Update README.md

README.md

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 design and prototyping of a FabLab-able polypropylene melt blowing setup for distributed manufacture of filter media
 
-## literature
-dimensional analysis of process and discussion of nozzle types (1988): https://pubs.acs.org/doi/pdf/10.1021/ie00084a021
-
-Exxon patent showing slot-style melt blowing nozzle: https://patents.google.com/patent/US3849241A/en?oq=3%2c849%2c241
+## challenges
+- primary air
+    - hundreds of SCFM, 200+ C, ??? psig
+- tooling
+- process control
+    - various temperatures
+    - air flow
+    - feedstock extrusion
+- safety
+    - 200 C + pipes and tooling: burn hazard
+    - Mach 0.5+ air: loud!
+    - fibers: Class III/Div 1 explosion hazard?
+- feedstock
+    - recycled? pelletized virgin PP?
+    - heat + extrude + pump?
 
-UTenn study on melt blowing: https://onlinelibrary.wiley.com/doi/epdf/10.1002/pen.760302202
-
-good discussion of primary air system requirements: http://jmppnet.com/journals/jmpp/Vol_3_No_1_June_2015/9.pdf
+## literature
+- dimensional analysis of process and discussion of nozzle types (1988): https://pubs.acs.org/doi/pdf/10.1021/ie00084a021
+- Exxon patent showing slot-style melt blowing nozzle: https://patents.google.com/patent/US3849241A/en?oq=3%2c849%2c241
+- UTenn study on melt blowing: https://onlinelibrary.wiley.com/doi/epdf/10.1002/pen.760302202
+- good discussion of primary air system requirements: http://jmppnet.com/journals/jmpp/Vol_3_No_1_June_2015/9.pdf
 
 ## melt blowing videos
-video of a polypropylene filter melt blowing line in action: https://www.youtube.com/watch?v=cGQ4piMNQ_E
-
-fast construction of a melt blowing line for covid-19 masks: https://www.youtube.com/watch?v=p9lchhBE7cE
-
-a smaller line with a built-in rotary mask die cutter: https://www.youtube.com/watch?v=KgWHNQo7j1k
-
-pilot-scale melt blowing machine making a mess: https://www.youtube.com/watch?v=SOFfpGnMjOc
+- video of a polypropylene filter melt blowing line in action: https://www.youtube.com/watch?v=cGQ4piMNQ_E
+- fast construction of a melt blowing line for covid-19 masks: https://www.youtube.com/watch?v=p9lchhBE7cE
+- a smaller line with a built-in rotary mask die cutter: https://www.youtube.com/watch?v=KgWHNQo7j1k
+- pilot-scale melt blowing machine making a mess: https://www.youtube.com/watch?v=SOFfpGnMjOc