September 18, 2009
Take Control of Press Production
Quality Control Devices are More Important than Ever
I recently read an article in the PIASC Weekly Update newsletter that is the basis for this Blog entry. In the article, Ken Rizzo, PIA Director of Technical Services, observed that across the industry there still exists a significant lack of process controls for offset printing. Many steps are taken in the pre-press area to assure quality control, but once the job reaches the pressroom, printers are plagued by print quality problems and excessive color variation to the point that effective color management may not be achievable. Here are some areas to help regain control of the pressroom.
Pressroom Color Control.
Solid ink density and tonal value control are key to process control and print quality. Dot gain is often the only point considered here, but it should be remembered that density loss is equally possible and needs to be measured. The fastest method of checking ink density for gain or loss is with a reflective color densitometer. These units can provide information not only on the solid ink density, but also dot gain/loss, tonal value change, density change, ink trap, dot area, hue error, grayness, brightness and other critical variables.
It is also equally important to control the actual color itself. The easiest color check is a visual, comparing colors to known standards from a PANTONE color formula guide. If spot colors are not being used, check the process colors on the color bar to the CMYK in the PANTONE color formula guide. Remember, an out-of-date guide will not yield accurate colors. Always view color under the proper lighting conditions as well.
A quality, properly mixed fountain solution is essential to maintain ink-free non-image areas and maximum color fidelity. Follow the manufacturer’s recommended mixing instructions and then check the pH and conductivity with a properly calibrated meter. One of the most important factors in fountain solution consistency is to mix the concentrate with treated water (distilled, deionized or RO). If the water is inconsistent, the fountain solution will be equally inconsistent. Check the pH and conductivity at the start, middle and end of each shift and log the readings, making notation of any problems being encountered on press.
A clean printing job starts with a clean press. Blankets and rollers need a deep cleaning between jobs to remove residual ink components (pigments as well as the carrier varnishes) from the pores of the rubber. It is obvious that residual pigments will cause color contamination, but residual varnishes that are allowed to dry in the pores will harden the rubber and decrease blanker and roller life. Print quality suffers and customers can be lost. Roller rubber condition can be easily monitored with a Shore A Durometer. Check rollers at least once a week with a durometer. A good two-step roller wash will keep rollers in a “like-new” condition longer and will also help minimize water soluble glaze build-up. A water-miscible wash should be utilized for general blanket and roller cleaning. A flush with plain hot water can’t hurt anything.
Pressures
Pressures should always be set to the minimum to get the job done. Minimum pressures increase plate life, bearing life, roller life and blanket life. Excess pressures cause premature press wear as well as causing unwanted dot gain and uneven solids. It is important to check pressures between ink and water form rollers and plate, plate-to-blanket and blanket and impression cylinder. Check with the press manufacturer for recommended pressure settings. Roller pressures are generally set with ink stripes – the wider the stripe, the greater the pressure. Blanket pressures must be checked on the press. It does no good to measure a blanket thickness off press. When the blanket is tightened on the press, it loses thickness. A quality blanket packing gauge should be used.
The All-Important Paper
All the care and monitoring in the world will not yield a good image if the paper isn’t ready for printing or if it isn’t as specified by the customer. Be certain to test the paper for temperature and moisture content and make sure it has the proper gloss factor as specified by the customer.
