ZD Non-Destructive Stiffness Measurement

ZD TSI measurement – the stiffness measurement in the thickness direction of the paper sheet – completes the three engineering directions that describe paper sheet performance. Because ZD properties are so sensitive to process changes and product performance and bond damage (and its quick!) the test can give information that other tests cannot.

This measurement can be used to monitor the overall quality of manufactured paper and can be correlated with Scott Plybond and ZD tensile tests. The effect of starch addition and indication of where starch resides in the sheet, minimising damage in calenders, cross machine press profiles, forming studies and more. It can also be related to delamination tendency in mediums and flutings. Did we mention it’s fast!

51b30f_75ec123d306a4c17a6000954601dc9e01. On-line ZD paper stiffness measurement

Aim: To implement a non-destructive, continuous measure of zd stiffness (single sided) for process use on paper machines compatible with current Aurelia on-line measurement platforms.


  • Starch addition measurement (zd thickness profile efficiency)
  • Recognise and control delamination issues
  • Fast and real time paper machine forming setup
  • Improved non-destructive prediction of paper SCT, ZD tensile and Scott Plybond, bending stiffness
  • 3-D characterisation of manufactured paper.
  • Determination of fibre quality


Traditionally, non-destructive zd measurement has been made using high frequency (1 MHz) ultrasonic pulses directed through the thickness of the paper sheet with a transmitter on one surface and a receiver on the other. The zd stiffness is calculated using the propagation time of the ultrasonic pulse through the paper.

The technique has a number of difficulties when considered for application to on-line, continuous measurement:

  • The measurement is double sided (the paper is required to pass through a nip)
  • Signal coupling into and out of the sheet is poor.
  • Data rates (due to the pulse measurement) are low and signal noise is high

An alternative method that is compatible with Aurelia’s existing in-plane, on-line ultrasonic tester platform is suited to zd measurement on the paper machine.

Paper Physics

Waves propagated through a sheet of paper in the plane of the sheet are generally dispersive. However, due to the relationships between in-plane and out-of-plane stiffness for paper it is possible to determine the ZD stiffness of the paper sheet from ultrasonic waves propagated in the plane of the paper sheet.

This relationship is remarkably robust and is shown in where a number of US linerboard machines show how well the ZD stiffness can be estimated from measurements of MD or CD plane stiffness and the MD or CD flexural stiffness.


  • The measurement of ZD stiffness on the paper machine opens the way for real-time machine set-up providing information related to forming, wet pressing, drying restraint, wet straining and calendering. As the zd stiffness is very sensitive to refining, the level of refining control can also be monitored.
  • Overall paper quality can be estimated using an extension of Baum’s equivalent stiffness circle– in this case an equivalent stiffness sphere gives important information related to principal axis stiffness and overall paper and fibre quality.
  • On-line and continuous prediction of properties such as ZD tensile and Scott Plybond become possible and delamination tendency can be estimated