Methods of and systems for controlling the expansion of cellular plastic insulation in the manufacture of insulated conductors

A method of controlling the expansion of cellular insulation in the manufacture of an insulated conductor having a conductive element and a layer of cellular insulation extruded thereabout, which comprises the steps of:

• generating a first preheat control voltage as a function of line speed wherein the line speed is greater than a predetermined minimum speed;
• applying the first preheat control voltage to a preheat power supply to generate a preheat voltage;
• applying the preheat voltage across a selected length of the bare conductive element to heat the element to the initial temperature;
• advancing the preheated conductive element through an extruder whereat a layer of cellular insulation is extruded thereabout to form an insulated conductor;
• advancing the insulated conductor through an air gap whereat the layer of cellular insulation expands about the conductive element;
• moving the insulated conductor into a cooling bath which is movable relative to the extruder at a quench-point to stop the expansion of the layer of cellular insulation;
• measuring the coaxial capacitance of the insulated conductor subsequent to the step of moving;
• generating an error signal which is the difference between the measured capacitance and a standard capacitance value;
• generating a set speed signal which represents that an operating line speed for advancing the conductive element through the extruder has been attained;
• generating, in response to the set speed signal, a second preheat control voltage as a function of line speed and the error signal;
• applying the second preheat control voltage to the preheat power supply to generate a preheat voltage in relation to the line speed and the error signal;
• applying the preheat voltage to the selected length of the bare conductive element previously preheated to the initial temperature to alter the temperature of the conductive element;
• generating, in response to the set speed signal, a quench-point control voltage as function of the error signal;
• and
• applying the quench-point control voltage to a servo amplifier which facilitates movement of the cooling bath relative to the extruder to adjust the quench-point of the insulated conductor.

A system for controlling the expansion of cellular insulation in the manufacture of an insulated conductor having a conductive element and a layer of cellular plastic insulation extruded thereabout, which comprises:

• an extruder;
• means for preheating a bare conductive element to an initial temperature;
• means for advancing the preheated conductive element through the extruder whereat a layer of cellular insulation is extruded thereabout to form an insulated conductor;
• means for quenching the insulated conductor at a point to stop the expansion of the layer of cellular insulation;
• means for measuring the coaxial capacitance of the insulated conductor subsequent to being quenched by the quenching means;
• means for generating an error signal which is the difference between the capacitance measured by the measuring means and a standard capacitance value;
• and
• means for adjusting the temperature of the conductive element as a function of line speed and the error signal and adjusting the point of quenching the insulated conductor as a function of the error signal whereby the adjusting of the temperature and the point of quenching controls the expansion of the layer of cellular insulation.

A system for controlling the expansion of cellular insulation in the manufacture of an insulated conductor having a conductive element and a layer of cellular plastic insulation extruded thereabout, which comprises:

• a preheat power supply;
• a cooling bath;
• means for generating a first preheat control voltage as a function of line speed wherein the line speed is greater than a predetermined minimum speed;
• means for applying the generated first preheat control voltage to the preheat power supply to generate a preheat voltage;
• means for applying the preheat voltage across a selected length of the bare conductive element to heat the element to the initial temperature;
• an extruder;
• means for advancing the preheated conductive element through the extruder whereat a layer of cellular insulation is extruded thereabout to form an insulated conductor;
• means for advancing the insulated conductor through an air gap whereat the layer of cellular insulation expands about the conductive element;
• a cooling bath mounted for movement relative to the extruder;
• means for moving the cooling bath relative to the extruder;
• means for moving the insulated conductor into the cooling bath at a quench-point to stop the expansion of the layer of cellular insulation;
• means for measuring the coaxial capacitance of the insulated conductor subsequent to the step of moving;
• means for generating an error signal which is the difference between the measured capacitance and a standard capacitance value;
• means for generating a set speed signal which represents that an operating line speed for advancing the conductive element through the extruder has been attained;
• means for generating, in response to the set speed signal, a second preheat control voltage as a function of line speed and the error signal;
• means for applying the second preheat control voltage to the preheat power supply to generate a preheat voltage in relation to the line speed and the error signal;
• means for applying the preheat voltage to the selected length of the bare conductive element previously preheated to the initial temperature to alter the temperature of the conductive element;
• means for generating, in response to the set speed signal, a quench-point control voltage as function of the error signal;
• and
• means for applying the quench-point control voltage to the means for moving the cooling bath relative to the extruder to adjust the quench-point of the insulated conductor.