in which Δ T is the change in temperature in °C, and Δ P is the change in pressure (1 bar = 14.503 psi). In addition, the extruder rpm and the geometry of the discharge screw elements
can drive the melt temperature even higher.
Feeding systems
Feeders set the throughput rate to the twin-screw extrusion system. The extruder-screw rpm is independent from the feed rate
and is used to optimize compounding and devolatilization efficiencies. Feedstocks can be pellets, granules, fibers, powders,
and/or liquids. Delivery mechanisms include vibratory trays, belts, single screw, and twin-screws for solids; and piston or
gear pumps for liquids. As previously stated, the pressure gradient in the process section of the twin-screw extruder is zero
for much of the process, which allows downstream unit operations to be performed.
Figure 9 (All figures are courtesy of the author.)
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Loss-in-weight feeders measure the flow rate for the feeder via a precision load cell (see Figure 9). As the feeder discharges material to the extruder, the speed of the delivery mechanism
adjusts to maintain the desired feed-rate set point. For loss-in-weight controls to work properly, the feeder must be optimized
mechanically with regard to the hopper, auger mechanism, and mounting/interface hardware with the extruder. Multiple feeders
facilitate introducing ingredients at different positions along the length of the process section. Material handling and refill
systems that integrate the appropriate degree of isolation and containment are integrated into the system design.
The feed rate, in combination with the screws' rpm, is integral to the mixing intensity that the materials experience during
the HME process. A high feed rate with a low screws rpm will result in a gentle mixing effect as compared with a low feed
rate and high screws rpm. Because the feed rate is independent from the screws rpm, the HSEI twin-screw extruder has a wide
operating window.
Downstream operations
A wide variety of downstream systems are available after the extruder. Pellets or shapes may be extruded and wound, or cut
to length. Film and lamination systems are used to combine melt extrusion with substrates for transdermal applications.
Figure 10 (All figures are courtesy of the author.)
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For any HME process, the melt stream must be converted from the circular shape into the final product. The basic die design
needs to account for varying material paths and residence times within the die based upon the extrudate dimensions and materials
being processed (see Figure 10). Advanced dies would also have some sort of adjustment to maintain a dimensionally stable
product. A variety of end products can be produced via hot-melt extrusion.
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