Precision surface-replicating thermoplastic injection molding method and apparatus, using a heating phase and a cooling phase in each molding cycle

A method for injection molding thermoplastic articles within an injection mold having at least one mold cavity formed between opposing inserts made from high thermal conductivity material having at least 50,000 compressive yield strength, said inserts 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 reservoirs being heated or cooled under control of an electronic process control unit, at least one hot side reservoir being maintained at a higher fluid temperature than a maximum surface temperature of said inserts, and at least one cold side reservoir being maintained at a lower fluid temperature than a minimum surface temperature of said inserts, said minimum surface temperature being less than a temperature at which the thermoplastic is a form-stable solid, comprising the steps of:

• a. substantially uniformly heating opposing part-forming surfaces of said inserts by said fluid circulating from said hot side reservoir and maintaining open a return line to said hot side reservoir at least until injection is completed, until an actual mold temperature as measured by a temperature sensor mounted within said inserts reads at least a setpoint temperature chosen to retard solidification rate of said thermoplastic, said thermoplastic being amorphous, said setpoint chosen to retard solidification rate is at least equal to a glass transition temperature of said thermoplastic, and said temperature at which the thermoplastic is a form-stable solid is the glass transition temperature of said amorphous thermoplastic, and said heat transfer fluid temperature supplied by the hot side reservoir is at least 25 degrees centigrade higher than the glass transition temperature of the thermoplastic, such that said actual mold temperature as measured by a temperature sensor mounted within said inserts is always substantially less than said hot side reservoir circulating fluid temperature;
• b. injecting molten thermoplastic into the mold cavity formed within the injection mold, while said actual mold temperature is at least equal to said setpoint temperature chosen to retard solidification at least until the mold cavity has been completely filled with said molten thermoplastic and is also further pressurized to a desired level of melt pressure within said mold cavity;
• c. transferring from a heating phase to a cooling phase by stopping flow of heat transfer fluid from the hot side reservoir and starting flow of heat transfer fluid supplied from the cold side reservoir, said heat transfer fluid temperature supplied from the cold side reservoir being at least 25 degrees centigrade below the glass transition temperature of the thermoplastic;
• d. substantially uniformly cooling opposing part-forming surfaces of said inserts by said heat transfer fluid circulating from said cold side reservoir until said actual mold temperature is below the temperature at which the thermoplastic is a form-stable solid, and said actual mold temperature as measured by a temperature sensor mounted within said inserts being always substantially greater than said cold side reservoir circulating fluid temperature;
• e. opening the mold along a parting line and removing the molded thermoplastic product and transferring from a cooling phase to a heating phase by stopping flow of heat transfer fluid from the cold side reservoir and starting flow from the hot side reservoir, 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 inserts, at least one temperature sensor mounted within said inserts, said inserts being fabricated from materials having at least 50,000 psi compressive yield strength so as to resist deformation by injection molding pressures, said material being selected from low alloy tool steels or copper alloys or aluminum alloys, said inserts each being maintained in thermal conductive contact with a heat transfer fluid circulating within a channel entering to and exiting from the injection mold by a supply line and a return line;
• b. a fluid control unit having at least two heat transfer fluid reservoirs in fluid communication with the injection mold by said supply lines and return lines, control valve means for opening or closing said supply line and return line under control of an electronic process control unit, with at least one hot side reservoir being maintained at a higher fluid temperature than a maximum surface temperature of said inserts, and at least one cold side reservoir being maintained at a lower fluid temperature than a minimum surface temperature of said inserts, said inserts being substantially uniformly heated or cooled with minimal point-to-point temperature differences;
• c. the electronic process control unit capable of receiving temperature readings from said at least one temperature sensor mounted within at least one of said inserts and from any temperature sensors provided within said supply lines and return lines and said reservoirs, and said heat transfer fluid reservoirs 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, and at least until injection is completed said hot side return line being maintained open, according to a sequence based upon comparison of said temperature readings from said temperature sensor mounted within at least one of said inserts against a 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.