Packaging Tablets into Bottles

A look at the differences between counting methods and technologies.

Dale Macy, Lead R&D Engineer, DT Packaging Systems

Slat fillers often rely on empty-pocket detection systems to ensure that tablets enter slat cavities before bottle filling. Photo courtesy DT Packaging Systems.

There are several options for accurately counting moderate to high volumes of pharmaceutical products such as solid-dose tablets, capsules, and gel caps. Each has its advantages, disadvantages, and most appropriate applications. They can be categorized according to the physical delivery system and the electronic counting or verification systems that they employ.

PHYSICAL DELIVERY SYSTEMS

Physical delivery systems for these products include slat fillers, machines with vibratory feeders, rotary slat fillers, and pocketed-wheel counters. Slat fillers deliver the highest volumes and speeds and the most efficient use of space, while machines with vibratory feeders offer gentler product handling, some inspection options, and require few or no change parts. Output volumes per station from vibratory feed machines range from

1000 to 5000 tablets per minute, while the output from slat fillers is up to 40,000 tablets per minute. Pocketed-wheel fillers fall somewhere in between in terms of both production volume and floor space usage; they do require some change parts.

There are two major types of slat fillers. One type is machinery with a number of pocketed slats or bars that run across a machine and circulate around like a belt. The other is the so-called rotary slat filler, in which the slat is shaped like a disk, and the pockets or cavities are on the outer edge.

The benefits of slat fillers are high-speed filling, moderate to gentle product handling (though somewhat less than with rotary slat fillers), high fill density, and flexible cavity design. Slat cavity design can be critical, especially with odd-shaped tablets. The tablet needs to fit well into the cavity so the filler can deliver high fill efficiencies with a minimum of missed or empty cavities. The tablet should enter and exit the cavity smoothly, and the cavity should deliver the correct number of tablets, usually one. This is true of both traditional slat fillers and rotary slat fillers. 

Traditional slat fillers have the advantage of a larger fill area, meaning higher cavity-fill efficiencies, and gentler product handling. On the other hand, rotary slat fillers have the ability to continue running one wheel longer to make up for a missed cavity. They do not rely on a physical pattern or number of slats to produce any given count. Even so, rotary slat fillers still rely on cavity design to achieve a high level of efficiency.

COUNT VERIFICATION METHODS

Several inspection or electronic counting methods have been used with slat fillers, including optical sensors, vision systems, empty-cavity detection systems, and a new high-speed scanning technology that counts tablets in multiple lanes after they fall from the slats.

Rotary slat fillers use an optical detector called a frame sensor that times the duration of a passing object. If the time recorded matches the programmed range of possible readings that can be caused by a passing tablet, it is considered a count. The machine continues to run until the count in all of the bottles is satisfied. If one bottle lags behind the others in the count, the others wait until the count for that one has been satisfied. This process reduces the number of rejected containers, but also slows the process and reduces production volume.

The most common inspection device for traditional slat fillers is an empty-cavity detection system. This device looks through small holes at the bottoms of the slat cavities to determine whether a tablet has filled that cavity or not. If the cavity is empty, the corresponding container is marked for rejection during the bottle index process. A potential source of inaccuracy in this system is that a jammed cavity can occur, in which the product gets stuck in a cavity and does not empty out into the container.

The latest counting technology developed in response to this problem is a product that can scan thousands of times a second to detect each tablet after it has left the slat. The scan data are compared with a preprogrammed product profile to verify a count. The counting method is different from the frame sensor in that the new technology measures the size of the passing tablet in width and length, as well as the opacity of the shadow created by the falling tablet. This device can simultaneously count all of the tablets falling from a slat at once and is currently programmable to count between 60 and 100 tablet rows at a time. That is, the same hardware can be used to count different numbers of slat cavities.

This technology is particularly useful in high-volume bottle-filling applications because it can handle high count density compared with the machine space used. In comparison, each wheel on a rotary slat filler requires an additional frame sensor and, therefore, greater machine width to accommodate more counting rows.

One brand of pocketed-wheel counter uses a combination of frame sensors to sense two dimensions of a passing tablet to determine a count. As with the rotary slat machine, the pocketed wheel continues to rotate until the proper count is achieved. That count can be set to any number. A number of these wheels can be assembled on a turret to form a rotary machine. An advantage of this arrangement is that there is a precount gate that allows the machine to continue counting and hold a number of tablets in a section over the bottle-free zone of the turret. One drawback, though, is that the turret must be slowed to give the pocketed wheel time to count out higher tablet counts. Also, the product is churned while getting moved into the pockets, so product handling is not as gentle. Maximum speeds are from 6000 to 18,000 tablets per minute.

Most vibratory feeders use optical sensors. One such sensor is available with an electrostatic detection system that adds the option of tablet inspection to the package. A common arrangement is a number of feed trays with 12 grooves or tracks from which the tablets are fed. Tablets fall off of the last feed tray in the line and through a set of optical arrays, one array for each track in the tray. Product is counted by timing the passage of the tablets as with the frame sensors above. The electrostatic system detects the tablets by comparing the disturbance to an electrical field against a preprogrammed profile and is able to determine the tablet�s mass and size.

Machines with vibratory feeders, often called electronic counters, have the advantages of easy cleaning, no need for product-specific change parts, good visibility of the product while it is running, and generally, the gentlest product handling. These machines handle a wide variety of products well and do not generally undercount. Electronic counters are the top choice for speed requirements in the range of 60 bottles per minute. They can be grouped together to double the output of the line. 

The best machine for a customer�s application depends on many factors, including production volume, count and product matrix, product durability, cost, validation, delivery options, and others. A customer in the market for a tablet counter needs to evaluate all of these factors and choose a solution that delivers the highest-quality output while meeting particular plant space and production needs. 

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