the process will cost you your product and possibly your customer. It comes down to a careful economic balance of capital cost vs. performance.
Drying PLA is no different from drying any other hygroscopic material except that it is temperature sensitive. Depending on the specific grade, most manufacturers recommend drying crystallized PLA at 150ºF-190ºF (65ºC-87ºC) using dehumidified air with a dewpoint of -40ºF. Amorphous PLA dries at a lower temperature.
Precise control of temperature, dewpoint, drying time, and drying airflow are critical to achieving the properties and performance intended. Temperature is most important. If the drying temperature is too low, the PLA pellets will not dry as readily. If the temperature is too high, the material may soften and agglomerate in the drying hopper.
Dewpoint is the next most important variable, since it determines how dry the material eventually becomes. If the dryer cannot produce a stable, low dewpoint, it doesn’t matter how long the resin dries; it will never reach the low moisture levels required for optimum property development.
Finally, the design of the drying hopper should be such that every pellet is exposed to the drying air for the time required. This means the hopper must be large enough to provide the necessary residence time, and airflow is adequate to create the proper temperature and dewpoint conditions for drying.
Crystallizing amorphous PLA regrind
Virgin PLA, like PET, is crystallized and does not need to be crystallized again before drying and processing. However, once the crystalline pellets are melt processed, the crystals are destroyed and the material reverts to its amorphous state and must be recrystallized by heating it past its glass-transition temperature of 140ºF (60ºC). As the temperature approaches the glass-transition point, the PLA begins to soften, which is why it needs to be constantly agitated to prevent agglomeration. As the temperature continues to rise, the PLA hardens again into its crystalline state. In this crystallized condition, it can be dried at higher temperatures like any other hygroscopic polymer.
Some processors, however, choose to eliminate the crystallization step and simply dry the amorphous regrind and the crystalline virgin together at temperatures as low as 130ºF (54ºC). This process can take twice as long as conventional drying, but it eliminates the need for a crystallizer.
There are three types of commonly used crystallizers. By far the most common is a vertical agitating hopper with convection heating. In these systems, the crystallizer directs heated air to the material through the bottom spreader cone of the hopper. Constant agitation prevents agglomeration during heating. Among their advantages is their space-saving design, relatively low cost, initiation of the drying process concurrent with crystallization, and integration with most conventional drying systems. On the downside, it can take up to 40 minutes for these to reach full production levels—but once this level is reached, the process is continuous. Energy use can also be an issue, but actual consumption varies widely depending on the application.
Horizontal conduction heating units rely on high pellet-to-metal contact area to heat the material, and constant particle motion to eliminate agglomeration. Advantages of