Keep Vendors in Check

The quality of the products purchased from anyone should be checked periodically. This keeps the suppliers and vendors honest. It shouldn't be that way but it is. I'll give you an example of a very big headache that occurred due to a supplier/vendor.

I was working on a fine paper machine and shortly after an outage a mysterious diagonal curl started appearing in the sheet. The thing with diagonal curl is that it's easy to diagnose but difficult to cure. The curl had not been there prior to the outage so it must have come from something that was changed out during the outage. The problem is that there are lots of the things changed out during an outage. So after a week or so of investigating the cause of the curl still couldn't be identified. The problem was that the curl was moving. It wasn't always in the same position. So whatever was causing the curl had to be a moving part. On a paper machine everything moves. So the next step was to send in the corporate engineers. The were as puzzled as the rest of us. Another week passes and still no answer. Next some paper machine experts are brought in, they are the ones that built most of this particular machine. They also supplied some parts that were routinely replaced. By this time it was about time for another outage. The paper machine is taken apart and things are be changed out as normal. Then an old machine tender is looking around at some of the headbox parts. He picks up the lexan sheet and notices something. He says, "these sheets feel flimsier than normal." He tells the machine manager who then tells the paper machine experts. They experts say no it can't be the lexan sheets, but for argument sakes lets put in a new set. Note that they supply the lexan sheets. So the machine is started back up and the diagonal curl is still there. A few days goes by and the machine tender keeps saying he thinks its the lexan sheets, there's something wrong with them. Finally the machine manager finds the old lexan sheets which had a few pieces broken off and says to put them back in. This takes about an hour and a half or so. The machine starts back up and the curl is gone. Hmmm.... The paper machine experts call back to their home office and are told that some changes to the lexan sheet production process had been made but that shouldn't have affected the sheets. After hearing of this change the mill manager makes a call and lets just say that they went back to the old way of making lexan sheets.

So trust your operators when they say something isn't right. And just because they say they're experts it doesn't mean they're always right.

Refining

Refining in the paper making processing refines the fiber. Sounds easy enough right? It's a little more complicated than that. Refining the fibers means cutting or brushing the fibers. Too much cutting and the fibers produce too many fines which is lost during the drainage process. So we want to keep the fibers long enough to provide the strength properties we are looking for while also cutting the fibers to a length that will produce an aesthetically pleasing sheet. Too long of fibers will produce knotting and blobs in the sheet, not very uniform. The brushing part of the refining process fibrillates the fiber to produce more surface area for bonding. The more bonding that takes place the stronger the sheet. Imagine a fiber starting as a long stick. After refining the stick may be cut in half or thirds. It also will have the outside bark stripped off and look like its been chewed up a bit. That's basically what refining does.

There are different types of refiners from cones to cylinders to discs. The most common seem to be the disc models. Pulp is pumped into the refiner through a set of discs. One disc is stationary while the other spins. The plates on the disc have bars which differ depending on the refiner model and the desired results. So plates cause more cutting while others provide more brushing. As the bars from the opposing plates cross the squeeze and cut the fibers. Operators adjust the load or amount of pressure on the plates to attain a desired freeness which is the measurement used to gauge refining. The freeness is usually tested by a lab person or a utility hand.

Different plate patterns are available for different purposes, as well as different fiber species. Hardwood plates are designed for more brushing since the fibers are already relative short. Hardwood refiners are also generally set in parallel since they usually only require one pass through the main refiners. Softwood fibers on the other hand are generally set in parallel since the fibers are longer. This gives the operator more flexibility in controlling the freeness.

This is a very brief overview of refining. There's much more to it. So if there is anything in particular you would like to know, just let me know.