Method and apparatus of detecting impurities in fluid

A system for forming a plastic article, said system comprising:

• an extruder for extruding resin, a detecting apparatus for detecting impurities in the resin, and a fluid passageway connected to said extruder and said apparatus,
• said detecting apparatus comprising a transparent member defining a fluid passage therein in-line with said fluid passageway such that resin flowing through said fluid passageway flows through said transparent member, the fluid passage having a longitudinal axis, an optical system comprising a light source oriented to direct an annular beam of light into said transparent member substantially along the longitudinal axis of said passage, whereby light will be reflected from impurities in the resin, and at least one optical detector oriented to receive, from a direction substantially perpendicular to said longitudinal axis, light emitted by said optical system and reflected from impurities in the resin.