US3621200A

US3621200A - Heating element and packaging machine equipped therewith

Info

Publication number: US3621200A
Application number: US772160A
Authority: US
Inventors: Ridley Watts Jr
Original assignee: American Packaging Corp
Current assignee: AMPAK Inc A CORP OF DE; American Packaging Corp
Priority date: 1968-10-31
Filing date: 1968-10-31
Publication date: 1971-11-16
Anticipated expiration: 1988-11-16

Abstract

A machine for thermoforming and skin packaging is equipped with electric heaters having very fast response and high efficiency. In the preferred embodiment, a corrugated Nichrome ribbon heater element is helically wound around an elongated cylindrical core of quartz or other material transparent to radiant energy with the corrugations in substantially point contact with the core. In an alternative embodiment, an uncorrugated ribbon heater element is helically wound around an elongated quartz core having a generally star-shaped transverse cross section. In both embodiments the heater element is stress relieved in the helical configuration and is in relaxed condition on the core. Convolution of the ribbon may be of varying concentration along the core to provide nonuniform heat distribution where that is desired.

Description

United States Patent [72] inventor Ridley Watts, Jr.

' Cleveland, Ohio [211 App]. No. 772,160 [22] Filed Oct. 31, 1968 [45] Patented Nov. 16, 1971 [73] Assignee American Packaging Corporation Hudson, Ohio Continuation-impart of application Ser. No. 726,513, May 3, 1968.

[54] HEATING ELEMENT AND PACKAGING MACHINE [56] References Cited UNITED STATES PATENTS 638,236 12/1899 Gold.... 338/296 X 638,388 12/1899 Gold. 338/296 X 1,147,388 7/1915 Gold 338/302 X 1,688,168 10/1928 Whittaker 219/353 X 2,261,496 11/1941 Happe et a1 219/552 X 2,530,806 1 H1950 Boxrud 338/296 X 2,747,072 5/1956 Lawser 219/538 X 2,965,868 12/1960 Eichler 219/347 UX 3,114,822 12/1963 Boland 219/352 X 3,398,263 8/1968 Ortiz et al..... 219/343 X 3,462,580 11/1967 Shibata et a1. 338/296 X 1,415,240 5/1922 Hynes 338/332 2,677,748 5/1954 Nayloru. 219/354 UX 3,218,437 11/1965 Laing 338/278 X 3,377,770 4/1968 Rorer 53/112 A FOREIGN PATENTS 26,704 8/1914 Great Britain 338/321 277,388 9/1927 Great Britain. 219/357 414,550 8/1946 Italy 219/539 306,940 7/1955 Switzerland 219/542 10,765 5/1913 Great Britain 338/282 633,110 12/1949 Great Britain 219/356 Primary E.\'aminer--A. Bartis Attorney-Watts, Hoffmann, Fisher & Heinke ABSTRACT: A machine for thermoforming and skin packaging is equipped with electric heaters having very fast response and high efficiency. in the preferred embodiment, a corrugated Nichrome ribbon heater element is helically wound around an elongated cylindrical core of quartz or other material transparent to radiant energy with the corrugations in substantially point contact with the core. in an alternative embodiment, an uncorrugated ribbon heater element is helically wound around an elongated quartz core having a generally star-shaped transverse cross section. in both embodiments the heater element is stress relieved in the helical configuration and is in relaxed condition on the core Convolution of the ribbon may be of varying concentration along the core to provide nonuniform heat distribution where that is desired,

HEATING ELEMENT AND PACKAGING MACHINE EQUIPPED THEREWITH CROSS-REFERENCES TO RELATED APPLICATIONS l. This is a continuation-in-part of application Ser. No. 726,513, filed May 3, 1968, by Ridley Watts, Jr., under the title, Heating Element and Packaging Machine Equipped Therewith."

2. U.S. Letters Pat. No. 3,501,886 issued Mar. 24, 1970 on an application filed Sept. 18, 1967 by Ridley Watts, Jr., et al.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to electric heaters and to mechanisms utilizing electric heaters and more particularly to a skin packaging machine equipped with such heaters.

2. The Prior Art Several types of electric heaters have been used in skin packaging and thermoforming equipment. All of these prior heaters have sufiered from the common disadvantage of slow response times. The result has been relatively long cycles. In order to shorten the cycle time, proposals have been made to maintain heaters in either a partially or wholly energized condition even when heat from them is not required or desired. This causes operator discomfort and is wasteful of electric power and heat.

The heaters which have been used in this include:

I. Heaters in which the heater element is encased within a quartz tube. Certain of these quartz heaters have relatively quick response time as a source of radiant heat but all quartz heaters have been very slow sources of convection heat. The reason for this is that the quartz tubes are relatively transparent to radiant energy and a considerable time element elapses before the tube itself becomes hot. It is necessary for the quartz tube to be hot before efficient convection heating can be achieved by passing air over the heated tube.

2. Metal-sheath-type heaters. In heaters of this type, a resistance element is embedded in an insulating material. The insulating material is surrounded by a metal sheath. These heaters reach their operating temperature only after relatively long periods of time. In addition, they are relatively large in cross-sectional area, both to provide sufficient mechanical strength to support their own weight and because of the materials which surround the heater element.

3. Coil heaters of spirally wound Nichrome or other high-resistance wire. The coils are supported at spaced locations by insulators. These heaters also must be of relatively large cross section to support their own weight. They also suffer from a disadvantage that in time the wire becomes stretched and sags.

4. Ribbons of resistance material which suffer from deficiencies quite similar to coil heaters. They, too, must be of relatively large cross-sectional areas to support their own weight.

Like coil heaters, the ribbon tends to become fatigued and stretched with repeated cycling and it then commences to sag. Expedients such as springs have been attempted to overcome this sagging problem but the springs in time become fatigued due to many factors including the heat to which they are exposed. Moreover, the springs tend to accelerate the stretching of the ribbon in that they maintain the ribbon under tension when heated and inhibit the return of the heater to its original length when it cools.

When a coil or ribbon heater commences to sag, advantages result. These disadvantages include nonuniform heat distribution, interference with articles positioned below the heater, and early failure of the heater elements.

type of equipment several dis- 2 SUMMARY OF THE INVENTION With the present invention, a very thin, flat, high-resistance ribbon is crimped to corrugate it. This corrugated ribbon is then helically wound around a core which is preferably a tube of quartz. This provides, for the first time, a highly efficient heater element which is mechanically supported throughout its length in a manner which not only does not interfere with its heat transfer characteristics but, in fact, enhances them. The advantages of this heater element include:

1. Extremely high surface area relative to the cross-sectional area of the ribbon. This not only contributes to extremely fast response time, but extremely efficient use of the resistance material both from the standpoint of the amount of material required and the amount of electricity required to produce the desired amount of heat energy. A principal reason for the efficiency as to use of electricity is that the heaters are energized when, but only when, needed.

2. A very high amount of surface area per unit volume of oven or the like is provided for extremely rapid and efficient heating of any given space.

. An exceptional source of radiant energy. Since the quartz core is essentially transparent to radiant energy, the helically wound heater element behaves as if it were, from the standpoint of radiant energy, suspended in air. if the reflector is positioned behind the element, all of the large surface area can radiate a given object being heated. That is, the object receives radiant heat from all heater element surfaces either directly or by reflection off the reflector.

4. It is an extremely efficient source of convection heat. When air is passed over the heater, the element corrugations cause turbulence and the air is in direct contact with the heater element. This produces a very efficient heat transfer from the element to the air passing over it.

5. Cycle times of machines equipped with the heaters are reduced as compared with the prior machine. The quick response, coupled with the convection efficiency produces this result.

6. Expansion and contraction of the heater element is, absorbed along the core without affecting the overall length of the heater or causing fatigue of the element. Each portion of a corrugation expands and contracts along its own axis. Since many portions of the corrugations are transverse to the core, it means that the expansion and contraction has both radial and axial vectors. The entire wound ribbon acts in the manner of a weak spring so that it can slide somewhat as expansion occurs, absorbing much of the expansion. in addition, some slight amount of harmless loosening around the core occurs when the heater is hot but the total effect is there is little change in position of the heater element on the core as it expands and contracts.

7. The heater has an extremely long life for several reasons.

' One is the heater element is fully mechanically supported throughout its length so that expansion and contraction does not, for the reasons described above, cause fatigue of the element. Another reason for the increased life is the fast response times due to the thinness of the heater. Tests have shown that the heater may be brought from an ambient temperature of about 70 to its full operating temperature of 1,700 F. or l.800 F. in 0.75 seconds or less. Because of this the heaters are shut off when not in use and heater degradation due to unneeded heating is eliminated.

8. It is quick to install and replace. Quartz coefficient of expansion. Accordingly, ment is cycled, the overall length of the heater is affected only nominally. Cuplike end caps are used as the terminals for the heater elements. The combination of relatively stable heater length and the end caps permits the heater to be mounted in a mounting resembling a carhas a very low as the heater eletridge fuse mounting. Thus, installation and replacement are efficient.

9. The watt density is selective. That is the watts of energy converted to heat need not be uniformly distributed along the heater. To accomplish this, the heater element is spatially distributed along the core by helically winding the ribbon at any pitch or pitches desired. Thus, the pitch of the heater convolutions need not be uniform from end to end. In that portion of the heater which has more element convolutions per unit of length, more watts of energy are used and more heat is produced.

10. The element is extremely inexpensive, being considerably less expensive to manufacture and install than any of the described prior heaters.

11. Minimized heat losses to surrounding and supporting structures. Since the crimped corrugations are transverse of the ribbon, and the ribbon is spirally wound, the base of each corrugation is essentially only in point contact with the core. This point contact minimizes element-tocore friction as the element expands. It also minimizes any heat-sink effect the core may have in absorbing heat energy from the resistance element, further contributing to the extremely high efficiency of the heater.

In an alternate form of the invention, an uncrimped coil is wound around the core of generally star-shaped cross section. Here, again, heater-core contact is essentially point contact and substantially all, if not all, of the advantages listed above are achieved.

Accordingly, a principal object of the invention is the provision of a new and improved electrical heat source.

Other objects and advantages of the present invention will become apparent from the following detailed description made with reference to the accompanying drawings which form a part of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a film packaging machine embodying the present invention;

FIG. 2 is a perspective view of the heater element assembly;

FIG. 3 is an elevational view ofa heater element embodying the present invention having portions shown in cross section and portions removed;

FIG. 4 is a cross-sectional view as seen from the plane indicated by the line 4-4 of FIG. 3; and,

FIG. 5 is a sectional view of a heater assembly of another form.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIG. 1 of the drawings, a film packaging machine is shown which is adapted to be supported on a table or stand. The machine shown in the drawings is, apart from the heaters and associated structure, the machine which is described and claimed in the above-cross-referenced U.S. Pat. No. 3,501,886. Accordingly, it will be described only briefly. The machine 10 has a rectangular base 12 with a control panel 14 on a front surface of the base. Two spaced, upright support columns 18, I9 extend from two rear corners of the base 12. A heating unit is supported by the support columns l8, 19 extending forwardly in cantilever fashion and spaced above the base I2.

In use, a sheet of card stock and products to be packaged are mounted on the base 10. A sheet of thermoplastic film is secured to frame 24. The heating unit 20 heat softens the film. The frame is then lowered, a vacuum is drawn, and ambient air pressure forces the film onto the card stock.

The heating unit 20 includes a rectangular hood 26 and a rectangular oven 28 carried beneath and surrounded by the hood 26. A blower and motor unit 29 at the rear of the oven 28 and hood 26 is provided to cool the heaters after each heat cycle. The hood is supported by the

support columns

18, 19 for vertical adjustment. For access, the oven 28 is pivotal relative to the base 12, to the position shown. In use the oven parallels the base 12.

The oven 28 includes a reflector 30 which also functions as a heating-element-supporting plate. The reflector 30 has a depending flange 30a at its periphery. Apertures 32 are provided in the reflector 30 to receive supporting brackets 34 for heating elements, and to provide passageways through the reflector 30 for air circulation. Heating elements 36 are supported on the lower surface of the reflector 30 by the brackets 34. The specific construction of the heating elements 36 is described in considerably greater detail presently.

The cycle time for a machine 10 of the type described depends upon the material and thickness of the film. Prior cycle times have been quite long. One reasonfor the relatively long heating cycles is that polyethylene or similar film is essentially transparent to infrared radiation and, accordingly, the heat transfer from the heating elements to the film must take place primarily by convection.

In prior art constructions, the heating elements typically reach an operating temperatures of approximately 1 ,800" F. in not less than 4 to 6 seconds. The cycle time required to elevate prior heaters from 70 F. to l,800 F. is considerably longer once they have been turned off.

According to the present invention, a new and improved high-speed air heater is provided which minimizes radiation losses and conductive heat losses from a heating element so that both radiantand convection-heat transfer is maximized. The speed of the heater is so fast the heating elements are cooled by the blower 29 each cycle to maintain cycle times constant whether the machine is used intermittently or virtually constantly.

The heating element is supported through its length by a member which is transparent to radiation or at least nonabsorbent of radiant heat. This provides a very large surface area of heating element per unit volume of relatively thin, resistance material. Further, the element cooperates with its support so that failure due to thennal stressing of the element does not occur, yet burning out of the element due to shorting is avoided.

A preferred form of a heater embodying the present invention is illustrated in FIGS. 3 and 4. The heater 36 includes a supporting body I01 including a tubelike core 102 and end caps I03, 104. The end caps 103, I04 are formed by an electrically conductive material and are connected to the reflector 30 by the electrically conductive, springlike brackets 34 similar to fuse clips.

A heater element 105 is helically wound about the support 102 and extends between the end caps I03, 104. The element 105 is preferably constructed of Nichrome or other high-resistance material. It is of rectangular, cross-sectional configuration and has a relatively small, cross-sectional area.

Corrugations or ribs 106 are formed in the ribbon I05 transversely of its length. When the ribbon 105 is wound about the support 102, the corrugations extend at a slight angle with respect to the axis of the helix. These corrugations provide an increase in mechanical and structural strength of the ribbon and other advantages described presently.

The illustrated ribbon 105 is wound on the support or core 102, rather snugly. The convolutions may be uniformly spaced. Conversely, if it is (for example) desired to have more heat at one end of the element than the other, one may provide more convolutions at the one end. Thus, one may control the watt density-that is, the heat distributionalong the heater. Varying coil distributions for achieving a desired heat distribution are shown in FIG. 1.

Once the ribbon has been wound, it is thermally set in place. This is accomplished by heating the element beyond its normal operating temperature. This stress relieves the ribbon. The ribbon, once so overheated, retains the helical configuration which has been provided in the winding operation, whether used in horizontal or vertical applications.

Each of the end caps 103, 104 includes a peripheral groove I10 receiving an end of the ribbon 105. A snapring III is disposed in the groove I10 and tightly engages the ribbon to insure electrical contact between the opposite ends of the ribbon and each end cap. As noted previously, the end caps are preferably formed of a suitable electrically conductive material so that a circuit through the ribbon 105 is established when the end caps are connected across an electrical power supply. Thus, the end caps serve as terminals and the need for conventional end terminals is eliminated. If the end caps are nonconductive the snaprings 111 serve as terminals.

As is best seen in FIGS. 4 and 5. the ribbon 105 is coiled about the support 102 so that each corrugation is in contact with the support 102. Because the corrugations are skewed relative to the helix axis and because the tube is cylindrical. the locations at which the ribbon 105 engages the support are defined by points of contact or, at most, relatively short line segments. Thus, the total area of engagement between the ribbon 105 and support 102 is extremely small and minimizes any heat-sink effect of the support.

When the ribbon 105 is connected across a power supply to elevate the temperature of the ribbon to approximately 1,800 F., thermal expansion of the ribbon occurs. Expansion of the ribbon results in increasing the length of the helix and increased size of the corrugations. The increased corrugation size is occasioned by radial vectors which have been explained previously. While there is some loosening of the ribbon on the core, it is hardly detectable visibly. The springlike construction substantially maintains ribbon-to-support contact and some axial movement of convolutions of the helix relative to the support I02 occurs. In this manner, thermal expansion of the resistance element is absorbed by the element itself without fatigue. The windings of the helix are spaced sufficiently far apart that contact between adjacent convolutions does not occur as a result of the thermal expansion.

The support 102, as noted previously, is elongatedpreferably generally tubelike or rodlike-and is preferably constructed of a quartz or other material which is substantially nonabsorbent of or transparent to infrared radiation; i.e. radiant heat. The exterior surface of the support 102 is smooth so that engagement between the convolutions of the ribbon 105 and the support is characterized by relatively low frictional forces. This permits free axial movement of the ribbon relative to the support 102 during thermal expansion and contraction without stressing the ribbon.

In addition to the previously noted functions of the corrugations in the ribbon I05, it should be noted that these corrugations additionally encourage turbulence in the air which flows across the heater 100. Furthennore, the corrugations provide radiating ribs and thus greater heat transfer areas than would be available utilizing a helical ribbon of the same size without corrugations.

FIG. 5 is very similar in performance to the construction of FIGS. 3 and 4. Here the support or core 150 is somewhat star shaped when viewed in cross section. This provides spaced contact points 151 for the convoluted ribbon 152. The ribbon 152 may be corrugated, but is shown as noncorrugated.

Although the invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed.

1. In a packaging machine for applying a heat-softenable film to a product:

a. a support structure defining a chamber open on a side;

b. a filmsupporting frame;

c. means connecting said support structure and said filmsupporting frame for relative movement toward and away from each other whereby said open side of said chamber is substantially closable by said frame when supporting a film;

d. at least one heater unit supported by said support structure in said chamber;

c. said heater unit comprising:

I. an elongated core member extending between spaced connections to said support structure;

2. a heater element member comprising a thin ribbon of electrical resistance material wound in a helical configuration about said core member, said ribbon of resistance material having been stress relieved in said helical configuration and being in a substantially relaxed condition on said core member;

. said members having confronting surfaces formed for engagement at a plurality of small area contact loca tions and spaced apart between said contact locations so that heat losses from said heater member by conduction to said core member are minimized and convective heat transfer to air in the vicinity of said heater element is maximized;

4. electrical connector means supported at opposite ends of said heater member and electrically connected to said heater member; and

f. said heater member being electrically resistance heated on completion of an electrical circuit therethrough and heating air surrounding said unit to create convective air currents in said chamber for heating and softening film carried on said film-supporting frame.

2. In a packaging machine as claimed in claim 1 wherein said core member is composed of a material which is substantially transparent to radiant heat whereby radiant heat transfer to said core member from said heater member is minimized.

3. In a packaging machine as claimed in claim I wherein said support structure includes a radiant heat reflector member and said heater unit is supported adjacent said reflector, said reflector operative to reflect radiant heat toward said open side of said chamber.

4. IN In a packaging machine as claimed in claim I wherein said ribbon is corrugated transverse to its length, said contact locations being defined by engagement between portions of said corrugations and said core member, the remaining surfaces of said corrugations disposed for convective heat transfer with air in said chamber.

5. In a packaging machine as claimed in claim 1 wherein said electrical connector means each comprises an end cap member supported on an end of said core member and a fastener member for clamping said heater element to said end cap.

6. In a packaging machine as claimed in claim 5 wherein said end caps each include a circumferential groove and said fastener member comprises a snapring resiliently clamping said element in said groove.

7. In a packaging machine as claimed in claim 1 wherein the pitch of said helical coils of said heater member varies along said core member so that the distribution of heat produced along said heater member can be controlled by varying the pitch of said coils.

8. In a packaging machine as claimed in claim I wherein said core member is of generally star-shaped cross-sectional configuration.

9. An electrically energized heater assembly comprising:

a. an elongated core member defining a relatively smooth substantially continuous outer peripheral surface and comprised at least in part of a material which is substantially nonabsorbent of radiant heat;

b. a ribbonlike heater element member of electrical resistance material formed in a helical configuration extending about said core member and supported on said core at spaced locations, said ribbonlike element having been stress relieved in said helical configuration and being in a substantially relaxed condition on said core member;

c. said ribbonlike element having transverse corrugations including first corrugation portions engaging said core member to define said spaced locations and second corrugation portions spaced from said core member;

d. said spaced support locations defined by small low friction contact areas on said members to minimize heat conduction from said heater member to said core member, to maximize the exposure of said heater element member to ambient atmosphere and to minimize the frictional engagement between said heater member and said core member;

e. said second corrugation portions spaced from said core member for transferring heat to said ambient atmosphere and creating a turbulent flow of said atmosphere about said assembly; and,

. electrical contact elements connected to said heater member adjacent ends of said core member.

10. An electrical heater as claimed in claim 9 wherein said core member material is quartz.

11. The assembly of claim 9 wherein said contact elements are a spaced pair of cuplike members telescoped over ends of the core member.

12. The assembly claimed in claim 9 wherein the pitch of said helically wound heater element member on said core member is varied proceeding along said core member whereby the quantity of heat transferred from said assembly is nonuniform along said assembly.

13. The assembly claimed in claim 12 wherein said heater element member is stress relieved in situ on said core member.

14. The assembly claimed in claim 9 wherein said core member and said heater element are unrestrained against thermal expansion and contraction and said assembly is devoid of support means compensating for thermal expansion or contraction.

15. A convection heater comprising:

a. a support structure defining a chamber having an open side;

b. at least one electrically energized heater unit in said chamber for transferring heat to air in said chamber;

c. means for detachably connecting said at least one unit to said support structure;

(1. said heater unit comprising:

. an elongated core member defining an electrically nonconductive exterior wall which is substantially nonabsorbent of radiant heat;

2. a ribbonlike electrical resistance heater member formed in a helical configuration extending about said core member and supported thereby, said heater member having been stress relieved in said helical configuration and being in a substantially relaxed condition on said core member;

. said ribbonlike member having a thickness which is extremely small as compared to its width and defining transverse corrugations;

4. said ribbonlike member engaging said core member at a series of small area low friction contact locations on a helical path along said core member to minimize conductive heat transfer to said core member and expose a maximum area of said resistance heated member to ambient air whereby to create turbulent circulation of air about said unit; and,

e. said heater member being freely slidable on said core member while supported thereby and said heater being devoid of means compensating for thermal expansion and contraction of said heater member relative to said core member.

16. An electrical heater assembly comprising:

a. an elongated core member;

b. a heater element member supported on said core member;

0. means for connecting said heater element member to an electrical power supply;

d. said heater element member comprising a ribbon of transversely corrugated electrical resistance material wound in a helical configuration about said core member, portions of corrugations lightly engaging said core member at spaced locations and with said ribbon unrestrained against movement along said core member; and,

c. said resistance material being stress relieved in said helical configuration whereby helical coils of said heater member are substantially relaxed when supported on said core member.

17. A heater assembly as claimed in claim 16 wherein coils of said helically configured heater member are nonuniformly distributed along said core member.

18. A method of convection heating comprising:

a. transversely corrugating a ribbon of resistance heating material;

b. winding said ribbon in a helical configuration on an elongated core member to supportingly engage said ribbon and said core at a plurality of spaced-apart small area locations with said ribbon being unrestrained against movement along said core member;

c. stress relieving said helically wound ribbon in situ on said core member by heating said ribbon substantially beyond the normal operating temperature thereof for relaxing said ribbon in said helical configuration for maintaining the ribbon coils distributed as desired along said core member;

d. completing an electric circuit through said ribbon and providing sufficient electrical power to said ribbon to elevate the temperature thereof from a room temperature level to an operating temperature of at least about l,700 F. in less than 4 seconds; and,

e. exposing major surfaces of said heated ribbon to said gaseous atmosphere and creating turbulent flows of heated gas about said ribbon and said core.

19. In a packaging machine for applying a heat-softenable film to a product:

a. a support structure defining a chamber open on a side;

b. a film-supporting frame;

c. said heater unit comprising:

1. an elongate core member extending between spaced connections to said support structure;

2. a heater element member comprising a thin ribbon of electrical resistance heating material helically wound about said core member;

3. said members having confronting surfaces formed for engagement at a plurality of small area contact locations and spaced apart between said contact locations so that heat losses from said heater member by conduction to said core member are minimized and convective heat transfer to air in the vicinity of said heater element is maximized;

4. electrical connector means connected to said heater element member comprising end cap members supported on opposite ends of said core member and fastener members for clamping said heater element member to said end caps;

f. said heater element member being electrically resistance heated on completion of an electrical circuit therethrough and heating air surrounding said unit to create convective air currents in said chamber for heating and softening film carried on said film-supporting frame; and g. said end cap members each including a circumferential groove and said fastener members each comprising a snapring resiliently clamping said heater element member in said groove.

20. An electrically energized heater assembly comprising:

a. an elongated core member defining a relatively smooth substantially continuous outer peripheral surface and comprised at least in part ofa material which does not absorb radiant heat;

b. a ribbonlike heater element member of electrical resistance heating material wound helically about said core and supported on said core at spaced locations;

c. said ribbonlike element having transverse corrugations including portions engaging said core member to define said spaced locations and portions spaced from said core member;

d. said spaced locations defined by small low friction contact areas on said members to minimize heat conduction from said heater member to said core member, to maximize the exposure of said heater element member to ambient atmosphere and to minimize the extent of the frictional engagement between said heater member and said core member;

e. said corrugation portions spaced from said core member transferring heat to said ambient atmosphere and creating a turbulent flow of said atmosphere about said assembly;

. electrical contact elements connected to said heater a snapring in each groove for tightly compressing a respective end portion of said heater element against the base of the respective groove. at least one of said cuplike members or said snaprings in electrical contact with said heater element member at said end portions.

Claims

1. In a packaging machine for applying a heat-softenable film to a product: a. a support structure defining a chamber open on a side; b. a film-supporting frame; c. means connecting said support structure and said filmsupporting frame for relative movement toward and away from each other whereby said open side of said chamber is substantially closable by said frame when supporting a film; d. at least one heater unit supported by said support structure in said chamber; e. said heater unit comprising: 1. an elongated core member extending between spaced connections to said support structure; 2. a heater element member comprising a thin ribbon of electrical resistance material wound in a helical configuration about said core member, said ribbon of resistance material having been stress relieved in said helical configuration and being in a substantially relaxed condition on said core member; 3. said members having confronting surfaces formed for engagement at a plurality of small area contact locations and spaced apart between said contact locations so that heat losses from said heater member by conduction to said core member are minimized and convective heat transfer to air in the vicinity of said heater element is maximized; 4. electrical connector means supported at opposite ends of said heater member and electrically connected to said heater member; and, f. said heater member being electrically resistance heated on completion of an electrical circuit therethrough and heating air surrounding said unit to create convective air currents in said chamber for heating and softening film carried on said film-supporting frame.

3. In a packaging machine as claimed in claim 1 wherein said support structure includes a radiant heat reflector member and said heater unit is supported adjacent said reflector, said reflector operative to reflect radiant heat toward said open side of said chamber.

3. said ribbonlike member haVing a thickness which is extremely small as compared to its width and defining transverse corrugations;

4. electrical connector means supported at opposite ends of said heater member and electrically connected to said heater member; and, f. said heater member being electrically resistance heated on completion of an electrical circuit therethrough and heating air surrounding said unit to create convective air currents in said chamber for heating and softening film carried on said film-supporting frame.

4. In a packaging machine as claimed in claim 1 wherein said ribbon is corrugated transverse to its length, said contact locations being defined by engagement between portions of said corrugations and said core member, the remaining surfaces of said corrugations disposed for convective heat transfer with air in said chamber.

4. said ribbonlike member engaging said core member at a series of small area low friction contact locations on a helical path along said core member to minimize conductive heat transfer to said core member and expose a maximum area of said resistance heated member to ambient air whereby to create turbulent circulation of air about said unit; and, e. said heater member being freely slidable on said core member while supported thereby and said heater being devoid of means compensating for thermal expansion and contraction of said heater member relative to said core member.

4. electrical connector means connected to said heater element member comprising end cap members supported on opposite ends of said core member and fastener Members for clamping said heater element member to said end caps; f. said heater element member being electrically resistance heated on completion of an electrical circuit therethrough and heating air surrounding said unit to create convective air currents in said chamber for heating and softening film carried on said film-supporting frame; and g. said end cap members each including a circumferential groove and said fastener members each comprising a snapring resiliently clamping said heater element member in said groove.

8. In a packaging machine as claimed in claim 1 wherein said core member is of generally star-shaped cross-sectional configuration.

9. An electrically energized heater assembly comprising: a. an elongated core member defining a relatively smooth substantially continuous outer peripheral surface and comprised at least in part of a material which is substantially nonabsorbent of radiant heat; b. a ribbonlike heater element member of electrical resistance material formed in a helical configuration extending about said core member and supported on said core at spaced locations, said ribbonlike element having been stress relieved in said helical configuration and being in a substantially relaxed condition on said core member; c. said ribbonlike element having transverse corrugations including first corrugation portions engaging said core member to define said spaced locations and second corrugation portions spaced from said core member; d. said spaced support locations defined by small low friction contact areas on said members to minimize heat conduction from said heater member to said core member, to maximize the exposure of said heater element member to ambient atmosphere and to minimize the frictional engagement between said heater member and said core member; e. said second corrugation portions spaced from said core member for transferring heat to said ambient atmosphere and creating a turbulent flow of said atmosphere about said assembly; and, f. electrical contact elements connected to said heater member adjacent ends of said core member.

15. A convection heater comprising: a. a support structure defining a chamber having an open side; b. at least one electrically energized heater unit in said chamber for transferring heat to air in said chamber; c. means for detachably connecting said at least one unit to said support structure; d. said heater unit comprising:

16. An electrical heater assembly comprising: a. an elongated core member; b. a heater element member supported on said core member; c. means for connecting said heater element member to an electrical power supply; d. said heater element member comprising a ribbon of transversely corrugated electrical resistance material wound in a helical configuration about said core member, portions of corrugations lightly engaging said core member at spaced locations and with said ribbon unrestrained against movement along said core member; and, e. said resistance material being stress relieved in said helical configuration whereby helical coils of said heater member are substantially relaxed when supported on said core member.

18. A method of convection heating comprising: a. transversely corrugating a ribbon of resistance heating material; b. winding said ribbon in a helical configuration on an elongated core member to supportingly engage said ribbon and said core at a plurality of spaced-apart small area locations with said ribbon being unrestrained against movement along said core member; c. stress relieving said helically wound ribbon in situ on said core member by heating said ribbon substantially beyond the normal operating temperature thereof for relaxing said ribbon in said helical configuration for maintaining the ribbon coils distributed as desired along said core member; d. completing an electric circuit through said ribbon and providing sufficient electrical power to said ribbon to elevate the temperature thereof from a room temperature level to an operating temperature of at least about 1,700* F. in less than 4 seconds; and, e. exposing major surfaces of said heated ribbon to said gaseous atmosphere and creating turbulent flows of heated gas about said ribbon and said core.

19. In a packaging machine for applying a heat-softenable film to a product: a. a support structure defining a chamber open on a side; b. a film-supporting frame; c. means connecting said support structure and said film-supporting frame for relative movement toward and away from each other whereby said open side of said chamber is substantially closable by said frame when supporting a film; d. at least one heater unit supported by said support structure in said chamber; e. said heater unit comprising:

20. An electrically energized heater assembly comprising: a. an elongated core member defining a relatively smooth substantially continuous outer peripheral surface and comprised at least in part of a material which does not absorb radiant heat; b. a ribbonlike heater element member of electrical resistance heating material wound helically about said core and supported on said core at spaced locations; c. said ribbonlike element having transverse corrugations including portions engaging said core member to define said spaced locations and portions spaced from said core member; d. said spaced locations defined by small low friction contact areas on said members to minimize heat conduction from said heater member to said core member, to maximize the exposure of said heater element member to ambient atmosphere and to minimize the extent of the frictional engagement between said heater member and said core member; e. said corrugation portions spaced from said core member transferring heat to said ambient atmosphere and creating a turbulent flow of said atmosphere about said assembly; f. electrical contact elements connected to said heater member adjacent ends of said core member, said contact elements comprising cuplike members telescoped over opposite ends of the core member, each of said cuplike members having a peripheral groove, end portions of the heater element member projecting into a respective groove; and, g. a snapring in each groove for tightly compressing a respective end portion of said heater element against the base of the respective groove, at least one of said cuplike members or said snaprings in electrical contact with said heater element member at said end portions.