Blown film autogauging: does its performance justify the price?

The goal of achieving 'perfectly' flat film appears closer than ever, thanks to automatic profile control systems. But do they hit the mark when you balance quality improvements against the six-figure price tags?

When making blown film, flatness is a real virtue. As an important indicator of quality, flat film can make for happy customers. But given the numerous and often shifting causes of gauge variation, even the best extrusion equipment has not guaranteed immunity from film that lacks acceptable gauge uniformity. Oscillating haul-offs may go far to distribute any variation, but they don't fix it. Today, however, a growing chorus of equipment suppliers touts automatic profile control as a consistent way to combat transverse-direction gauge-variation problems. And their systems--most of them quite new--don't aim to improve film quality alone. Automatic profile controllers also have the potential to yield savings in material costs by facilitating downgauging and scrap reduction.

The various systems on the market all use different control approaches to cut gauge variation. Even seemingly similar systems, such as several based on air rings, are each somewhat distinct. But as much as the systems may vary, all the suppliers report similarly impressive results. They all claim that automatic profile control typically results in a 50% reduction in point-to-point gauge variation. These tighter tolerances potentially give rise to a bottom-line bonanza in increased manufacturing efficiencies and customer satisfaction. "Autoprofile has worked for us and enabled us to get into new markets," says Chuck Hansen, technical manager at W&H customer Blako Industries of Dunbridge, Ohio.

At least six suppliers now sell blown film autoprofile systems in North America. The first was commercially introduced by Windmoeller & Hoelscher in early 1988. Together, the suppliers count 183 autogauging systems in place worldwide, mostly in Europe. In North America, only 12 systems from W&H and a handful from other suppliers have been sold.

Regardless of the number of choices, though, autoprofile control still doesn't come cheap. Complete systems usually cost well over $100,000, including the accompanying computer system. These high prices make payback a big issue. "In commodity markets, the payback is probably too long," admits Kiefel president Steve Engel, echoing sentiments expressed by all the suppliers.

So far, an evaluation of the autoprofile systems' cost-effectiveness has been hampered by the paucity of realworld operating data. Some systems are, after all, still in their infancy, and suppliers can offer only lab test-run results. Others are running at processors who shroud their operations in secrecy. Fortunately, W&H and Battenfeld Gloucester customers have stepped forward to discuss their experience with autoprofiling and to provide some "hard" data. The suppliers themselves, meanwhile, have been candid in their assessments of where these costly systems make the most sense.

ALL IN THE DIE

As much as they differ on the surface, all autoprofiling systems do share a conceptual underpinning: make the circumferential gauge profile more uniform. All must first sense the thickness variations around the bubble's circumference. They all use computers to process the measurements and trigger some sort of control response, whether it be thickening the thin spots or vice versa.

The differences between systems arise in the specifics of those three steps--thickness sensing, number crunching, and control actuation. The most profound differences appear in the third step--the physical alteration of film gauge.

Windmoeller & Hoelscher has controlled thickness at the die level since its first system came on the market. The original Optifil P system employed compressed air to cool sections of the die wall and thicken corresponding thin sections of the adjacent bubble. All but 14 of the existing 115 Optifil lines still rely on this older approach.

For the past few months, however, W&H has taken a new tack in switching to a more direct sort of thermal control. By placing cartridge heaters inside the new Optifil P-2 die (first shown at K'92), the die wall sections are heated more intensely in those areas corresponding to thick spots on the bubble. The number of heaters, and thus control zones, varies with the die size. A 25 in. die, for example, would contain 132 heaters. Sales manager Andrew Wheeler says the P-2 system can optimize the profile faster than the earlier approach, and electric heating also provides an operating-cost advantage over compressed air.

W&H does not recommend its autogauging system for high-stalk applications like HMW-HDPE, citing inherent control difficulties imposed by the higher blow-up ratio, which could produces poor results. "Picking a spot on the bubble and tying it back to a point on the die is more difficult," Wheeler says, though his opinion is not shared by some of his competitors.

Another die-level control approach comes from Reifenhauser, which introduced the Reicoflex system in 1989. The first system had piezoelectric actuators located around the die circumference. These expanded and contracted on an electrical signal to selectively deform a flexible die lip, changing the die gap and altering melt flow to smooth out variations. At NPE '91, the company introduced the Reicoflex II version, which uses lower cost stepper motors to push a rod that deforms the die lip. Besides lower cost, this approach offers a greater range of lip deflection and maintains control if power is lost to the system (PT, Aug. '91, p. 43). So far, no Reifenhauser autoprofile systems have appeared in the U.S., although 40 are running in Europe, says v.p. Matthew Bangert.

Lastly, Filmaster has developed an automatic die based on thermal expansion. It uses a series of wedges that expand to change the die gap, much as an automatic profile control on a flat die would do. None have yet been sold, reports sales v.p. Frank Goffreda.

MORE THAN HOT AIR

Most of the newer systems, by contrast, use segmented air rings to change the bubble profile. This approach has already become the most popular choice in terms of offerings though not in sheer numbers of working systems. Even these air-ring autoprofile systems bear little resemblance to one another.

One of them, for instance, goes the thermal control route, controlling the temperature of air impinging on the bubble. Battenfeld Gloucester's Autoprofile system positions heaters within the vanes that delineate the dual-lip air ring's segments. By locally boosting air temperature, the air ring seeks to thin out any thick spots. In those zones where thickness is already within the acceptable range, the heaters simply don't fire up. Sales manager William Hellmuth points out that this system has no moving pans, a feature he says is likely to cut down on maintenance.

The rest of the air-ring autoprofile approaches rely on controlling the volume of unheated air that falls on the various segments of the bubble's circumference. Increasing air-flow rate thickens thin spots while reducing air flow thins out thick ones. Suppliers of these air-volume systems proclaim the ability of their systems to implement control without adding heat to an extrusion process often starved for more cooling power and sensitive to any limitation on output rate.

Alpine's entry into autogauging, the Profile Optimization System (POS), features a second air ring dedicated solely to profile control. Stepper-motor-controlled valves can deliver varying amounts of chilled or ambient air through nozzles to individual bubble segments. The "rest" position for each of the valves is halfway open. So the detection of a thin spot on the bubble would cause a valve to open partially from this intermediate position while a thick spot would trigger a valve closing. The separation of the POS cooling ring from the regular air ring allows the company to tackle high-stalk bubbles with the POS placed near the blow-out point, a positioning the company says gives better control. This way, gauge is influenced at the last possible location before the frost line "to correct for any undesirable environmental effects," says sales engineer John Wise. In low-stalk applications, the segmented ring sits just below a conventional air ring.

Kiefel's Pro-Con system, developed in conjunction with Germany's Plast-Control, also relies on a segmented dual-lip air ring fitted with motorized valves. Here, the motors move a "stopper," to selectively impede air flow to sections of bubble. Sales engineer Gunther Neumann notes, however, that at any given time, 80-90% of the air still reaches the bubble. The air ring's bottom lip incorporates raised, typically diamond-shaped, outcroppings to direct air flow at the bubble. Kiefel also tackles high-stalk bubbles, using just the segmented air ring in the conventional position.

Reifenhauser has also come out with an air-ring-based system called Reicoflow. In fact, the company now favors this approach over its better known Reicoflex autoprofile system, at least in this country. Vice president Matt Bangert explains that the relatively high price and complex retrofit procedures associated with the mechanical die-based system don't allow it to compete effectively in the North American marketplace today. "Technically, we still think Reicoflex is the better approach," says Bangert. But Reicoflow costs only half as much.

Although Reifenhauser is the fourth company to introduce an air-ring based autoprofile system, Bangert says Reifenhauser holds the original patent on air-ring control of bubble thickness and even licenses the technology to Alpine and Kiefel. Reifenhauser's design differs from the others in that the air ring's top lip sits on three stepper-motor stanchions. These can tilt that lip on a bias, allowing large volumes of air to affect relatively wide swaths of the bubble. Gross adjustments can be made with the top lip, leaving fine control to the air ring's lower lip. This strategy works to increases the range of adjustment by letting the top lip bring big variations down low enough for the segmented lower lip to take over, explains product manager Hector Marchand. The lower lip, with its air-flow control valves, is similar to those of the other systems.

WHAT THEY MEASURE AND HOW

There are fewer differences between systems when it comes data collection and processing. Nearly all the suppliers use capacitance gauging, a contact system that determines film gauge by its effect on an electric field generated by a capacitor. The sensing heads oscillate around the bubble, scanning up to 100 times/sec. The lone gauging dissenter, Battenfeld Gloucester, prefers a non-contacting sensor. So it mounts a nuclear beta gauge downstream of the nips. The other suppliers argue that the capacitance sensors provide the fastest, most reliable, and least temperature-sensitive readings. Wheeler explains that capacitance gauging doesn't take measurements from the layflat. So it doesn't have to wait for oscillating hauloff to complete a revolution before sensing an entire profile.

Dissimilarities again start to crop up on the number-crunching end of things. On the hardware side, the systems either run on 386-type PC platform--as Alpine, Kiefel, Reifenhauser and W&H do--or they work on proprietary hardware like Gloucester's Extrol. Suppliers also take different approaches to processing the thickness readings. Kiefel, for instance, says it uses 100% of the measured points in coming up with what it calls a "true profile." Gloucester also says it doesn't smooth out its data. Other suppliers prefer to filter out what they call "noise," throwing out measurements that they believe indicate defects too fleeting or small to fix. W&H, for instance, processes 100% of the readings. But it counts as relevant and tries to correct only those measurements within two standard deviations, or "two sigma," of the mean. "Using three sigma, you end up compensating for defects that might no longer be there," Wheeler argues.

HOW THEY PERFORM

Despite all these dissimilarities among the various systems, there seems to be more than one successful way to automatically control a bubble profile. Indeed, the current crop of autogauging systems do appear to share an important performance benchmark: Each supplier claims roughly a 50% reduction in gauge variation through its autoprofile system. In fact, the systems may perform much better. At the K'92 show, blown film lines from several vendors made film at thicknesses under 1 mil with |+ or -~3% variation. The suppliers say comparable new lines without autoprofiling would easily have resulted in more than twice that level of thickness variation.

And in the real processing world, Royal Precision Plastics, a recent entry into the medical film market and user of a Battenfeld Gloucester system, reports excellent early results from a new coex line in Powell, Tenn. During runs at relatively slow rates of 300-350 lb/hr, the line has been turning out three-layer, 3-mil LLDPE film with gauge variation "well below |+ or -~1%," says blown film manager Carl Prince. He notes that this level of variation may grow slightly when the company ramps up to full production rates. Even so, Prince says he will be happy with the system because "it's right on the money."

Likewise, Blako Industries, a custom extrusion house with a two-year-old W&H Optifil P line, has seen considerable improvement over the gauge uniformity it previously attained without autogauging. On a wide variety of polyethylene films, the company has seen gauge variation of less than |+ or -~2% on good runs, according to technical manager Hansen. And during less-than-ideal runs, the variation has commonly dropped below |+ or -~4%. Previously purchased lines, he says, would hold |+ or -~12-15% on the best products and greater than |+ or -~15% on more difficult runs. Hansen says the system has performed best in the 2-5 mil range. He should know--Blako's products range all the way from 0.8 to 8 mils.

As a result of cutting variation, Blako solved some roll conformity problems it had experienced on some critical products. "All of a sudden the complaints went away," Hansen says, recalling bagginess, wrinkles, and poor unwinding as headaches he sometimes had to deal with in the past.

Most importantly, Hansen notes that his converting customers have reported significant drops in their scrap rates. And that improvement says more about film quality than any numbers on gauge variation. "This is not just a tolerance game," explains Hansen. For Blako at least, the real test of autoprofiling has been the customer's pragmatic indicator of quality--how well the film converts. "Customers say they want flat film, and they want to see how it runs. If they like it, they buy. Otherwise, forget it."

NO PANACEA 

Autoprofiling may fix roll flatness problems, but these systems are by no means a panacea for all that can go wrong in film production. For example, none of the systems address the machine-direction variations that can throw average thickness out of kilter. So the suppliers must pair their autogauging systems with gravimetric feeding to keep average thickness under control.

Battenfeld Gloucester's Hellmuth notes that other gauge-band variations are so narrow relative to the width of the control zone that they can't be fixed. That zone is about 2 in. wide in Gloucester's case, too wide to go after problems like a nick or speck of dirt on the die lip. "Some perceive autogauging systems as a quick fix for all the problems that have been nagging them," Hellmuth says. "But these systems are really for processors that have a handle on their causes of gauge variation and have identified them as broad-band." He cites examples of broad-band variation such as nonuniform heater bands or improperly adjusted die gaps. Uneven dies or air rings might also be the culprit. Or the resin's flow characteristics might change from batch to batch and adversely affect tolerances, Hellmuth notes. The list goes on and on--in blown film, lots can go wrong. Ironically, all these problems can be corrected in other ways. "The things automatic profile control does can be addressed by a good operator," Hellmuth concedes. "But how many of those do you have?" So another advantage of autoprofiling systems springs from their assiduous and consistent nature in fixing gauge problems. "It's like having someone always watching," Hellmuth says.

A QUESTION OF PAYBACK

There may be little argument about whether autoprofile systems impart film-quality benefits. But ultimately, the systems must be judged not just on quality but also on economics. It remains to be seen how many processors will find that improved quality adds enough to the bottom line to justify the capital expense. After all, prices range from just over $100,000 for the least expensive air-ring system from Gloucester up to $500,000 for a large, fully-featured Optifil P-2 die as part of a W&H turnkey system. How quick the payback, of course, depends on what type and how much film you make.

This payback issue may be especially troublesome for those with slim profit margins. One maker of a commodity-type film rejects even the prospect of a one-year payback period for an autoprofiling system. "I'm in a competitive business," he says. "I just can't wait that long."

Kiefel's Engel points out that the systems may be easiest to justify as part of a brand-new line, especially a coex one, where the autoprofile control adds proportionally less cost. Battenfeld Gloucester's Hellmuth agrees: "The systems are short money on a new line and a bigger investment as a retrofit."

Nevertheless, all the autoprofile systems offer opportunities for retrofitting on existing lines--some more so than others. The die-based systems, Optifil and Reicoflex, just aren't designed as add-ons despite a smattering of successful retrofits for each, according to W&H and Reifenhauser. But the air-ring systems lend themselves to easier retrofitting. Even Blako's Hansen, who says his company is considering a second Optifil P line, has not ruled out air-ring autoprofile retrofits for its existing lines. "The key is that they are less expensive," he notes.

Suppliers do report autoprofile systems going both on older lines and new ones. Battenfeld Gloucester, for example, says it has sold eight systems for monolayer and coex lines, five as retrofits. The balance tilts far to the side of new turnkey installations at W&H and Reifenhauser, which have done only a small percentage of retrofits to other vendors' downstream equipment.

POTENTIAL FOR DOWNGAUGING

One way autoprofile control can pay for itself is by facilitating downgauging. A lower average thickness with less variation can yield a film as strong and reliable as one with a higher average gauge marked by greater fluctuations. The mathematics behind such a "target shift" are clear enough: W&H's Wheeler estimates that a processor can typically save about 6-7% of total material costs from downgauging with automatic profile control. He adds that there's no reason the savings couldn't stretch up to 10% or so in some cases.

But downgauging can't always be counted on as source for savings. Oftentimes, the customer, not the film producer, has the final say in setting gauge. "We haven't seen too much in the way of downgauging," says Blako's Hansen. "Most of our customers won't a llow it." He doesn't, however, rule out the possibility of thinner films in the future. And at least two of his customers have already bought into the concept by specifying gauges 10% below their previous levels.

With medical films, resistance to downgauging may be more strident from government regulators and buyers alike. Carl Prince of Royal Precision Plastics says, "Downgauging won't help me. It would take years before thinner films would be accepted in our market."

LESS STARTUP SCRAP

More certain material savings come from autoprofile systems' ability to shorten start-up or changeover times, the suppliers say. This consideration can't be overemphasized at multi-product houses like Blako, which may have 50 different orders a month. Getting on-line faster means less scrap and more on-tolerance product to sell. W&H says its new Optifil P-2 with electrically heated die zones system can optimize gauge in 12 min, half the time of its previous system. There's some indication that air-ring autoprofile systems may take longer. Royal Precision Plastics says its Battenfeld Gloucester system has taken up to 2 hr to achieve minimum gauge variation. "It's a little slower zeroing-in than I'd expected, but it's incredibly accurate once it does," says Prince. Suppliers of the other air-ring systems have cited times of up to 20 min.

Vertically integrated processors with in-house converting operations can save even more from scrap reductions. W&H's Wheeler explains that autogauging has often helped cut scrap rates to nil because uniform film simply converts better. He estimates that saving for a 20-million-lb/yr plant with integrated extrusion, printing and bag making could amount to at least $240,000 annually (with PE at 30|cents~/lb). He used an extremely conservative estimate of 4% total scrap rate for extrusion, printing and bag making, when it can actually run as high as 15-18% for the three processes, he says.

Beyond material savings through downgauging or scrap reduction, autoprofile control potentially can cut costs in more hidden but no less dramatic ways. Consider the possible product-liability costs for a faulty medical product. "It only takes one bad product to get you in trouble," says Royal's Prince. "For medical film, I'd go with autogauging." In this industry where zero defects is a necessity, autoprofile systems provide the means to attain documentable "perfection."

Moreover, heightened quality in the form of tight profile control can also pay off by opening new markets. Autogauging has helped Blako, for instance, win 10 new accounts in markets that previously had eluded the company. Its expanded customer base now includes some high-end laminating accounts who've been benefiting from reduced variation on 2-mil LLDPE films. "We've been very happy with our ability to compete in these new markets," Hansen says.

NOT FOR EVERYONE

The consensus among suppliers and processors alike is that autoprofile control is not for everyone. "We don't have any systems out there making trash bags," says W&H's Wheeler. "Their margins are too low, even if they do save money on material costs."

Bangert of Reifenhauser's also concedes that autoprofiling might not be appropriate for the "average processor." While the company continues actively marketing the less costly Reicoflow autoprofile system in North America, Reifenhauser is also looking to less expensive technology to control gauge. Recently, for example, the company decided to concentrate more on its "isothermal die." It employs an annular heat pipe near the die lip to improve gauge uniformity through an equalize melt temperature. At roughly $50,000 it improves variation by 25-30%, for more affordable gauge control, according to Bangert.

In the end, then, autoprofile systems mostly make sense where quality alone provides the justification. It's for high-end producers who see the direct effects of better film quality by securing new markets--and, conversely, for those who could suffer the greatest from poor quality.