PATTERN SELECTION

3.1 THEORY OF PATTERN SELECTION

Determining the pattern to be used on a pulpstone depends upon many variables such as stone composition, wood quality, motor size, type of grinder, stone surface speed, freeness range desired, type of paper for which pulp will be used, and condition of sharpening equipment. The prime consideration in pattern selection is to select a pattern, which will produce the proper or "target" pulp freeness.

The theory of pattern selection can be reduced to a formula. The basic grinding formula is Q = Ps in which Q is pulp quality, and Ps is specific grinding pressure. This can be further simplified to read Ps = CSF or specific grinding pressure controls freeness. As Ps increases, the deforming pressure on the wood fibres increases, increasing the rate of heating of the lignin, which holds the fibres in the wood. The faster softening of the lignin will cause longer fibres and more fibre bundles to be released from the wood causing a rise in pulp freeness  (faster drainage or lower Schopper Riegler).

The formula Ps = CSF can be changed to read LW = CSF or land width equals freeness. Land width refers to the width of the land of the stone pattern measured at 90º to the axis of the groove. These directly control specific grinding pressure and thus pulp quality. The above formulas assume constant motor load

The theoretical grinding area of a pulpstone for any given burr pattern can be calculated by using the following formula

The following table illustrates the grinding areas for different burr patterns on a 67" x 54"(1700x1370mm) pulpstone, assuming a constant groove depth of .040" (1.0mm).

In the table below we see the effect that a change in groove depth has on the pulpstone grinding area for a single burr pattern. This illustrates the importance of precisely controlling burr tooth penetration into the stone

Pattern/Depth Grinding Area

3.2 THE FOLLOWING RULES ARE BASIC TO PATTERN SELECTION:

1. A narrow land width produced by a higher pitch burr will produce a higher freeness.

   
   Fig.13 Narrow Land Width = High CSF

2. To lower freeness increase the land width by using a lower pitch burr.

   
   Fig.14 Wide Land Width = Low CSF

3. To apply more energy to the stone to improve pulp quality, without changing freeness, increase land width by using a lower pitch burr.
4. Coarser grit stones require wider land width produced with lower pitch burrs.
5. Grinders having larger motors require wider land width to produce equal pulp freeness.
6. Grinders having higher surface speed require wider land width to produce equal pulp freeness.

3.3 GRIT SIZE

Particular attention must be paid to the average grit size of the pulpstone to make certain not to select a pattern having too narrow a land width. The width of the pattern at the base of the land should be at least 5 grit diameters in width in order for the land to be strong enough to support the grinding load.

Fig.15 Minimum Pattern Width

The following table lists the highest (finest) pitch burr, which can be used for the largest grit size in a stone specification.

Occasionally operators will observe that when they change to higher pitch burrs, the resulting pulp produced is of lower freeness. This indicates that the burr selected is too fine and that the lands of the pattern are breaking down because there is not at least 5 grit diameters of width at the base of the pattern to provide adequate support for the applied load. Frequent sharpening will be required to keep an excessively narrow pattern producing an adequate volume of pulp the proper freeness level. The result of using too fine a pattern will be shortened stone life.

3.4

The lead angle of the pattern has a lesser affect on pulp freeness but plays a role in determining fibre length, slenderness and fibrillation. this is the "L8S" factor of pulp quality. Lead will have a bearing on the amount of regrinding of the fibre once it has been separated from the wood. A higher lead angle produces a long grinding zone, which results in a more slender fibre. A pattern having a higher lead angle will produce a lower freeness pulp having shorter fibres, less shives and a higher fines content. Slightly lower production rates will result due to the increased regrinding of the fibres. increasing the lead angle will reduce the grinding area of the stone, although to a much lesser degree than changing the pitch of the burr.

3.5

A burr that is used more than once will effect the grinding area of the pulpstone by causing the grooves to become wider. The widening of the grooves decreases land width thereby increasing freeness of the pulp. The result of using a burr more than once can be broken stone patterns as illustrated below.

Fig. 16a. Dull Burr Widens Grooves

The diagram below illustrates the widening of the grooves when a burr is used successive times. After 3 sharpenings with the same burr at the same depth the width of the land area becomes less than 5 grit diameters and too weak to support the grinding load ( See Sect 3.3 ). The result is breakdown of the pattern and the need for more frequent sharpenings. More frequent sharpenings shorten stone life.

Fig. 16b. Using a Burr More Than Once Widens Grooves

3.6

The following list of pulp grades and the most widely used burrs for these grades will give a starting point in the selection of burrs. Due to the many variables listed at the beginning of this section trial work is necessary to determine the ultimate burr pattern.