Printing Blankets — Got Ya Covered
The Offset Blankets, Print Quality and Sheetfed Movement study examined 12 blankets, all used in print runs under the same conditions at GATF. Press, paper, ink, fountain chemistry, speed, plate and temperature were identical. In the first print run, GATF's press operators carefully used a torque wrench. Although suppliers recommend a variety of torque settings, all blankets were tensioned to 38-foot pounds—an acceptable middle ground torque.
Also, all blankets were printed at 0.006˝ squeeze between the plate and the blanket. GATF's second shift press operators were then asked to print two of the blankets without a torque wrench or packing gauge. Twenty random samples were pulled for print attribute measurement, and 30 consecutive sheets were measured with the Mitsubishi Digital Register Analysis—a machine vision system that carefully measures relative position of triads of dots.
The findings showed that print attributes are only slightly affected by the blanket, all other things being equal. Results regarding sheet movement were inconclusive and may be a topic for a future study. However, torque setting greatly affected tone value and could be used as another tool to control and adjust the process.
GATF also explored several blanket issues with the participating blanket manufacturers. Representatives who troubleshoot problems discussed customer feedback regarding packing latitude, blanket life, printer expectations, torque and packing frequency among printers, and the effect of blanket washing on blanket life. Interestingly the manufacturers estimated that only 10 percent to 50 percent of printers even use a torque wrench when fastening the blankets.
In comparing the printability of different blankets with all other conditions equal, Offset Blankets, Print Quality and Sheetfed Movement also documents trapping and print contrast, discusses sheet movement from unit to unit affecting tone value, and shows the effect of various blankets on printing solids and halftones.