Weigh Watchers
ADVANCEMENTS IN CHECKWEIGHERS KEEP PACKAGERS WITHIN TOLERANCE.
By Bridget McCrea
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Thompson Scale's Sonic 350 checkweigher |
Getting the accurate amount of product into a package is something consumers have come to expect. It also helps packagers stay on the right side of net weight content laws, which require that the content of packages agree with the weight listed on their labels.
Packagers stay on top of this weighty issue with the help of checkweighers, which are scales that weigh products in individual packages as they move along the packaging line. They evaluate an item's mass while it's in motion, eliminating the need to halt the momentum and pull the package off the line in order to get an accurate weight. The weight determined by the checkweigher is compared to a pre-determined “appropriate” or “acceptable” weight; and the machine reports any discrepancies between the two.
Usually positioned after the primary packaging machinery and before the case-packing or related equipment, a checkweigher is a remarkably versatile piece of machinery. “Our customers range from firms that put diapers into plastic bags for retail sale, to the U.S. Army, which packages extruded blocks of explosives,” says Alan Vaught, vice president of operations at Houston-based Thompson Scale Co., “and everything in between.”
WIDE VARIETY
Highly customizable, checkweighers can be configured to meet highly specific needs. A pharmaceutical firm that's weighing individual tablets or bottles containing those tablets, for example, would require a high-rate, small-package checkweigher whose accuracy level is fine-tuned down to milligrams. A manufacturer of soil or concrete, on the other hand, would use a heavy-duty, industrial checkweigher designed to handle such products and masses of 20 to 100 pounds.
Checkweighers come in different types, the most popular of which is the conveyor or “belt” variety, which weighs products as they proceed down the packaging line. Rack or “chain” checkweighers comprise two or more chains that straddle a scale used to weigh the items, and stop-motion systems temporarily halt a product's movement while a device picks it up, weighs it and then releases it back onto the line.
The weighing units also come in different sizes, from lightweight to heavyweight models. The lower end of the scale can generally handle packages weighing less than 20 pounds, while mid-range models manipulate 20- to 80-pound items. Heavyweight checkweighers tackle packages weighing 100 pounds or more. Jack Jones, vice president of sales at Alpha Checkweighers in Exton, Pa., says selection depends on the size and weight of the package.
“Companies give us a particular size and weight, and we first try to fit those requirements to a standard model,” Jones says. “If there isn't a fit, then we'll design a checkweigher around those parameters.”
CAREFUL CONSIDERATION
Installing a checkweigher requires that an existing conveyor be cut to make room for the new equipment, which then uses its own conveyors during the weighing process. Charged with checking discrete packages or single packages, the machinery is positioned after the cartoner or filling line (for liquids) and prior to case-packing.
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"A CHECKWEIGHER IS NOT A |
A close relative of the checkweigher is the case weigher, which checks a case's weight after it has been filled with the individual packages. Take a carton of cereal, for example. The standard checkweigher would make the initial check for acceptable tolerances as the carton makes its way down the packaging line. A case weigher would then verify that the larger box indeed contained, say, 24 cartons of cereal, each of which was the proper weight.
Kyle Thomas, marketing manager at Hi-Speed Checkweigher in Ithaca, N.Y., says checkweighers are most often used by food or food-related companies and pharmaceutical firms. Their usefulness goes beyond just weighing individual packages, says Thomas, who points to the packaging of pre-fabricated furniture and ready-to-assemble toys as suitable applications.
“A checkweigher can verify whether or not the small kit of screws and parts is included in the box of materials,” says Thomas, whose firm also makes checkweighers that measure the amount of propellant being infused into aerosol cans and those that weigh bags of disposable diapers to make sure that the item count is correct.
Checkweighers can also be customized to handle various shapes and sizes of packaging. A curvy beverage container that moves at a rate of 350 packages per minute, for example, would require a specialized, high-speed checkweigher that can seamlessly transport and accurately weigh the bottles at the same time.
Other checkweighers are designed to handle the work at even greater speeds. One example is the recently introduced Ramsey Teorema global checkweigher from Minneapolis-based Thermo Electron Corporation. The machine-with side-to-side conveyor transfers at in-feed and out-feed, controlled feeding and a high-rate reject mechanism-allows weighing accuracies of plus or minus 0.7 grams at speeds in excess of 600 cans per minute.
In other applications, such as when meat or poultry is moving along the line, the checkweigher must be sanitary. That means it must have non-corrosive surfaces capable of withstanding regular wash-downs to remove bacteria from exposed areas.
WEIGHING THE COSTS
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The model CW-10 from Alpha Checkweighers |
Checkweighers cost anywhere from $13,000 to $25,000, depending on the individual firm's packaging requirements. For $13,000, companies can purchase a basic, chain-driven (as opposed to belt-driven) checkweigher designed to handle rigid packages (such as bottles or cartons). Small-package, high-accuracy systems typically cost $17,000 to $22,000, while heavyweight machines with in-feed and out-feed conveyors that can handle up to 100-pound packages cost about $25,000.
In return for their investment, companies get more accurate pictures of exactly what is moving off their production lines and gain improved quality control and better vital statistics about the production operation. “A checkweigher is not a piece of packaging machinery,” Jones explains. “It's a piece of quality control equipment.”
Checkweighers also represent a packaging-related piece of equipment that is in an evolutionary mode, as machinery manufacturers strive for higher levels of integration with the rest of the operation.
“There are several large corporations that are helping to drive the standards for that communication and integration,” says Thompson Scale's Vaught. “It's going to be the focus for the next few years.”
To match the right checkweigher with the right application, consider the reliability of the equipment and availability of pricing and spare parts. Be sure to quote an accurate environment for the equipment. If it's going to be washed down six times a day, for instance, don't tell the manufacturer that it's “washed infrequently.” And check out what other firms in your industry are using before making a final decision.
“Base your decision on your firm's specific requirements, even if they add a bit more cost to the job,” Jones advises, noting that stainless steel construction may be very important to a company in the food service industry but not as imperative for a company in the automotive sector. “Figure out exactly what you need, then find the right checkweigher to fill those requirements.”
Bridget McCrea is a Florida-based writer who has been covering manufacturing and technology topics for 10 years.
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BY BEN MIYARES Most pieces of packaging machinery, in time, spin off their own little cottage industry of insider terms. Checkweighers are no exception. Here are a few of my personal favorites. Accuracy: The difference between the known weight and the displayed weight of sample being weighed. The presumption here is that the known weight was determined by a previous weigh-in on an accurate scale. Checkweigher: Interestingly enough, not a scale for measuring the weight of your checkbook. Rather, it's a system that weighs product as it moves along the production/packaging line. Ounce, pound, etc.: Obsolete units of measurement (known as the “Imperial” or “English” standard) not understood in most places on Earth other than the United States and parts of the United Kingdom. The European Union will require metric-only labeling of all products by 2010. Slack fill: A fill amount below the net weight statement on the package. Good checkweighers have a low tolerance for slack fills while at the same time guarding against “over fills” or “giveaways,” the meaning of which should be obvious. Weighment: A single weighing. Checkweigher accuracy, according to Hi-Speed Checkweigher, is “the standard deviation of the weighments of a single item weighed on the checkweigher several times.” See “Principals of Checkweighing,” at www.hispeedcheckweigher.com/poc.pdf . |
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Ben Miyares , vice president for industry relations at the Packaging Machinery Manufacturers Institute, has been providing packaging market analyses and commentary since 1963. |
