Thermography process and apparatus

In a thermography process running on thermographic equipment wherein sheets bearing image areas overlaid with thermoplastic powder particles are transported successively at a first transport rate through a heating chamber heated at a first rate of energy input to fuse together such particles on each sheet, and then through a cooling chamber for cooling fused image portions on the sheets, in which process periods of non-use occur with the thermographic equipment readied for processing such sheets but awaiting supply of them to be processed, the method which comprises during said non-use periods maintaining in the heating chamber a temperature sufficient for fusing said particles onto such sheets whenever passed through the heating chamber at said first transport rate, holding substantially closed passageways for sheets transport that normally are open at the infeed and the outfeed ends of said heating chamber, thus obstructing escape of heat from the heating chamber, and driving through said heating chamber the upper flight of an endless conveyor provided for transporting the sheets therethrough, the conveyor's speed reduced to a second transport rate slower than said first transport rate but sufficient to prevent damage to the conveyor by the heating of it as it passes through the heating chamber.

In a thermography machine including a heating chamber containing electrical heating means for heating sheets passed successively through said chamber to a temperature sufficient to fuse together and to each sheet thermoplastic powder particles overlying image areas of the sheet, a cooling chamber wherein each heated sheet is cooled to solidify fused image portions thereon and conveyor means for transporting the sheets through said a motor variable in speed for driving said conveyor means, heat control means for activating said heating means to maintain continually in said heating chamber a preset temperature suited for fusing said powder particles onto such sheets during passage of the sheets through the heating chamber, a control circuit to be disposed in a state establishing a standby mode of operation during non-use of the machine when the machine readied for processing sheets stands awaiting supply of sheets to be processed, and a speed control responsive to disposal of said control circuit in standby mode for causing said motor to drive said conveyor means at a reduced speed.

A thermography machine including a heating chamber, a cooling chamber, and a flight of an endless conveyor extending and movable through the chambers for transporting therethrough sheets bearing thermoplastic powder particles to be fused together in said chamber to form raised image portions on the sheets, said heating chamber having infeed and outfeed end openings that normally are open for passage therethrough of said conveyor flight with sheets thereon, each of said openings having a door movable across it to obstruct escape of heat from said heating chamber during standby periods of the machine's operation in which the machine though readied for processing such sheets stands awaiting supply of sheets to be processed, door positioning means for moving each of said doors to and away from closed position across the related chamber end opening, each said positioning means including a lever having an arm carrying and swingable to displace the door, means normally biasing said lever to a position in which the door is held away from the related chamber end opening, and means operable in each of said standby periods for displacing the lever so that it moves the door to closed position.

In a thermography machine including a heating chamber containing electrical resistance heaters for heating sheets passed successively through said chamber to a temperature sufficient to fuse together and to each sheet thermoplastic powder particles overlying image areas of the sheet, a cooling chamber wherein each heated sheet is cooled to solidify fused image portions thereon, conveyor means for transporting the sheets through said chambers, and heat control means for activating said heaters to maintain continually in said heating chamber a preset high temperature suited for fusing said powder particles onto such sheets during passage of the sheets through the heating chamber, said heat control means including a thermocouple positioned in said heating chamber and adapted to respond to a preset high temperature therein, a thermocouple heat control rendered operative to transmit current energizing said heating means in response to a temperature deficiency sensed by said thermocouple, and a proportional heat control operative to vary on/off cycles of the power supplied from an A.C. power supply line to said thermocouple heat control and thus control the wattage output to said heating means in proportion to variations of the voltage in said supply line.