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