passing said purge composition through such plastics processing equipment until said {HEAD ~ purge composition} exiting {TAIL ~ such plastics processing equipment} appears clean 
exiting
passing said purge composition through such plastics processing equipment until said { HEAD ~ purge composition } exiting { TAIL ~ such plastics processing equipment } appears clean
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH