Checkweighers: Weighing the Options

Checkweighers can hardly be considered interchangeable. They vary greatly in size, function, performance, and price, depending on the applications they are used for and the requirements of the end-user. As such, pharmaceutical companies looking to add or replace a checkweigher in a plant must carefully consider exactly what they expect from the equipment.

Will the machine need to weigh contents before packaging or to weigh the package after filling? Often a technology that is optimal for one use is not necessarily optimal for another.

How accurate must the checkweigher be? Some models have tighter tolerances than others, and some drug products can withstand a greater margin of error than others.

Will it be used for every product or package as part of the production line, or will it be an off-line unit used as an occasional quality assurance/quality control measure or for a potentially problematic batch? In-line, every-product checkweighers tend to be larger, more complex, and more expensive than those that are off-line and used occasionally, so users must determine their needs before buying.

Will the checkweigher need to be networked with other equipment? Will it need to provide data analysis of everything that it weighs? Will it need to be validated? The more these requirements come into play, the more complex a machine may need to be.

ACCURACY

The KKE 2500 capsule checkweigher from Bosch includes an automatic fault clearance system and integrated data recording and evaluation.

Weigh cells are a popular checkweighing method, and often the most accurate, says Bill Kohl, product manager for capsule fillers and checkweighers at Bosch Packaging Technology (Minneapolis). "Our hard-gelatin capsule checkweigher uses a weigh cell," he says. "Other capsule checkweighers use capacitance, which sends an electrical charge across the capsule, measures the density, and converts it to weight. We actually weigh the capsules, which is a more reliable and more accurate method. The deviation for our weigh cell is 2 mg. It has a much tighter tolerance range than a capacitance weighing system, and the end result is a higher acceptance rate of filled capsules."

The configuration of the weigh cell is important, he says. "Something unique to our machine is that our weigh cells hang," he notes. "We surround the capsule placed in the weigh cell. Any powder that may have normally built up in a weigh cell would fall out. There are no dust deposits on the weigh cell and the electrostatic charge is not affected."

Thermo Ramsey (Minneapolis) offers a dynamic load cell, says Don Bina, marketing communications director. "All of our weighing is done dynamically, in-line," he says. "As the product passes a photo eye, the photo eye sends a signal to the scale to start weighing the product. As the product passes through, it is weighed thousands of times. The calculated average is digitized, the compression of the load cell times the rate of product across the load is devised, and it is averaged out as a digital readout. It is done very accurately, ±50 mg for the pharmaceutical industry. If a plastic container has 100 tablets and one is broken, we can detect it, and the bottle is rejected off-line. The checkweigher can now be used in everything from case sealers down to the finite weighing of blister packs."

Mark D'Onofrio, vice president and general manager, Lock Inspection Systems (Fitchburg, MA), says users must consider both accuracy and speed requirements, as sometimes increasing one means decreasing the other. "Our most accurate checkweigher can handle up to 200-g bottles, with accuracy to ±50 mg at speeds up to 100 bottles per minute. It can handle up to 300 bottles per minute, but then the accuracy decreases to some degree," he says.

To ensure high accuracy, D'Onofrio says that Lock's Weighchek checkweigher "takes raw signal data from the weigh cell and sends it into five digital filters. The output signal is processed at 400,000 samples per signal to ensure high accuracy."

IN-LINE VERSUS OFF-LINE

If an in-line system is needed, a number of logistical questions arise. How will the checkweigher be integrated with other equipment, and how will product be transported to and from the weigher in a way that ensures that accuracy will not be compromised?

"Getting bottles to and from the checkweigher can be difficult. Often, bottles are not stable because they have narrow diameters," says D'Onofrio. "If the bottle is not stable on the weigh cell, you can't be sure that you are getting good accuracy. It all starts with applications engineering. Companies that have done well with pharmaceutical checkweighers have experience with transfer systems that can take care of that problem."

D'Onofrio says that "the conveyor medium used at almost all pharmaceutical plants is the tabletop chain with a large plastic chain belt. Butting that up against a checkweigher creates a big gap. One way to resolve this is with a side-belt transfer mechanism. The vertical belt system grabs the bottle, carries it across the gap, and places it smoothly on the checkweigher infeed. Also, a timing screw can bridge the gap and create proper spacing."

To counter similar concerns with prepackaging weighing, Bosch has come up with a way to get capsules from the capsule filler to the checkweigher, says Kohl. "The transfer used to be done more or less by gravity, and the capsules would often fall 12 in. into the checkweigher. But if the capsule were brittle, it could break and spill on the weigh cell or in the machine," he says. "In the case of potent drugs, that could mean an exposure issue for the workers on the line. Now, we positively control the capsule. It is grabbed with mechanical fingers that advance it to the next stop, and a star wheel advances it to the weigh cell. Others use pneumatics that can become expensive because they require a lot of compressed air. Our unit uses very little compressed air, just for the accept/reject flaps."

There is no requirement, however, that 100% of pharmaceuticals be checkweighed, and some firms may not wish to have an in-line system and deal with networking and integration. If a manufacturer wants a checkweigher only as a quality assurance tool, it may want to consider an off-line model.

"We get a lot of panic sales from people who have gotten out-of-specification tablets and do not want to spend money on a big checkweigher," says Paul E. Schaa, president, AC Compacting LLC (North Brunswick, NJ), which distributes off-line checkweighers made by CI Electronics (Salisbury, UK). "The customers we get do not have to weigh everything they produce. In fact, some say that if you do that, there is something wrong with your process. Off-line models are for when you think you might have a bad batch, or are doing clinical trial manufacturing, and have to make sure the weight is dead-on. We offer it as an added-value benefit for contract houses for an added level of security."

Those that don't want a checkweigher as part of their line should strongly consider having one off-line, he says, because it is too costly to throw out a batch perceived to be bad when further verification might find it to be acceptable. "Tablet presses are accurate, but I've seen circumstances in which they don't always produce uniform tablets," Schaa says. "If the product shows variability, the checkweigher can save a batch. When tablet presses fail, these work."

ANALYSIS AND VALIDATION

With the increased emphasis on recordkeeping, users may find they need a checkweigher that can produce real-time data. Similarly, there is more need for checkweighers to be validated.

"More communication output data are required, as is validation of the systems," says Bina. "These newer products have validation procedures geared up for the pharmaceutical market. And we can provide validation services in our plant or at the customer's plant as part of aftermarket service."

Even the smaller, less complex models are often validatable now. Schaa says the validation for off-line models is "similar to how you'd validate a scale. You put a calibrated weight on and take it off, see if the checkweigher verifies it, and see if it zeros itself out after the weight's off. Some take samples of tablets and run them, but then you get attrition. If you keep using the same tablets, they will degrade and not give an accurate reading."