RADIANT HEATER OF SHORT RESPONSE TIME

Abstract: Plastic web to be fed into opposite dies for forming articles must be heated and softened by heater means in front of the forming machine, even if said machine is combined with an extruder supplying said plastic web. The invention provides a radiant heater adapted to be sensitively responsive to manual or automatic control so as to more suitably regulate a temperature of the plastic web in comparison with the conventional heater comprising quartz tubes each having a coiled nichrome wire. The heater of the invention comprises an extended thin nichrome electrical resistance heater element plate (30) arranged in a zig-zag on a support plate (11) and insulating plate (13).

Description: The present invention relates to a radiant heater to be preferably used for heating plastic web to be fed to a molding machine. There are various formed articles such as buckets and various vessels, cabinets for electric or electronic appliances, inner boxes of refrigerators and the like, which are formed by feeding a plastic web previously suitably heated and softened between separated male and female dies which are then moved to engage with each other for molding. The temperature, at which the web is to be heated, is very important in order to attain satisfactory molding and varies depending on the kind of the thermoplastics material used, such as polyvinyl chloride, polystyrene, and acrylonitrile/Butadiene/styrene copolymer. Said temperature, for instance for high impact polystyrene, is about 140°C. A typical heating device for the forming machine comprises a plurality of quartz tubes arranged side by side so as to heat the web as uniformly as possible. Each tube contains therein a coiled wire of electrically high resistant material which must be hard to be oxidised, such as nickel alloy added with about 20% chromium which is usually called nichrome. Such a heating device is usually arranged above the plastic web to be supplied to the molding machine, and as occasion demands, further another heating device is arranged below said plastic web. In order to carry out efficient forming, the plastic web must be heated as fast as possible, and at the same time it must be avoided that the plastic sheet is excessively heated in order to attain satisfactory molding. Thus, heat generation of the heater is to be controlled by energisation and deenergisation of the heater, or by adjustment of voltage or current of electric energy supplied thereto, or by employing the above two methods in combination, manually or according to a programmable controller with detecting the temperature of the plastic web. Such control is, however, not always easy, as well known by those skilled in the art. Even if deenergisation of the heater or decrease of electric energy thereto is made well before the temperature of the plastic sheet reaches at the desired temperature, it often becomes too high, as explained later in more detail. In other words the conventionally used heater is too slow in response to said control. In CH-A-620 076 there is disclosed a radiant heater having all the features of the preamble of Claim 1 to be used in a vacuum oven. An object of the invention is, thus, to provide a radiant heater of a short response time so as to be preferably used for heating plastic web to be fed to a molding forming machine. Another object is to provide such heater of simple construction, to be readily assembled. Such objects may be attained according to Claim 1, which will be appreciated for those skilled in the art when studying the explanation of a preferred example to be made hereafter in reference to the accompanying drawing, by providing a radiant heater comprising a support plate; an electrically insulating plate extending over the surface of one side of said support plate: an electrical resistance heater element comprising a plurality of elongated thin plate members extending longitudinally above said insulating plate and spaced transversely thereof, being electrically connected at the free ends thereof by a transversely extending thin plate member of the same material so as to form said heater element in a zig-zag; and a plurality of cup insulators, transversely arranged in two rows with a space between each adjacent pair that said element plate members are held therebetween. Each of said element plate members is longitudinally bent so that the transverse section thereof is substantially V-shaped and preferably each of said cup insulators has a body portion, a leg portion protruded so as to pass through said insulating plate and said support plate to be fixed thereto and a head portion forming an annular groove between said head portion and said body portion. In the drawings: Fig. 1 is a plan view of a preferred embodiment of the invention partly cut away, Fig. 2 is a section taken along a line II-II in Fig. 1, Fig. 3 is a similar but fragmentary view to a larger scale, Fig. 4A is a graph of temperature against time for the conventional heater and for the plastic sheet heated thereby, and Fig. 4B is a similar graph relating to the heater according to the invention. In Figs. 1 to 3, the radiant heater according to the invention comprising a table plate 11 having four side walls 12 forming a rectangular pan and an insulating plate 13 made of mica and extended over the bottom wall of said pan-like table plate 11. A cup insulator made of sintered alumina represented generally by 20 has a body portion 21, a leg portion 22 downwardly protruded therefrom and a head portion 23. Between said head portion 23 and said body portion 21, there is formed a neck or an annular groove 24, as best shown in Fig. 3. Said cup insulator 20 is mounted on the insulating plate 13 by penetrating the leg 22 through a hole formed in said plate 13 and the table plate 11 and by fixing said leg 22 to said plate 11 with a snap ring 25. As best shown in Fig. 1, a plurality of cup insulators 20 (eight are in this embodiment) are transversely arranged, leaving a space between adjacent insulators, in each of two rows (total sixteen insulators). An elongated thin plate represented generally by 30 and made of nichrome is longitudinally bent 31 so that the transverse section there of is V-shaped as best shown in Figs. 2 and 3. A plurality of such elongated thin plates 30 (seven are in this embodiment) are transversely arranged side by side, leaving a space between every adjacent two, which are electrically connected at the ends by nichrome thin plate 40 so as to form a heater element extending in zig-zag from the left-top corner to the right-bottom corner of the table plate and consequently of the insulating plate in Fig. 1. Said heater element is preferably coated with black paint of a good infrared radiation property. Said connecting thin plate 40 is preferably bent so that the sloped half thereof may closely be fixed to the sloped surface of the elongated thin plate for instance by welding. Of course it is possible to form said heater element by punching without using the separate connector piece 40. It is possible to mount the elongated thin plate 30 between the adjacent two cup insulators 20 of two rows by engaging the side edges of said elongated plate with the annular grooves 24 of the insulators and then fixing said transverse connector thin plates 40. It is also possible to mount a heater unit, formed in one piece by previously welding the transverse thin plates to the longitudinal thin plates, by placing the former on the latter and pushing yieldable edge portion of the longitudinal thin plate 30 into the annular groove 24 of the relevant cup insulator 20. A pair of terminals 50, 50, each of which comprises a screwed bolt 51, an insulating sleeve 52 surrounding said bolt 51 at the middle portion, an upper nut 53 and lower nuts 54, are arranged at opposite corners of the plates 11, 13 and mounted by inserting said screwed bolt into through hole formed in said both plates. Each of the free ends of the heater element is formed with a slit so as to engage with said bolt 51 and electrically connect therewith by careful tightening of said nut 53. Between the lower nuts 54, 54 a lead wire from the electric source not shown is electrically engaged. It is preferable to provide a bar 55 downwardly protruded from the table plate 20 for readily mounting the heater to a machine frame not shown. A sensor or thermocouple 61 detecting the temperature of the heater element is arranged preferably at the centre of the heater device and fixed directly to the heater element. Said sensor 61 has a lead 62 connected therewith so as to send electrical signal representing the temperature to a temperature controller not shown. In Fig. 4A, changes of temperatures of the quartz tube heater, as referred to in the beginning of the specification as typical heater for the forming machine, and of the plastic web heated thereby in course of time, during which said heater was controlled so as to prevent the overheating, are shown by curves respectively in dotted and solid lines. The two dot phantom line represents targeted temperature of the plastic web to be heated. Three tubes of 12mm diameter and 500mm length were arranged side by side, leaving a space of a few cm between any adjacent two and energised by applying a voltage of 200V. Due to the quartz tube absorbing heat, the phantom curve rising up to the point (30 seconds) where the heater is to be controlled, in reply to the signal of temperature detected by the sensor, is inaccurate and the phantom curve slowly falls down due to the absorbed heat of the tube, as a result of which the solid curve representing the plastic sheet temperature does not fall down so soon so that there is inevitably caused a hatched overheated area beyond the targeted temperature line When the heater is controlled earlier than that, the plastic web can not be sufficiently softened. In either case satisfactory molding formation can not be attained. In contrast with the above, when using the four heater units each as illustrated in the drawings and of 122mm x 122mm, which were so arranged as to form a regular square and applied with voltage of 21V for reasons of safety, as shown in Fig 4B the temperature of the heater and consequently of the unit plastic web rises up sooner and more sharply so that said heater could be controlled 20 seconds after the energisation thereof and the solid line representing the temperature of the heated plastic web may be kept in compliance with the targeted temperature line.

Claims: A radiant heater comprising a support plate (11) having an obverse overlaid with an electrically insulating plate (13); a plurality of elongated electrical resistance thin plate members (30) arranged above the insulating plate (13) in parallel with each other and leaving a space between each adjacent pair, each adjacent two plate members (30) being electrically connected at the free ends thereof by a transverse thin plate (40) so as to form as a whole a heater element in the form of zig-zag configuration; and a plurality of cup insulators (20) planted to the support plate (11) and transversely arranged in two rows so as to hold each of the elongated thin plate member (30) in pairs, characterized in that each of said elongated thin plate members (30) is longitudinally bent (31) so that the transverse section thereof is substantially V-shaped, and that each cup insulator (20) has an annular groove (24) for retaining the opposed side edges of the elongated thin plate member (30) by using the annular grooves of the adjacent two cup insulators (20). The radiant heater according to claim 1, characterized in that each cup insulator has a body portion (21), a leg portion (22) protruded so as to pass through said insulating plate (13) and said support plate (11) to be fixed thereto, and a head portion (23), and that the annular groove (24) is provided between said head portion (23) and said body portion (21). The radiant heater according to claim 1 or claim 2, characterized in that the support plate (11) is rectangular and said heater element extends in zig-zag from a point adjacent one corner of said rectangular support plate (11) to another point adjacent the opposite corner thereof and a terminal (50) is mounted respectively at each of said points so that the two ends of said zig-zag heater element are respectively electrically connected with said terminals (50). The radiant heater according to any one of claims 1 to 3, characterized in that a bar (55) is mounted on said support plate (11) at the reverse side thereof for attachment of said heater to a supporting structure.

Inventors: ASANO, KAZUO
Applicants: KABUSHIKI KAISHA ASANO KENKYUSHO
Application year: 1986
Main classification: H05B3
IPC codes: H05B3