The Basics of Checkweighers

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Check out our  Pharmaceutical Checkweigher  section for more information or to find manufacturers that sell these products.

A checkweigher is an automated machine or system that checks the weight of packaged items as they pass through the production line. Checkweighers are used at the end of the production process to ensure that the weight of a package is within specified limits. Units that are outside the established tolerances can be moved out of the line automatically. A checkweigher weighs and classifies the items by weight zone (e.g., acceptable, overweight, underweight). They often incorporate additional checking devices such as metal detectors and X-ray machines to enable the checking of other package details.

Checkweighers are also known as belt weighers, in-motion scales, in-line scales, and conveyor scales. A checkweigher is a dynamic scale, weighing while the product is in motion, as opposed to static scales. Dynamic weighing is a challenging technology. Checkweighers must obtain an accurate reading within a fraction of a second. Static scales do not need to react as quickly and have much longer “settling times.”

The Value of Checkweighing

Checkweighers are sometimes called the “policemen” of the packaging line. The advantage of checkweighing is in achieving 100% sampling of all packages, rather than intermittent off-line manual sampling practices that might check less than 1% of all units in a fast-moving line. This 100% sampling is key to the quality control process. Valuable uses for checkweighers include:

• checking for under or overweight units
• enabling compliance with net contents laws for packaged goods
• insuring compliance with customer, agency, and government specifications
• verifying count-by-weight by checking for a missing item in a case of packaged items
• saving costs by reducing product giveaway by using checkweigher totals to determine adjustments to the filling machine (too often manufacturers “over- fill” to assure the required amount is in the package)
• classifying products into weight grades
• monitoring and measuring production line efficiency
• maintaining product quality standards

The Department of Commerce National Institute of Standards and Technology (NIST) publishes Handbook 133, which defines the net contents laws on packages for processors, retailers, and wholesalers. The handbook defines details on testing and sampling, and specifies the Maximum Allowable Variation (MAV) for an underweight package. For prepackaged items commonly weighed by checkweighers, the MAV varies according to package weight, and the NIST Handbook 133 defines the MAV in weight gradations for prepackaged goods. For more information on the most recent edition of Handbook 133, published in 2005, go to: ts.nist.gov/WeightsAndMeasures/h1334-05.cfm or contact the U.S. Government Printing Office at this toll free number: (866) 512-1800. State Weights and Measures offices actually regulate and enforce the net contents laws as defined by NIST.

Key Components and Terms

A checkweigher utilizes a series of conveyor belts. An infeed belt adjusts up or down the speed the package is moving, to bring it to the correct speed for weighing. The infeed belt is sometimes used as an indexer, setting the gap between products (called package spacing pitch) to the ideal distance for weighing. Belts or chains may then position the product onto the weigh belt, which is mounted on a weight transducer, usually a strain gauge or servo-balance load cell. The scale section of the checkweigher is called the weigh pan. The length of the weigh pan is critical in determining both the weigh time and the maximum package size that can be weighed. The weigh time can range from 60 milliseconds to more than 350 milliseconds.

The target weight of a package is the nominal or desired product weight. The tare weight is the weight of the package before any product is added. Dynamic, in-motion checkweighers take multiple weight readings or samples to form an accurate weight over a given period of time. Many systems use a trigger from an optical or ultrasonic device to signal the movement of the package onto the center of the weigh bed to start weight-sampling for a given duration.

A reject conveyor is often included in the system to enable out-of-tolerance packages to be removed from the normal flow. The rejecter mechanism can be a simple pneumatic pusher or diverting arm, which pushes the rejected unit sideways from the belt. The checkweigher can incorporate a bin to collect the out-of-tolerance units.
Checkweighers can be built with metal detectors, x-ray machines, bar code scanners, temperature sensors, and more. In some plants the checkweighing system may include an integral printer or an interface to the factory PLC or supervisory control system. They can be engineered and built in many sizes and shapes and can run at very high speeds (line rates) of more than 100 meters/minute. They can also be installed in a variety of ways, such as in ovens or refrigerators or hanging from ceilings. The installation environment and the kind of cleaning to be used determine the design and materials of construction of each system. Systems that will be cleaned with harsh chemicals will be constructed with all stainless steel parts and labeled for “full washdown.”

Checkweigher Accuracy

Checkweigher accuracy is comprised of the linearity and repeatability of the system. Linearity measures the closeness in weight between an actual package and a test package. The difference or error is expressed as the mean error. Checkweighing repeatability is expressed with standard deviation, describing the weight variance when weighing the same mass multiple times. The load cell used in the weighing system is a key determiner of accuracy. Higher accuracy systems are more expensive but may offer cost savings over a short payback period by reducing overfilling and product giveaway as well as the cost of reworking underweight packages.

A number of factors can affect checkweigher accuracy, including the manufacturing and packaging process and the factory environment. Environmental factors that can affect load cell accuracy include temperature fluctuations, air currents, and vibration or factory “noise.” Debris and dust can affect the tare setting of the checkweigher. Checkweighing systems will vary in how well they are equipped to handle such hazards.

Selecting the Right Checkweigher for Your Application

Before selecting a checkweighing system for your application, there are some key points to review with a potential supplier to determine how well the proposed system will meet your requirements.

• Factory Environment: Will the system be used in a refrigerated or heated area or in an area where the temperature will vary? Can it withstand moisture or condensation? How about wet washdown or washdown with caustic cleaning chemicals? Will the controls meet the National Electrical Manufacturers Association (NEMA) code to be as dust-tight and drip-proof as your factory requires? Will the system be used in a hazardous area that requires a certain explosion proof standard?
• Accuracy Level Needed: As mentioned above, the load cell used in the weighing system is the key determiner of the accuracy of the checkweighing system. Additional factors affecting accuracy include the stability of the item to be weighed (the footprint of the product to be weighed is important—more surface area touching the scale is better) and the line speed (the higher line speeds are more of a challenge for maintaining accuracy). Define the accuracy level needed in your system and review with the needs with a potential supplier.
• Line rate: Checkweighers can handle a wide range of rates—from one to several hundred items per minute. A longer package requires the line to run at a faster rate to maintain the same item rate. As mentioned above, higher line rates usually result in decreased accuracy. A compromise between high accuracy and high line rate may be needed.
• The Package: It is important to match the package to be weighed to the correct type of checkweighing system. Systems may be designed specifically for cans, bags, cases, bottles or heavy items vs. light items. A top heavy, unstable product with a small footprint may require a certain type of system. Handling the package correctly is an important part of checkweighing.

Checkweighing is an effective quality control tool. Quality personnel must specify acceptable weight limits for each package as well as the required level of accuracy expected of the system for each product and each line. With proper preventive maintenance, the checkweighing system can help to ensure that the end customer receives a product that is on-weight and complete.

Check out our  Pharmaceutical Checkweigher  section for more information or to find manufacturers that sell these products.