Process and mold for molding foamed plastic articles

A method of molding foamed articles by injecting a molten synthetic resin material with a foaming agent contained therein from an injection means, including one or more nozzle means, into a cavity of a mold means, the method comprising the steps of:

• closing the mold means;
• prepressurizing the mold cavity through a port with gas so as to substantially prevent diffusion of the blowing gas produced from the foaming agent;
• injecting an amount of molten synthetic resin material containing a foaming agent into the cavity, the amount being insufficient to fill the cavity;
• venting a portion of the prepressurization gas from the mold cavity through the port during the injection step while maintaining the pressure at the port at a level which substantially prevents diffusion of the blowing gas produced from the foaming agent such that diffusion of the blowing gas is inhibited as this portion of the prepressurization gas is vented and the mold cavity is being filled with the molten resin material;
• drawing a vacuum at the port following the completion of the injection step so as to vent the remaining prepressurization gas and blowing gas from the mold cavity and allow expansion of the injected foamable resin material within the mold cavity, the vacuum drawing the resin material throughout the mold cavity;
• solidifying the molded article;
• and
• ejecting the article from the mold means.

A mold for forming structural foamed articles comprising:

• a mold assembly including a fixed mold portion and a movable mold portion, said fixed and movable mold portions having respective first and second surfaces with side margins which abut when the mold is closed, the first and second surfaces having central portions which define a mold cavity therebetween;
• gas flow means for delivering gas to and venting gas from the mold cavity, said gas flow means comprising an annular groove formed in the side margin of the first surface and surrounding the central portion of the first surface, plural gas flow channels formed in the side margin of the first surface and communicating between the mold cavity defining central portion of the first surface and the annular groove, said fixed mold portion being provided with a single external gas supply and gas venting opening and defining an internal gas passageway communicating between plural locations of the groove and the gas supply and venting opening such that a change in pressure at the opening is substantially immediately communicated to various spaced locations of the mold cavity;
• and
• sealing means for sealing the space between the first and second surfaces at a location which is preipheral of the groove;
• and
• vacuum means for selectively applying a vacuum to the venting opening.