these are trouble-free operation once the process window has been identified, and their lower energy demand. But the initial equipment cost can be high.
The third type, fluid bed crystallizers, is essentially a modified pellet classifier that uses heated air to crystallize regrind. Typically used for pellets, these have also been used successfully for ground flake. They have been used extensively for crystallizing PET and PET copolymers. There is no known commercial experience with PLA to date; however, successful laboratory tests have been performed with all grades of semicrystalline PLA.
Relatively new on the scene is infrared crystallizing and drying. Material is metered into a horizontal tube, where it is conveyed lengthwise. Residence time is determined by the rotational speed of the drum, and the rotating action agitates the material while its surface is irradiated with infrared energy. Among the advantages of these are a short startup time (generally 10- to 20-minute residence), potential energy savings, and the ability to be coupled to a finishing dryer for low-ppm moisture level requirements.
These have their downside, too. Vacuum conveying can draw moisture-laden air from the chamber interior. They can be sensitive to flake size distribution (smaller flakes may melt), and results can depend on ambient air conditions and starting material moisture levels. They also aren’t cheap and take more floor space than other options.
Clearly there are plenty of options when dealing with PLA, and as more processors develop more experience with the material, expect the body of knowledge about this in-demand material to increase quickly.
About the author: Jamie Jamison is product manager, dryers, at The Conair Group, a leading supplier of auxiliary equipment for plastics processors. —Edited by Matt Defosse