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Keeping wide-mouth PET jars in spec

When blow/trim technology is used to make wide-mouth PET jars, the neck finish is blown into its shape rather than injection molded. Sophisticated quality-check tools come in handy.

Two different quality-check tools are used by Graham Packaging to make sure the wide-mouth PET jars it’s making are consistently in spec.
Two different quality-check tools are used by Graham Packaging to make sure the wide-mouth PET jars it’s making are consistently in spec.

True to its commitment to help customers gain competitive advantage, two years ago custom blow-molded plastic container manufacturer Graham Packaging Co. L.P. built a production facility immediately adjacent to the filling plant of an internationally known food processor (who prefers to remain unnamed). The co-location is a win-win strategy for both parties. Benefits range from saving the freight on empty bottles to the stability of a long-term supply agreement to the agility of just-in-time manufacturing.

The PET containers Graham produces for the customer are all specialty items, but one type is even more specialized than the others: 30- and 32-oz hot-fill, wide-mouth jars for peanut butter and other viscous foods. The challenge is two-fold: not only is the neck finish too large for standard preforms and blowmolders, but also its threads have to be perfectly executed or else its cap won’t screw on.

Graham’s strategy to overcome these hurdles is to use its proprietary blow-trim technology in tandem with two quality-check tools from Agr International—OptiCheck™ vision-based inspection and Process Pilot® Automated Blowmolder Control. This cost-effective approach not only makes the need for a custom blowmolder unnecessary. It also allows the plant to control the blowmolding process to a level of unmatched precision—while assuring that every container can be sealed flawlessly to protect the integrity of its contents.

Describing the through-the-wall production process, Graham Plant Manager Don Waud says, “Fifteen minutes after we blow a bottle it’s filled and going to the case packer.” The close-coupled operation promotes a steady communication flow between the container manufacturer and the customer.

The wide-mouth jars must consistently meet specifications, a responsibility with which Graham is very familiar. As a global packaging leader, it is at the forefront of the migration to plastic from glass, paper, and metal.

“We see the plastic jar in the food area as a major growth segment,” notes David Piccioli, Director, Global PET Technology Development for the York, Pa.-headquartered company.

To facilitate the shift to plastic, Graham has developed a robust technology portfolio that supports the cost-effective manufacturing of custom, value-added packaging for the branded product market. The wide-mouth jar is a classic example of the way Graham is using both in-house and Agr technology to overcome production obstacles. Let’s begin with how Graham’s blow/trim technology works. It’s been around for a while, and Graham is by no means the only PET container maker with some version of it. But it hasn’t been described very fully in the pages of Packaging World, so it’s worth a look.

The most significant difference between blow/trim and conventional blow molding used for most PET beverage bottles is that the threaded neck finish in a PET bottle undergoes no change during the blow molding process. The shape given to the threaded neck finish during injection molding is the shape it retains all the way through to when the consumer drinks the beverage.

This is not the case with blow/trim. The threaded neck finish is produced by the same blowing action as the sidewal of the jar. Then, in a secondary operation, the portion of the jar above the newly blown threads, which is referred to as the “moil,” is cut off by a trimmer and recycled.

Graham produces PET jars whose neck finish diameters are either 63 or 83 mm. Remarkably, it also uses the same Sidel blow molder to make beverage bottles whose neck finish diameter is just 43 mm. It’s a simple matter of switching out the custom molds used for the wide-mouth jars and replacing them with molds used routinely for stretch blow molding of PET bottles.

In addition to making both container formats—wide-mouth jar and conventional beverage bottle—on the same blowmolder, the blow-trim process allows the customized jar line to run at normal production speeds, in synch with the filling line. “The fact that the blow-trim process works with existing blowmolder equipment allows us to be very competitive,” says Graham Plant Manager Don Waud.

According to Piccioli, blow/trim is no walk in the park. “The process is more challenging than traditional reheat stretch blowmolding. It is much more difficult to control a blown neck finish than an injection-molded neck finish.”

Material distribution in the body and thickness in the thread region is where the challenge lies. Neck finish is especially critical. Any interference or variation in the thread dimension makes it impossible to achieve the proper torque setting when applying the injection-molded caps on the filling line. After capping and the hot-fill process, the jars are dipped in cooling water to hasten the drawing of a vacuum. A flaw in the finish—a shift in thread size or the presence of excess material, for example—can compromise the seal. The consequences—allowing cooling water into the jar, creating the potential for contamination, product spoilage, or leakage—are all unacceptable.

To ensure that each and every jar on the conveyor to the filling line is defect-free, Graham installed Agr’s OptiCheck as “the last line of defense” before the containers go through the wall to the customer facility. The compact OptiCheck is an inspection and measurement system that offers both in-line gauging of critical finish dimensions and defect detection for seal surface-related problems.

With its ability to measure finish dimensions inline to an accuracy of 0.002 inches, OptiCheck is well suited to the needs of blow/trim containers. Its finish gauging module uses a proprietary configuration of multiple cameras, LED-based backlighting, and high-speed imaging to measure the critical dimensions. Robust image analysis algorithms accurately identify flaws and blemishes, such as whiskers left from trimming, which had been a frequent but hard-to-detect defect on the Graham line.

“We are looking at the diameter of the neck, the height of neck, and multiple dimensions of the wide-mouth jars, so that after filling the cap and tamper band can be fully engaged,” Graham Process Technician Josh Chesley points out. This is where Agr’s OptiCheck comes in mighty handy. A long-standing relationship with Agr throughout Graham’s network of 72 North American plants, coupled with a rigorous qualification process, convinced Waud that the OptiCheck would function effectively.

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