Desiccant Dryer Dew Point Calibration and Karl Fischer Titration Verification
Dew point monitoring alone fails to guarantee dry resin; coulometric Karl Fischer titration per ISO 15512 Method B is mandatory to prevent hydrolytic scission.
Polycarbonate Splay
Meaning
Molding cosmetic defects appear as surface streaks or silver marks on a finished plastic part, indicating thermal or chemical issues during the process. In clear polymer molding, polycarbonate splay occurs when moisture or trapped gas is carried along the mold surface during the injection phase. This results in localized silver streaks that destroy the optical clarity of the molded part.
It is the most common reason for part rejection in lighting or display applications.
Chemical Origin
Insufficient resin drying is the leading cause of this surface defect. High melt temperatures trigger hydrolysis when water is present in the polycarbonate splay, releasing carbon dioxide gas which forms microbubbles in the melt stream. These bubbles are stretched against the cold mold wall during filling, leaving the characteristic silver streaks.
Mold Geometry
Sharp corners and restricted gates can create high shear stress that also causes splay. If the resin is forced through a gate that is too small, localized shear heating can degrade the polymer and release gas. This mechanical splay looks identical to moisture splay, which can make troubleshooting difficult.
Technical Adjustment
Reducing the injection speed or lowering the melt temperature are common ways to address shear induced splay. If the splay persists, the processor must check the dryer operation and verify that the dewpoint is low enough.