Abstract: A running web of thermoplastic material is uniformly relaxed across its width by monitoring the tension in its marginal sections and by adjusting the corresponding heaters adjacent the path of the marginal sections. The median section of the web is relaxed in response to generation of a signal which denotes the tension profile across the entire web and is obtained as a result of the processing of signals which denote the monitored tension in marginal sections of the web. Those sections of the web wherein the tension is higher are heated less intensively than the sections wherein the tension is lower.

Abstract: A pellet injector for injecting pellets of condensed gas, such as hydrogen, tritium or deuterium, into a reactor comprises an arrangement of pipe guns in a cylindrical arrangement. Each pipe gun which may be made from stainless steel, has a freezing section and three axially spaced metal rings of a heat conductive metal arranged in good thermal contact with the outer surface of the pipe gun at the freezing section thereof. Each of the rings is thermally connected to a cryostat, for example by means of a copper wire. The center ring is cooled to a temperature below the condensing temperature of the gas so as to condense the gas in the freezing section of the pipe gun to form a pellet therein, and the temperature of the adjacent outer rings are maintained at temperatures exceeding the temperature of the center ring so as to obtain a pellet of a desired size. The pellets formed in the pipe guns may be expelled therefrom by a pressurized gas force pulse and injected into a reactor.

Abstract: In a fluidized-bed granulating apparatus for continuously producing granular particles from a powdered raw material, while a granulated product is continuously exhausted in a substantially constant amount from the bottom or bottom side wall of a fluidizing chamber and is introduced into a classifying section where the granulated product is classified by classifying means thereby selecting and delivering the granules granulated to particle size greater than the desired size and returning ungranulated material to the fluidizing chamber, the pressure drop of the fluidized bed is detected to adjust the charging rate of the powdered raw material into the fluidized bed and the moisture content of the material to be granulated in the bed is detected to adjust the feed rate of a binding liquid, thereby producing a granulated product of a desired particle size.

Abstract: An apparatus for liquefying a thermoplastic, high-polymer material has a supply container for the thermoplastic material, heating means for heating the material in the supply container, a base part disposed beneath the supply container and containing a collecting trough for the liquefied thermoplastic material and control means regulating the heating means in dependence upon a comparison between a predetermined desired value and an actual value detected by a sensor. In order to be able to follow even rapid changes of the operating conditions the amount of liquefied thermoplastic material conveyed out of the collecting trough is detected as actual value and supplied to the control means; a suitable measure of this amount is in particular the speed of rotation of a pump serving as conveying means to convey the liquefied material from the collecting trough.

Abstract: A method and an apparatus for measuring and controlling foam sheet blow-up in a thermoformer oven. Control of the oven operation is achieved through monitoring the measured blow-up of a material in the thermoformer oven at a sensing location in a forming station receiving the material from the over in order to regulate the thickness of the plastic material prior to its conveyance into said forming station. This is effected by varying the rate of advance and dwelling time of the material in the oven, and optionally, also concurrently regulating the temperature in the oven to optimize control over material blow-up.

Abstract: A method and an apparatus for measuring and controlling foam sheet blow-up in a thermoformer oven. Control of the oven operation is achieved through monitoring the measured blow-up of a material in the thermoformer oven at a sensing location in a forming station receiving said material from said oven in order to regulate the thickness of the plastic material prior to its conveyance into said forming station.

Abstract: An injection nozzle for an injection molding machine having a heating device for heating its injection nozzle body (3) protruding from the front end of the heating cylinder (1) thereof. The heating device is composed of a plurality of heating members (7, 10) longitudinally arranged side by side. The temperatures of the different nozzle body sections heated by the respective heating members are individually detected (9, 11), the heating members (7, 10) being individually controlled in accordance with the temperatures thus detected. The construction enables the different nozzle body sections to be individually set to a desired, optimum temperature. The nozzle front end section 3b, for example, which is in contact with a metallic mold and whose temperature is consequently apt to be lowered can be kept high, and the nozzle middle-rear section 3a, which is near the heating cylinder and whose temperature is consequently apt to be raised, can be kept low.

Abstract: A spinner assembly for a cotton candy machine. The spinner assembly includes a tubular wall member having an upright wall provided with slots. A tubular heater element is supported by the heater element in close proximity with the slots. The wall member is of aluminum material. A dielectric anodized coating on the wall member insulates the wall member from the heater element.

Abstract: A method for identifying measured quantity proportions for the length difference between an optical element and the envelope for the optical lead when forming an optical lead, which has the envelope loosely receiving the optical element and the method includes extruding the envelope around the element and then passing the lead through at least one cooling path. The speed of the envelope is identified both proceeding as well as following the cooling path, and the desired quantities are formed by different uses of the two measured speeds.

Abstract: Machined steel blocks 1 and 2 have ridges 20 facing valleys in the opposite block. Polypropylene tape 3 to be heat-treated has sliding contact transversely over each ridge on each block. The blocks are heated by heaters 4. The gap between the blocks is adjustable to suit the material being processed. The gap is large enough so that the tape passing between the blocks is not constrained to deflect from coplanarity by more than its own thickness.

Abstract: The heat-sink capacity of a thick-walled article being cured with a hot liquid supplying heat to one side of the wall, allows the point of least cure (PLC) to receive a higher rate of cure equivalents even after the hot liquid is replaced with a cold liquid. In the mean while, the other side of the wall continues to be heated. Because such a change-over is effected before the PLC has received the minimum number of cure equivalents required to effect a cure at the PLC (a "pre-cure change-over"), a valving arrangement is provided which allows the change-over to be effected without permitting a pressure drop in the bladder sufficient to permit the tire to "blow" because it is as yet uncured when the change-over is effected. Such a pre-cure change-over cannot be made when steam is replaced with water at ambient temperature.

Abstract: An extrusion die-set is disclosed for extruding plastic film, in which the nozzle-lips (6,8) are heated, for the purpose of allowing production speed to be raised without losing film-quality. The heaters include aluminum rings (62,82) embedded in grooves in the dies near the nozzle lips. The surfaces (66,67,86,87) of the ring that face away from the nozzle lips are cut away, so that heat transfer to the bulk of the dies (60,80) is reduced. The disclosed heaters eliminate the problem of hot-spots and cold-spots around the circumference of the nozzle. The nozzle lips can run several degrees hotter than the bulk of the dies, using the disclosed heaters.

Abstract: An extrusion die-set is disclosed for extruding plastic film, in which the nozzle-lips (6,8) are heated, for the purpose of allowing production speed to be raised without losing film-quality. The heaters include aluminum rings (62,82) embedded in grooves in the dies near the nozzle lips. The surfaces (66,67,86,87) of the ring that face away from the nozzle lips are cut away, so that heat transfer to the bulk of the dies (60,80) is reduced. The disclosed heaters eliminate the problem of hot-spots and cold-spots around the circumference of the nozzle. The nozzle lips can run several degrees hotter than the bulk of the dies, using the disclosed heaters.

Abstract: An apparatus for molding and curing a resin matrix composite material under pressure and heat by a use of a bag provided in an autoclave for covering the material and applying the pressure to the material.

Abstract: An injection molding machine includes a numerical control system in communication with a host computer, the numerical control system having incorporated therein a computer, wherein the injection molding machine can itself start and stop in operation on the basis of stored data indicating that the injection molding machine is in a state capable of being operated or in a state incapable of being operated. Pre-processing is executed in response to a start-up signal from the host computer (steps S4 through S9). If a malfunction or abnormality happens during the pre-processing, the location and thus the existence of the abnormality is stored in memory of the numerical control system. After completion of the pre-processing, if the stored abnormalities are judged to be restorable on the basis of the storage contents in the memory, an abnormality restoration processing is executed (S17) and, subsequently, automatic operation is carried out (S11).

Abstract: A device for executing a method of molding expandable thermoplastic polymer beads in a mold to form articles of foamed material comprising: pulsing steam through at least one of the cavities to contact and expand the beads in the cavity for a finite period of time and drawing the steam amongst the beads by providing reduced pressure to at least one of the cavities for the same period of time; and pulsing mist through the expanded beads by providing mist to at least one of the cavities and drawing the mist through the expanded beads by providing reduced pressure to at least one of the cavities for the finite period of time of predetermined duration to form articles of foamed material.

Abstract: Method and apparatus are disclosed for inducing dimensional stability in cured composite structures using acoustic emission analysis to identify the appropriate level and extent of coincident thermal conditioning. Temperature of the composite structure is lowered at a bounded rate to a minimum temperature typical of the intended operating environment. Passive stress wave acoustic emissions of the composite are analyzed to generate a signature of stress relaxation events over time. When the rate of stress relaxation events decreases below a predetermined percentage of the maximum rate, and remains below that level for a period of time sufficient to predict operational stability, thermal conditioning is terminated.

Abstract: In a method of operating a screw extruder having a drive motor in which the speed of rotation of the drive motor is controlled, in order to achieve as constant a quality of product as possible, the speed of rotation is controlled depending on the specific energy input in such a manner that the specific energy input is maintained substantially constant. A control depending on the product temperature may advantageously be superimposed on the control keeping the specific energy constant in the form of a cascade control. A screw extruder for carrying out the method has a housing in which a screw is mounted, a drive motor having a variable speed for continuously driving the screw, feed devices for supplying basic products, additives and the like, and a control device for controlling the speed of rotation of the screw.

Abstract: In an injection molding machine combined with a peripheral apparatus, control units capable of interchanging signals are provided for the molding machine and the peripheral apparatus. Each control unit comprises a memory device storing set molding data applied with numbers or identifications. When the control unit of the molding machine sends the set data number or identification to the control unit of the peripheral apparatus to read out from the memory devices, the set molding data are formed when the numbers or identifications coincide with each other.

Abstract: A control process for foaming and fusing plastics in a foaming and fusing device, and a fusing device for carrying out the process, in which the control process works in cycles, each cycle consists of phases which are at least an injection phase, a heating phase, a fusion phase, a cooling phase and a mould release phase, such phases being started and completed depending on control criteria comprising a time period, a pressure or a temperature limit value for the conditions evolving in the device such control criteria belonging to a certain cycle comprising a control criteria set, and with each cycle quality values of predetermined characteristics determined by measurements taken from the respective plastic part or an agglomerate cake situated in the device, a new set of control criteria being generated for each new cycle from prior control criteria depending from the quality values measured with such prior cycle of the control process, wherein the quality values of each cycle are combined to a deviation which

Abstract: A pipe of thermoplastic polymer, e.g. polyvinylchloride is extruded by an extruder comprising an extrusion die (2) having a moulding end part (2a) with heating elements (11) located about said end part. The wall thickness of the pipe is controlled in order to obtain a pipe of a substantially even wall thickness. For said control the wall thickness of the extruded pipe is measured at 360 points equally distributed over the circumference and moreover the average is determined in eight measuring sectors comprising 45 measuring points per sector. The average of the measuring values of one measuring sector is compared with the circumferential average of the 360 points, the deviation of the desired wall thickness in said one sector being counter balanced by the action of the heating elements. In a preferred embodiment a polyvinylchloride pipe obtained by means of a two screw extruder presents an even wall thickness over the circumference.

Abstract: In a heat displacing type die adapted to extrude a sheet of molten resin, a heat displacing unit provided with a heater is secured to an upper die block. A lip gap is defined between a flexible lid and a stationary lip respectively provided for the upper and lower die blocks. The lower end of a bolt extending through the heat displacing unit is normally in contact with or secured to the flexible lip while the upper end is secured to the upper portion of the heat displacing unit. In a modified embodiment the bolt is air cooled.

Abstract: A curing press for tires is operated with a minimum curing time which will provide the near-optimum number of cure equivalents at the point of least cure without locating it. An average temperature for a particular period during the year is "set" in a programmable controller ("PC") based on minimum and maximum temperature measurements made in the curing room during the day and night over a preselected period of time. The actual ambient temperature is measured at the press, which temperature is essentially that of the green tire just before it is loaded in the prss. The PC controls operation of the press, and is used to provide a near-optimum but variable curing period, depending upon the difference between the "set" temperature and the actual (green tire) temperature, and whether it is greater than, or less than the "set" temperature, to provide a near-perfect cure without tracking the point of least cure.

Abstract: Composites such as graphite reinforced PMR-15 resin are cured in a heated press by following a temperature schedule and changing from low to high pressure after the start of resin cross linking. A pressure control and method to time application of high pressure uses temperature and dielectric sensor outputs and a semi-empirical model of the resin electrical conductivity. The model estimates the future variation of resin conductivity and determines time-to-go to a maximum in electrical conductivity. A signal to apply high pressure is generated when the estimated time to maximum conductivity falls below a threshold. On-line fitting of the model to sensor data as it is gathered provides a mechanism for adapting the control to the peculiarities of a given batch of resins and a particular cure schedule.

Abstract: This invention relates to an apparatus for regulating the heating of a thermoplastic band used in a station for thermoforming upwardly open containers, the apparatus being characterized in that, in a zone of the softened thermoplastic band, zone surrounded by a mesh of rigidified matter, the degree of deformation of this zone under the action of a predetermined mass is measured, the measured value of the degree of deformation is compared with a predetermined reference value corresponding to a good aptitude to thermoforming, and the heating of the band is regulated as a function of the value of deformation measured so that the deformation corresponds to the reference value.

Abstract: The calender rolls of the present invention are comprised of relatively thin-walled hollow shells which are supported at each end by shafts having bores which extend into the cavities formed in the rolls. Fittings located at the ends of the shafts are rotatable relative to them so that by tying the fittings together they do not rotate along with the rolls. The fittings at one of the ends of the rolls are connected to a source of wet steam through a pressure regulator and the fittings at the other ends carry probes which extend through the shafts and curve downwardly towards the bottoms of the rolls. A valve regulates the amount of material which flows out of the cavities through the probes. The method by which the rolls are used comprises introducing wet steam into the cavities, maintaining its pressure at a constant level.

Abstract: An extruder control system and method controls melt temperature to maintain viscosity of the melt at the die. A constant volumetric flow to the die is delivered by a gear pump, the die is a fixed orifice, and pressure between the gear pump and the die is indicative of viscosity. Changes in pressure are sensed and compensating changes in temperature are effected to return the pressure and, hence, the viscosity to its previous value.

Abstract: A method of producing a thermoplastic foil with reduced thickness tolerances in which close to the head the temperatures of the mass flow emerging from the heat at various regions corresponding to tempering sectors are detected, the temperature measurements are averaged and the averages are used to form a set point value with which the individual averages are compared and the comparison used to generate a correction signal which is applied to the temperature controller of the respective sector.

Abstract: An apparatus for jointing CV cables includes a lock mechanism which closes a split portion of a mold, used for extrusion-molding and opens the split portion by heat when crosslinking is performed.

Abstract: An injection molding apparatus for automatically presetting parameters of injection conditions by performing trial injection. A time (T0) and an injection quantity (C0) are obtained (step 124), wherein a drive current (I2) of a servo motor (M2) for performing trial injection reaches a drive current (i0, step 123) corresponding to a maximum injection pressure. Parameters (A, B, D) of the injection conditions stored in a memory, are read out in accordance with the detected time (T0) and the detected injection quantity (C0) to automatically set the injection conditions (step 131).

Abstract: An arrangement for the production of vitrified slag includes a rotating cooling drum provided with an internal cooling and on whose surface molten-slag-receiving recesses are provided. The site of impact of the slag is approximately at the uppermost point of the cooling drum. In order to ensure the vitrification of the slag with the flowing of molten slag off the cooling drum being reliably prevented, and an economic production of solidified slag being ensured, the recesses are separated by webs in the circumferential direction of the cooling drum and in the end region of the cooling drum are designed to be closed in the axial direction thereof. A plurality of cooled counter rolls contacting the webs by their surfaces and aligned with their axes parallel to the cooling drum are provided. They are arranged after the site of impact of the slag in the rotation direction of the cooling drum and one behind the other in its circumferential direction.

Abstract: A molten resin to be injected into a mold cavity in use of an injection molding machine has a temperature, i.e. viscosity difference between at the injection starting point and the injection completing point. In order to eliminate or make small this difference, a high temperature and highly pressurized gas, preferably an inert gas, is preliminarily supplied to and sealed in the mold cavity and the pressure of the supplied gas is continuously controlled in accordance with the position of the screw during the injection molding operation thereby to maintain the temperature of the supplied gas in the mold cavity to be higher than the softening temperature of the molten resin to be injected.

Abstract: A control apparatus for a vulcanizing machine in which vulcanizing is effected by introducing a vulcanizing medium into a rubber made product placed in a molding die and then discharging the same therefrom is disclosed, wherein said control apparatus includes a vulcanizing sequence timer adapted to an output function signal in response to a vulcanizing operation start signal on the basis of vulcanizing data comprising preset vulcanizing steps and functions, said vulcanizing data being read by said vulcanizing sequence timer, and a plurality of control valves for controlling introduction and discharge of a vulcanizing medium in accordance with said function signal outputted from the vulcanizing sequence timer whereby sequential function of the vulcanizing machine is automatically carried out.

Abstract: A heater is provided for heating an unwound portion of a web supported at a holder on a rotatable roll. A first pair of rollers between the holder and heater pull the web from the roll. A second pair of rollers advance the web after it passes through the heater.

Abstract: A heating and cooling jacket for the distributor of a injection-molding machine comprises tubular inner and outer sleeves and possibly an intermediate sleeve which defines compartments filled with a high-density particle compacted mass around the cooling tube turns which hug the inner sleeve and electric heater turns which hug the intermediate sleeve.

Abstract: A magnetic audio or video tape is smoothed and/or otherwise treated by causing it to pass through one or more calenders wherein a large-diameter primary calender roll is heated from within and the major part of its peripheral surface is surrounded by the running tape. The primary roll rotates about a fixed axis and is surrounded by a cluster of two or more smaller-diameter secondary calender rolls each of which has a viscoelastic peripheral layer and each of which can be individually biased against the adjacent portion of the running tape with a selected force. The magnetic film on the tape is out of contact with the secondary rolls as well as with the guide rollers which cause the tape to advance toward the peripheral surface of the primary roll and thereupon toward a takeup reel.

Abstract: A viscous sealing material is applied to a communications cable by being forced out of a container by means of a heatable pressure plate having a bore and a pump adjacent to the bore; the sealing material is conveyed by the pump through a conduit to a chamber, in which the sealing material is introduced under pressure into and around an already stranded cable core; the pressure plate includes a plurality of preferably helically coil electrical heating elements separated from each other and projecting therefrom for penetrating into the sealing material in the container when the pressure plate applies pressure to the sealing material so that it is progressively liquified and can be pumped out of the container.

Abstract: A rotational apparatus for molding dolls including an automatic control mechanism, a mold holding mechanism, a robot mechanism having a mold cover opening and closing means for opening and closing the mold, mold releasing means for releasing a molded product from the mold, mold release detecting and cleaning means and a raw material charging means, a heating means and a cooling means.

Abstract: The invention provides an installation and method for the continuous production of materials using exothermically hardening binders, such as gypsum or cement, comprising a mixing and feeding station and a calibrating unit or press and also a control system for operating the installation.

Abstract: A moulding apparatus and a method of moulding for components of unvulcanized elastomeric material which forms a spew free component and provides for handling the component for subsequent use. The apparatus as shown in the Figures comprises a first mould member having a mould cavity for part of the component and a feed port for material and a second complementary mould member which includes a detachable component carrying insert. The two mould members and are relatively movable to provide two different volumes for the mould cavity, the change to the smaller volume being used to return some material through the feed port and to complete moulding of the component.

Abstract: The calender rolls of a calender are provided with heat insulating protective shields which can be laterally pivoted open and with heat insulating end covers or closures at the ends of the rolls. At least one of the rolls is provided with a blower device for generating air flows or jets which can be regulated over the length of such roll to equalize the temperature over the length of this roll.

Abstract: At least one roll of the calender is heated and provided with heat insulating shields pivotably mounted on the sides of the calender and heat insulating end covers disposed at the ends of the rolls. A counter roll (controlled deflection roll) cooperates with the heated roll in the direction of its pressure or pressing action. The counter roll is provided with a cooling device (blower device) which is capable of being regulated in zones for cooling the surface of the counter roll.

Abstract: Confectionery solution containing gelling agents such as agar-agar, tends ry markedly to form streamers or tails when being poured. The formation of tails or streamers is inhibited in accordance with the invention by the atmosphere in the environment of the pouring region being maintained at (a) a temperature at least equal to the temperature of the confectionery solution and (b) a moisture content at least equal to the equilibrium moisture of the confectionery solution.

Abstract: A controlled temperature hot melt adhesive dispensing system including three closed loop temperature control arrangements along a flow path for hot melt adhesive extending from an upstream adhesive source to a downstream adhesive dispenser. As disclosed, each closed loop temperature control arrangement includes a comparator which compares the temperature sensed by a temperature sensor with a setpoint temperature to produce a control signal for a heater in the temperature control arrangement. The temperature control loops are located along the adhesive flow path so that the second closed loop temperature controller is upstream from the first closed loop temperature controller; and the third closed loop temperature controller is upstream from both the first and second temperature controllers. A first feedback circuit scales and integrates the control signal from the first location to produce a first feedback circuit output which adjusts the setpoint temperature at the second location.

Abstract: A hot melt adhesive dispensing system including a hot melt adhesive dispenser, a hot melt adhesive tank, and a hose for coupling hot melt adhesive from the tank to the dispenser. The dispenser, the tank and the hose are heated by electrical heaters to maintain the adhesive in a suitable molten condition for dispensing and for flowing from the tank to the dispenser. When the hot melt adhesive system is first energized, the hose and tank heaters are coupled to a source of electrical power and the temperature at either the tank or the hose is sensed. When the sensed temperature reaches a suitable preselected level, the dispenser heater is then energized by coupling the heater to a source of electrical power. In this way, as the adhesive in the dispenser is heated from a solid to a molten form, it is free to expand into the hose, with the adhesive in the hose adjacent the dispenser having already been heated to a molten state.

Abstract: A method and apparatus for forming a decorative seal on envelopes and the like is disclosed. The envelopes to be embossed are intermittently transported along a conveyor past various processing stations. At a first station, a quantity of heated molten seal material is deposited at a desired location on an envelope. The envelope is next transported to a first cooling station where a stream of cooled air is directed against the deposited molten material for partial cooling. At a subsequent stamping position, a cooled embossed die is applied to the seal material to form an embossed decorative pattern therein. The envelope is then transported to a refrigerated chamber which accelerates solidification of the stamped seal material to form an embossment. The timing of the intermittent movement of the conveyor is controlled in accordance with the quantity, temperature, and type of seal material used to form the embossment.

Abstract: An apparatus produces granules from two-phase mixtures wherein an initial substance is enriched with crystal nuclei in a temperature controlled precrystallizer and the resultant precrystallized melt is fully crystallized on a cooling surface. A droplet former is disposed over the cooling surface and includes inner and outer telescoping tubes. The inner tube communicates downstream with the precrystallizer and has a plurality of outlet orifices for conducting the precrystallized melt outwardly therethrough. The outer and inner tubes are arranged for relative rotation. The outer tube has a plurality of discharge orifices which are intermittently alignable with the outlet apertures in response to relative rotation between the tubes for discharging drops of precrystallized mass onto the cooling surface.

Abstract: A system is disclosed for extruding, drawing, vacuum molding or processing, or the like hot processing of plastomers or elastomers, which can conveniently utilize the hot air from the cooling process of its heated component parts. The system makes use, to heat the extruder cylinder, of ventilated electric thermal units wherein ventilation is performed in closed circuit fashion. In particular, arrangements are made to convey the hot air from the cited electric thermal units, through specially provided ducting, to areas of the system where application of heat is required.

Abstract: An apparatus for embossing a high resolution relief pattern such as a phase hologram or diffraction grating from a mold onto a thermoplastic material is disclosed.The apparatus comprises first and second adjacent disks adapted to rotate in opposite senses, the disks serving to define an embossing zone therebetween in which the circumferences of the disks exert pressure on one another. The first disk serves to transport the thermoplastic material to the embossing zone. To preheat the thermoplastic material, a preheating band of limited length is mounted under tension on the circumference of the first disk, the preheating band being resistively heated by a longitudinally flowing electric current. A resistively heated band-shaped embossing mold is mounted on the circumference of the second disk. The embossing mold and the thermoplastic carrier come into contact with one another in the embossing zone.

Abstract: In a plastic injection apparatus comprising a barrel, an axially reciprocable, rotatable, plastic working screw in the bore of the barrel, a nozzle for injecting melted plastic from out of the front of the bore into a mold, and a plurality of axially spaced heater bands ringing the barrel over its length, the heat control for such apparatus is improved by subdividing the barrel into rear and front parts, impeding outward flow of heat from the rear part by the use of thermal insulating material and a heat reflective surface, and promoting outward flow of heat from the front part by the use of cooling means and of a tubular barrel cover which encloses such front part and has inner and outer surfaces which are heat absorptive and heat reflective, respectively. In one embodiment, the mentioned thermal insulating material is incorporated in a tubular insulating shroud enclosing the heater bands in the rear part and of greater I.D. than the O.D. of such bands.