Patent GB-900517-A (GB)
Title: Electric resistance surface heating devices
IPC: H05B3/00
Filed: 1957-07-24
Abstract: 900,517. Electric resistance heating films or foils. EISLER, P. July 18, 1958 [July 24, 1957], No. 23480/57. Class 39 (3). [Also in Groups VI, XXXV and XXXVI] A dispensible heating film forming an electric resistance of large surface and minute thickness is associated with a food package or eating appliance and has terminal areas for connection to a low voltage, e.g. 12-48 volts, not dangerous to touch. The heater is a flimsy metal pattern on a plastic film and forms the whole or part of a dispensible food package or may be placed on or in dispensible or permanent food containers, e.g. boxes, bottles, glasses, pans or dishes, food covers or table mats. The heating film for a food package is not intended for re-use. The resistance may be of thin titanium, tin, aluminium, or of lacquered steel, or may be a plastic film with a carbon or graphite filler, or a plastic film with a metal pattern deposited by vacuum evaporation or otherwise. The metal foil may be crimped and protected against contact with the food by a covering film of plastic. A bag 14, Fig. 1, for potato crisps has within it a metal foil heating loop 12 with its ends separated by an insulating strip 13 adhered thereto or bent from the bag, the top of which must be torn to provide access to terminals and to form an air vent during heating. Current is supplied through a detachable spring clip (see Group XXXVI). The foil 12 may be perforated or may be a metal strip slotted at opposite edges alternately to give a zigzag pattern supported by an insulating tape adhered thereto centrally along its length. A bag may be made having metal resistance pattern on a film of plastic, e.g. of P.V.C., paper, regenerated cellulose, polyester, polycarbonate, polythene or irradiated polythene. Aluminium foil is patterned by any printed circuit process, e.g. printing, etching, punching slots or by expansion and may be anodized. In Fig. 5, the foil has a long central slot 26 and apertures 25. The plastic film 31 has transverse reinforcing strips 29 and a thin continuous metal line 32 at each side of the foil, so that when the film is cut at C and folded at F to form a bag, current is passed through the notched lines 32 to seal the plastic film edges together. The foil may be coloured or decorative or may form lettering, and when it is outside the bag may be protected by a layer of plastic, plastic foam, paper varnish, or lacquer. When the heating foil is inside the bag, the outer side may be covered with metal foil. Such layers or metal foils may have on one side a selfadhesive coating for attachment to a surface, e.g. inside or outside a metal canister or other food container. For higher temperatures thin asbestos paper, glass fibres or P.T.F.E. may be the protecting layer. A sodium silicate adhesive may be used for bonding the metal foil to an insulating layer. The heater may be placed loosely in a canister and may be used for defreezing food, boiling, baking or frying. In Fig. 8 the heater 36 is formed as a tubular immersion heater with the top cut at a slope for easy pouring. A forked wedge connector 38 is inserted to lift the heater from the bottom of a bottle 37 to allow pouring of the liquid, which is thus heated during flow. Slices of food or biscuits 52 may be wrapped in a heating film which may extend between the slices; or the slices be wrapped separately in a long foil 53 with contact-strips 55. Slices, e.g. of meat, may be heated singly and separated at tear-off lines 54, or the wrapped slices may be placed separately between electrodes in a container and connected in series (Fig. 11, not shown). The heating film may be porous or perforated to allow escape of steam. The food may be heated more at the top or the bottom and may be squeezed out of such packages. In Fig. 12, a long polythene film carries a perforated aluminium foil 63 with intermediate bus-bars 64 which may be torn along a central perforated line 68 into lengths each of which carries a transverse polythene tape 65 so that on folding around a slab of food, the extending parts of the tape 65 may be folded over the ends of the slab to enclose it entirely. The food may be placed in trays on a conveyer each tray carrying at one edge a removable blade for severing the heating strip at 68. A paok having as top and bottom thin paper or plastic films or a heating foil may be reinforced by making its sides of corrugated cardboard, stiff plastic foam, honeycomb construction or paper mache and may be placed in a cardboard tray. Each pack may have two current paths connected in parallel for rapid heating and then be switched by timing or a temperature-sensitive device to connection in series for keeping warm. An outer tray may carry one or more heating films and a lamp indicating passage of current. Current may be supplied through a small transformer serving also as a heater on the tray or as a support. A preliminary rapid heating at a higher voltage may be arranged, and may be ended by a temperature-sensitive device. The packs may be arranged in a coin-operated vending device with warming lamps. A paper strip may carry two terminal end portions of a heating foil within a container and access to the terminals is obtained by pulling them out of a slot in the container by the outer end of the paper strip. A fuse may be formed by narrowing the width of a metal line of the heating pattern or by a bridge of solder between two lines held under tension and broken by'development of vapour pressure during cooking. The fuse may be a conductive adhesive comprising graphite or metal powder in a binder softening at a critical temperature. A heating film cornprising a patterned foil on a plastic film coated on the other side with unpatterned metal has the latter connected to earth; and the plastic may be chosen to melt at a critical temperature. The earthing current may operate a fuse or cut-out elsewhere. The earth may be a metal box and the adhesive holding the foil heater thereon may melt at the critical temperature. A fuse may be " exploded " by steam from a little water in a capsule. Temperature-sensing control elements may be incorporated in the food pack and be made a layer of the heating film. A drinking " straw " may be made from heating film and may have several contacts co-operating with contacts on a bottle cap. A heating film may be shaped to an egg or egg cup or may be attached to an egg crate and have a timer in the supply cable. When dry food is to be moistened during cooking, the liquid may be in a plastic bag attached to the film and having a seal breaking at a required temperature; or the dry food is placed in porous or perforated heating film coated with gelatine, sugar or chocolate soluble in hot water. A metal heating foil on a film of gelatine may surround a liquid or solid concentrated food or a sausage. Food is steamed by placing it in a partly perforated heating film in a container holding water or ice, and the steam may pass through the package to an outlet. The heating film is spaced from the food for roasting or grilling. The film may be stuck on soup plates, teapots, tea cosies or other table ware, or may form an eating appliance. The Provisional Specification describes an arrangement in which the supply is derived from the secondary windings of a transformer which is designed to act as a hot-plate to provide auxiliary heat for the package. The device also acts as a temperature-sensitive circuit element and it is used as a means of regulating the heating current through the foil. Specification 900,518 is referred to.