An investigation of the effect of forming variables on handsheet strength properties showed that stock dilution, shear gradients and controlled initial drainage are factors that have major effects on sheet structure and properties. A better understanding of the causes of changes in paper strength properties resulted from the introduction of a new concept of basic sheet properties.
One basic property is the specific tensile strength. This represents the average tensile strength throughout a sheet,in contrast with the standard tensile strength, which is generally a measure of strength in the weakest part of the test samples. The well-known loss of tensile strength that occurs when handsheets are formed from stock at higher concentrations is shown to be caused mainly by small-scale substance variability, since the specific tensile strength is essentially constant over the same range of concentrations.
The effect of substance variability on other strength properties was examined by means of uniform base layer sheets with superimposed substance spots. The spots were used to obtain a known and reproducible pattern of substance variability. Notwithstanding the increased substance of the spotted sheets, they were found to be physically weaker in all properties except tearing strength. Substance variability was found also to be responsible for the reciprocal dependence of tearing strength on bursting and tensile strengths.
As a test of the practical importance of dilution and shear gradients, Fourdrinier machine trials were run in which the water removal capacity was increased considerably by the application of fan-produced vacuum under the forming zone. Sheet properties were found to be still improving up to the maximum flow box dilution or speed set by other machine limits such as drying and stock pumping.