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 Post subject: How can a doctor blade flaw be constant?
PostPosted: Tue Jul 19, 2011 7:00 pm 
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Joined: Fri Aug 27, 2010 6:00 pm
Posts: 148
Dear All,

Can somebody explain how a doctor blade flaw can be constant?

By definition, the doctor blade is moving all the time during the printing, and the paper on the web is also in constant motion, and the chances of an exact repeat must be virtually nil.

Is it then possible for a doctor blade to damage the surface of the cylinder, and thus the flaw is doctor-blade-caused? Would such a scratch normally be erased with the next cleaning and re-chroming?

If so, it casts all those doctor blade lines in a new light, since instead of being dismissed as once-off accidentals, they might be constant flaws.

(I'm working with 1936-1937 photogravure material, in case that is relevant).

Sorry if this reads like a novice question... I guess it is!

Cheers, and thanks for any insight.

Robin R


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 Post subject: Transfer from previous board: original post 1810
PostPosted: Tue Jul 19, 2011 7:05 pm 
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Joined: Fri May 08, 2009 6:00 pm
Posts: 88
These flaws are caused by particles of ink adhering to the db. Faulty mixing the problem.
Although unlikely, the flaws could be repititious - if the adhering ink lasts long enough - the db travels back and forward over the same area!
Something extremely hard would have to be caught up by the db to cause significant damage. Theoretically it could happen - but it is as unlikely as a stone under your windscreen wiper


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 Post subject: Transfer from previous board: original post 1823
PostPosted: Mon Jul 25, 2011 7:00 pm 
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Joined: Fri Aug 27, 2010 6:00 pm
Posts: 148
Thanks Robin, I guess that's pretty well how I saw it. Not sure about your windscreen wiper analogy - I seem to get those bits under mine quite often! Cheers, Robin.


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 Post subject: Transfer from previous board: original post 1824
PostPosted: Tue Jul 26, 2011 7:00 pm 
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Joined: Thu Jun 25, 2009 6:00 pm
Posts: 11
Who ever said a doctor blade flaw could be "constant"?

Of all the errors found on stamps they are probably the least constant! They are probably the result of ink problems on the cylinder itself most likely caused by particles of dirt, even minute drops of water. But to cut a long story short, if you imagine that the doctor blade is analogous to a windsceen wiper and you think how "constant" and "consistent" each wipe is ,then I think you'll get some idea of the almost total NON-CONSTANCY" of doctor blade flaws!


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