Gas assisted injection molding combining internal and external gas pressures

In a gas-assisted injection molding method for producing an injection molded part, including the steps of providing a mold having a part defining cavity formed by a stationary mold portion and a movable mold portion, clamping mating faces of said stationary and movable mold portions in abutting contact with one another, and injecting a quantity of a flowable thermoplastic material into said mold cavity under pressure, to form an injection molded part, the improvement comprising the steps of:

• introducing a first quantity of gas into said quantity of flowable thermoplastic material at a mold location corresponding to a mold parting line defined by intersecting mold portions to form a first gas channel therealong, said first quantity of gas in said gas channel pressing said thermoplastic material against said mold parting line to form a gas seal between said mold parting line and said mold cavity;
• introducing a second quantity of gas into said cavity along an underside of said molded part, said second quantity of gas being retained within said cavity by said thermoplastic material sealing said mold cavity along said mold parting line;
• and
• containing said second quantity of pressurized gas within said mold cavity via said gas seal as said injection molded part cools thereby forming a Class A appearance side on said injection molded part.

In an injection molding apparatus including stationary and movable mold portions defining a mold cavity, and mating means for moving mating faces of said mold portions into abutting contact with one another, a method for producing an injection molded part having a sink-free appearance side, comprising the steps of:

• introducing a flowable thermoplastic material into said mold cavity to form an injection molded part;
• introducing a first quantity of gas into said mold cavity so as to form a gas channel in said thermoplastic material, said first quantity of gas biasing said thermoplastic material against a mold parting line formed at an intersection of said mold portion mating faces to form a gas seal between said mold parting line and said mold cavity;
• and
• introducing a second quantity of pressurized gas through at least one gas inlet located in a first of said mold portions, thereby allowing said second quantity of gas to be uniformly dispersed along an underside of said formed injection molded part to hold said formed injection molded part against an inner surface of a second of said mold portions;
• said step of introducing a second quantity of gas preceding said step of introducing a first quantity of gas.

In a gas-assisted injection molding method for producing an injection molded part, including the steps of providing a mold having a part defining cavity formed by a stationary mold portion and a movable mold portion, clamping mating faces of said mold portions in abutting contact with one another, and injecting a quantity of a flowable thermoplastic material into said mold cavity under pressure, to form an injection molded part, the improvement comprising the steps of:

• introducing a first quantity of gas into said quantity of flowable thermoplastic material at a mold location corresponding to a mold parting line defined by intersecting mold portions to form a first gas channel therealong, said first quantity of gas in said gas channel pressing said thermoplastic material against said mold parting line to seal said mold cavity;
• and
• introducing a second quantity of gas into said cavity along an underside of said molded part, said second quantity of gas being retained within said cavity by said thermoplastic material sealing said mold cavity along said mold parting line;
• said step of introducing a second quantity of gas preceding said step of introducing a first quantity of gas.