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