Slitters : History, How They Work, Advantages & Disadvantages
The earliest forms of paper making involved pulp and screens, then later it was discovered that paper could be made from rags and wood pulp. The advent of the printing press in the 1400s created a much greater demand for paper, along with a desire for higher-quality paper that could withstand the rigors of machinery. Paper mills powered by water increased quality and production, but it was the introduction of steam technology that really propelled the industry forward. Finally, paper could be produced at high volumes at a lower cost.
Almost as soon as 19th-century inventors began creating paper on large rolls, other inventors began puzzling over how to cut the rolls to size. The slitter machine was invented to solve this problem. In our article, A New Look at Paper Slitter Rewinders (seen here), we note that since the 1800s, a long string of patents have been filed with the U.S. Patent and Trademark Office, improving on the roll converting process, but the basic idea behind a slitter/rewinder has remained the same for more than 100 years.
How slitter/rewinders work
Roll slitters are used to cut a large roll of material into smaller rolls. These specialized machines are used by a number of industries, including paper, plastics, foil, and film, and they handle anything from very thin foil or cigarette paper to thick board or plastic materials. The process of running materials through a slitter/rewinder is known as converting a roll, or roll conversion, as the end result is essentially an entirely new roll.
Slitter rewinders are heavy manufacturing machines. They can vary in size widely, but typically are large enough to cut down a roll as small as a matter of inches, as with tape and adhesives all the way up to 1370-inch wide and as big as 80 inches in diameter. There are roll converting machines built to handle rolls weighing up to 13,000 pounds. Installing and running one of these large industrial machines is a sizeable investment, costing about $1 - $4 million completely installed, and it's estimated that the average slitter/rewinder in use today in North America is 35 years old.
Operators load a roll of paper (or any appropriate substrate) called the "parent roll" into the front end of the slitter rewinder, which is called the unwind stand. The unwind stand holds the roll in place and allows it to spin while the material is pulled off into the slitter. The roll is held in the unwind stand using one of two methods. The first method requires chucks that insert into each end of the roll's core and mechanically expand to grab it. The second method involves shafts - a bar is inserted through the entire length of the core and is equipped with expanding gripping elements that grab the core. The shaft extends beyond the ends of the roll and the exposed shaft ends are then lowered into a bearing system that locks the shaft in place and allows it to rotate so that the material can be un-wound into the slitter/rewinder. The operator then manually feeds the material (called "the web") into the slitter/rewinder through a series of rollers and across the cutting area which can contain razor blades, straight blades, or circular blades, depending on the type of materials that's being cut. The material is then attached to new cores on the rewind stand and pulled onto the new cores. This pulling action is what causes tension on the web material.
Two critical elements that affect the quality of the finished rolls produced on a slitter/rewinder include how well the it can control the tension of the material (Tension Control) and how well it can control the alignment of the material as it is rewound on to the new cores (Edge Guiding.) The tension needs to be carefully managed not only to avoid breaking the web while converting the roll, but to also ensure that the finished rolls have consistent in-wound tension. Poor tension control and/or edge alignment during the slitting and rewinding process are major causes of problems in any downstream operations where the converted rolls may be used. These problems include registration issues, web breaks, or edge guiding problems. The alignment of the material as it is rewound can be problematic since even small variations in the material alignment can cause coning or wavy edges in finished rolls. These inconsistencies make it more difficult for downstream uses of the rolls to control the edges as it runs through other web fed operations including printing, embossing, coating, laminating, sheeting, and other materials.
Depending on the basis weight of the parent roll, most s/r's require about an hour to set up, cut, rewind and remove the finished rolls, and can require two or three technicians to operate.
Slitters have been the industry standard for converting rolls for more than a century. They are used constantly to trim or cut rolls. Because it's the industry-standard equipment, there is a small army of workers who know how to operate, calibrate and maintain slitter/rewinders of all shapes and sizes.
There are specialty models that can help meet the needs of various industries – turret winders that can reduce downtime, or slitting machines designed for specific uses such as cutting very thin film, or very thick plastic.
In general these machines work very well for slitting many small rolls from a parent – like tape rolls for example -- or for applications where converting one roll into another with a different sized core is required. They are also helpful where a change in diameter is required.
Limitations of paper roll slitters
There are some drawbacks to slitter rewinders, as well. They are slow and expensive, and quite often produce inconsistent results. Since the equipment is often old, it's difficult to maintain consistent tension which can cause breaks, and the process forces the roll to lose its original mill wind.
Even more problematic and time consuming is the process of trying to avoid imperfections brought on by picking & paper dust.
When paper is made it has a rough side and a smooth side. This difference is simply a result of the process of making paper. Even though these differences are very small, the side of the paper is printed on can have significant affect on the quality of the final printed product. Printers prefer printing on the smooth side, as it produces a higher quality finished product with fewer imperfections. Printing on the rough side often causes "picking" – bits of fibre from the paper are pulled from the surface, causing imperfections in the finished product when run through a printing press. Using a slitter/rewinder causes the smooth side of the paper to invert on the finished rolls coming of the rewind, so that the rough side will be fed into the printer (a less desirable situation). Additionally, slitting creates a lot of paper dust which gets on to the surface of the paper and can be a major source of imperfections in the finished printed product. Finished rolls risk being rejected due to these issues. So, after a roll goes through a slitter/rewinder for sizing, it sometimes has to go through the rewinding process a second time so that the paper orientation is correct. This process further degrades the roll and is more time consuming and expensive for the processor.
While slitter/rewinders are ideal for applications where it's necessary to convert many smaller rolls from a single parent and/or where core or diameter changes are required, it falls short when a large roll simply needs to be cut down to a narrower size. The time needed for setup and running one for jobs like this is prohibitive, particularly for quick-turn orders. Just one simple quick-turn order can throw a production schedule into chaos, particularly if there is no "spare" machine available to run the order.
The maximum roll width any given slitter/rewinder can handle is set by the machine dimensions. This creates a paradox: Large capacity machines are expensive but can run large rolls well. However, large machines don't run narrow rolls particularly well, and small capacity machines are limited in the size of rolls they can handle. Paper slitters are all built to cut only up to a certain size, so some slitter/rewinders are simply too small to handle the big jobs that are required in modern mills.
Additionally, paper slitters are very dangerous. According to Health and Safety Executive, "Reelers and slitters cause the most machine accidents in the paper and board industry, (ie, those accidents involving contact with moving machinery or the material being produced), and can often result in serious injury and sometimes death."
Next generation solutions for converting rolls
Manufacturers continue to develop improvements for slitter/rewinders, and a quick patent search shows that the technology is being improved upon on a regular basis. But the nature of unwinding a roll, cutting it, and rewinding it again means that the process can only be improved upon so much. Slitter/rewinders will never be truly "fast" – a slow pace is built into the DNA of a slitter.
As technology improves, and demand gets higher, there are new alternatives. RollRazor takes unwinding and winding out of the equation entirely to deliver a process that's fast – from an industry average of one hour to load and convert a roll on a slitter rewinder to a complete cycle time of under six minutes – accurate, maintains the original mill wind and web orientation and creates no paper dust. See for yourself how it works. RollRazor - Slitter Rewinder & Slitter Machine Replacement