HIya,
late posting, but on the initial topic... to be clear. "Scratching" is something I think of as a press problem. Like a piece of paper in paper path dragging on the sheet as it passes by. What you're talking about is post production scratching, particularly on direct mail. (as opposed to posters, brochures etc) This is essentially an adhesion problem and it is most pronouced in over-rich blacks. Give desingers a spec for rich black: 100 K, 60C, 50 M, 40 Y. This adds up to 250 and is a very solid rich black. Setting a color to "Registration" or 100% of everything is incorrect on any press. Regarding adhesion, the liquid toner ink adheres to the surface of sheet with heat and pressure, and the excess oil is removed. A number of things contribute to a good final piece. A good substrate is first. Some sheets have chemicals from the mill which casue a resist to the inks, which is why they have HP coating and certification. Papers withouth certification can work fine. You have to test for yourself. Use scotch tape on a printed job, pull it off and see how much ink is left behind. If it comes off clean as a whistle, you have a problem. Next up is heat and pressure. If there is a real problem, change papers to see if it improves. I you have to, calibrate your ITM temperture, and bump it up 3-10 degrees to improve adhesion. (put back down to a spec of 160 after a trouble job) Also, lie to the press about how thick the paper is. Tell it that it is .32mm when it really measure .41, for example. this increases the pressure on the sheet. Also, set the press in "Heavy Substrate Mode" in blanket/machine parameters. This changes the duplexer to run slower, but prevents the sheets from scuffing each other as they pass in the duplexor. This slows your machine down, but give heavy coverage a chance to cure. Also, do not overhandle sheets right off of the press. let them sit, untouched for 15 minutes before jogging. Then, if your customer has to have a perfect piece after being kicked around the post office, upsell them to UV coating. Yes, an offline UV coater is great. Finally, make a call to HP and have you Field Service Tech look at your press around this issue. You may need to change your ozone filters, or make sure the more subtle environmental issues (too much oil in the ink) are looked into. Good luck.