Method and Apparatus of Gas-Assisted Injection Molding of Thick Parts
Gas-assisted injection molding is used to form molded parts having relatively thick cross-sections, such as a support section of a chair back and seat portion of a chair. Gas is injected into a mold cavity using the same nozzle and screw injector that injects molten resin into the mold cavity. The injected gas creates a void in the molten resin that allows a reduced amount of resin to be used to fill a mold cavity without sacrificing the integrity of the molded part.
1 . A method of forming a thick part formed of plastic material, comprising:
injecting molten resin through a nozzle into a mold cavity for a period between 10-15 seconds, the mold cavity defined by an interior surface; and
injecting gas into the mold cavity for a period between 30-50 seconds to press the molten resin against the mold surface, wherein the injected gas functions to form a void area in the part.
2 . The method of claim 1 wherein the molten resin is injected for a period of approximately 12 seconds.
3 . The method of claim 1 wherein the gas is injected for a period of approximately 40 seconds to form a void in the injected molten resin.
4 . The method of claim 1 further comprising waiting a period between 10-15 seconds after commencement of molten resin injection into the mold cavity before injecting the gas into the mold cavity.
5 . The method of claim 4 wherein the gas is not injected until after a delay time of approximately 12 seconds.
6 . The method of claim 1 further comprising injecting the gas at a gas pressure between 700-800 psi.
7 . The method of claim 6 wherein the gas pressure is approximately 800 psi.
8 . The method of claim 1 further comprising injecting the molten resin with a pressure between 750-850 psi.
9 . The method of claim 8 wherein the molten resin is injected at a pressure of 800 psi.
10 . The method of claim 1 wherein the gas is injected into the mold cavity through the nozzle.
11 . The method of claim 1 wherein the mold cavity defines a part having a thickness between 1-3 inches.
12 . The method of claim 11 wherein the thickness is approximately 2 inches.
13 . The method of claim 11 wherein the part formed is that of a seat and chair back portion of a chair.
14 .- 18 . (canceled)