Patent ID: 6290882
Filing Date: 2001-09-18
Classification: B29C,B29D,Y10S

Abstract:
A method for injection molding thermoplastic articles having no objectionable knitline at a meeting of meltfronts within a useable portion of said thermoplastic article having been molded within an injection mold having at least one mold cavity formed between opposing mold cavity support blocks,said mold cavity support blocks being constructed of high thermal conductivity metal alloy, said mold cavity support blocks being constructed of sufficient thickness to resist unacceptable mechanical deflection when maximum melt pressure is being exerted during a packing stage of a molding cycle, said mold cavity support blocks being maintained in thermal conductive contact with a heat transfer fluid circulating within channels entering to and exiting from the injection mold by means of supply lines and return lines plumbed to a fluid control unit having at least two heat transfer fluid supply systems capable of being heated or cooled under control of an electronic process control unit,at least one hot side supply system being maintained at a higher fluid temperature than a settable maximum surface temperature of said mold cavity support blocks, said maximum surface temperature being greater than a temperature at which the thermoplastic is a form-stable solid, andat least one cold side supply system being maintained at a lower fluid temperature than a settable minimum surface temperature of said mold cavity support blocks, said minimum surface temperature being less than a temperature at which the thermoplastic is a form-stable solid,said mold cavity having a plurality of substantially opposing edge gates suitable for feeding in injected molten thermoplastic so as to reduce meltflow distance and thereby reduce aspect ratio during filling of said mold cavity, so as to produce a meeting of meltfronts within a useable portion of said thermoplastic article when injection is started substantially simultaneously from each of said plurality of substantially opposing edge gates,comprising the steps of:a. heating opposing partforming surfaces of said mold cavity support blocks by said fluid circulating from said hot side supply system, until an actual mold temperature as measured by a temperature sensor mounted therein reads at least a predetermined setpoint temperature settably chosen to retard solidification rate of said thermoplastic;b. injecting said molten thermoplastic from each of said plurality of substantially opposing edge gates, into the mold cavity formed within the injection mold, while said actual mold temperature is at least equal to said predetermined setpoint at least until said meeting of meltfronts within a useable portion of said thermoplastic article has been attained during the filling of the mold cavity with said molten thermoplastic;c. transferring from a heating phase to a cooling phase;d. cooling the mold until said actual mold temperature is below the temperature at which the thermoplastic is a form-stable solid;e. opening the mold along a parting line and removing said thermoplastic article and transferring from a cooling phase to a heating phase, in preparation of the start of the next injection molding cycle.