus/minus two percent variance.  This has turned up the ante for the pressmen in achieving consistent density.  Since we have a closed-loop inking system, we have managed to keep this controlled in a narrow window.

The problem however has arisen now that our plates wear after about 300,000 impression and put us out of tolerance in our 40% screen.  This is in spite of the fact that our plates are rated to last 500,000 impressions.

Given that our runs are over 5 million, the question begs, is there a way to continue using our current less expensive offset printing plate (Kodak Sword Excel) and not deal with plate wear?  The answer it seems is yes.  Baking our plates will give us considerably longer plate life.

The Plate Wear Test

Contrary to what we thought, printing plates do not seem to hold their image to the last and then drop off in quality.  The loss in the 40% is gradual from the very first impression.  Here is a graph of scans showing dot gain over 500,000 impressions.

Plate wear is steady in 80% and declines in 40%

The graph here shows our dot gain in the 40 and 80% screens.  Notice how the 40% gradually decreases.  We were told the life of the plate was 500,000 impressions.  However reality showed that we dropped out of ISO tolerance (plus or minus 2% dot gain/loss) after only 300,000 impressions.  The dot loss was gradual, but steady.

Throughout this test we tried to achieve the following:

  • Consistent density from beginning to end of run
  • Done on 8 plates of four color process
  • Consistent variables - water settings, speed, paper type
Now look at a graph showing what happened when we baked our plates.  This is a series of scans of the 40% screen over 2,000,000 copies.

Plate wear is negligible after 2 million copies.

Notice there is virtually no dot loss.  We could probably go much longer on these plates too, provided they didn't crack on us.

Conclusion

From what we have learned, plates that are not baked undergo instant and gradual dot loss starting from the 1/2 tones.  The payback will come to us quickly since we have very long runs.  Even more striking though is the fact that dot loss occurs much earlier than the declared plate life.  For those trying to hold ISO standards or are looking for superior quality, there is more to consider than just run length.

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