The biggest limitation of traditional tablet coaters is the amount of time and solution required to achieve uniformity. Typically, for a cosmetic coating (i.e., instant-release coating) using a conventional oater, tablet weight must increase 4–5% to achieve a uniform coat. The newest drum coaters, however, achieve the same coverage with only a 1.5–2% weight gain, primarily due to an advanced mixing system and pan configuration. This improved uniformity and mixing effect results in better distribution of the coating solution on the tablet surface. Thus, with a lower weight gain, manufacturers are able to achieve the color specification required.
The mixing system of a high-efficiency coater uses patented spiral baffles to move tablets constantly, as shown in Figure 1. One of the baffles moves tablets to the back of the pan while the other brings them forward.In 10 min or less, manufacturers can attain tablet homogeneity in each batch as was demonstrated in a mixing experiment with white, red, and green tablets (see Figure 2). In this experiment, an approximate 500 mL sample was taken from the front, middle, and back of the mixer after 2, 5, and 10 min, and the percentage of white, red, and green tablets in each location was calculated.
As shown in Table I, the tablets in the middle were almost perfectly mixed after 2 min, and all zones were completely mixed after 10 min. The tablet pan may seem like a basic component of a coater, but its size and shape affects coating uniformity as well as the number of defects in a batch, and it plays an important role in increasing process efficiency.
Traditional tablet pans are short in length and large in diameter as shown in Figure 3a. This geometry leads to thick tablet beds. A zone in the middle of the traditional thick tablet bed,
however, is slow moving, and tablets in that area do not get exposed to the surface of the bed as often. High-efficiency coaters, in comparison, have coating pans that are longer in length but smaller in diameter, as shown in Figure 3b. Figure 4a shows the slow-moving
zone in the traditional tablet pan.
In the high-efficiency coater, shown in Figure 4b, the tablet bed is spread wider, and there is no slow-moving zone. The tablets are in constant motion, so they enter the spray zone more often, which results in better coating uniformity and improves efficiency because less coating solution is wasted and thus less time is required compared to a traditional tablet bed.
The smaller diameter pan also helps prevent twinning (i.e., tablets sticking together in the tablet bed). These tablets must be sorted out and often are counted as rejects, thus affecting overall batch efficiency.
High-efficiency tablet coaters improve efficiency by reducing waste of coating material through eliminating spray drying. Traditional tablet coaters send airflow from the top or side of the
coater, across the spray zone, through the tablet bed and out (see Figure 5).
Bringing hot air into direct contact with spray as it is airborne, however, will dry part of the solution before it hits the tablet surface.
While running a conventional coater, one can see a cloud-like effect where the coating spray meets turbulent air, which spreads the solution out and causes it to cover the pan and spray
arms instead of the tablets. This wastes coating material and causes loss of tablets when they stick to the pan or get trapped in the machine’s filter system.
With advanced coating technology, airflow enters the coater from beneath the tablet bed, and it is distributed over the entire length of the pan as shown in Figure 6. All airflow through the tablet bed is in the direction of flow, leaving no hot air in the spray zone and making spray drying almost non-existent.
In addition to eliminating spray drying on the pan and spray arms, the lack of hot air in the spray zone ensures that droplets are wet when they hit the tablet surface. When airflow partially dries a droplet of coating material, it has a tendency to bead on the table pan instead of disperse on the tablets. Wet droplets, however, disperse well and so require less coating solution to achieve coating uniformity.
Manufacturers want to increase the length of campaigns (i.e., the number of batches run continuously). Because high-efficiency coaters reduce the amount of wasted coating solution left
on the tablet pan and spray arms, manufacturers can run a campaign longer without stopping to clean. Less cleaning also helps reduce wash water use and improve waste management.
A typical coater wash cycle uses 400 gallons of water. Because newer coaters require fewer leanings, manufacturers potentially can save thousands of gallons of water and reduce wastewater processing each month.
All coaters have dust collectors or filter systems to catch solidified solution that is lost during processing. Since high-efficiency coaters lose less solution than conventional coaters, filters can be changed less often, thus saving time and cost of maintenance as well as the cost of expensive filters.
In conclusion, advanced tablet coating technologies offer advantages that can lower the cost per tablet and increase processing efficiency, yielding a significant return on investment for manufacturers.