Heat seal temperature control

A heat sealing circuit comprising:

• (a) a heat sealing element;
• (b) an energization circuit coupled for intermittently applying electrical energy to said heat sealing element;
• (c) circuit means for monitoring an electrical parameter associated with at least one of said energization circuit and said sealing element, the value of said parameter being a function of heating element temperature, and
• (d) means for controlling the duty cycle of said intermittent energization circuit in response to said monitoring means.

An electrical heat sealing circuit comprising:

• (a) an electrically conductive heater element comprising a material whose resistivity is a function of temperature;
• (b) an energization circuit coupled to apply electrical energy to cause current flow through said heater element;
• (c) a sensing resistor in series with said heater element;
• (d) means for monitoring voltage drop across said sensing resistor, and
• (e) control circuitry responsive to said monitored voltage drop for adjusting the duty cycle of said energization circuit to stabilize heater element temperature at a predetermined value.

A method for maintaining and stabilizing temperature of an electrically conductive heater element coupled to an electrical energization circuit, said heater element having a resistivity which varies as an increasing function of temperature, said method comprising the steps of:

• (a) monitoring electrical current flow through said heater element, and
• (b) influencing operation of said energization circuit in response to said current monitoring step, as follows:
• (i) interrupting power applied to said heater element in response to a reduction in said monitored electrical current, and
• (ii) continuing application of electric power to said heater element in response to said current being greater than a predetermined value.

An electrical apparatus for effecting control of temperature of a sealing heater element, said apparatus comprising:

• (a) the electrically conductive heater element;
• (b) an electrical energization circuit coupled to intermittently apply electrical energy to said heater element, and
• (c) means for sensing electrical current flow through said heater element, and
• (d) control circuitry coupled between said current sensing means and said electrical energization circuitry, said control circuitry being operable to:
• (i) maintaining application of electrical power to said heater element in response to an increase in sensed electrical current, and
• (ii) interrupt electrical power to said heater element in response to a decrease in said sensed current through said heater element.

Apparatus for effecting packaging utilizing a web of inter connected heat-sealable bags, said apparatus comprising:

• (a) apparatus for advancing a succession of said bags along a path;
• (b) structure defining a loading station along said path past which said bags are successively advanced for loading;
• (c) structure defining a sealing station downstream along said path from said loading station;
• (d) a heat sealer element;
• (e) means for selectively moving said heat sealer element to a location proximate a bag when located at said sealing station to effect a heat seal of said bag;
• (f) an energization circuit coupled for intermittently applying electrical power for heating said sealer element;
• (g) a sensing resistor in series with said sealer element;
• (h) means for monitoring electrical current through said sensing resistor and said sealer element;
• (i) control means interposed between said sensing resistor and said energization circuit for controlling the duty cycle of said circuit as follows:
• (i) interrupting application of power to said sealing element in response to a reduction in sensed current, and
• (ii) continuing application of power to said sealer element in response to said current being of at least a predetermined value, and
• (j) structure defining a bag detaching station downstream along said path from said sealing station.

Apparatus and circuitry for effecting heat sealing, said apparatus and circuitry comprising:

• (a) an electrically resistive heater element comprising a material whose resistivity increases as a function of temperature;
• (b) a current sensing resistor in series with said heater element;
• (c) an energization circuit coupled for causing electrical current to flow intermittently through said heater element and said sensing resistor by intermittent application of a desirably fixed electrical potential;
• (d) means for monitoring electrical current through said sensing resistor;
• (e) control means coupled between said sensing resistor and said energization circuit for decreasing the duty cycle of said energization circuit in response to a decrease in said sensed current, and for increasing or maintaining the duty cycle of said energization circuit in response to said current being above a predetermined value;
• (f) means for sensing the magnitude of said electrical potential;
• (g) means responsive to said sensed potential to regulate operation of said energization circuit to compensate for undesirable changes in said desirably fixed potential to maintain said heater element at a substantially constant temperature notwithstanding undesirable potential changes;
• and
• (h) means for adjusting said predetermined current value.

A method for heat sealing layers of a thermoplastic material, said method comprising the steps of:

• (a) moving an electrically heatable resistance heating element proximate an area to be sealed prior to application of electric power to said heating element;
• (b) applying electric power to heat said heating element to effect a seal;
• (c) monitoring temperature of said heating element by measuring current flowing through said heating element in response to the application of a substantially constant voltage;
• (d) cutting off electric power to said heating element in response to said heating element having reached a predetermined temperature as indicated by said measured current;
• (e) maintaining said heating element proximate said area to be sealed subsequent to said electric power cut-off;
• and
• (f) moving said heating element away from said sealed area.

A system for maintaining a resistive heating element near a predetermined temperature, said system comprising:

• (a) a transformer having a primary and a secondary;
• (b) a circuit including the transformer secondary and said resistive heating element for applying electric current to said heating element;
• (c) means for sensing the amount of electric current flow in said heating element;
• (d) means for selectively coupling and decoupling said primary of said transformer to an external source of alternating electrical voltage;
• (e) tickle pulse means for initiating coupling of said transformer primary to said voltage source in response to a pulse generated by said tickle pulse means;
• (f) comparator means coupled to said coupling and decoupling means and responsive to said current sensing means for causing the decoupling of said voltage source from said transformer primary in response to the sensing of said current being less than a predetermined amount, and
• (g) said tickle pulse means comprising means for generating a subsequent pulse at a time subsequent to said decoupling.

A system for temperature stabilization of an electrically heated mainly resistive heating element, said system comprising:

• (a) a transformer having a primary and a secondary;
• (b) a sensing resistor whose resistance is known and small compared with the resistance of said heating element and means for sensing current flow through said sensing resistor;
• (c) means for conductively coupling said heating element and said sensing resistor in series to said transformer secondary in a circuit without substantial other resistive impedance;
• (d) means for selectively coupling and decoupling said transformer primary to a source of alternating electrical voltage;
• (e) means for initiating coupling of said primary to said voltage source, and
• (f) means associated with said coupling and decoupling means and responsive to current sensed in said sensing resistor being below a predetermined level to decouple said voltage from said transformer primary.