Paper flies off a paper machine at speeds often over 70 miles per hour (96.5 kph), or rather, 5-7,000 fpm. One minute of downtime on a paper machine that might spew paper over 100 yards long can cost the manufacturer thousands of dollars. Restarting a paper machine that was stopped due to paper web breakage, quality problems, or mechanical breakdown can take 20 to 60 minutes after the problem has been corrected. In other words – downtime is to be avoided at all costs.
Duncan and a team of dedicated people at Flow in the 1980’s and 90’s mastered the art of paper slitting with pure waterjets (no abrasive). Waterjet is still used today. The 24/7 slitting of paper was such a demanding application in terms of up-time that it drove the development of very long lasting, reliable pumps and system redundancy. If you look at the picture below, you’ll notice that there are actually two cutting heads at each slitting location. This allows for quick switchover from one head to another if the waterjet orifice needs changing. The paper line does not need to stop to do this.
The redundant pump was also developed. Even though in these applications our intensifier pumps have a mean time between maintenance of over 1,500 hours, the paper mills needed even more assurance that the waterjet cutting process would not be the cause of downtime. Therefore, Duncan and the engineers developed a redundant pump. Redundant pumps have two intensifiers on top, but only one is used at any given time. If maintenance is required (such as a seal change on one intensifier) the system can quickly be switched over to the other intensifier. Then when the paper mill shuts the paper machine off, maintenance on the waterjet, and the entire paper machine for that matter, is performed. Paper machines might run 360 days a year. It’s all about uptime when you’re making a continuous web of product.
Why use a waterjet? Why not a rotary die cutter (like a pizza cutter), rotating saw, or reciprocating blade of some type?
Waterjet allows for the converting of the large roll to smaller rolls directly on the paper machine. The waterjet puts no cutting force on the paper and is compact in size. These capabilities can eliminate the need for a rewinding process; a machine that can cost millions of dollars.
The paper itself after cutting is in better condition, has more crepe, and be made of a more absorbent or softer product.
Moisture – you get more moisture on the paper by touching it or breathing on it than you do when we waterjet cut it. How is that? The waterjet stream is very small and moving very fast. The paper is thin and moving extremely fast as well. The kerf width and the waterjet pass right through the paper into the catcher tank for easy disposal and there is no time, nor enough water for the tissue paper or coated paper to absorb the water. Amazing and true.
Edge trimming – An important ability of waterjet is trimming of the outside edge off the paper. As you can imagine, it would be hard to trim a very thin sliver of paper with a mechanical device – but waterjet erodes a thin kerf width and doesn’t care whether it is the very edge of the paper or the middle of the paper.
Tail cutting – when the paper machine has to be restarted, a strip of paper must be threaded through the machine. The waterjet can be used very effectively to cut the ever enlarging triangular shape as the paper is fed through the rollers.
I have always been fascinated in the pure waterjet cutting of hydroscopic (absorbent) materials like paper and diapers. Still amazes me that we can cut so fast, so clean, without moisture – even though we’re using water.
By Chip Burnham