Slit-sealing wider HMW-HDPE film slowly gains acceptance.

Slit-Sealing Wider HMW-HDPE Film Slowly Gains Acceptance

Slit-sealing HMW-HDPE film is desirable from a capital-investment viewpoint in making products such as T-shirt bags; yet, until now, it has been difficult to implement. One limiting factor has been the strength of the side weld. Because of the high modulus of HMW-HDPE, the film has a tendency to wrinkle--thus the difficulty in making a strong seal.

Yet, as HMW-HDPE continues to penetrate film markets--particularly T-shirt bags--some machinery manufacturers report that some converters are successfully running HMW-HDPE bags three-up from one wide roll. David Nunes, sales manager of Alpine American Corp., Natick, Mass., says that slit-sealing HMW-HDPE "has come into vogue in the past three or four years."

Equipment manufacturers did not specify if their customers were using 100% HMW-HDPE for T-shirt production, although a few said that their machines were capable of handling it. Processors contacted would not comment on whether they were using an additive.

While achieving a strong side seal with HMW-HDPE has been difficult, it is not an insoluble problem, says Nunes. The real reluctance to slit-sealing HMW-HDPE film stemmed from marketing reasons: "People didn't want to risk finding out they had bad slit seals. They wanted to be as sure as possible that they made a good product."

The ability to slit-seal HMW-HDPE film enables much higher outputs, says Nunes. Processors could graduate from a 4-in. die used for one-up tubing, to a 9-or 10-in. die, he claims. "Output would increase significantly--especially with our PEAC cooling system, with which you can get maximum utilization of output with the blown film process. If you can then convert the film to make a good product, you are that far ahead." Given the claimed feasibility today of accomplishing strong side welds, the economics of slit-sealing HNW-HDPE film far outweigh type extrusion. For example, a one-up line costing $250,000 that produces 150 lb/hr of film would generate an amortized capital cost of about 21^/lb of material over a year; a $550,000 three-up line producing 600 lb/hr would have a cost per lb of about 11^.

SKEPTICS REMAIN

A number of industry experts remain skeptical that slit-sealing 100% HMW-HDPE film is practical. Werner Paulhardt, sales manager of plastic bag machines for Windmoeller & Hoelscher Corp., Providence, R.I., says that virtually none of the T-shirt bag machines supplied by W&H for grocery sacks are equipped with slit-sealing units. He claims that, during the last decade, major bag makers have invested in small extruders producing narrow tubing. "I don't believe that there are any miracles out there that will slit-seal 100% HMW-HD material to compete with a grocery bag that has been made with narrow tubing. If temperature is not controlled on a slit-sealed bag, there is a tendency to get a zipper effect right next to the seam." Even if it is possible to slit-seal 100% HMW-HD material properly and strongly enough to compete with unslit narrow tubing, slit-sealed 100% HMW-HD bags might encounter some market resistance, he adds.

One alternative, of course, is blending LLDPE into the HMW-HDPE. Paulhardt claims that blends of up to 20% produce seals that are competitive with non-slit grocery sacks. Similarly, Rick Knittel of Sano Corp., Passaic, N.J., knows of no converters that are slit-sealing 100% HMW-HDPE for T-shirt applications. "We have seen blends with LLDPE and we have seen slit-sealing of coextruded product," he says. Yet he adds that blending LLDPE in a grooved-feed extruder could be difficult.

One processor notes that the lower cost of LLDPE has created an advantage for blending it into HMW-HDPE in the past, but the narrowing price difference between the two materials could mean less blending in the future.

Yet even some processors who do not run HMW-HDPE for T-shirt bags admit that it is being slit-sealed successfully, and that there is potential there. According to Michael Snowden of Equitable Bag in Florence, Ky., the biggest improvement has been in the resins themselves, which could lead to more successful slit-sealing of HMW-HDPE in the future. "They're a different generation of products today. They're tougher materials, and therefore the seals are that much better."

'PROVEN TECHNOLOGY'

David Nunes of Alpine, on the other hand, thinks that slit-sealing HMW-HDPE has found a place in the market. He calls it a proven technology that is "absolutely here to stay." He claims that big differences in the economics of producing T-shirt bags on Alpine's Elba bag machine have been achieved by slitting 60-in. rolls of HMW-HDPE film down to three 20-in. widths.

The Elba unit is equipped with a round plate that removes the wrinkles in the film before it is slit and sealed. The knife is heated by a PID temperature controller that accurately regulates the degree of heating. Because the knife is cooler at the tip that first comes in contact with the film, it gradually heats and seals, reportedly providing a more consistent weld. Each blade has its own temperature controller; the blades are said to be thoroughly hardened, and the depth and angle of each knife is fully adjustable. After the film is slit-sealed, it is re-inflated and gusseted.

The Elba bag machine slit-seals HMW-HDPE at a rate of about 250 ft/min, according to Nunes, who adds that slit-sealing is most suitably performed in-line with a bagmaking operation. One obstacle to slit-sealing in-line with an extruder is roll build-up, which is due to beading. And some processors consider the bag machines to be the limiting factor in line speeds. According to one processor, "You typically can extrude the film a lot faster than you can convert it."

Yet customers report good results in the quality of the slit-seal. One processor slit-sealing HMW-HDPE T-shirt bags on the Elba machines is Vanguard Plastics, St. Louis, Mo. According to Donald Van Nortwick, Vanguard's engineering manager, "HMW-HDPE is a little trickier to slit-seal because of its high viscosity--it doesn't really flow together." He claims that the side-sealing equipment on the Alpine Elba machine is able to handle the material successfully, although he refuses to comment on rates or whether the company is using an additive to make the material slit and seal more easily.

One possible alternative to slit-sealing in-line with the bag machine was a laser slit-seal system proposed a few years ago by FMC Corp., Green Bay, Wis. (see PT, Feb. '85, p. 51). The laser unit reportedly allowed film to be slit-sealed in-line with the extruder because it did not produce as thick a weld as with hot-knife sealing, eliminating roll build-up problems. Larry Clayton, sales manager of FMC Corp., says that, although the unit was effective, it was too expensive, at $150,000 for a dual-head unit, to make an impact on the market.

HIGHER LINE SPEEDS CLAIMED

Even though slit-sealing appears to be mainly restricted to in-line operation with bag machines, higher line speeds reportedly are still attainable. According to Larry Clayton, FMC Corp.'s Model 1150 bag machine can slit-seal 100% HMW-HDPE for T-shirt bag production at 340 ft/min, which he claims is the fastest machine for that application on the market. (Although he says that FMC has successfully slit-sealed 100% HMW/HDPE film at these speeds in its plant, he adds that he has not done that in a commercial installation.) "We invented the bag machine first, then we realized that the bag machine would work faster than we were able to slit-seal," says Clayton. "So then we had to modify the slit-sealers to make them work at higher speeds." This involved using longer blades to increase the dwell time, while slitting at a lower temperature to avoid crystallizing the film and making a brittle seal. More attention was also given to proper tension control of the film going through the slit-sealer.

One company that claims to offer stand-alone slit-seal units that have been successful with HMW-HDPE film is Atlanta Mesco Inc., Doraville, Ga. AMI offers two units: a hot-air slit-sealing unit and a welding unit that does not use hot air.

The hot-air model is designed for heavier gauge films of 1 mil or more. It uses a hot-air shoe that lays down a seal that will then be slit by a heated blade. The unit is said to warm, soften, and press the area to be welded at relatively low temperatures. Lower-temperature welding is said to prevent the material from thinning out along the seal, which causes weakness and zipper-like tearing. The blade is designed with a "tilt" edge shape, said to create an optimum welding condition. The welding slit-seal unit is the same basic design without the hot-air shoe. It is recommended for thinner film.

Both free-standing units can be used in different configurations. They can be put behind the bag machine on the floor or mounted on the extrusion tower. In a typical off-line operation, the tube will come out of the extruder and into the slit-sealer, which will form two lanes of tubing that then feed into a two-up bag machine.

According to AMI sales engineer Timothy Lewis, the welding unit can run at up to 400 ft/min on 0.7-mil HMW-HDPE film. The stand-alone unit allows customers to buy wider rolls of material, which can then be slit down to smaller tubes. It reportedly offers cost savings in addition to providing flexibility. Also, the unit is said to provide a stronger, smaller bead that makes the weld almost imperceptible.

The units are priced according to width. A 100-in.-wide hot-air unit with one cut costs about $15,000; one with two cuts would run $19,000. The 100-in. welding slit-seal system with one cut would be $14,000; two cuts, $17,000.