Method of forming a plastic container

Claim 1

A method of forming a foamed thermoplastic container having a bottom wall, a sidewall joining said bottom wall and extending upwardly and outwardly from said bottom wall, said sidewall having an inner surface which is free of protuberances, a lip defining an opening into said container, and a nesting ledge extending outwardly from said sidewall and disposed imtermediate the upper end of said sidewall and said lip, said method comprising the steps of: A. clamping a heated, foamed, cellular thermoplastic sheet between male and female mold members, said male and female mold members when brought into telescoping relationship with each other defining the boundaries of a cavity therebetween, said cavity including a bottom region in which the bottom wall of the container is formed, a side region in which the entire sidewall of the container is formed, a nesting ledge region in which the nesting ledge is formed and lip region in which said lip is formed, the side surface of the male mold member which defines one boundary of the side region of the cavity is joined to a bottom surface of the male mold member which defines one boundary of the bottom region of the cavity, said side surface continuously sloping outwardly from the bottom surface and being substantially smooth and uninterrupted; B. relatively moving said male and female mold members into final forming position to stretch said sheet into said cavity so that said sheet is compressed in the bottom region and the lip region of the cavity and incompletely fills the side region of said cavity; and C. applying a vacuum to opposite sides of the sheet in the side region of the cavity to expand the sheet into conformity with substantially the entire cooperating surfaces of the mold members which define said side region to thereby form a container in which the sidewall has a greater thickness and a lower density than the lip region and the compressed bottom region.

Claim 2

A method of forming a foamed thermoplastic container having a bottom wall, a sidewall joining said bottom wall and extending upwardly and outwardly from said bottom wall, said sidewall having an inner surface which is free of protuberances, a lip defining an opening into said container, and a nesting ledge extending outwardly from said sidewall and disposed imtermediate the upper end of said sidewall and said lip, said method comprising the steps of: A. clamping a heated, foamed, cellular thermoplastic sheet between male and female mold members, said male and female mold members when brought into telescoping relationship with each other defining the boundaries of a cavity therebetween, said cavity including a bottom region in which the bottom wall of the container is formed, a side region in which the entire sidewall of the container is formed, a nesting ledge region in which the nesting ledge is formed and lip region in which said lip is formed, the side surface of the male mold member which defines one boundary of the side region of the cavity is joined to a bottom surface of the male mold member which defines one boundary of the bottom region of the cavity, said side surface continuously sloping outwardly from the bottom surface and being substantially smooth and uninterrupted; B. relatively moving said male and female mold members into final forming position to stretch said sheet into said cavity so that said sheet is compressed in the bottom region and the lip region of the cavity and incompletely fills the side region of said cavity; and C. applying a vacuum to opposite sides of the sheet in the side region of the cavity to expand the sheet into conformity with substantially the entire cooperating surfaces of the mold members which define said side region to thereby form a container in which the sidewall has a greater thickness and a lower density than the lip region and the compressed bottom region.