Method for the injection molding of plastic articles using fluid pressure

A method for injection molding a plastic article in a fluid assisted injection molding system, the method comprising the steps of pressurizing a charge of fluid at a predetermined pressure level no greater than a nominal plastic injection pressure;

• injecting a quantity of molten plastic through an injection flow path from an injection molding machine of the system into a mold cavity at the nominal plastic injection pressure;
• communicating the charge of pressurized fluid to the flow path during the step of injecting through an orifice immediately adjacent the flow path, the orifice having a sufficiently small dimension and the predetermined pressure level of the charge of fluid being sufficiently high at the orifice to resist entry of the molten plastic into the orifice, the pressure of the injected plastic in the flow path adjacent the orifice and the small dimension of the orifice preventing the fluid charge from entering the flow path at the orifice prior to completion of the step of injecting molten plastic;
• the fluid charge traveling through the orifice and into the flow path upon substantial completion of the plastic injection in a self-executing fashion in response to a reduction of the pressure of the plastic in the flow path below the predetermined pressure level of the fluid charge whereby residual plastic in the machine for the next molded article is not contaminated with the fluid;
• containing the fluid under pressure within the article until it has set in the mold cavity;
• and venting the fluid to ambient.