Improvements in or relating to heat sealing apparatus

摘要

1,032,039. Wrapping-machines. AMERICAN MACHINE & FOUNDRY CO. March 15, 1963 [March 16, 1962], No. 10395/63. Heading B8C. [Also in Division B5] Heat sealing apparatus in a wrapping machine comprises a travelling heating element 46, means 18 for presenting a succession of wrapped articles to the element 46 for engagement and movement therewith whilst at least part of their wrapping is heat sealed by the element 46, a stationary system constituting a mechanical analogue of the element 46 and including means adapted to simulate the conditions to which the element 46 is subjected in operation, and means responsive to said system to maintain the actual heat sealing temperature of the element 46 substantially constant at a desired temperature. In apparatus for sealing wrappers of bread loaves, a conveyer 40, Figs. 1A, 2 (both not shown), 3 and 4 (not shown), comprises unheated cross-bars 44 and chains 42. The bars 46 are electrical resistance heating elements having a " non-stick " coating, e.g. a copolymer of tetrafluoroethylene and hexafluoropropylene, and a crowned wrapper engaging surface. The bars 46 are energized by brush assemblies 76, 78 comprising a U- or V-shaped spring 86, Fig. 5 (not shown), having a shoe 88 to ride on a conductor rail 82, Fig. 3. A master heater bar assembly 100, Figs. 3 and 4 (the latter not shown), comprises a stationary heater bar 106 having an electrical resistance corresponding closely to that of the bars 46. A thermal detector 110 is secured by a winding 112 to the bar 106. A shaft 114 having a pair of vanes 116 extending tangentially therefrom is rotated in synchronism with the conveyer 40 to induce a flow of air across the bar 106 proportional to the speed of the bars 46 to simulate the convention losses experienced by the bars 46. The end flaps are sealed by similar assemblies 123, 124. The assembly 124 comprises a taut, driven endless wire mesh belt 126 having a " non-stick " coating and is heated by electrical resistance radiant heating elements 154, each having an oblong oval configuration, Fig. 6 (not shown). Curved reflectors 167 are placed behind each heater 154. The sole backing for the belt 126 along the wrapper engaging run thereof is a rod 169 which allows the belt to tilt thereabout if the end surface of the loaf is inclined to the vertical. Thermal sensing units 166, 1661, Fig. 2 (not shown), are provided on the outer runs of the assemblies 123, 124, respectively. The unit 166, Figs. 7 and 8 (both not shown), is suspended from a bracket 168 and comprises a yoke 174 pivotally mounted on a block 170 and having an insulating block 180 held therein by bolts 182 having their shanks journalled in sleeves 184 in the yoke arms. A shoe 186, covering a cutaway 185 in the block 180, has a thermistor 190 secured to the inside surface thereof and flatly engages the belt 126 at all times due to the gimballed mounting of the block 180. Apparatus 194, Fig. 1A (not shown), may be provided to supply end labels to the loaves and further sealing assemblies 195, 196 are preferably provided. The assemblies 195, 196 are similar to the assemblies 123, 124 and comprise belts 198, 1981, respectively, engaged by thermal sensing units 216, 2161. The loaves are held in contact with the conveyer 40 by a canvas belt 224, Fig. 1B (not shown), mounted on a frame 220 and bearing downwardly thereon. After heating, side belts 226, Fig. 2 (not shown), backed by plates 234 maintain pressure on the end folds. When the loaves leave the conveyer 40 they pass on to a conveyer 250, Fig. 1B (not shown), the upper run of which is supported by a table 252. Cooling apparatus 254 comprising refrigerated plate 256 and a table 258 is provided. In operation, loaves from a slicing machine 302, Fig. 9 (not shown), are delivered by conveyer flights 2, Fig. 1A (not shown), and transferred by a pusher bar 4 between spaced guide plates 7 to a lifter table 8 in the course of which a web of wrapping material is draped about the leading side of the loaf and first end folds are produced. The table 8 moves upwardly, plates 14, produce second end folds and downward movement of a lap roller 12 completes the draping of the wrapping material. The load is moved between third and fourth folding plates 16, 22 by conveyer flights 18 and concurrently therewith the web is severed by a knife 20 and rollers 26, 28 complete the bottom overlap. The flight 18 moves the loaf on to the conveyer 40 with which the flights 18 are synchronized so that the bottom overlap of the loaf wrapper rests on one of the bars 46, the bottom overlap being heat sealed during conveyance of the loaf. The power input to the bars 46 through the assemblies 76, 78 is periodically cut in and out by a relay 278, Fig. 9 (not shown), acting in response to a controller III responsive to the temperature of the bar 106 as sensed by the thermistor 110. During operation of the slicer 302 a relay 294 is energized through a switch 300 to cut in a bucking voltage transformer 292 which reduces by a certain percentage the input voltage to the bar 106 as compared to the line voltage to the bars 46, whereby the relay 278 remains energized for longer periods of time to slightly increase the power input to the bars 46. This compensates for the fact that the bar 106 is not subjected to heat losses directly to travelling articles as are the bars 46. When the slicer 302 is stopped, the relay 294 is de-energized and the input to the bars 46, 106 through lines 280, 108, respectively, becomes equal to prevent overheating of the bars 46. The power input to the assemblies 123, 124 is controlled by relays 268, 282, respectively, operated by controllers I, IV responsive to the belt temperatures sensed by the units 166, 1661 to maintain the belts 126, 1261 at the desired temperature. Similarly, the belts 198, 1981 are maintained at the desired temperature by relays 274, 288 operating in response to controllers II, V responsive to the units 216, 2161, respectively.