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TAPPI T 538 Roughness (Scheffield)

Standard:

TAPPI

Method:

TAPPI T 538

Title:

Roughness of paper and paperboard
(Sheffield method)

Scope

TAPPI T 538 Roughness is a measurement of the air flow between the specimen (backed by flat glass on the bottom side) and two pressurized, concentric annular lands that are impressed into the sample from the top side. The rate of air flow is related to the surface roughness of paper or paperboard.

Summary

The measuring head, which has concentric annular lands, is dead-weight loaded against the specimen, which is supported by a flat glass surface. Air pressure is supplied to the zone between the annular rings that form the lands, and the flow rate of air that leaks between the surface of the paper and the metal lands in contact with the specimen is measured. The purpose of this test is to measure the extent to which the surface of a specimen deviates from a plane, as affected by the depth, width and number of departures from that plane. The measured flow rate of the leakage of air is an indirect measurement of surface roughness. This method does not read absolute roughness, but indicates the degree of roughness for comparison. The effective measuring range of this method is from 0 to approximately 445 Sheffield units.

Significance

Surface roughness has an important influence on the printing quality, as uniform ink coverage of the paper can be achieved only when the deepest depressions of the paper surface under impression are separated from the printing plate by not more than ink film thickness on the plate. Roughness also affects properties such as the coefficient of friction, gloss, and coating absorption.

This method describes a procedure for obtaining surface roughness data quickly, using a measuring head with air passages that can easily be inspected for blockage, and easily cleaned.

In addition to the air that leaks between the lands and the surface of the specimen, there is some leakage through the specimen due to its porosity and also to the roughness of the underside of the sheet. The degree to which the test results are affected is related to the air permeance and compressibility properties of the specimen.

The air pressure that is supplied to the zone between the annular rings does not remain constant for the wide range of air flows that are within the capability of this test. Table 1 [In the method] shows the relationship between pressure and flow.

The force with which the annular rings (lands) are loaded against the sample increases as the air pressure decreases. This is due to the force component (pressure times effective pressure area) that acts in an upward direction, opposite to the direction of the force of the dead weight.

If significant thickness variations are present in a specimen that is otherwise relatively smooth, the measured air flow will be higher than that which is measured on a similar surface that has uniform thickness.

 

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