GB1576142A

GB1576142A - Curved corrugated fibreboard structure and method of and apparatus for producing the same from initially flat blank

Info

Publication number: GB1576142A
Application number: GB27033/77A
Authority: GB
Original assignee: Nissan Motor Co Ltd
Current assignee: Nissan Motor Co Ltd
Priority date: 1976-07-09
Filing date: 1977-06-28
Publication date: 1980-10-01
Also published as: FR2357366B1; JPS537882A; DE2730997A1; FR2357366A1; DE2730997C2; JPS5523534B2; AU503583B1

Description

(54) CURVED CORRUGATED FIBERBOARD STRUCTURE AND METHOD OF AND APPARATUS FOR PRODUCING THE SAME FROM INITIALLY FLAT BLANK (71) We, NISSEN MOTOR COM PANY, LIMITED, a corporation organized under the laws of Japan, of No. 2, Takaramachi, Kanagawa-ku, Yokahama City, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to a corrugated fiberboard structure having a curved cross section and to a method of and an apparatus for producing such a corrugated fiberboard structure from a fiat blank.

While a corrugated fiberboard structure provided by the present invention may find a wide variety of practical applications especially where insulation of heat and/or sound and/or dampening out of mechanical vibrations is a serious requirement, such a fiberboard structure will prove useful particularly as interior linings for walls, floors and ceiling panels of residential or office buildings or as facings, trims and interlayers for various structural members of automatic vehicles, ships, boats or aircrafts, for its stiffness, excellent heat and sound absorbing abilities, light-weight construction, low production and installation costs and, inter alia, adaptability to compoundcurved surfaces.

Where a corrugated fiberboard structure is to be utilized as any of the above mentioned materials and the member to which the fiberboard structure is to be attached has a curved surface, it is required to have the fiberboard curved locally or Th to so as to conform to' the curved surface of the member. A corrugated fiberboard for use with such a member is therefore preliminarily deformed to have a curved cross section usually by the use of a powdered hot-press. If, in this instance, the fiberboard structure is deformed for a sharp bend, a furrow-like or ridge-like crease tends to be produced along the axis about which the fiberboard structure has been curved or bent.

Such a crease impairs the aesthetic and accordingly commercial values of the product. The fiberboard structure formed with the crease may be covered with an extra web or coating overlaying the creased spot of the structure so that the crease is concealed by the web or coating. The extra web or coating to be used for this purpose must have a thickness which is sufficient to take up the depth or height of the furrow-like or ridge-like crease in the fiberboard. Such a thick web would make the resultant fiberboard construction unduly bulky and unwieldy and would further impair the commercial value of the article. The present invention contemplates elimination of these problems.

It is, accordingly, an important object of the present invention to provide an improved corrugated fiberboard structure having at least one three-dimensionally curved portion and free from a crease in the curved portion.

It is another important object of the present invention to provide a method of producing such a curved corrugated fiberboard structure from an initially flat blank.

It is still another important object of the present invention to provide an apparatus suitable for putting the method into practice.

In accordance with one aspect of the present invention, there is provided an at least partially, and preferably three-dimensionally, curved corrugated fiberboard structure comprising at least one pair of spaced parallel linerboards and at least one corrugated interlayer board interposed between the linerboards and bonded at the tops of its ridges to the respective inner faces of the linerboards, characterized by at least one portion having a curved cross section and having a thickness smaller than the thick ness of the remaining portion of the fiberboard structure, the thickness of said portion having the curved cross section being stepwise reduced from said remaining portion.

In accordance with another aspect of the present invention, there is provided a method of producing an at least partially, and prefer ably three-dimensionally, curved corrugated fiberboard structure from an initially flat blank having at least one pair of spaced parallel linerboards and at least one corrugated interlayer board interposed between the linerboards and bonded at the tops of its ridges to the respective inner faces of the linerboards, comprising inelastically compressing at least one predetermined portion of said blank to a .thickness smaller than the thickness of the remaining portion of the blank by a stepwise reduction, and inelastically deforming said predetermined portion to have a curved cross section.

In accordance with still another aspect of the present invention, there is provided an apparatus for producing an at least partially, and preferably three-dimensionally, curved corrugated fiberboard structure from an initially flat blank having at least one pair of spaced parallel linerboards and at least one corrugated interlayer interposed between the linerboards and bonded at the tops of its ridges to the respective inner faces of the linerboards, comprising 'two rigid clamping members one of which has a partially recessed wall portion and the other of which has a partially protruded wall portion, .the clamping members being movable relative to each other into and out of predetermined relative positions having their respective partially recessed and protruded wall portions located closest to and in faceo4ace relationship to each other and forming therebetween a clearance having a configuration which is in conformity to the desired configuration of the fiberboard structure to be obtained, the above mentioned clearance having at least one reduced portion which has a cross-seotion curved from the remaining portion of the clearance and which is smaller in size than the remaining portion of the clearance, said reduced portion of said clearance being reduced stepwise from said remaining portion of the clearance. By preference, the apparatus having the above described general construction may further comprise heating means for heating each of the clamping members.

The drawbacks of prior-art techniques to produce a curved corrugated fiberboard structure and the features and advantages of a curved corrugated fiberboard according to the present invention and of a method of and an apparatus for producing such a fiberboard structure in accordance with the present invention will be more clearly understood from the following description taken in conjunction with the accompanying drawings in which:: Fig. 1 is a front elevation view showing, partly in section, a representative example of a prior-art apparatus to produce a curved corrugated fiberboard structure from an initially flat blank; Fig. 2 is a cross sectional view showing, to an enlarged scale, a portion of a curved corrugated fiberboard structure produced by the use of the prior-art apparatus illustrated in Fig. 1; Fig. 3 is a schematic sectional view showing, also to an enlarged scale, the clamping members or female and male dies forming part of a preferred embodiment of the apparatus according to the present invention; and Fig. 4 ,is a view similar to Fig. 2 but shows a portion of a curved corrugated fiberboard structure produced by the use of the apparatus including the clamping members or die blocks illustrated in Fig. 3.

Before entering into detailed description of the present invention, description will further be made regarding conventional techniques used to produce a curved corrugated fiberboard structure from an initially flat blank.

Referring to Fig. 1 of the drawings, a priorart apparatus for producing a curved corrugated fiberboard structure is constituted by a down-stroke type hydraulic hot-press 10 which comprises a lower stationary female die block 12 and an upper movable male die block 14 positioned above the stationary female die block 12. The movable die block 14 is mounted on a die carrier 16 which is fastened to the plunger of a hydraulic cylinder 18 supported on a top cross member of a frame structure 20. The frame structure 20 has a pair of spaced parallel vertical ports respectively forming vertical guide rails for the die carrier 16, though not shown. The upper die block 14 is thus vertically movable toward and away from the lower stationary die block 12 positioned below the die block 14.The die blocks 12 and 14 have embedded therein nestings of fluid circulating pipes 22 and 24, respectively, which are in communication with a source 26 of hot fluid through valved conduits 28 and 30, the conduit 30 for the movable male die block 14 being partially flexible so as to be partially movable relative to the hot fluid source 26 which is held stationary.

The lower stationary female die block 12 has an upper wall portion partially recessed to form therein a cavity or depression 32 which is open at the upper end of the die block 12 and which is configured in conformity to a desired overall configuration of a curved corrugated fiberboard structure to be obtained. On the other hand, the upper movable male die block 14 has a land 34 which protrudes downwardly and which is configured in such a manner that a certain clearance 36 having a uniform thickness throughout the total area thereof is formed between the partially recessed upper face of the lower die block 12 and the partially protruding lower face of the upper die block 14 when the upper die block 14 is moved into a predetermined lowermost position thereof above the lower die block 12 as indicated by phantom lines. The configurations of the respective upper and lower faces of the lower and upper die blocks 12 and 14 and accordingly the configuration of the clearance 36 to be formed between the die blocks 12 and 14 are determined to conform to a desired overall shape and a desired thickness of a curved cor rugated fiberboard structure which is to be produced from a blank 318. The blank 38 is constituted by a flat, double-faced or singlewall corrugated board structure which consists of a pair of spaced parallel linerboards 40 and 40' and a corrugated interlayer board 42 which is sandwiched between the linerboards 40 and 40' and which is glued or otherwise bonded at the tops of its ridges to the respective inner faces of the linerboards 40 and 40'.

For the formation of a curved corrugated fiberboard structure from such a blank 38, the die blocks 12 and 14 are warmed to a suitable predetermined temperature by the hot fluid through the fluid circulating pipes 22 and 24 from the hot fluid source 26 and the blank 38 is placed on the lower stationary female die block 12 with the upper movable male die block 14 positioned at a suitable spacing above the lower die block 12 as shown. The hydraulic cylinder 1;8 is then actuated to move the die carrier 116 downwardly until the upper movable male die block .14 reaches the pre determined lowermost position thereof forming the clearance 36 between the two die blocks 12 and 14.When the upper die block 14 is thus brought into mating engagement with the lower die block 12 across the blank 38, the blank is closely clamped and pressed upon between the respective upper and lower faces of the lower and upper die blocks 12 and 14 and is inelastically deformed into the configuration conforming to the entire configuration of the clearance 3.6 between the die blocks 12 and 14 with the agency of the heat transferred from the die blocks to the blank 318. The upper movable die block 14 is thereafter raised away from the lower stationary die block 12 and the curved corrugated fiberboard structure (not shown) which is now formed on the lower die block 12 is removed from the die block 12 for a subsequent cycle of operation.

When .the initially flat blank 38 is locally deformed in the above described fashion, the linerboard on the inner side of the curved blank is forced to locally shrivel and tends to produce therein a furrow-like or ridge-like crease along the axis about which the particular linerboard has been curved or bent as indicated at 44 in Fig. 2 in which the linerboard 40 is shown to be formed with a furrow-like crease. If the curved corrugated fiberboard structure thus formed with the crease 44 in the linerboard 40 is to be used with the outer face of the linerboard 40 left uncovered, the presence of the crease 44 would critically impair the aesthetic and accordingly the commercial values of the fiberboard structure.If an additional web or coating is to be applied to the outer face of the linerboard 40 for the purpose od concealing the crease 44, it is necessary that such a web or coating be thick enough to assuredly cover and conceal the creased spot. Such a thick web or coating makes the resultant corrugated fiberboard construction unduly bulky and unwieldy and would further impair the commercial value of the product, as previously discussed. A prime object of the present invention is to make it possible to produce a curved corrugated fiberboard structure which is free from a crease in the curved portion.

In a method according to the present invention, the portion or each of the portions to be curved in a three-dimensional fashion of an initially flat blank composed of at least one pair of spaced parallel linerboards and at least one corrugated interlayer board sandwiched and bonded between the linerboards is inelastically compressed to have a thickness smaller than ;the thickness of the remaining portion of the blank before the particular portion is to be inelastically deformed to have a curved cross section or, preferably, while the portion is being inelastically deformed to have a curved cross section. The portion to be curved of the blank is compressed preferably by applying a mechanical force to the blank in a direction substantially normal to the faces of the initial blank. The blank is preferably kept heated while the blank is being compressed and deformed.

An apparatus to put the method into practice in accordance with the present invention 'may be constructed and arranged largely similarly to the prior-art apparatus shown in Fig.

I but is characterize] over the prlor-arr apparatus by clamping members which are respectively constituted by female and male die blocks 46 and 48 shown in Fig. 3. Similarly to the female and male die blocks 12 and 14 of the prior-art apparatus shown in Fig. 1, the damping members or female and male blocks 46 and 48 of the apparatus according to the present invention have partially recessed and protruded wall portions 50 and 52, respectively, which are substantially coextensive with each other.These female and male die blocks 46 and 48 are movable relative to each other into and out of predetermined relative positions having their respective partially recessed and protruded wall portions 50 and 52 located closest and in face-to-face relationship to each other and forming therebetween a clearance 54 which has a configuration conforming to the desired configuration of a curved fiberboard structure, a portion of which is shown in Fig. 4.While the partially recessed and protruded wall portions of the female and male die blocks 12 and 14, respectively, of the priorart apparatus shown in Fig. 1 are so shaped that the clearance 36 formed therebetween is uniformly thick throughout the area of the clearance 36, the clearance 54 formed between the respective partially recessed and protruded wall portions 50 and 52 of th clamping members or female and male die blocks 46 and 48 of the apparatus according to the present invention is shaped in such a manner as to have at least one portion 54a which has a cross section curved from the remaining portion of the clearance 54 and which is smaller in size than the remaining portion of the clearance 54 so that the size ll of the particular portion 54a is less than the 12 of the remaining portion of the clearance 54.Fig. 4 shows a portion of an example of a partially curved corrugated fiberboard structure produced from an initially flat blank clamped between the die blocks 46 and 48 thus shaped, wherein the corrugated fiberboard structure is shown being of the doublefaced or single-wall type consisting of two spaced parallel linerboards 56 and 56' and a corrugated interlayer board 58 interposed between the linerboards 56 and 56' and bonded at the tops of its ridges to the respective inner faces of the linerboards 56 and 56'. The fiberboard structure further has a portion 60 which has been clamped in the reduced portion 54a of the clearance 54 between the die blocks 46 and 48 and which is therefore curved from the remaining portion of the fiberboard structure and clamped to a thickness smaller than the thickness of the remaining portion of the fiberboard structure.

The portion 54a of the clearance 54 is reduced stepwise as at 50a from the remaining portion of the clearance 54 so that the com- pressed and curved portion 60 of the corrugated fiberboard structure produced between the die blocks 46 and 48 is stepwise thinned from the remaining portion of the fiberboard structure as will be seen from Fig. 4. Such a configuration of the corrugated fiberboard structure is advantageous to exploit the original stiffness of a corrugated fiberboard construction .because the fiberboard structure is compressed over a minimum area. The stepwise reduced portion 54a of the clearance 54 between the die blocks 46 and 48 will form a dull edge 62 between the compressed and curved portion and the remaining portion of the corrugated fiberboard structure produced between the die blocks, as shown in Fig. 4.For this reason, the stepped surface should be formed on that die block which is to be in contact with that face of the fiberboard structure which is to be left uncovered when the fiberboard structure is in use. Furthermore, the reduced portion 54a of the clearance 54 between the die blocks 46 and 48 and accordingly the compressed and curved portion 60 of the resultant fiberboard structure may be uniformly thick throughout the areas of the portions or may be continuously thickened or thinned depending upon the radii of curvature of the curves cross sections of the portions.

Experiments have revealed that it is preferable that the thickness of the compressed and curved portion 60 of a fiberboard structure obtained by the present invention be within the range of from about one ninth.to about five ninths of the thickness of the remaining portion of the fiberboard structure. The clearance 54 between the die blocks 46 and 48 of the apparatus according to the present invention is therefore preferably so dimensioned that the size 1l of the reduced portion 54a of the clearance 54 is within the range of about one ninth to about five ninths of the size 12 of the remain ing portion of the clearance 54.If, thus, the initial flat blank to produce a curved corrugated fiberboard structure in accordance with the present invention is 5 millimeters thick and if the size 12 of the clearance 54 between the die blocks 46 and 48 is selected at 4.5 millimeters by way of example, then the size II of the reduced portion 54a of the clearance 54 may be within the range of from about 0.5 millimeter to about 2.5 millimeters. More preferably, the thickness of the compressed and curved portion 60 of a fiberboard structure to be obtained in accordance with the present invention is about one third of the thickness of the remaining portion of the fiberboard.The size li of the reduced portion 54a of the clear ance 54 between the die blocks 46 and 48 of the apparatus according to the presen.t inven tion should preferably be determined accord ingly with respect to the size l2 of the remain ing portion of the clarance 54.

In order to maintain the inelastic deforma- tion of a curved corrugated fiberboard, each of the clamping members or die blocks 46 and 48 of the apparatus according to the present in vention may be provided with heating means such as, for example, a nesting ob a hot-fluid circulation pipe communicating with a suitable source of hot fluid or an eleotric conductor connected to a suitable source of electric power, though not shown in the drawings. Such heating means is embedded in or otherwise in heat conductive contact with each of the die blocks for keeping each die block heated to a pre determined temperature when the die blocks are in use.

While the present invention has been described in connection with a corrugated fiberboard structure of the double-faced or singlewall type having two linerboards and a single corrugated interlayer board, it should be born in mind that the gist of the present invention is applicable to corrugated fiberboard structures of any other types having three or more linerboards and two or more corrugated interlayer boards.

WHAT WE CLAIM IS: 1. An at least partially curved corrugated fiberboard structure comprising at least one pair of spaced parallel linerboards and at least one corrugated interlayer board interposed between the linerboards and bonded at the tops of its ridges to the respective inner faces of the linerboards, characterized by at least one portion having a curved cross-section and having a thickness smaller than the thickness of the remaining portion of the fiberboard structure, the thickness of said portion having the curved cross section being stepwise reduced from said remaining portion.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

the clearance 54 and which is smaller in size than the remaining portion of the clearance 54 so that the size ll of the particular portion 54a is less than the 12 of the remaining portion of the clearance 54. Fig. 4 shows a portion of an example of a partially curved corrugated fiberboard structure produced from an initially flat blank clamped between the die blocks 46 and 48 thus shaped, wherein the corrugated fiberboard structure is shown being of the doublefaced or single-wall type consisting of two spaced parallel linerboards 56 and 56' and a corrugated interlayer board 58 interposed between the linerboards 56 and 56' and bonded at the tops of its ridges to the respective inner faces of the linerboards 56 and 56'.The fiberboard structure further has a portion 60 which has been clamped in the reduced portion 54a of the clearance 54 between the die blocks 46 and 48 and which is therefore curved from the remaining portion of the fiberboard structure and clamped to a thickness smaller than the thickness of the remaining portion of the fiberboard structure.

The portion 54a of the clearance 54 is reduced stepwise as at 50a from the remaining portion of the clearance 54 so that the com- pressed and curved portion 60 of the corrugated fiberboard structure produced between the die blocks 46 and 48 is stepwise thinned from the remaining portion of the fiberboard structure as will be seen from Fig. 4. Such a configuration of the corrugated fiberboard structure is advantageous to exploit the original stiffness of a corrugated fiberboard construction .because the fiberboard structure is compressed over a minimum area. The stepwise reduced portion 54a of the clearance 54 between the die blocks 46 and 48 will form a dull edge 62 between the compressed and curved portion and the remaining portion of the corrugated fiberboard structure produced between the die blocks, as shown in Fig. 4.For this reason, the stepped surface should be formed on that die block which is to be in contact with that face of the fiberboard structure which is to be left uncovered when the fiberboard structure is in use. Furthermore, the reduced portion 54a of the clearance 54 between the die blocks 46 and 48 and accordingly the compressed and curved portion 60 of the resultant fiberboard structure may be uniformly thick throughout the areas of the portions or may be continuously thickened or thinned depending upon the radii of curvature of the curves cross sections of the portions.

Experiments have revealed that it is preferable that the thickness of the compressed and curved portion 60 of a fiberboard structure obtained by the present invention be within the range of from about one ninth.to about five ninths of the thickness of the remaining portion of the fiberboard structure. The clearance 54 between the die blocks 46 and 48 of the apparatus according to the present invention is therefore preferably so dimensioned that the size 1l of the reduced portion 54a of the clearance 54 is within the range of about one ninth to about five ninths of the size 12 of the remain ing portion of the clearance 54.If, thus, the initial flat blank to produce a curved corrugated fiberboard structure in accordance with the present invention is 5 millimeters thick and if the size 12 of the clearance 54 between the die blocks 46 and 48 is selected at 4.5 millimeters by way of example, then the size II of the reduced portion 54a of the clearance

54 may be within the range of from about 0.5 millimeter to about 2.5 millimeters. More preferably, the thickness of the compressed and curved portion 60 of a fiberboard structure to be obtained in accordance with the present invention is about one third of the thickness of the remaining portion of the fiberboard.The size li of the reduced portion 54a of the clear ance 54 between the die blocks 46 and 48 of the apparatus according to the presen.t inven tion should preferably be determined accord ingly with respect to the size l2 of the remain ing portion of the clarance 54.

In order to maintain the inelastic deforma- tion of a curved corrugated fiberboard, each of the clamping members or die blocks 46 and 48 of the apparatus according to the present in vention may be provided with heating means such as, for example, a nesting ob a hot-fluid circulation pipe communicating with a suitable source of hot fluid or an eleotric conductor connected to a suitable source of electric power, though not shown in the drawings. Such heating means is embedded in or otherwise in heat conductive contact with each of the die blocks for keeping each die block heated to a pre determined temperature when the die blocks are in use.

While the present invention has been described in connection with a corrugated fiberboard structure of the double-faced or singlewall type having two linerboards and a single corrugated interlayer board, it should be born in mind that the gist of the present invention is applicable to corrugated fiberboard structures of any other types having three or more linerboards and two or more corrugated interlayer boards.

WHAT WE CLAIM IS: 1. An at least partially curved corrugated fiberboard structure comprising at least one pair of spaced parallel linerboards and at least one corrugated interlayer board interposed between the linerboards and bonded at the tops of its ridges to the respective inner faces of the linerboards, characterized by at least one portion having a curved cross-section and having a thickness smaller than the thickness of the remaining portion of the fiberboard structure, the thickness of said portion having the curved cross section being stepwise reduced from said remaining portion.
2. A corrugated fiberboard structure as set

forth in claim 1, in which the thickness of said portion having the curved cross section is substantially uniform throughout the area of the portion.
3. A corrugated fiberboard structure as set forth in claim 1, in which the thickness of said portion having the curved cross section is varied with the radius of curvature of said curved cross section.
4. A corrugated fiberboard structure as set forth in claim 1, in which the thickness of said portion having the curved cross section is within the range of from one ninth to five ninths of the thickness of the remaining portion.
5. A method of producing an at least partially curved corrugated fiberboard structure from an initially flat blank having at least one pair of spaced parallel linerboards and at least one corrugated interlayer board interposed between the linerboards and bonded at the tops of its ridges to the respective inner faces of the linerboard, comprising (1) inelastically com- pressing at least one predetermined portion of said blank to a thickness smaller than the thickness of the remaining portion of the blank by a stepwise reduction, and (2) inelastically deforming said predetermined portion to have a curved cross section.
6. A method as set forth in claim 5, in which said blank is compressed simultaneously while the blank is being deformed to have the curved cross section.
7. A method as set forth in claim 6, in which said blank is compressed by applying a mechanical force to the blank in a direction sub ,stantially normal to the faces of the initial blank.
8. A method as set forth in claim 7, in which said blank is pressed between two matingly shaped, respectively concave and convex surfaces spaced apart from each other to form therebetween a clearance which is configured conformingly to the desired configuration of the fiberboard structure to be obtained and which has at least one portion smaller than the remaining portion of the clearance, said predetermined portion of the blank being clamped in said reduced portion of the clearance.
9. A method as set forth in claim 7, in which said predetermined portion of the blank is compressed so that the thickness thereof is substantially uniform throughout the area of said predetermined portion.
10. A method as set forth in claim 7, in which said predetermined portion of the blank is compressed so that the thickness thereof is continuously varied with the radius of curvature of said curved cross section.
1lull. A method as set forth in claim 7, in which said predetermined portion of the blank is compressed so that the thickness thereof is within the range of from one ninth to five ninths of the thickness of said remaining portion.
12. A method as set forth in claim 10, in which the thickness of said predetermined portion is approximately one third of the thickness of said initially flat blank.
13. A method as set forth in claim 7, further comprising keeping said blank heated while said predetermined portion of the blank is being compressed and deformed.
14. An apparatus for producing an at least partially curved corrugated fiberboard structure from an initially flat blank having at least one pair od spaced parallel linerboards and at least one corrugated interlayer interposed between the linerboards and bonded at the tops of its ridges to the respective inner faces of the linerboards, comprising two rigid clamping members one of which has a partially recessed wall portion and the other of which has a partially protruded wall portion, the clamping members being movable relative to each other into and out of predetermined relative positions having their respective partially recessed and protruded wall portions located closest and in a face-to-face relationship to each other and forming therebetween a clearance havi.ng a configuration conforming to the desired con figuration of the fiberboard structure to be obtained, said clearance having at least one reduced portion which has a cross section curved from the remaining portion of the clear nance and which is smaller in size than said remaining portion, said reduced portion of said clearance being reduced stepwise from said remaining portion of the clearance.
15. An apparatus as set forth in claim 14, in which said reduced portion of said clearance is substantially uniformly sized throughout the area of the reduced portion.
16. An apparatus as set forth in claim 14, in which said reduced portion of said clearance has a size varying with the radius of curvature of the cross section of the reduced portion.
17. An apparatus as set forth in claim 14, in which said reduced portion of said clearance has a size within the range of between one ninth and five ninths of the size of the remaining portion of the clearance.
18. An apparatus as set forth in claim 17, in which said size of said reduced portion of said clearance is about one third of the size of the remaining portion of the clearance.
19. An apparatus as set forth in claim 14, further comprising heating means operative to keep each of said clamping members heated when the clamping members are in operation.
20. An at least partially curved corrugated fiberboard structure substantially as hereinbefore described with reference to and as illus traced in Fig. 4 of the accompanying drawings.
21. A method of producing an at least partially curved corrugated fiberboard structure from an initially flat blank, substantially as hereinbefore described with reference to and illustrated in Figs. 3 and 4 of the accompanying drawings.
22. An apparatus for producing an at least partially curved corrugated fiberboard structure from an initially fiat blank, substantially as hereinbefore described with reference to and illustrated in Fig. 3 of the accompanying drawings.