Impact of the Fused Deposition (FDM) Printing Process on Polylactic Acid (PLA) Chemistry and Structure

Comment: This is a very useful and potentially high-impact paper - I hope the authors plan to submit it for peer review soon. I had a paper accepted less than one week ago by Rapid Prototyping Journal that contained a discussion about the need and lack of good literature on this topic for PLA. A lot of good work exists for FDM-processed ABS, but very little to none for other common materials such as PLA, nylon, and various forms of PET. PLA is becoming a much more important material as additive manufacturing becomes more widely used, primarily because it has superior material properties to ABS, it is far more environmentally friendly, and has much lower industrial danger risk than ABS or other petrochemical polymers. While this work is very useful and well-written, I have a few questions/suggestions for improvement for the final version of the paper. 1. Overall: PLA is extremely sensitive to humidity and processing temperature. I have had entire new rolls of PLA in my lab scrapped due to long exposure to humidity above 60%. Could you add a brief discussion of this and talk about some of the processing implications of the humidity? 3-4 sentences will suffice. And please include your processing conditions (temperature and humidity, if available) for the samples you made in this study. 2. Introduction (optional but nice): Could you provide a figure demonstrating the FDM process? Not every reader understands the mechanics of the process and if you have space, it would be nice to include. 3. Introduction: Can you provide a reference for the glass transition and melting temperatures? Does Reference (6) cover this? If so, please make it more clear. Makerbot filament usually contains additives that give it slightly different properties (higher melting temperature and higher brittleness from my experience) than PLA filament from Makergeeks or Hatchbox or other sources. Did you use Makerbot-brand filament or another brand? This is relevant to understanding your results. 4. Materials and Methods: In your description of the processing parameters (resolution, speed, etc), could you provide a few sentences explaining your choice of parameters? The nozzle diameter is also a very relevant bit of information. In my experience with both PLA and Makerbots, Makerbots usually do not do well with PLA with a nozzle diameter less than 0.5mm. Also, can you give your filament retraction distance? 5. Figure 4 (pg 10): The defects in the surface likely were caused by extrusion problems and temperature instability that I have observed affects Makerbot printers more than other varieties. Using a large nozzle (+0.5), slowing print speed, and using tool steel nozzles (as opposed to brass ones) seems to help with this. Replacing the nozzles every 100 hours or so helps as well (I always use a new set of nozzles for each batch of experimental samples for my studies). A brief discussion of this would be great. You can talk about it here or in the discussion section. Once again, congratulations on an excellent study. Please email me when it is published, as I want to add this paper to my catalog of references. Feel free to contact me with any questions.