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Unique stretch-blow system for custom shapes.

Husky Injection Molding Systems Ltd. of Bolton, Ontario, introduced its long-awaited PET stretch-blow molding system in May, presenting a paper at the SPE ANTEC in Detroit and showing the first commercial machine at Plast-Ex in Toronto. The shape of this system is different from the two generations of prototypes before and is unique in the industry. Whereas other stretch-blow machines are termed either "one-stage" (fully integrated) or "two-stage" (reheat-blow), Husky's new ITS (Integrated Two-Stage) system is something in between. Husky's ITS system includes the company's standard preform injection molder integrated with components for reheating and stretch-blowing. The links between the two are a single control system and a side-entry robot that takes preforms out of the press and orients them on transport fingers.

This technology is said to provide the flexibility to mold unusual asymmetrical shapes with larger neck openings (up to 4.3 in.) than other stretch-blow systems. That's because Husky offers higher clamp tonnage in the preform press (175-440 tons) than is available in comparable one-stage PET bottle machines (80-150 tons). For preform molding, the Husky unit offers a choice of three standard injection presses with either reciprocating-screw or two-stage injection and ability to handle eight to 18 cavities.

The ITS matches preform and bottle cavitation more efficiently than one-stage models, Husky notes. For example, models from Nissei ASB Co. in Atlanta, and Aoki Mfg. Co. (rep. by Formex, Inc. in Dayton) must match preform and bottle cavities one-for-one. But since it takes roughly four times longer to mold a preform than to stretch-blow a bottle, Husky's ITS models have up to four preform cavities per blowing cavity. The ITS is designed to mold 900 parts/hr with one blow cavity, 1600-1800/hr with two cavities, and 2700-3600/hr with three cavities.


The first-generation prototype of the ITS began five years ago with a more linear arrangement of injection and blowing stations. It went to Constar Plastics Inc. (then Sewell Plastics) in Atlanta for development. It was redesigned three years ago with the stretch-blow unit relocated perpendicular to the preform press, on the side away from the operator. Two developmental second-generation models were built, one for Eastman Kodak Co. in Rochester, N.Y., to make an oblong copy-toner bottle with an off-center square neck; the other for Metrolina Plastics Corp. (a unit of spice-maker C.F. Sauer Co.) in Richmond, Va., to make rounded spice jars. Eastman says Husky's machine was the only one in the world that could make Kodak's unusual part at the required rate. And Metrolina pays the ITS the ultimate compliment--it has ordered two more of the next generation and says, "Generation 2 achieves over 95% uptime."

The third-generation ITS is much refined and simplified. It eliminates hydraulic clamping and is entirely electric servo-driven downstream of the preform molding stage. This results in faster, more accurate motions in "a cleaner environment with no potential for oil leaks," Husky says. The stretch-blow station is relocated to the operator side, improving machine access and simplifying robotic transport of preforms to the carrier pallets.

After the robot places the preforms upside-down over rotating carrier fingers, the preforms pass through a conditioning oven where they are heated by convection and infrared radiation. The ITS has eight oven segments, arranged in a horseshoe between the injection press and the stretch-blow unit. Each oven has a stack of nine quartz infrared elements, which can be independently positioned to tailor the heating profile to the parison shape.

For best orientation results, the inside of a thick preform must be hotter than the outside, so fans blow ambient air to cool the outer surface of preforms. By contrast, Husky sources say, in a one-stage stretch-blow machine, just-molded preforms are stretch-blow while heat is concentrated in the center of the preform wall, so "the PET cannot achieve the same degree of orientation and additional material must be used to maintain strength."

From the ovens, a servo-driven shuttle moves carriers with conditioned preforms into the 55-ton stretch-blowing station. Patented servo drives elongate the stretch rods with controlled velocity and position. A servo drive also actuates the clamp via a double-acting eccentric cam, providing a 1.8-sec dry cycle--nearly 50% faster than previous hydraulic clamps, Husky says.

The injection and stretch-blow units are synchronized by a single Siemens 115U programmable logic controller, which also stores setups. The operator control interface is an IBM-compatible PC with color-graphic CRT.


If the new machine has any fault, users say it is that its highly engineered complexity may limit applications to those where it's truly economical. "It's a high-tech machine requiring highly skilled operations people," says James Hurd, senior manufacturing engineer for Eastman Kodak. "Changeover from one part to another is relatively time-consuming and takes expert technical knowledge." Metrolina v.p. Gib Stevenson agrees: "It's the most precision demanding machine I've ever seen. It's not for rookies, but its so trouble free that I wouldn't hesitate to use it for a dedicated bottle." Husky says its quick-mold-change features allow a blow-station tool change in 1 hr and a complete tool change in 3 hr. Users say the blow-station changeover can take as little as 30 min, but the complete tool change takes at least 8 hr.

The ITS makes a broad range of shapes from 1.3 oz to 1.3 gal, up to 7.5 in. diam. and 14.6 in. high, with 0.7-4.3 in. necks. It costs $700,000-$1 million with tooling, depending on options.
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Title Annotation:Stretch-Blow Molding
Author:Schut, Jan H.
Publication:Plastics Technology
Date:Jul 1, 1992
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