Injection molding apparatus and method to reduce birefringence

A molding apparatus for making a molded part comprising:

• a mold core extending between an upper surface and a lower surface;
• the upper surface of the mold core defining a runner for transferring liquid material to a part cavity to form the molded part, with a gate providing fluid communication between the runner and the part cavity;
• the mold core including an inner surface defining a plunger cavity extending into the mold core from the upper surface;
• a first pin including a first shaft extending through a first bore in the mold core to a plunger slidably disposed within the plunger cavity and sealingly engaging the inner surface to draw the liquid material into the plunger cavity;
• and
• a second pin including a second shaft extending through a second bore in the mold core to the upper surface of the mold core to engage one of the runner or the sprue or the molded part.

A method for making a molded part with a molding apparatus comprising:

• providing a mold core having a generally rectangular block shape extending between an upper surface and a lower surface;
• transferring liquid material through a runner to a part cavity within the upper surface of the mold core to fill the part cavity with the liquid material;
• conveying the liquid material through a gate between the runner and the part cavity;
• hardening the liquid material within the part cavity to form the molded part having the shape of the part cavity;
• pulling a first pin through a first bore in the mold core away from the gate;
• pulling a plunger through a plunger cavity in fluid communication with the gate by the pulling of the first pin through a first bore in the mold core;
• drawing a quantity of the liquid material from the gate into the plunger cavity by moving the plunger through the plunger cavity before the liquid material within the part cavity has completely hardened, and thereby reducing the pressure of the liquid material within the gate.