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Sep 1, 2010 12:00 AM
I have been asked many times, “What is the proper way to control ink color during the press run? Should solid ink density or gray balance be used?” To properly answer the question, a series of press runs under very controlled conditions recently was conducted using three different types of paper and two variables, to ascertain the benefits of two control points.
The test procedure used a new Heidelberg Speedmaster 105XL 6-color sheetfed press with a color management system called Image Control. The basic press was set to manufacturer's standards in terms of roller settings, packings, back cylinder pressure, etc. Fresh ink was used as well as new blankets and fresh fountain solution. A customized test form with color bars utilizing solids and gray patches in every ink zone was used for this test. The ink was Toyo HyPlus XM, which conforms to ISO 2846-1. The water-based coating is one of the more popular on the market, which is NiCoat low curl gloss. Profiles were obtained for the No. 1 coated stock in advance of the test itself. To evaluate the control mechanism, the press was set at top speed, which is 18,000 iph. In an effort to get as much data as possible, samples were pulled approximately every 400 sheets (about a minute interval), scanned and data transferred. To insure that the press was at optimum print conditions, the press was warmed up by running blank stock before running the actual tests.
For the first test, we used NewPage 80-lb. Signature True Gloss Text, which is considered a high quality No. 1 sheet. After consideration of many variables, it was decided that during the press run we would introduce an intentional change: an increase in solid ink density in magenta by 0.2 points and a decrease in cyan by 0.3 points. Then, we'd determine how each control point, namely density or gray balance, detected and then adjusted to the change. The change occurred at approximately the 5,500 sheet point. What this is doing is mimicking a press stop, which could be for a blanket wash, a major trip off, a blanket smash, etc.
The graph below shows what happened during the production run when solid ink density was used as the control point.
As expected, the solids were most sensitive to solid ink density changes. We achieved a basic control within 400 sheets, there was some slight over control, and chroma (C*) reflected solid ink density rather well. In regard to gray balance control, the correction was slower, namely about 800 sheets, and there were continued corrections. In terms of makeready, both methods were similar from the OK to the production run. Solid ink density obtained a faster correction when a variable was introduced.
Thanks for assistace with this project to Heidelberg USA's Chuck Koehler, RIT's Steve Suffoletto, PIA/GATF's Joseph Suffoletto, CMU's Dr. Tony Stanton, and NewPage's Dr. Bruce Blom.
Raymond J. Prince, NAPL partner consultant, is a leading expert in pressroom technical and operational issues. Contact him at (605) 941-1492 or firstname.lastname@example.org.