Fig. 1. Cumulative variation for commercial paper machines
These variations in property occur at various time scales, but for time scales longer than say two hours, where we would hope to be able to actively control, the variation is still considerable at around 15-20%. Figure 1 shows a comparison of variation for liner board machines from a number of supplier markets plotted against the speed of the variation.
Apart from the extent of property variation within a grade run, it is disturbing to note that the variation increases from run to run. That is the paper machine has difficulty meeting quality requirements from run to run for the same grade (over many weeks). Figure 2 gives an example taken from a Kraft liner board machine comparing variation in a run with variation over a number of runs of the same grade.
Fig. 2. Property distribution for consecutive makings of the same grade on a commercial liner board machine.
So what does this all mean in terms of money? The purpose of paper quality is to ensure that paper that is manufactured is “fit for purpose”. That is, it meets the requirement and expectation of the customer. In practical terms we can more accurately say, that the paper at least meets the requirement of the customer. No matter to what criteria paper is manufactured (for example a grade property aim) it is sold on an expected minimum. If there is variation in property, the manufacturer must ensure that all (or in practice, most) of that variation exists above the expected minimum performance. If the extent of variation can be minimised (the distributions as shown in Figure 2 narrowed) then the average of the variation (the “aim”) can be lowered. There lies the potential for savings!
The benefit from this “narrowing” or minimisation of variation can achieved in one way: move the aim properties of the paper down while still meeting the minimum requirement. How to do this? Well, there are two main ways to achieve a lower aim, either: lower the basis weight or use less expensive materials (say replace virgin fibre with recycled fibre, increase filler content or lower starch addition) to make the paper.
Of these, the best return is obtained by lowering the basis weight. In a controlled environment this would mean letting the basis weight “float” over some range. Another way of looking at this is that the paper would no longer be sold by weight but sold by area. Of course for both approaches it is important to consider ALL the properties that would be affected. In any case, up to 10% down weighting might be a possibility while still meeting the minimum performance requirement. You can do the maths on potential savings.
As an example of the sort of things you can include in your calculations:
Transport from the mill (per square meter)
Paper machine production rate (square meters/hr)
Material cost per square meter of production
As you can see the concept is remarkably straight forward, with high and nearly immediate returns. All we need is an on-line property measurement device, like AIP’s PQM-3, and some knowledge of how to change and control properties on the paper machine in real time. Simple!