Microcellular Foaming in a Continuous Process

The pressure drop rate is correlated to the thermodynamic instability necessary to generate the nuclei and, mainly, affects foam morphology. The gas concentration is directly correlated to the availability of gas necessary for ceU growth, influencing the final density of the foamed plastic [72]. The pressure drop rate in the die (—dp/dt) is expressed as follows [24]  

Compared with the batch foaming process, relatively few publications feature continuous microcellular foaming of PLA in extrusion or injection molding equipment [3, 5, 73-79]. 

CO2 contents (1.8-9.3 wt%), processing temperatures (90-100°C), and also by adding 0.5 wt% talc into the resin are summarized in Table 17.5. 

Different behaviors can be seen with the data listed in Table 17.5. The concentration of gas had a limited impact on the density reduction of neat PLA at lower CO2 content. Below 5 wt% CO2, a smaller number of cells were nucleated and both the density and the cell population density remained almost constant. However, above 7 wt% CO2, lower density foams with a large number of nucleated cells were produced. The density remained constant while the cell population density slightly increased with CO2 content. Since higher gas concentration favors the nucleation of large number of cells, the results imply that a critical gas concentration of 7 wt% is required to achieve low-density foams. The highly expanded foams had a high open-cell content. 

Interestingly, the narrow processing window associated with this critical threshold, that is, low-density foams and 

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