Reduced-knitline thermoplastic injection molding using multi-gated non-sequential-fill method and apparatus, with a heating phase and a cooling phase in each molding cycle

A method for injection molding optically transparent 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 copper alloy having at least 40 BTU/square foot/foot/degree Fahrenheit conductivity and at least 50,000 psi loadbearing compressive yield strength, 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 maximum surface temperature of said mold cavity support blocks, and
• at least one cold side supply system being maintained at a lower fluid temperature than a 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,
• 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 chosen to retard solidification rate of said thermoplastic, said thermoplastic being optically transparent and amorphous, said predetermined setpoint chosen to retard solidification rate is at least equal to a glass transition temperature characteristic of said thermoplastic, and said temperature at which the thermoplastic is a form-stable solid is the glass transition temperature characteristic of said amorphous thermoplastic, and said heat transfer fluid temperature supplied by the hot side supply system is at least 25 degrees centigrade higher than the glass transition temperature characteristic of the thermoplastic,
• said mold cavity formed between opposing mold cavity support blocks being pre-enlarged before start of injection, by increasing a mold cavity height dimension by means of increasing distance between said opposing mold cavity support blocks, so as to reduce aspect ratio during filling of said mold cavity and thereby to reduce geometric constrictiveness during filling of said mold cavity;
• b. injecting into and compressing said molten thermoplastic within said pre-enlarged mold cavity formed between opposing mold cavity support blocks before start of injection, by decreasing a mold cavity height dimension by means of decreasing distance between said opposing mold cavity support blocks,
• said decreasing a mold cavity height dimension being done either during or after injecting said molten thermoplastic substantially simultaneously from each of the 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 by stopping flow of heat transfer fluid from the hot side supply system and starting flow of heat transfer fluid supplied from the cold side supply system, said heat transfer fluid temperature supplied from the cold side supply system being at least 25 degrees centigrade below the glass transition temperature characteristic of the thermoplastic;
• 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 the molded optically transparent thermoplastic product and transferring from a cooling phase to a heating phase by stopping flow of heating transfer fluid from the cold side supply system and starting flow from the hot side supply system, in preparation of the start of the next injection molding cycle.

An apparatus for injection molding thermoplastic articles, comprising:

• a. an injection mold having at least one mold cavity formed between opposing mold cavity support blocks, said injection mold comprising:
• (i) a runner system maintained in fluid communication between an injection molding mackine and a plurality of substantially opposing edge gates suitable for feeding in injected molten thermoplastic so as to reduce meltflow distance and thereby reduced aspect ratio during a filling phase of a molding cycle;
• (ii) said mold cavity support blocks being constructed of high thermal conductivity copper alloy having at least 60 BTU/square foot/foot/degree Fahrenheit conductivity and at least 50,000 psi loadbearing compressive yield strength, 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 said molding cycle,
• (iii) passageways for circulating heat transfer fluids within said mold cavity support blocks being maintained in thermal conductive contact with said heat transfer fluids entering to and exiting from said injection mold by means of supply lines and return lines plumbed to a fluid control unit;
• b. said fluid control unit having at least two heat transfer fluid supply systems in fluid communication with said injection mold by said supply lines and return lines, with
• 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, and
• at 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;
• c. an electronic process control unit capable of receiving temperature readings from at least one temperature sensor mounted within at least one mold cavity mold cavity support block and from any temperature sensors provided within said supply lines and return lines and said supply systems, and said heat transfer fluid supply systems being capable of being heated under control of said electronic process control unit,
• such that said process control unit can open and close control valves governing flow of heat transfer fluid within said supply line and said return lines maintained in fluid communication between said fluid control unit and said injection mold, according to a sequence based upon comparison of said temperature readings from said temperature sensor mounted within at least one mold cavity mold cavity support block against a predetermined series of settable temperatures and related time values,
• thereby determining the start and stop of a heating phase and a cooling phase within each individual injection molding cycle.