Core selection - the difference between success and failure.

Published: February 01, 1996, By Boyle, Edward

Proper core selection can mean the difference between converting success or failure. Here are things to consider when making your choice.

A commitment to producing the highest quality finished product should be at the heart of every successful converting operation. But the ability to deliver on that promise may literally rest with its core.

Scott Pleune, marketing manager/paper and film industries for Sonoco Products Co., Industrial Products Div., Hartsville, SC, explains that proper core selection can not only improve production efficiencies at your own facility, but it can greatly diminish the likelihood of problems at your customer's plant.

Consequently, he recommends a core based on precise specifications, taking into account core surface, straightness, moisture content, strength, dimensional consistency, and packaging. Sonoco reports that its total systems approach has saved customers thousands of dollars in increased productivity and reduced waste.

"New thinking about the role of cores in total systems performance has largely displaced the traditional notion of the core as a commodity item," says Pleune. "Failure of the core to perform properly during winding, transportation, or unwinding can jeopardize thousands of dollars worth of product. This may cost the manufacturer, as well as the converter, in over-expenditures throughout the total system."

Pleune suggests that for today's converters and manufacturers, specifying interior diameter, length, and flat-crush is no longer enough to assure trouble-free productions. Core-to-core consistency, end-cut, ovality (roundness), straightness, surface, cleanliness, moisture, beam stiffness, radial strength, and dynamic load strength must also be considered for a variety of reasons:

* Dimensional consistency is critical to roll quality and to the operation of automated core-handling equipment.

* End-cut quality has an impact on an end-user's perception of product quality. Core ends should be flat and square; free of burrs, dust, and other debris; and should be sealed when cleanliness is critical.

* Ovality is especially critical for very thin materials, as any core that is out-of-round can stretch the substrate, producing an inconsistent product or even a web break.

* Straightness is critical for uniform winding tensions, to prevent wrinkling, and to ensure smooth running on unwinding reel stands.

* Uncontrolled core moisture during manufacturing may result in core shrinkage, which can cause a loose roll and make product use difficult.

* Flat crush and beam strength have been the standard strength criteria for film and paper mill cores. While flat crush remains an important characteristic in core design, innovations have also allowed Sonoco to develop cores geared to radial crush and dynamic load strength, which are more relevant specifications for both types of cores, says Pleune.

Radial Crush

For years, film manufacturers have known that radial crush, not flat crush, is the important specification for designing film cores that won't crush. This is because radial crush more closely approximates the type of loading placed on a film core.

However, the technology had never been able to efficiently measure radial crush, much less design tubes to a known radial crush specification. Pleune says his company is now capable of designing cores to a known radial strength requirement, which can significantly improve total system costs by reducing the frequency of crushed cores and subsequent downtime in film manufacturing plants and converting operations.

The Industrial Products Div. of Sonoco Products Co. is the nation's leading producer of high-performance, cost-effective paper and plastic cores, cones, and tubes used for a variety of applications in the paper, film, converting, construction, and textile industries.

Roll Surface Requirements

The two key surface variables of a core are waviness and roughness. Excessive waviness can result in uneven tensions, air entrapment, and spiral marking of the film. Excessive roughness, such as that caused by a gritty spot on the core surface, can cause pimpling throughout the roll.

Waviness is the long-term variation in the core surface. Pleune compares the core surface to that of a road: Waviness describes whether the road is flat or hilly. Roughness or smoothness describes where the road is smooth or bumpy. Waviness is more important than roughness, because it works through many winds of thin film. Roughness problems normally disappear after a few wraps.

Waviness is measured with laser or stylus (like the needle of a record player) instruments. These instruments "feel" 100 mm of core surface, including the spiral seam. The difference between the highest and lowest spot on the core is its waviness reading.

The most common defect caused by a wavy core will show up around the spiral seam of the core. Marking of the film, tension spots, and even film stretching can result. Severe waviness can also lead to air being trapped between the core and the film. These raised spots or dips can work through many wraps of film found on the core.

"Ultimately," says Pleune, "the straighter and smoother the core, the more usable film there will be on each roll. Similarly, the fiber core manufacturer of the '90s must be able to deliver cores that can perform under much greater loads and stresses than ever before. The cores must provide for smooth, effective winding and contribute to good roll structure, even with 50-in. roll diameters, lengths up to 125 in., and roll weights exceeding five to six tons."

For their part, the pressrooms and converters then expect these rolls of paper to run trouble-free, with no vibration, web flutter, or breaks at speeds that may exceed 3,000 fpm.

Historically, flat crush performance has been the standard in describing core performance, says L.W. Roper, product manager/paper industry for Sonoco's Industrial Products Div. What does flat crush really measure? It is the strength of the core that allows handling or shipping of the core without collapsing.

"Flat crush performance alone is not the answer to these extreme demands," says Roper. "Winding and unwinding forces now exerted on the core require higher strengths in many dimensions. The dynamic strength of the cores combined with the critical speed performance are very important attributes. Understanding these total requirements, and designing the proper care to give optimum performance, is the challenge of the 90s."

Choosing the Right Core

Since roll performance is enhanced by using the right core, the question becomes, "How do I know which core to use?" At a minimum, several core characteristics are required by the printer and converter:

* I.D. consistency. The correct inner diameter is required for the core to fit properly on the chuck or shaft, which is vital for proper unwinding and to maintain the proper web tension during the printing process. If the core I.D. is too small, the paper is often torn and compacted on the I.D., which causes roll bounce. If the I.D. is too large, the core won't fit properly on the chuck or shaft, resulting in chewout.

* Dynamic load strength. In the printing process, greater dynamic load strength is required to carry the weight of the larger roll on the unwind stands. Cores must have load-bearing strength capability to provide good roll structure. If the core is destroyed, it won't have enough integrity to run properly on the press, which can lead to chewout, poor registration, or even web breaks.

* Critical speed. When the web is expiring, the core must be capable of withstanding the critical speed encountered, thus preventing the vibration that can cause web breaks and excessive paper waste. The widest cores now have diameters as large as 6 in., enabling them to perform at or beyond designed press speeds.

* Stiffness. In wide web printing in particular, a core must provide increased stiffness so that it remains straight without deflection. This prevents vibration when the web is expiring, which results in web flutter and may cause a web break and excessive paper waste.

Redefining Fiber Core Performance

The need for redefining fiber core performance attributes, in particular, has developed largely because of five changes that have been evolving in the paper and printing industries: wider web widths; faster press speeds; larger diameter rolls; surface-center-drive, single-drum winders; and the elimination of metal pipes and ends.

"All of these changes are aimed at increasing productivity for the pressrooms and converters - the ultimate customers," says Roper. "One of the consequences of these changes is the need for higher-strength core designs. The challenge to improve performance has extended to the core manufacturer. As a result, cores must be designed to enhance overall performance by addressing these operational issues that occur with the newer winding and unwinding technology."

Today and in the future, adds Roper, many other factors - such as the ability to resist vibration at high winding and unwinding speeds and dynamic strength performance - will be the keys to purchasing cores.

Sonoco's HQ Series of cores is a line of advanced, high-performance products - with standard cores for normal operations to high-strength products for the increased demands of wide web gravure printing. The HQ cores are highly specialized, with varying degrees of flat crush strength, dynamic load strength, and stiffness.

"The different levels of these required characteristics determine the type of core to be used," Roper explains.

How important is the right core? The cost of a roll of paper or film is constantly increasing in today's market, says Pleune, approaching tens of thousands of dollars per roll - as compared to a core that costs just a few dollars. Yet, the core serves a critical roll as the substrate carrier, used for winding, transportation, storing, and then unwinding the paper roll.

"If the core fails during any of these steps, the paper roll will not run properly, jeopardizing thousands of dollars worth of product," warns Pleune. "Choosing the right core is crucial, because the right core ensures optimum performance through every phase of production."