Matching Paper to Press

Although paper quality points like brightness, whiteness, shade, gloss, opacity, and basis weight are reported by mills and widely followed by printers because they lead pricing, strength data like tear and tensile—or, more importantly, their index or modulus and elongation—seems to have faded completely off the scene, but could be helpful in setting tensions or identifying breaks.

Though most pressrooms will have some form of basis weight scale, almost none have brightness or opacity meters. Products labeled "ultrasmooth" and likewise do not convey the data or the linkage to performance that could be so helpful to pressrooms. Fortunately, interest in measuring smoothness—or at least quantifying it—is rising, so an explanation of the technique follows.

Smoothness can influence registration, line screen selection, speed, blanket pressure, and many other press conditions. As pressrooms become more diverse in commercial printing, and paper is dumped on the receiving dock with just its label (specified elsewhere, in most cases), the pressman really needs information on strength and smoothness for proper set-up. Most failed jobs are due to poor smoothness, where images and text mottle and uniform reproduction is impossible.

Roughness is defined as the deviation of the surface of interest (paper) from the surface of a reference plane (metal or glass plate, or electrical signal), and smoothness is its inverse. Unfortunately, smoothness measurement development has been split along national lines for decades so that there are a half-dozen types used worldwide.

However, it seems that one method, Parker Print Surf (PPS), is emerging as the leader in global standards, particularly in Asia and Europe. In the U.S., "Sheffield" is considered a more widely used standard, but most mills also measure PPS.

For decades, pressrooms ran letterpress technologies and struggled with on-line corrections due to changes in the roughness of the paper. A young scientist named Dr. John Parker working for Bowater's U.K. works decided to simulate the process in a different way. His methodology measures the rate of air leakage between the paper surface and a reference plane pressed against it, like Sheffield and others. But PPS sought to simulate the nip pressure and packing hardness of letterpress or offset printing presses in order to better predict performance in commercial printing conditions.

PPS provides a roughness value as the average depth of the pits in the paper surface at the test pressure in micrometers, unlike Sheffield, which produces its own units. This can be used to determine ink film thickness in letterpress and blanket specifications in offset. For offset, different backing materials can be used. Clamping pressures can be varied (to better reflect nip pressure at the print couple or impression plate). The reference plane is also secured to the instrument so that it remains air-tight during operation. Modern units manufactured by Testing Machines of New York (TMI) and Lorentzen & Wettre (L&W) of Sweden incorporate all sorts of new digital output, and cost about $20,000. Sheffield units cost a bit less.

Smoothness can be instrumental in pressrooms that use photo polymer plates (letterpress and flexography) for predicting press performance. Conditions in the press nip can be matched easily in the tester. Offset operations often have to adjust line screens downward when using rough paper or going from coated to lower weights or uncoated.

Pressrooms utilizing coldset offset use blanket piling; if a shaftless press is used, drive speed variation holds the sheet. In operations where a single shaft pulls the sheet through the units or tower, non-uniform smoothness in the sheet can cause the latter to wander, and mis-register occurs.

Operators can change the nip pressures at the print couple by varying blanket compression, especially if using heatset offset technology. Lots of on-line alignment equipment is available and employed to watch wandering sheets. It should be noted that ink tack has probably more influence in offset operations at the print couple.