Don't ignore your film core.

Staying competitive in today's film-extrusion arena means producing high-quality products at the fastest possible output rates. But productivity and quality goals are tough to meet unless each and every part of the extrusion line pulls its weight - even the often-overlooked film core.

Indeed the core represents only a fraction of the film roll's final value. Yet the core can have an impact far beyond its cost. Consider that the money saved on low-end cores, which tend to have a higher potential for failure and for damaging film, pales in comparison with the expense of production stoppages, returned product, and scrapped rolls. A $5 core that causes 50 lb of damaged film at $1.50/lb costs far more than a $9 core that brings about little or no film damage.

As today's wider, faster, and thinner webs place more stringent physical demands on cores, awareness of specification issues has become more vital than ever. Properly specified cores help ensure the maximum film yield/roll without failing or causing film defects; poorly specified ones can do just the opposite.

And contrary to popular wisdom, there is much more to film core specification than citing an ID, length, and flat-crush strength. A host of other factors - like dimensional consistency, roundness, radial strength, straightness, and surface quality - should play a role in any informed core selection.

DEMAND CONSISTENCY

For cores to fit your winding machines properly day in and day out, consistent core dimensions are essential. Look at the diameters, for instance: If the ID is too large, film tension will cause the core to sep in the chucks or on the mandrel. If the ID is too small, or out of round, the core may not fit on the mandrel or chuck in the first place. When the OD varies, so does winding tension, resulting in loose rolls that won't please buyers. OD variation can also signal a more subtle problem: variations in core-wall thickness, which reduce core strength.

As for length, variations here can cause big problems too. When winding film all the way to the core edge, a short core will make the film protrude over that edge - often cause for a roll to be scrapped. And as core extensions, or the length of core left bare at either end of the roll, decrease due to ever-widening webs, there is less room for error all the time.

With today's automated equipment, dimensional consistency becomes even more crucial. For example, automatic core-handling equipment requires almost no variation in core length since most such equipment is set up to gang-wind cores of a specific size. Excessive variation from that set-up length results in downtime as operators try to compensate for length variations. In fact, with automated equipment, any sort of dimensional problems that require operator attention have the potential to shut down lines. Or looked at another way, choosing the wrong film core can endanger the productivity that expensive automated solutions were supposed to enhance in the first place.

Getting around dimensional problems is simple for anyone who takes the time to complete a few easy steps. First, develop accurate ID, OD, and length specifications and tolerances - preferably with the help of your supplier. Second, verify the ongoing appropriateness of those specifications as your products and manufacturing methods evolve. Third and perhaps most important, be sure the dimensions of cores are monitored and documented to ensure that your specs are met continuously.

MEASURING ROUNDNESS

Core manufacturers speak of roundness in terms of concentricity and "ovality." A core with an out-of-round OD can wobble on the slitter, resulting in inconsistent tension on the film as it is wound. This flaw affects all winding applications, but it's especially important for very thin films such as those under 7[mu]. Here, the wobble can make the film stretch, resulting in an inconsistent gauge. A core with severe ovality can even break webs.

For general-application film cores, you can test for roundness by simply rolling the core on a flat surface. If it seems to lope rather than roll smoothly, it is probably out of round. For high-end cores - those used for magnetic media, medical films, and other more demanding applications - a Total Indicator Runout (TIR) device can be used to measure OD ovality. In this procedure, a core is placed in a cradle formed by two sets of rollers. A dial indicator positioned in line with the rollers reveals any ovality by turning up the high and low points on the core.

MANY MEASURES OF STRENGTH

Film core strength is tough to peg since it derives from so many factors, including wall thickness, ID, material choice, and construction method. In the case of fiber cores, spiral wind angle also plays a role. One thing is for sure: As roll sizes and line speeds have increased, so too has the risk of core failure - and the related financial and safety losses. As just one example, high winding speeds and heavy loads make cores vulnerable to the shear stress that commonly occurs when the chuck does not penetrate far enough into the core.

Core manufacturers and customers have long used a flat-crush test as the principal way to measure overall core strength. As its name implies, this test measures strength by determining the load needed to crush a core section between two plates. While flat-crush strength is an excellent indicator of a core's ability to withstand transportation and handling, it does not provide the best forecast of a core's strength under winding.

Radial-crush strength, by contrast, does reveal a core's ability to stand up to the pressures that really count - those of the wound film. Until recently, there was no practical way to measure the radial crush. Just this year, however, Sonoco patented a device for duplicating the radial forces experienced by a core during winding. This hydraulic device exerts radial pressure on the core OD while simultaneously inhibiting the core from going out of round. Using data from the new test and the results of other research, Sonoco will soon be able to offer cores designed to customer-specified radial strength requirements.

Another important measure of likely core performance is beam strength - an indication of stiffness that becomes especially important with long or wide cores. Some film producers now have their own beam-strength tests, which typically involve suspending a weight from the center of a core. Evaluation can be on the basis of the core's ability to support a specific weight, or cores can be tested with increasing weight levels until failure. Both test methods have some limitations, however, since they are testing point loads rather than the uniform loading a core experiences during winding.

STRAIGHT TALK

Core straightness is a sign of core quality that mostly concerns producers of high-value and thin films. Straight cores promote uniform winding tension and thus prevent film from wrinkling.

Cores used to wind films for such high-end applications as magnetic media or medical packaging can be measured using the TIR stand, the same device that checks for ovality. In this case, the test employs three sensors, one on each core end and one on the core center. As the core is rotated, the sensors will detect the height variations that indicate warpage.

Another more precise method of uncovering warpage is to measure any gaps between the surface of the rotated core and a set of machined precision rollers. Called the Clearance Method, this test more closely simulates actual film winding conditions.

ON THE SURFACE

Two characteristics, waviness and roughness, together describe variations in a core's surface. To better understand these, think of the core surface as a road: Waviness is whether the road is flat or hilly, while roughness is whether the pavement is smooth or bumpy.

One common mistake in evaluating cores for polyester film, for instance, is to assume that a slick core surface ensures trouble-free winding. Surface waviness, however, is a more important measure since it relates to the long-term dips and valleys on the core surface. These irregularities can cause film-tension fluctuations, allowing air pockets to be trapped or surface indentations to mark the film. Typically, waviness defects cause far more scrap than roughness defects.

But don't count out the benefits of eliminating roughness. A slick surface reduces short-term variations and markings on thin-gauge films. Low surface roughness values can also speed winding startups by protecting film from damage on the first plies of the roll.

As films increase in value and decrease in gauge, the importance of surface smoothness grows. Manufacturers of audio, video and other polyester films need sophisticated cores with consistently smooth outer surfaces that are free of imperfections because even tiny defects can carry through several hundred feet of film. Pimpling, stretching, and marring can result, greatly reducing a film's quality and driving up scrap costs. Fortunately, there are instruments that quantify surface smoothness. Stylus devices, which travel over the core like a record player arm, measure waviness and roughness down to the micron.

ENDING UP

While end-cut appearance may have no direct impact on core performance, it is nonetheless significant as a general indication of manufacturing quality. Remember, end cuts are the only part of the core that customers see on a wound roll so they should be flat, square, and free of burrs and other debris from the cutting process.

To determine end-cut quality, place the core end on a level surface. A flat end-cut will have little or no visible space between it and the level surface. A square end-cut core will stand at exactly 90[degrees] from that surface. For applications where cleanliness is critical - as it is in medical films - core ends can even be sealed by your supplier.

Other, less quantifiable measures are important cues to core quality:

* Fiber cores must be manufactured so they will have consistent, stable moisture content. Moisture variations will cause a core to shrink, resulting in a loose roll. Barring the availability of simple moisture testers, one way to make sure you receive cores with the proper moisture content is to ask your supplier to provide SQC documentation from the manufacturing process.

* Cores should contain adequate adhesive. Properly glued cores maintain the required strength to withstand high-speed production and the tension it can produce. If you see any defects such as "dog ears" - a condition in which the exposed spiral ply lifts away from the core - adhesion problems are likely.

* Finally, cores should look good. A core with no visible imperfections is a sign of good craftsmanship.

Cores and Stripes

Although they are not strictly a part of the core, startup stripes deserve a film-core specifier's scrutiny because they are not all created equal.

Tape stripes leave a "ledge" on the surface of the core, which can result in film markings. The best alternative: Adhesive material should be applied directly to the core surface. This way there are no right angles at the end of the stripe, and thus no ledges that can result in surface marks on the film.