Pelletization is a downstream process for hot-melt extrusion where the melt stream is pumped through the die, cooled and formed
into a pellet, typically between 0.5 and 5 mm. In strand pelletization, "spaghetti" strands are extruded and cooled on a stainless
steel or FDA-approved plastic-belt conveyer. The feedrolls of the pelletizer pull the strands and push them into the cutting
assembly where cylindrical pellets are produced. Die-face pelletization is also common, where the pellets are cut at the die
face and conveyed/cooled by various methods, including chilled air chimneys and vibratory towers. Because many pharmaceutical
formulations tend to smear at the die face, this method should be tested prior to implementation. Smaller pellets can be used
for direct capsule-filling, and in-line spheronization is also possible (see Figure 11).
Figure 11 (All figures are courtesy of the author.)
For the production of flat film or sheets such as those used in transdermal or dissolvable-film applications, the process
melt material is distributed in the die and cooled on rolls. The roll surface is maintained at the desired temperature by
pumping a liquid (typically water or oil) through internal cooling channels. The molten material solidifies onto the roll
as it cools. Take-off units are available in several different roll-stack configurations such as vertical and/or horizontal
arrangements. For many flat products, the nip force across the roll face is used to "squeeze" the extrudate between the rolls.
Unwind stations can be used to laminate the film onto a substrate. The final product is then either wound or cut to length.
Shape extrusion occurs when the process melt is directly extruded into a part with specific dimensions. The extrudate can
be a simple rod, or complex shape, referred to as a "profile." The extruded profile is formed in the die, sized by calibration
tooling, and then conveyed and supported on a conveyer with auxiliary air cooling devices. A belt puller then feeds the product
to an on-demand or flywheel-type cutter. In this manner, for example, a 3 mm-diameter by 1 mm-length tablet might be produced.
Twin-screw extruders are becoming the manufacturing methodology of choice to continuously mix APIs with pharmaceutical grade
polymers. Twin-screw extruders are highly flexible and efficient mixing devices suited for producing wide ranging pharmaceutical
products that demand consistency and high quality. Understanding the extrusion process, as well as integration of the machine
system, is necessary to ensure content uniformity for both compounds produced via hot-melt extrusion.
Charlie Martin is a general manager at Leistritz North America, tel. 908.685.2333, ext 616, email@example.com
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