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