Measuring fourdrinier table stock activity helps formation

On fourdrinier forming sections, stock activity on the table is necessary to improve sheet formation and drainage rate. However, too little or too much activity can cause poor formation.

Stock activity is actually something that can be observed during operation through the use of a low angle, bright strobe light flashing at a high frequency.

Stock activity starts as soon as the jet lands on the forming board. Activity is later influenced by the type of foils (blades) with which the fourdrinier is equipped. The stock consistency, foil geometry, spacing, machine speed, and forming fabric tension all play a role in determining how much activity there will be on the table.

Trials should be conducted with different foils to optimize formation and drainage. When performing these trials, it is helpful to use a stock activity scale to compare different foil arrangements. The following illustration is a scale, from 1 to 10, that can be used to compare stock activity levels.

Scale of table stock activity, from wave through microturbulence to stock jumping

The following picture montage also helps show the relative differences of activity.

Examples of stock turbulence seen using a a high frequency strobe light at an oblique angle

High activity is normally a good thing early on the table, i.e. the upstream end of the fourdrinier. Some high speed machines will actually have significant stock jump over the first few foil boxes. The stock will separate from the table and land back onto the wire at a later point. As the sheet forms though, activity level should decrease so as not to disrupt the partially formed sheet.

There are many considerations and alternatives when setting up a fourdrinier table - too many to cover here. This brief article is only an introduction to this particular scale of stock activity. The best source of information for setting up and optimizing your table foils is your current foil supplier or your Valmet representative.