I worked as a roving process engineer for a major forest products company that had paper mills across the US and Canada. One of my first assignments was to identify the cause of a repeated buildup of “hairballs” in the secondary headbox of a paper machine that made liner. Liner is the brown paper that forms the outer surface of corrugated boxes. The headbox is a mechanism that produces a precise jet of paper fibers suspended in water. At the other end of the machine, it becomes dry paper.
This paper machine was suffering from unscheduled downtime due to clearing these hairballs every few weeks. This had been going on for all of its 20-year existence. The hairballs looked like sisal rope that had been unraveled and randomly tangled back into something roughly the size and shape of a half-smoked cigar. They repeatedly filled every one of the several hundred flow straightener tubes in that headbox. This unusual phenomenon had been accepted as a quirk of the machine until the new mill manager heard about it and insisted that it be dealt with. That’s how I came to be invited to that mill.
My first move was to solicit the opinions of all the people involved in the day-to-day operation of the machine, as the root causes of many production problems are often correctly identified by the operators. It was clear that the hairballs were made exclusively of paper fibers, but where they came from was anyone’s guess.
During scheduled downtime, we used a video crawler to inspect the headbox feed piping. There were a few short deadlegs, but nothing serious enough to be a source of fiber tangling. Other than some minor boron-based white deposits inside the piping (a product of the local water supply), the chemistry was within accepted ranges, as was the fiber supply from the mill. Nor was the machine being operated outside its design envelope. I was scratching my head along with everyone else.
Then I saw something unusual. During a walkthrough, I came upon the pressure screen that fed the headbox in question. It was disassembled for maintenance. A pressure screen is used to protect the headbox from any debris larger than the screen holes. It’s a little like an outsized top-load washing machine. The main difference is that, instead of using a central agitator, internal hydrofoils were used to rotate inside the screen basket to keep the screen holes from blinding over with paper fibers.